Classification of hydrous meteorites (CR, CM and C2 ungrouped) by phyllosilicate fraction: PSD-XRD modal mineralogy and planetesimal environments

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ScienceDirect Geochimica et Cosmochimica Acta 149 (2015) 206–222 www.elsevier.com/locate/gca

Classification of hydrous meteorites (CR, CM and C2 ungrouped) by phyllosilicate fraction: PSD-XRD modal mineralogy and planetesimal environments K.T. Howard a,b,c,⇑, C.M.O’D. Alexander d, D.L. Schrader e, K.A. Dyl f a

Kingsborough Community College of the City University of New York, 2001 Oriental Blvd., Brooklyn, NY 11235, United States b American Museum of Natural History, United States c The Natural History Museum, London, United Kingdom d Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW Washington, DC 20015-1305, United States e Smithsonian Institution, National Museum of Natural History, Washington, 10th & Constitution, NW Washington, DC 20560-0119, United States f Department of Applied Geology, Curtin University, Perth, WA 6845, Australia Received 20 April 2014; accepted in revised form 26 October 2014; available online 4 November 2014

Abstract The relative differences in the degree of hydration should be reflected in any classification scheme for aqueously altered meteorites. Here we report the bulk mineralogies and degree of hydration in 37 different carbonaceous chondrites: Renazzo-like (CR), Mighei-like (CM), and ungrouped (type 2) samples. This is achieved by quantifying the modal abundances of all major (phases present in abundances >1 wt.%) minerals using Position Sensitive Detector X-ray Diffraction (PSD-XRD). From these modal abundances, a classification scheme is constructed that is based on the normalized fraction of phyllosilicate (total phyllosilicate/total anhydrous silicate + total phyllosilicate). Samples are linearly ranked from type 3.0 – corresponding to a phyllosilicate fraction of <0.05, to type 1.0 – corresponding to a total phyllosilicate fraction of >0.95. Powdered meteorite samples from any hydrated carbonaceous chondrite group can be ranked on this single classification scale. The resulting classifications for CRs exhibit a range from type 2.8 to 1.3, while for CMs the range is 1.7–1.2. The primary manifestation of aqueous alteration is the production of phyllosilicate, which ceased when the fluid supply was exhausted, leading to the preservation of anhydrous silicates in all samples. The variability in hydration indicates that either accretion of ices was heterogeneous or fluid was mobilized. From the bulk mineral abundances of the most hydrated samples, we infer that the initial mass fraction of H2O inside of their parent body(ies) asteroids was <20 wt.%. Bulk carbonaceous chondrite mineralogy evolved towards increasingly oxidizing assemblages as the extent of bulk hydration increased. This is consistent with the escape of reducing H2 gas that is predicted to have been produced from water during hydration reactions. Ó 2014 Elsevier Ltd. All rights reserved.

1. INTRODUCTION ⇑ Corresponding author at: Kingsborough Community College

of the City University of New York, 2001 Oriental Blvd., Brooklyn, NY 11235, United States. E-mail address: [email protected] (K.T. Howard). http://dx.doi.org/10.1016/j.gca.2014.10.025 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved.

Aqueously altered meteorites are predominantly carbonaceous chondrites that appear to be samples of C-complex asteroids, with the exception of Tagish Lake (C2 ungrouped) that likely derives from a D-type asteroid

K.T. Howard et al. / Geochimica et Cosmochimica Acta 149 (2015) 206–222

(Hiroi et al., 2001). All of these rocky bodies experienced variable amounts of aqueous processing (e.g., McSween, 1979a,b; Zolensky et al., 1997; Morlok et al., 2006; Rubin et al., 2007) where reactions between H2O and anhydrous components formed phyllosilicates (e.g., Fuchs et al., 1973; Barber, 1985; Tomeoka and Buseck, 1985; Tomeoka et al., 1989; Abreu and Brearley, 2010). Accretion of hydrous chondrites may have provided Earth with its liquid water and other volatiles (e.g., Alexander et al., 2012; Marty, 2012), which profoundly influenced development of Earth’s key systems (e.g., plate tectonics) and the origin and evolution of its life. Understanding the processes that led to the hydration of asteroids is, therefore, an important step in explaining the origin of Earth-like planets. Accurate determination of the extent of hydration of carbonaceous chondrite (CC) samples will help to facilitate this understanding. Recently, hydration has been quantified using bulk H abundances and transformed into a classification scheme (Alexander et al., 2013). Here we present another related, direct measure of bulk hydration that is used to create a similar classification scheme: total phyllosilicate abundances determined by Position Sensitive Detector X-ray Diffraction (PSD-XRD) studies of meteorite powders. PSD-XRD allows us to arrive at a complete modal mineralogy (for phases present in abundances greater than 1 wt.%), and we use bulk mineral abundances, in samples with different degrees of hydration, to infer the underlying controls on the observed mineralogy and the conditions under which hydration likely occurred. By comparing bulk mineral abundances and bulk H data, not only do we check for self-consistency between the two techniques, but we can try to constrain a fundamental control on the nature of hydration reactions: the initial water mass fractions inside of parent body asteroids. Van Schmus and Wood (1967) proposed a now ubiquitously applied classification scheme for chondrites. They assigned type 3 to a sample that is completely unmodified by parent body processes. Decreasing petrologic types from 3 to 1 indicates increasing degrees of aqueous alteration; increasing petrologic types from 3 to 6 indicates increasing thermal metamorphism. As it is currently applied, samples showing no evidence for aqueous alteration or thermal metamorphism are type 3.0, samples in which matrix is mostly hydrated and chondrules are partially altered are type 2, and samples where matrix is hydrated and chondrules are largely altered are type 1. Such classifications are relatively coarse and of limited use in cross-comparisons with other high-resolution isotopic and geochemical parameters; improving this situation to more easily facilitate cross-comparisons is one of our main motivations. Classification of meteorites is usually carried out using a combination of optical light microscopy, scanning electron microscopy (SEM) and electron microprobe analysis. However, even with these techniques, classification is not always straightforward because of the complexity and fine-grained nature of the primary materials and alteration products. For instance, it can prove difficult to distinguish between altered and unaltered matrices. As a result, meteorites may be classified as aqueously altered (type 2) by optical

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light microscopy and SEM, despite Transmission Electron Microscope (TEM) revealing that the fine-grained materials are mostly pristine, e.g., the Renazzo-like carbonaceous (CR) chondrites MET 00426 and QUE 99177 that were reclassified from 2 to 3 (Abreu and Brearley, 2010). Even when using SEM, the extent of hydration can be underestimated when early stage fluid attack is cryptic/topotactic, making the alteration difficult to identify without using TEM (Eggleton, 1984). TEM resolves even the subtlest evidence for hydration, but relating observations at the TEM scale to the degree of bulk sample hydration requires large extrapolations. Mid infrared (MIR) spectroscopy can resolve differences in relative hydration and offers new ways of classifying meteorites (e.g., Beck et al., 2010, 2014; McAdam et al., 2013, 2014). The calibration and interpretation of the IR spectra can benefit greatly from constrained mineral abundances for the studied meteorites, which makes PSD-XRD and spectral techniques complementary. PSD-XRD also allows for characterization of larger sample volumes than often used in MIR studies (e.g., Beck et al., 2014), ensuring more representative sampling. This is important from the perspective of matching sample spectra to asteroids. Ultimately, we intend that IR spectra will be measured on all the powders studied by PSD-XRD and with measured bulk H contents, allowing for integrated cross calibration of the three classification techniques (and for a database to be created). Prior to Alexander et al. (2013), there were two main attempts to provide higher resolution classification schemes for Mighei-like carbonaceous (CM) chondrites (Browning et al., 1996; Rubin et al., 2007). Both of these approaches rely on multiple petrographic and geochemical parameters to measure relative degrees of alteration. Where the same samples are studied, these schemes are broadly consistent. This consistency is not surprising since the underlying rationale of both approaches is based on the work of McSween (1979b) and Tomeoka et al. (1989), who demonstrated that more advanced alteration is associated with more Mg-rich matrices and serpentine compositions. These expectations are supported by thermodynamic studies (Dyl et al., 2006; Zolotov, 2014), which show that Fe-rich phases are rapidly altered during the early stages of hydration (forming cronstedtite and Fe-rich serpentines). Magnesium-rich phases (mostly in chondrules) alter more slowly to form more Mg-rich serpentines. The progressive alteration scales of Browning et al. (1996) and Rubin et al. (2007) assume that samples with more Mg-rich matrices have progressed further towards complete hydration, and use this as a key parameter in evaluating the relative degree of aqueous alteration. A higher resolution petrographic scheme for classification of CRs has also recently been proposed (Harju et al., 2014). Given the different compositions and mineralogies of CRs, this CR scheme adopts different chemical proxies than those used for CMs (Rubin et al., 2007), resulting in a classification scheme that while similar is not the same. Variations in lithology have been noted in Ivuna-like (CI) carbonaceous chondrites (e.g., Morlok et al., 2006; King et al., 2014), but all members of this small group are classified as type 1 and no further attempts at

