39Ar methods revisited for dating fine-grained K-bearing clay minerals

Chemical Geology 354 (2013) 163–185

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The K-Ar and 40Ar/39Ar methods revisited for dating fine-grained K-bearing clay minerals Norbert Clauer ⁎ Laboratoire d'Hydrologie et de Géochimie de Strasbourg, (CNRS-UdS), 1 Rue Blessig, 67084 Strasbourg, France

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Article history: Received 30 January 2013 Received in revised form 20 May 2013 Accepted 23 May 2013 Available online 7 June 2013 Editor: U. Brand Keywords: K-Ar and 40Ar/39Ar dating methods K-bearing clay minerals Burial and hydrothermal alterations Indirect dating of ore deposits Fault gouge dating 39 Ar recoil and step heating

a b s t r a c t The strengths and weaknesses of the two Ar isotopic methods (K–Ar and 40Ar/39Ar) were evaluated on the basis of respective recent applications mainly on low-temperature K-bearing illite-type clay minerals. This review includes a presentation of basic, analytical and technical aspects for both methods, as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization. Whenever possible, the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates. The comparative review examines stratigraphic dating of glauconites, indirect dating of low-temperature ore deposits, dating of burial-related illitization, and dating of polyphased tectono-thermal activity, more specifically of fault gouges. Some pending questions such as the necessary encapsulation due to 39Ar recoil and its restoration into step-heating patterns are also raised, together with the new potential of Ar-dating of nanometric illite crystals. Weakness of the K–Ar method is in its pioneering status that makes many believe that it is no longer accurate, because of its traditional analytical aspects, and of the K determinations leading to somewhat large uncertainties. However, precise evaluation of varied applications points to a K–Ar method having probably larger applicability in sedimentary to diagenetic environments than the 40Ar/39Ar method. The drawbacks become less important if the method is applied to nanometer-sized clay minerals in diagenetic to low-grade metamorphic environments. In this instance, the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations, if mathematically justified, can be reduced by duplicate analyses. Weakness of the 40Ar/39Ar method is in its basics such as the 39Ar recoil, the necessary encapsulation, the reintegration of the 39Ar into the step-heating patterns, and the meaning of the step-heating patterns that are more suggestive of variable 39Ar “reservoirs” created among the clay particles by irradiation than of meaningful geologic ages. If the K–Ar method is the preferred method for dating diagenetic clay processes such as glauconitization, illite crystal nucleation and growth, or low-temperature hydrothermal activities, then the 40Ar/39Ar method has more potential in dating low-temperature tectono-thermal activities, and in detailing mixtures of multi-generation illite. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles and basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Characteristics of the K–Ar method . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Characteristics of the 40Ar/39Ar method . . . . . . . . . . . . . . . . . . . . . . 2.3. The not-valid concern about Ar loss by clay particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Technical aspects of K–Ar and 40Ar/39Ar dating of clay material 3.1. Sample preparation, size fractionation and characterization of the separated fractions 3.2. Analytical aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ar dating of low-temperature minerals . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Glauconite dating and stratigraphic applications . . . . . . . . . . . . . . . . . . 4.2. Indirect dating of ore deposits . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Dating illitization during sediment burial and in overpressured hydrocarbon reservoirs 4.4. Dating of polyphased tectono-thermal events . . . . . . . . . . . . . . . . . . .

⁎ Tel.: +33 390 24 04 33, +33 680 01 80 49(mobile); fax: +33 390 24 04 02. E-mail address: [email protected]. 0009-2541/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2013.05.030

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5.

Ar tracing of low-temperature crystallization and deposition processes . . . . . . . 5.1. Natural and experimental alteration and weathering . . . . . . . . . . . . 5.2. Origin of deposition patterns in recent marine sediments . . . . . . . . . . 5.3. Burial illitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Ar isotopic information of crystallization processes . . . . . . . . . . . . . . . . 6.1. K–Ar dating of crystal nucleation and growth . . . . . . . . . . . . . . . . 6.2. K–Ar tracing of the regional extent of illitization . . . . . . . . . . . . . . 6.3. Ar dating of gouge clay minerals and tracing faulting activation and reactivation 7. A few more pending questions . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. The problem of sample encapsulation in the 40Ar/39Ar method . . . . . . . . 7.2. Restoring and modeling the 39Ar recoil . . . . . . . . . . . . . . . . . . . 8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The K–Ar (40K/40Ar) method is a pioneering dating technique based on the natural decay of 40K into 40Ar (e.g., Dalrymple and Lanphere, 1969; Faure, 1986). It has been applied soon to dating low-temperature, fine-grained illite-type separates (Wasserburg et al., 1956), and then routinely to various K-bearing clay types (e.g., Lipson, 1958; Hurley et al., 1960; Bailey et al., 1962; Perry, 1974). In addition to dating clay minerals, stratigraphic applications were also extended to zeolites (Bernat et al., 1970) and sedimentary alunite (Shanin et al., 1968; Bird et al., 1990). However, some authors raised interpretation problems that soon launched the still alive concern about preferential loss of radiogenic 40Ar due to particle size reduction with the smallest crystals being often the “youngest” in some pioneering studies (e.g., Amirkhanov et al., 1958; Obradovich, 1965). In this respect, the comment by Brown and Miller (1969): “Much still remains to be learned on the interpretation of isotopic ages, and the realization that the isotopic age is not necessarily the geologic age of a rock has led to an over-skeptical attitude by some field geologists” appears to be still valid except that skepticism extended meanwhile to other specialists of Earth Sciences. In fact, it has been shown since the '70s (e.g., Thompson and Hower, 1973) that unexpected low ages relative to decreasing grain size do not result from a preferential “mechanical” release of radiogenic 40Ar due to decreasing crystal size and correlative increasing crystallographic defects in the selected mineral fractions, but relate to variations in the mineral composition. Occurrence of proportionally more authigenic particles in progressively smaller size fractions is an obvious analytical reality that is visualized by the automatic reduction of the apparent age of mixtures with increasing authigenic/detrital ratios. K–Ar dating of nanometric (b0.02 μm) illite crystals with more radiogenic 40Ar than coarser micrometric (>0.1 μm) crystals confirmed analytically the lack of mechanical loss of 40Ar due to particle size decrease (Clauer et al., 1997). The 40Ar/39Ar method is a variant of the K–Ar method based on the transformation of 39K into 39Ar by fast neutrons generated in a nuclear reactor. It is routinely applied to coarse-grained plutonic and metamorphic mineral separates, and comparisons of its merits with those of the K–Ar method were examined soon (Dalrymple and Lanphere, 1971). Its fundamental aspects and analytical benefits have also been evaluated (McDougall and Harrison, 1999). The method has recognized practical advantages such as the measurement of the amounts of both the radioactive and radiogenic isotopes on the same aliquot. Beside this analytical advantage, which reduces and even avoids uncertainties due to potential sample heterogeneity, even if limited for small-sized clay-rich aliquots, a more convincing benefit is in its highly precise indirect determination of the K content by measuring the 39Ar produced by neutron irradiation of 39K. However, as will be

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detailed later, neutron irradiation has also technical side effects, namely 39Ar recoil due to neutron flow during irradiation, with a variable intensity depending mainly on the power of the neutron flow. The extent of the 39Ar recoil depends also on the size of the analyzed material, especially at the micrometric scale, which is precisely that of clay particles, and on the occurrence of defects in crystal structures (Villa, 1997), which is also somehow common in clay crystals. The K–Ar method is often overlooked today because of a general decreased interest for basic geochronological information, and of the incorrect assumption that its technical basics are outdated, which has been shown to not be the case any longer (Farley et al., 2013). These claims do not, anyway, prohibit its application to the study of mineral alteration in sedimentary basins and other low-temperature settings, as will be demonstrated in this comparative evaluation. The overall potential utility of the K–Ar method still goes far beyond the pioneering stratigraphic use, and it certainly needs to be revisited and compared to its 40Ar/39Ar “daughter” method in varied applications to smallsized, low-temperature K-bearing clay minerals. A review of the strengths and weaknesses of both Ar-based isotopic methods with an evaluation of their recent applications appears therefore timely. Recent advances in both methods will be described and persistent, decades-old myths regarding the K–Ar method will be rebutted. When possible, the explicit advantages and weaknesses of each method will be evaluated by comparing results obtained by both methods on the same mineral separates. Unfortunately such comparisons suggested by Kapusta et al. (1997) are still not numerous. 2. Principles and basics As for any isotopic dating method, closed-system behavior of the selected minerals is required for the two K–Ar and 40Ar/39Ar dating methods. This requirement is, of course, especially crucial for Ar, which is not bonded to other atoms or ions in a mineral structure like most of the classical cations of the basic isotopic dating methods, such as Sr, Nd, or Pb. Instead, Ar is only “trapped” mechanically in the mineral structures due to its larger radius than the radioactive 40K, thus making it particularly susceptible to loss by alteration changes related, or not, to temperature increase. The closed-system requirement is also critical, for the same reasons, in studying small, microto nanometer-sized clay crystals. In the case of K-rich high-temperature silicate minerals, such as micas, microcline, and even hornblende and pyroxene, the overall behavior of K relative to Ar is characterized by a cooling interval during which radiogenic 40Ar is not accumulated, regardless of the K content. This cooling interval generally considered to be short relative to the mineral's lifespan (e.g., Faure and Mensing, 2005) is typical for igneous or high-grade metamorphic minerals, while there is none for

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clay-type minerals. Clays never crystallize at a temperature, which would allow radiogenic 40Ar to be released during crystallization. However, that does not mean that temperatures above the retention point are not produced by further metamorphic and/or tectonic activity, but the materials are no longer clay minerals at those conditions. Thermally modified, they are micas with a reset K–Ar system (e.g., Clauer and Chaudhuri, 1995, 1998; Meunier et al., 2004). Another specific aspect of clay genesis is that these minerals often precipitate during slow, long-lasting and/or repetitive crystallization processes characterized by a progressive increase of K content, and therefore of radiogenic 40Ar. This can be the case during burial of sedimentary strata in a basin or during migration of mineralizing fluids through porous sedimentary sequences, representing an alternative to “instantaneous” thermally-induced crystallization, as is common during faulting activity for instance. The determined isotopic age corresponds then to an “integrated age” of the lasting crystallization process rather than to an instantaneous K accumulation episode with further decay into radiogenic 40Ar. The measured “age” could also result from successive events in the case of repetitive fluid-mineral interactions, and therefore repetitive crystallization episodes with potential K and radiogenic 40Ar gains and/or losses during each episode. The analytical result, whatever the uncertainty, does not correspond then to a strict “geological” age, but to an integrated, combined accumulation and/or release of the two isotopes in undetermined quantities. Long-lasting illitization, for instance, starts theoretically with mineral phases depleted in K, and therefore in radiogenic 40Ar, that scavenge progressively greater quantities of K decaying into radiogenic 40Ar. Consequently, the K–Ar “age” of a pure illite resulting from such a slow process will be theoretically closer to the end of the process than to the start, as the low K and radiogenic 40Ar contents of the initial mineral nuclei are overwhelmed by further accumulations of both. The paradox is, in this case, the use of an isotopic dating method set to generate ages of instantaneous events to evaluate lasting processes. The initial 40Ar/36Ar ratio during crystallization of any datable mineral is assumed to be identical to that of the present-day atmospheric ratio, ca. about 299 (Nier, 1950; Lee et al., 2006; Renne et al., 2009; Mark et al., 2011). This 299 value for the atmospheric 40Ar/36Ar ratio is a reasonable proxi as the secular variation of the atmospheric Ar-isotopic composition is not precisely known. The isotope 40Ar is not expected to be recycled on a geologically short-time scale, because it is chemically inert and, in contrast to its sister isotopes 36Ar and 38 Ar, it is primarily a product of the radioactive decay of 40K. In fact, the amount of the initial 40Ar, and consequently of 36Ar, is generally very low in K-rich minerals relative to the total accumulated amount of 40Ar. Uncertainty in the initial 40Ar/36Ar ratio has then only a minor-to-negligible influence on the age calculated for old to very old (>100 Ma) rocks or minerals. It can be sometimes higher than its atmospheric value of 299 in the case of low-temperature minerals, because of possible 40Ar supply by basin fluids in the range of 0.01–1.0 mg/g (Kendrick et al., 2002). Occurrence of such slight excesses in 40Ar were confirmed by the 40Ar/36Ar ratio of 305.1 ± 2.0 (2σ) determined in recent smectite-type clay minerals from the Galapagos Spreading Center in the Pacific Ocean (Clauer, 2006). 2.1. Characteristics of the K–Ar method A limited external supply of radiogenic 40Ar to nucleating crystals is not of a major concern in the final age calculation as most measured 40 Ar depends directly on the K content. In fact, the 40Ar/36Ar ratio of initially trapped Ar in the crystallizing mineral phases can be controlled by comparing the contents of the radiogenic 40Ar and K or 40 K (Harper, 1970), or by using isochron 40Ar/36Ar vs. 40K/36Ar patterns. In both cases, the initial of the arrays should cut either at the origin for contents of both K and radiogenic 40Ar, or at a 40Ar/36Ar ratio close to the atmospheric 299 value. Addition of non-radiogenic 40Ar to any mineral can, at least partly, be due to contamination by

