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Diagenetic calcite from the Chazyan Group (Vermont): an example of aragonite alteration in a greenhouse ocean
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Sedimentary Geology 121 (1998) 277–288
Diagenetic calcite from the Chazyan Group (Vermont): an example of aragonite alteration in a greenhouse ocean Kenneth J. Tobin Ł , Kenneth R. Walker Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA Received 15 September 1997; accepted 9 March 1998
Abstract Marine diagenetic calcite with both a calcitic (low-to-intermediate Mg) and aragonitic origin was examined from the middle Ordovician buildups of the Chazyan Group in Vermont. All marine phases have elevated Sr (up to 1800 ppm) compared with that observed from marine precipitates in other middle Ordovician units. Stromatoporoids (labechiids), which were originally aragonitic, have higher Sr values than phases with an original calcite mineralogy (trilobites, marine cement). Additional evidence supporting precursor mineralogy interpretations includes elevated Mg values (up to 3.6 mole% MgCO3 ) and the presence of microdolomite in interpreted calcitic phases. Originally aragonitic precipitates have lower Mg values and most significantly lack microdolomite. This study demonstrates the presence of elevated Sr values in marine precipitates that formed during a period when calcite, not aragonite, was the dominant physiochemically precipitated calcium carbonate mineralogy that formed from sea water. Elevated Sr is attributable to at least a partially open system diagenetic stabilization of biogenic aragonite. 1998 Elsevier Science B.V. All rights reserved. Keywords: calcite; aragonite; greenhouse; stromatoporoids
1. Introduction For over twenty years it has been realized that the mineralogy of abiotic marine carbonate has shifted during the Phanerozoic between high-Mg calcite=aragonite, icehouse, and lower Mg calcite, greenhouse eras (Sandberg, 1975, 1983; Mackenzie and Pigot, 1981). These shifts in facilitated carbonate mineralogy are commonly known as the Sandberg curve, which divides the past 600 m.y. into two greenhouse and three icehouse eras. ReŁ Corresponding
author. Present address: Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 085441003, USA. E-mail: [email protected]
cent work suggests that for the middle Ordovician, which marks the climax of the early-to-mid Paleozoic greenhouse era, that the MgCO3 composition of abiotic marine calcite ranged from 2 to 5 mole% (low-to-intermediate Mg calcite; Tobin and Walker, 1996, 1997; Tobin et al., 1996). Biotically mediated calcite precipitation is capable of overriding physiochemical secular trends in mineralogy. Organisms that secrete high-Mg calcite (e.g. echinoderms) and aragonite (e.g. gastropods) are common throughout the Phanerozoic (Milliken and Pigott, 1977; Wilkinson, 1979). The purpose of this study is to examine the alteration of an allochem interpreted to have had an originally aragonitic mineralogy, in this case stromatoporoids (Stearn and Mah, 1987; Stearn, 1989),
c 1998 Elsevier Science B.V. All rights reserved. 0037-0738/98/$ – see front matter PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 5 5 - 4
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although, we do not dispute the findings of other researchers that suggest that certain types of stromatoporoids are high-Mg calcite (e.g. Rush and Chafetz, 1991). Additionally, we point out how alteration of this allochem can generate a distinctive chemical signal (elevated Sr) that is readily identifiable against a backdrop of greenhouse abiotic precipitates, which typically have low Sr contents.
2. Geologic setting We examined more than 100 samples from organic buildups at two localities on Isle La Motte in Lake Champlain, Vermont (Fig. 1). We examined samples from the Day Point, Crown Point, and Valcour formations. Organic buildups from the Crown Point Formation consist of boundstones, which contain stromatoporoids, calcareous algae, tabulate corals, sponges, and bryozoans (Pitcher, 1964; Kapp, 1975). Pelamatozoan grainstones are the dominant
Fig. 1. Map showing location of Goodsell Ridge and Fisk Quarry localities.
lithology adjacent to buildups. Kapp (1975) suggested that the Crown Point Formation was deposited in a relatively shallow, open-marine, somewhat agitated setting, an interpretation supported by more recent work (Hampt and Droser, 1990; Bechtel, 1993). The overlying Valcour Formation represents deeper water conditions with bryozoans and algae dominant in the buildups from this unit (Pitcher, 1964). These units range in age from late Llanvirn to early Caradoc (Fig. 2). Based on vitrinite reflectance, fluid inclusion homogenization temperature, and stable isotopic data Friedman (1987) suggested that the Chazyan carbonates were subjected to maximum burial temperatures in excess of 100ºC and a maximum burial depth of 5 km, a conclusion consistent with conodont-color alteration data (Epstein et al., 1977; Harris, 1979).
3. Methods Nearly 100 thin sections were examined petrographically. Selected thin sections were polished and examined with a CITL Cold Cathode Luminescence 8200 mk3 microscope (voltage 10 kV, beam current 150–180 µA, and chamber pressure 180–200 mTorr). The cathodoluminescence of areas that were analyzed with the electron microprobe was documented with photography both before and after analysis. Isotopic samples of carbonate cement were obtained by drilling areas on billets, matched as closely as possible with the accompanying thin section, with a microscope-mounted microdrill. Samples were roasted at 380ºC for 1 h to remove volatile organic matter and were reacted off-line with 100% H3 PO4 at 25ºC for at least 12 h (after the method of McCrea, 1950). Overall reproducibility of data, as obtained by analysis of lab standards, is 0.2‰ for Ž13 C and Ž18 O. Isotopic ratios were measured on a VG-903 isotope ratio mass spectrometer. Carbonate values are all reported relative to the PDB standard. Wavelength dispersive electron microprobe analysis for Mg, Ca, Mn, Fe, and Sr in samples of calcite was performed on polished thin sections using a Cameca SX-50 Electron Microprobe with an accelerating voltage of 25 kV, a beam current of 10 nA, and a defocused beam of 10 to 20 µm in width
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Fig. 2. Chronostratigraphic and biostratigraphic relationships for the Chazyan Group (* D units examined in this study).
to minimize sample volatilization. Standard count times were: Ca D 10 s, Mg D 20 s, Mn, Fe, and Sr D 60 s with corresponding detection limits of Mg D 0.1 mole% MgCO3 , Mn and Fe D 100 ppm, and Sr D 200 ppm. Calibration error was monitored by analysis of well-characterized carbonate standards at the beginning and end of each day. In addition, the Mg distribution in selected diagenetic phases was observed with backscatter X-rays and elemental X-ray map images.
4. Petrography and cathodoluminescence 4.1. General characteristics of calcite cements Syntaxial overgrowths, which have both a turbid and translucent appearance, are the most abundant calcite cement in echinoderm-dominated microfacies. The equant and bladed-to-fibrous calcite cements that are dominant in microfacies with a paucity of echinoderms are the focus of this study. These cements formed in intergranular, intragranular, shelter, and moldic porosity. Additionally, when present in the same sample equant calcite cements post-date bladed-to-fibrous calcite cements. Translucent equant calcite is the most common type of calcite cement in the Chazyan buildups and ranges in size from <0.01 mm up to several mm’s.
