Speciation of sulfur in the insoluble organic matter from carbonaceous chondrites by XANES spectroscopy

Earth and Planetary Science Letters 300 (2010) 321–328

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Speciation of sulfur in the insoluble organic matter from carbonaceous chondrites by XANES spectroscopy F.-R. Orthous-Daunay a,⁎, E. Quirico a, L. Lemelle b,c, P. Beck a, V. deAndrade d, A. Simionovici e, S. Derenne f a

Laboratoire de Planétologie de Grenoble, Université Joseph Fourier - CNRS, BP 53, Bâtiment D de Physique, 38041 Grenoble Cedex 9, France Laboratoire de Sciences de la Terre, Université de Lyon, Ecole Normale Supérieure de Lyon, 46 allée d'Italie, France CNRS, UMR5570, USR3010, France d ID21 Beamline, European Synchrotron Radiation Facility (ESRF), 6 rue J. Horowitz, BP220, 38043 Grenoble Cedex, France e Laboratoire de Géologie des Chaînes Alpines, Université Joseph Fourier - CNRS, Maison des Géosciences, 1381 rue de la Piscine, 38400 Saint Martin d'Hères, France f UMR CNRS 7618 BioEMCo Université Pierre et Marie Curie 4, place Jussieu 75252 Paris Cedex 05, France b c

a r t i c l e

i n f o

Article history: Received 3 June 2010 Received in revised form 7 October 2010 Accepted 8 October 2010 Available online 17 November 2010 Editor: T. Spohn Keywords: insoluble organic matter S K-edge XANES aqueous alteration carbonaceous chondrites oxidation sulfur speciation

a b s t r a c t Sulfur speciation in a comprehensive set of insoluble organic matter (IOM) samples extracted from 3 CI (Orgueil, Alais, Ivuna), 5 CM (QUE97990, Murchison, Murray, QUE99355, Cold Bokkeveld), 1 CR (Renazzo) and ungrouped C2 Tagish Lake chondrites has been studied by K-edge XANES micro-spectroscopy. Five main sulfur groups were identified: (1) sulfides, (2) aliphatic sulfur, (3) heterocyclic organic sulfur, (4) oxidized organic sulfur and (5) sulfates. The IOM of the 3 CIs and the extensively altered Cold Bokkeveld CM exhibit a higher abundance of heterocyclic versus aliphatic organic sulfur compared to the other CMs and compared to the Renazzo and Tagish Lake chondrites. This suggests greater thermal heating on the parent body of CIs, consistent with the higher temperatures experienced by these chondrites (~ 70 °C for the most altered clasts of Cold Bokkeveld; 100–150 °C for CIs). Alais may have experienced more heating than Ivuna and Orgueil. The IOM of CIs contains oxidized organic sulfur, suggesting the presence of a mild-temperature oxidation process on the parent body. Among type 2 chondrites other than Cold Bokkeveld, no significant variation of the sulfur speciation was detected. This suggests tenuous oxidation processes and a low-temperature aqueous alteration (b ~35 °C) on the parent body. The global chemical variations between the CR and CM groups (e.g. H/C elemental ratio, alkyl groups concentration) reported in earlier studies appear to be more the result of chemical variations among the accreted precursors than of post-accretional processes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The insoluble organic matter (IOM) in carbonaceous chondrites is a macromolecular polyaromatic material. The astrophysical context and the physical routes of its formation (interstellar medium or protoplanetary disk) are still a matter of debate (Alexander et al., 2007; Remusat et al., 2006, 2010). Another matter of debate lies in the interpretation of the large chemical, isotopic and structural variations of IOM that have been reported among and within chondrites of various chemical classes (Alexander et al., 1998, 2007). These variations could be accounted for by heterogeneities in the polyaromatic material present in the protoplanetary disk (Quirico et al., 2009), but they could also be the result of post-accretion processes. Such processes include thermal metamorphism observed in type 3 or higher chondrites (Alexander et al., 2007; Bonal et al., 2006, 2007; Busemann et al., 2007; Quirico et al., 2003, 2009), poorly defined heating events as observed in metamorphosed CMs (MCMs) (Yabuta