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present-day atmospheric Ar adsorbed on the mineral surfaces during preparation, handling and purification. Adsorption of such atmospheric 40Ar decreases the accuracy of the generated data, but its effect can be reduced by preheating the minerals at 80–100 °C for several hours in vacuo before Ar extraction and analysis. The isochron approach is seldomly used for K–Ar isotope applications on K-rich clays, mainly because the 40Ar/36Ar and 40K/36Ar ratios are high to very high. This moves the data points away from the intercept of the isochron line with the abscissa of the diagram and, consequently, increases the analytical uncertainty of this initial value (Shaffiqullah and Damon, 1974). The paradox is that precise measurements need the best evaluation of the radiogenic 40Ar, which moves the data points away from intercept of an isochron line, and therefore deteriorates the analytical precision of its intercept. The other critical aspect of the use of isochrons is that it needs a precise determination of the atmospheric 40Ar that has to be deduced from measured 40Ar to obtain the amount of radiogenic 40Ar. Some studies reported isochrons with abnormally low or even negative intercept values (e.g., Langley, 1978). As minerals or rocks cannot yield negative initial 40Ar/36Ar ratios, the lines with such intercepts can only correspond to geologically meaningless mixing lines. Mussett and McCormack (1978) promoted the use of 3-dimensional plots that have the theoretical advantage of avoiding such assumptions. Harper (1970) also presented theoretical examples with negative intercepts from 40Ar contents relative to K contents, suggesting that such values may result from 40Ar loss irrespective of K contents. A likely cause for lines with negative intercepts in a 40Ar vs. K (40K) or a 40Ar/36Ar vs. 40K/36Ar isochron diagram is therefore either mechanical mixing of heterogeneous materials, or differential loss of 40Ar by minerals during recrystallization (see discussion in Clauer and Chaudhuri, 1995). 2.2. Characteristics of the 40Ar/39Ar method In addition to the practical advantage of measuring simultaneously the radioactive and radiogenic isotopes on the same aliquot with the same mass spectrometer, the 40Ar/39Ar method has the advantage of a highly precise, indirect determination of K by measuring 39Ar produced by neutron irradiation of 39K. This 39K isotope amounts to 93.26% of the whole K, while 40K, which is used for the calculation of the whole K in the case of the K–Ar method, represents only 0.01%. Another potential advantage of the 40Ar/39Ar method is in the extraction of 40Ar and 39Ar isotopes from a mineral by incremental stepwise heating, with assignments of potential individual ages for each temperature step. However, as suggested recently for illite materials, the interpretation of step-heating patterns is not as straightforward as claimed usually, because the heating steps do not necessarily outline meaningful ages for the given release temperatures (Clauer et al., 2012b). Harrison (1983) identified eight restrictions for the interpretation of step-heating diagrams: (1) multiple episodic loss, (2) slow cooling of the minerals, (3) mixed phases, (4) resolution of the age spectrum, (5) excess 40Ar, (6) recoil artifacts, (7) grain distribution and size, and (8) phase changes while in the vacuum extraction system. At least, five of these concerns (namely 1, 3, 4, 6 and 7) potentially apply to clay-material dating. The major limitations of dating clays are essentially related to the small size of the minerals that are often of different origin, which induces variable recoil of 39Ar, and therefore questionable age spectra. The total amount of this recoil needs, for instance, to be determined precisely for each analysis by encapsulating the samples, breaking the capsule under vacuum before starting the Ar extraction, and integrating the amount of recoiled 39Ar into the global amount of Ar released by fusion (e.g., Hess and Lippolt, 1986; Foland et al., 1992). Halliday (1978), Kunk and Brusewitz (1987), and many others have identified and documented the amount of 39Ar recoil relative to the particle-size of clay minerals. The intimate mechanism responsible for this recoil has not yet been clearly explained, but relates to the

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impact of neutrons on the 39K and its subsequent displacement (= recoil) within or out of the mineral structure. What is clear, is that its extent depends on the intensity of the irradiation, the size of the irradiated clay material and, in the case of in situ determinations by laser ablation (Kelley, 1995), the type of the studied rock. Application of the 40Ar/39Ar method to clay material includes also necessary corrections for the production of 39Ar from neutron irradiation of 42Ca (when present in the analyzed aliquot) that needs to be deduced from total 39Ar measured, knowledge of the neutron flux with necessary measurement of CaF2 and K2SO4 salts that have to be prepared, irradiated and analyzed for correction calculations. In summary, the 40 Ar/39Ar method also has various technical inconveniences and analytical uncertainties. 2.3. The not-valid concern about Ar loss by clay particles The conventional wisdom that radiogenic 40Ar is preferentially lost by clay-type particles when their size decreases, irrespective of K content, results from early studies (e.g., Hurley et al., 1960, 1963; Obradovich, 1965) that often showed decreasing K–Ar ages of glauconite-type separates relative to their size. They were obtained during pioneering applications of the K–Ar method to clay material, when knowledge of the mineralogy, crystallography, genesis mechanisms of the selected material, and when the suitability of sample preparation and isotopic dating methods were in their infancy. If Ar retention does not apply to small clay-type mineral particles, all the results published during about three decades by both the K–Ar and 40Ar/39Ar methods on fine-grained clay materials would have to be trashed, which seems not to be the case from what will be discussed below. The assertion that no inert mechanical Ar release occurs with decreasing grain size is based on studies on natural clay material and laboratory experiments that critically evaluated the Ar behavior in small mineral crystals. It has been shown that in separated, naturally occurring nanometric illite crystals from bentonites, the smallest crystals with sizes of b 0.02 μm in the a-b crystallographic dimension and of 0.002 μm in the c dimension are often older than the coarser crystals that grew from these small nuclei (e.g., Clauer et al., 1997; Honty et al., 2004; Środoń et al., 2006). The resulting age differences in the nanometric fractions cannot be due to detrital contaminants as commonly argued, because the crystals were extracted from altered volcanic ash devoid of detrital components. In fact, older ages in the smallest crystals are not universal; they depend, in fact, on the illitization process that modified progressively the initial smectite of the bentonites into illite (Clauer, 2006). However, relative to this issue of increased “inert” loss of radiogenic 40 Ar with decreasing crystal size, some attention must be directed to possible “active” Ar release from clay crystals. Such loss may be induced by any post-crystallization temperature increase of any origin (burial, tectonic, metamorphic) that is potentially capable of modifying the previously crystallized crystal. Any temperature increase beyond crystallization temperature impacts the Ar contents of the authigenic as well as of the detrital particles, and therefore decreases the initial age of both. This decrease and possible change in the K contents correlates with the size of the mineral crystals, and with the size of the chemical system in which the change occurred. The smaller the crystals, the more visible will be the mineralogical and chemical changes, with more extensive changes expected in open chemical systems due to increased interactions with external fluids.

of clay size, needs to be based on a well-controlled and reproducible mineral separation. These aspects have already been discussed in length (e.g., Clauer and Chaudhuri, 1995; Clauer and Lerman, 2012); they are applicable to all isotopic studies of clay material, with a special attention to the impact of varied crushing methods on the final data. The impact of crushing coarse framework minerals of rocks into small clay-sized grains was examined by Liewig et al. (1987) who compared K–Ar ages of clay fractions from the same sandstones after classical crushing and after freezing-thawing disaggregation (Fig. 1). The diagram shows a clear age increase related to increasing feldspar content in the fractions obtained from rock crushing, which is lacking after freezing-thawing the same rock samples. Identical results were published recently on similar sandstones (Clauer and Liewig, accepted for publication). Ultrasonic disaggregation represents also an improved alternative to grinding, but complementary experiments are needed to control the effect of the power of the sonic probe or bath, and which duration of the disaggregation is the most appropriate. In order to evaluate best the potential of the Ar methods applied to fine minerals from Earth surface and sub-surface environments, some basics of material separation are reiterated here, as they are different from techniques applied to coarse-grained mineral separates. Granulometric fractionation is the most widely used separation method for clay material from any type of sedimentary or low-grade metasedimentary rocks, including gravity sedimentation in deionized water and ultra-centrifugation. It is inherent to these routine techniques that the coarser the size of the separated clay fractions, the higher are the chances for occurrence of detrital, contaminant grains that have the potential to bias the isotopic results (e.g., discussion in Clauer and Chaudhuri, 1995). As just discussed, grinding initial framework grains such as micas and K-feldspars produces artificial mixtures of clay-sized fractions with “artificial” ages; the examples are numerous in the literature (e.g., Reuter, 1987; Glasmann, 1992; Matthews et al., 1994). Alternatively, the presence of other minerals than clays in clay-rich size fractions does not automatically mean that the measured isotopic age is biased: feldspar, for instance, can form in low-grade metamorphic conditions concomitantly with clay minerals, as clay-sized crystals. In fact, age biases occur only if the fine contaminants are representative of coarser detrital minerals, and the potential for incorporating such ground detrital grains is greater when analyzing coarse size fractions (b 2 μm) from ground whole rocks, than finer sub-fractions (b0.1 μm or even smaller) from disaggregated rocks. From analytical point of view, with a systematic separation of very fine clay fractions (in the tenth or hundredth of micrometer range), the challenge becomes realistic that the smallest nanometric crystals represent nucleating crystals and the coarser the growing equivalents (Nadeau et al., 1984). A

3. Technical aspects of K–Ar and 40Ar/39Ar dating of clay material 3.1. Sample preparation, size fractionation and characterization of the separated fractions A relevant characterization of fine-grained clay-type minerals, mainly illite or glauconite in the present case but also alunite or zeolites

Fig. 1. K–Ar ages of clay-sized fractions depending on the sample preparation, grinding or freezing-thawing, and therefore on the K-feldspar/illite ratio (after Liewig et al., 1987).

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continuous growth at this scale theoretically produces younger ages for the coarser crystals, and older for the smaller crystals that stopped growing earlier. This concept is based on the crystal-growth theory of Eberl et al. (1998), which predicts that the growth rate is proportional to crystal size, indicating that the coarser fractions may be younger than the finer because they add new material at a faster rate than the finer crystals. This was confirmed by the first K–Ar ages of bentonite beds from the East-Slovak Basin, but not in the nearby Zempleni Mountains where illite crystallized rapidly from hot hydrothermal fluids (Clauer et al., 1997). X-ray diffraction (XRD; e.g., Bailey, 1980; Brindley and Brown, 1984; Moore and Reynolds, 1997) techniques for identification and quantification of minerals present in datable size fractions are probably more critical for establishing the similarity of clay mineral assemblages than for other minerals. The illite crystallinity index (Kish, 1991; Kübler, 1997), and polytype identification (Bailey, 1988), also obtained by XRD analysis, may be equally helpful. As most of the conversion from smectite- to illite-type minerals occurs in the diagenetic domain and during the transition into low-grade metamorphism, peak decomposition (e.g., Lanson and Besson, 1992) or structure simulation (Reynolds, 1985), as well as precise XRD peak locations (Środón, 1984) also represent improved tools for identification of clay types. Lastly, XRD helps to identify and even quantify potential non-clay contaminants in separates. Volume-weighted and area-weighted mean thicknesses of the finer crystals can be calculated, and the crystal growth reaction paths evaluated by “Mudmaster” and other simulation programs (Eberl et al., 1998). The 2 M1 illite polytype is often, if not always, assigned to a detrital origin and the 1 M polytype to an authigenic origin in sedimentary sequences, and it is therefore often claimed that discrimination of the polytypes (polymorphic types produced by stacking sequence variations) of illite is of critical importance in isotopic dating of clay-sized fractions (e.g., Velde, 1985). Pevear (1999) published a model on this assumption that he combined with K–Ar ages, reporting a positive correlation between K–Ar ages and 2 M illite amounts. This correlation indicates that higher 2 M illite contents increase the bias with the age expected for the authigenic illite. In fact Pevear's (1999) correlation resembles the correlation published long before by Hunziker et al. (1986) on illite from Glarus region in the Alps that they interpreted similarly, and that by Bechtel et al. (1999) on illite from Kupferschiefer units in Germany and Poland. Conversely, Clauer and Liewig (accepted for publication) showed recently that the theory of 2 M illite being of systematic detrital origin in diagenetic-to-hydrothermal conditions does not apply automatically, as 2 M illite polytype can also crystallize in high pressure-high temperature (HP-HT) oil-bearing reservoirs at temperatures slightly above 125 °C. Such low-temperature crystallization of 2 M illite was confirmed by its synthesis at room temperature, together with 1 M illite (Flehmig, 1992). Isotopic dating of clay crystals also needs examination of their morphology and typology by scanning (SEM) and transmission (TEM) electron microscopy, and comparison of the observed shapes with the XRD data. Studies of metasedimentary rocks have shown that cleavages, for instance, dissect rocks and isolate microlithon volumes consisting of variable framework components that can be of variable size and origin. Dating the newly formed sheet silicates of such volumes can give valuable insights into the thermo-structural dynamics of the host rocks, as they are subjected to combined heat and deformation effects. However, such attempts are realistic only if the smallest sheets of the cleavages can be isolated without initial contaminating micas of the microlithons or the cleavage planes. Such sorting requires careful SEM observation of rock chips and size-fraction separates for selection of the best-suited samples and/or separation of the appropriate size fractions to be analyzed, such as in recent studies of gouge minerals (Zwingmann et al., 2011; Haines and Van der Pluijm, 2012). Straight particle edges are often considered to be typical for authigenic sheet silicates, whereas irregular edges are rather identifying