This cement has either a drusy or non-drusy fabric. Drusy equant calcite is typically fine-grained (generally <0.01 mm) and commonly occludes relatively small former intergranular, intragranular, and moldic voids (Fig. 3a,c). Coarse-grained (non-drusy) cement can occlude the entire former porosity of a void (typically large moldic) and also is present in fractures (Fig. 3b). Equant cement has complex CL zonation. Concentric and sector zonation is present with luminescence spanning the spectrum from nonluminescent to bright luminescent (Fig. 3d,f). Luminescence of fringe cement is variable. Conversely, pore-central equant calcite is most commonly dull luminescent (Fig. 3f). Less common is bladed-to-fibrous calcite. Most bladed-to-fibrous calcite has a turbid appearance (Fig. 3c,e). This cement has a dark gray or light brown color in transmitted light. Turbid bladed-tofibrous cement is typically radiaxial fibrous, although some fascicular-optic and even radial fibrous turbid cement is present. Less common is translucent bladed-to-fibrous calcite (Fig. 3e). Translucent bladed-to-fibrous calcite is the initial cement present within shelter voids on certain substrates (trilobites, brachiopods) forming a thin fringe of cement. This cement also is present in some intragranular voids. Some translucent bladed-to-fibrous cement has a radial fibrous fabric; crystals are generally <100 µm in length, and this cement is similar to that described
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Fig. 3. Plane-polarized transmitted light and cathodoluminescence photomircographs of calcite cements from the Chazyan Group. (a) Equant (drusy) calcite (D) occluding intergranular porosity. Allochems include bryozoans (B) and echinoderm grains (E). (b) Equant (non-drusy) calcite occluding moldic and fracture (F) porosity. (c,d) Paired plane-polarized transmitted light and cathodoluminescence photomicrographs. Bryozoan intragranular porosity filled with micrite (M), turbid bladed-to-fibrous calcite (T), and equant (drusy) calcite (D). Micrite and turbid bladed-to-fibrous calcite is mainly dull luminescent. Equant (drusy) calcite exhibits nonluminescent to bright luminescent concentric zonation. (e, f) Paired plane-polarized transmitted light and cathodoluminescence photomicrographs. Gastropod moldic=intragranular void filled with translucent bladed-to-fibrous calcite (arrow), turbid bladed-to-fibrous calcite (T), and equant (drusy) calcite (D). Translucent bladed-to-fibrous calcite has nonluminescent cores and dull luminescent rims, turbid bladed-to-fibrous calcite is mainly dull luminescent, and equant calcite is dull to bright luminescent.
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by Wilson and Palmer (1992). When both translucent and turbid bladed-to-fibrous calcite is present in the same sample it is the translucent cement that predates the turbid cement (Fig. 3e,f). Translucent bladed-tofibrous cement has nonluminescent (or very dull luminescent) crystal cores and dull luminescent rims with minor scattered moderate to bright luminescent patches throughout the cement (Fig. 3f). Conversely, turbid bladed-to-fibrous calcite is more uniformly dull luminescent (Fig. 3d,f). 4.2. Replacement calcite and moldic calcite cements Replacement and dissolution of metastable allochems is particularly common in the Chazyan Group. Both allochems with an original aragonitic mineralogy (stromatoporoids, evidence presented later, Fig. 4a; gastropods, Fig. 4b) and calcitic mineralogy (trilobites, Fig. 4c; bryozoans, Fig. 4d; although some aragonitic bryozoans have been described, McKinney, 1971) are either replaced by turbid replacement calcite or, in the case of all gastropods and some trilobites, are selectively dissolved and resulting molds are occluded by translucent equant or bladed-to-fibrous calcite cement. Replacement calcite in all types of these former allochems can be distinguished from cement by the following characteristics (compare Fig. 4d,e with Fig. 3a–c). (1) Replacement calcite is typically equant in crystal habit, fine grained (<0.01 mm), and lacks a drusy fabric. (2) Replacement calcite is typically turbid due to the presence of organic inclusions (pseudopleochroism) and relict skeletal material whereas most calcite cement is generally translucent in appearance. (3) Particularly in the case of bryozoans, areas of incomplete replacement where relict intragranular porosity filling turbid marine cement and=or internal sediment (micrite) can be recognized (Fig. 4f). Replacement calcite has a CL that ranges from nonluminescent to dull luminescent.
5. Minor element and stable isotope data Minor element data was obtained from two samples of the Crown Point and one of Valcour Formation. Replacement calcite after an aragonitic precur-
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sor (stromatoporoid, Crown Point) has relatively low MgCO3 (n D 16, 0.2 to 1.5 mole%, Fig. 5), variable Sr (below detection limit of 200 ppm to 1600 ppm, Fig. 5), and no microdolomite (n D 3). Conversely, turbid replacement calcite after a calcitic precursor (trilobite, Crown Point), and turbid bladed-tofibrous calcite (Crown Point, Valcour), have variable MgCO3 (n D 10, 0.2 to 3.6 mole%; n D 56, 0.3 to 2.8 mole%) and generally lower Sr (below detection limit of 200 ppm to 1200 ppm; below detection limit to 900 ppm) relative to other analyzed phases (Figs. 6 and 7). Additionally, the above phases have significant microdolomite (n D 1, 0.3 vol.%; n D 23, 0 to 5.0 vol.%; Fig. 8). Translucent equant (drusy) and bladed-to-fibrous calcite cements associated with altered stromatoporoids (Crown Point) have low MgCO3 (n D 13, <0.1 to 1.5 mole%; n D 23, <0.1 to 1.4 mole%, Figs. 5 and 6), variable Sr (below detection limit to 1700 ppm; below detection limit to 1800 ppm, Figs. 5 and 6), and no microdolomite (n D 6, n D 7). Equant (nondrusy) calcite cement (Crown Point, Valcour) has low MgCO3 (n D 39, 0.3 to 0.9 mole%) and Sr (below detection limit to 800 ppm) values (Fig. 7). In all the phases examined Mn concentrations are low (below detection limit of 100 ppm to 300 ppm; Figs. 5–7). Replacement calcite (both types) have Fe values near or below the detection limit of 100 ppm with a maximum of 400 ppm for trilobite replacement calcite (Figs. 5 and 6). Translucent bladed-to-fibrous and equant (drusy) calcite has low Fe ranging from below detection limit to 200 ppm (Figs. 5 and 6). Turbid bladed-to-fibrous calcite has variable and generally more elevated Fe (below detection limit to 1600 ppm; Fig. 7). Conversely, equant (non-drusy) calcite generally has low Fe near or below detection limit but one analysis of 700 ppm is present (Fig. 7). Mixed replacement calcite and equant calcite cement .n D 7/ have a Ž13 C of 0 to 0.4‰ and a Ž18 O of 6.1 to 7.0‰ PDB (Fig. 9). Translucent equant (drusy) calcite .n D 4/ has similarly a Ž13 C of 0 to 0.3‰ and a Ž18 O of 6.7 to 7.9‰, as well as equant (non-drusy) calcite and equant calcite in veins (respectively: n D 7, n D 5; Ž13 C D 0.1 to 0.7‰, C0.1 to 1.0‰; Ž18 O D 6.5 to 8.3‰, 7.1 to 8.3‰; Fig. 9). Finally, Chazyan brachiopods .n D 15/ analyzed by Qing and Veizer
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Fig. 4. Plane-polarized transmitted light photomicrographs of cement and replacement calcite from the Chazyan Group. (a) Stromatoporoid (S) replaced by turbid calcite. In contrast is void filling cement, which has a translucent equant appearance. (b) Close-up of a moldic void after gastropod (arrows), which is occluded with equant calcite. (c) Moldic void after trilobite filled with equant calcite cement (arrow). (d) Bryozoan (B), which is locally replaced by turbid replacement calcite. Note contrast with void filling translucent equant calcite (E). (e) Close-up of turbid microspar, which replaces a stromatoporoid. (f) Moldic void after bryozoan filled with replacement calcite with relict micritic internal sediment (arrows), which was deposited before replacement.