⁎ Corresponding author. E-mail address: [email protected] (F.-R. Orthous-Daunay). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.10.012

et al., in press), impact induced metamorphism (shock) and aqueous alteration (Alexander et al., 2007; Cody and Alexander, 2005; Yabuta et al., 2005). While the effect of thermal metamorphism on the composition and structure of IOM is unambiguous, the potential impact of aqueous alteration is open to debate. Cody and Alexander suggest that IOM undergoes a low-temperature oxidation process, resulting in the gradual disappearance of the CH2/CH3 alkyl functional groups (Cody and Alexander, 2005). As already mentioned, variations in the composition of insoluble organic matter could also be accounted for by variations in the composition of the precursors accreted. Chemical heterogeneity of the organic precursors is possible given the miscorrelation of the H/C elemental composition of IOM with its maturation grade in type 3 chondrites (Quirico et al., 2009). There is also some evidence that CIs and type 2 chondrites experienced a range of temperatures over 20–150 °C (e.g. Bullock et al., 2005; Guo and Eiler, 2007; Zolensky et al., 1989) and such mild heating combined with fluid circulation could lead to processes other than low-temperature oxidation. In this paper, sulfur speciation is studied to characterize the extent of hydrothermalism processes on IOM extracted from chondrites that were not subject to extensive thermal metamorphism (petrologic

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types 1 and 2). Sulfur is a minor element of particular interest because its oxidation state in terrestrial samples provides a precious indication of the nature and extent of both alteration and heating processes. In the case of bitumens, weathering mostly results in the disappearance of aliphatic sulfur and the formation of oxidized sulfur like sulfones or sulfonic acids (Sarret et al., 1999). In kerogens, catagenesis heating leads to an increase of aromatic sulfur (Kelemen et al., 2007). Organic sulfur in chondrites has received little attention. Cooper et al. (1997) reported detection of sulfonic acids in the soluble fraction of Murchison. Remusat et al. (2005) detected SO2 and H2S as the more abundant gases from the pyrolysis of Orgueil and Murchison, respectively. The former was interpreted as the combustion product of highly oxidized sulfurs (e.g. sulfates) and the latter as the result of the combustion of reduced organic sulfur. These results were consistent with a S-XANES study focused on Orgueil and Murchison and which suggested that Orgueil contains a higher aromatic/aliphatic organic sulfur ratio than Murchison (Remusat et al., 2005). In this paper, we present a study of sulfur chemical speciation on a comprehensive series of IOM extracted from samples of 3 CI (Orgueil, Ivuna, Alais), 5 CM (QUE97990, Murchison, Murray, QUE99355, Cold Bokkeveld), 1 CR (Renazzo) and ungrouped C2 Tagish Lake chondrites. Sulfur speciation determined from K-edge XANES measurements is confronted with the nature and extent of postaccretion processes that affected these chondrites, provided by both petrology and mineralogy (Bullock et al., 2005; Endress and Bischoff, 1996; Rubin et al., 2007), and physical and chemical parameters derived from measurements or models (Benedix et al., 2003; Clayton and Mayeda, 1999; Guo and Eiler, 2007; Zolensky et al., 1989).

The demineralization protocol we used is similar to the procedure described in Gardinier et al. (2000). It consists first in expelling soluble low-weight molecules, then removing carbonates by HCl attack, silicates by HF/HCl attack and finally encaged soluble molecules. Each acid attack was followed by rinsing with distilled ultrapure water and neutralization (Fig. 1). A set-up specially designed and built for extracting IOM from very small amounts of sample (b1 mg) was used (Fig. 2). Small matrix fragments were separated from the bulk chondrite under a binocular and deposited on a Teflon grid with a 0.5 × 0.5 μm2 mesh. The grid was placed in a reaction chamber, where liquids were simultaneously injected and expelled by mean of a peristaltic pump (typical flow ~ 90 μl s− 1). A first tank contained the liquids necessary for the operation while a second tank was used for trash. Both tanks and the reaction chamber were maintained in an argon atmosphere during the whole demineralization process (~8 hours). The IOM was finally recovered from the grid and prepared for XANES measurements. Some IOM grains were deposited in the holes of an Au grid located on a 4-μm thick Ultralene foil and then covered by another similar Ultralene foil. This sample was centered in the hole of a Poly Ether Ether Ketone (PEEK) sample holder. Ultralene and PEEK are industrial polymers free of sulfur traces, ensuring contamination-free samples. The sample was positioned at an angle of 60° with respect to the incoming beam and 30° with respect to the fluorescence detector axis. Each sample was pre-aligned using a video-microscope located inside the X-ray microscope chamber and the mechanical stage.

