# Carbon isotopic variation in ureilites: Evidence for an early, volatile-rich Inner Solar System

Jean-Alix J-A Barrat, Pierre Sansjofre, Akira Yamaguchi, Richard

Greenwood, Philippe Gillet

► To cite this version:

Jean-Alix J-A Barrat, Pierre Sansjofre, Akira Yamaguchi, Richard Greenwood, Philippe Gillet. Carbon isotopic variation in ureilites: Evidence for an early, volatile-rich Inner Solar System. Earth and Planetary Science Letters, 2017, 478, pp.143 - 149. 10.1016/j.epsl.2017.08.039. hal-01919244

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# Carbon isotopic variation in ureilites:
## Evidence for an early,
## volatile-rich Inner Solar System

by

Jean-Alix Barrat<fnref n="1" />*, Pierre Sansjofre<fnref n="1" />, Akira Yamaguchi<fnref n="2" />,³,

Richard C. Greenwood⁴, and Philippe Gillet⁵

Submitted to *EPSL*, 7/5/2017, revised manuscript, 30/8/2017.

### Abstract

We analyzed the C isotopic compositions of 32 unrecciated ureilites, which represent mantle debris from a now disrupted, C-rich, differentiated body. The δ¹³C values of their C fractions range from -8.48 to +0.11‰. The correlations obtained between δ¹³C, δ¹⁸O and Δ¹⁷O values and the compositions of the olivine cores, indicate that the ureilite parent body (UPB) accreted from two reservoirs displaying distinct O and C isotopic compositions. The range of Fe/Mg ratios shown by its mantle was not the result of melting processes involving reduction with C (“smelting”), but was chiefly inherited from the mixing of these two components. Because smelting reactions are pressure-dependent, this result has strong implications for the size of the UPB, and points to a large parent body, at least 690 km in diameter. It demonstrates that C-rich primitive matter distinct from that represented by carbonaceous chondrites was present in some areas of the early inner Solar System, and could have contributed to the growth of the terrestrial planets. We speculate that differentiated, C-rich bodies, or debris produced by

<footnote n="1">¹Laboratoire Geosciences Océan (UMR CNRS 6538), Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané, France.</footnote>

<footnote n="2">²National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan. ³Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo 190-8518, Japan. ⁴Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom. ⁵Earth and Planetary Science Laboratory (EPSL), Ecole Polytechnique Fédérale de Lausanne, Institute of Condensed Matter Physics, Station 3, CH-1015 Lausanne, Switzerland</footnote>

<footnote>*Corresponding author.</footnote>
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their disruption, were an additional source of volatiles during the later accretion stages of the
rocky planets, including Earth.

# 1. Introduction

The origin of volatiles in the inner Solar System, including Earth, remains controversial. It is widely held that water and other volatiles were delivered late in the development of the terrestrial planets (Marty, 2012; Alexander et al., 2012; Rubie et al., 2015). *An alternative view suggests that* the inner planets accreted volatile-rich, *so that* phases such as water and other light elements, vital to the origin of life, were mainly indigenous (Halliday, 2013). Here we look at the origin of the ureilites, C-rich meteorites from a single differentiated body (Downes et al., 2005) that formed in the inner Solar System (Yamakawa et al., 2010; Warren, 2011; Barrat et al., 2016a; Budde et al., 2017), and hence are potentially representative of some of the materials from which the terrestrial planets accreted.

The ureilites are peridotites that constitute the largest sampling of the mantle of a body other than the Earth and so provide a unique opportunity to constrain the differentiation processes that took place within a protoplanet. Geochemical studies have shown that the UPB accreted no more than 1.6 My after initial Solar System formation (Budde et al., 2015). It was heated by decay of short-lived isotopes, principally $^{26}$Al, resulting in the segregation of a S-rich core (e.g., Warren et al., 2006; Rankenburg et al., 2008; Barrat et al., 2015) and the formation of a trachyandesitic crust (Bischoff et al., 2014; Barrat et al., 2016b). This differentiation event occurred between 3 and 7 My, and was terminated by the catastrophic breakup of the body as the result of a large impact event (Goodrich et al., 2015).