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Fig. 5. Replacement calcite after aragonite precursor and associated cements. (A) MgCO3 versus Fe, (B) MgCO3 versus Sr composition of calcite from the Chazyan Group. Analyses below MgCO3 detection limit (<0.1 mole%) are plotted on the x-axis. Percentage of analyses for Fe below the detection limit (100 ppm) is indicated in the inset. Explanation of symbols: replacement calcite after an aragonitic precursor (squares); drusy equant calcite associated with aragonitic allochems (crosses); translucent bladed-to-fibrous calcite associated with aragonitic allochems (upright arrows).
(1994) have Ž13 C D 0 to 0.6‰ and Ž18 O D 6.4 to 8.2‰ (Fig. 9). Note that bladed-to-fibrous calcite was confined to small zones that proved difficult to drill without inclusion of adjacent phases and hence was not analyzed. Data from the Crown Point Formation are indicated in Fig. 9b.
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Fig. 6. Replacement calcite after calcite precursor and associated cement. (A) MgCO3 versus Fe, (B) MgCO3 versus Sr composition of calcite from the Chazyan Group. Analyses below MgCO3 detection limit (<0.1 mole%) are plotted on the x-axis. Percentage of analyses for Fe below the detection limit (100 ppm) is indicated in the inset. Explanation of symbols: replacement calcite after a calcitic precursor (crossed squares); drusy equant calcite associated with calcitic allochems (triangles).
6. Low-to-intermediate Mg calcitic marine precipitates Turbid bladed-to-fibrous calcite has variable MgCO3 and significant microdolomite, characteristics present in altered Ordovician marine cements from other units (Tobin et al., 1996; Tobin and Walker, 1996, 1997). The highly variable MgCO3
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Fig. 8. Mg X-ray maps with light gray representing calcite and darker areas microdolomite (arrows). (a) Turbid fibrous calcite. (b) Replacement calcite after a possible trilobite.
Brand, 1989), exhibits a similar range of MgCO3 and microdolomite compositions. Additionally, turbid bladed-to-fibrous calcite typically have a significant degree of chemical heterogeneity as reflected by variable and generally elevated Fe concentrations, particularly when compared with least-altered marine cements from other Ordovician units that have Fe values 100 ppm (Tobin et al., 1996; Tobin and Walker, 1996, 1997). Significantly, these observations document the diagenetic susceptibility and metastability of abiotic and biotic marine calcite phases, which is contrary to the commonly assumed stability of greenhouse marine calcite precipitates (Sandberg, 1983; James and Choquette, 1984).
7. Stromatoporoids: an aragonitic marine precipitate Fig. 7. Other calcite phases. (A) MgCO3 versus Fe, (B) MgCO3 versus Sr composition of calcite from the Chazyan Group. Analyses below MgCO3 detection limit (<0.1 mole%) are plotted on the x-axis. Percentage of analyses for Fe below the detection limit (100 ppm) is indicated in the inset. Explanation of symbols: non-drusy equant calcite (plusses); micrite (diamonds), and turbid bladed-to-fibrous calcite (circles).
compositions measured in turbid bladed-to-fibrous calcite are likely due to the mixture of calcite and microdolomite within the zone analyzed by the electron beam. During the middle Ordovician the original mineralogy of marine cement is thought to be low-to-intermediate Mg calcite with 2 to 5 mole% MgCO3 (Tobin and Walker, 1997). Trilobite replacement calcite, an allochem thought to have an intermediate-Mg calcite mineralogy (McAllister and
Stromatoporoids analyzed in this study are from the Crown Point Formation and consist of labechiids from zone II of Kapp and Strean (1975). Unlike the above turbid marine diagenetic phases altered stromatoporoids lack microdolomite, have low MgCO3 (1 mole%; Fig. 5), and have variable but generally elevated Sr values (up to 1600 ppm; Fig. 5) compared with Ordovician marine precipitates interpreted to have a low-to-intermediate Mg calcite mineralogy (Section 6; Tobin et al., 1996; Tobin and Walker, 1996, 1997). Based on these observations we suggest that the examined Chazyan stromatoporoids originally had an aragonitic mineralogy, which is consistent with previous original mineralogy interpretations of the labechiids (Stearn and Mah, 1987; Stearn, 1989; Stock, pers. commun.).
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Fig. 9. Ž 13 C and Ž 18 O data from the Chazyan Group. Data from this study are black, and data from Qing and Veizer (1994) are gray. Data include: brachiopods (triangles); mixed replacement calcite and equant calcite (stars); drusy equant (crosses) non-drusy equant calcite (plusses), and equant calcite in veins (ð-signs).
8. Stable isotopic and depositional constraints on environment of diagenetic alteration Both (1) mixed replacement calcite and translucent equant calcite and (2) equant (drusy) calcite have Ž13 C (0 to 1‰) and Ž18 O values ( 6 to 8‰) that are similar to the stable isotopic compositions of meteoric calcite from other low latitudinal Ordovician carbonate platforms (Rao, 1990; Steinhauff, 1993; Tobin and Walker, 1994; Kher and
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Parker, 1995; Wu and Wu, 1996). Given the shallow depositional setting of particularly the Crown Point Formation (Kapp, 1975; Hampt and Droser, 1990), the unit from which most of the isotopic data were obtained (Fig. 9b), it is conceivable that the alteration of at least some of the metastable precipitates in the Chazyan buildups occurred in a meteoric diagenetic setting. While the middle Ordovician is a period in which there is limited evidence for a terrestrial biota (Gray and Shear, 1992) that could have locally elevated pCO2 immediately below subaerial exposure surfaces due to respiration, this period is marked by an elevated atmospheric pCO2 (Berner, 1991, 1994; Yapp and Poths, 1992) that could have facilitated dissolution in the meteoric realm. Alternatively, brachiopods analyzed by Qing and Veizer (1994) from the Chazyan Group overlap in isotopic composition with the above phases and an argument could be advanced that these brachiopods and the above observed diagenetic phases formed and were altered by sea water. However, Qing and Veizer (1994) exclude Chazyan brachiopod values from their well preserved band of stable isotopic values for the Ordovician. Additionally, the 2‰ range in brachiopod Ž18 O values is not characteristic of well-preserved marine precipitates, which typically exhibit a range of 1‰ (Carpenter et al., 1991; Tobin and Walker, 1997). Consequently, it is likely that the stable isotopic values from the diagenetic phases analyzed in this study do not preserve or reflect a marine signature. An additional alternative is that the Chazyan isotopic values reflect a burial diagenetic signature. However, aragonite (and calcite) stabilization is typically an early diagenetic process (e.g. Land et al., 1967; Gavish and Freidman, 1969) and it is rare for these phases to persist for protracted periods in the burial diagenetic realm (Choquette and James, 1987).