2. Experimental

2.3. X-ray microscope SXM at ID21 - ESRF

2.1. Samples

XANES spectra were acquired on the ID21 scanning X-ray microscope (SXM) of the European Synchrotron Radiation Facility (ESRF) in Grenoble (Lemelle et al., 2008; Susini et al., 2002). The spectral resolution of this microscope is 0.25 eV and the spatial resolution reaches 0.5 μm. In this study, the X-ray beam was unfocused and a 50 μm pinhole was used. The pinhole size was selected in order to maximize the probed area for our IOM grains. Therefore, only one final spectrum is obtained for each meteorite that corresponds to an average of 10 spectra obtained at the same location and measured with a 2 s (10 × 0.2 s) dwell time. The microscope was operated close to the sulfur K-edge at ~ 2472 eV and the energy was scanned between 2450 eV and 2510 eV in 0.15 eV energy steps. For

Chondrites were selected from different chemical groups: CR2 [Renazzo], CM [Queen Alexandra Range (QUE) 97990, Murchison, Murray, Queen Alexandra Range (QUE) 99355, Cold Bokkeveld], CI (Orgueil, Ivuna, Alais) and the ungrouped C2 Tagish Lake (Table 1). With the exception of two Antarctic chondrites, the studied samples came from falls and did not suffer terrestrial weathering. The aqueous alteration degrees of CM chondrites are taken from Rubin et al. (2007). CI chondrites have a petrologic type 1, with an increasing degree of aqueous alteration in the order Alais bb Ivuna b Orgueil (Endress and Bischoff, 1996). Renazzo is the least altered chondrite of the series. Tagish Lake is a complex brecciated object which cannot be simply related to CI or CM groups and which presents two main carbonate-rich and carbonate-poor lithologies (Nakamura et al., 2003; Zolensky et al., 2002).

2.2. IOM extraction

Table 1 Meteorite sample set. Name

Class

Petrologic type

Find/fall

Provider

Tagish Lake Renazzo QUE97990 Murchison Murray QUE99355 Cold Bokkeveld Alais Ivuna Orgueil

C2 CR CM CM CM CM CM CI CI CI

– 2 2.6 2.5 2.4/2.5 2.3 2.2 1 1 1

Fall Fall Find Fall Fall Find Fall Fall Fall Fall

Private MNHN JSC-NASA MNHN MNHN JSC-NASA MNHN MNHN MNHN MNHN

Private: private seller; MNHN: Museum National d'Histoire Naturelle, Paris-France; JSCNASA: Johnson Space Center, NASA, Houston — USA. Petrologic types provided by Rubin et al. (2007).

Fig. 1. The experimental set-up used to extract insoluble organic matter from small amounts of chondrites (b 1 mg). See text (Section 2.2) for details on the procedure.

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Gaussian function was maintained constant during the fitting procedure and only the peak intensity and the position of the arctangent edge were free parameters. The fits converged properly and the position of the arctangent edge was found to be identical for all IOM, consistent with the study of Prietzel et al. (2003) (Fig. 2). Therefore, the relative contributions of the different components could be derived to estimate the variations of the different S-bearing chemical groups within our series of IOM, after correction for the energy dependence of the white line area (Xia et al., 1998) (Fig. 4). A particular concern was to quantify the relative abundance of aliphatic versus heterocyclic reduced sulfur, however this is somewhat difficult because of the spectral proximity of the white line positions and the width of these sulfur groups. Sarret et al. (1999) concluded that such quantification on the sole basis of the analysis of K-edge spectra was highly uncertain. However, both forms appeared as distinct peaks in several IOM samples. Moreover, the recent study by Kelemen et al. (2007) derived this relative abundance by both K-edge XANES and Xray photoelectron spectroscopy and found a fair agreement. 4. Sulfur speciation in chondritic IOM Fig. 2. Spectral decomposition of XANES spectra with an arctangent function for the edge and 9 Gaussian curves for the white lines of sulfides, aliphatic disulfide and monosulfide, heterocyclic sulfur, sulfoxides, sulfites, sulfones, sulfonates and sulfates.