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detrital particles variously affected by dissolution and erosional processes (e.g., Hunziker et al., 1986 and many others since). In summary, an appropriate separation and characterization of clay-rich size fractions is not absolute mandatory, but a precise identification of the selected size fractions definitely helps to determine how clay minerals were generated in the rocks, which will strengthen the interpretation of their isotopic data, and that whatever the dating method. 3.2. Analytical aspects Even if never clearly stated, many claims that the K–Ar method is outdated point to the fact that the analytical uncertainty is suspectedly large, far more than of the 40Ar/39Ar method. In fact, these comments generally miss the real meaning of the errors generated by measuring isotopic ages. If K–Ar age calculations provide larger (= less precise) errors than the 40Ar/39Ar method, this uncertainty is a mathematical and a technical aspect of the analytical procedure that has not necessarily a determinate impact on the geologic reliability of the ages. On the other hand, Ludwig (2003) stated that: “The uncertainty of a date is as important as the date itself”. This divergence raises the problem of how each investigator solves the precision problem of the data he releases and the accuracy problem of the equipment he uses. There are varied ways to address these aspects, and the purpose here is not to evaluate or rate them, but to recall some analytical and mathematical aspects for both methods. One might, for instance, agree that a K–Ar age of 100 ± 5 (2σ) Ma is not geologically less accurate than a 40Ar/39Ar age of 100 ±0.5 (2σ) Ma, which clearly raises the duality between precision and accuracy of geochronological data that was clearly and precisely discussed recently by Schoene et al. (2013). In combining both for the two methods concerned here, one might agree that if the K–Ar method is less precise than the 40Ar/39Ar method, it is not necessarily less accurate. In the theoretical example given above, the errors strictly mean that if a duplicate analysis is made on the same powder aliquot, the new result has to fit within either ± 5% of the previous result by the former method, or ±0.5% of the previous age by the latter method. This could probably be controlled more often by duplicate analyses. The precision (= uncertainty) is then not to be related with the geological meaning of the “absolute” 100-Ma age, which alternatively also depends on how the selected sample is representative of the dated event. In other words, the most important aspect of any analytical determination is probably more in how well the selected fractions represent the physical and chemical conditions that prevailed at the time the authigenic mineral phase crystallized, and in the reproducibility and accuracy of the extraction and separation procedures (e.g., Engels and Ingamells, 1970) then in the calculated laboratory error. This aspect was addressed by Clauer et al. (1992a) who pointed to the impact investigators may have on results depending on the selected techniques used for isotopic dating of low-temperature minerals. Two decades later this concern is still valid, keeping also in mind that even applying the best-suited methods to produce geologically meaningful ages, lithological and mineralogical characteristics may be such that the dating attempt can be misleading. If authigenic mineral phases cannot be insolated, even by separating size fractions at the nanometer scale, direct dating of a specific clay mineral may not be possible, which is still the case for shales (Clauer et al., 1997). In situ 40Ar/39Ar dating with a laser microprobe or of separated grains may also not be successful either, in terms of geological meaningful ages (e.g., Bell, 1985; Bray et al., 1987; Smith et al., 1993). In addition to the constraints imposed by mineral preparation and separation, undisputable control of the internal reproducibility of the Ar analytical procedures includes: (1) periodical runs of international standard minerals that are also measured in other laboratories, (2) random runs of duplicates of some of the studied samples, and (3) periodical determination of the blank of the extraction line and of the mass spectrometer. As an example, random duplicate analyses taken from recently published K–Ar data of clay minerals of varied ages show that the

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Table 1 K–Ar results of samples of varied origin and of the duplicate analyses (from Clauer, 2011; Nierhoff et al., 2011; Surace et al., 2011). The differences between initial analysis and duplicates are evaluated in % and the 2σ errors of each analysis are also given in % of the determined age.

From Clauer (2011) C18 (30 mn) Duplicate C18 (96 h) Duplicate C18 (45 mn) Duplicate C18 (45 mn) Duplicate C18 (45 mn) Duplicate From Nierhoff et al. (2011) E9 (FT) — 1.0–2.0 Duplicate E10 (RD) — b2 Duplicate E33 (FT) — b0.4 Duplicate B10 (FT) —b0.4 Duplicate E89 (FT) — 0.4–1.0 Duplicate From Surace et al. (2011) C74 Duplicate C17 Duplicate C20 Duplicate C44 Duplicate

40

40

K2 O

radio

Age in Ma

Δ in duplicate

2σ

(%)

(%)

(10−6 cm3/g)

(±2σ)

(%)

(%)

5.56

96.1 95.9 96.7 94.8 59.73 29.29 81.82 79.69 85.09 76.42

97.84 97.23 108.70 110.32 1.21 1.19 4.37 4.59 14.45 14.94

477.0 474.4 460.5 466.5 60.3 59.4 54.3 57.7 122.8 126.8

(10.7) (10.6) (10.1) (10.4) (2.8) (4.6) (1.5) (2.3) (3.2) (3.6)

95.68 97.91 94.65 97.53 96.21 94.42 87.90 88.83 97.21 92.84

73.69 74.65 71.94 70.82 77.30 71.60 78.35 77.85 82.91 73.75

428.3 433.3 331.2 326.3 294.4 303.7 335.2 349.0 350.5 336.1

(9.8) (9.7) (8.8) (7.1) (6.4) (6.8) (8.0) (8.2) (7.6) (7.6)

45.4 45.8 25.4 23.6 73.5 30.5 9.2 6.0

0.98 0.99 1.29 1.37 2.77 2.60 0.80 0.81

16.3 16.5 9.2 9.7 12.5 12.5 6.6 6.7

(0.6) (0.6) (0.2) (0.2) (0.2) (0.2) (0.2) (0.2)

6.43 0.61 2.46 3.53

4.73 6.14 7.49 6.72 6.59 6.28 6.64 6.19 1.86 4.37 6.86 3.74

Ar

reproducibility is better than 2.2% for about half of the determinations, therefore below the 2.5–3% uncertainty that is routinely given in the publications. This does not distract from two values of twenty-eight being higher than 4.0% (Table 1), while more than half of the duplicate analyses fall below 3% reproducibility. In fact, half of the reproducibility data are lower than half of the routine uncertainty value. Another aspect that shall be mentioned briefly is how representative are the international standards periodically measured in each laboratory for control of both short-term precision and/or long-term accuracy. The purpose is not to debate if such or such widely used mineral is best appropriate, or if natural minerals are the best suited because of their potential mineral heterogeneity. The international standard “Glauconite-Odin” (GL-O), which was questioned in several circumstances for its possible heterogeneity, was analyzed continuously at the Centre de Géochimie de la Surface at Strasbourg, France (CNRS/Strasbourg University) for several decades, because the laboratory was involved in its initial testing (Odin et al., 1982), and because it appeared to be the most appropriate available standard for a laboratory mainly dedicated in isotope dating and tracing to sedimentary clay-type minerals. During the last decade, about thirty publications were released from this place, all mentioning average values of several measurements of GL-O during the successive studies. About one hundred and forty aliquots were measured for their 40 Ar content, ranging from 24.38 to 24.86 × 10−6 cm3/g with an average of 24.62 ± 0.24 × 10−6 cm3/g (2σ), which represents a long-term precision better than 2%, and an accuracy better than 1% relative to the mean value set at 24.85 ± 0.24 × 10−6 cm3/g (2σ). 4. Ar dating of low-temperature minerals 4.1. Glauconite dating and stratigraphic applications The critical aspect of any dating attempt is the isotopic homogeneity of the selected minerals or mineral mixtures. In this respect,

radio

Ar

0.5 1.3 1.5 6.3 3.3

1.2 1.5 3.2 4.1 4.2

1.2 5.4 0 1.2

2.2 2.2 2.2 2.2 4.6 7.7 2.8 4.0 2.6 2.8 2.3 2.2 2.7 2.2 2.2 2.2 2.4 2.3 2.2 2.3 3.7 3.6 2.2 2.1 1.6 1.6 3.0 3.0

glauconite appeared soon to be the most appropriate candidate for stratigraphic attempts, because it is easily recognizable by its green color and pellet shape, and because it progressively approaches mineral and chemical homogeneity during exposure to seawater on the marine seafloor (e.g., Obradovich, 1965). However, this application was variably successful as the obtained ages was not systematically of stratigraphic use. Very early, Burst (1958a, 1958b) raised the problem of crystal heterogeneity in glauconite grains used for stratigraphic application, followed by Sardarov (1963) who questioned the preservation of the radiogenic 40Ar in the grains. The extensive study of glauconite as a stratigraphic chronometer by Odin and collaborators (e.g., Odin and Matter, 1981; Odin, 1982b) as well as by others (e.g., Montag and Seidemann, 1981; Harris et al., 1984; Smith et al., 1993), was a determining step not only in explaining why some of the ages were off the stratigraphic expectations, but also in decrypting its genesis. For instance, Odin (1982a,b) and others showed that glauconite becomes a reliable stratigraphic record only when the pellets have a smooth surface with the highest possible K content, that is to say more than 6 wt.% K (Odin and Matter, 1981; Odin and Dodson, 1982). Clauer et al. (1992b) and Stille and Clauer (1994) completed the description of the glauconite genesis by investigating the isotopic evolution of progressively glauconized detrital smectite in recent West African off shore sediments. They found that: (1) initial detrital Fe-rich smectite incorporated progressively K from marine environment, and (2) accumulation of radiogenic 40Ar started only when the glauconitic grains were in isotopic equilibrium with the environment. One of the most convincing examples of glauconite stratigraphic dating is probably the Rb–Sr and K–Ar investigation by Odin and Hunziker (1982) on glauconite separates from the Albian and Cenomanian of Normandy (France). They obtained a K–Ar isochron age of 98.6 ± 2.1 Ma for Albian glauconite fractions, and K–Ar and Rb-Sr isochron ages of 93.0 ± 1.4 and 93.5 ± 1.6 Ma, respectively, for Cenomanian glauconite fractions. A similar result was reported

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by Rousset et al. (2004) on Albian glauconite of southeastern France at 97.9 ± 0.4 and 97.9 ± 3.5 Ma for K–Ar and Rb–Sr isochron data, respectively. These K–Ar and Rb–Sr isochron ages preclude any further pervasive recrystallization during burial of the sequence either by heat flux or fluid flow, as the two radiogenic isotopes (40Ar and 87 Sr) behave differently during such processes. It also rules out any detectable grain heterogeneity, at least any visible impact of detrital initial grains on the determined ages. 40 Ar/39Ar studies of glauconite separates reported systematic recoil 39 of Ar due to irradiation (Yanase et al., 1975; Brereton et al., 1976; Foland et al., 1984, 1992; Klay and Jessberger, 1984; Smith et al., 1993, 1998). Interestingly, glauconite is as highly crystalline as illite, but it forms at low temperature in marine environments (b 40 °C) and contains Fe2+, which induces crystallographic defects, whereas Al3+-rich illite precipitates preferentially at higher temperature (>75 °C). In addition to potential crystallographic defects, the low-temperature genesis may be another deleterious characteristic that needs to be taken into account when irradiating glauconite pellets in a nuclear reactor at a temperature necessarily higher than the crystallization temperature of the mineral. Not only 39Ar may be recoiled but radiogenic 40Ar may be released as well, as reported by Jourdan et al. (2007) for coarse-grained sanidine, plagioclase and biotite. Beyond these technical aspects, Smith et al. (1993) showed that 40Ar/39Ar and K–Ar ages and the associated errors (Table 2) obtained from encapsulated individual glauconite pellets fused by laser microprobe compare well with those of a large number of pellets from the same samples. They also demonstrated that encapsulation is necessary, and they illustrated natural heterogeneity of individual extracted glauconite pellets. Ar-age heterogeneities of both methods point to the naturally erratic evolution of glauconite grains in the marine environment as suggested by Odin and his collaborators (e.g., Odin, 1982b), rather then to analytical artifacts. This sample heterogeneity being inherent to low-temperature clay-type minerals, the problem is then its extent relative to the analytical uncertainty in order to set reliable limits to the numerical ages and their geological meaning. Theoretically, it will decrease when the amount of analyzed pellets increases. In summary, variable glauconite ages, either older or younger than the expected stratigraphic reference, clearly depend on the purity of the glauconite pellets, as shown again by Clauer et al. (2005). Unexpected K–Ar ages for glauconite are not due to the use of the Ar methods. As is the case for other minerals in other environments, they are rather due to inappropriate grain separation and mineral characterization than to analytical problems. Expected or not, they appear very useful either in providing stratigraphic ages, identifying incomplete glauconitization, or illustrating further diagenetic recrystallization. Application of the 40 Ar/39Ar method to glauconite material by individual grain laser probe fusing has the advantage of introducing supplementary selection criterions, namely determining the degree of glauconitization of