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subdividing them have been made. The scheme proposed here is independent of chemical group and based on total phyllosilicate abundances that have been measured for a range of CCs by PSD-XRD. 2. SAMPLES AND TECHNIQUES 2.1. Sample selection We have quantified mineral abundances in some of the most often-studied CR, CM and C2 ungrouped samples, in addition to less well known Antarctic finds (Table 1). All of the samples have been previously analyzed for their bulk H, C and N elemental and isotopic compositions (Alexander et al., 2012, 2013). Aliquots of most of the powders studied here have also been analyzed for their bulk O isotopes (Schrader et al., 2011; Howard et al., 2013). Many of these same powders have also had mid-infrared (MIR) spectra recorded (McAdam et al., 2013). For the CM and ungrouped C2 samples, all determinations are based on 0.2 g bulk powders that were aliquots of much larger powdered and well-mixed samples made from chips that were typically P1 g in mass. For the CRs, sample availability restricted modal determinations to sample volumes that were typically around 0.1 g; again these were aliquots of larger volumes (0.3–0.4 g) of well-mixed powders. The representativeness of these sample volumes is described below. Care was always taken to avoid analyzing fusion crust, and where possible all samples were chips of fresh meteorite interiors. 2.2. Terrestrial weathering Terrestrial weathering has affected to some degree all of the Antarctic samples in this study. We do not resolve crystalline Fe-oxides other than (pre-terrestrial) magnetite, but rusts are often X-ray amorphous and contribute only to X-ray fluorescence in PSD-XRD patterns. Schrader et al. (2011) provide estimates of oxide abundances (magnetite and rusts) in thin sections of the CRs studied here that allow us to estimate its abundance/contribution to X-ray fluorescence relative to that from other Fe-bearing amorphous materials (e.g., amorphous Fe–(Mg)-silicates that are likely to be primary and unrelated to weathering). Weathering can also produce sulfates and carbonates. By studying interior samples, we attempted to minimize the chance of these weathering products being included, and where obvious ‘white crusts’ or severe rusty discoloration were evident on chip surfaces, they were removed before powdering the samples. 2.3. Sample preparation All the samples were first crushed to a coarse powder with a grain size of 100–150 lm. During the crushing of the CRs, a magnet was used to separate the coarse magnetic metal fraction (>100–150 lm). Both the coarse metal fraction and the powder were then weighed in order to determine the abundance of metal in weight percent. The reason for extracting the coarse metal was to prevent large grains

potentially biasing the powder aliquots used to determine the PSD-XRD modal abundances. For the CRs, the modal abundances were later normalized to include the extracted metal fraction. Prior to the XRD measurements, the aliquots were gently ground by hand in an agate mortar and pestle to a maximum grain size of 35 lm. For XRD analyses, the samples were packed into circular aluminum wells with volumes of 180 mm3 for CM analyses and 100 mm3 for the CR samples. To avoid inducing preferred alignments of platy crystals parallel to the top surface of the holder, each well was packed using the sharp edge of a spatula in an attempt to produce a high-degree of randomness in grain orientations (c.f., Cressey and Batchelder, 1998). 2.4. Modal mineralogy by XRD Analyses used a Nonius PDS 120 powder diffraction system consisting of an INEL curved PSD within a static beam-sample geometry. A germanium monochromator in the primary beam selects only CuKa radiation, horizontal and vertical slits define the beam size at the sample, and diffracted X-rays are detected simultaneously around 120° of arc by the PSD. The ability to collect data simultaneously over 120° of diffraction allows for whole pattern profiling, which is important when de-convolving the mineralogy of fine-grained and complex polyphase samples like hydrous CCs. This instrumentation overcomes the effects of preferred orientation and non-constant sample-area irradiation that make the task of phase quantification by conventional scanning diffractometry extremely difficult. Phase quantification uses the pattern fitting method of Cressey and Schofield (1996). Phase quantification is made possible by the PSD array mostly because there are no moving parts (only the sample spins in its own plane), meaning identical analytical conditions can be obtained for analyses of meteorite mixtures and the mineral standards used in phase quantification (flux variations are monitored and corrected for). The PSD-XRD technique and pattern fitting approach has been demonstrated to be accurate to within 1–3 vol.% in international round robins that tested the phase quantification of blind mixtures (Madsen, 1999; Madsen et al., 2001). Application of this technique to meteorites has been described in numerous publications where errors are shown to be 1–3% for anhydrous phases and 2–4% for phyllosilicates (Bland et al., 2004; Menzies et al., 2005; Howard et al., 2009, 2010, 2011; Dunn et al., 2010), and we follow an identical methodology for the new measurements presented here. The indicated errors bars on all plots are 5 vol.%, reflecting the structural complexity of meteoritic serpentines that complicate matches to the standards used in pattern fitting (Howard et al., 2009, 2010, 2011). As previously, errors in the modal abundances reported here were estimated by varying the pattern fit factor of a mineral standard until the fit was obviously much poorer (Howard et al., 2009, 2010, 2011). 3. RESULTS CRs and CMs (and CIs too) can readily be distinguished from the overall profile of their respective PSD-XRD pat-

Table 1 Modal mineralogy and resulting classifications of CR, CM, and C2 ungrouped meteorites. The oxide (magnetite plus rusts) abundances are from Schrader et al. (2011) based on examination of thin sections. The maximum abundance of amorphous silicate inferred is equal to the total amorphous Fe-bearing material minus oxide abundances from Schrader et al. (2011). In the classification of the CRs, the amorphous Fe-(Mg)-silicate is assumed to be hydrous and is combined with the total resolvable phyllosilicate before calculation of the phyllosilicate fraction (using Eq. (1)) and assignment of sub-type (see text for discussion). GRO 03116 contains abundant amorphous material but this is inferred to be mainly rusts; the sample has a high weathering grade, and the studied powders were red in color. Pyroxene abundances are usually modeled with enstatite (En. 92) although we have a selection of pyroxene standards from which to select the best structural matches for use in pattern fitting. Sample

38

PCA 91082*,^ GRA 95229*,^ LAP 02342*,^ QUE 99177*,^ MET 00426*,^ GRA 06100^ GRO 03116^ Al Rais GRO 95577*,^ Dhofar 1432*,^ QUE 97990** Y 791198** Murchison (n = 2)** Murray** Mighei (n = 3)** ALHA 81002** Nogoya (n = 2)** Nogoya* Cold Bokkeveld (n = 2)** QUE 93005 (n = 2)** LON 94102* ALH 85013* LEW 88001* LEW 90500* TIL 91722* DOM 08013* LAP 03718* MAC 88101* DNG 06004* LAP 02336* MET 00432* LAP 02333* ALH 83100** MET 01070 (n = 3)** SCO 06043 (n = 2)** WIS 96100 (n = 2) Essebi** *

Bells (W)

CR2 CR2 CR2 CR3 CR3 CR2 CR2 CR2 CR1 CR2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2 CM2/1 CM1 CM1 C2ung CM2/ C2ung CM2/ C2ung

Be A A/B Be B B B/C Fall B Mod Be Find Fall Fall Fall Ae Fall Fall Fall A/Be Ce A Ce B B/Ce B/C Be Be A/B B B B Be Be B/Ce Fall Fall

Olivine Fo100

Fo90

26.5% 37.6% 43.7% 36.7% 36.9% 40.1% 26.7% 12.2% 5.2% 34.0% 8.7% 3.8% 7.2% 17.3% 9.5% 7.0% 8.9% 5.9% 2.9% 3.3% – 9.8% 9.7% 15.2% – – – – – – – – 4.9% 5.5% 3.3%

– – – – – – – – – – 10.3% 4.5% – – – 6.2% – – 1.1% 7.8% 7.8% – 3.4% – 12.3% 12.8% 9.7% 13.1% 10.6% 11.6% 3.3% 12.2% 3.8% 2.2% 3.0% 11.0% 14.5% – –

11.9%

Fo80 Fo60 Fo40

– – – – – – – – – – 3.7% 8.8% 4.0% 0.0% 0.0% 1.5% 4.5% – 4.4% – – – – – – – – – – – – – – – – 3.8% –

Pyroxene Calcite Gypsum Magnetite Sulfide Fe,Ni Metal

Cronstedtite MgFeserpentine

X-ray amorphous Fe-rich

Oxide (magnetite + rust) by petrography

Total Phyllosilicate Proposed Alexander Phyllosilicate fraction* type et al. (2013) types

– – – – – – – – 5.7% 0.0% 0.9% 1.0% 1.2% 0.0% 1.2% 0.0% 1.1% 2.0% 1.0% 1.7% 1.1% 4.0% 3.0% 1.0% 3.9% 3.4% 1.1% 2.8% 3.4% 4.2% 0.0% 0.9% 1.2% 1.9% 1.7%

. – – – – – – – – – – – – – – – – – 0.8% – 2.2% 1.9% – 3.7% 9.3% – – – – – – – – – –

–

15.3% 8.8% 7.1% 1.5% 1.3% 11.4% 16.8% 60.0% 57.9% 22.2% 40.4% 37.4% 22.2% 25.0% 26.7% 50.8% 32.4% 64.3% 54.1% 61.3% 47.5% 40.7% 42.8% 41.7% 43.8% 57.7% 42.8% 53.3% 25.9% 56.7% 82.8% 51.7% 62.4% 61.2% 66.0% 73.5% 74.5%

18.5% 7.8% 10.8% 14.1% 24.0% 5.8% 14.0% – – 13.3% – – – – – – – – – – – – – – – – – – – – – – – – – – –

2.0% 7.0% 4.0% 3.0% 6.0% 6.0% 6.0%

0.8%

– – – – – – – – 9.5% – 26.6% 33.5% 50.3% 49.1% 47.9% 26.8% 43.4% 20.2% 23.3% 20.8% 29.9% 37.1% 29.0% 27.7% 16.7% 10.2% 24.4% 21.2% 30.4% 15.0% 0.0% 21.6% 24.2% 26.3% 21.6% – 0.0%

3.0% 3.0% – – – – – – – – – – – – – – – – – – – – – – – – – – –

15.3% 8.8% 7.1% 1.5% 1.3% 11.4% 16.8% 60.0% 67.4% 22.2% 67.0% 70.9% 72.5% 74.0% 74.6% 77.6% 75.8% 84.4% 77.4% 82.1% 77.4% 77.8% 71.8% 69.4% 60.5% 67.9% 67.1% 74.5% 56.3% 71.7% 82.8% 73.3% 86.7% 87.5% 87.6% 73.5% 74.5%

0.35 0.11 0.16 0.14 0.22 0.13 0.28 0.74 0.89 0.38 0.69 0.72 0.76 0.76 0.79 0.80 0.80 0.93 0.82 0.87 0.87 0.86 0.78 0.78 0.76 0.76 0.74 0.81 0.63 0.78 0.93 0.76 0.90 0.92 0.93 0.82 0.82

2.3 2.8 2.7 2.8 2.6 2.8 2.5 1.6 1.3 2.3 1.6 1.6 1.5 1.5 1.4 1.4 1.4 1.2 1.4 1.3 1.3 1.3 1.4 1.4 1.5 1.5 1.5 1.4 1.7 1.4 1.1 1.5 1.2 1.2 1.2 1.4 1.4

1.9

1.7%

–

11.4%

55.1%

–

66.5%

0.82

1.4

2.3

– – – – – – – – – – – – – – – – 1.2% – 2.5% – – – – – – – – – – – – – – – –

– – – – – – – – – – 0.4% – 3.9% – 3.6% 0.4% – – 0.6% – – – – – – – – – – – – – – – –