energy calibration, the X-ray microscope was used with a 200 μm pinhole focused on a CaSO4 standard. The X-ray detector used is this study is an energy-dispersive high purity Ge detector (Princeton Gamma-Tech) available on ID21 facilities. Energy values were further measured with respect to the position of the white line of the sulfur Kedge spectrum of CaSO4, which was assigned an energy of 2482.6 eV. Representative S compounds (Sigma Inc.) in various oxidation states were used as standards (Table 2). Sample stability under the X-ray beam was carefully checked by adjusting the incident beam intensity. The final XANES spectra were obtained by summing ten low integration time spectra (0.2 s) in order to monitor photoreduction kinetic, if any. The count rate was kept below 5000 counts s−1. Note that measurements on the whole series of chondrites were performed in fluorescence mode. 3. Analysis of S-XANES spectra Each XANES spectrum was corrected for the incident beam intensity. The region below the edge was fitted by a straight line which was subtracted from the spectrum in order to bring the baseline to zero. The spectrum was then normalized to 1 at the height of the edge jump (Fig. 2). The interpretation of the XANES spectra is a critical issue, which has been reviewed extensively by Sarret et al. (1999). Two main methods are proposed in the literature: (i) fitting by a linear combination of standard materials spectra (Kelemen et al., 2007; Sarret et al., 1999) and (ii) fitting with an arctangent curve and Gaussian features peaking at the white line energies of the main Sbearing moieties covering the whole range of oxidation (Prietzel et al., 2003; Xia et al., 1998). The first method requires the use of a large set of standards and assumes that the content of the different chemical forms of sulfur can be derived from a simple linear decomposition. The second method uses spectral profiles which are not necessarily realistic and does not strictly account for the variations of the edge position with the oxidation state. Its advantage is however to be selfconsistent and it does not depend on the nature and the extent of a set of standards. In this study, we employed the second method. We first located, within the whole set of data, the peaks which point to the main chemical forms of sulfur present in the samples (Table 2, Fig. 3). Each of these sulfur forms was thus represented by a Gaussian curve, while the edge was represented by an arctangent curve. The width of each

In the set of spectra we measured, individual peaks fall into four main energy ranges, corresponding to 5 major groups of compounds: 1) inorganic sulfides, 2) aliphatic organic sulfur, 3) heterocyclic sulfur, 4) organic oxidized functions and 5) inorganic sulfates. The low-energy band (2.4703 keV) is undoubtedly assigned to inorganic sulfides. The energy of this peak does not match exactly the energy of commonly used sulfide standards (e.g. pyrite). Nevertheless, its intermediate position between the -1 and -2 oxidation state reveals the presence of reduced sulfur within sulfides. Regarding the well described mineralogy of these chondrites, the sulfides are likely pentlandite or troilite. The aliphatic and heterocyclic sulfur range can be divided into three categories of S-bearing species by crescent oxidation degree: disulfides (+0.2 oxidation state), thioethers and thiols (+0.5) and heterocycles (+0.5 and +1). Because the energy positions of these compounds are quite close and hard to distinguish, we consider aliphatic sulfur and heterocyclic sulfur spectral components, with positions derived from the spectra of the Alais CI. Aliphatic sulfur thus refers to disulfides, thioethers and thiols, with respective oxidation number ranging from 0.2 to 0.5 and associated white lines peaking from 2.472 to 2.473 keV. Heterocycles are of two kinds: aromatic pentacycles from the thiophene family with one sulfur atom at an oxidation number of 1 and six atoms cycles from the thianthrene family where the two sulfur atoms break aromaticity. The thianthrene derivatives bear a couple of 0.5 oxidation number sulfur atoms with a white line position exceptionally higher in energy than in more oxidized atoms. Note that with the exception of thiols, which are side functional groups, all sulfur bonds are structural. That is to say, this energy range probes sulfur bonded inside the carbonaceous skeleton of the IOM macromolecules. The oxidized organic functions detected in our samples include sulfones and sulfonic acids. Sulfones are a sulfonyl group bearing functions where sulfur has an oxidation number of five. They are associated with the feature peaking at 2.4801 eV (Table 2). This functional group involves two oxygen atoms bonded to a single sulfur atom, located inside a carbon chain. Sulfonic acids are identified by a diagnostic feature peaking at 2.4816 eV (Table 2). They are terminating functions with an oxidation number equal to five, such as a sulfur atom bonded to three oxygen atoms. In this manuscript, we will deal with oxidized organic sulfur and will not attempt to quantify the respective abundance of sulfones and sulfonic acids. The most oxidized sulfur atoms in our dataset are found in sulfates. These species have white line positions in the highest energy range of the spectra (Table 2). The oxidation number of the sulfur atom is here six. Sulfates are salts and are easily dissolved in water. The fact that

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Table 2 Standard compounds and white line position in keV. Mineral or function