Table 2 Comparison of glauconite K–Ar data by Odin (1982a,b) and 40Ar/39Ar data by Smith et al. (1993) on the same mineral fractions. Sample #

Irradiation

K–Ar (±Ma)

40

132a

Air In vacuo Air In vacuo In vacuo Air In vacuo Air In vacuo Air

40.8 (1.2)

60.4 41.6 60.5 40.9 37.6 84.4 58.1 113.9 83.7 147.2 155.8 132.7 92.1 138.9 442.5

234a 49 L 110a 553a GL-Ol GL-Od 482a 582a

Air In vacuo In vacuo In vacuo

44.2 (0.7) 44.4 (1.2) 52.6 (1.2) 89.8 (3.6) 95.0 (1.1) 95.0 (1.1) 131 (4) 473 (17)

Ar/39Ar (±Ma) (1.9) (1.2) (0.9) (1.3) (0.2) (4.6) (3.1) (0.9) (1.1) (4.8) (5.7) (4.3) (1.0) (0.7) (1.6)

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individual pellets, as well as distinguishing between incomplete evolution, varied deposition genesis and potential recrystallization effects. 4.2. Indirect dating of ore deposits Chronological information on sediment-hosted ore deposits is essential for the reconstruction of their genesis and further evolution. The direct dating of ore minerals such as sulfides or oxides remains a challenge by routine dating methods because of both the technical difficulty of measuring very small amounts of radiogenic isotopes due to very low parent to daughter element ratios, and the limited knowledge of some of the initial isotopic compositions of the host minerals. Uranium minerals are among the few ore minerals known to contain significant amounts of radiogenic Pb isotopes, but they are also known to lose frequently intermediate isotopes of the uranium decay chain when exposed to near surface conditions. These shortcomings launched attempts to develop other approaches based on authigenic minerals associated with the ores, or alternative methods, such as the Sm-Nd method (Maas, 1989; Chesley et al., 1991; Turpin et al., 1991). In recent years, dating of ores and reconstruction of their evolution were often complemented by isotopic dating of varied associated minerals, especially clays, from rock volumes hosting the ores that potentially recorded disturbance episodes related to ore emplacement. The K–Ar and 40Ar/39Ar methods were applied since the 70s' on authigenic clays and feldspars that crystallized in connection with ore deposition (Ineson and Mitchell, 1972; Pop et al., 1980; Hearn and Sutter, 1985), but only few studies have been published since, except on clay minerals associated with varied uranium deposits in Canada and Australia (Gustafson and Curtis, 1983; Clauer et al., 1985; Brockamp et al., 1987; Kotzer and Kyser, 1995; Kyser et al., 2000; Laverret et al., 2010). Alteration halos enriched in illite were described around uranium ores in the Athabasca and Thelon deposits of Canada (Percival et al., 1993; Renac et al., 2002; Quirt, 2003; Laverret et al., 2006) and in the McArthur basin of Australia (Patrier et al., 2003). These halos are supposed to have recorded the whole history of the host rocks (Beaufort et al., 2005), and therefore to reveal potential benchmarks in the complicated evolution of Proterozoic uranium deposits. Although clay minerals are common in and around unconformitytype uranium deposits, their use as tracers is not unequivocal, mainly because they can be of polygenic origin. It is then difficult to sort them out on the basis of size separation only, because they may result from burial-related interactions that were not necessarily involved in the ore genesis, or from circulating hydrothermal fluids that generated the ores, along unconformities for instance. The current ore-genesis model of uranium deposits is conceptualized as deposition by local basement fluids entering the cover sediments during larger migration cycles (e.g., Kotzer and Kyser, 1995; Derome et al., 2005; Kister et al., 2006). A K–Ar and δ18O isotopic study supported by a mineralogical investigation of illite separates from the sandstone cover and underlying basement rocks close to and distant from Shea Creek uranium deposit (Athabasca Basin, Canada) illustrates this concept (Laverret et al., 2010). In the barren sediments and basement away from deposit, illite occurs mainly as coarse-grained lath-shaped particles of the cis-vacant 1 M polytype, while fine-grained particles of the transvacant 1 M type were observed next to and in the uranium mineralized strata. The tectonic-induced hydrothermal activity that contributed to illite crystallization was multi-episodic at about 1450 Ma (1455 ± 9 and 1451 ± 11 Ma), 1350 Ma (1352 ± 17 Ma) and 1235 Ma (1234 ± 18 Ma), on the basis of isochron plot calculations (Fig. 2A and B). The illite crystallization episodes occurred contemporaneously with the tectono-thermal activity responsible for the concentration of the associated uranium oxides, which were dated independently by the U–Pb method in the Shea Creek deposits and elsewhere in the Athabasca Basin (Cumming and Krstic, 1992; Alexandre and Kyser, 2003).

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Fig. 2. A: K–Ar isochron diagram with the data points of the illite sepatates from size fractions of altered basement rocks fitting two isochrons. B: K–Ar isochron diagram with the data points of the illite separates from inner-halo sandstones giving two isochrons (after Laverret et al., 2010).

The lack of a relationship between illite polytypes and crystallization ages is a new contribution as it indicates that precipitation of the cis-vacant or trans-vacant illite did not relate to a specific event, but rather to varied physical and chemical crystallization conditions during the same event, depending on the physical properties of the host rocks close and far from uranium precipitation, and on the chemical characteristics of the interactive fluids. The change in contemporaneous illite polytypes (Laverret et al., 2006, 2010) relates to an increasing δ18O with distance from the uranium concentrations, which probably reports a progressively decreasing crystallization temperature, combined with a changing δ18O of the interstitial fluids due to variable water–rock interactions in the rocks. The cis-vacant 1 M polytype crystallized at slightly lower temperatures and away from uranium concentrations than the equivalent trans-vacant 1 M polytype identified next to the uranium ores. Both precipitated contemporaneously within analytical uncertainty. Initial results of in situ laser 40Ar/39Ar dating of illite particles from thin sections of uranium hosting rocks showed highly scattered results, even greater than the corresponding K–Ar data of clay particles extracted from the same rock samples (Bell, 1985; Bray et al., 1987), that may result from uncontrolled 39Ar recoil loss due to the in situ technique. Scattered loss due to recoil was confirmed by Alexandre et al. (2009) on clay material of Athabasca unconformity-type uranium deposits in Canada, for which the 40Ar/39Ar methodology is very detailed. However, the report lacks a specific description of the encapsulation technique, which may have some importance as will be discussed in a later section. The mineralogical description of the analyzed illite was also approximate as no specific XRD data could relate it to probable occurrence of high-temperature sericite or muscovite. The mineral was just identified to be of “pre-ore or syn-ore alteration” origin, without supporting arguments except the ages (Alexandre et al., 2009). Also, disturbed step-heating age spectra, either hat-shaped

or indicating superficial radiogenic 40Ar losses, do not satisfy current concerns about the limitations of illite step-heating patterns (Clauer et al., 2012b). The problem becomes then whether data interpretations should rely on cumulative probability diagrams of the various 40Ar/ 39 Ar ages calculated using the individual step ages and the error margins, or on precise mineralogical and petrographical analyses and observations. 4.3. Dating illitization during sediment burial and in overpressured hydrocarbon reservoirs Since the mid-1960s, illitization was used extensively for recording the thermal evolution of sedimentary basins. It is generally considered to occur continuously, mainly driven by temperature increase and interaction with interstitial fluids during progressive burial. However, despite recent progress in evaluating the duration and understanding of illitization in sedimentary sequences, there is still a debate about how to quantify its progress and detail its timing, because the available isotopic ages are often obscured by variable amounts of detrital minerals in the size fractions (e.g., Aronson and Hower, 1976; Burley and Flisch, 1989; Ehrenberg and Nadeau, 1989; Glasmann et al., 1989; Clauer et al., 1999). The available illitization models of a dissolution-precipitation or a solid-state alteration (e.g., Altaner and Ylagan, 1997) are basically not helpful for evaluating timing and duration. An alternative approach to routine analysis for extraction of timing and duration of illitization is to model both. Except for a few successful applications (e.g., Elliott et al., 1991), convincing proposals are still lacking. In fact, it seems difficult to formulate a general model, because the illitization extent and intensity depend on the specific evolution of each individual basin, with results potentially varying from basin to basin (e.g., Środoń and Eberl, 1987). Not much has

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171

Fig. 3. A: K–Ar isochron diagram of the b0.4 μm size fraction from cores of the northern North Sea. The numbers of the data points refer to the sample numbers in Table 2 of Clauer and Liewig (accepted for publication). B: K–Ar isochron diagram of the b0.4 μm size fractions from the samples sampled in the central North Sea about 500–600 km away (after Clauer and Liewig, accepted for publication). The light gray dots are representative of data points off the isochrons.

been published on age records of illitization at basin scale that integrate the location and depth of the studied material, the changing impact of the chemical composition and temperature of the interacting fluids, the thermal heat flux, and the sedimentation rate of the sediments. The only case showing variable illitization intensity and timing within a single basin appears to be the recent description of illitization in bentonite beds of the East Slovak Basin (Clauer et al., accepted for publication-b). Understanding the impact of increasing pressure on illitization still needs to be improved in clastic reservoirs of structural basins, such as of the North Sea, that has variable and often anomalous high pressures together with high porosities relative to their present-day depth of burial. Among the potential causes of pressure increase in reservoirs is the volume increase due to gas generation from source rocks, clay dehydration, and thermal cracking of the oil (Isaksen, 2000). Growth of cements, especially quartz overgrowth, in pore spaces of sediments is most often cited to generate overpressure as a function of temperature and associated with a net reduction in pore volume, if the fluids remain trapped (e.g., Bjørkum, 1996; Bjørkum and Nadeau, 1998, for many North-Sea reservoirs). Illitization is also supposed to induce geopressure (Kerr and Barrington, 1961; Bruce, 1984; Gautier et al., 1987; Freed and Peacor, 1989; Nadeau et al., 2002), but controversial studies report that it could have no significant impact (Bethke et al., 1988; Giles et al., 1998; Swarbrick et al., 2000).

Fig. 4. Backstripping of burial vs. time in three wells (C1, D1 and E1) from the central North Sea estimated from vitrinite reflectance and modeled pressure change vs. time. The temperature variation relative to pressure variations is shown for well C1. The K–Ar ages obtained on the fine illite fractions of the sandstones in Fig. 3 are reproduced in the diagrams (after Clauer and Liewig, accepted for publication).

When clay-particle size increases in samples of overpressured reservoirs from the Brent and Fulmar formations of respectively the northern and central North Sea, illite/smectite mixed-layers and minor chlorite are mixed with increasing kaolinite (Clauer and Liewig, accepted for publication). Long illite laths or fibers were observed in both formations, mixed with euhedral kaolinite particles, irregular and electron-dense grains of probable detrital origin, or rounded grains from which small authigenic laths protrude. The K–Ar data of the b 0.4 μm fractions of the northern Brent samples plot on two isochrons in a conventional

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Fig. 5. Isochrons of illite from Rotliegend (Permian) sandstones of northern Germany. The light gray symbols belong to the upper isochron and the dark gray to the lower isochron, whereas the white symbols are off the two lines. (After Clauer et al., 2012a.)

diagram with ages of 76.5 ± 4.2 and 40.0 ± 1.5 Ma and initial 40Ar/36Ar ratios close to the atmospheric value. The data points of the same b0.4 μm fractions of the Fulmar Formation located about 550–600 km to the south in the Viking Graben, also fit two isochrons with close to identical ages at 76.6 ± 1.4 and 47.9 ± 0.5 Ma (Fig. 3A and B). Most of the coarser fractions (>0.4 μm) plot above the two isochrons indicating the presence of detrital components. The two dated episodes of illitization relate to pressure increase in the reservoirs (Fig. 4), which is indirectly supported by different K–Ar results from a reservoir at hydrostatic pressure. In the diagrams that relate the pressure changes in the reservoirs with either temperature or depth, the timing of illitization given by the K–Ar ages is close to the estimated timing of the pressure changes in the reservoirs that were calculated independently. In both the northern and central North Sea, the K–Ar ages of the older illite generation remain unaffected after crystallization despite continued temperature increase, potentially because overpressure inhibits illite recrystallization probably due to the lack of fluid renewal to fuel the illitization reaction, and/or because of a correlative trapping of the radiogenic 40Ar into the mineral structures even with increasing temperature (Clauer and Liewig, accepted for publication).

Fig. 6. 40Ar/39Ar step heating patterns of varied illite-rich size fractions of Rotliegend (Permian) sandstones from northern Germany (after Clauer et al., 2012b).

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4.4. Dating of polyphased tectono-thermal events

Fig. 7. K–Ar ages of clay-rich size fractions from metapelites of anchimetamorphic (labeled AMP) and epimetamorphic (EMP) degrees and from metatuffs of anchimetamorphic (AMT) and epimetamorphic (EMT) degrees. The fractions isotopically homogeneous plot horizontally (after Reuter, 1987).

