The bulk valence state of Fe and the origin of water in chondrites

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ScienceDirect Geochimica et Cosmochimica Acta 211 (2017) 115–132 www.elsevier.com/locate/gca

The bulk valence state of Fe and the origin of water in chondrites S. Sutton a,b,⇑, C.M.O’D. Alexander c, A. Bryant a, A. Lanzirotti a, M. Newville a, E.A. Cloutis d a

Center for Advanced Radiation Sources, 5640 S. Ellis Avenue, University of Chicago, Chicago, IL 60637, USA b Department of Geophysical Sciences, 5640 S. Ellis Avenue, University of Chicago, Chicago, IL 60637, USA c DTM, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA d Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canada Received 28 September 2016; accepted in revised form 13 May 2017; available online 19 May 2017

Abstract There is abundant petrologic evidence for the oxidation of Fe during the aqueous alteration of chondrites, and water must have been the oxidant for this process. The H2 lost from the chondrite parent bodies as a result of Fe oxidation would have been isotopically very light, enriching any residual water in D. The extents of the D enrichments will have depended on the fractions of water consumed and the temperatures during Fe oxidation. Here we have estimated the likely ranges of water consumed by Fe oxidation in the CI, CM, CR and LL parent bodies, as well as the likely range of changes in water H isotopic compositions this would have produced. We first used Fe XANES to determine the Fe valences of bulk meteorite powders in Orgueil (CI1), a number of CMs and CRs that experienced varying degrees of alteration, and Semarkona (LL3.00). The total ranges of bulk Fe valences we obtained were: Orgueil 2.77, CMs 2.40–2.63, CRs 1.46–2.54, and Semarkona 2.10. Combining previous estimates of the present water/OH contents of our samples with the present bulk Fe valences and an estimated range of initial bulk Fe valences, we estimate the likely ranges of fractional water losses to have been: Orgueil 15–26%, Semarkona 73–83%, CMs 23–48%, and CRs 39–62%. The associated maximum and minimum changes in the H isotopic compositions of the remaining water were estimated assuming the equilibrium H2-H2O isotopic fractionation factor, Rayleigh fractionation of the H2, and oxidation temperatures of 0–200 °C. Using previous estimates of the water H isotopic compositions in the chondrites, the ranges of estimated dD values for the initial chondritic waters are: Orgueil
672‰ to
422‰, CMs
676‰ to
493‰, CRs
527‰ to
56‰, and Semarkona
527‰ to 154‰. The CI, CM, CR and ordinary chondrites all accreted water with similar H isotopic compositions that were distinct from the compositions of comets or Saturn’s moon Enceladus. Thus, the carbonaceous chondrites are unlikely to have come from comets or from bodies that were scattered into the Asteroid Belt from comet forming regions by orbital migration of the giant planets. If the carbonaceous chondrites did form in the outer Solar System, as some models predict, it was probably not beyond 7 AU. However, based on water isotopic compositions at present it is equally plausible that the carbonaceous chondrites formed in the inner Solar System. Ó 2017 Elsevier Ltd. All rights reserved.

1. INTRODUCTION ⇑ Corresponding author at: Building 434A, Advanced Photon

Source, Argonne National Laboratory, Argonne, IL 60439, USA. E-mail addresses: [email protected] (S. Sutton), [email protected] (C.M.O’D. Alexander), [email protected] (A. Bryant), [email protected] (A. Lanzirotti), [email protected] (M. Newville), [email protected] (E.A. Cloutis). http://dx.doi.org/10.1016/j.gca.2017.05.021 0016-7037/Ó 2017 Elsevier Ltd. All rights reserved.

Alteration by water was a fundamental process that affected almost all chondrites, even though in many cases it was subsequently overprinted by thermal metamorphism (e.g., Brearley, 2006). The oxidation of Fe played a very important role during this aqueous alteration. The process of oxidation, particularly of metal, during alteration is most

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obvious in the least altered CR2s (Weisberg et al., 1993; Noguchi, 1995), Semarkona (LL3.0) (Hutchison et al., 1987; Alexander et al., 1989; Krot et al., 1997) and Paris (CM2) (Hewins et al., 2014). This is because in all of them the conversion of metal and sulfide to magnetite and clay minerals is incomplete. In all other CMs and all CIs, the alteration of metal has essentially gone to completion. The oxidant in this process was the water responsible for the aqueous alteration through reactions such as: 3Fe þ 4H2 O ¼ Fe3 O4 þ 4H2 :

ð1Þ

The likely consequences of this oxidation are that: (1) the water/rock ratios during alteration were higher than would be inferred from their present H2O/OH contents, (2) the internal pore pressures associated with the buildup of H2 generated by the oxidation would have led to the catastrophic disruption of asteroids unless the H2 was able to escape (i.e., there was open system behavior at least for a time), (3) the loss of this H2 would have led to increasingly oxidizing conditions, and (4) the potential H isotopic mass fractionation associated with the generation and loss of the H2 could have dramatically changed the H isotopic compositions of the remaining water in chondrites (Wilson et al., 1999; Rosenberg et al., 2001; Alexander et al., 2010, 2012; Le Guillou et al., 2015). To quantitatively assess the consequences of the Fe oxidation requires the bulk initial and final Fe valence states of the altered chondrites. The initial Fe valence states must be estimated from what is known of the least altered meteorites and other primitive extraterrestrial materials (e.g., Ogliore et al., 2010). The current bulk Fe valence states of chondrites (i.e., after alteration) have yet to be measured in any systematic way. The number of wet chemical analyses of primitive chondrites in which the abundances of Fe in its different valence states have been determined are limited (Jarosewich, 1990). Beck et al. (2012) applied Fe K edge Xray absorption near edge spectroscopy (XANES) to 500 (diameter)  50 (thick) lm CM matrix sections and documented the centroids and intensities of pre-edge peaks, from which they inferred bulk matrix mineralogies but not bulk matrix valence states. Le Guillou et al. (2015) applied Fe L edge XANES methods to CR matrix focused ion beam (FIB) sections and reported the Fe valences to be 2.64–2.75. However, since matrix only makes up 60 vol.% of CMs and 30 vol.% of CRs, it is unlikely that the Fe valences in the matrix samples analyzed were representative of the bulk Fe in the chondrites; in particular, more reduced Fe associated with non-matrix components will not have been analyzed. Here we report the bulk Fe valence states of a number of CMs and CRs that experienced a range of alteration, as well as one CI, Orgueil, and one ordinary chondrite, Semarkona. The bulk Fe valences of these chondrites were determined by Fe-XANES. Fe-XANES is attractive for this purpose because it determines the average valence state of all Fe atoms in the sample, regardless of host material. In our approach, whole meteorite chips were ground to produce representative bulk chondrite samples for these measurements. Combining the valence results with estimates of the initial valences at the time of accretion and current

water/OH contents, we estimate the changes in water contents and H isotopic compositions as a result of the oxidation during the aqueous alteration of these meteorites. We also explore the implications for the origin of the water in the chondrites and the origins of the chondrites themselves. 2. SAMPLES AND TECHNIQUES 2.1. Samples The meteorites analyzed are listed in Table 1 along with their petrologic types and weathering grades (finds only). All of the samples had previously been powdered (<106 lm). All of the samples have also been analyzed for their bulk H, C and N contents and isotopic compositions (Alexander et al., 2012, 2013), as well as the abundances and C and O isotopic compositions of their carbonates (Alexander et al., 2015), and in many cases have had their bulk mineralogies determined (Howard et al., 2015). The samples were selected to cover the full range of degrees of alteration, and for finds to be weathering grade A or B. In preparing the bulk meteorite powders of the CRs Renazzo and Queen Alexandra Range (QUE) 99177, it was necessary to magnetically separate metal grains (5.99

Table 1 The meteorites analyzed, their petrologic types, and weathering grades (finds only). Pet. Type1 CI Orgueil CM ALH 84042 ALH 85013 Banten Bells W DNG 06004 GRO 95566 LEW 85312 LEW 87022 LEW 87148 LEW 90500 Murchison Nogoya II QUE 99355 SCO 06043 II CR GRO 95577 Renazzo QUE 99177 OC Semarkona3 1

Pet. Type2

Weath.