–

–

31.6% 36.8% 28.3% 35.9% 30.1% 33.0% 34.6% 8.4% 2.2% 19.0% 7.5% 9.8% 8.3% 5.6% 6.9% 4.5% 3.9% 0.0% 4.9% 1.4% 3.8% 3.4% 6.7% 4.9% 6.5% 8.6% 14.2% 4.8% 23.2% 8.5% 3.2% 10.4% 0.7% 0.0% 0.0% 1.4% 2.1%

–

–

2.9%

–

– – – 0.9% 0.2% 0.2% 0.2% 10.9% 7.0% 0.5% 0.6% 0.7% 1.1% 1.4% 2.3% 1.3% 2.2% 5.3% 2.0% 1.5% 2.4% 1.6% 1.9% 1.9% 1.9% 0.3% 2.2% 2.5% 4.0% 1.3% 8.4% 1.8% 1.7% 1.8% 1.6% 6.5% 5.2%

4.7% 3.2% 3.4% 5.3% 3.5% 7.1% 4.4% 6.1% 9.3% 5.9% 0.9% 0.6% 1.8% 1.8% 1.9% 1.4% 2.3% 2.3% 3.0% 2.2% 5.4% 1.6% 2.9% 3.1% 0.9% 1.6% 1.7% 1.5% 2.2% 1.6% 2.2% 1.4% 1.0% 1.2% 2.7% 3.5% 3.9%

–

4.5%

11.6%

3.6% 6.2% 7.1% 5.6% 4.2% 2.7% 3.6% 5.0% 0.0% 0.2% 0.2% 0.2% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.2% 0.0% 0.0% 0.4% 0.0% 0.0% 0.0% 0.0% 0.0%

–

2.3 2.5 2.5 2.4 2.6

2 1.3 1.7 1.5 1.6 1.5 1.6 1.3 1.2 1.1 1.3 1.5 1.8 1.4 1.6 1.6 1.5 1.8 1.6 1.4 1.8 1.6 1.7 1.5 1.1 1.2 1.2

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Weathering grade

*

XRD analyses were performed on samples that are aliquots from the same powders as bulk H data were reported for in Alexander et al. (2013). These samples were first reported on in Howard et al. (2011) and bulk H data for these are reported in Alexander et al. (2013). ^ These XRD measurements are of aliquots from the same powders that Schrader et al. (2011) reports bulk O-isotope data for. Bulk H contents for different powders of the same meteorites are reported in Alexander et al. (2013). **

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terns. This is true even when the patterns were acquired rapidly on small sample volumes; this would be sufficient classification for curatorial purposes. For more detailed studies, quantification of mineral abundances, and the degree of hydration (for assignment of sub-type) is necessary, and this is described below. XRD patterns for CRs are dominated by high intensity diffraction from olivine and pyroxene. In samples like MET 00426, these phases show no evidence for peak broadening in XRD patterns, which is a common feature of even minor aqueous alteration (Fig. 1). In pattern fitting, we model olivine compositions in CRs (Table 1) using the best fitting standard selected from Fo100, Fo90, and Fo80 standards. Solid solution in olivine compositions is evidently more complex than this in CRs, but low incident beam angles (used to resolve phyllosilicate) and fluorescence complicates further de-convolutions. XRD patterns for Al Rais and GRO 95577 show high intensity diffraction peaks from phyllosilicate and magnetite (Fig. 2). Minor peaks from FeNi-metal and sulfide are also evident in all samples. Using copper radiation, it is not possible to resolve accurately the contributions of different sulfide phases in bulk powders of CR (or CM) chondrites. As discussed in detail in Howard et al. (2009, 2011), sharp peaks at 12° and 25° (corresponding to 0 0 1 and 0 0 2 planes for a 0.7 nm layer structure) dominate PSDXRD patterns of CMs. These peaks are distinct from the reflections in the pattern that correspond to fine-grained phyllosilicate, e.g., at 19° and 61° (Fig. 2). At higher angles in the XRD patterns, the major peaks correspond predominantly to magnetite, olivine and pyroxene. Minor peaks are sulfide and calcite, with occasional weak diffraction from sulfate and FeNi-metal. Tochilinite is only detected in the least hydrated CMs (Howard et al., 2011). Without a phase pure standard tochilinite, abundances cannot be resolved by PSD-XRD, but the phase may be present in abundances of up to 3–4 vol.% based on mass balance calculations (McSween 1987; Howard et al., 2009; Zolensky et al., 1993). Previously (Howard et al., 2011), we described phyllosilicate in Essebi as distinct from the main CM population in that evidence for diffraction from well–crystalline phyllosilicate was absent, and this is also the case for Bells and WIS 96100. 3.1. Mineral abundances in CR chondrites The CR samples contain 5–44 vol.% olivine and 2– 37 vol.% pyroxene. The ratio of olivine to pyroxene averages 1, which reflects the predominance of FeO-poor chondrules in CRs (Schrader et al., 2011). FeNi-metal (<1–7 vol.%) and sulfide (3–9 vol.%) are the next most abundant phases. Calcite is present in very low abundances and is often not detected (<1%) in the CRs. Magnetite abundances are also low (<1 vol.%) in samples other than Al Rais and GRO 95577. Less altered CR chondrites, such as MET 00426, QUE 99177, and PCA 91082, contain abundant Fe-rich amorphous material (Table 1). This is evident in their PSD-XRD patterns where, after subtraction of crystalline phases, large residual X-ray counts remain that are not associated with peaks, but only elevated back-

grounds (Fig. 3). These residual counts can represent more than 20% of the total X-ray counts and are fluorescence from Fe-rich, amorphous material. In CCs, amorphous Fe-rich materials are known to be FeNiS, ‘rusts’ (FexOx, ±OH, ±xH2O), and Fe-(Mg)-silicates. Glass in chondrule mesostasis typically contains less than a few percent FeO, and most often its composition is plagioclase-like (Kurahashi et al., 2008); therefore, it will not contribute significantly to X-ray counts from fluorescence, which are produced by materials in proportion to their Fe contents. The PSD-XRD derived modal abundances can be converted into an approximation of bulk chemistry (e.g., Bland et al., 2004; Howard et al., 2010; Table 2). Converting our mineral abundances into bulk compositions results in large overestimates of SiO2 (6+13 wt.%) and MgO (6+16 wt.%) abundances, and large underestimates of FeO (610 wt.%) contents, compared to literature values (Kallemeyn et al., 1994). The average Mg/Si ratio (1.07) that we calculate from the inferred modal abundances is close to the solar value (Lodders, 2001). Fluorescence is extreme in these samples, and structural variations accompanying olivine solid solution are slight and easily masked. The best structural fit for pattern fitting bulk olivine abundances are usually Fo100 or Fo90 standards, but electron microprobe analysis shows that CC olivine is often more Fe-rich and in part this explains differences between bulk compositions estimated from the PSD-XRD mineral abundances and those measured directly (Kallemeyn et al., 1994). Considering QUE 99177, MET 00426 and PCA 91082 (samples with the greatest abundances of amorphous materials, excluding the weathered GRO 03116) and focusing on S and H2O contents, we can infer the likely phases that are contributing to X-ray counts from amorphous Fe-rich material. If we assumed that all amorphous material detected in these samples was sulfide, the bulk FeO contents would be close to the measured values, but S abundances would average 11 wt.% (Table 2b); measured values for CRs are typically around 3–5 wt.% (e.g., Wasson and Rubin, 2009). If it were assumed that all amorphous materials were FeOH (Table 2c), bulk Fe contents would again be close to the measured values and the H2O contents would be 2–3 wt.%, close to the values reported in QUE 99177 (3.2 wt.%), MET 00426 (4.2 wt.%) and PCA 91082 (5.1 wt.%) by Alexander et al. (2013). However, petrographic studies by optical light microscopy and SEM (Schrader et al., 2011) appear to exclude the possibility of such large contributions to X-ray counts from rusts – the maximum volume of oxides (magnetite plus rusts) reported in these three samples is <6 vol.% in MET 00426. TEM studies note abundant amorphous Fe-(Mg)-silicate in CR matrices and chondrule rims (e.g., Abreu and Brearley, 2010; Keller and Messenger, 2012; Le Guillou and Brearley, 2014). If we assume that all amorphous material is Fe-(Mg)-silicate with a composition like that of the average of matrix material reported in MET 00426 and QUE 99177 by Abreu and Brearley (2010), more reasonable bulk S (2–3 wt.%) contents are obtained. As discussed below, assuming that the amorphous silicate is hydrous (13 wt.% H2O equivalent to serpentine) gives estimated H2O contents

K.T. Howard et al. / Geochimica et Cosmochimica Acta 149 (2015) 206–222 Ol.,Pyx.

STOE Powder Diffraction System

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Fig. 1. PSD-XRD patterns for the CR chondrites. (A) MET 00426, and (B) Al Rais. In our classification scheme, MET 00426 is a type 3.0 and Al Rais is a type 1.5. Identified phases (matched using the ICCD database) are indicated and dashed lines are tentative peak assignments. The pattern for MET 00426 is dominated by forsteritic olivine and magnesian pyroxene (enstatite), with minor peaks from FeNi-metal and sulfide. Olivine and pyroxene yield sharp, high intensity diffraction peaks with no evidence for broadening that accompanies fluid attack. Peaks from magnetite dominate high angle regions of the pattern for Al Rais, along with minor peaks from olivine, pyroxene and sulfide. At low angles, diffraction from phyllosilicates is evident, most prominently in a strong peak at 12° and a weaker peak at 25° (2hCuK1) that are indexed as 0 0 1 and 0 0 2 planes for a 0.7 nm layer structure (e.g., FeMg-serpentine). Noise in these patterns is because they were collected over short time periods.

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Fig. 2. PSD-XRD patterns for the CM chondrites. (A) DNG 06004, and (B) QUE 93005. In our classification scheme, DNG 06004 is a type 1.7 and less hydrated than QUE 93005, which is a type 1.3. Identified phases (matched using the ICCD database) are indicated and dashed lines are tentative peak assignments. Sharp peaks from magnetite and lesser peaks from olivine and pyroxene, along with weak peaks from sulfide, dominate high angle regions of the patterns. Low angle regions are dominated by high-intensity peaks at 12° and 25° (2hCuK1), which are indexed as 0 0 1 and 0 0 2 planes for a 0.7 nm layer structure consistent with well–crystalline serpentine. Diffuse hk reflections appear in the patterns, most noticeably at 19° and 61° and correspond to finer grained phyllosilicate. As discussed in the text, the high intensity phyllosilicate diffraction peaks are assigned to cronstedtite and the diffuse reflections to FeMg-serpentine. These patterns show what we have described previously (Howard et al., 2011, Fig. 1) that as the degree of hydration increases, the full width half maximum (FWHM) of the phyllosilicate 0 0 1 and 0 0 2 diffraction peaks increases and the broad hk reflection grows in size. Note that the DNG 06004 pattern appears noisy as it was collected over only a short period of time.