Group formula

Standard compound

Structural formula

White line peaks

S S2

Pyrrhotite Pyrite

Fe[0,8-1]S FeS2

2.4701 2.4718

b

Sulfides

2.4725

b

Elemental

S

Sulfur

S8

d

G. peaka

2.4694 2.4718

c

2.4725

c

e

2.4704

H2N HO Alkyl-disulfides

Cystine

HO

S

S

2.4727

O

d

2.4724

e

O

R-S-S-R′

2.4721

NH2 Aryl-disulfides

Diphenyl dissulfide

2.4724

S

d

2.4725

e

S O

Cysteine

Thiols

HO

SH NH2

R-SH

SH

O Glutathione

NH

HO

Thioethers

R-S-R′

Methionine

b

2.4731

d

2.4733

d

2.4733

e

2.4735

d

2.4735

e

2.4735

c

O

NH

OH

2.4729

NH2

O

O

O

2.4735

S

HO

CH3

NH2 S Aromatic rings

Dibenzothiophene

2.4743

S Cyclic-thiothers

Thianthrene

2.4742

d

2.4743

e

2.4767

b

2.4758

c

2.4760

2.4783

b

2.4787

c

2.4783

2.4801

b

2.4802

c

2.4801

2.4811

b

2.4810

b

2.4813

c

S O Sulfoxides

R-SO-R′

Dimethyl sulfoxide

S

H 3C Sulfites

SO3

Potassium sulfite

CH3

KSO3

H 2N Sulfones

R-SO2-R′

O

Methionine sulfone

O

OH Sulfonates

R-SO3

Sodium methane sulfonate

CH3

S

O

O-

O

S O

CH3 O

Cysteic acid

O

S

OH

O HO Sulfonic acids

NH2 OH

R-SO3H

O

O

Anthraquinone sulfonic acid

S

2.4816

O 2.4816

O

b

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Table 2 (continued) Mineral or function

Group formula

Standard compound

Thioesters

R-OSO3

Sodium dodecyl sulfate

Structural formula

2.4826

OO CH3

G. peaka

White line peaks

S

c

O

O 2.4830

Sulfates a b c d e

SO4

Sodium sulfate Magnesium sulfate Gypsum

NaSO4 MgSO4 CaSO4(H2O)2

2.4822 2.4824 2.4822

d

2.4828

e

d d

Position of the Gaussian curve associated with a chemical class. Spectra from the ID21 database. Mean value from Prietzel et al. (2003) and ref. therein. Measured during this study. Mean value from Lemelle et al. (2008) and ref. therein.

they are detected in our samples might be due to protective embedding of salt particles by macromolecules that resisted acid attacks and/or by oxidation of insoluble sulfides during storage. We also cannot exclude the presence of organic sulfates. Examination of the spectra of the IOM series reveals variability of sulfur speciation among the different meteorite groups (Fig. 3). The IOM extracted from CI chondrites contains highly oxidized organic functional groups, which do not appear in CM, Renazzo and Tagish Lake chondrites. This rules out a possible terrestrial weathering origin since all IOM was extracted and stored by similar processes. Sulfates are detected in CIs and Cold Bokkeveld and to a lesser extent in Murray. The intensity of the sulfide peaks is higher in CMs than in CIs, including Cold Bokkeveld. The latter meteorite thus contains both reduced and oxidized minerals. Another remarkable difference is the higher intensity of S heterocycle features in CIs than in CMs, with again the exception of Cold Bokkeveld. In CMs, this feature only appears as a shoulder of a broad band peaking at the energy position of aliphatic sulfur. Cold Bokkeveld is the only CM with well separated aliphatic and heterocyclic sulfur peaks. Some variations are also observed among CIs. Alais appears slightly different than Orgueil and Ivuna, since this meteorite has similar peak intensities for aliphatic and heterocyclic sulfur. We also note that the sulfide abundance is lower in Ivuna than in Orgueil. This result is consistent with the measurements of (Burgess et al., 1991), but appears not fully consistent with the classification scheme of degree of aqueous alteration (Alais bb Ivuna b Orgueil) proposed by Endress and Bischoff (1996), based on carbonate compositions. From our point of view, several factors may account for this lower abundance, for instance a lower sulfide content in the initial mineralogical assemblage. There is no significant evidence to question the reliability of the alteration sequence derived by Endress and Bischoff (1996). The fitting analysis confirms naked eyes observations and reveals more subtle differences (Figs. 5 and 6). Sulfides are less abundant in CI than in other chondrites, while S-heterocycles, sulfoxides and sulfones are more abundant (Figs. 5 and 6). The Renazzo CR contains less sulfates and more sulfides than the CM, CI and Tagish Lake chondrites, consistent with their less aqueously altered mineralogy.