2.1 2.3 2.5

A A

2.6 2.5

A/B A/B B B A B

1.1 1.2 1.4 1.7 1.9 1.8 1.6 1.8 1.4 1.3 1.6 1.6 1.1 1.5 1.2

2.3 2.4 2.3 2.5 2.2 2.3 2.0

1.3 2.5 2.4

2.0 2.4 2.8

B B/C B B

3.00

Petrologic classifications from Alexander et al. (2013) on a scale from 1 to 3. 2 Petrologic classifications from Alexander et al. (2013) and Rubin et al. (2007) for CMs, and Harju et al. (2014) for CRs on scales from 2 to 3. The classification for Bells and LEW 85312 are uncertain on this scale using the approach of Alexander et al. (2013). CIs must either be classified as 2.0 on this scale or remain unclassified. 3 Petrologic classification from Grossman and Brearley (2005).

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wt.% and 4.88 wt.%, respectively) that would not pass through the 106 lm sieve. To avoid potential ‘nugget’ effects, these metal grains were not recombined with the remaining powders. The bulk Fe valence states of the chondrites were determined on 30 mg aliquots of the bulk meteorite powders. Prior to the XANES measurements, the meteorite aliquots and standards (Table 2) were further crushed to a grain size of <30 lm and dispersed between layers of Scotch tape. The only exception to this was for tochilinite and serpentine standards extracted from a thin section of the CM Maribo using a focused ion beam (FIB) technique. The 20  20 lm FIB foils were mounted on Cu TEM grids and the XANES spectra collected as for the powdered samples. Fifteen valence standards were used. Six of the standards were obtained from the Winnipeg mineral collection (http://psf.uwinnipeg.ca/Samples_Directory/SAMPLES% 20DIRECTORY.pdf): nontronite (NON101), saponite (SAP103), serpentine (SRP111), chamosite (CHM102), greenalite (GRE001), and San Carlos olivine (OLV003). The curatorial characterization included determination of total Fe by X-ray fluorescence (XRF), ferrous Fe by wet chemistry and ferric Fe by subtraction. Based on the accuracies of these analyses (S. Mertzman, private communication), the Fe valence uncertainty (1r) is estimated to be 0.01 for nontronite and 0.02 for the other five. Three standards were certified Alfa Aesar compounds (FeO, Fe2O3, and FeS) with an estimated Fe valence uncertainty of 0.01. The Fe metal standard is pure metal foil from EXAFS Materials, Inc. Based on unobservable oxidized Fe in the X-ray absorption fine structure (XAFS) spectrum of this material, a reasonable upper limit for oxidation is 2% and we therefore ascribe a liberal valence uncertainty of 0.01 (1r). Two San Carlos pyroxenes were used. San Carlos orthopyroxene (OPX) has been found to have 0% Fe3+ by electron microprobe analyzer (EMPA) and 6% by Mo¨ssbauer (Dyar et al., 1989); we ascribe a valence uncertainty

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of 0.03 (1r). San Carlos clinopyroxene (CPX) has been found to have 0% Fe3+ by EMPA and 21% by Mo¨ssbauer (Dyar et al., 1989); we ascribe a valence uncertainty of 0.05 (1r). The ferrihydrite standard was synthesized by Hansel et al. (2003); we use a valence of 3.0 with a 0.01 (1r) uncertainty based on the expectation that the material is fully oxidized and unlikely to be reduced from its original state. Two minerals were FIBed from the Maribo meteorite by M. Zolensky (NASA-JSC), tochilinite and serpentine. Tochilinite is a ferrous Fe mineral and we ascribe a valence uncertainty of 0.03 (1r). Vollmer et al. (2014) report Fe valence values of 2.5–2.6 based on scanning transmission X-ray microscope (STXM) L-edge analyses of layered tochilinite-serpentine intergrowths in Maribo. These values are likely dominated by the serpentine consistent with the valence of terrestrial serpentine and cronstedtite, and with the observation that our XANES spectra show the absorption edge of the Maribo serpentine at higher energy (more oxidized) than that of the tochilinite. We use a valence uncertainty of 0.03 (1r) for the Maribo serpentine valence. Our cronstedtite standard is a powdered terrestrial sample from Llallagua, Bolivia (CRO101 in the Winnipeg collection; Smithsonian NMNH #193910). Stoichiometric cronstedtite has a Fe valence of 2.50. Measured valences for terrestrial cronstedtite are 2.57 (Mackenzie and Berezowski, 1981; Gole, 1982; Burns and Fisher, 1991; Hybler et al., 2002), and grains from CM meteorites (Murchison, Murray and Cold Bokkeveld) fall within a tight range of 2.45–2.54 (Zega et al., 2003). The Winnipeg valence determination for CRO101 is 2.78. We use the average of the stoichiometry and XRF/wet chemistry values (2.64) with a relatively large uncertainty of 0.07 (1r). Two other sulfide standard spectra were used, troilite and pentlandite, collected by Westphal et al. (2009) at the Advanced Light Source (LBNL, beamline 10.3.2). These are ferrous minerals with valence uncertainties that are expected to be small (0.01, 1r).

Table 2 The standards used in the linear combination fitting of the meteorite Fe-XANES spectra.

Ferrihydrite Nontronite Fe3O4 Saponite Serpentine Serpentine Cronstedtite Chamosite Greenalite Olivine Orthopyroxene Clinopyroxene FeO Pyrrhotite Tochilinite Troilite Pentlandite Fe-metal

Fe valence

Origin

Reference

3.00 2.96 2.67 2.63 2.62 2.60 2.64 2.44 2.38 2.04 2.05 2.10 2.00 2.00 2.00 2.00 2.00 0.00

Synthetic Allentown, PA Alfa Aesar Griffith Park, CA King City, CA Maribo meteorite Llallagua, BO Chamois, FR La Union, SP San Carlos, NM San Carlos, NM San Carlos, NM Alfa Aesar Alfa Aesar Maribo meteorite Berkeley XANES library Berkeley XANES library Synthetic

C. Hansel (WHOI) a; NON101 1317-61-9 a; SAP103 a; SRP111 M. Zolensky (NASA-JSC) a; CRO101 a; CHM102 a; GRE001 a; OLV003 Carnegie Inst. Wash. Carnegie Inst. Wash. 1345-25-1 17422 M. Zolensky (NASA-JSC) Westphal et al., 2009 Westphal et al., 2009 EXAFS Materials, Inc.

(1) (1) (3) (2) (2) (3) (7) (2) (2) (2) (3) (5) (1) (1) (3) (1) (1) (1)

a = HOSERLab Mineral Database, University of Winnipeg

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2.2. Fe-XANES measurements Fe-XANES spectra were collected in transmission mode at the GSECARS 13BMD station at the Advanced Photon Source (APS, Argonne National Laboratory, Argonne, IL USA). The APS storage ring was operated at 7 GeV with 100 mA current. The X-rays were monochromatized using a water-cooled Si(111) double-crystal monochromator. Harmonic rejection was achieved with both a Pt-coated mirror, pitched at 3 mrad, and by detuning the second crystal of the monochromator to reduce the total intensity by about 30%. The Pt-coated mirror was also used to focus the beam in the vertical to 0.5 mm, and slits were used to define the horizontal beam size to 3 mm. Further details of the equipment and X-ray optics of this beamline are described by Shen et al. (2005). The incident and transmitted X-ray intensities were monitored with N2-filled ion chambers (10 cm length). X-ray absorption fine-structure (XAFS) spectra were collected by scanning the monochromator energy from 7010 eV to 7500 eV using 5 eV steps before the main edge, 0.25 eV steps within
15 eV and ˚ -1 steps in photo+25 eV of the main edge, and 0.06 A electron wavenumber above the main edge. At each energy point, the signals from the ion chambers were recorded for three seconds. Several scans were collected on each sample and merged to give the total XAFS spectrum. The first derivative peak of Fe metal foil (7112 eV) was used to calibrate energy for the system. Figs. 1 and 2 show the Fe K XANES spectra for the standards and meteorite samples used in this study. 2.3. Bulk valence determinations The bulk Fe valence of the standards for which this value was determined was performed through a combination of X-ray fluorescence analysis to determine total Fe.