K.T. Howard et al. / Geochimica et Cosmochimica Acta 149 (2015) 206–222

A

213

of 2–4 wt.%, improving the agreement between calculated and measured bulk H2O contents (Alexander et al., 2013). The ability to model both residual X-ray counts and bulk compositions using amorphous Fe-(Mg)-silicate means that it cannot be ruled out that this material is a dominant component of pristine CC matrices. In samples such as Al Rais and GRO 95577, zero counts remain in residuals after pattern fitting, i.e., we do not detect amorphous Fe-bearing material, suggesting that Fe-(Mg)-silicate is destroyed by aqueous processing in the CR parent body, which is consistent with the presence of abundant phyllosilicates (60 vol.%) in these samples. 3.2. Mineral abundances in CM (and C2 ungrouped) chondrites

B

Fig. 3. Modeling of the PSD-XRD for MET 00426 (A) without, and (B) with FeO-rich amorphous silicates. The XRD pattern recorded for MET 00426 is shown in red, the models from which the modal mineralogies were calculated are in blue, and the residuals after subtracting the models from the XRD patterns are in green. (A) This model is the product of single-phase standard patterns for crystalline phases mixed in relative proportions determined by pattern fitting. The green residual background (actual pattern minus model pattern) remains well above zero. These residual X-ray counts are not associated with peaks but are raised backgrounds and are produced by fluorescence induced by Cu radiation interacting with Fe-rich X-ray amorphous materials. The phases and fit proportions used in construction of the model (A) are: olivine – Fo.90 (13 vol.%); Fo.80 (10 vol.%); pyroxene (19 vol.%); FeNi-metal (23 vol.%); sulfide (12 vol.%); magnetite (3 vol.%). This leaves 25% of the X-ray intensity in the XRD pattern unaccounted for. (B) Background fluorescence is proportional to Fe content (Menzies et al., 2005), such that a contribution from a hypothetical amorphous material of a given Fe content can be modeled. Here a 25% contribution from an amorphous material with a similar composition to the amorphous silicate reported in the matrix of MET 00426 (Abreu and Brearley, 2010; average Fe  50 wt.%) is added to the model and can account for the residual X-rays. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The CM samples contain 3–23 vol.% olivine, <1– 23 vol.% pyroxene and from 56 to 89 vol.% total phyllosilicate (mean 75 vol.% ± 1.3); later we address variations in phyllosilicate phases, but here for the purposes of classification, it is only necessary to consider the total volume of phyllosilicate. Magnetite (1–8 vol.%), sulfide (1–5 vol.%) and calcite (<1–6 vol.%) are minor components of CMs, and gypsum is also present in abundances of up to 0.8 vol.% (Table 1). These data significantly extend the ranges in phyllosilicate and anhydrous component abundances in comparison to those reported in Howard et al. (2011). However, even though we have now determined modal abundances for >30 CMs, in addition to those in Howard et al. (2009), the average modal abundance of total phyllosilicate has not changed. In terms of bulk mineral abundances, the C2 ungrouped samples share overlapping ranges with the CM2 samples, except for magnetite that is much more abundant in Bells (4.5 vol.%), Essebi (5 vol.%) and WIS 96100 (6 vol.%) than is typical for CMs (1– 2 vol.%). 4. DISCUSSION At the scale of our modal determinations (minimum 100 mg), analyses of multiple aliquots of an individual CC sample generally yield modal abundances that are within error of each other. For example, in measurements of three 200 mg aliquots of a Mighei powder, from a single large chip, the standard deviations (1r) in the determined mineral abundances were <2 vol.% (Howard et al., 2009). For three 200 mg aliquots of Nogoya and two 200 mg aliquots of Murchison, QUE 93005, MET 01070, SCO 06043 and WIS 96100, the modal mineral abundances determined here are also all within error of each other. This suggests that the volume of sample we typically characterize is representative of the larger bulk powders from which the aliquots were taken. Based on the masses of material measured, 100–200 mg aliquots of even larger homogenized samples, our PSD-XRD determinations are likely to be far more representative of individual meteorites than petrographic studies. A typical thin section that is 2 cm2 and 30 lm thick (0.0012 cm3) comprises around 30 mg of sample, of which only the surface 2–3 microns are resolved by

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Table 2 CR chondrite bulk compositions estimated from modal mineral abundances in Table 1. Stoichiometric compositions are used for modeling all phases except for amorphous FeMg-silicate as described below. PCA 91082 (%)

GRA 95229 (%)

LAP 02342 (%)

QUE 99177 (%)

MET 00426 (%)

GRA 06100 (%)

GRA 03116 (%)

A FeO MgO SiO2 S H2O

27 33 39 2 1

18 39 41 2 1

20 39 39 2 1

27 33 35 3 0

23 36 38 2 0

19 38 39 4 1

20 36 40 2 2

B FeO MgO SiO2 S H2O

34 28 31 11 1

22 36 37 6 1

26 35 34 7 1

33 30 31 9 0

35 28 29 13 0

22 36 36 7 1

28 31 34 9 2

C FeO MgO SiO2 S H2O

32 29 32 2 4

20 37 38 2 2

24 36 35 2 2

31 31 33 3 2

32 30 30 2 3

20 37 37 4 2

26 32 35 2 3

(A) Bulk compositions resulting from the modal mineralogies in Table 2, where the Fe-bearing amorphous component is modeled using silicate: Fe(50%)Mg(20%)Si(30%) (This is the average composition of amorphous FeMg-silicate from the matrix of QUE 99177 reported by Abreu and Brearley (2010)). If the amorphous material is assumed to be hydrated equivalently to a serpentine (13% H2O), bulk H2O contents in MET 00426, QUE 99177 and PCA91082 would be 3.2 wt.%, 4.1 wt.% and 5.1 wt.%, respectively. (B) Bulk compositions resulting from the modal mineralogies in Table 2, where the Fe-bearing amorphous component is modeled using iron sulfide: Fe(36%) S(64%). (C) Bulk compositions resulting from the modal mineralogies in Table 2, where the Fe-bearing amorphous component is modeled using ferrihydrite: Fe2O3(89%)H2O(11%).

SEM, corresponding to a sample mass of around 1.2 mg (assuming an average density of 3 g/cm3). 4.1. Breaking with tradition: the rationale for a new classification scale In the original Van Schmus and Wood (1967) classification scheme, meteorites could only be designated as type 1, 2 or 3, which led to most CMs being classified as type 2 because they were identified as being intermediate between fully altered and unaltered. Classification simply as type 2 is coarse given that it has long been recognized that CM2 samples vary considerably in their degree of alteration, with nearly completely altered CM samples being classified as type 1 (e.g., Zolensky et al., 1997). It is now possible and desirable to have a higher resolution classification scheme, but our data clearly show that almost all CMs are >50% altered. On a 1–3 scale that is a linear function of alteration, all CMs will be classified as <2.0. To try to preserve the traditional type 2 classifications, Rubin et al. (2007) and Harju et al. (2014) have compressed the CM and CR alteration scales to span from 2.0 to 3.0 and have thereby abandoned the original type 1 and type 1/2 classifications for the most altered CM and CR samples, i.e., CM1 and CR1 samples became types CM2.0 and CR2.0. This also necessitates either a break with tradition and the reclassification of CI chondrites as CI2.0s, or that the type 1 designation be set aside for CIs only. The latter is contrary to the principle of a petrologic classification

scheme that should be independent of chemical group and makes no allowance for the possibility that an only partially altered CI may one day be found. We strongly believe in the principle that any petrologic classification should be independent of chemical group and that a scale of 1.0 to 3.0 provides the resolution that our new classification tools require. We also believe that petrologic type should be a simple linear function of the degree of alteration, which the Rubin et al. (2007) scale for CMs is not (Alexander et al., 2013), even though this means that almost all CMs will no longer be designated as type 2s. It is important to emphasize that petrologic criteria can and should be part of our new classification scheme. The fact that there are strong linear correlations between the Rubin et al. (2007) classifications for CMs and those based on H content (Alexander et al., 2013) and phyllosilicate fraction shows that petrologic criteria (e.g. Weisberg et al., 2006) can be used for classification in the absence of PSD-XRD and bulk H data. 4.2. Classification of hydrous CCs by phyllosilicate fraction (PSF) The primary effect of aqueous alteration in CCs is the production of phyllosilicates that predominantly belong to the serpentine family of minerals, along with significant volumes of saponite-like materials. Since our rationale for assigning a petrographic subtype is to define the relative degree of hydration, we consider that the phyllosilicate frac-

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tion (PSF) of the total silicate is an accurate discriminator. We considered using the total fraction of all secondary minerals, but there is some ambiguity in whether to assign sulfides as primary or secondary, for instance (Brearley and Martinez, 2010; Maldonado and Brearley, 2011; Schrader et al., 2013), and magnetite abundances may be particularly sensitive to initial metal abundances (Dyl et al., 2006). Currently, fundamental questions remain about the nature of the original anhydrous material accreted by the parent bodies of hydrous CCs, particularly with regard to their fine-grained matrices (McSween, 1987; Browning and Bourcier, 1998; Chizmadia and Brearley, 2008; Abreu and Brearley, 2010). This uncertainty complicates the accurate determination of alteration pathways and means that assumptions are required in order to use geochemical parameters to estimate the relative variations in bulk sample hydration. The one assumption here is that alteration took place entirely in the parent bodies. Irrespective of the anhydrous mineralogy or reaction pathways that hydration reactions followed, the phyllosilicate fraction is a normalized ratio tracking conversion of anhydrous silicates to phyllosilicates (Eq. (1)). Phyllosilicate fraction ðPSFÞ ¼

total phyllosilicate total anhydrous silicate þ total phyllosilicate

ð1Þ

In keeping with the traditional scheme of Van Schmus and Wood (1967), the resulting value is converted to a ranking of petrographic type ranging from 3.0 to 1.0 along a linear scale where each sub-type is defined on the basis of 5 vol.% increments in total phyllosilicate abundances. Therefore, a phyllosilicate fraction of <0.05 results in classification as type 3.0 (effectively a completely anhydrous sample), a phyllosilicate fraction of 0.50 gives a type 2.0 classification and a fraction of >0.95 results classification as type 1.0 (effectively completely hydrous). Hence, the petrologic type provides both a relative and absolute measure of the degree of alteration. Any hydrated CC can be placed on this scale, regardless of the chemical group to which it belongs (and including ungrouped/anomalous samples), meaning that inter-group comparisons of the degree of hydration are possible. Note that it is not our intention to replace the thin section based classification of meteorites (e.g., Rubin et al., 2007; Harju et al., 2014). We are adding an approach to allow for classification of large numbers of powdered samples. Other critical parameters in classification, such as shock stage and degree of weathering, can only be resolved in thin section, just as only petrography can directly describe the processes responsible for hydration. Indeed, a simple transformation converts the Rubin et al. (2007) and Harju et al. (2014) scales to ours (Eq. (2)). New classification^ ¼ 0:96  Rubin CM or Harju CR classification  0:81:

ð2Þ

^On 1–3 scale as for classification by phyllosilicate fraction (PSF) or bulk H content. In our classification, we reserve type 1.0 for currently hypothetical samples with >95 vol.% phyllosilicate;

215

Alexander et al. (2013) also reserved type 1.0 for as yet to be discovered samples with higher H abundances than currently known. The bulk H abundance reserved for type 1.0 by Alexander et al. (2013) is 1.44 wt.% – corresponding to the H content of stoichiometric Mg-serpentine. Serpentine is the dominant phyllosilicate in many chondrites, but 100% serpentine is an unlikely end product of aqueous alteration from the perspective of the bulk chemistry of chondrites and thermodynamic studies (Zolotov, 2012). Nevertheless, in a plot of water content versus phyllosilicate fraction, Alexander et al. (2013) found that CM chondrites roughly fall on a line connecting anhydrous silicates and Mg-rich serpentine. It remains an open question whether the ‘end point’ of aqueous alteration would ever be 100 vol.% phyllosilicates, rather than a more diverse mineralogy similar to CIs that contain nominally anhydrous secondary phases such as magnetite and calcite (King et al., 2014). Under the classification scheme proposed here, once anhydrous silicates are exhausted, the normalized phyllosilicate fraction equals 1.0 and a sample is considered 100% aqueously altered. Just as the type 1.0 end member is currently hypothetical, the type 3.0 end member might also be considered to be so: lithification must involve thermal annealing and/or fluid-assisted cementation, which might explain evidence for cryptic alteration observed by TEM in even the most primitive samples (Abreu and Brearley, 2010; Le Guillou and Brearley, 2014). 4.3. Proposed classifications of samples In Table 1, 37 different meteorite samples from the CM, CR and C2 groups are classified and ranked on a single scale derived from the fraction of phyllosilicates. CRs define a range in petrographic types from 2.8 to 1.3 – the CR fall Al Rais is classified as a type 1.6 and is more altered than all other CRs except for GRO 95577, which is a type 1.3. CMs define a range in relative hydration from petrographic sub-type 1.7–1.2. Mighei, the type member for the group, is a type 1.4. Fall samples are all type 1.5–1.4, corresponding to phyllosilicate fractions of 0.7–0.8. Antarctic samples span a range from less altered to much more altered than the fall samples studied here. Therefore, residence in Antarctic ice does not seem to affect bulk mineral abundances, at least when internal chips of low weathering grade samples are considered. Bulk H, C, and N isotopes also show no apparent evidence for significant effects of Antarctic weathering (Alexander et al., 2012). This means that correlations in the other bulk properties of these CCs with our modal abundances must largely reflect pre-terrestrial processes. 4.4. Comparison of classifications by phyllosilicate fraction to previous schemes Fig. 4 is a plot of the abundance of crystalline phyllosilicate in vol.% versus bulk H contents (Alexander et al., 2013). The high degree of correlation (R2 = 0.82) indicates that both are suitable for use in an integrated classification scheme. However, one difference that emerges from this plot is that we do not resolve sufficient crystalline phyllosi-

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Fig. 4. Bulk H content versus total phyllosilicate abundance (vol.%). The strong correlation is to be expected given that phyllosilicates are the dominant carrier of OH. Both measures yield correlated classifications of the CCs. In the least altered CRs, PSDXRD does not resolve sufficient crystalline phyllosilicate to explain their bulk H contents and they plot off the trend line that passes through the bulk of the samples. This trend line is a linear regression through the CM population projected backwards to the origin. If the resolved amorphous component is assumed to be hydrous (with the equivalent H2O content of Mg-serpentine: 13 wt.%) and is added to the total phyllosilicates, the least altered CRs plot nearer to the observed trend line (open circles on plot). The implication is that the Fe-bearing amorphous material may be hosting OH.

licate in the least altered CR samples to explain their bulk H contents. This is evident in Fig. 4 where the minimally altered CR samples plot away from the trend line that is defined by the main CM population and projected to the zero point. The more altered CRs fall on this trend but the least altered ones do not. However, if we combine the abundance of amorphous material with our total phyllosilicate abundances, these least altered samples would plot on or nearer to the trend line (here we are assuming amorphous silicate contained 13 wt.% H2O – equivalent to serpentine). This suggests that the amorphous Fe-rich silicate in CRs is hydrous, as has been shown previously in MET 00426 (Le Guillou et al., 2011; Le Guillou and Brearley, 2014). Counting amorphous silicate as hydrous and treating this as ‘phyllosilicate’ in the classification of CRs yields subtypes that are more in line with those of Alexander et al. (2013) based on bulk H contents. Consequently, we have included the amorphous material in the phyllosilicate fractions for our petrologic classification of the CRs that are presented in Table 1. For CMs, most of our classifications agree with those in Alexander et al. (2013), the average difference in our classifications being <1 sub type and the standard deviation <2 subtypes. This is because the errors in our analyses can be up to 5 vol.%, or the equivalent of 1 sub type, while Alexander et al. (2013) may have included water hosted in minor phases other than phyllosilicates. However, in two CMs, LON 94102 and MET 00432, we underestimate hydration relative to Alexander et al. (2013) by 5 and 6 sub types, respectively. These samples are weathered, and

perhaps we are not resolving amorphous FeOH that is contributing to bulk H abundances. Bells(W) and Essebi are falls with geochemical and petrographic affinities to CMs (Metzler et al., 1992; Clayton and Mayeda, 1999; Mittlefehldt, 2002) but they have anomalous C, H and N isotopic compositions and, based on bulk H, are the least altered CMs (i.e., Essebi is type 2.3 and Bells(W) type 1.9 in the scheme of Alexander et al., 2013, compared to a typical CM like Murchison which is a type 1.5). Our classifications of Bells(W) and Essebi as type 1.4 suggest that these are much more typical of average CMs in their degrees of bulk hydration. We have also studied WIS 96100 (CM) and GRA 06100 (CR); heating and shock (Abreu and Bullock, 2013; Abreu et al., 2014) appear to have affected bulk H contents and H, C, N, isotopes in these samples so that they do not adhere to the trends defined by the unheated samples (Alexander et al., 2013). Although we may underestimate phyllosilicate abundances and some material may have been destroyed in heated samples, dehydration was not accompanied by complete phyllosilicate destruction in these meteorites. We resolve modal abundances of phyllosilicates indicative of the degree of aqueous alteration typically found in CM (WIS 96100: type 1.4) and CR (GRA 06100: type 2.7) groups. Fig. 5 shows the phyllosilicate fraction (and bulk H contents) versus the petrologic types of Rubin et al. (2007) and Harju et al. (2014). For CMs, there exists a high degree of correlation (R2 = 0.88) between the phyllosilicate fraction and the Rubin et al. (2007) petrologic types. Our assignments agree to within 1–2 subtypes of the Rubin et al. (2007) classifications once their values are converted to the 1–3 scale (using Eq. (2)); this results in the CM samples Rubin et al. (2007) classify as types 2.6–2.0 being types 1.6– 1.2 in our scheme. For a more limited number of CR samples, there also exists a correlation (R2 = 0.81) between the phyllosilicate fraction and the petrologic types from Harju et al. (2014). Our classifications of the least altered CRs agree with petrographic studies to within 2 sub types (Harju et al., 2014), and for the more altered CRs Al Rais and GRO 95577 (Weisberg et al., 1993; Weisberg and Huber, 2007), our classifications also agree to within 2 sub types of Harju et al. (2014) after their values are converted to our proposed 1–3 scale using Eq. (2). The most hydrated CR sample GRO 95577 is an example of why the normalized ratio is a better measure of hydration than simply total phyllosilicate abundances. The sample appeared more oxidized than the other powders and contains larger volumes of magnetite, carbonate and sulfide. If we were to consider only the total volume of phyllosilicate, GRO 95577 would be ranked much closer in its degree of hydration to that of Al Rais, something that petrographic studies rule out (Weisberg et al., 1993; Weisberg and Huber, 2007). 4.5. Cross applicability of the proposed scheme It is desirable to have a means of classification of meteorites that can be widely applied by the community; this is one of the great strengths of petrographic classification

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also ultimately be suitable for classification purposes (Garenne et al., 2014), but the contributions from the breakdown of phases other than phyllosilicates must be taken into account. 4.6. Correlations in modal mineral abundances Fig. 6 shows the relationship between the combined volume abundances of olivine + pyroxene versus that of total phyllosilicates, or the dominant pre-cursors versus products of aqueous alteration. The observed inverse correlation (R2 = 0.95) indicates that, not surprisingly, the phyllosilicates were formed at the expense of olivine + pyroxene. This provides a clear justification for our rationale of using the fraction of total phyllosilicate to compare to the degree of hydration. In less altered CM samples (types 1.7–1.4), traditionally classified as CM2, the chemistries of minor phases have been shown to vary systematically with the degree of hydration (Rubin et al., 2007) but, at the resolution of PSD-XRD, the abundances of minor phases show no obvious relationship to the degree of bulk hydration. Nevertheless, the most altered samples (<1.4), traditionally classified as CM1, do tend to contain the greatest abundances of minor phases, such as magnetite, sulfide, calcite and sulfates. 4.7. Evolution of bulk mineralogy during hydration of CC materials The conditions under which hydration reactions occurred remain poorly constrained and it is yet to be resolved if alteration took place in open or closed systems (Clayton and Mayeda, 1999; Young et al., 1999; Lauretta Fig. 5. (A) Comparison of petrographic classifications of CM (Rubin et al., 2007) and CR (Harju et al., 2014) chondrites with classifications based on the phyllosilicate fraction. (B) Petrographic classifications versus bulk H content. The lines drawn through the CM data do not intercept with a phyllosilicate fraction or bulk H content of 0 at type 3.0, indicating the non-linearity of the petrographic classifications. For CRs, there are a limited number of samples common to both studies and data scatter significantly, but the observed trend appears more linear. That CR samples plot below CM samples assigned the same sub-type in petrographic schemes shows the lack of integration of the petrographic schemes that are specific to chemical group.