Timing of regional extent of illitization was also estimated in Rotliegend (Permian) sandstone gas reservoirs of northern Germany (Clauer et al., 2012a). The size fractions from core samples collected mostly to the ESE of the city of Bremen at depths between 4596 and 5330 m consist mainly of illite (90–100%) occurring as flakes coating detrital framework minerals and laths/fibers invading the pore space. Most K–Ar values fit two isochrons with ages of 191 ± 8 and 178 ± 1 Ma and initial 40Ar/36Ar ratios close to the atmospheric value (Fig. 5). Microthermometric analysis of fluid-inclusions in quartz and calcite reveal two types of percolating fluids: a highly saline fluid (19% NaCl equivalent) at temperatures from 185 to 150 °C, and a slightly saline fluid (2.6% NaCl equivalent) at lower temperatures of 170 to 145 °C. These temperatures are higher than paleotemperatures calculated on the basis of a present-day burial gradient of 30.5 °C/km, therefore suggesting an epithermal illitization with the older illite crystallizing at slightly higher temperatures than the younger illite. One of the few combined K–Ar and 40Ar/39Ar studies of dating authigenic illite from hydrocarbon-bearing resevoirs was also conducted on Rotliegend (Permian) sandstones from northern Germany (Clauer et al., 2012b). Illite yields K–Ar ages between 220 and 160 Ma with 220–200 Ma ages that are dominant in the horst areas, and 180– 160 Ma ages that relate to illite from reservoirs in the graben areas (Zwingmann et al., 1999). Some of the samples analyzed by K–Ar were packed in quartz vials under vacuum and irradiated. The 39Ar recoil from irradiation ranges from about 20% of the total 39Ar in the b 0.2 μm size fractions to about 10% in the 2–6 μm size fractions. Total 40Ar/39Ar gas ages of the illite fractions are systematically older, but comparable to the K–Ar ages of the same aliquots (almost within analytical uncertainty for some). Examination of the step-heating patterns suggests the occurrence of two to three generations of authigenic illite mixed with detritals (Fig. 6). However, these three generations of illite seem to occur in all the different mineral separates, suggesting either that the size separation was not made efficiently or that more has to be known about how 39Ar recoils within and outside illite particles during irradiation. 39Ar release patterns gave "staircases" suggesting erratic redistribution in the mineral structures so that only “apparent” ages were obtained by step heating. This might be the actual contribution of the step-heating 40 Ar/39Ar technique to the interpretation of global K–Ar and 40Ar/39Ar ages of clay-type separates, even if it is not yet proven that the plateaus give or not meaningful ages. Alternatively, successive illite authigenic generations can be differentiated from detrital components, and can even be quantified in the varied analyzed size fractions.

Precise geometrical and time constraints are essential to detail the evolution of fold-and-thrust belts. Since the initial attempt by Hoffman et al. (1976), many K–Ar isotopic dating studies of incipient metamorphic processes and of slaty cleavages in silici-clastic rocks were reported to constrain the timing and duration of such events (e.g., Kralik, 1982; Kligfield et al., 1986; Clauer et al., 1995; Kirschner et al., 1995; Zhao et al., 1999). Difficult and inconsistent interpretations due to incomplete isotopic resetting of pure mineral separates and/or incomplete separation of mineral assemblages with mixed ages were also published (e.g., discussion in Clauer and Chaudhuri, 1998). Again, size fractionation procedures are appropriate to separate small authigenic from larger particles, often detrital in origin. They are especially helpful in studies of mineral fractions from rocks that were subjected to a metamorphic grade of 300 °C and above, which are temperatures known to reset the K–Ar system of metapelites (Leitch and McDougall, 1979). Geochronological studies of clay-rich size fractions from fine-grained pelites that were slightly metamorphosed, especially in fold and thrust belts, make challenging to understand how nucleation of authigenic minerals differs from alteration of the initial rock minerals that are often similar in size, morphology and chemical composition. Recent detailed mineralogical studies supplied new, pertinent information on how clay crystals organize in fault gouges (Haines and Van der Pluijm, 2012). However, such information does not necessarily solve the difficulties of the isotopic dating aspects, as the gap between in situ particle identification and ages of extracted particles needs to be closed. Numerous dating studies published on the Rhenish Massif during the last three decades have identified the region as an appropriate area for the study of deformed and metamorphosed shale- to slate-type rock units that have experienced a complex polyphased evolution. In examining different rock lithologies, Reuter (1987) attributed the variable K–Ar ages of clay-rich grain-size fractions from metapelites as due to detrital influences, especially when recrystallized only to an anchimetamorphic degree, by comparison with the ages of associated tuffs devoid of detritals, which can be considered to have recorded true geological K–Ar ages. An appropriate way to address the K–Ar homogenization process in varied size fractions of a sample is to compare the isotopic ages of the size fractions. In Reuter's (1987) study, the anchimetamorphic metapelite (labeled AMP in Fig. 7) produced an oblique array, whereas epimetamorphic recrystallization (EMP in Fig. 7) provided a bench-like, broken array with an identical age for the smallest fractions (b0.63 μm) that is geologically meaningful. Different size fractions of various samples giving the same K–Ar age are necessarily homogeneous and result from authigenesis, which is the only process capable of producing homogeneous crystals with identical ages. The size fractions of the associated tuffs subjected to either anchi- or epimetamorphic thermal conditions (AMT and EMT in Fig. 7) fit a horizontal line of the same age, which indicates that they were isotopically homogeneized ahead of the metapelites and that their age, which is identical within analytical uncertainty to the ages of the small epimetamorphic size fractions, is geologically meaningful. Reuter and Dallmeyer (1987) documented the recoil effect on the redistribution of 39Ar of the same non-encapsulated mineral fractions of the eastern Rhenish Massif, showing that 39Ar recoil (loss) occurs preferentially from particles with large surface areas and irregular grain edges, the same interpretation being suggested again later (Van der Pluijm et al., 2001). All illite fractions from a sample subjected to an anchimetamorphic grade at a temperature of about 100–150 °C yield systematically older 40Ar/39Ar data compared to the corresponding K–Ar data, whereas well-crystallized illite from a sample at an upper anchimetamorphic grade of a temperature of about 150–200 °C gave 40Ar/39Ar data within analytical uncertainty of the K–Ar ages, independent of the particle size. The authors concluded that the 40Ar/39Ar method should be preferentially applied to well-crystallized illite and mica-type particles at least 10 μm in size, which appears to be an

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Fig. 8. K–Ar, total gas 40Ar/39Ar ages and step-heating patterns of Alpine phengites (from Liewig et al., 1981b). Of interest are the uncertainties and accuracies of the K–Ar and total-gas 40Ar/39Ar determinations in the frame of the discussion about the overall precision of the two methods discussed earlier.

appropriate size (e.g., Clauer et al., 2012b), but which is significantly larger than the 2 μm size limit of diagenetic clay particles of interest here. The claim about the most-adapted crystallite size for 40Ar/39Ar dating was confirmed by similar conventional K–Ar and total-gas 40 Ar/39Ar analyses of unsealed low-grade metamorphic clay-sized material from varied places (Burghele et al., 1984; Hunziker et al., 1986; Kligfield et al., 1986). Mica-rich schists and calcshists of the Schistes Lustrés from northern Cottic Alps were fractionated into phengite-enriched size fractions and K–Ar dated (Liewig et al., 1981a). Overthrusted during Alpine orogenic activity by obducting ophiolitic units, these rocks were subsequently affected by several intense deformational events since Cretaceous time (Caron, 1979). The structural evolution was concomitant with metamorphic transformations starting with blue-schist parageneses at 300– 400 °C and 4–8 kbars during the Cretaceous, continuing with recrystallizations into either the same facies or the green-schist facies during the Upper Eocene (e.g., Chatterjee, 1971). The pervasive synschistous structures contain at least two generations of phengites, whose K–Ar ages fit into four isochrons giving two ages in the eastern Alpes at 50.2 ± 2.7 Ma and 49.1 ± 2.2 Ma with initial 40Ar/36Ar ratios close to the atmospheric value, as well as at 57.6 ± 2.8 Ma and at about 54 Ma, again with initial 40Ar/36Ar ratios close to the atmospheric value, in the western

Alps. 40Ar/39Ar analyses were also made on some of these phengites (Liewig et al., 1981b) with some of the step-heating patterns illustrated here together with the K–Ar and 40Ar/39Ar total-gas ages (Fig. 8). The flat step-heating pattern with identical K–Ar and 40Ar/39Ar total-gas ages at respectively 49.0 ± 1.2 and 49.1 ± 0.3 Ma (Fig. 8A), corresponds to a pure authigenic phengite fraction that crystallized probably during the third deformational event. The second pattern with a flat intermediate step-heating pattern (50% of the cumulative 39Ar) probably illustrates the intermediate deformational event (Fig. 8B) with a low-temperature step at about 40 Ma and a start of a staircase towards the higher release temperatures up to about 100 Ma. Interestingly, the K–Ar and 40Ar/39Ar total-gas ages are identical again at respectively 55.6 ± 1.4 and 55.3 ± 0.4 Ma. The third case shows, after an initial step (of about 15% 39Ar) still at about 40 Ma, a progressive and continuous age increase (staircase) of the release steps up to 130 Ma (Fig. 8C). Again, the interesting aspect is the concordance of the K–Ar and 40Ar/39Ar total-gas ages at 63.8 ± 1.6 and 63.4 ± 0.6 Ma, respectively. It is clear from this comparison that the 40Ar/39Ar method is more informative than the K–Ar method in distinguishing the late 40 Ma event in all the mineral separates, in confirming a main event at about 55 Ma, and in detailing mineral domains of the previous high-temperature precursors with variable staircases. The two ages of about 40 and 55 Ma were

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Fig. 9. Comparison of the illite mineralogy and variations of the K–Ar systematic relative to burial depth in varied basins (after Clauer and Chaudhuri, 1996).

confirmed by 40Ar/39Ar in situ dating (Agard and Monié, 2002), but the intriguing aspect is that both Ar-methods provide almost identical total-gas ages, whatever the size of the pure analyzed fractions (mostly 80– 100 μm) and the shape of the step-heating patterns. This identity shows in turn that constrained analyses by both methods of coarse and slightly metamorphosed sheetsilicates give identical 40Ar/39Ar total-gas and K– Ar ages. 5. Ar tracing of low-temperature crystallization and deposition processes 5.1. Natural and experimental alteration and weathering Since the review of Clauer and Chaudhuri (1995), apparently only a few isotopic studies have been concerned with the alteration of the K–Ar signature of clay minerals from weathering environments. The review of this section will, therefore, mainly focus on an experimental leaching of the K–Ar system of clay- and mica-type minerals, and on the 40Ar/39Ar method applied to non-clay mineral phases in continental weathering environments. One study to mention is that tracing the origin of micas in soils of South Carolina (Naumann et al., 2012). Despite pedogenesis, K–Ar “dating” of mica remnants was used to distinguish between K structurally held in the mica structure from that participating in pedogenic mineral-water reactions by leaching moderately the clay-sized mica fractions. Another type of leaching can provide useful information about the type and origin of illite, namely based on the potential organic akylammonium cations have to replace K stochiometrically in dioctahedral mica structures. Identification of an illite population of any origin is possible in comparing the K–Ar ages of the residual particles to that of the initial, non-exchanged illite crystals (Chaudhuri et al., 1999; Clauer, 2011). The experiments showed that: (1) similar

K–Ar ages of untreated and alkylammonium-exchanged particles identify an homogeneous trioctahedral illite population, and (2) different illite K–Ar ages before and after alkylammonium leaching indicate a variable chemical composition of the exchanged illitic material with occurrence of dioctahedral 2:1 layers in the trioctahedral crystals, or the reverse. No 40Ar/39Ar dating study of clay minerals crystallized or altered in weathering profiles seems to have been published recently. Alternatively, the method has been applied to authigenic small-sized and lowtemperatured alunite, jarosite, hollandite and cryptomelane from weathering environments, by step-heating analysis of single grains and control of the 39Ar recoil (e.g., Dammer et al., 1999; Vasconcelos, 1999; Polyak et al., 2006). These applications appear very useful for paleoclimatologic and archeologic studies.