Ureilites are restites formed after extraction of magmas and S-rich metallic melts (e.g., Scott et al., 1993; Warren et al., 2006; Barrat et al., 2016b). They consist chiefly of olivine and pyroxene, and are remarkably rich in C [1-6 wt% (Goodrich et al., 2015), graphite (Fig. 1) and diamond]. Olivines in ureilites have distinctive Mg-rich rims and veins that formed as a result of localized reduction reactions with adjacent C phases. These reactions probably took place during the disruption of the UPB (Miyamoto et al., 1985; Warren and Huber, 2006). Apart from these rims and veins, olivine cores and pyroxene grains are quite uniform in Mg# number [= 100 Mg/(Mg+Fe), atomic] within any individual ureilite sample. However, collectively their overall compositional variation is very large, as shown by the olivine-core Mg#, which ranges from 74 to 97 (Downes et al., 2008). The Fe/Mn ratios and $\Delta^{17}$O values are correlated with this parameter. These variations cannot be explained by igneous fractionation. Instead, it has been
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argued that the Mg# range is the result of reduction reactions involving C (“smelting”, e.g., Singletary and Grove (2003), Goodrich et al. (2007, 2013)) that took place during the differentiation of the UPB, pre-igneous oxidation of metal by H₂O from ice (Sanders et al., 2017) or, alternatively, was inherited from the accreted materials, and preserved despite significant partial melting (Warren and Huber, 2006; Warren, 2012).

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In this paper, we report new high-quality C isotopic data obtained on a large number of unrecciated ureilites, with the aim of constraining the role of C during the differentiation of the UPB, the size of this body, and finally of discussing the diversity of volatile sources in the inner Solar System.

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## 2. Samples and analytical procedure

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We analyzed 32 unpaired meteorites collected in Antarctica and in the Sahara. The details of the samples used in this study, all unrecciated ureilites, are given in Table 1. Their olivine cores cover nearly the full range of compositions known for these meteorites (Fo = 74–97 %). Polymict ureilites have deliberately been excluded from this study, as have samples of the rare “Hughes” group ureilites. However, one sample (MET 01085) is not a peridotite, but a pyroxenite devoid of olivine.

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Samples (0.25 - 1 g) were powdered using a boron-carbide mortar and pestle. About 250 mg of powder for each sample was digested on a hot plate heated to 125°C, using sequential mixtures of 10 N HF/14 N HNO₃, 14 N HNO₃ and 6 N HCl. This procedure ensures a perfect dissolution of all the phases, except graphite and diamond, which are not oxidized by these acids at this temperature. The resulting clear solution was carefully pipetted off, and the C residues rinsed twice using ultrapure water and then dried. Our procedure has the advantage of directly removing all terrestrial carbonates and all terrestrial C in the samples. Carbon isotopic compositions were determined at the Pôle Spectrométrie Océan (PSO), Institut Universitaire Européen de la Mer (IUEM, Plouzané, France). 0.1 to 0.2 mg of the C powder was introduced in tin capsules and δ¹³C was determined following flash combustion using an EA-IRMS Flash-EA 2000 analyzer coupled to a Thermo Fisher Delta-V Plus mass spectrometer. The oxygen flux was optimized at 240 ml/min for a flash combustion of 15 s, to ensure a complete oxidation of the sample. Isotopic data are reported as parts per thousand deviation (‰) from the Pee Dee belemnite reference material (PDB): δ¹³C = 1000 [((¹³C/¹²C)sample)/(¹³C/¹²C)PDB - 1]. Most measurements were determined in duplicate with an external reproducibility better than 0.2 ‰ (2 σ). An in-house graphite standard (LCgraphite), kindly provided by Pierre Cartigny (IPG,
Paris) has been repeatedly analysed during the course of this study in order to check the reproducibility and our calibration. These analyses are in excellent agreement with previous results obtained by the Institut de Physique du Globe, Paris (Table 1).