5. Precursor heterogeneity and post-accretion processes 5.1. Thermal and oxidation processes in CIs and Cold Bokkeveld The studied CIs are the only chondrites within the studied series that contain oxidized organic sulfur. They also have, with Cold Bokkeveld, a higher content of heterocyclic organic sulfur with respect to aliphatic organic sulfur, than other CMs, Renazzo and Tagish Lake.

A first mechanism for the specific sulfur speciation in CIs and Cold Bokkeveld could be the temperature conditions on the parent body. Recent “Clumped-isotope” thermometry of carbonates determined parent body temperatures of 20–30 °C for Murchison (Guo and Eiler, 2007), consistent with earlier estimations (Benedix et al., 2003; Clayton and Mayeda, 1999; Zolensky et al., 1989). The same study determined parent body temperatures for Cold Bokkeveld within the same range for two splits, but a higher value of 71 °C for a third split. These measurements are consistent with the fact that Cold Bokkeveld is a brecciated CM with clasts that experienced different alteration histories. Unlike most CMs, its whole rock D/H ratio presents large heterogeneities, with low and high D/H ratios (Kerridge, 1985). Its mineralogical characteristics are also different than those of most CMs, for example the large Mg-rich serpentine fraction in the matrix (Howard et al., 2009) and the Fe speciation dissimilar to what is observed in other CMs (Beck et al., 2010). Furthermore, infrared spectroscopic measurements on its IOM point to significant differences with other CMs (Orthous-Daunay et al., 2010). The XANES spectrum at the S K-edge points to the presence of sulfides and sulfates and definitely supports the fact that Cold Bokkeveld is a breccia containing clasts with different natures and extents of postaccretion alteration (Bischoff, 1998; Metzler et al., 1992). Parent body temperature estimations for CIs are older and less accurate, but it is worth noting that CI temperatures reach higher values than CM temperatures in all studies, spanning the range 100–150 °C (Bullock et al., 2005; Clayton and Mayeda, 1999; Zolensky et al., 1989). Therefore, the high heterocyclic sulfur content observed in the more heated objects makes the action of a thermally activated cyclization process on the parent body plausible. In this respect, while a sole lowtemperature oxidation process cannot account for the higher abundance of heterocyclic sulfur in Alais, a higher heating on the parent body can. Finally, the lack of oxidized organic sulfur in Cold Bokkeveld suggests that oxidation under chondritic conditions is a “mild temperature” oxidation process (T N ~ 100 °C). CIs are described as volatile-rich chondrites, compared to CMs which are described as objects with intermediate volatile abundance (Eiler and Kitchen, 2004). There is strong evidence that CIs experienced more intense aqueous alteration events than CMs. The high content of smectites and ferrihydrite indicates a higher degree of transformation of an initially anhydrous protolith (Brearley and Jones, 1998; Tomeoka and Buseck, 1988). 53Mn–53Cr chronometry on carbonates consistently indicates the longest duration of the aqueous alteration process in CIs, assuming this is a continuous process (de Leuw et al., 2009). In CMs, aqueous alteration leads to the decrease of volatile species abundance and a heating process is expected to have a similar effect on volatiles. Therefore, the fact that CIs experienced more post-accretion processing than CMs and that

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Fig. 4. The area of the white line of the K-edge XANES spectra of the standards used in this study, plotted against peak position. The curve shows the proximity of the data for aliphatic and heterocyclic sulfur, but no significant overlap. This curve was used to calculate the relative abundance of the different species from the area of the Gaussian curves used for the spectral decomposition. The number after each standard name refers to the oxidation number.

organic heterocyclic sulfur is higher in Alais than in Ivuna and Orgueil. This fact is inconsistent with the lower grade of aqueous alteration of the former (Endress and Bischoff, 1996). 5.2. Precursor heterogeneity in CMs and Renazzo

Fig. 3. XANES spectra of the IOM of the chondrites studied. CI chondrites exhibit oxidized organic sulfur and a higher aromatic versus aliphatic sulfur abundance. With the exception of Cold Bokkeveld, the most altered CM, the XANES spectra of CM, CR and Tagish Lake chondrites are fairly similar.

they contain more volatiles supports the idea that they come from different protoliths (Eiler and Kitchen, 2004). Note that another possible interpretation is based on the effects of terrestrial weathering on asphaltenes (Sarret et al., 1999). Under such conditions, an increase of the abundance of heterocyclic organic sulfur would be observed, along with oxidized organic sulfur. However, the physical and chemical parameters at play in the evolution of sulfur speciation have not been identified and terrestrial conditions are likely very different from those acting in a parent body. Therefore, it is unclear if these naturalistic observations can be used as a proxy for chondritic conditions. Indeed, in the case of CIs, the abundance of