5.2. Origin of deposition patterns in recent marine sediments Mineralogical, chemical and stable isotope compositions are generally used as indicators for the origin of inorganic materials from presentday marine sediments (Chamley, 1989). A deposition history can also be reconstructed by evaluating differences in the heavy mineral assemblages in coarse-grained clastic sediments (e.g., Davis and Moore, 1970). Alternatively, K–Ar dating can help identify the origin of the fine-grained clay-type minerals by establishing the age of K-bearing components, especially illite, in marine depocenters where various coastal sedimentary processes, oceanic current transfers, or even iceberg discharges may have operated (e.g., Jantschik and Huon, 1992; Bond et al., 1992). Local or regional variations in such processes may obscure the search for the origin of detrital fine-grained inorganic components, for instance by differential flocculation of clay particles at the fluvial/marine interface (Meunier, 2005). Smectite may also be partially disintegrated in estuarian environments into nanometric particles by

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delamination (Brockamp, 2011) that potentially alters the isotopic signature. Among the suitable index minerals for reconstructing recent transport pathways are feldspars, heavy minerals and clay-sized materials, but the results remain sometimes equivocal. For instance, in the case of the German Bight in the southeastern North Sea, the clay minerals of the recent marine sediments derived from submarine erosion and redeposition of Pleistocene material during the Holocene transgression according to Zöllmer and Irion (1996), with only a minor contribution from the rivers Elbe, Weser and Ems, on the basis of XRD data. Using the K content, K/Rb ratio and K–Ar age of illite from similar North Sea sediments of the German Bight, Zuther et al. (2000) showed that the river-borne suspension load appeared much more significant in the same area. The b2 μm fractions of sediments from the Elbe and Weser rivers and the southeastern German Bight provide an interesting, complementary information to the controversial question about the origin and distribution of clays in this near-shore marine area (Brockamp and Clauer, 2012). They contain smectite, chlorite, illite and kaolinite within a 100-km wide zone, are variably mixed by currents parallel to the coast, and are transported during flood tide into the estuaries, where they mix with suspended fluvial material. Their K/Rb ratios and K–Ar ages are especially useful for modeling this mixing, and explaining the origin and distribution of the clay material (e.g., Pache et al., 2008). Similar attempts to identify mineral origins were published using 40 Ar/39Ar dating of muscovite in the Red and Yangzte River systems (Van Hoang et al., 2010). VanLaningham and Mark (2011) published 40 Ar/39Ar step-heating ages of two cardinal minerals of sediments. These step-heating ages were considered as robust indicators of the cooling/crystallization ages of the K-bearing minerals that were supplied to a depositional center. Such an application is of special interest for environments that do not provide sand-sized sediment archives (e.g., distal terrigenous sedimentary deposits), when information about source changes through time prevails source identification. In summary, the two Ar methods are of use to trace the origin of detrital components of varied grain size in present-day and ancient marine sediments. 5.3. Burial illitization Minerals deposited in sedimentary basins are basically of varied provenance, which is visualized by variable K–Ar ages of the K-bearing minerals (e.g., Weaver and Wampler, 1970). However, the varied sources of the original detrital minerals, especially of the clay-sized particles, may be confounded by continental erosion, weathering and riverine transport that tend to reduce the scatter of their K–Ar signature. After deposition in basinal sedimentary sequences, progressive burial favors K addition to the smectitic end-member of the smectite–illite mixings (e.g., Dunoyer de Segonzac, 1970; Boles and Franks, 1979; Środoń and Eberl, 1987; Jennings and Thompson, 1986; Šucha et al., 1993; Furlan et al., 1996). This addition of K induces a decrease in the K–Ar signature that is amplified by a concomitant escape of radiogenic 40Ar resulting from alteration of the associated detrital clay material. Deeper in the sedimentary strata, incorporation of K by the authigenic particles theoretically increases as well as escape of radiogenic 40Ar from detrital components due to temperature increase, the combination of both accelerating the decrease of the K–Ar signature (e.g., Hunziker et al., 1986; Burley and Flisch, 1989; Hassanipak and Wampler, 1996). Concerning the illitization mechanism, McCarty et al. (2008) showed in Gulf Coast sediments that nucleation of smectite-rich particles occurred on the surface of detrital particles or grains and not in the interlayer sites of the already present detrital sheet-silicates. This observation explains, at least partly, why separation of pure authigenic illite from associated detrital particles is especially difficult in shale-type sediments and why attempts to date authigenic nanometric illite crystals from shales remain unsuccessful (e.g., Clauer et al., 1997). It also tends to prove that authigenesis of new

illite-type particles in the general smectite-to-illite trend is independent of the alteration of associated detrital material, except if the overall reaction is a dissolution of the detritus followed by a crystallization of a new generation of illite/smectite mixed-layers. A few studies report fairly continuous K–Ar data for burial-induced illitization in sediments of the Gulf Coast (Perry and Hower, 1970; Aronson and Hower, 1976; Awwiller, 1994), the North Sea (Glasmann et al., 1989; Darby et al., 1997), and the Mahakam Basin in eastern Borneo (Clauer et al., 1999). Comparison of the K–Ar results obtained on various illite-rich size fractions of shales from these basins highlights a very consistent behavior of the K–Ar system in the clay-sized fractions independent of the stratigraphic age of the studied rocks, the type and evolution of the basin, or the changing mineralogy of the clay separates (Fig. 9; Clauer and Chaudhuri, 1996). These coherent data suggest that the K–Ar method is the most appropriate and best-adapted tracer of illitization, because radiogenic 40Ar of any analyzed size fraction from any rock type of any sedimentary basin only depends on the amount of K present, independent of the origin of the particles, authigenic and/ or detrital. No variation of the initial 40Ar/36Ar ratio relates to potentially varied origins like changing initial 87Sr/86Sr, 143Nd/144Nd, or 207Pb/204Pb ratios. In other words, whatever the origin of the mineral separates, their deposition age, their evolution, and the detrital/authigenic composition of the separated size fractions, the K–Ar system systematically reproduces the radioactive K to radiogenic 40Ar ratio. One might, however, be aware that this is true only in shales, in which the deposited detrital particles result from a long and complex process including continental weathering, fluviatile transport and coastal mixing, which ends up with mechanically “homogeneous” fine-grained mineral mixtures of detrital particles on which authigenic crystals will nucleate and grow. Mathematical simulation of theoretical K–Ar ages for detrital/ authigenic clay-rich mixtures appears as an attractive alternative method for tracing the illitization process, when the separation of the authigenic K-bearing clay crystals from associated detrital components becomes almost impossible. Mossmann (1991) first published a model that evaluated the age of the authigenic component of clay-sized mixtures, followed by Środoń (1999) and more rencently by Szczerba and Środoń (2009). Lerman et al. (2007) and Clauer and Lerman (2009) explored an alternative approach in combining the changing K–Ar system of the progressively aggrading authigenic and degrading detrital minerals, by simulating kinetic gain and loss rates of K and radiogenic 40Ar from both components relative to burial depth, on the basis of available analytical results. Only this latter approach takes into account the progressive change of the detrital components, which K–Ar system cannot yield the same K/radiogenic 40Ar ratio, and consequently the same age during burial, and therefore during increasingly thermal conditions. If based on this concept, the application has real potential for decrypting the mineralogical/crystallographical aspects of illitization and the combined authigenesis/alteration mechanism of progressively buried clay mixtures. The Mahakam Basin in eastern Borneo provides another interesting aspect of K–Ar application to progressively buried sediments, by comparing of how illitization extends in associated sandstones and shales relative to depth (Furlan et al., 1996; Clauer et al., 1999). The entire stratigraphic interval (from 18 to 0 Ma) shows a pronounced, continuous decrease in the K–A age of the clay fractions with depth from about 80 Ma for the pure detrital end-member at the subsurface to about 20 Ma for the almost pure authigenic fraction of the sandstones at 4000 m depth. The evolution of the K–Ar system is different for the clay material of the associated shales for which the K–Ar age decrease ranges from about 80 Ma at subsurface to 55 Ma at 1700 m depth, whereas it remains about constant below (Fig. 10). If the K–Ar age of the progressively buried clay mixtures results from continuous addition of K and variable release of radiogenic 40Ar in both rock types, as assumed above, they should decrease continuously, which is not the case for the shales. The almost constant K–Ar pattern of the b0.4 μm fractions in the deeper Mahakam shales can be explained in simulating

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small rates of K addition and 40Ar escape, each about 1 %/Ma. Such reduced escape of radiogenic 40Ar and reduced K supply in the shales below 1700 m depth can both be explained by a lower porosity and/ or permeability due to progressive compaction. In this case, the relationship between K–Ar age and stratigraphic depth does not represent a strict “steady state” as claimed for the Gulf Coast sediments (Morton, 1985), but rather a continuous diagenetic process with significantly reduced addition and release rates, and therefore more limited rock-fluid interactions. The K–Ar ages of deposited clay-rich mixtures are consistently, in all published studies, either older or equal to the stratigraphic reference age of the deepest stratigraphic units, and the challenge in K–Ar dating illite of a burial induced process is that the measured K and radiogenic 40 Ar contents, and therefore the K–Ar value of any size fraction taken randomly in the sedimentary sequence, represents an integral of a lasting process. The difficulty is then evaluating the combined behavior of K and radiogenic 40Ar. It can be argued that the K released from detrital components is taken up by the nearby authigenic illite, but Furlan et al. (1996) showed in the buried sedimentary sequence of the Mahakam Delta that K was released the most by the detrital components at a depth where the least nucleation of illite particles was observed. An alternative concept for burial-driven illitization in sedimentary basins is that it consists of repeated, temporary episodes driven by episodic heat flows associated with hydrothermal-like fluids. Lampe et al. (2001) suggested such epithermal conditions for a spatially confined thermal anomaly in sandstones of the structural context of the upper Rhine Graben. Their concept and mathematical model assume that a lateral flow of relatively “hot” fluids was driven through a confined aquifer. Similar to the approach of Ziagos and Blackwell (1986), the model introduces kinetic aspects of kerogen degradation and vitrinite reflectance systematics. Such episodic fluid flows could have been initiated by sudden permeability changes produced by periodically reactivated local faults associated with episodic regional rifting during a regular progressive burial. It is clear that in such cases, the K–Ar ages of illite-enriched size fractions will not range over long time periods but will cluster at the times such fluid flows occurred. Crystallization temperature of such “punctuated” events has to be high enough not to be reset by further burial-induced temperature increase. One should also remember that if the temperature induced from further evolution of authigenic illite in the sediments is higher than that of the K–Ar measured illite, the K–Ar system of the punctuated illite will be affected by the further evolution and will yield ages that can be lower than the true crystallization age, as radiogenic 40Ar will have a tendency to escape. 6. Ar isotopic information of crystallization processes A permanent challenge in dating clay-type minerals is the purification of the authigenic crystals that are often mixed with detrital components of varied nature, clays, micas, feldspars, etc, in most sedimentary sequences. This request also implies that their crystallization process is well known and understood, which is not yet the case especially at the elemental and atomic scale, as needed for any type of dating method applied to any type of mineral. Recent progress in understanding the intimate physico-chemical aspects of illitization comes from extraction, and mineralogical and chemical characterization, of extremely small crystals that were called “fundamental” by Nadeau et al. (1984) because their thickness is in the nanometric scale, consisting only of some illite layers of illite/ smectite mixed-layer structures. They have no specific “fundamental” characteristics, except their size. Nanoparticles of illite (b0.02 μm) are then theoretically among the first to nucleate from a supersaturated solution at diagenetic temperatures (Eberl et al., 1998), and dating of such nanometric particles, mainly by the K–Ar method, has opened new complementary potential for evaluation of kinetic rates and of changing mechanisms of burial-induced illitization, especially in bentonite beds

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Fig. 10. Distinct evolution relative to burial depth of the K–Ar ages for b0.4 μm size fractions from sandstones and shales of the Mahakam Delta basin (after Clauer et al., 1999).

(Clauer et al., 1997; Środoń et al., 2002; Honty et al., 2004; Środoń et al., 2006; Clauer et al., accepted for publication-a). The improvement in studying such small particles is precisely in their size, which allows considering them to be representative of initial nucleation for the smallest and further growth for the coarser. The technical aspects of their separation are heavy duty, starting with separation of the b2 μm fraction from whole rock by gravity sedimentation in deionized water after disaggregation and treatment with sodium acetate, hydrogen peroxide and sodium dithionite according to Jackson (1975). The b0.2 μm fraction is then recovered by high-speed ultracentrifugation, and serves for separation of several nanometric sub-fractions by continuous-flow high-speed ultracentrifugation after infinite swelling, following Środoń et al. (1992). Varied tools exist to identify more precisely the extracted nanometric illite-type crystals: their size may ultimately be controlled by transmission electron microscopy, whereas the crystal thickness distribution (CTD) can be obtained from first-order X-ray reflections using the Bertaut-Warren-Averbach method (Drits et al., 1998) and the “MudMaster” program (Eberl et al., 1996). Crystal-growth mechanisms that reproduce the measured CTDs of the size fractions can also be simulated using the “Galoper” program (Eberl et al., 2001). These complementary analytical methods are helpful to better identify the nucleation and growth processes. Nucleation and progressive growth of these nuclei depend on the immediate physical and chemical environmental conditions that may be identified by elemental and isotope geochemistry, and therefore contribute to understanding of the process. However, one caveat is critical: it is never certain that the separated size fractions do not sometimes consist of or contain aggregates of smaller crystals, or of authigenic crystals grown on detrital components. Most of the presently available studies were conducted on illite/smectite mixed-layers from bentonite beds because these rocks are theoretically almost always devoid of detrital particles. A few K–Ar results were published on sandstones, also by the Rb–Sr method (Clauer et al., 2003). More attention