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## 3. Results and discussion

### 3.1. δ¹³C systematics and the size of the UPB

Ureilitic C yielded δ¹³C ranging from -8.48 to +0.11 ‰ (Table 1), in agreement with earlier results ranging from -11.1 to ≈+0.5 ‰, (Grady et al., 1985; Grady and Pillinger, 1986; Russell et al., 1993; Smith et al., 2001; Hudon et al., 2004; Downes et al., 2015; Miyahara et al., 2015). Many of our samples (e.g., the Antarctic ureilites found before 2000) were analysed previously, but a comparison with our results is unfortunately not straightforward. Most of the literature results are only available in conference abstracts, with few, if any, details of the analytical procedures used and data sometimes appearing only in figures (e.g., Hudon et al., 2004). In a δ¹³C vs. olivine core composition plot, the previous results are scattered and no clear systematic behaviour is apparent (Fig. 2). However, results obtained by Hudon et al. (2004) show that C in ureilites with ferroan olivines have a tendency to be heavier than in those with the most forsteritic olivines. In contrast, our analyses, with the exception of one sample (Elephant Moraine (EET) 83225), fit hyperbolae in δ¹³C versus olivine core composition plots (Figs. 2-4). The reasons for the spread in the literature data are unclear. Earlier results were mostly obtained by the stepped combustion technique on small whole-rock samples (e.g., Grady et al., 1985). Analytical biases are possible and could mask possible relationships. Alternatively, the spread could be indicative of strong intra-sample heterogeneities in δ¹³C. However, the sole detailed in-situ study of a ureilitic clast failed to detect large C-isotopic variations (Miyahara et al., 2015), and weakens this possibility.

The hyperbolae in δ¹³C versus olivine core composition plots obtained in this study (Figs. 3-4) provide strong constraints on the differentiation processes of the UPB. These results are pertinent to the three competing processes for the evolution of the UPB: i) was the range in Mg#-values measured in ureilitic olivine cores the result of smelting; (ii) the pre-igneous oxidation of metal by H₂O from ice; or (iii) simply inherited from the accreted materials?

In the case of smelting, C reacts with silicates containing iron and eventually partially melts, as illustrated by the two following reactions (Singletary and Grove, 2003; Warren and Huber, 2006):

6 Mg₁.₅Fe₀.₅SiO₄ (olivine Fo₇₅) + 3 C →
$$132 \qquad 3 \mathrm{Mg}_2\mathrm{SiO}_4 \text{ (forsterite)} + 1.5 \mathrm{Mg}_2\mathrm{Si}_2\mathrm{O}_6 \text{ (enstatite)} + 3 \mathrm{Fe} + 3 \mathrm{CO} \quad (1)$$

$$133 \qquad 6 \mathrm{Mg}_{1.5}\mathrm{Fe}_{0.5}\mathrm{SiO}_4 \text{ (olivine } \mathrm{Fo}_{75}) + 3 \mathrm{C} + \mathrm{CaO}_{\text{(melt)}} \rightarrow$$

$$134 \qquad 4 \mathrm{Mg}_2\mathrm{SiO}_4 \text{ (forsterite)} + \mathrm{CaMgSi}_2\mathrm{O}_6 \text{ (diopside)} + 3 \mathrm{Fe} + 3 \mathrm{CO} \quad (2)$$

If redox reactions were involved, they produced not only Mg-rich silicate assemblages but also converted the indigenous solid C to CO gas, which was then lost to space. Such reactions are controlled by the composition of the silicate assemblages (i.e., Mg#), pressure and temperature, and are blocked if the pressures are too high (Warren and Huber, 2006; Warren, 2012). In other words, redox reactions can affect about half the mantle of a 200 km diameter C-rich body, but much lower fractions for larger bodies. Furthermore, to yield internal pressures prohibitive of smelting for the most ferroan ureilitic compositions, the body would have to have been at least 690 km in diameter (Warren and Huber, 2006; Warren, 2012). Thus, if it can be demonstrated that C phases remained stable during partial melting, a strong constraint on the size of the UPB would be provided.

Carbon isotopic compositions are fractionated by redox reactions with CO formation preferentially consuming ¹²C, the **lighter** isotope (see Fig. 20 in Chacko et al., 2001). Such reactions are likely to produce residues enriched in ¹³C. Therefore, C after smelting should display δ¹³C equal **to** or higher than the initial value.