Fe2+ was determined by wet chemistry and Fe3+ as the difference between total and ferrous iron (Mertzman, 2000). The bulk Fe valence of each analyzed meteorite powder was extracted from the Fe-XANES spectrum using two semi-independent XAFS analysis methods: (a) pre-edge peak centroid, and (b) linear combination fitting (LCF) of full spectra using standard spectra. The LCF was conducted with two different standard approaches: all 15 standard spectra and combinatorial taking 4 of the 15 standards at a time. Thus, the end result was three bulk Fe valence values for each meteorite powder and these were combined by weighted averaging. These semi-independent methods have different potential systematic errors so that general agreement between the results would enhance reliability. (1) Pre-edge Peak Method: Fe K XANES spectra of materials with mixed Fe2+/Fe3+ valence exhibit peaks in the pre-edge spectral region due to 1s-3d electron transitions. The excitation of these ‘forbidden’ transitions is made possible by orbital mixing. The energy of the pre-edge peak is valence dependent, the Fe3+ peak being at higher energy than that produced by Fe2+ (e.g., Waychunas et al., 1983). In mixed state spectra, both peaks occur with intensities that are dependent on the proportions of Fe in each state. This behavior has been extensively exploited in XANES determinations of Fe valence (e.g., Bajt et al., 1994; Delaney et al., 1998; Bonnin-Mosbah et al., 2001; Dyar et al., 2001; Galoisy et al., 2001; Wilke et al., 2001; Dyar et al., 2002; Berry and O’Neill, 2004; Wilke et al., 2004; Metrich et al., 2006; Berry et al., 2008; Cottrell et al., 2009; Kelley and Cottrell, 2009; Righter et al., 2013). The spectra were fit using the PAN routine written by R. Dimeo (http://www.ncnr.nist.gov/staff/dimeo/panweb/pan.html) for IDL. A damped harmonic oscillator (DHO) was used for fitting the background (e.g., Cottrell et al., 2009), and Lorentzian functions were used for fitting the Fe2+ and Fe3+ peaks. The energy fit range was limited to 7108–

Fig. 1. Fe K XANES spectra for the valence standards used in this study.

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Fig. 2. Fe K XANES spectra for the 19 bulk chondrite powders for which bulk Fe valences were determined in this study.

7120 eV, and the full widths at half maximum (FWHM) of the Lorentzians were fixed at 1.8 eV. The Fe valence determination was based on the centroid energy computed as the area-weighted mean of the energies of the two fitted peaks. The standard mineral spectra for nontronite, olivine, chamosite, saponite and orthopyroxene (standards with wellcharacterized Fe valence), together with their known Fe valences, were used to calibrate ‘‘valence vs. centroid energy” assuming a linear correlation (Fig. 3). A correlation coefficient of 0.97 was obtained leading to a typical standard error of estimate of 0.06 in Fe3+/RFe. The pre-edge method is typically only applicable to mixtures of ferrous and ferric iron. The presence of metal can compromise the ability to extract the two relevant peaks because its absorption edge spectrum can obscure the ferrous and ferric peaks. (2) Full-Spectrum, Linear Combination Fitting (LCF) Method: The second approach was to fit the full XANES spectrum (Fig. 4) with a linear, least squares routine using spectra for the 15 standard minerals with known compositions, including Fe valence states (Table 2). Fitting was done with Athena, an X-ray absorption spectroscopy analysis program written for IDL as part of the IFEFFIT package (Newville, 2001). Because there is the potential for the LCF results to depend on the suite of standards used, two fitting procedures were used: (1) fitting with all 15 standard spectra, and (2) fitting with all possible combinations of the 15 standard spectra taken four at a time. Athena provides the proportions of Fe in each phase, the uncertainties in these proportions and the value of a goodness of fit factor R that is essentially a measure of residuals between the fitted and measured spectra, where P ðdata
fitÞ2 R¼ : ð2Þ P data2

The sums are calculated over the spectral channels within the region of fitting, in this case
20 eV to +190 eV relative to the absorption edge energy. Typical values of R in this work were 0.001, which means that the average deviation between the measured spectrum and the fit was at the few percent level. For the ‘‘combinatorial” procedure, there were 1925 possible 4-standard combinations and the valence results for each were sorted in order of increasing R-factor. Two valence results are reported: (1) all 15-std fit, and (2) the average of the best 50 fits of the combinatorial procedure. For each, the relevant R-factor values are listed. The proportion of the Fe associated with each standard given by a fit was multiplied by the known Fe valence of that standard to determine the Fe valence contribution of that standard. These contributions were then summed to obtain the bulk Fe valence of the unknown. The uncertainty in the estimated bulk valences for the unknowns derives from two sources: the uncertainties in the valences of the standards and the uncertainties in the phase proportions from the LCF results. We obtained the uncertainty in the proportion of each standard by adding the fractional values of these two quantities in quadrature and multiplying by the proportion. Valence uncertainties for the standards were generally small compared to the uncertainties from the LCF fits. A Monte Carlo (MC) approach was used to evaluate the uncertainty in the bulk valence estimates based on these proportion uncertainties. For each standard in each fit, a random number generator was used to select a Fe proportion within the associated uncertainty range, these were summed for the 15 standards in each fit, and the proportions were scaled by a constant to sum to unity. These proportions were then multiplied by the associated standard valences and the products summed to produce a bulk Fe

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Fig. 4. Two examples of the linear combination fitting method (15 standards) for Banten (top) and Semarkona (bottom). Measured spectra are in red and fitted spectra are in blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Fe valence calibration curve (middle) for the pre-edge peak method based on centroid energy and using five standards. The pre-edge peak spectra (after background subtraction) for Banten (top) and Semarkona (bottom) are shown as well as the fitted ferric (blue) and ferrous (red) peaks. Arrows point to the associated positions on the calibration curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

valence. This MC generation was performed 50 times and the standard deviation of the 50 results taken as the bulk valence uncertainty. These uncertainties ranged from 0.04 to 0.11. In order to test the linear combination procedure, we made and analyzed two physical mixtures, an oxidized mixture (Mix1) Fe3+-rich and a reduced mixture (Mix2) dominated by Fe2+. The known weights, valences and Fe

concentrations for each mineral standard were used to calculate the proportions of Fe contributed by each phase as well as their contributions to Fe valence and the nominal bulk valence. Mix1 consisted of the following standards (wt.%): 3.39 pyrrhotite, 11.70 olivine, 22.71 serpentine, 1.10 magnetite and 61.11 cronstedtite with predicted bulk Fe valence of 2.59. Mix2 was (wt.%) 9.92 Fe(1-x)S, 83.85 olivine, and 6.23 orthopyroxene with predicted bulk Fe valence of 2.02. For each mixture, four XANES spectra were collected and then merged. For one meteorite sample, Lewis Cliffs (LEW) 90500, we studied the sensitivity of the bulk valence results to the precise suite of standards used, simply by repeating the linear combination fitting procedure with different input standards suites. We also fitted multiple XANES spectra from this meteorite to provide some insight into the reproducibility of the linear combination fitting. 3. RESULTS Table 3 shows the results of the physical mixture study. The nominal Fe contributions from each phase in Mix1 (the ferric- or Fe3+-rich sample) was dominated by cronstedtite (87%), whereas Fe in Mix2 (the ferrous- or Fe2+-rich sam-

2.07 (3)

0.722 (3) 1.344 (4) 0.00 (3) na na na

2.02 (2)

0.328 (2) 0.672 (2) 0.00 (2) na na na 0.962 (6) 1.034 (7) 0.026 (2) na na na 0.481 (2) 0.507 (0) 0.013 (0) na na na (2) (2) (2)

2.53 (2)

(5) (5) (7)

9.92 (5) 83.85 (5) 6.23 (5) na na na (2) (1)

0.34 0.14 na 0.42 0.34 1.29 (1) (1)

(1) (1) (2)

2.59 (1)

(2) (3) (1)

0.17 0.07 na 0.16 0.13 0.49 (2) (1)

0.105 0.046 na 0.084 0.054 2.303 (1) (0)

(5) (5) (5) Bulk Valence

Nominal valence Fraction of Bulk Fe

0.053 0.023 na 0.032 0.020 0.872 (5) (5)

3.39 11.70 na 22.71 1.10 61.11 Pyrrhotite Olivine Clinopyroxene Serpentine Magnetite Cronstedtite

Fitted Fe fractions Nominal valence Fraction of Bulk Fe Fitted valence

Mixed portions (wt.%)

Mix2 (Ferrous dominated)

Mixed portions (wt.%)

Fitted Fe fractions Mix1 (Ferric dominated) Standard

Table 3 Summary of the results in the physical mixtures study. The fitted Fe fractions were determined using the linear combination fitting method (see text). na = ‘‘not applicable”.