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has still to be paid to explain misapplications reported on shales (Clauer et al., 1997; Środoń and Clauer, 2001). 6.1. K–Ar dating of crystal nucleation and growth The rationale for separating and analyzing nanometric illite-type crystals from illite/smectite mixed-layers is that they normally consist of non-swelling coherent illite domains that are enriched in K, while smectite-type interlayers are the shared expandable interfaces between adjacent illite crystals. Basically, the smaller the illite domains, the larger is the proportion of swelling interlayers that occur in the mineral structure. Dispersion by infinite osmotic swelling allows “cleaning” of the swelling smectite-type interlayers that then become “smectite-reacting” surfaces of illite crystals, while the illite interlayers remain collapsed. Growth of illite-type nucleating crystals depends on temperature, thermal-kinetic parameters and accessibility of the particles to interstitial fluids in the sedimentary and associated volcanic beds. For instance and as already discussed, nucleating particles that remain small by stop growing, yield theoretically the oldest K–Ar ages relative to particle-size distribution (Clauer et al., 1997 and more since). Alternatively, during crystal growth that proceeded episodically, not continuously, the smallest nanometer-sized crystals yield potentially younger K–Ar ages than the associated coarser micrometer-sized particles because they nucleated last. Therefore, K–Ar dating of nanometer particles from any bentonitic illite/smectite mixed-layer does not automatically provide unequivocally meaningful K–Ar ages that decrease when crystal size increases. Also, different size fractions of a sample can consist of particles of similar mineral types but of variable ages, resulting from episodic and variable K supply. Such varied mineral types cannot be distinguished by the traditional analytical techniques such as XRD or wet chemical analysis. On the other hand, K–Ar ages also record indirect information about the growth mechanism and reaction rate(s). For instance, crystal growth may have remained constant or it varied over time (Clauer, 2006), and the cluster of the K–Ar ages will be located variably on a theoretical evolution sketch that relates particle size to time (Fig. 11). The data may plot either: (1) towards the middle of the theoretical time span of illitization when the crystal growth rate represented by the line drawn diagonally across the diagram was about constant; or (2) close to the beginning of the theoretical time span, along the upper curve, when the rate decreased or was interrupted; and (3) towards the end of the time span, along the lower curve of the diagram, when the rate increased. Nucleation and growth of nanometric particles could also have been episodic resulting in the mixing of several generations of particles of variable sizes, the smallest yielding the younger ages in this case. In summary, the K–Ar ages of bentonite nanoparticles record complicated illitization histories in sedimentary strata that may have occurred with constant or varied growth rates, during one or successive episodes of growth, implying a constinuously increasing or a more episodically variable temperature. If such scenari occur in individual bentonite beds of a given stratigraphic age, the interpretation will be necessarily complicated, raising questions about how to detail the complex evolution of a sedimentary basin, but also spontaneously about sample preparation, especially size fractionation and analysis.

metamorphic mineral and organic-matter alterations in rocks of varied compositions. Evidence that high temperatures promote the diagenetic precipitation of illite or the conversion of illite/smectite mixed-layers into illite comes from experimental work as well as geologic studies in locations with well-documented geothermal regimes (e.g., Frey et al., 1980), and/or local temperature increases resulting from circulation of hot solutions at depth (e.g., Lampe et al., 2001; Timar-Geng et al., 2004; Meunier and Velde, 2004). The formation of illite under different physical and chemical conditions has been variably attributed to such processes as direct mineral precipitation associated with local or remote dissolution of K-containing micas and feldspar, and/or uptake of K + ions from interstitial solutions by existing illite/smectite mixed-layers, which induces a structural rearrangement of the precursor crystal. In both cases, it may be agreed that the process can be identified as illitization, even if the term appears more appropriate for a progressive than an instantaneous crystallization. Whatever the process and even if attributed to local dissolution of detrital K-feldspars and mica in the coarser fractions of the sediments or to K import from outside the rock volume, the variable sources of K are still not well established (e.g., Freed and Peacor, 1992; Harris, 1992; Milliken, 1992; Pollastro, 1993). Based on experimental studies, the upper temperature limit of illite stability was reported as ≤250 °C (relative to muscovite) or 360 °C (Yates and Rosenberg, 1996; Rosenberg, 2002). Progress in evaluating the duration of regionally spread illitization by K–Ar isotope dating comes from recent interest for the study of nanometer-sized fundamental illite crystals from bentonite beds. The comparison of the stratigraphic ages of these crystals from the East Slovak Basin with the onset of illitization outlines a general association with the Middle Miocene subsidence occurring between 13.5 and 11.5 Ma ago. A sketch of the timing of illitization in the different bentonite beds shows that in most illite/smectite mixed-layers, the smallest crystals (white symbols in Fig. 12) precipitated the latest, not following the rule of continuous growth after nucleation, and that consequently the process had to be episodic. Continuous growth with the smallest crystals being the oldest is only observed in one sample (Ptr 47). The

6.2. K–Ar tracing of the regional extent of illitization Clay materials initially deposited in sediments are used to display a continuous recrystallization during burial, from early diagenesis starting soon after sediment deposition, to late diagenesis occurring during deep burial, and finally to anchi-metamorphism. The rate and extent of mineral transformation depend on factors such as local or regional temperature and pressure gradients, chemistry of the interacting fluids hosted by the rocks or migrating occasionally, lithology of the host rocks, and duration of diagenesis (Środoń and Eberl, 1987; Franks and Zwingmann, 2010). During recent decades, significant progress has been made in the understanding of diagenetic-to-low-grade

Fig. 11. Theoretical crystal growth measured by K incorporation of nanometric illite-rich fractions relative to time (after Clauer, 2006). The small black dot in the lower left corner becomes larger during illitization from 1 to 4 arbitrary units. Three different pathways are envisioned as: (1) a straight line across the diagram pictures a constant growth, (2) a lower curve when the rate is slow at the beginning and increases, and (3) an upper curve when the rate is high at the beginning and then slows down.

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vertical bars in the diagram illustrate several periods of active illitization from 12.5 to 10 Ma, from 8.5 to 7 Ma, from 5 to 2.5 Ma and from 1.5 to present. These K–Ar ages of illite crystals from bentonite beds of varied depositional ages and spread over a larger geographic area, point to a recurrent illitization process with variable duration along different crystallization pathways, and to various degrees. These age spreads allowed calculation of subsidence rates in the entire studied area that range from less than 300 m/Ma to more than 500 m/Ma, as well as of the thermal gradient ranging from ~60 °C/km to less than 50 °C/km. In fact, low subsidence rates correlate with high thermal gradients and vice versa. Also, illitization lasted variably: short duration when initiated soon after sedimentation, and prolonged when initiated long after sedimentation (Clauer et al., accepted for publication-b). These successive illitization episodes reactualize the claim of “punctuated” diagenesis made by Morton (1985) for the Gulf Coast shales. Alternatively, they could also represent variable crystal-growth rates (Clauer and Lerman, 2009) due to temporarily reduced supply of K caused by the very low porosity of the bentonite beds, resulting from a limited access of the illite crystals by the interstitial fluids, unless slower elemental diffusion took temporarily over faster elemental advection.

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Fig. 12. Time spans of illitization of varied nano- to micrometric illite-rich size fractions from illite-smectite mixed-layers of bentonite beds from the East Slovak basin. The white symbols are for the nanometric size, whereas the grays are for the micrometric size. The large gray areas cover varied time spans during which illitization occurred in several samples. Th sample identification is to the right of the diagram and refers to Clauer et al. (accepted for publication-b).

6.3. Ar dating of gouge clay minerals and tracing faulting activation and reactivation Isotopic dating of clay minerals from gouges of initial or reactivated brittle faults in varied basement or cover terranes is also a pertinent and efficient application field for Ar dating of tectonic episodes (Lyons and Snellenberg, 1971; Zwingmann and Mancktelow, 2004). For instance and until recently, lack of isotopic dating of the motion of shallow faults from Taconian Allochtons of the northern Appalachians limited the comprehension of the structural evolution in the North American rift system, as reported by Tremblay et al. (2003). By combining clay mineralogy, crystal morphology and K–Ar isotopic dating of clay-rich size fractions from varied host lithologies of key faults of the Allochtons from southern Quebec, Canada, Sasseville et al. (2008) reported kinematic constraints on reactivated faults on the basis of K–Ar age clusters from illite of fault gouges at ~490 Ma, 465 to 450 Ma, ~410 Ma and ~355 Ma next to or in the fault system of the Saint Lawrence River. These four episodes were referred to the regional D1 and D2 Taconian deformational pulses, the D3 Late-Silurian–Early Devonian extensional deformation, and the D4 Acadian compressional event, respectively. Recognition of such discrete contractional pulses during the Taconian orogeny also suggested that folding and thrusting were initiated during the early Ordovician and culminated with the out-of-sequence imbrication of thrust stacks during the Middle to Late Ordovician. The 300 to 270-Ma K–Ar age range for clay fractions from numerous K-bentonite beds described in the Ordovician carbonate rock-dominated sedimentary sequence (Huff et al., 1986; Huff and Kolata, 1990) of the Appalachian tectono-structural context in the eastern United States was interpreted to reflect a geographically widespread illitization resulting from expulsion of K-rich, hot, saline fluids from deeper parts of the Appalachian Basin during the Alleghenian orogeny (Elliott and Aronson, 1987, 1993; Hay et al., 1988). Isotopic support for basin-wide migrations of hot saline fluids during this orogeny was provided by 40Ar/39Ar ages between 322 and 278 Ma of authigenic K-feldspars in Cambrian limestones (Hearn and Sutter, 1985; Hearn et al., 1987). K–Ar ages of nanometersized illite particles in K-bentonites collected close to the Allegheny Front in northwestern Georgia (Clauer et al., accepted for publication-a) suggest the occurrence of two hydrothermal overprints at 320 ± 2 and 285 ± 2 Ma on the basis of two isochrons (Fig. 13). The clay fractions with the older crystallization age consist of illite and illite-rich mixedlayers, whereas the younger are devoid of illite and illite-enriched mixed-layers. The two generations of illite yield very consistent δ18O (V-SMOW) values at 17 ± 1‰ for the older generation and at 21 ± 1‰ for the younger. The δ18O of the hot and saline interactive fluids

remained unchanged during local crystal growth, with δ18O values of 4 ± 1‰ and 8 ± 1‰, depending on the geographic location of the samples, the timing of illitization, and the assumed temperature of the interactive fluids. To alleviate the difficulty of separating physically the authigenic from detrital illite in a 40Ar/39Ar dating study of clay populations of ductile shear zones, Van der Pluijm et al. (2001) quantified the illite polytypes of the dated clay separates on the basis of X-ray analysis, considering the 2 M polytype to be detrital and the 1 M polytype to be authigenic, and compared them to the 40Ar/39Ar data of the same separates. Although promoting highly precise 40Ar/39Ar ages, the authors did not discuss the precision of the 2 M/1 M illite ratios determined mainly by iterative modeling (Grathoff and Moore, 1996). Also, their conceptual argument for a detrital origin of the 2 M illite is based on a minimum crystallization temperature of 280 °C (Środoń and Eberl, 1987), which is surely above the temperatures expected in fault gouges at shallow depth, but deeper fault systems can be subjected to higher temperatures, up to temperatures of 350 °C determined by Velde (1965) for authigenic 2 M illite crystallization. Precisely in another study, mixed 2 M and 1 M illite was reported in a K-rich bentonite unit that is basically devoid of detrital clay minerals of an Acadian cleavage zone in U.K., at a temperature approaching 250 °C (Merriman et al., 1995). This 2 M illite has therefore to be assumed to be authigenic; the mixture of the two illite polytypes giving a K–Ar age of 397 ± 7 Ma, almost concordant with the 40Ar/39Ar total fusion age of 418 ± 3 Ma. Van der Pluijm et al. (2008) presented a concept for 40Ar/39Ar dating clay fractions of the San Andrea Fault system that is based on the combined use of XRD, HRTEM and fabric goniometry to identify best the illite material and determine its geometric distribution in the gouges. This information, of interest for the understanding of how fault gouges operate and how and where illite forms, is not automatically critical for the interpretation of the ages that are exclusively based on 40Ar/39Ar step-heating determinations and the assumption that all the modeled 2 M illite is of detrital origin (Schleicher et al., 2006; Warr et al., 2007; Haines and van der Pluijm, 2012). In fact, the connection between ideally described fault gouges with these techniques and the isotope ages of the separated material is not straightforward. The 40Ar/39Ar step-heating patterns never show convincing plateau ages and the complex images of the fault gouges give an idea of the challenge to select the most appropriate coatings. Also, the advocated ages for the authigenic illite obtained from regression calculation