A number of important conclusions emerge from the strong link between the variation in C isotopes and the composition of olivine cores.

Firstly, the importance of the reduction reactions during the breakup of the UPB is highly variable (e.g., Singletary and Grove, 2003), with a full gradation in textures from ureilites displaying olivines with thin forsteritic rims (e.g., Asuka 881931) to ureilites containing Mg-rich olivines displaying rare domains of preserved (i.e., more Fe-rich) cores (e.g., Northwest Africa 2234). The conservation of a well-defined relationship between olivine core and C isotopic compositions indicates that primary C isotope values were unaffected, or not severely modified, during the disruption of the body.

Secondly, it was originally proposed by both Boynton et al. (1976) and Wasson et al. (1976) that C was introduced into ureilites after the main phase of UPB differentiation, or during the disruption of the body. Late addition of C has also been invoked by Day et al. (2017) based on a study of the Antarctic ureilite Miller Range (MIL) 091004. They observed a small area enriched in graphite (34 vol%) in a single polished section (1.35 cm²), and proposed that it was injected or smeared into the rock after silicate and Fe-Ni-S depletion, by late-stage C-rich fluid/melt infiltration. The composition of this fluid is unspecified. Alternatively, the
heterogeneous distribution of graphite at a centimeter scale is possible in ureilites, and can
account for the observations made by Day et al. (2017), without invoking C-rich fluids or melts.
Also significant is the lack of chemical differences between the silicate phases in the “C-rich
lithology” compared to the rest of the sample. MIL 091004 has a regular bulk composition,
which is similar to group-B ureilites as defined by Barrat et al. (2016), ruling out hypothetical
low degree partial melts as possible metasomatic agents as suggested by Day et al. (2017). A
true C-rich veinlet (~4.7 mm x ~0.3 mm in size) containing a complex network of S-rich, Fe-
S-rich, Al-rich and Si-rich areas, was described in the Yamato 74130 ureilite (Tomeoka and
Takeda, 1990). It demonstrates that, locally, a small amount of C was mobilized. The
importance of the mobilization of C into ureilites via melts or fluid, should not be
overemphasized: among the hundreds of ureilites examined so far, only this single C-rich
veinlet has been described. Furthermore, C-addition by fluids/melts, either before or during the
disruption of the body, would have totally erased the hyperbolic relationship between δ¹³C and
olivine core composition documented in this study and this is clearly not the case. Significantly,
among the 32 ureilites we have analyzed, only one sample (EET 83225) does not lie on this
trend. A detailed study of this ureilite is clearly warranted in order to understand its potentially
anomalous composition.

More significantly, C isotopes directly rule out redox reactions during the differentiation
of the UPB as the cause of the wide range of silicate compositions. If this had been the case, C
in Mg-rich ureilites would have been heavier than in more ferroan rocks. The trends seen in
Figures 3 and 4 show just the opposite: the ureilites with the most Mg-rich olivines always
display the lowest δ¹³C values (< -7 ‰) and consequently the isotopically lightest C.

Sanders et al. (2017) have proposed that the UPB accreted as a mixture of nebular ice
with high Δ¹⁷O, Mg-rich silicates with low Δ¹⁷O and metallic iron, and also assumed that the
UPB was large enough that smelting was inhibited. Water from the thawing ice then oxidized
the metal and formed secondary FeO-bearing minerals with high Δ¹⁷O values. These secondary
phases became incorporated into ureilite silicates during the metamorphism of the body before
the onset of the igneous processes. This model can potentially explain the correlations observed
between Fe/Mg, Fe/Mn and O isotopic compositions (Fig. 3). However, the effects of this pre-
igneous oxidation processes on the δ¹³C values is not obvious. Assuming that this model is
correct, the most ferroan ureilites incorporated the highest fractions of the pre-igneous
secondary phases, and display the heaviest C isotopic compositions. Thus, an increase of the
δ¹³C values could be a fingerprint of the oxidation process and of the aqueous alteration of
198 chondrites. However, available data on chondrites do not show such an effect (e.g., see analyses
199 given by Alexander et al., 2012), and weaken the model proposed by Sanders et al. (2017).