9. Early, translucent equant-to-bladed calcite cement: an example of high Sr values during a greenhouse era Based on the similarity in minor element and microdolomite content between turbid replacement microspar after stromatoporoids and translucent ce-
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ment that occluded primary intragranular porosity within stromatoporoids from the Crown Point Formation it is likely that precipitation of some of the early translucent calcite cements (both equant and bladed-to-fibrous) in this unit occurred in a meteoric setting and were genetically linked with stromatoporoid diagenesis. The stable isotopic composition of equant (drusy) calcite supports this interpretation. Of additional significance is the lack of microdolomite, low MgCO3 , and variable but generally elevated Sr values indicative of both precipitated and replacement calcite within the Crown Point stromatoporoids. If the water=rock ratio during meteoric stabilization of aragonite is low (closed system) then the Sr=Ca ratio of the diagenetic fluid can approach that of the dissolving aragonite (Pingitore, 1982). Conversely, under open system conditions the Sr2C =Ca2C ratio of the diagenetic fluid should be significantly lower. For example, let us assume that the original Sr content of the stromatoporoid replacement calcite is reflected by the highest observed value (1600 ppm) and K d,Sr D 0:05 (typically in settings with slow calcite formation; see Morse and Mackenzie, 1990) resulting in a molar Sr2C =Ca2C ratio in the diagenetic fluid that is constrained between 0.037 (closed system) and 0.0018 (open system). The corresponding diagenetic calcite Sr2C =Ca2C ratios are 1:8 ð 10 3 (closed system) and 9:2 ð 10 5 (open system) with equivalent Sr concentrations in calcite of 1600 ppm (closed system) and 80 ppm (open system). The high Sr content of some equant (drusy) calcite cement (up to 1700 ppm) implies formation of this phase under at least partially closed conditions (with respect to Sr) during aragonite stabilization. It is significant to note that later, non-drusy equant calcite mainly occluded pore-central porosity (as well as calcitic metastable phases) have significantly lower Sr values suggesting either (1) precipitation=stabilization under conditions that were genetically unrelated to aragonite transformation, or (2) formation=alteration of these phases occurred during aragonite transformation but under more open conditions than associated with precipitation of translucent equant (drusy) and bladed-to-fibrous calcite within intragranular stromatoporoid vugs. The Sr content of both altered and unaltered middle Ordovician marine cements from nearly coeval units else-
where in Laurentia and Baltica are typically lower (<200 ppm; Tobin et al., 1996; Tobin and Walker, 1996, 1997) than observed in Chazyan metastable calcite phases (<200 ppm to 900 ppm) supporting the premise that the stabilization of at least some of the Chazyan metastable calcite occurred during open system aragonite transformation. In summary, alteration of biotically generated aragonite should be considered as a potentially important mechanism that can account for high Sr values in diagenetic precipitates formed during greenhouse periods such as the Ordovician. It is suggested that detailed diagenetic characterization is needed to rule out this possibility. In the absence of such documentation interpretation of transient aragonite ‘blimps’ in calcite seas based on Sr values in micrite of >800 ppm (Lasemi and Sandberg, 1993) may not be warranted (e.g. Gao et al., 1996).
10. Summary The diagenetic alteration of biogenic aragonitic phases from the Chazyan buildups yield a distinctively elevated Sr signature preserved in diagenetic calcite. In formerly metastable low-to-intermediate Mg calcite marine phases Sr values are elevated relative to values from both altered and unaltered middle Ordovician marine cements elsewhere, although Sr values in former Mg-calcite phases are lower than in diagenetic calcite inferred to have had an originally aragonitic mineralogy. These results are interpreted to be indicative of alteration of all metastable components during an episode of stabilization of originally aragonitic allochems early in the diagenetic history of these buildups. Significantly, this study indicates that elevated Sr values in marine calcite cannot be automatically equated with physiochemical precipitation of aragonite.
Acknowledgements This study has been funded by NSF EAR9315651 (KRW and KJT). A. Patchen provided guidance with electron microprobe operation. J. Johnson made standard thin sections and M. Bennett polished thin sections for electron microprobe analysis. K.
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Howard facilitated collection of stable isotopic data. C. Mehrtens (University of Vermont) is thanked for providing KJT with an introduction to the Chazyan buildups in the field.
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Land, L.S., Mackenzie, F.T., Gould, S.J., 1967. Pleistocene history of Bermuda. Geol. Soc. Am. Bull. 78, 993–1006. Lasemi, Z., Sandberg, P., 1993. Microfacies and compositional clues to dominant mud mineralogy of micritic precursors. In: Rezak, R., Lavoie, D.L. (Eds.), Carbonate Microfacies, Frontiers in Sedimentary Geology. Springer-Verlag, Berlin, pp. 173–185. Mackenzie, F.T., Pigot, J.P., 1981. Tectonic controls of Phanerozoic sedimentary rock cycling. J. Geol. Soc. London 138, 183–196. McAllister, J.E., Brand, U., 1989. Geochemistry of some Ordovician and Devonian trilobite cuticles from North America. Chem. Geol. 78, 51–63. McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotemperature scale. J. Chem. Phys. 18, 849–857. McKinney, F.K., 1971. Trepostomatous Ectoprocta (Bryozoa) from the lower Chickamauga Group (Middle Ordovician), Wills Valley, Alabama. Am. Paleontol. Bull. 60, 195–337. Milliken, K.L., Pigott, J.D., 1977. Variations of oceanic Mg=Ca ratio through time — implications for the calcite sea. Geol. Soc. Am., Abstr. Progr. 7 (1), 64–65. Morse, J.W., Mackenzie, F.T., 1990. Geochemistry of Sedimentary Carbonates. Developments in Sedimentology 12, Elsevier, Amsterdam, 707 pp. Pingitore, N.E., 1982. The role of diffusion during carbonate diagenesis. J. Sediment. Petrol. 52, 27–39. Pitcher, M., 1964. Evolution of Chazyan (Ordovician) reefs of eastern United States and Canada. Bull. Can. Pet. Geol. 12, 632–691. Qing, H., Veizer, J., 1994. Oxygen and carbon isotopic composition of Ordovician brachiopods: implications for coeval seawater. Geochim. Cosmochim. Acta 58, 4429–4442. Rao, C.P., 1990. Marine to mixing zone dolomitization in peritidal carbonates: the Gordon Group (Ordovician), Mole Creek, Tasmania, Australia. Carbonates Evaporites 5, 153–178. Rush, P.F., Chafetz, H.S., 1991. Skeletal mineralogy of Devonian stromatoporoids. J. Sediment. Petrol. 61, 364–370. Sandberg, P.A., 1975. New interpretation of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy. Sedimentology 22, 497–538. Sandberg, P.A., 1983. An oscillating trend in Phanerozoic non-skeletal carbonate mineralogy. Nature 305, 19–22. Stearn, C.W., 1989. Specks in the microstructure of Paleozoic stromatoporoids. Mem. Assoc. Australas. Palaeontol. 8, 143– 148. Stearn, C.W., Mah, A.J., 1987. Skeletal microstructure of Paleozoic stromatoporoids and its mineralogical implications. Palaios 2, 76–84. Steinhauff, D.M., 1993. Sequence Stratigraphy and Comparative Diagenesis of Middle Ordovician Shelf to Shelf-Edge Limestones, the Copper Creek Allochthon, East Tennessee. Ph.D. Dissertation, University of Tennessee, Knoxville, 460 pp. Tobin, K.J., Walker, K.R., 1994. Meteoric diagenesis below a submerged platform: implications for Ž 13 C compositions prior to pre-vascular plant evolution, Middle Ordovician, Alabama, U.S.A. Sediment. Geol. 90, 95–111. Tobin, K.J., Walker, K.R., 1996. Ordovician low-to-intermediate