Fitted valence

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ple) was contributed roughly equally by pyrrhotite and olivine. The mineral proportions obtained by linear combination fitting results for Mix1 were significantly different from the nominal values. Specifically, the cronstedtite portion was low (49% compared to nominal 87%) and the proportions of the other four phases were correspondingly high (e.g., 17% compared to nominal 5% for pyrrhotite). It seems that the reason for this is that a slightly lower Rfactor could be obtained using this mixture. However, importantly, the bulk valence from the fits agrees well with the nominal value (2.53 compared to 2.59). For Mix2, the fitted proportions are close to the nominal values (e.g., 67% compared to 51% for olivine). The bulk valence was 2.07, which compared well with the nominal value of 2.02. The pre-edge results, 2.48 and 2.04 for Mix1 and Mix2, respectively, agreed well with the nominal and linear combination values. These results support the view that the bulk valences obtained from the linear combination fitting procedure are accurate and insensitive to the precise fitted mineral proportions since components with similar valence have similar XANES spectra in terms of absorption edge energy, for example. To further test the uncertainties associated with the linear combination procedure, the procedure was applied to four individual XANES spectra collected on the LEW 90500 samples, labeled A to D (Table 4). The reproducibility was quite good, with typical standard deviations of the Fe proportions in the individual components of 0.2 wt.%, and the results for all spectra compared well with the values obtained independently for the spectrum produced by merging them (labeled ‘‘Merge” in Table 4). The mean bulk valence obtained was 2.417 ± 0.015 (1r) and essentially the same as the merge valence of 2.413. This uncertainty value is a good estimate of the valence uncertainty associated with the linear combination fitting routine. Another test of the linear combination fitting robustness was to determine the bulk valence for LEW 90,500 using different combinations of the standards. Table 5 shows the results for 10 such combinations; troilite and pendlandite were added to the standards used in some cases. The R-factor values ranged from 0.001 to 0.003, with one outlier at 0.040. The 15-standard suite (suite A, Table 5) had the lowest R (best fit). The valences obtained were very consistent for all 10 combinations, including the high R suite, varying from 2.41 to 2.47 providing further support for the idea that the bulk valence method is relatively insensitive to the precise standards used. The standard deviation of these results was 0.016, a similar value as that obtained from the reproducibility test (Table 4). The valence results for the meteorite powders are presented in Table 6. Shown are multiple valence results for each meteorite: Linear combination fitting using all 15 standard spectra, the average of the best 50, 4-standard fits, and pre-edge peak fitting. Each of the linear combination results is also accompanied by its ‘‘goodness-of-fit” parameter R, an R range in the case of the 50 combinatorial results. The final column shows the weighted average valence using the three results for each meteorite. Samples are arranged in Table 6 in descending order of average valence, i.e., the order of decreasing oxidation.

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Table 4 The results of linear combination fitting of multiple spectra for LEW 90500. Standards

Merge

Metal Wu¨stite Pyrrhotite Olivine Clinopyroxene Orthopyroxene Tochilinite Greenalite Chamosite Serpentine Saponite Magnetite Cronstedtite Nontronite Ferrihydrite

Fe proportions (%) 1.5 1.6 13.0 12.7 0.0 0.0 14.0 14.0 0.0 0.0 1.1 1.3 0.0 0.0 6.5 6.7 11.2 11.2 7.8 8.0 13.3 13.5 10.7 10.6 0.2 0.4 10.8 10.6 8.6 8.5

Metal Wu¨stite Pyrrhotite Olivine Clinopyroxene Orthopyroxene Tochilinite Greenalite Chamosite Serpentine Saponite Magnetite Cronstedtite Nontronite Ferrihydrite

Valence contributions 0.000 0.000 0.260 0.254 0.000 0.000 0.280 0.280 0.000 0.000 0.022 0.026 0.000 0.000 0.155 0.159 0.204 0.273 0.273 0.210 0.350 0.355 0.286 0.283 0.006 0.011 0.320 0.314 0.258 0.255

Bulk valence

A

2.413

2.420

B

C

D

Mean

1r

1.2 12.7 0.0 13.8 0.0 1.3 0.0 6.8 11.3 7.9 13.1 10.7 0.2 10.8 8.8

1.6 13.4 0.0 14.2 0.0 0.6 0.0 6.4 11.3 7.7 13.3 10.8 0.0 10.9 8.5

1.3 12.8 0.0 13.9 0.0 1.2 0.0 6.6 11.2 7.9 13.3 10.6 0.4 10.8 8.7

1.4 12.9 0.0 14.0 0.0 1.1 0.0 6.6 11.3 7.9 13.3 10.7 0.3 10.8 8.6

0.2 0.3 0.0 0.2 0.0 0.3 0.0 0.2 0.1 0.1 0.2 0.1 0.2 0.1 0.1

0.000 0.254 0.000 0.276 0.000 0.026 0.000 0.162 0.276 0.207 0.345 0.286 0.006 0.320 0.264

0.000 0.268 0.000 0.284 0.000 0.012 0.000 0.152 0.276 0.202 0.350 0.288 0.000 0.323 0.255

0.000 0.256 0.000 0.278 0.000 0.024 0.000 0.157 0.273 0.207 0.350 0.283 0.011 0.320 0.261

0.000 0.258 0.000 0.280 0.000 0.022 0.000 0.158 0.275 0.206 0.350 0.285 0.007 0.319 0.259

0.000 0.007 0.000 0.003 0.000 0.007 0.000 0.004 0.001 0.003 0.004 0.003 0.005 0.004 0.005

2.420

2.410

2.420

2.417

0.015

Table 5 Valence results using different groups of standards in the linear combination fitting procedure for LEW 90500 (X = standard used). Standard suite

A

B

C

D

E

F

G

H

I

J

Valence result R factor Valence mean Valence std. dev. Metal Wu¨stite Pyrrhotite Olivine Clino-pyroxene Ortho-pyroxene Tochilinite Greenalite Chamosite Serpentine Saponite Magnetite Cronstedtite Nontronite Ferrihydrite Troilite Pendlandite

2.410 0.0008 2.435 0.016 X X X X X X X X X X X X X X X

2.430 0.0013

2.470 0.0018

2.430 0.0022

2.430 0.0014

2.450 0.0021

2.430 0.0014

2.430 0.0398

2.440 0.0013

2.430 0.0032

X X X X X X X X

X X X X X X X X

X X X X X X X X

X

X

X

X

X

X

X X X X X X

X X X X X X

X X X X X X

X X X X X X

X X X X X X

X X X X X X

X X X X

X X X X

X

X X X X

X

X X X X

X

X X X X

X

X

X X

X

X X

X X

X X

X X

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123

Table 6 Summary of the Fe valences determined by various approaches to fitting the Fe-XANES spectra. One sigma uncertainties in parentheses. See text for details.

Orgueil (S) LEW 85312 ALH 85013 GRO 95566 Bells W LEW 87148 Murchison GRO 95577 ALH 84042 DNG 06004 Banten LEW 87022 QUE 99355 LEW 90500 SCO 06043 II Nogoya II QUE 99177 Semarkona Renazzo

CI1 CM2 CM2 CM2 CM2 anom. CM2 CM2 CR1 CM2/1 CM2 CM2 CM2 CM2 CM2 CM1 CM2 CR2 LL3.0 CR2

15-standard valence

15-standard R factor

Combinatorial valence

Combinatorial R factor range

Pre-edge valence

Weighted mean valence

2.67 2.68 2.61 2.55 2.53 2.61 2.55 2.53 2.53 2.50 2.51 2.51 2.51 2.42 2.37 2.46 2.29 1.93 1.93

0.00112 0.00012 0.00073 0.00039 0.00046 0.00234 0.00040 0.00034 0.00350 0.00051 0.00014 0.00055 0.00091 0.00082 0.00066 0.00138 0.00072 0.00016 0.00050

2.78 2.60 2.59 2.58 2.64 2.56 2.52 2.55 2.49 2.47 2.44 2.45 2.44 2.42 2.37 2.39 2.09 2.09 1.88