schemes. PSD-XRD modal determinations of meteorites are not routinely carried out by many labs. Fortunately, phyllosilicate abundances can be inferred by other much more widely used techniques. Beck et al. (2010, 2014) have shown that phyllosilicate abundances determined by PSDXRD correlate with inferred alteration trends observed in 2–25 lm transmission IR spectra of CMs and, to a lesser extent, CRs. Recent results also demonstrate a correlation with the 12 lm absorption feature in reflectance spectra (McAdam et al., 2013, 2014). Constrained modal mineral abundances provide a means of further calibrating these spectroscopic approaches. In principle, H2O contents measured by methods such as thermo gravimetric analysis may

Fig. 6. Olivine + pyroxene versus total phyllosilicate. The strong inverse correlation suggests that, not surprisingly, the phyllosilicates formed by alteration of anhydrous olivine + pyroxene. Minimally altered CRs plot in the bottom right, the main cluster of CM samples have almost pervasively hydrated matrices and variably hydrated chondrules. The most altered samples have pervasively hydrated matrices and chondrules. Open circles are the CR samples if amorphous Fe-(Mg)-silicate is counted as phyllosilicate.

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et al., 2000; Bland et al., 2009; Pulguta et al., 2010). Models must reconcile evidence for advanced aqueous alteration with the primitive chemistry of hydrous carbonaceous chondrites that usually preserve relatively uniform bulk compositions for most elements, irrespective of the extent of alteration (Bland et al., 2005). It must also be explained why CC compositions are relatively uniform and any variations observed in their bulk compositions do not appear to correlate with their expected elemental solubilities in aqueous solutions. The extremely limited permeability of primitive materials is one means to explain isochemical aqueous alteration (Bland et al., 2009). However, while most altered chondrites currently have very low permeabilities, this is after the swelling associated with silicate hydration during alteration, as well as compaction, so it is not certain that the permeabilities of chondrites were as restricted early in the evolution of their parent bodies. As important as reconciling the lack of variation in bulk chemistry of CCs is explaining why the extent of hydration is so variable. This variability would seem to imply either a redistribution of water within the parent bodies or very heterogeneous accretion of ices. Whatever the cause, the extent of alteration experienced by a meteorite also produced distinct mineral assemblages. Below, we attempt to infer the underlying controls on the mineralogies of samples that experienced different degrees of hydration. It is also apparent that the water supply must have been exhausted prior to complete alteration, since all samples preserve at least some anhydrous components (something that is also true of CIs). To understand why fluid supply ceased, it is necessary to know what the initial water mass fractions of asteroids are. Water mass fraction is also a fundamental parameter to constrain in order to evaluate the nature of hydration reactions, the likelihood of fluid flow and even the formation locations of the asteroids. Inferences about the initial water mass fractions of hydrous asteroids can be drawn from comparison of modal mineralogies with bulk H contents (Alexander et al., 2013) and thermodynamic models (Dyl et al., 2006; Zolotov, 2014). To facilitate the discussion, we use the same approach as previously used by Howard et al. (2009, 2011), which is briefly described below, to determine the abundances of the main phyllosilicate minerals cronstedtite and MgFe-serpentine in CMs. TEM observations show, and expectations from crystal structure/chemistry predict, a preference for cronstedtite to form platy crystals while MgFe-serpentines appear fine-grained and complexly intergrown/disordered by TEM (Lauretta et al., 2000; Howard et al., 2011). Hence, here we attribute high intensity X-ray diffraction peaks to cronstedtite and diffuse reflections to MgFe-serpentine (Howard et al., 2009, 2011). Following this approach, we estimate the abundance of cronstedtite to vary from 10 to 50 vol.%, and MgFe-serpentine from 22 to 82 vol.%. These ranges in abundances are shown in Fig. 7, along with the abundances of other major phases, plotted relative to the degree of bulk hydration as defined by the fraction of phyllosilicate (Eq. (1)). The changes in phyllosilicate and other mineral abundances with increasing alteration illustrated in Fig. 7 suggest a likely alteration sequence as outlined below.

Fig. 7. Selected phase abundances versus bulk hydration in CMs. In this plot, the degree of hydration is defined by the phyllosilicate fraction (PSF; Eq. (1)). Note that cronstedtite abundances plateau at 35 vol.%; in contrast, the abundance of MgFe-serpentine continuously increases as bulk hydration progresses. Magnetite abundances increase after cronstedtite production appears to cease, and this is interpreted to reflect an exchange of Fe in cronstedtite for Mg, with magnetite incorporating the Fe3+ expelled from cronstedtite. Tochilinite is not included on the plot since we cannot quantify its abundance owing to a lack of a pure-phase standard. Sulfide abundances (not shown) scatter widely, but are generally greatest in the most hydrated samples.

4.7.1. Low bulk hydration (types 3.0–2.3) We do detect trace amounts of magnetite in the least altered samples in which only faint reflections from phyllosilicates can be observed. This is consistent with petrologic observations that indicate that during the very early stages of alteration oxidation of metal forms magnetite (3Fe(s) + 4H2O(1,g) ! Fe3O4 + 4H2(g)) (Palmer and Lauretta, 2011). These minimally altered samples all contain amorphous Fe-(Mg)-silicate that may be hydrous and a form of proto-phyllosilicate. The petrogenesis of amorphous Fe-(Mg)-silicate, including the setting(s) of its formation and hydration, remains to be defined. We continue to detect amorphous Fe-(Mg)-silicate in samples with up to 15–20 vol.% crystalline phyllosilicates, but in more altered samples it is absent, suggesting that advanced parent body processing destroys this material. 4.7.2. Medium bulk hydration (types 2.2–1.4) If conditions are oxidizing (e.g., CMs), hydration of amorphous FeMg-silicates and matrix olivine along with the oxidation of metal forms cronstedtite, e.g., 2Fe (metal) + (Fe 2+ ,Mg)2SiO4 + 5H 2O(l) ! (Fe,Mg)2Fe 3+ 2 , SiO5(OH)4 + 3H2. During formation of cronstedtite, if sulfur is present, tochilinite [6Fe0.9S(Mg, Fe2+)(OH)2] also forms. Subsequently, hydration of Mg-rich olivine and

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pyroxenes, including those in chondrules, forms MgFeserpentines [(Mg,Fe)3Si2O5(OH)4], and tochilinite decomposes to form Fe,Ni-sulfides such as pentlandite. This explains our detection of tochilinite in only the least altered CMs. As hydration progresses, the alteration reactions systematically increase the abundance of MgFe-serpentine (Fig. 7). Partially re-crystallized tochilinite that is intimately intergrown with serpentines may also contribute to diffuse reflections in PSD-XRD patterns. In contrast, cronstedtite abundances cluster at 35 vol.% early in hydration and remain constant or decline slightly as hydration increases (Fig. 7). Magnetite abundances steadily increase with hydration (Fig. 7). These variations can be explained by exchange of Mg, released from Mg-rich silicates, for Fe in cronstedtite, yielding MgFe-serpentine plus magnetite. 4.7.3. High bulk hydration (types 1.3–1.0) At high degrees of bulk hydration, dissolution of sulfide and reaction of cronstedtite make additional S and Fe available, leading to further increases in the abundances of Fe-oxides (e.g., magnetite; Fig. 7) and possibly also sulfates and ferrihydrite, i.e., CI-like (King et al., 2014). Saponite also becomes abundant in the most altered meteorites, suggesting that during late stage alteration the activities of Si and Al increase, perhaps because pyroxene begins to alter, and, at the same time, dissolution of serpentine also expels Al and Si to fluid. The evolution of the bulk mineralogy is towards production of increasingly oxidized assemblages as the extent of bulk hydration increases. At the same time, some implicated hydration reactions (e.g., the magnetite and cronstedtite forming reactions above) are associated with production of H2 gas while other reactions are suggested to be concurrently producing CH4 (Guo and Eiler, 2007). In order not to inhibit the production of oxidized mineral assemblages as bulk hydration progressed, these reducing gases (H2 and CH4) must have efficiently escaped from the meteorites. 4.8. Initial water mass inside of hydrous asteroids Parent body temperatures during hydration reactions are generally thought to have been low, perhaps as high as 70 °C in CMs and lower for CRs (Guo and Eiler, 2007). At such low temperatures, cronstedtite is not predicted to form until the water mass fraction reaches 0.5, but it is more efficiently formed at even higher water mass fractions, up to 0.8 (Dyl et al., 2006; Zolotov, 2014). Typically the upper limit for the initial water mass fraction in asteroidal materials is considered to be 0.5 (Clayton and Mayeda, 1999; Zolotov, 2014). However, if initially only matrix is involved in the alteration reactions, the effective water/rock ratios could be much higher than the bulk and allow for early formation of cronstedtite. A bulk water mass fraction of 0.5 corresponds to an asteroid with a bulk H content of 5.5 wt.%, which is almost four times the present content of even the most altered of the unheated CMs (Alexander et al., 2013). Bulk mineral abundances can be converted to an estimate of the bulk composition in order to place an upper

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Table 3 Estimated initial mass fraction of H2O inside of a hydrous asteroid such as the CM parent body. We convert modal mineral abundances to bulk chemistry. By assuming the most reduced silicate compositions possible are accreted, we can estimate the minimum abundance of O contributed to the bulk composition from anhydrous silicates. We then assume that all other O was contributed to the bulk by H2O to arrive at an estimate of the initial H2O content. Element Bulk Bulk O Minimum composition* content O from accreted silicate** at.% wt.% wt.% O Mg Si Fe Ca Al H