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Mg calcite from Sweden: an example of marine stabilization and implications for Ordovician stable C–O marine isotopes. Sedimentology 43, 719–735. Tobin, K.J., Walker, K.R., 1997. Ordovician oxygen isotopes and paleotemperatures. Palaeogeogr., Palaeoclimatol., Palaeoecol. 129, 269–290. Tobin, K.J., Walker, K.R., Steinhauff, D.M., Mora, C.I., 1996. Fibrous calcite from the Ordovician of Tennessee: preservation of marine oxygen isotopic composition and its implications. Sedimentology 43, 235–251. Wilkinson, B.H., 1979. Biomineralization, paleoceanography,
and the evolution of calcareous marine organisms. Geology 7, 524–527. Wilson, M.A., Palmer, T.J., 1992. Hardground and hardground faunas. Univ. Wales, Aberystwyth, Inst. Earth Stud. Publ. 9, 131 pp. Wu, Y., Wu, Z., 1996. Two-phase diagenetic alteration of carbonate matrix: Middle Ordovician limestones, Taiyuan City area, China. Sediment. Geol. 106, 177–191. Yapp, C.J., Poths, H., 1992. Ancient atmospheric CO2 pressures inferred from natural goethites. Nature 355, 342–343.
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Diagenetic calcite from the Chazyan Group (Vermont): an example of aragonite alteration in a greenhouse ocean Kenneth J. Tobin Ł , Kenneth R. Walker Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA Received 15 September 1997; accepted 9 March 1998
Abstract Marine diagenetic calcite with both a calcitic (low-to-intermediate Mg) and aragonitic origin was examined from the middle Ordovician buildups of the Chazyan Group in Vermont. All marine phases have elevated Sr (up to 1800 ppm) compared with that observed from marine precipitates in other middle Ordovician units. Stromatoporoids (labechiids), which were originally aragonitic, have higher Sr values than phases with an original calcite mineralogy (trilobites, marine cement). Additional evidence supporting precursor mineralogy interpretations includes elevated Mg values (up to 3.6 mole% MgCO3 ) and the presence of microdolomite in interpreted calcitic phases. Originally aragonitic precipitates have lower Mg values and most significantly lack microdolomite. This study demonstrates the presence of elevated Sr values in marine precipitates that formed during a period when calcite, not aragonite, was the dominant physiochemically precipitated calcium carbonate mineralogy that formed from sea water. Elevated Sr is attributable to at least a partially open system diagenetic stabilization of biogenic aragonite. 1998 Elsevier Science B.V. All rights reserved. Keywords: calcite; aragonite; greenhouse; stromatoporoids
1. Introduction For over twenty years it has been realized that the mineralogy of abiotic marine carbonate has shifted during the Phanerozoic between high-Mg calcite=aragonite, icehouse, and lower Mg calcite, greenhouse eras (Sandberg, 1975, 1983; Mackenzie and Pigot, 1981). These shifts in facilitated carbonate mineralogy are commonly known as the Sandberg curve, which divides the past 600 m.y. into two greenhouse and three icehouse eras. ReŁ Corresponding
author. Present address: Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 085441003, USA. E-mail: [email protected]
cent work suggests that for the middle Ordovician, which marks the climax of the early-to-mid Paleozoic greenhouse era, that the MgCO3 composition of abiotic marine calcite ranged from 2 to 5 mole% (low-to-intermediate Mg calcite; Tobin and Walker, 1996, 1997; Tobin et al., 1996). Biotically mediated calcite precipitation is capable of overriding physiochemical secular trends in mineralogy. Organisms that secrete high-Mg calcite (e.g. echinoderms) and aragonite (e.g. gastropods) are common throughout the Phanerozoic (Milliken and Pigott, 1977; Wilkinson, 1979). The purpose of this study is to examine the alteration of an allochem interpreted to have had an originally aragonitic mineralogy, in this case stromatoporoids (Stearn and Mah, 1987; Stearn, 1989),
c 1998 Elsevier Science B.V. All rights reserved. 0037-0738/98/$ – see front matter PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 5 5 - 4
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although, we do not dispute the findings of other researchers that suggest that certain types of stromatoporoids are high-Mg calcite (e.g. Rush and Chafetz, 1991). Additionally, we point out how alteration of this allochem can generate a distinctive chemical signal (elevated Sr) that is readily identifiable against a backdrop of greenhouse abiotic precipitates, which typically have low Sr contents.
2. Geologic setting We examined more than 100 samples from organic buildups at two localities on Isle La Motte in Lake Champlain, Vermont (Fig. 1). We examined samples from the Day Point, Crown Point, and Valcour formations. Organic buildups from the Crown Point Formation consist of boundstones, which contain stromatoporoids, calcareous algae, tabulate corals, sponges, and bryozoans (Pitcher, 1964; Kapp, 1975). Pelamatozoan grainstones are the dominant
Fig. 1. Map showing location of Goodsell Ridge and Fisk Quarry localities.
lithology adjacent to buildups. Kapp (1975) suggested that the Crown Point Formation was deposited in a relatively shallow, open-marine, somewhat agitated setting, an interpretation supported by more recent work (Hampt and Droser, 1990; Bechtel, 1993). The overlying Valcour Formation represents deeper water conditions with bryozoans and algae dominant in the buildups from this unit (Pitcher, 1964). These units range in age from late Llanvirn to early Caradoc (Fig. 2). Based on vitrinite reflectance, fluid inclusion homogenization temperature, and stable isotopic data Friedman (1987) suggested that the Chazyan carbonates were subjected to maximum burial temperatures in excess of 100ºC and a maximum burial depth of 5 km, a conclusion consistent with conodont-color alteration data (Epstein et al., 1977; Harris, 1979).