0.00011–0.00016 0.00009–0.00019 0.00049–0.00076 0.00036–0.00052 0.00030–0.00046 0.00039–0.00062 0.00043–0.00063 0.00030–0.00049 0.00071–0.00104 0.00043–0.00068 0.00013–0.00022 0.00042–0.00065 0.00035–0.00057 0.00070–0.00097 0.00055–0.00082 0.00057–0.00087 0.00043–0.00063 0.00027–0.00054 0.00051–0.00109

2.68 2.58 2.55 2.58 2.58 2.51 2.59 2.56 2.55 2.53 2.55 2.50 2.51 2.49 2.48 2.40 nd 2.27 nd

2.77 2.63 2.58 2.57 2.57 2.56 2.54 2.54 2.51 2.50 2.47 2.47 2.47 2.44 2.40 2.40 2.19 2.10 1.91

(11) (4) (7) (5) (5) (7) (5) (5) (8) (6) (4) (7) (8) (6) (6) (0) (7) (6) (10)

(2) (6) (6) (3) (8) (4) (3) (3) (3) (5) (2) (4) (4) (4) (5) (5) (8) (8) (4)

(6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6)

(2) (3) (3) (3) (3) (3) (2) (3) (2) (3) (2) (3) (3) (3) (3) (3) (5)/1.77 (5)a (4) (8)/1.46 (8)a

nd = not determined due to high metal interference. a Corrected for metal removed during crushing. See text for details.

The agreement between the different valence estimates in Table 6 is generally good; the correlation between the preedge and 15-standard valences (Fig. 5) has a correlation coefficient of 0.50. On average, the difference between the 15-standard and pre-edge results is 0.06. The largest difference is for Semarkona (0.36) with the pre-edge method indicating a more oxidized state than the linear combination

(2.27 vs. 1.93, respectively). The combinatorial results are in general agreement with the 15-standard and pre-edge averages. The R-factors are generally similar for the three different linear combination fittings for a given sample. Of the meteorites studied, Orgueil is the most oxidized with a bulk valence of 2.77. The bulk valences for the CRs range from 1.93 in Renazzo to 2.53 in GRO 95577. However, the measured valences for Renazzo and QUE 99177 must be corrected for the removal of metal during crushing (5.99 wt.% and 4.88 wt.%, respectively). Assuming a bulk Fe content for CRs of 24.0 wt.% and 5.7 wt.% Ni in the metal (Kong et al., 1999), adding back the metal reduces the bulk valences to 1.46 for Renazzo and 1.77 for QUE 99177. The bulk valence for Semarkona is 2.10. For the CMs, the Fe valences are all very similar (2.40–2.63; Fig. 5). However, there is a weak inverse correlation with bulk H content, a proxy for the extent of alteration. This trend is apparent in both falls (open symbols) and Antarctic finds (filled red symbols). 4. DISCUSSION

Fig. 5. Correlation plot for Fe valences determined by the two methods, pre-edge peak centroid (vertical axis) and linear combination fitting with 15 standards (horizontal axis). The dashed line is the 1:1 perfect correlation line. Error bars are 1r.

Bulk wet chemical analyses that included the determination of Fe contents in its various valence states have only been reported for four of the meteorites we have measured – the bulk Fe valences determined by wet chemistry for Orgueil, Banten, Murchison and Semarkona are 2.5, 1.99, 1.99, and 1.79, respectively (Jarosewich, 1990). The wet chemistry valence for Orgueil is quite similar to our value (Table 1), but the values for the CMs are significantly lower than we obtained (Table 1). Jarosewich (1990) reported no Fe3+ (as Fe2O3) for the two CMs, which is surprising since Fe3+–bearing serpentines (e.g., cronstedtite) are major min-

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eral in CMs (Brearley and Jones, 1998; Zega et al., 2003; Brearley, 2006; Zega et al., 2006; Howard et al., 2015). Jarosewich (1990) did report significant Fe2O3 contents in the Antarctic CMs ALH 83100 and ALH 83102, giving bulk valences of 2.30. These two meteorites are paired with ALH 84042, although Alexander et al. (2013) has questioned this pairing. The wet chemical valences determined for these two finds are closer to those we have obtained for all our CM (Table 1). The fact that Jarosewich (1990) only reported significant Fe2O3 in CMs for two Antarctic finds raises the question of whether all the Fe2O3 is a product of weathering. However, the valences that we have determined for the CMs are similar for falls and finds. Jarosewich (1990) did not report any Fe2O3 for Semarkona either, despite it containing magnetite, maghemite and Fe3+–bearing smectite (Hutchison et al., 1987; Alexander et al., 1989; Krot et al., 1997; Choi et al., 1998; Grossman et al., 2000). Hence, it seems likely that Jarosewich (1990) underestimated the bulk Fevalences of Banten, Murchison and Semarkona. In a simplistic picture of progressive aqueous alteration one might expect the fine-grained matrix to alter first, followed in sequence by isolated metal/sulfide grains, metal/sulfide in chondrules, chondrule mesostases and finally chondrule phenocrysts. Hence, the expectation would be that the bulk Fe valence should increase with the degree of alteration, although the changes in bulk Fe valence as chondrule silicates begin to alter may be relatively modest due to their fairly low Fe contents. This is consistent with Orgueil being the most oxidized of the chondrites measured, and with the increase in oxidation state from Renazzo (CR2) to Grosvenor Mountains (GRO) 95577 (CR1). Despite having similar water contents (Alexander et al., 2013) and petrologic evidence that QUE 99177 is less altered than Renazzo (Abreu and Brearley, 2010; Le Guillou et al., 2014), QUE 99177 has a higher bulk valence (Table 1). This could be due to some Antarctic weathering of the Fe-metal in QUE 99177. Antarctic weathering should be much less problematic for GRO 95577 and the Antarctic CMs since they contained little Fe-metal prior to falling to Earth. Thus, the apparent slight decrease in oxidation state with increasing alteration in the CMs (Fig. 6) is surprising. In principle, reaction between Fe oxides and organic C could have reduced the bulk valence if conditions in the more altered chondrites allowed, but there is no evidence that this ever occurred. At present, we have no explanation for the apparent slight decrease in bulk valence with increasing degree of alteration. 4.1. Consumption of water by oxidation of Fe The prime motivation for our measurements of the bulk Fe valences was to determine the amount of water that was consumed when the Fe was oxidized during aqueous alteration. The water (wt.%) consumed by the oxidation is given by W H 2 O ¼ W Fe ðV f
V i Þ  0:5  18=55:85

ð3Þ

where WFe is the bulk Fe content (wt.%), and Vf and Vi are the current and initial Fe valences. The fractional amounts

Fig. 6. Plot of Fe valence vs. bulk H content for CM meteorites in this study. The bulk H content is a proxy for the degree of alteration. The apparent inverse correlation between Fe valence and H content is at present a mystery. Finds are red squares, falls are open circles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of water consumed needed to calculate the range of changes in H isotopic compositions in the next section can then be calculated by combining WH2O with literature estimates of the current water contents (see Table 7). The current valences have been determined here (Table 6), and the bulk Fe contents of the chondrites are well established (Table 7). Unfortunately, there are no unaltered CIs, CMs, CRs or OCs with which to determine prealteration valences, and therefore the initial bulk Fe valences must be estimated. The less altered CRs (e.g., QUE 99177 and Renazzo) are more metal-rich than highly altered CRs (e.g., GRO 95577) and must be closer in composition to the pre-alteration material, although there has been some alteration of matrix even in these meteorites (Le Guillou et al., 2014; Le Guillou and Brearley, 2014). For the CRs, the upper limit for the initial bulk Fe valence is given by Renazzo (1.47), but it has seen alteration of its matrix and some chondrules (Weisberg et al., 1993; Le Guillou et al., 2014, 2015). In the absence of unaltered meteorites, we must estimate the initial valence states. One way that this can be done is to use the long recognized (e.g., Larimer and Anders, 1970) correlations between bulk Fe/(Mg,Si) and Ni/(Mg,Si) ratios (Fig. 7). These correlations suggest that all chondrites experienced fractionation of metal with similar compositions (Ni/Fe (wt.)  0.0638) from silicates (Mg/Mg + Fe (at.)  0.93). Sulfur does not seem to have taken part in this fractionation. Being a highly volatile element, the S in chondrites was probably primarily accreted in matrix, and is now mostly in sulfide that presumably formed by sulfidation of metal. The formation of sulfides, either in the nebula or in the parent bodies, will have increased the bulk Fe valence and reduced the amount of water that must have been con-

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125

Table 7 The bulk Fe, Ni and S contents, the range of estimated initial bulk Fe valences, the estimated current bulk water contents and range of H isotopic compositions, and the range of calculated water fractions consumed by oxidation during aqueous alteration, and the range of initial H isotopic compositions depending on whether the water was lost by reaction with Fe at 0 °C or 200 °C. CI Orgueil CM ALH 84042 ALH 85013 Banten Bells W DNG 06004 GRO 95566 LEW 85312 LEW 87022 LEW 87148 LEW 90500 Murchison Nogoya II QUE 99355 SCO 06043 II CR GRO 95577 Renazzo QUE 99177 OC Semarkona a b c

Fe (wt.%)a

Ni (wt.%)a

S (wt.%)a

Init. Val.