200 The isotopic diversity recorded by ureilites was more likely inherited from the materials
201 that accreted to form the UPB. The chemical, O and C isotopic variations displayed by ureilites
202 are compatible with the involvement of two components which can be constrained using the
203 two arms of the hyperbolic trends. The first (A) displays the heaviest C (δ¹³C ≈ 0 ‰), δ¹⁸O ≈ 8
204 ‰, Δ¹⁷O ≈ -0.5 ‰, and corresponds to the ureilites with the most ferroan olivines (Mg# ≈ 75,
205 Fe/Mn ≈ 50). The isotopic compositions of the second one (B) are δ¹³C ≈ -8 ‰, δ¹⁸O ≈ 5 ‰,
206 Δ¹⁷O ≈ -2.5 ‰, and are defined by the ureilites with the more Mg-rich olivines (Mg# > 95,
207 Fe/Mn < 10). Because ureilites are restites, abundances of most elements before extraction of
208 the core and silicate partial melting cannot be directly deduced. However, the curvatures of the
209 hyperbolas indicate that the second component (B) was by far the richer in C than the first (A).
210 The exact nature of these two components is unclear, but they were most likely to have been
211 primitive chondritic materials, both relatively rich in C. However, they were certainly unlike
212 any known carbonaceous chondrites, as demonstrated by their stable Cr, Ti and Mo isotope
213 systematics (Yamakawa et al., 2010; Qin et al., 2010; Warren, 2011; *Budde et al., 2017*), C and
214 N isotope systematics (Downes et al., 2015) and by their slightly negative Tm anomalies,
215 similar to non-carbonaceous chondrites and inner Solar System bodies (Barrat et al., 2016a).

216 Because redox reactions involving C were inhibited during the differentiation of UPB,
217 this body was certainly much larger than generally thought (e.g., Goodrich et al., 2015 and
218 references therein), and was at least 690 km in diameter, as calculated by Warren (2012).
219 Moreover, the discovery of unusually “large” diamonds, displaying sector zoning, in the
220 Almahata Sitta ureilite was interpreted as evidence for static growth inside the UPB (Miyahara
221 et al., 2015). If correct, the UPB was at least ~1000 km in diameter, and could have been much
222 larger. Additional constraints on the equilibrium conditions of these diamonds, and on the size
223 of the UPB, could be provided by the nature and compositions of their inclusions.
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## 3.2. Implications for the compositions of the planetary building blocks and the origins of the volatile elements in the Inner Solar System.

228 Whatever the exact size of the UPB, our results have two important implications. Firstly,
229 they show that volatile-rich materials distinct from known carbonaceous chondrites were
230 present in some areas of the early inner Solar System, and were the main ingredients of at least
one large rocky protoplanet. Similar materials could have contributed to the growth of other early embryos **that** were involved in the building of terrestrial planets. While a contribution of such materials at the percent level would not be detected with O, Cr, Ti isotope compositions usually used in discussions on the nature of planetary building blocks (e.g., Fitoussi et al., 2016), even at such low levels, they would still have been important carriers of volatile elements, especially C. Secondly, it is well accepted that less than 1 % of the mass of the Earth was added after segregation of **its** core on the basis of platinum-group element abundances (e.g., Chou, 1978). This late veneer is often thought to have essentially been composed of CI and CM carbonaceous chondrite-like material, which in turn can account for a large proportion of the terrestrial volatiles (e.g., Marty, 2012; Alexander et al., 2012; Rubie et al., 2015). However, the volatile abundances of the bulk Earth remain poorly constrained, notably because some of the phases involved are siderophile at high pressures, and could have been partly sequestered by the core, as exemplified by C (Hirschmann, 2016). Consequently, we don't know at present the proportion of terrestrial C in the mantle and core. The average $\delta^{13}$C value of the Earth's upper mantle ($\delta^{13}$C ≈ -5 ‰, Marty et al., 2013) is very similar to average CM carbonaceous chondrites ($\delta^{13}$C= -3.5 ‰), and if not modified by mantle processes, is at first glance consistent with the view that the carbonaceous chondrites were the main source of Earth's volatiles. However, it does not preclude the involvement of other components (Wang et al., 2016; Li et al., 2016). Indeed, the destruction of the UPB produced a multitude of debris, which on average display the same $\delta^{13}$C value ($\delta^{13}$C= -4.3 ‰, n=32) as the Earth's mantle, and so potentially could have been an ingredient **of** the late veneer. Interestingly, the distribution of volatiles in the bulk Earth displays a large negative nitrogen anomaly (Fig. 5, e.g., Marty, 2012). Because nitrogen is siderophile at high pressures, this element could have been partly sequestered by the core (Roskosz et al., 2013). **If** correct, this would suggest a decoupling with C, which is not well understood at present. Alternatively, the high C/N ratio of the bulk Earth could be partly inherited from some non-chondritic components of the late veneer (Hirschmann, 2016). Significantly, ureilites are poor in nitrogen and display very high C/N ratios (Fig. 5). The recent discovery in a chondrite of graphite-bearing igneous clasts from a possible Moon-sized body (Hiyagon et al., 2016), indicates that the UPB was not the sole C-rich differentiated body. Volatile-rich materials, similar in composition to ureilites, may have been more important than has so far been considered. Thus, the involvement of C-rich differentiated objects during the accretion of the terrestrial planets should now be seriously re-evaluated.