3. Methods Nearly 100 thin sections were examined petrographically. Selected thin sections were polished and examined with a CITL Cold Cathode Luminescence 8200 mk3 microscope (voltage 10 kV, beam current 150–180 µA, and chamber pressure 180–200 mTorr). The cathodoluminescence of areas that were analyzed with the electron microprobe was documented with photography both before and after analysis. Isotopic samples of carbonate cement were obtained by drilling areas on billets, matched as closely as possible with the accompanying thin section, with a microscope-mounted microdrill. Samples were roasted at 380ºC for 1 h to remove volatile organic matter and were reacted off-line with 100% H3 PO4 at 25ºC for at least 12 h (after the method of McCrea, 1950). Overall reproducibility of data, as obtained by analysis of lab standards, is 0.2‰ for Ž13 C and Ž18 O. Isotopic ratios were measured on a VG-903 isotope ratio mass spectrometer. Carbonate values are all reported relative to the PDB standard. Wavelength dispersive electron microprobe analysis for Mg, Ca, Mn, Fe, and Sr in samples of calcite was performed on polished thin sections using a Cameca SX-50 Electron Microprobe with an accelerating voltage of 25 kV, a beam current of 10 nA, and a defocused beam of 10 to 20 µm in width
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Fig. 2. Chronostratigraphic and biostratigraphic relationships for the Chazyan Group (* D units examined in this study).
to minimize sample volatilization. Standard count times were: Ca D 10 s, Mg D 20 s, Mn, Fe, and Sr D 60 s with corresponding detection limits of Mg D 0.1 mole% MgCO3 , Mn and Fe D 100 ppm, and Sr D 200 ppm. Calibration error was monitored by analysis of well-characterized carbonate standards at the beginning and end of each day. In addition, the Mg distribution in selected diagenetic phases was observed with backscatter X-rays and elemental X-ray map images.
4. Petrography and cathodoluminescence 4.1. General characteristics of calcite cements Syntaxial overgrowths, which have both a turbid and translucent appearance, are the most abundant calcite cement in echinoderm-dominated microfacies. The equant and bladed-to-fibrous calcite cements that are dominant in microfacies with a paucity of echinoderms are the focus of this study. These cements formed in intergranular, intragranular, shelter, and moldic porosity. Additionally, when present in the same sample equant calcite cements post-date bladed-to-fibrous calcite cements. Translucent equant calcite is the most common type of calcite cement in the Chazyan buildups and ranges in size from <0.01 mm up to several mm’s.
This cement has either a drusy or non-drusy fabric. Drusy equant calcite is typically fine-grained (generally <0.01 mm) and commonly occludes relatively small former intergranular, intragranular, and moldic voids (Fig. 3a,c). Coarse-grained (non-drusy) cement can occlude the entire former porosity of a void (typically large moldic) and also is present in fractures (Fig. 3b). Equant cement has complex CL zonation. Concentric and sector zonation is present with luminescence spanning the spectrum from nonluminescent to bright luminescent (Fig. 3d,f). Luminescence of fringe cement is variable. Conversely, pore-central equant calcite is most commonly dull luminescent (Fig. 3f). Less common is bladed-to-fibrous calcite. Most bladed-to-fibrous calcite has a turbid appearance (Fig. 3c,e). This cement has a dark gray or light brown color in transmitted light. Turbid bladed-tofibrous cement is typically radiaxial fibrous, although some fascicular-optic and even radial fibrous turbid cement is present. Less common is translucent bladed-to-fibrous calcite (Fig. 3e). Translucent bladed-to-fibrous calcite is the initial cement present within shelter voids on certain substrates (trilobites, brachiopods) forming a thin fringe of cement. This cement also is present in some intragranular voids. Some translucent bladed-to-fibrous cement has a radial fibrous fabric; crystals are generally <100 µm in length, and this cement is similar to that described
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Fig. 3. Plane-polarized transmitted light and cathodoluminescence photomircographs of calcite cements from the Chazyan Group. (a) Equant (drusy) calcite (D) occluding intergranular porosity. Allochems include bryozoans (B) and echinoderm grains (E). (b) Equant (non-drusy) calcite occluding moldic and fracture (F) porosity. (c,d) Paired plane-polarized transmitted light and cathodoluminescence photomicrographs. Bryozoan intragranular porosity filled with micrite (M), turbid bladed-to-fibrous calcite (T), and equant (drusy) calcite (D). Micrite and turbid bladed-to-fibrous calcite is mainly dull luminescent. Equant (drusy) calcite exhibits nonluminescent to bright luminescent concentric zonation. (e, f) Paired plane-polarized transmitted light and cathodoluminescence photomicrographs. Gastropod moldic=intragranular void filled with translucent bladed-to-fibrous calcite (arrow), turbid bladed-to-fibrous calcite (T), and equant (drusy) calcite (D). Translucent bladed-to-fibrous calcite has nonluminescent cores and dull luminescent rims, turbid bladed-to-fibrous calcite is mainly dull luminescent, and equant calcite is dull to bright luminescent.
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by Wilson and Palmer (1992). When both translucent and turbid bladed-to-fibrous calcite is present in the same sample it is the translucent cement that predates the turbid cement (Fig. 3e,f). Translucent bladed-tofibrous cement has nonluminescent (or very dull luminescent) crystal cores and dull luminescent rims with minor scattered moderate to bright luminescent patches throughout the cement (Fig. 3f). Conversely, turbid bladed-to-fibrous calcite is more uniformly dull luminescent (Fig. 3d,f). 4.2. Replacement calcite and moldic calcite cements Replacement and dissolution of metastable allochems is particularly common in the Chazyan Group. Both allochems with an original aragonitic mineralogy (stromatoporoids, evidence presented later, Fig. 4a; gastropods, Fig. 4b) and calcitic mineralogy (trilobites, Fig. 4c; bryozoans, Fig. 4d; although some aragonitic bryozoans have been described, McKinney, 1971) are either replaced by turbid replacement calcite or, in the case of all gastropods and some trilobites, are selectively dissolved and resulting molds are occluded by translucent equant or bladed-to-fibrous calcite cement. Replacement calcite in all types of these former allochems can be distinguished from cement by the following characteristics (compare Fig. 4d,e with Fig. 3a–c). (1) Replacement calcite is typically equant in crystal habit, fine grained (<0.01 mm), and lacks a drusy fabric. (2) Replacement calcite is typically turbid due to the presence of organic inclusions (pseudopleochroism) and relict skeletal material whereas most calcite cement is generally translucent in appearance. (3) Particularly in the case of bryozoans, areas of incomplete replacement where relict intragranular porosity filling turbid marine cement and=or internal sediment (micrite) can be recognized (Fig. 4f). Replacement calcite has a CL that ranges from nonluminescent to dull luminescent.