H2Ocurr (wt.%)b

dDcurr (‰)c

FH2O lost

dDinit (‰)

18.2

1.04

5.9

1.34/2.00

12.1


587/
373

0.15/0.26


672/
422

21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0

1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20

3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3

0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27 0.76/1.27

11.3 10.1 8.2 6.8 7.8 8.9 7.5 9.8 10.5 8.9 8.6 11.8 9.5 11.4


467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421
467/
421

0.26/0.35 0.29/0.39 0.32/0.42 0.38/0.48 0.34/0.44 0.32/0.42 0.37/0.47 0.28/0.38 0.28/0.37 0.30/0.40 0.32/0.42 0.23/0.33 0.29/0.39 0.24/0.34


615/
502
632/
513
647/
523
676/
543
655/
527
645/
522
669/
540
629/
509
626/
509
637/
514
646/
523
606/
493
633/
511
609/
495

24.0 24.0 24.0

1.36 1.36 1.36

1.3 1.3 1.3

0.41/0.65 0.41/0.65 0.41/0.65

11.0 3.3 3.5

34/208 34/208 34/208

0.39/0.44 0.43/0.58 0.53/0.62


328/
56
467/
86
498/
169

18.5

1.02

2.3

0.71/1.20

0.9

799/1210

0.73/0.83


527/154

The elemental compositions come from the compilation in Table 2.1 of Hutchison (2004). The estimated water contents are from Alexander et al. (2013). The estimated water H isotopic compositions are from Alexander et al. (2012).

sumed to produce the current valence states of the chondrites. To calculate the initial bulk Fe valence, we first assume a metal Ni/Fe (wt.) = 0.0638 during metal-silicate fractionation and use the bulk Ni contents of the chondrites to calculate how much Fe in metal (Feval = 0) they retained and accreted. The Fe not in the metal is assumed to be FeO (Feval = 2). We then assume that all the bulk S in the chondrites reacted with the metal to form FeS (Feval = 2). Labidi et al. (2017) determined the S speciation in 13 CMs, including several studied here. They reported that elemental S made up  10% of the total S that they extracted from CMs, with a further 25% in sulfate that may have formed by oxidation of elemental S in the CM parent body. The majority of the S (65%) was in sulfide. The total S yields reported by Labidi et al. (2017) were about two thirds of those reported in the literature for bulk CMs, so the true abundances of the different S species in the CMs is still somewhat uncertain. Also, no similar studies have been conducted on the other meteorites studied here. Until there is more comprehensive data on S speciation in chondrites, assigning all S to sulfide would seem to be the most conservative approach. It is not clear when this metal-silicate fractionation occurred, although the metal and silicate compositions are reasonably close to those of chondrules (except for enstatite chondrites). Nor can we rule out that there was modification of metal and silicates in the nebula after the fractionation. There is no indication in the CI, CM, CR and LL chondrites studied here that conditions became

much more reducing after metal-silicate fractionation. Hence, the initial valence states calculated above are probably minimum estimates, and are listed as such in Table 7. All the chondrites are now much more oxidized than our minimum estimates of the initial valence states, and petrologic evidence indicates that at least some of the oxidation took place in the parent bodies. However, it is possible that there was some oxidation prior to accretion and we need to try to constrain the likely upper limits for the initial Fe valence states. There is an independent means for estimating Semarkona’s Fe valence at the time of accretion that is afforded by the close relationship between the ordinary chondrite groups. While even the metal-rich H chondrites saw some parent body aqueous activity (Grossman et al., 2000), the Fe in the H chondrites is much less oxidized than in Semarkona. The S contents of the ordinary chondrites are all very similar. Assuming that the sulfidation of the ordinary chondrite metal was a late-stage process, combining the metal and Fe in sulfide gives an average Ni/Fe (wt.) = 0.089 for the H chondrites analyzed by Jarosewich (1990). If this was the Ni/Fe ratio of the metal Semarkona accreted, the initial metal content can be calculated from the bulk Ni content of Semarkona. With subsequent sulfidation of this metal, the bulk Fe valence is 1.20, a nearly 70% increase over the minimum initial valence estimated above, and we consider it to be the upper limit for the initial Fe valence for Semarkona. Using a hand magnet, Kong et al. (1999) generated metal and nonmagnetic separates from three Antarctic

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Fig. 7. The correlations between (a) Ni/Si vs. Fe/Si and (b) Ni/Mg vs. Fe/Mg for bulk chondrites. The correlations with non-zero xaxis intercepts indicate that all chondrites experienced metalsilicate fractionation with similar bulk metal (Ni/Fe (wt.)  0.0638) and silicate (Mg/(Mg + Fe) (at.)  0.93) compositions. Data are from the compilation of Hutchison (2004) (see Table 2.1).

CRs (EET 92042, PCA 91082 and Y793495) that they then measured for their major and trace element compositions. On average, the amount of bulk Fe in the separated metal is 14.3 ± 3.0 wt.% (Kong et al., 1999). The ‘nonmagnetic’ material may contain metal that is too fine grained to be extracted and/or has been altered. Using the levels of Os, Ir, Co and Ni in the  84.7 wt.% of ‘nonmagnetic’ material to gauge its pre-alteration metal content, a further 6.6 ± 0.5 wt.% Fe is or was originally present as metal in the bulk meteorites. This leaves 3.1 wt.% Fe in FeO, assuming a bulk Fe content of 24.0 wt.% (Table 7) and that no Fe3+ was present prior to accretion. Converting 2.3 wt.% Fe in metal to FeS (to account for the bulk S) results in a bulk Fe valence of 0.45, which is very similar to our minimum estimate. Taking into account all the upper uncertainty limits, the initial Fe valence estimated in this way could have been as high as 0.65, which is 60% higher than our minimum estimate and we take it as our upper limit. Unfortunately, it is not possible to make similar estimates for the CM and CI chondrites. However, the least

altered CM, Paris, contains significant amounts of metal in its least altered lithologies (Hewins et al., 2014). If, as we suggest, chondritic porous interplanetary dust particles (CP-IDPs) or comet Wild 2 dust are good analogs for the pre-alteration CI material, then they also originally contained significant metal. Metal is less abundant in CPIDPs than Wild 2 dust (Ogliore et al., 2010), but this could reflect a bias amongst CP-IDPs towards less dense (metalpoor) particles because they survive atmospheric entry better as well as oxidation during and/or after atmospheric entry. The average valences for CP-IDPs and Wild 2 dust measured by Ogliore et al. (2010) are 1.98 and 1.52, respectively. In the absence of other constraints, we take the upper limit for the valence of the pre-alteration CI material to be 2. Given the lack of quantitative constraints for the CM chondrites, to estimate the upper limits on their initial Fe valences we have assumed that after apportioning all S to FeS 50% of the remaining Fe is FeO (Table 7). This is somewhat arbitrary, but it increases the initial Fe valence for the CMs, relative to our minimum estimates, by about the same amount (67%) as for Semarkona and the CRs. Using the ranges of initial Fe valence values estimated above (see also Table 7), the best estimates for the current Fe valences and their uncertainties (weighted mean valence, Table 6) and the bulk Fe contents (Table 7), we can estimate the likely maximum and minimum amounts of water consumed by oxidation. Given the current water contents of the meteorites estimated by Alexander et al. (2013) (Table 7), then 15–26% of the initial water in Orgueil was consumed in the oxidation of Fe, and 73–83% was consumed in Semarkona (Table 7). Similar estimates of water consumption for the CMs and CRs vary considerably (23–48% and 39–62%, respectively). For the CRs, the highest estimated consumption fractions are for QUE 99177, but as discussed earlier its measured bulk Fe valence may have been affected by Antarctic weathering of its metal. Thus, for the CRs we favor a somewhat more restricted range of water consumption fractions of 0.39–0.58. The range of water consumption fractions in the CMs and CRs reflect at least in part the different degrees of alteration experienced by their different members. Alexander et al. (2012) argued that in the CM and CR chondrites Fe oxidation largely occurred in open hydrological systems before the swelling associated with the alteration of silicates, compaction and loss of H2 caused the permeability to decrease dramatically. The observed water/rock ratios would have been established as the systems became closed. If correct, then by including samples that have seen more and less alteration than for the bulk parent body we have overestimated the range of likely water consumption fractions. Unfortunately, we do not know what the average degree of alteration (or water/rock ratio) was in the CM and CR parent bodies. However, it seems likely that the most altered members of the two groups analyzed (the CM Nogoya and the CR GRO 95577) provide minimum estimates of the water fractions consumed, which are for the CM Nogoya, 23–33%, and for the CR GRO 95577, 39– 44%. There are, of course, a number of uncertainties in these estimates that are hard to assess at present. The pre- and