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# 4. Conclusions

The data presented here demonstrates that the C isotopic composition of ureilites correlates with the Fe/Mg ratios of the cores of their olivines. This observation has important implications for the differentiation processes on their parent body, *for its* size, and for the origin of the volatile elements in the inner Solar System.

Various models have been put forward to explain the origin of the wide range of compositions of the ureilitic silicates. *Carbon* isotopic systematics are not consistent with the recent model of low-temperature oxidation of metal by water derived from ice (Sanders et al., 2017), nor with the more widely proposed smelting model, which assumes reduction of silicates by C during melting (e.g., Singletary and Grove, 2003; Goodrich et al., 2007). The C and O isotopic compositions indicate that the chemistry of the ureilitic mantle has recorded the involvement of two C-rich reservoirs characterized by distinctive $\delta^{13}$C, $\delta^{18}$O, $\Delta^{17}$O and Fe/Mg ratios. The imperfect mixing of these two reservoirs during the accretion of the UPB [merging of two planetesimals as suggested by Sanders et al. (2017)] can explain the $\delta^{13}$C range and the mass independent variation in their O isotopic ratios that correlates with their Fe/Mg ratios. The inhibition of the reactions of reduction of the silicates by C indicates that the UPB was much larger than 690 km in diameter, in agreement with thermodynamic modelling by Warren (2012), and the discovery of diamonds too large to be formed upon impact (Miyahara et al., 2015), indicative of a body much larger than Ceres.

Although ureilites are in some ways unusual (same O isotopic compositions as carbonaceous chondrites, but distinct Cr and Ti isotope systematics), the accretion of at least one such large C-rich body was certainly not fortuitous. It indicates that C-rich primitive matter, distinct from carbonaceous chondrites, was present in the early Solar System. The direct involvement of ureilite-derived material, or material from other UPB-like bodies, may have played a significant role in the accretion histories of the terrestrial planets and contributed to their volatile element inventories.

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## 292 Acknowledgement

Some of the ureilites from the Sahara were kindly provided by Bruno and Carine Fectay, and Peter Marmet. Samples from Antarctica were kindly provided by the Meteorite Working Group (NASA) and the National Institute of Polar Research, Tokyo. 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 in the Department of Mineral
298 Sciences of the Smithsonian Institution and Astromaterials Curation Office at NASA Johnson
299 Space Center. We are grateful to Pierre Cartigny for sharing the IPGP in-house graphite
300 standard, Albert Jambon and Cyrena Goodrich for their comments made on a previous version
301 of this paper, Sasha Verschovsky for discussions on step-heating procedures and Pascale Barrat
302 for her help. We thank Derek Vance for the editorial handling, Paul Warren, James Day and Ed
303 Scott for their helpful and constructive reviews. This work was funded by a grant from the
304 Programme National de Planétologie (CNRS-INSU) to the first author.