5. Minor element and stable isotope data Minor element data was obtained from two samples of the Crown Point and one of Valcour Formation. Replacement calcite after an aragonitic precur-
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sor (stromatoporoid, Crown Point) has relatively low MgCO3 (n D 16, 0.2 to 1.5 mole%, Fig. 5), variable Sr (below detection limit of 200 ppm to 1600 ppm, Fig. 5), and no microdolomite (n D 3). Conversely, turbid replacement calcite after a calcitic precursor (trilobite, Crown Point), and turbid bladed-tofibrous calcite (Crown Point, Valcour), have variable MgCO3 (n D 10, 0.2 to 3.6 mole%; n D 56, 0.3 to 2.8 mole%) and generally lower Sr (below detection limit of 200 ppm to 1200 ppm; below detection limit to 900 ppm) relative to other analyzed phases (Figs. 6 and 7). Additionally, the above phases have significant microdolomite (n D 1, 0.3 vol.%; n D 23, 0 to 5.0 vol.%; Fig. 8). Translucent equant (drusy) and bladed-to-fibrous calcite cements associated with altered stromatoporoids (Crown Point) have low MgCO3 (n D 13, <0.1 to 1.5 mole%; n D 23, <0.1 to 1.4 mole%, Figs. 5 and 6), variable Sr (below detection limit to 1700 ppm; below detection limit to 1800 ppm, Figs. 5 and 6), and no microdolomite (n D 6, n D 7). Equant (nondrusy) calcite cement (Crown Point, Valcour) has low MgCO3 (n D 39, 0.3 to 0.9 mole%) and Sr (below detection limit to 800 ppm) values (Fig. 7). In all the phases examined Mn concentrations are low (below detection limit of 100 ppm to 300 ppm; Figs. 5–7). Replacement calcite (both types) have Fe values near or below the detection limit of 100 ppm with a maximum of 400 ppm for trilobite replacement calcite (Figs. 5 and 6). Translucent bladed-to-fibrous and equant (drusy) calcite has low Fe ranging from below detection limit to 200 ppm (Figs. 5 and 6). Turbid bladed-to-fibrous calcite has variable and generally more elevated Fe (below detection limit to 1600 ppm; Fig. 7). Conversely, equant (non-drusy) calcite generally has low Fe near or below detection limit but one analysis of 700 ppm is present (Fig. 7). Mixed replacement calcite and equant calcite cement .n D 7/ have a Ž13 C of 0 to 0.4‰ and a Ž18 O of 6.1 to 7.0‰ PDB (Fig. 9). Translucent equant (drusy) calcite .n D 4/ has similarly a Ž13 C of 0 to 0.3‰ and a Ž18 O of 6.7 to 7.9‰, as well as equant (non-drusy) calcite and equant calcite in veins (respectively: n D 7, n D 5; Ž13 C D 0.1 to 0.7‰, C0.1 to 1.0‰; Ž18 O D 6.5 to 8.3‰, 7.1 to 8.3‰; Fig. 9). Finally, Chazyan brachiopods .n D 15/ analyzed by Qing and Veizer
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Fig. 4. Plane-polarized transmitted light photomicrographs of cement and replacement calcite from the Chazyan Group. (a) Stromatoporoid (S) replaced by turbid calcite. In contrast is void filling cement, which has a translucent equant appearance. (b) Close-up of a moldic void after gastropod (arrows), which is occluded with equant calcite. (c) Moldic void after trilobite filled with equant calcite cement (arrow). (d) Bryozoan (B), which is locally replaced by turbid replacement calcite. Note contrast with void filling translucent equant calcite (E). (e) Close-up of turbid microspar, which replaces a stromatoporoid. (f) Moldic void after bryozoan filled with replacement calcite with relict micritic internal sediment (arrows), which was deposited before replacement.
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Fig. 5. Replacement calcite after aragonite precursor and associated cements. (A) MgCO3 versus Fe, (B) MgCO3 versus Sr composition of calcite from the Chazyan Group. Analyses below MgCO3 detection limit (<0.1 mole%) are plotted on the x-axis. Percentage of analyses for Fe below the detection limit (100 ppm) is indicated in the inset. Explanation of symbols: replacement calcite after an aragonitic precursor (squares); drusy equant calcite associated with aragonitic allochems (crosses); translucent bladed-to-fibrous calcite associated with aragonitic allochems (upright arrows).
(1994) have Ž13 C D 0 to 0.6‰ and Ž18 O D 6.4 to 8.2‰ (Fig. 9). Note that bladed-to-fibrous calcite was confined to small zones that proved difficult to drill without inclusion of adjacent phases and hence was not analyzed. Data from the Crown Point Formation are indicated in Fig. 9b.
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Fig. 6. Replacement calcite after calcite precursor and associated cement. (A) MgCO3 versus Fe, (B) MgCO3 versus Sr composition of calcite from the Chazyan Group. Analyses below MgCO3 detection limit (<0.1 mole%) are plotted on the x-axis. Percentage of analyses for Fe below the detection limit (100 ppm) is indicated in the inset. Explanation of symbols: replacement calcite after a calcitic precursor (crossed squares); drusy equant calcite associated with calcitic allochems (triangles).
6. Low-to-intermediate Mg calcitic marine precipitates Turbid bladed-to-fibrous calcite has variable MgCO3 and significant microdolomite, characteristics present in altered Ordovician marine cements from other units (Tobin et al., 1996; Tobin and Walker, 1996, 1997). The highly variable MgCO3
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Fig. 8. Mg X-ray maps with light gray representing calcite and darker areas microdolomite (arrows). (a) Turbid fibrous calcite. (b) Replacement calcite after a possible trilobite.
Brand, 1989), exhibits a similar range of MgCO3 and microdolomite compositions. Additionally, turbid bladed-to-fibrous calcite typically have a significant degree of chemical heterogeneity as reflected by variable and generally elevated Fe concentrations, particularly when compared with least-altered marine cements from other Ordovician units that have Fe values 100 ppm (Tobin et al., 1996; Tobin and Walker, 1996, 1997). Significantly, these observations document the diagenetic susceptibility and metastability of abiotic and biotic marine calcite phases, which is contrary to the commonly assumed stability of greenhouse marine calcite precipitates (Sandberg, 1983; James and Choquette, 1984).
7. Stromatoporoids: an aragonitic marine precipitate Fig. 7. Other calcite phases. (A) MgCO3 versus Fe, (B) MgCO3 versus Sr composition of calcite from the Chazyan Group. Analyses below MgCO3 detection limit (<0.1 mole%) are plotted on the x-axis. Percentage of analyses for Fe below the detection limit (100 ppm) is indicated in the inset. Explanation of symbols: non-drusy equant calcite (plusses); micrite (diamonds), and turbid bladed-to-fibrous calcite (circles).
compositions measured in turbid bladed-to-fibrous calcite are likely due to the mixture of calcite and microdolomite within the zone analyzed by the electron beam. During the middle Ordovician the original mineralogy of marine cement is thought to be low-to-intermediate Mg calcite with 2 to 5 mole% MgCO3 (Tobin and Walker, 1997). Trilobite replacement calcite, an allochem thought to have an intermediate-Mg calcite mineralogy (McAllister and
Stromatoporoids analyzed in this study are from the Crown Point Formation and consist of labechiids from zone II of Kapp and Strean (1975). Unlike the above turbid marine diagenetic phases altered stromatoporoids lack microdolomite, have low MgCO3 (1 mole%; Fig. 5), and have variable but generally elevated Sr values (up to 1600 ppm; Fig. 5) compared with Ordovician marine precipitates interpreted to have a low-to-intermediate Mg calcite mineralogy (Section 6; Tobin et al., 1996; Tobin and Walker, 1996, 1997). Based on these observations we suggest that the examined Chazyan stromatoporoids originally had an aragonitic mineralogy, which is consistent with previous original mineralogy interpretations of the labechiids (Stearn and Mah, 1987; Stearn, 1989; Stock, pers. commun.).