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post-alteration oxidation states of S are particularly important. We have simply assumed that all S is in troilite, but CMs, at least, seem to have accreted some of their S in its elemental form (Labidi et al., 2017). All of the above estimates of water consumption would be lower limits if there was significant oxidation of S, Ni and/or organics during alteration, but their present oxidation states are not well known. It is unclear whether the CIs contained indigenous sulfate as there has clearly been at the very least some terrestrial remobilization (Gounelle and Zolensky, 2001), but CMs do appear to contain some pre-atmospheric sulfate (Labidi et al., 2017). To illustrate how important the oxidation state of S could be, if all the S in CI chondrites was in sulfate prior to atmospheric entry the maximum likely fraction of water consumed would increase from 26% to almost 60%. There have been no reports of sulfate in CRs and Semarkona that we are aware of. There is little evidence for oxidation of organics to CO, CO2, etc. in the CI, CM or CR chondrites. However, the IOM in Semarkona has clearly seen higher temperatures than these other meteorites (Busemann et al., 2007; Cody et al., 2008; Quirico et al., 2009) and, therefore, some oxidation of its organics cannot be ruled out. Oxidation of P, Cr and Si that was originally in metal almost certainly occurred during alteration, as did the formation of carbonate, but their relatively low abundances mean that they will not have had a very significant influence on the amounts of water consumed during alteration. 4.2. Initial H isotopic compositions of chondritic water The consumption of such large water fractions could potentially have significantly increased the D/H ratio of the remaining water and, therefore, the ratios of the bulk meteorites. The likely ranges of changes in H isotopic compositions can be estimated by assuming equilibrium H2OH2 H isotopic fractionation when H2 is generated by the oxidation of Fe, and Rayleigh loss of the H2 after its formation (Alexander et al., 2010). In this case, the change in the D/H ratio of the water, R, is given by R=R0 ¼ f a
1 1

127

of the water will change as a function temperature and fraction of water remaining is illustrated in Fig. 8. Based on these assumptions, including a potential temperature range of 0–200 °C, along with the range of water fractions consumed and the range of estimated current water dD values (see Table 7), we have calculated the likely maximum and minimum pre-alteration water dD values for the various chondrites (Table 7). The ranges of initial water dD values are all significantly lighter than estimated by Alexander et al. (2012). This is particularly true for Semarkona whose water H isotopic composition decreases from a range of 800–1200 ‰ to between -527 ‰ and 154 ‰. This brings the initial H isotopic composition of Semarkona water closer to the ranges for the CI and CM chondrites, especially given that the higher initial metal contents and, therefore, consumption of larger water fractions for Semarkona seem more likely. The same is true for the CR chondrites. Finally, it should be pointed out that an implicit assumption in the above estimates has been that all the water originally present reacted during alteration, either by oxidizing the Fe or by forming hydrous silicates. However, some primitive asteroids clearly retain some water ice (Campins et al., 2010; Rivkin and Emery, 2010; Jewitt, 2012). There is little evidence that the chondrites retained unreacted water, but if there was any it would almost certainly have sublimed to space prior to atmospheric entry. If some water did remain unreacted in any of the chondrites, we will have overestimated the change in the water H isotopic compositions associated with oxidation of Fe. Of the various chondrite groups, the possibility that they retained some unreacted water seems most likely for the CIs. They have experienced essentially complete alteration and it is unlikely that they accreted exactly the right amount of water to achieve this. Thus, for Orgueil, at least, we have probably overestimated the change in its water H isotopic composition.

ð4Þ

where R0 is the initial D/H ratio, f is the fraction of water remaining, and a is the equilibrium H2O-H2 fractionation factor. Here we have adopted the equilibrium H2O-H2 H isotopic fractionation factor of Suess (1949), which is 1000lna ¼ 1000lnðRH 2 O =RH 2 Þ ¼

467; 600
303:9 T

ð5Þ

with the temperature T being in Kelvin. Temperatures between 0 °C and 200 °C cover most of the range of alteration temperatures that have been estimated for Semarkona and the CI, CM and CR chondrites (Brearley, 2006). The fractionation factor given by Equation (4) is for water vapor. However, for simplicity and because the liquidvapor isotopic fractionation factors are relatively small (<10% of the values given by Eq. (3): Horita and Wesolowski, 1994), we have not converted the fractionation factors to those for liquid water even when temperatures are below 100 °C. The predictions for how the D/H ratio

Fig. 8. The estimated change in H isotopic composition of the residual water as a function of the fraction of water remaining by Fe oxidation and the temperature of oxidation. See the text for details about how the changes in isotopic composition were calculated.

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4.3. Implications Two dynamical models of giant planet migration, the Grand Tack (Walsh et al., 2011) and Nice (Levison et al., 2009) models, predict that objects from the outer Solar System would have been implanted into the Asteroid Belt, primarily in the outer part of belt where the parent bodies of the carbonaceous chondrites are thought to dominate. The Grand Tack model would have taken place while the gas disk was still present and would have implanted objects into the Asteroid Belt that formed beyond the orbit of Jupiter (5 AU) and perhaps as far out as 15 AU from the Sun. The Nice model would have taken place some 500– 600 Ma later, and would have scattered a smaller number of objects into the Asteroid Belt that formed in the comet-forming regions (20 AU from the Sun: Brasser and Morbidelli, 2013). The D/H ratio of water in the solar nebula is expected to have increased with increasing radial distance from the Sun (e.g., Jacquet and Robert, 2013; Yang et al., 2013) due to radial mixing of D-rich interstellar water in the outer reaches of the disk with D-poor water produced by isotopic equilibration with D-poor solar H2 in the warm inner disk. The existence of such a radial gradient in the nebula is consistent with the fact that the water in comets, as well as Saturn’s Moon Enceladus, tend to be quite D-rich relative to terrestrial and solar H isotopic compositions (Fig. 9). The H isotopic compositions of chondritic water potentially provide a means of testing the predictions of the dynamical models. There has long been speculation that chondrites, particularly carbonaceous chondrites, may have cometary origins ¨ pik, 1968; Campins and Swindle, 1998; Brown et al., (e.g., O 2013). Recently, for instance, estimates of Orgueil’s orbit

Fig. 9. Comparison of the H isotopic compositions of water in Oort Cloud and Jupiter Family (JFC) comets, Saturn’s moon Enceladus, the estimated ranges for the accreted water in CI, CM, CR and LL chondrites, as well as the measured composition of water in phyllosilicates in the R chondrites (RC). Sources of data: Solar - (Geiss and Gloeckler, 1998); Earth - (Le´cuyer et al., 1998); Comets - (Eberhardt et al., 1995; Bockele´e-Morvan et al., 1998; Meier et al., 1998; Hutseme´kers et al., 2008; Villanueva et al., 2009; Brown et al., 2012); Enceladus - (Waite Jr et al., 2009); R chondrites - (McCanta et al., 2008).

