Carbon isotopic ratios of calcite grains in the LAP 031166 CM chondrite: Implications for possible
link between CM and cometary ices.
Wataru Fujiya1, Kohei Fukuda2, Paula Lindgren3, Martin R. Lee4, Mizuho Koike5, and Yuji Sano5
1
Ibaraki University, 2-1-1 Mito, Bunkyo, Ibaraki 310-8512, JAPAN. (wateru.fujiya.sci@vc.ibaraki.ac.jp),
2
University of Tokyo, 3Lund University, 4University of Glasgow, 5Atmosphere and Ocean Research Institute,
University of Tokyo.
Introduction: Carbonate minerals are ubiquitous in CM chondrites, which formed by aqueous alteration in the CM chondrite
parent body. Previous studies have shown that C isotopic ratios of CM carbonates are highly variable at whole-rock scales aswell as among individual grains in single meteorites [e.g., 1-3]. Despite extensive studies on C isotopic ratios of CM carbonates,
the reason for variability in 13C variation and the origin of the carbonate C remain poorly understood.
Here we report C isotopic ratios of calcite (CaCO3) in the LAP 031166 CM chondrite (CM 2.1), and discuss the origin of thecarbonate C and a possible link between ices in CMs and in comets. The O isotopic ratios of calcite in this meteorite werereported by Lindgren et al. [4].
Experimental: Carbon-isotope analysis was performed with the NanoSIMS 50 installed at Atmosphere and Ocean ResearchInstitute, the University of Tokyo, on the calcite grains whose O isotopic ratios were measured previously. Negative secondary
ions of 12C-, 13C-, 18O-, 12C14N-, and 28Si- produced by rastering a Cs+ primary ion beam (20-30 pA, ~1 m in diameter) over 6 x
6 m2 sized areas were detected simultaneously with five electron multipliers.
Results and discussion: The 13CPDB and 18OSMOW values of the calcite grains in LAP 031166 are shown in Fig. 1, together
with literature data of Ca-carbonate (calcite and aragonite) in the Murchison, Nogoya, and Paris CM chondrites [3,5,6]. The 13Cand 18O values of the CM carbonates are highly variable, ranging from ~20 to 80‰ and from ~15 to 40‰, respectively, andthey do not correlate with each other.
Alexander et al. [1] have suggested that the range in 13C can be explained by variable formation temperatures (0-130 oC) of CMcarbonates which are isotopically equilibrated with gaseous C species dominated by CO (or CH4). However, this model implies
that the 13C and 18O values of the CM carbonates would correlate, which is clearly not the case as demonstrated by this work
(Fig. 1). Instead, the observed 13C range must be produced under nearly constant temperatures.
Here we propose a modified version of the model outlined by Alexander et al. [1], where we consider mixing of CO2 and CO
gases with higher and lower 13C values, respectively. Because the equilibrium CO/CO2 ratios are <<1 for fO2 higher than theiron-magnetite buffer, CO should be converted to CO2 and the CO/CO2 ratios of gas phases would have decreased with time. Asthe CO/CO2 ratios decreased, CO2 as well as carbonates acquired lower 13C values. Given 1000ln(calcite-CO2) of ~10-14 at
0-30 oC [7], where  is a C isotopic fractionation factor, the CM carbonates with the highest 13C values of ~80‰ wereequilibrated with CO2 with 13C ~ 70‰. The 13C value of the trapped CO in Murchison is ~-30‰ [8]. The lowest 13C valuesof the CM carbonates are ~20-30‰, which is common for all the CM chondrites analyzed (Fig. 1). Therefore, the gaseous Cspecies which coexisted with the CM carbonates with the lowest 13C values of ~20-30‰ would have been dominated by CO2,i.e., most of CO was converted to CO2. If correct, the 13C value of the bulk gaseous C species (CO2 + CO) would be ~10‰.
Using the above numbers, mass balance calculation suggests that a CO/CO2 mole ratio of the gaseous C species which coexistedwith the CM carbonates with the highest 13C value of ~80‰ would be ~1.5, which may reflect the CO/CO2 ratio of ice accreted
onto the CM parent body. The carbonate C/H2O mole ratios of most CMs range from ~0.01 to 0.1 [1]. These carbonate C/H2O
ratios may set lower limits on the CO2/H2O ratio of the CM ices because of possible CO2 loss. Thus, CO2 accounts for at least
1 % of the CM ices.
The inferred CO/CO2 and CO2/H2O mole ratios of the CM ices are shown in Fig. 2, together with those of cometary ices [9].
Unfortunately, only upper limits on the CO/CO2 ratios are obtained for most of the observed comets. Nevertheless, the CO/CO2
ratios of these comets seem lower than unity, and thus, lower than those of the CM ices. Although the possible similarity between
ices in CMs and in some comets cannot be ruled out, these observations suggest that ices in the two classes of bodies had adifferent origin, which is consistent with contrasts in the D/H ratios of water between CMs and comets [10].
References: [1] Alexander C. M. O’D. et al. 2015. Meteorit. Planet. Sci. 50:810. [2] Tyra M. A. et al. 2012. Geochim.
Cosmochim. Acta 77:383. [3] Fujiya W. et al. 2015. Geochim. Cosmochim. Acta 161:101. [4] Lindgren P. et al. 2017. Geochim.
Cosmochim. Acta 204:240. [5] Fujiya W. et al. 2016. Abstract #1712. 47th LPSC. [6] Vacher L. G. et al. 2017. Geochim.
Cosmochim. Acta 213:271. [7] Chacko T. et al. 2001. Rev. Mineral. Geochem. 43.1. [8] Yuen G. et al. 1984. Nature 307:252.
[9] Ootsubo T. et al. 2012. Astrophys. J. 752:15. [10] Alexander C. M. O’D. et al. 2012. Science 337:721

Fig. 1. 13C and 18O values of Ca-carbonates in CM
chondrites. Errors are 2.

Fig. 2. CO/CO2 and CO2/H2O mole ratios of theinferred CM ices and comets. Data with arrows
represent upper limits. JFCs: Jupiter-family comets,
OCs: Oort cloud comets. Only comets which wereobserved within 2.5 AU from the Sun are shown,
where H2O effectively sublimes from the nucleus ofthe comet.