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of mixtures containing varied amounts of 2 M illite are not as convincing as portrayed. Using the basic relationship between 2 M illite content and exp(λt) − 1 (instead of the 40Ar/39Ar ages), Schleicher et al. (2010) “demonstrated” the occurrence of two generations of illite related to two faulting events at 8.0 ± 1.3 and 4.0 ± 4.9 Ma, and evaluated the ages of two 2 M illite generations to be at 77.3 ± 20.3 Ma and at 48.9 ± 11.0 Ma. Beyond the fact that both data sets are not analytically accurate as they significantly overlap, the younger authigenic illite generation with the highest analytical uncertainty would probably not exist because the data points would be part of the upper line if the necessary uncertainties in the amounts of 2 M illite had been taken into account. It also cannot be denied that the scatter in the “ages” of the 2 M illite could simply be due to a varied alteration resulting from thermal impact of the faulting process. The same 40Ar/39Ar dating concept was applied on varied size fractions of fault-gouge clays from northeastern Tibet (Duvall et al., 2011) with a more convincing regression line between exp(λt) − 1 and the amount of detrital 2 M illite, but with step-heating patterns displaying large amounts of 39Ar recoil not restored back into the mineral structures, that are therefore not really pertinent. The authors have not, again, considered that the thermal stress affecting the gouge material had to modify the K–Ar system of the detrital 2 M illite, the same way it affects it during burial (Clauer and Lerman, 2012), signifying that the given “true” age of the detrital illite remains questionable. Jaboyedoff and Cosca (1999) published a similar concept in determining the 40Ar/39Ar ages of illite-rich size fractions from a carbonate-rich sedimentary sequence of the Swiss Préalpes. The amount of the authigenic illite in the dioctahedral sheetsilicates was increasing relative to the trioctahedral equivalent sheetsilicates with increasing metamorphic grade, explained by a probable dissolution/ reprecipitation process. The plot of the authigenic illite amount relative to the 40Ar/39Ar total age of the different size fractions provides a near-linear curve with an extrapolated age of about 27 Ma for 100% dioctahedral phyllosilicates. The authors interpreted this age as the time of incipient metamorphism, which is supported by independent biostratigraphic constraints. Alternatively, they did not insist on the age of the detrital sheetsilicates, but mentioned the changes in the degassing properties of progressively metamorphosed mixtures of detrital mica and authigenic illite. Surace et al. (2011) studied the “Centovalli Line” in the Central Alps that is characterized by ductile-brittle deformations developping along a fault system with a general E-W trend. The b2 μm size fractions of fault gouges consists mainly of illite, smectite and chlorite, together with varied mixed layers. They used the Patissier© software (Jaboyedoff and Thélin, 2002) to combine and integrate at once the full width at half maximum (FWHM) of the 10 Å illite diffraction peak, the average number of

layers in a coherent diffraction domain (N), the average number of consecutive layers of illite in a fundamental particle (Nfp), the amount of smectite layers in illite/smectite mixed-layers (%S), and the K–Ar age of illite. This mineralogical/crystallographic/geochronological information depicts a progressive trend in the thermal conditions of the tectonic pulses from anchizone to diagenetic conditions. The K–Ar ages of b2 and b 0.2 μm illite-rich fractions and K-feldspar separates, alltogether range between 14.2 ± 2.9 and roughly 0 Ma, with major pulses at about 12, 8, 6 and close to 0 Ma, which on the basis of these ages and a typical average thermal gradient of 25–30 °C/km, indicates that the faults were initiated or reactivated at depths between 4 and 7 km. If they were active until now, which seems to have been the case as Quaternary lake sediments were faulted, then the average exhumation rate was approximately 2.5–3.0 km for the last 12 Ma, with a mean velocity value of 0.4 mm/y. A similar timing was reported for episodic faulting activity in the Aar and Gotthard massifs about 30–50 km to the north of the Centovalli Line (Pleuger et al., 2012). In fault gouges as well as in other geologic environments, the primary goal of investigations utilizing dating techniques, especially the K–Ar or 40Ar/39Ar method, should be to establish the best possible correlation between age and complementary interpretative information provided by supplementary analyses of the sample fractions or grains. These techniques may show how clay particles are organized in fault gouges, and/or how their mineral structures vary. Their integration is justified because the physical, crystallographic-mineralogic or chemical responses of clay material differ in varied physical-chemical conditions and therefore impact variably their isotopic systems. It is clear that the meaning and reliability of the final age will depend mostly on the characterization of the analyzed size fraction or mineral grain. Despite some inconsistencies, application of both Ar methods, but especially the 40Ar/39Ar method, appears best suited in dating fault activation and reactivation. However, whatever the complementary methods, routine or upgraded, that are applied to describe, identify and quantify the illite-type material to be dated, their results must provide determining arguments for interpretation of the isotopic ages. 7. A few more pending questions The present comparison of the two Ar-dating methods as applied to clay material shows that: (1) they are complementary (not antagonistic), (2) neither is presently outdated, and (3) they have distinct fields of applications in clay geochronology and offer data that are complementary to clay crystallography. However, while the conventional K–Ar method is of convenient and straightforward use, it is still not the case for its 40 Ar/39Ar variant, especially about: (1) the most appropriate encapsulation technique, (2) the most reliable restitution for the recoiled 39Ar into the crystals when using the step-heating application, and (3) the meaning of the step-heating “ages”. 7.1. The problem of sample encapsulation in the 40Ar/39Ar method

Fig. 13. Isochron diagram of the nano- to micrometric size fractions from K-bentonites of the northeastern Appalachian orogen of the North-American continent (after Clauer et al., accepted for publication-a).

A basic question for dating clays by the 40Ar/39Ar method using micro-encapsulation remains: i.e. which is the most efficient and appropriate encapsulation technique? In fact, encapsulation is rarely described in detail in the available publications. Dong et al. (1995, 1997) centrifuged clay particles in water to compact them into “pills” before sealing them in vials. Clauer et al. (2012b) stored dry particles in a foil, compacted them by hand, and sealed them in quartz vials. If the specific surface areas of the particles have potentially varied impacts on the magnitude of the 39Ar recoil depending on which of these two preparation techniques is preferred, it may well be that the best encapsulation technique should include a centrifugation step of the micro-vials in order to compact and orient the wet mineral particles, considering that the incident angle between incoming neutrons and the c-axis of clay particles could also influence the 39Ar

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recoil (Lin et al., 2000). A comparative evaluation of both techniques would definitely help to better standardize this technical aspect. 7.2. Restoring and modeling the

39

Ar recoil

Restoring the recoiled 39Ar into 40Ar/39Ar total-gas ages of clay-sized crystals is simple, but it is not as straightforward in the case of stepheating determinations, especially in the perspective of Harrison's (1983) concerns about the resolution of the age spectra and the potential 39Ar recoil artifacts. Numerical models explaining the possible reasons for 39Ar recoil from clay material were published (York et al., 1992; Dong et al., 1995; Lin et al., 2000) that show that all simulations to restor the recoiled 39Ar depend somehow on the concept of each investigator, as there is a need to decide which crystallographic model works best to explain the behavior of the recoiled 39Ar. 40 Ar/39Ar dating studies of diagenetic to low-grade metamorphic illite-rich size fractions (e.g., Hunziker et al., 1986; Reuter and Dallmeyer, 1987; Dong et al., 1995, 1997) all report a negative correlation between amount of recoiled 39Ar and either grain size or illite crystallinity. As the illite crystallinity index reflects particle thickness and crystallographic order (e.g., Eberl and Velde, 1989), these correlations imply that the amount of recoiled 39Ar depends on the increasing content of “coherent domains” in growing crystals, that is to say an increase in the relative proportion and size of domains with interlayered K relative to those without interlayered K (with smectitic interlayers) in illite/smectite mixed-layered structures. For instance, Dong et al. (1995) envisioned that : (1) neutron-induction distributes 90% of the 39Ar recoil within the particles and only 10% outside, (2) 39Ar recoil is only relevant for low-temperature heating steps, and therefore (3) 39Ar only recoils from low-retentive sites of illite-type particles. Others related recoil quantities to the higher surface area of the smallest size fractions, which is also pertinent as it is based on analytical data (e.g., Reuter and Dallmeyer, 1987; van der Pluijm et al., 2001). Lin et al. (2000) linked it to 39Ar backscattering, as the calculated estimates were generally consistent with the analytical data. York et al. (1992) suggested a correlation between 39Ar recoil and its implantation ability, but the model failed to explain re-implantation. Of interest is the fact that their model was compatible with the K–Ar analyses. Most individual step-heating plateaus published by Clauer et al. (2012b) on illite-rich size fractions from Rotliegend (Permian) gasbearing sandstones do not reach the theoretical 50% of the total amount of 39Ar that is considered to be a geologically meaningful cut off (Fig. 6). The short plateaus could reflect illite populations that released variable amounts of 39Ar as a result of variable “39Ar reservoirs” depending on specific characteristics (i.e., different crystallographic features still to be identified). This is based again on the concerns of Harrison (1983) and on a recent study of Kula et al. (2010) who examined the stepheating data of artificial mixtures of two populations of muscovite and biotite. The authors reported that rocks with multiple mica populations produce complex age spectra that yield geologically meaningless ages caused by simultaneous degassing of various mineral “reservoirs” during step heating. When compared to the K–Ar data, the information from 40Ar/39Ar incremental step-heating patterns appear to identify mixtures of detrital and authigenic minerals, as well as variable proportions of distinct generations of authigenic particles. Gas-release patterns even allow evaluation of the amounts of each category of illite by the extents of the combined plateaus with amounts of 39Ar up to 85% of the total released 39Ar (Fig. 6). In Clauer et al. 's (2012b) example, the patterns apparently identify three illite generations of variable crystal sizes that yield ages identical to the K–Ar ages, but which cannot be interpreted as such when using the classical criteria of the 40Ar/39Ar method. In summary, 40Ar/39Ar incremental heating data of various size fractions from a previously undifferentiated illite population might be of help in discriminating mineral generations that resulted from multistage crystallization.

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8. Conclusions The above comparative evaluation of the strengths and weaknesses of the two Ar isotopic methods (K–Ar and 40Ar/39Ar) is based mainly on recent applications on low-temperature K-bearing illite-type clay minerals, together with the basics, technical and analytical aspects of both. Beyond prerequisites that include appropriate and well-controlled mineral preparation, particle size fractionation and mineral characterization, this review evaluates stratigraphic dating of glauconites, indirect dating of low-temperature ore deposits, dating of burial-related illitization, and dating of polyphased tectono-thermal activity, more specifically of fault gouges, by both methods. Pending questions such as the necessary encapsulation due to 39Ar recoil and its restoration into step-heating patterns are addressed, together with the new potential of Ar-dating of nanometric illite crystals. There are differences in the potential of the two Ar methods with the K–Ar method having probably a larger applicability in low-temperature environments than the 40Ar/39Ar method, both being still limited by challenging prerequisites that are mostly due to the low-temperature small-sized mineral crystals of sub-surface environments. Whatever the sample preparation, it needs to be carefully controlled and whatever the characterization methods, they need to end with the best possible mineral identification. The “success” of an isotopic study of mineral mixtures consisting of nanometer- to micrometer-sized crystals is never guaranteed and depends considerably more on the selected material than on the isotopic method used and the analytical reproducibility and/or accuracy of each method. The weaknesses of the K–Ar method are: (1) its pioneering status incorrectly suggesting that it is no longer accurate, (2) the relative uncertainty in the K determinations, and (3) the amount of sample powder used, to a lesser extent. However, these drawbacks become less determining if the method is applied to small crystals such as nanometersized clay minerals. Nanometric extractions are generally homogeneous mineralogically, and the relative uncertainty given by the age calculations, if justified mathematically, is often smaller than the results when they are constrained by duplicate analyses. The weaknesses of the 40Ar/ 39 Ar method are in its basics: (1) the recoil of 39Ar, (2) necessary encapsulation, (3) approximate reintegration of the 39Ar into the step-heating patterns, and (4) meaning of the step-heating patterns. The K–Ar method is definitely preferred for dating diagenetic clay processes such as glauconitization, illite crystal nucleation and growth, or low-temperature hydrothermal activities. The 40Ar/39Ar has apparently a greater potential for dating tectono-thermal activities and in detailing multi-generation illite crystal mixtures, not meaning in turn than the other method is outdated in each of these topics. Acknowledgments This review is partly based on personal publications resulting from a four-decade fruitful collaboration with many colleagues. I am indebted to all who added to the essence of our progressively improved understanding. Some contributed more than others to this Ar playground and among so many, I would like to dedicate special thanks to S. Chaudhuri of Kansas State University, Manhattan Kansas, USA, N. Liewig of the department and now of the Institut Pluridiscilinaire Hubert Curien from the University of Strasbourg, France, J. Środoń of the Polish Academy of Sciences at Krakow, Poland, A. Tremblay of the Université du Québec à Montréal (UQAM), Canada, and O. Brockamp of the Bremen Universität, Germany. A special thank is also due to my scientific icone G. Millot who was the head of the department when I started as a young scientist; without him none of this would have happened. Another hartfelt thank is dedicated to my colleague and friend R. Ferrell Jr. of Louisiana State University at Baton Rouge, USA, whose most appreciated review was necessary to erase the French-biased writing. Many students contributed to completion of an endless number of projects; they were the data producers and the active participants of

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numerous brain storming sessions; they are collectively thanked for this, and for the topics and questions they raised. H. Zwingmann presently at CSIRO, Australia, deserves special thanks; he was the driving person for the installation of the 40Ar/39Ar method at our department. At last but not least, the technical team consisting of R. Wendling, D. Tisserant and R. Winkler who maintained the equipments, blew glass lines, encapsulated vials, repaired leaks, prepared chemical analyses and made Ar extractions when needed, was the daily working elite of the team; they deserve special thanks for their undeflectable involvement. Two reviewers helped keying and improving the topics and related discussions. I am very indebted to them as they raised constructive discussions, even if not firmly agreeing in all launched ideas. My final thanks are for U. Brand, the editor of the journal for his support and kind help in the review round. References Agard, P., Monié, P., 2002. 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