showed that it was consistent with a cometary origin (Gounelle et al., 2006), and the CMs Maribo and Sutter’s Mill were found to have similar orbits to comet 2P/Enke (Tubiana et al., 2015). However, the H isotopic compositions that we estimate for the water accreted by CI and CM chondrites are quite distinct from the compositions of water in any measured comets (Fig. 9). Thus, it is very unlikely that the CIs and CMs come from comets or from bodies that formed in the same regions as comets and were scattered into the Asteroid Belt early in Solar System history. Alexander et al. (2012) came to a similar conclusion for these meteorites groups as well as for Tagish Lake. Tagish Lake has been spectroscopically linked to the D-type asteroids (Hiroi et al., 2001) that Levison et al. (2009) considered amongst the most likely candidates for implantation by the Nice model. The estimated initial isotopic range for CR chondrite water does overlap with those of the least deuterated comets. However, a cometary link for the CRs becomes less likely if, as argued above, the initial CR water H isotopic compositions most likely fell toward the lighter end of the range estimated here. The present D-rich compositions of water in Semarkona (Deloule and Robert, 1995; Deloule et al., 1998; Grossman et al., 2002; Alexander et al., 2012; Piani et al., 2015) and the R chondrites (McCanta et al., 2008) are also consistent with a cometary origin. Indeed, it has been suggested that, contrary to all models, these chondrites accreted more interstellar water than the carbonaceous chondrites (Deloule and Robert, 1995; Deloule et al., 1998; Piani et al., 2015), although there is no evidence that they accreted higher abundances of any other presolar materials in their matrices (Huss and Lewis, 1995; Davidson et al., 2014). The more mundane alternative that this paper set out to test is that all the chondrites accreted water with similar dD values and that the D-rich compositions are the result of parent body processes, first as a result of the well documented oxidation of Fe that took place during alteration (e.g., Hutchison et al., 1987; Alexander et al., 1989; Weisberg et al., 1993; Noguchi, 1995; Krot et al., 1997; Hewins et al., 2014) and later through exchange with D-rich organics (Alexander et al., 2010, 2012; Bonal et al., 2013). Our measurements of bulk Fe valence states and estimates of the extent of H isotopic fractionation associated with the oxidation of Fe show that at the very least the oxidation would have led to significant D enrichment of the residual water. If, as we have argued, the pre-alteration water H isotopic compositions in the CRs and Semarkona were most likely to be near the lighter ends of the estimated isotopic ranges, then the water compositions were similar in all the chondrites measured here. While it seems unlikely that the parent bodies of any of the chondrites were scattered into the Asteroid Belt from the comet-forming regions of the disk, whether the carbonaceous chondrites formed between 5 AU and 15 AU as the Grand Tack predicts is less clear. Assuming that Enceladus formed in Saturn’s subdisk towards the end of Saturn’s growth, then in the context of the Grand Tack model Enceladus would have formed between 3 AU and 7 AU from the Sun. Since the estimates of the D/H of water in the chondrites are much less than for Enceladus (Fig. 9),

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Alexander et al. (2012) concluded that these chondrites probably formed no furthur from the Sun than 3–7 AU. However, it is possible that Enceladus formed from the debris of a scattered trans-Neptunian object that broke up when it was captured by Saturn (Charnoz et al., 2011). In this case, Enceladus’s isotopic composition can no longer be used as a constraint on the radial gradient of the D/H of nebular water. Saturn’s largest moon Titan could not have formed in this way, and Alexander (2017) used the N isotopic composition of Titan’s atmosphere to also argue that the carbonaceous chondrites formed 3-7 AU from the Sun if there was a Grand Tack. In the Grand Tack, the ordinary chondrites are inner Solar System in origin. If, as seems plausible, Semarkona had an initial water isotopic composition that was similar to that of the carbonaceous chondrites, this would suggest that either the D/H gradient in nebular water was very flat out to 7 AU or the carbonaceous chondrites also formed in the inner Solar System. There is an additional complication that needs to be considered and that is that the CI-CM-CR chondrites probably formed 1–2 Ma after the ordinary chondrites (e.g., Fujiya et al., 2013; Sugiura and Fujiya, 2014; Schrader et al., 2016). The temperature gradient in the disk will have been largely controlled by the accretion rate of material onto the Sun, which will have decreased dramatically with time. In this case, the region where temperatures were high enough to enable H2O-H2 isotopic equilibration in the nebula will have moved sunward between the times of ordinary chondrite and CI-CM-CR accretion. Consequently, it seems likely that water D/H ratios will have risen with time at a given distance from the Sun, strengthening the case for carbonaceous chondrites not forming much beyond the formation locations of the ordinary chondrites. 5. SUMMARY AND CONCLUSIONS There is abundant petrologic evidence for the oxidation of Fe during the aqueous alteration of chondrites, and water must have been the oxidant for this process. Oxidation of Fe by water would have generated considerable amounts of H2 that must have escaped to space to prevent disruption of the chondrite parent bodies. By analogy with H2 generated by low temperature serpentinization on Earth, the H2 lost from the chondrite parent bodies would have been isotopically very light. Therefore, the water left behind would have become more D-rich, with the extent of D enrichment depending on the fraction of water consumed and the temperature. Here we report our attempts to estimate how much water was consumed during alteration in the CI, CM, CR and LL parent bodies, and how much this may have changed the isotopic compositions of the residual water that is now preserved in the hydrated silicates of these meteorites. Our first step was to use Fe XANES of bulk meteorite powders and standards to determine the bulk Fe valences of Orgueil (CI1), a number of CMs and CRs that experienced varying degrees of alteration, and Semarkona (LL3.00). Several linear combination strategies were used to fit the spectra of the standards to the bulk meteorite spectra and the pre-edge peak method was also applied.

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Bulk Fe valences were reasonably consistent for the various analysis strategies. The total ranges of bulk Fe valences we obtained were: Orgueil 2.77, CMs 2.40–2.63, CRs 1.46– 2.54, and Semarkona 2.10. Combining previous estimates of the present water/OH contents of our samples with the present bulk Fe valences and a range of assumed initial bulk Fe valences, the estimated ranges of water fractions consumed by Fe oxidation are: Orgueil 15–26% (up to 59% if all S is in indigenous sulfate), Semarkona 73–83%, CMs 23–48%, and CRs 39–62%. The ranges in estimates for the CMs and CRs reflect to a significant degree the variations in the degrees of alteration and, therefore, their water/OH contents. The changes in the H isotopic compositions of the water that would have been associated with these levels of water consumption were estimated assuming the equilibrium H2-H2O isotopic fractionation factor, Rayleigh fractionation of the H2 produced by the oxidation of Fe, and oxidation temperatures of 0-200 °C. Using previous estimates of the water H isotopic compositions in the chondrites, the estimated dD values for the chondritic waters prior to Fe oxidation are: Orgueil -672 ‰ to -422 ‰, CMs -669 ‰ to -493 ‰, CRs -498 ‰ to -56 ‰, and Semarkona -527 ‰ to 154 ‰. Since oxidation of Fe by water can occur at low temperatures (producing larger isotopic fractionations) and there are reasons to think that our upper estimates for water consumption by oxidation in Semarkona and the CRs are more likely, it appears that the CI, CM, CR and LL chondrites all accreted water with fairly similar H isotopic compositions. While the uncertainties in the estimated pre-alteration water H isotopic compositions are large, these isotopic compositions are significantly depleted in D relative to comets. Thus, the carbonaceous chondrites are unlikely to come from comets or from bodies that were scattered into the Asteroid Belt from comet forming regions by orbital migration of the giant planets. If the carbonaceous chondrites did form in the outer Solar System, as some models predict, it was probably not beyond 7 AU and the gradient in water H isotopic compositions must have been very flat. However, based on water isotopic compositions at present it is equally plausible that the carbonaceous chondrites formed in the inner Solar System. ACKNOWLEDGEMENTS We thank Mike Zolensky (NASA-JSC) for producing and providing the Maribo FIB sections. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy- GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. EAC thanks the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Space Agency (CSA), the Canada Foundation for Innovation (CFI), the Manitoba Research Innovations Fund (MRIF), and the University of Winnipeg for funding operation of the Planetary Spectrophotometer Facility (PSF).

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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). 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. We thank A. Westphal (University of California – Berkeley) for making spectra from the Berkeley XANES Library available. Three anonymous reviewers helped significantly improve this manuscript.

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