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Fig. 9. Ž 13 C and Ž 18 O data from the Chazyan Group. Data from this study are black, and data from Qing and Veizer (1994) are gray. Data include: brachiopods (triangles); mixed replacement calcite and equant calcite (stars); drusy equant (crosses) non-drusy equant calcite (plusses), and equant calcite in veins (ð-signs).
8. Stable isotopic and depositional constraints on environment of diagenetic alteration Both (1) mixed replacement calcite and translucent equant calcite and (2) equant (drusy) calcite have Ž13 C (0 to 1‰) and Ž18 O values ( 6 to 8‰) that are similar to the stable isotopic compositions of meteoric calcite from other low latitudinal Ordovician carbonate platforms (Rao, 1990; Steinhauff, 1993; Tobin and Walker, 1994; Kher and
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Parker, 1995; Wu and Wu, 1996). Given the shallow depositional setting of particularly the Crown Point Formation (Kapp, 1975; Hampt and Droser, 1990), the unit from which most of the isotopic data were obtained (Fig. 9b), it is conceivable that the alteration of at least some of the metastable precipitates in the Chazyan buildups occurred in a meteoric diagenetic setting. While the middle Ordovician is a period in which there is limited evidence for a terrestrial biota (Gray and Shear, 1992) that could have locally elevated pCO2 immediately below subaerial exposure surfaces due to respiration, this period is marked by an elevated atmospheric pCO2 (Berner, 1991, 1994; Yapp and Poths, 1992) that could have facilitated dissolution in the meteoric realm. Alternatively, brachiopods analyzed by Qing and Veizer (1994) from the Chazyan Group overlap in isotopic composition with the above phases and an argument could be advanced that these brachiopods and the above observed diagenetic phases formed and were altered by sea water. However, Qing and Veizer (1994) exclude Chazyan brachiopod values from their well preserved band of stable isotopic values for the Ordovician. Additionally, the 2‰ range in brachiopod Ž18 O values is not characteristic of well-preserved marine precipitates, which typically exhibit a range of 1‰ (Carpenter et al., 1991; Tobin and Walker, 1997). Consequently, it is likely that the stable isotopic values from the diagenetic phases analyzed in this study do not preserve or reflect a marine signature. An additional alternative is that the Chazyan isotopic values reflect a burial diagenetic signature. However, aragonite (and calcite) stabilization is typically an early diagenetic process (e.g. Land et al., 1967; Gavish and Freidman, 1969) and it is rare for these phases to persist for protracted periods in the burial diagenetic realm (Choquette and James, 1987).
9. Early, translucent equant-to-bladed calcite cement: an example of high Sr values during a greenhouse era Based on the similarity in minor element and microdolomite content between turbid replacement microspar after stromatoporoids and translucent ce-
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ment that occluded primary intragranular porosity within stromatoporoids from the Crown Point Formation it is likely that precipitation of some of the early translucent calcite cements (both equant and bladed-to-fibrous) in this unit occurred in a meteoric setting and were genetically linked with stromatoporoid diagenesis. The stable isotopic composition of equant (drusy) calcite supports this interpretation. Of additional significance is the lack of microdolomite, low MgCO3 , and variable but generally elevated Sr values indicative of both precipitated and replacement calcite within the Crown Point stromatoporoids. If the water=rock ratio during meteoric stabilization of aragonite is low (closed system) then the Sr=Ca ratio of the diagenetic fluid can approach that of the dissolving aragonite (Pingitore, 1982). Conversely, under open system conditions the Sr2C =Ca2C ratio of the diagenetic fluid should be significantly lower. For example, let us assume that the original Sr content of the stromatoporoid replacement calcite is reflected by the highest observed value (1600 ppm) and K d,Sr D 0:05 (typically in settings with slow calcite formation; see Morse and Mackenzie, 1990) resulting in a molar Sr2C =Ca2C ratio in the diagenetic fluid that is constrained between 0.037 (closed system) and 0.0018 (open system). The corresponding diagenetic calcite Sr2C =Ca2C ratios are 1:8 ð 10 3 (closed system) and 9:2 ð 10 5 (open system) with equivalent Sr concentrations in calcite of 1600 ppm (closed system) and 80 ppm (open system). The high Sr content of some equant (drusy) calcite cement (up to 1700 ppm) implies formation of this phase under at least partially closed conditions (with respect to Sr) during aragonite stabilization. It is significant to note that later, non-drusy equant calcite mainly occluded pore-central porosity (as well as calcitic metastable phases) have significantly lower Sr values suggesting either (1) precipitation=stabilization under conditions that were genetically unrelated to aragonite transformation, or (2) formation=alteration of these phases occurred during aragonite transformation but under more open conditions than associated with precipitation of translucent equant (drusy) and bladed-to-fibrous calcite within intragranular stromatoporoid vugs. The Sr content of both altered and unaltered middle Ordovician marine cements from nearly coeval units else-
where in Laurentia and Baltica are typically lower (<200 ppm; Tobin et al., 1996; Tobin and Walker, 1996, 1997) than observed in Chazyan metastable calcite phases (<200 ppm to 900 ppm) supporting the premise that the stabilization of at least some of the Chazyan metastable calcite occurred during open system aragonite transformation. In summary, alteration of biotically generated aragonite should be considered as a potentially important mechanism that can account for high Sr values in diagenetic precipitates formed during greenhouse periods such as the Ordovician. It is suggested that detailed diagenetic characterization is needed to rule out this possibility. In the absence of such documentation interpretation of transient aragonite ‘blimps’ in calcite seas based on Sr values in micrite of >800 ppm (Lasemi and Sandberg, 1993) may not be warranted (e.g. Gao et al., 1996).
10. Summary The diagenetic alteration of biogenic aragonitic phases from the Chazyan buildups yield a distinctively elevated Sr signature preserved in diagenetic calcite. In formerly metastable low-to-intermediate Mg calcite marine phases Sr values are elevated relative to values from both altered and unaltered middle Ordovician marine cements elsewhere, although Sr values in former Mg-calcite phases are lower than in diagenetic calcite inferred to have had an originally aragonitic mineralogy. These results are interpreted to be indicative of alteration of all metastable components during an episode of stabilization of originally aragonitic allochems early in the diagenetic history of these buildups. Significantly, this study indicates that elevated Sr values in marine calcite cannot be automatically equated with physiochemical precipitation of aragonite.
Acknowledgements This study has been funded by NSF EAR9315651 (KRW and KJT). A. Patchen provided guidance with electron microprobe operation. J. Johnson made standard thin sections and M. Bennett polished thin sections for electron microprobe analysis. K.
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Howard facilitated collection of stable isotopic data. C. Mehrtens (University of Vermont) is thanked for providing KJT with an introduction to the Chazyan buildups in the field.
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