50.13 10.47 10.58 6.85 0.29 0.23 20.40

44.60

29.24

Maximum O from H2O

Initial H2O

wt.%

wt.%

15.38

17.31

* This composition is for the following CM1 mineralogy (vol.%): forsterite, 7%; magnetite, 2%; calcite, 1.7%; sulfide, 2%; cronstedtite, 26%; MgFe-serpentine, 62%. ** Assuming no FeO.

limit on initial water mass fractions inside of the parent body(ies) of CMs (Table 3), assuming that all initial water was consumed by reactions with anhydrous material. We assume that the initial assemblage of anhydrous silicates accreted by their parent body was as reducing as possible (i.e., no FeO) to arrive at an estimate of the minimum initial abundance of O bound in silicates at the onset of aqueous alteration (29 wt.%). Subtracting this value from the bulk O content reconstructed from an average of the CM1 bulk mineralogies reported in Table 1 (45 wt.%) yields the total O derived from H2O (15 wt.%), corresponding to an initial H2O content of 17 wt.%. The same calculation using bulk compositions from Kallemeyn and Wasson (1981) gives a similar result with an estimated initial H2O content of 18 wt.%. The initial H2O content (17 wt.%) that we calculate from the bulk mineralogy is less than half of the 47 wt.% estimated by Clayton and Mayeda (1999) in their model to explain the O-isotope compositions of Murchison phyllosilicates. Comparing this value (17 wt.%) to the abundance of H2O bound in phyllosilicate in a CM1 (10.5 wt.%; Alexander et al., 2013), and it is apparent that 7 wt.% H2O has been consumed by oxidation of metal and other phases during alteration, presumably forming H2 gas at the same time. Consumption of 7 wt.% H2O by oxidation of Fe would yield 0.4 wt.% of H2 gas. The potential significance of the production and escape of H2 gas from a planetesimal will depend on the rate and timing of its production. Gas production and escape after parent body compaction and/or hydration reactions reduce permeability and porosity, may have consequences that are likely to be destructive and result in extensive fracturing, as has been suggested previously (Wilson et al., 1999; Rosenberg et al., 2001).

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5. CONCLUSION By determining modal mineral abundances in hydrous CCs, we have shown that the volume fraction of silicates in phyllosilicates provides both an assumption-free measure of the degree of hydration and a basis for the petrologic classification of these CCs. We have integrated our results with the schemes of Alexander et al. (2013) and the traditional 1–3 scheme (Van Schmus and Wood, 1967) as follows: a type 3.0 sample contains a normalized volume fraction of phyllosilicate <0.05, a type 1.0 sample contains a phyllosilicate volume fraction >0.95. Assignment of petrologic types between 1.0 and 3.0 are a linear function of the phyllosilicate volume fraction. Increasing bulk hydration is most evident in: (1) Increases in the total abundance of phyllosilicate; (2) Increases in the fraction of phyllosilicate relative to the total abundance of anhydrous olivine + pyroxene; (3) Increases in the proportion of MgFeserpentine; and (4) Increases in the apparent structural complexity of MgFe-serpentines. Oxide abundances are greatest in the most hydrated samples, and the evolution of bulk chondrite mineralogy is towards increasingly oxidized assemblages. This requires the escape of reducing gases (H2 and CH4) produced during hydration reactions, implying that the planetesimal environment was an open system, at least with respect to gas. The evolution of the bulk mineral abundances in hydrous CCs is consistent with a declining supply of fluid during hydration reactions. Variations in bulk hydration suggest that either ices were heterogeneously accreted or that fluid was able to flow in the asteroid parent bodies. As is evident from the number of meteorites that we have studied, PSD-XRD provides a means of rapidly classifying large volumes of samples and results in petrologic classifications that are related to mineral assemblages that systematically vary with increasing bulk hydration. These mineralogical variations should also produce recognizable spectral features that are indicative of the extent of aqueous alteration; as a result, the possibility exists that advances in spectrographic techniques will enable an integration of classification schemes applicable to meteorites in the laboratory and remotely observed asteroids. ACKNOWLEDGEMENTS K.T.H. was supported by NASA Cosmochemistry grant NNX14AG27G and thanks Ashley King, Paul Schofield and the UK-Cosmochemical Analysis Network (UK-CAN) at the Natural History Museum, London, for access to the PSD-XRD. C.A. was partially supported by NASA Astrobiology grant NNA09DA81A and by NASA Cosmochemistry grant NNX11AG67G. K.A.D. acknowledges the support of the Australian Research Council via the Australian Laureate Fellowship Program. For supplying the many samples that were necessary for this work, the authors would like to thank: the members of the Meteorite Working Group, Cecilia Satterwhite and Kevin Righter (NASA, Johnson Space Center), Tim McCoy and Linda Welzenbach (Smithsonian Museum for Natural History), Laurence Garvie (Arizona State University), Sara Russell, Caroline Smith and Deborah Cassey (Natural History Museum, London). US Antarctic meteorite samples are recovered by the Antarctic Search for Meteorites (ANSMET) program, which has been funded by NSF and NASA,

and characterized and curated by the Department of Mineral Sciences of the Smithsonian Institution and Astromaterials Curation Office at NASA Johnson Space Center. The efforts of Corentin Le Guillou and two anonymous reviewers are appreciated. Associate Editor Trevor Ireland is thanked for handling this work.

REFERENCES Abreu N. M. and Brearley A. J. (2010) Early solar system processes recorded in the matrices of two highly pristine CR3 carbonaceous chondrites MET 00426 and QUE 99177. Geochim. Cosmochim. Acta 74, 1146–1171. Abreu N. and Bullock E. (2013) Opaque assemblages in CR2 Graves Nunataks (GRA) 06100 as indicators of shock-driven hydrothermal alteration in the CR chondrite parent body. Meteor. Planet. Sci 48, 2406–2429. Abreu N. M., Bland P. A. and Rietmeijer F. J. M. (2014) Effects of shock metamorphism on the matrix of CR chondrites: GRA 06100. Lunar Planet. Sci. 45. #2753 (abstr.). Alexander C. M. O’D., Bowden R., Fogel M. L., Howard K. T., Herd C. D. K. and Nittler L. R. (2012) The provenances of asteroids and their contributions to the volatile inventories of the terrestrial planets. Science 337, 721–723. Alexander C. M. O’D., Howard K. T., Bowden R. and Fogel M. L. (2013) The classification of CM and CR chondrites using bulk H, C and N abundances and isotopic compositions. Geochim. Cosmochim. Acta 123, 244–260. Barber D. J. (1985) Phyllosilicates and other layer structured materials in stony meteorites. Clay Miner. 20, 415–454. Beck P., Quirico E., Montes-Hernandez G., Bonal L., Bollard J., Orthous-Daunay F. R., Howard K. T., Schmitt B., Brissaud O., Deschamps F., Wunder B. and Guillot S. (2010) Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids. Geochim. Cosmochim. Acta 74, 4881–4892. Beck P., Garenne A., Quirico E., Bonal Montes-Hernandez G., Moynier F. and Schmitt B. (2014) Transmission infrared spectra (2–25 lm) of carbonaceous chondrites (CI CM CV– CK CR C2 ungrouped): Mineralogy water and asteroidal processes. Icarus 229, 263–277. Bland P. A., Cressey G. and Menzies O. N. (2004) Mineralogy of carbonaceous chondrites by X-ray diffraction and Mo¨ssbauer spectroscopy. Meteorit. Planet. Sci. 39, 3–16. Bland P. A., Alard O., Benedix G. K., Kearsley A. T., Menzies O. N., Watt L. and Rogers N. W. (2005) Volatile fractionation in the early Solar System and chondrule/matrix complementarity. Proc. Natl. Acad. Sci. 102, 13755–13760. Bland P. A., Jackson M. D., Coker R. F., Cohen B. A., Webber J., B- W., Lee M. R., Duffy C. M., Chater R. J., Ardakani M. G., McPhail D. S. and Benedix G. K. (2009) Why aqueous alteration in asteroids was isochemical: High porosity – high permeability. Earth Planet. Sci. Lett. 287, 559–568. Brearley A. J. and Martinez C. (2010) Ubiquitous exsolution of pentlandite and troilite in pyrrhotite from the TIL 91722 CM2 carbonaceous chondrite: A record of low temperature solid state processes. Lunar Planet. Sci. 41. #1689 (abstr.). Browning L. and Bourcier W. (1998) Constraints on the anhydrous precursor mineralogy of fine-grained materials in CM Carbonaceous chondrites. Meteorit. Planet. Sci. 33, 1213–1220. Browning L. B., McSween, Jr., H. Y. and Zolensky M. E. (1996) Correlated alteration effects in CM carbonaceous chondrites. Geochim. Cosmochim. Acta 60, 2621–2633. Chizmadia L. J. and Brearley A. J. (2008) Mineralogy aqueous alteration and primitive textural characteristics of fine-grained