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Table 1. Details of ureilite samples studied, olivine core compositions (Singletary and Grove, 2003; Downes et al., 2008; Warren, 2012; Barrat et al., 2015 and references therein) and C isotopic compositions (this work). Errors given here are calculated from the difference between the duplicates.

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<table><thead><tr><th></th><th>source</th><th>split</th><th>Olivine cores</th><th>δ<sup>13</sup>C (‰)</th><th>Remarks</th></tr><tr><th></th><th></th><th></th><th>Mg#</th><th>Fe/Mn (atomic)</th><th></th></tr></thead><tbody><tr><td>A 881931</td><td>NIPR</td><td>,71</td><td>78.7</td><td>48.1</td><td>-4.89 ± 0.02 n=2</td></tr><tr><td>ALH 77257</td><td>NIPR</td><td>,105</td><td>86.1</td><td>29.1</td><td>-7.30 ± 0.01 n=2</td></tr><tr><td>ALH 81101</td><td>MWG</td><td>,63</td><td>78.9</td><td>41.8</td><td>-0.84 ± 0.01 n=2</td></tr><tr><td>ALH 82130</td><td>MWG</td><td>,43</td><td>95.2</td><td>9.1</td><td>-7.73 ± 0.04 n=2</td></tr><tr><td>EET 83225</td><td>MWG</td><td>,37</td><td>88.3</td><td>22.1</td><td>-4.42 ± 0.06 n=2</td></tr><tr><td>EET 96042</td><td>MWG</td><td>,48</td><td>81.3</td><td>38.2</td><td>-6.43 ± 0.03 n=2</td></tr><tr><td>GRA 95205</td><td>MWG</td><td>,64</td><td>79.2</td><td>45.2</td><td>-1.74 ± 0.07 n=2</td></tr><tr><td>GRO 95575</td><td>MWG</td><td>,46</td><td>78.6</td><td>44.9</td><td>-1.87 ± 0.03 n=2</td></tr><tr><td>LAP 03587</td><td>MWG</td><td>,10</td><td>74.7</td><td>56.6</td><td>-0.28 ± 0.02 n=2</td></tr><tr><td>LAR 04315</td><td>MWG</td><td>,46</td><td>81.9</td><td>39.8</td><td>-7.80 ± 0.03 n=2</td></tr><tr><td>MET 78008</td><td>NIPR</td><td>,69</td><td>75.9</td><td>49.5</td><td>-1.46 ± 0.11 n=2</td></tr><tr><td>MET 01085</td><td>MWG</td><td>,23</td><td>88.8 (px)</td><td></td><td>-6.89 ± 0.01 n=2, no olivine</td></tr><tr><td>PCA 82506</td><td>MWG</td><td>,125</td><td>79.2</td><td>44.3</td><td>-6.40 ± 0.15 n=2</td></tr><tr><td>Y 74130</td><td>NIPR</td><td>,55</td><td>77.0</td><td>48.0</td><td>-1.71 ± 0.14 n=2</td></tr><tr><td>Y 790981</td><td>NIPR</td><td>,85</td><td>77.5</td><td>46.5</td><td>-1.77 ± 0.23 n=2</td></tr><tr><td>Y 791538</td><td>NIPR</td><td>,109</td><td>91.3</td><td>18.0</td><td>-7.49 ± 0.04 n=2</td></tr><tr><td>Y 981810</td><td>NIPR</td><td>,76</td><td>78.3</td><td>50.2</td><td>+0.11 ± 0.03 n=2</td></tr><tr><td>NWA 2234</td><td>NIPR</td><td></td><td>81.5</td><td>37.3</td><td>-7.76 ± 0.01 n=2</td></tr><tr><td>NWA 2236</td><td>NIPR</td><td></td><td>96.8</td><td>7.0</td><td>-7.54 ± 0.07 n=2</td></tr><tr><td>NWA 4471</td><td>J.A. Barrat</td><td></td><td>78.1</td><td>50.1</td><td>-3.32 n=1</td></tr><tr><td>NWA 4852</td><td>J.A. Barrat</td><td></td><td>87.5</td><td>25.2</td><td>-7.14 ± 0.23 n=2</td></tr><tr><td>NWA 5555</td><td>P. Marmet</td><td></td><td>90.8</td><td>19.5</td><td>-8.20 ± 0.02 n=2</td></tr><tr><td>NWA 5602</td><td>J.A. Barrat</td><td></td><td>79.0</td><td>46.3</td><td>-2.39 ± 0.03 n=2</td></tr><tr><td>NWA 5884</td><td>J.A. Barrat</td><td></td><td>78.6</td><td>47.4</td><td>-0.90 ± 0.02 n=2</td></tr><tr><td>NWA 6056</td><td>J.A. Barrat</td><td></td><td>84.8</td><td>31.8</td><td>-7.01 ± 0.05 n=2</td></tr><tr><td>NWA 7349</td><td>J.A. Barrat</td><td></td><td>76.5</td><td>51.3</td><td>-1.09 n=1</td></tr><tr><td>NWA 7630</td><td>J.A. Barrat</td><td></td><td>79.1</td><td>46.6</td><td>-2.91 ± 0.01 n=2</td></tr><tr><td>NWA 7686</td><td>J.A. Barrat</td><td></td><td>91.0</td><td>19.1</td><td>-8.41 ± 0.01 n=2</td></tr><tr><td>NWA 7880</td><td>J.A. Barrat</td><td></td><td>78.6</td><td>49.2</td><td>-1.17 ± 0.01 n=2</td></tr><tr><td>NWA 8049</td><td>J.A. Barrat</td><td></td><td>84.3</td><td>34.5</td><td>-8.48 n=1</td></tr><tr><td>NWA 8172</td><td>J.A. Barrat</td><td></td><td>76.9</td><td>53.6</td><td>-1.15 ± 0.03 n=2</td></tr><tr><td>NWA 10900</td><td>J.A. Barrat</td><td></td><td>77.3</td><td>52.4</td><td>-0.31 ± 0.10 n=2</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td>-4.27 32 samples</td></tr><tr><td colspan="6"><strong>Standard</strong></td></tr><tr><td>LCgraphite</td><td>IPGP</td><td></td><td></td><td>this work</td><td>-18.41 2 σ = 0.19, n=6</td></tr><tr><td></td><td></td><td></td><td></td><td>IPGP</td><td>-18.42 reference value</td></tr></tbody></table>