IOM in Renazzo and CMs other than Cold Bokkeveld (simply referred to as CM-wt-CB below) show similar S-XANES spectra. The relative abundance of heterocyclic versus aliphatic organic sulfur is similar and no oxidized organic sulfur is detected. In contrast, this IOM presents some variations in global elemental composition and chemical structures (Alexander et al., 2007; Cody and Alexander, 2005; Orthous-Daunay et al., 2010). These variations can be the result of post-accretion processes on the parent body and/or inherited by processes which acted in the solar nebula. Disentangling these two processes requires an accurate knowledge of the conditions in the parent body. As a first constraint, sulfur speciation does not point to any thermal heating nor oxidation processes at play on the parent body. CR and CM-wt-CB chondrites in this study span a rather broad range of extent of aqueous alteration. Renazzo is definitely the least altered object. Aqueous alteration was at play for a short period, although it was very mild and restricted to the matrix (Brearley and Jones, 1998; de Leuw et al., 2010). In CMs, the extent of aqueous alteration is quantified by the petrologic types (Rubin et al., 2007). Petrologic types are defined from petrologic and mineralogical characteristics. They do not strictly provide the physical and chemical parameters relevant to the physical conditions. Nevertheless, they likely reflect the water/rock ratio and the duration of the process (de Leuw et al., 2010). Mineralogical models provide more data on the physical conditions, such as the temperature on the parent body, and also the Eh and pH parameters (Bullock et al., 2005; Guo and Eiler, 2007; Zolensky et al., 1989). The pH of the water in which carbonates formed appears to range between 7 and 12 with increasing aqueous alteration. The organochemical mechanisms which possibly modified IOM in chondrites are still largely unknown. As pointed out above, heating and oxidation processes are known to transform coals and kerogens under laboratory or natural conditions, but the relevant conditions cannot be extrapolated to those acting in a chondritic parent body. The lack of any significant variation of sulfur speciation among CMs of different petrologic types is likely inconsistent with the action of an oxidizer-

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327

Fig. 5. Relative abundances of the 5 main sulfur speciations (sulfides, aliphatic and heterocyclic reduced organic sulfur, oxidized organic sulfur and sulfates) for each chondrite studied.

bearing fluid at low temperature. Thus, even if petrologic types do not fully account for the complex chemical reactions between fluids and organic matter, they quantify the duration and the water/rock ratio and therefore the content of oxidizing agents in the ice accreted along with minerals. If a low-temperature oxidation process were at play on the parent body, we think that the chemical features of its impact

would appear gradually. From our point of view, the global chemical differences (H/C ratio, alkyl abundance, etc.) between the IOM extracted from Renazzo and CMs are mostly due to variations in the compositions of the organic precursors accreted. The similarities of the sulfur K-edge spectra suggest that all these chondrites experienced similar parent body temperatures in the range 20–35 °C. 5.3. Puzzling Tagish Lake Tagish Lake is a complex object and cannot be strictly classified either as a CI or as a CM (Baker et al., 2002). Its low density, high porosity, oxygen bulk composition, noble gases and volatile abundances and its mineralogical characteristics indicate that it is a unique object, with possible links with micrometeorites and the Kaidun chondrite (Nakamura et al., 2003; Noguchi et al., 2002; Zolensky et al., 2002). The matrix of Tagish Lake contains abundant sulfides, for which the nature and compositions would point to an intermediate stage of parent body fluid circulation among CMs. The S-XANES spectrum of Tagish Lake IOM appears consistent with this classification, as neither heating nor oxidation processes are observed. The IOM of Tagish Lake has however a polyaromatic structure dissimilar to that of Renazzo, CMs (Murchison, Murray, Cold Bokkeveld) and CIs (Orgueil, Ivuna, Alais) (Matrajt et al., 2004). Nuclear magnetic resonance measurements show that aliphatic carbon chains are less abundant than in Orgueil and Murchison (Cody and Alexander, 2005). However, infrared measurements by Flynn et al. (2001), Matrajt et al. (2004), Orthous-Daunay et al. (2010) do not report a dramatic lower abundance of the aliphatic functional groups. These results are accounted for by the fact that Tagish Lake is a complex breccia containing different forms of insoluble organic matter, reflecting different alteration histories before the accretion of the parent body and/or chemical variations in the organic precursors accreted. Note also that the lack of an oxidation process as indicated by the S-XANES spectra is consistent with the high abundance of diradicaloid species (Binet et al., 2004). 6. Conlusions

Fig. 6. The relative deviation with respect to the mean of the abundance of each sulfur speciation, plotted for each group of chondrites. CIs clearly plot apart from other chondrites. Renazzo appears to be the most reduced chondrite.