K.T. Howard et al. / Geochimica et Cosmochimica Acta 149 (2015) 206–222 rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002. Geochim. Cosmochim. Acta 72, 602–625. Clayton R. N. and Mayeda T. K. (1999) Oxygen isotope studies of carbonaceous chondrites. Geochim. Cosmochim. Acta 63, 2089– 2104. Cressey G. and Batchelder M. (1998) Dealing with absorption and micro absorption in quantitative phase analysis. Int. Union Crystallogr. Newsletter 20, 16–17. Cressey G. and Schofield P. F. (1996) Rapid whole-pattern profile stripping method for the quantification of multiphase samples. Powder Diffr. 11, 35–39. Dunn T. L., Cressey G., McSween H. Y. and McCoy T. J. (2010) Analysis of ordinary chondrites using powder X-ray diffraction: Modal mineral abundance. Meteorit. Planet. Sci. 45, 127–138. Dyl K., Manning C. E. and Young E. D. (2006). Modeling aqueous alteration of CM carbonaceous chondrites: Implications for cronstedtite formation by water–rock reaction. Lunar Planet. Sci. 37. #2060 (abstr.). Eggleton R. A. (1984) Formation of iddingsite rims of olivine: A transmission electron microscope study. Clays Clay Min. 32, 1– 11. Fuchs L. H., Olsen E. and Jensen K. J. (1973) Mineralogy crystal chemistry and composition of the Murchison (C2) meteorite. Smithsonian Contrib. Earth Sci. 10, 1–39. Garenne A., Beck P., Montes-Hernandez G., Chiriac R., Toche F., Qurico E., Bonal L. and Schmitt B. (2014) The abundance and stability of “water” in type 2 carbonaceous chondrites (CICM and CR). Geochim. Cosmochim. Acta 137, 93–112. Guo W. and Eiler J. M. (2007) Temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the CM chondrites. Geochim. Cosmochim. Acta 71, 5565–5575. Harju E. R., Rubin A. E., Ahn I., Zielgler K. and Wasson J. T. (2014) Progressive aqueous alteration of CR carbonaceous chondrites. Geochim. Cosmochim. Acta 139, 267–292. Hiroi T., Zolensky M. E. and Pieters C. M. (2001) The Tagish Lake Meteorite: A possible sample from a D-type asteroid. Science 293, 2234–2236. Howard K. T., Benedix G. K., Bland P. A. and Cressey G. (2009) Modal mineralogy of CM2 chondrites by X-ray diffraction (PSD-XRD). Part 1: Total phyllosilicate abundance and the degree of aqueous alteration. Geochim. Cosmochim. Acta 73, 4576–4589. Howard K. T., Benedix G. K., Bland P. A. and Cressey G. (2010) Modal mineralogy of CV3 chondrites by position sensitive detector X-ray diffraction (PSD-XRD). Geochim. Cosmochim. Acta 74, 5084–5097. Howard K. T., Benedix G. K., Bland P. A. and Cressey G. (2011) Modal mineralogy of CM chondrites by X-ray diffraction (PSD-XRD): Part 2. Degree nature and settings of aqueous alteration. Geochim. Cosmochim. Acta 75, 2735–2751. Howard K. T., Benedix G. K., Bland P. A., Gibson J., Greenwood R. C., Franchi I. A. and Cressey G. (2013) Non-progressive aqueous alteration of CM carbonaceous chondrites: The perspective of modal mineralogy and bulk O-isotopes. Lunar Planet. Sci. 44. #2520 (abstr.). Kallemeyn G. W. and Wasson J. T. (1981) The compositional classification of chondrites—I the carbonaceous chondrite groups. Geochim. Cosmochim. Acta 45, 1217–1230. Kallemeyn G. W., Rubin A. E. and Wasson J. T. (1994) The compositional classification of chondrites: VI. The CR carbonaceous chondrite groups. Geochim. Cosmochim. Acta 58, 2873–2888.

221

Keller L. P. and Messenger S. (2012) Formation and processing of amorphous silicates in primitive carbonaceous chondrites and cometary dust. Lunar Planet. Sci. 43. #1880 (abstr.). King A. J., Schofield P. E., Howard K. T. and Russell S. S. (2014) Modal mineralogy of CI and CI-like chondrites by position sensitive detector X-ray diffraction. Lunar Planet. Sci. 45. #1861 (abstr.). Kurahashi E., Kita T. N., Nagahara H. and Morishita Y. (2008) 26 Al-26Mg systematics of chondrules in a primitive CO chondrite. Geochim. Cosmochim. Acta. 72, 3865–3882. Lauretta D. S., Hua X. and Buseck P. R. (2000) Mineralogy of finegrained rims in the ALH81002 CM chondrite. Geochim. Cosmochim. Acta 64, 3263–3273. Le Guillou C. and Brearley A. (2014) Relationships between organics water and early stages of aqueous alteration in the pristine CR3.0 chondrite MET00426. Geochim. Cosmochim. Acta 131, 344–367. Le Guillou C., Brearley A., Remusat L. and Bernard S. (2011) Aqueous alteration of organic matter and amorphous silicate in pristine chondrites: A multiscale study. Mineral. Mag. 75, 1291. Lodders K. (2001) Solar system abundances and condensation temperatures of the elements. Astrophys. J. 591, 1220–1247. Madsen I. C. (1999) Quantitative phase analysis round robin. Int. Union Crystallogr. Newsletter 22, 3–5. Madsen I. C., Scarlett N. V. Y., Cranswick L. M. D. and Lwin T. (2001) Outcomes of the international union of crystallography commission on powder diffraction round robin on quantitative phase analysis: Samples 1a to 1h. J. Appl. Crystallogr. 34, 409– 426. Maldonado E. M. and Brearley A. J. (2011) Exsolution textures in pyrrhotite and alteration of pyrrhotite and pentlandite in the CM2 carbonaceous chondrites Cresent Mighei and ALH 81002. Lunar Planet. Sci. 42. #2271 (abstr.). Marty B. (2012) The origins and concentrations of water carbon nitrogen and noble gases on Earth. Earth Planet. Sci. Lett. 313– 314, 56–66. McAdam M. M., Sunshine J. M., Howard K. T. and McCoy T. J. (2014) Aqueous alteration on asteroids: Linking the mineralogy and spectroscopy of CM and CI chondrites. Icarus 245, 220– 332. McAdam M. M., Sunshine J. M., Howard K. T., Kelly M. S. and McCoy T. J. (2013) Fe and Mg compositional variations of CM/CI meteorites and dark asteroids. Lunar Planet. Sci. 44. #1048 (abstr.). Menzies O. N., Bland P. A., Berry F. J. and Cressey G. (2005) A Mossbauer spectroscopy and X-ray diffraction study of ordinary chondrites: Quantification of modal mineralogy and implications for redox conditions during metamorphism. Meteorit. Planet. Sci. 40, 1023–1042. Metzler K., Bischoff A. and Stoffler D. (1992) Accretionary dust mantles in CM chondrites: Evidence for solar nebular processes. Geochim. Cosmochim. Acta 56, 2873–2897. Mittlefehldt D. (2002) Geochemistry of the ungrouped carbonaceous chondrite Tagish Lake the anomalous CM chondrite Bells and comparison with CI and CM chondrites. Meteorit. Planet. Sci. 37, 703–712. Morlok A., Bischoff A., Stephan T., Floss C., Zinner E. and Jessberger E. K. (2006) Brecciation and chemical heterogeneities of CI chondrites. Geochim. Cosmochim. Acta 70, 5371– 5394. Palmer E. and Lauretta D. (2011) Aqueous alteration of kamacite in CM chondrites. Meteoritics Planet. Sci. 46, 1587–1607. Pulguta J., Schubert G. and Travis B. J. (2010) Fluid flow and chemical alteration in carbonaceous chondrite parent bodies. Earth Planet. Sci. Lett. 296, 235–243.

222

K.T. Howard et al. / Geochimica et Cosmochimica Acta 149 (2015) 206–222

McSween H. Y. (1979a) Are carbonaceous chondrites primitive or processed – A review. Rev. Geophys. Space Phys. 17, 1059–1078. McSween H. Y. (1979b) Alteration in CM carbonaceous chondrites inferred from modal and chemical variations in matrix. Geochim. Cosmochim. Acta 43, 1761–1770. McSween H. Y. (1987) Aqueous alteration in carbonaceous chondrites: Mass balance constraints on matrix mineralogy. Geochim. Cosmochim. Acta 51, 2469–2477. Rosenberg N. D., Browning L. and Bourcier W. L. (2001) Modeling aqueous alteration of CM carbonaceous chondrites. Meteorit. Planet. Sci. 36, 239–244. Rubin A. E., Trigo-Rodriguez J. M., Huber H. and Wasson J. T. (2007) Progressive aqueous alteration of CM carbonaceous chondrites. Geochim. Cosmochim. Acta 71, 2361–2382. Schrader D. L., Franchi I. A., Connolly H. C., Greenwood R. C., Lauretta D. S. and Gibson J. M. (2011) The formation and alteration of the Renazzo-like carbonaceous chondrites I: Implications of bulk-oxygen isotopic composition. Geochim. Cosmochim. Acta 75, 308–325. Schrader D. L., Connolly, Jr., H. C., Lauretta D. S., Nagashima K., Huss G. R., Davidson J. and Domanik K. J. (2013) The formation and alteration of the Renazzo-like carbonaceous chondrites II: Linking O-isotope composition and oxidation state of chondrule olivine. Geochim. Cosmochim. Acta 101, 302–327. Tomeoka K. and Buseck P. R. (1985) Indicators of aqueous alteration in CM carbonaceous chondrites: Microtextures of a layered mineral containing Fe, S, O and Ni. Geochim. Cosmochim. Acta 49, 2149–2163. Tomeoka K., McSween H. Y. and Buseck P. R. (1989) Mineralogical alteration of CM chondrites: A review. Proc. NIPR Symp. Antarct. Meteorites 2, 221–234. Van Schmus W. R. and Wood J. A. (1967) A chemical-petrologic classification for the chondritic meteorites. Geochim. Cosmochim. Acta 31, 747–765.

Wasson J. T. and Rubin A. E. (2009) Composition of matrix in the CR chondrite LAP 02342. Geochim. Cosmochim. Acta 73, 1436– 1460. Weisberg M. K. and Huber H. (2007) The GRO 95577 CR1 chondrite and hydration of the CR parent body. Meteorit. Planet. Sci. 42, 1495–1503. Weisberg M. K., Prinz M., Clayton R. N. and Mayeda T. K. (1993) The CR (Renazzo-type) carbonaceous chondrite group and its implications. Geochim. Cosmochim. Acta 57, 1567–1586. Weisberg M. K., McCoy T. J. and Krot A. N. (2006) Systematics and evaluation of meteorite classification. In Meteorites and the Early Solar System II (eds. D. S. Lauretta and H. Y. McSween). University of Arizona Press, Tucson, pp. 19–52. Wilson L., Keil K. and Love S. J. (1999) The internal structures and densities of asteroids. Meteorit. Planet. Sci. 34, 479–483. Young E. D., Ash R. D., England P. and Rumble, III, D. (1999) Fluid flow in chondritic parent bodies: Deciphering the compositions of planetesimals. Science 286, 1331–1335. Zolensky M. E., Mittlefehldt D. W., Lipschutz M. E., Wang M.-S., Clayton R. N., Mayeda T. K., Grady M. M., Pillinger C. T. and Barber D. J. (1997) CM chondrites exhibit the complete petrologic range from type 2 to 1. Geochim. Cosmochim. Acta 61, 5099–5115. Zolensky M., Barrett R. and Browning L. (1993) Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites. Geochim. Cosmochim. Acta 57, 3123–3158. Zolotov M. Yu. (2012) Aqueous fluid composition in CI chondritic materials: Chemical equilibrium assessments in closed systems. Icarus 220, 713–729. Zolotov M. Yu. (2014) Formation of brucite and cronstedtitebearing mineral assemblages on Ceres. Icarus 228, 13–26. Associate editor: Trevor Ireland