Source abbreviations are: IPGP= institut de Physique du Globe de Paris, MWG= NASA meteorite working group, NIPR= National Institute of Polar Research, Tokyo, UBO= Université de Bretagne Occidentale, Brest.

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<figure><img src="image_2.png" /></figure>

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Fig. 1. Reflected light view of C grains (here graphite) in NWA 8049, a typical unrecciated ureilite (G: graphite; Ol: olivine).
<figure><img src="image_3.png" /><figcaption>Figure 2. δ¹³C vs. olivine core compositions (Mg#) from ureilites. Data from this study and the literature.</figcaption></figure>

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<figure><img src="image_4.png" /><figcaption>Figure 3. δ¹⁸O, Δ¹⁷O, δ¹³C, Fe/Mn ratios in olivine cores vs. olivine core compositions (Mg#) from unrecciated ureilites. Oxygen isotopic compositions from Clayton and Mayeda (1996). See Table 1 for the sources of other data. Mixing lines and curves between A and B, the two endmembers, are drawn in red. The errors are equivalent to the size of the data points.</figcaption></figure>

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<figure><img src="image_5.png" /><figcaption>Figure 4. Plot of δ¹³C vs. Fe/Mn ratios in olivine cores from unreCCIated ureilites (see Table 1 for the source of the data). The errors are equivalent to the size of the data points.</figcaption></figure>

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<figure><img src="image_6.png" /></figure>

Figure 5. Volatile abundances of Haverö and ALH 82130 (Grady et al., 1985; Grady and Pillinger, 1986; Rai et al., 2003a and b), two representative ureilites, compared to the bulk Earth composition, normalized to carbonaceous chondrite (Marty, 2012).
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Keywords: carbon isotopes, ureilite, planetary differentiation, volatiles

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