This study reports sulfur speciation measurements by K-edge XANES micro-spectroscopy of insoluble organic matter samples extracted from a comprehensive set of 3 CIs (Orgueil, Ivuna, Alais), 5 CMs (QUE97990, Murchison, Murray, QUE99355, Cold Bokkeveld),

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1 CR (Renazzo) and the ungrouped C2 Tagish Lake chondrite. The conclusions are the following: • 5 main sulfur groups have been identified: (1) sulfides, (2) aliphatic organic sulfur, (3) heterocyclic organic sulfur, (4) oxidized organic sulfur and (5) sulfates. • CIs and Cold Bokkeveld IOM experienced a chemical transformation by mild thermal heating in their parent body, resulting in a high heterocyclic organic sulfur content. • Cold Bokkeveld appears to be a complex brecciated chondrite that accreted clasts with different thermal histories. • CIs are the only chondrites that have experienced a “mild temperature” (100–150 °C) oxidation process. • The temperatures experienced by CIs in their parent body addresses the unresolved issue of the survival of their highly volatile content when subjected to major aqueous and thermal alteration. This strongly suggests that the ice present in CI protoliths had a chemical composition very different from those of CMs. • No evidence of a low-temperature oxidation process is reported for Renazzo, Tagish Lake and other CMs other than Cold Bokkeveld. Our data support the idea that they all experienced a parent body temperature b ~ 35 °C. Tagish Lake is a complex breccia, with heterogeneous insoluble organic matter which deserves more systematic investigations. Acknowledgments The Museum National d'Histoire Naturelle (Paris France) and the Working Meteorite Group (Johnson Space Center NASA USA) are warmly thanked for having provided us with valuable samples. We also thank the ESRF (Grenoble France) and the staff of the ID21 beamline for making the measurements possible. This work has been funded by the Centre National d'Etudes Spatiales (CNES-France). References Alexander, C.M.O.D., et al., 1998. The origin of chondritic macromolecular organic matter: a carbon and nitrogen isotope study. Meteorit. Planet. Sci. 33, 603–622. Alexander, C.M.O.D., et al., 2007. The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter. Geochim. Cosmochim. Acta 71, 4380–4403. Baker, L., et al., 2002. The oxygen isotopic composition of water from Tagish Lake: its relationship to low-temperature phases and to other carbonaceous chondrites. Meteorit. Planet. Sci. 37, 977–985. Beck, P., et al., 2010. Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids. Geochim. Cosmochim. Acta 74, 4881–4892. Benedix, G.K., et al., 2003. Carbonates in CM2 chondrites: constraints on alteration conditions from oxygen isotopic compositions and petrographic observations. Geochim. Cosmochim. Acta 67, 1577–1588. Binet, L., et al., 2004. Diradicaloids in the insoluble organic matter from the Tagish Lake meteorite: comparison with the Orgueil and Murchison meteorites. Meteorit. Planet. Sci. 39, 1649–1654. Bischoff, A., 1998. Aqueous alteration of carbonaceous chondrites: evidence for preaccretionary alteration — a review. Meteorit. Planet. Sci. 33, 1113–1122. Bonal, L., et al., 2006. Determination of the petrologic type of CV3 chondrites by Raman spectroscopy of included organic matter. Geochim. Cosmochim. Acta 70, 1849–1863. Bonal, L., et al., 2007. Organic matter and metamorphic history of CO chondrites. Geochim. Cosmochim. Acta 71, 1605–1623. Brearley, A.J., Jones, R.H., 1998. Chondritic meteorites. Planetary Materials. Mineralogical Soc America, Washington. Bullock, E.S., et al., 2005. Mineralogy and texture of Fe-Ni sulfides in CI1 chondrites: clues to the extent of aqueous alteration on the CI1 parent body. Geochim. Cosmochim. Acta 69, 2687–2700. Burgess, R., et al., 1991. Determination of sulfur-bearing components in C1 and C2 carbonaceous chondrites by stepped combustion. Meteoritics 26, 55–64. Busemann, H., et al., 2007. Characterization of insoluble organic matter in primitive meteorites by microRaman spectroscopy. Meteorit. Planet. Sci. 42, 1387–1416. Clayton, R.N., Mayeda, T.K., 1999. Oxygen isotope studies of carbonaceous chondrites. Geochim. Cosmochim. Acta 63, 2089–2104.

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