Evolution of Fluid Composition Inferred from Calcite inthe Yamato 791198 CM Chondrite.
Wataru Fujiya (wataru.fujiya.sci@vc.ibaraki.ac.jp)1, Yuto Aoki1, Takayuki Ushikubo2, and Ko Hashizume2.
1Faculty of Science, Ibaraki University, 2Kochi Institute for Core Sample Research, JAMSTEC.
Introduction:
Carbonate minerals are ubiquitous in CM chondrites and formed byaqueous alteration in the CM chondrite parent body [1]. Previous studies
have revealed a large variation in C isotope ratios of CM carbonates withd13C values ranging from 20 to 90‰ (Fig. 1), however, the reason for thisvariability has remained poorly understood [2-9].
To trace the change in C isotope ratios of carbonates with progressivealteration, here we conducted in-situ C- and O-isotope measurements by
Secondary Ion Mass Spectrometry (SIMS) within single carbonate grains in
Yamato 791198 (CM 2.4).

Oxygen isotopic compositions:
The O isotope ratios significantly vary within individual grains (Fig. 3). For
example, the grain in Fig. 1 has variable d18O values of 37‰ (A), 27‰ (B),and 25‰ (C). The O isotope ratios plot on a single trend line in an O threeisotope plot with a slope of 0.61, reflecting a progressive O-isotope exchange
between initially 16O-poor water and 16O-rich rock [10,11]. This d18O change
demonstrates that the crystal grew from a relatively dark CL core (A) to a red
CL periphery (C). Interestingly, Mg, Mn, and Fe abundances do not changemonotonically with the crystal growth (Fig. 4).

Fig.
3:
Oxygenisotopic
compositions of calcite grains inY791198. Data obtained for thesame grain are represented by
the same color.

Fig. 1: d13C and d18O variation of calcite
in the LAP 031166 CM chondrite [8].
Note that each data point represents
composition of one calcite grain.

Experimental:
We obtained Cathodoluminescence (CL) images of seven calcite (CaCO3)
grains with an FE-EPMA. Then, we measured O isotope ratios of the calcite
grains with the CAMECA ims-1280HR ion microprobe using a focused Cs+
primary ion beam of 30 pA and 3-4 mm in diameter. Next, we measured C
isotope ratios of the calcite grains using a focused Cs+ primary ion beam of120 pA, 6-8 mm in diameter. The C-isotope measurements were performed
just upon the points where the O isotope ratios were measured (if possible),or adjacent regions to those for the O-isotope measurements. Finally, we
obtained quantitative chemical compositions and X-ray maps of Mg, Si, Ca,Mn, and Fe of the calcite grains using the FE-EPMA.

Fig. 4: Minor element abundances of calcite grains in Y791198. Colors as Fig. 3.
Carbon isotopic compositions:
The d13C values of the calcite grains range from 17 to 32‰ (Fig. 5). Thisrange is much smaller than that observed for different carbonate grains orbulk meteorite samples by previous studies [2-9]. Especially, the d13C
variations within individual grains are only 4‰ at most except for one grain.

Results and discussion:
CL images and chemical compositions of calcite grains:
Fig. 2 shows representative calcite grains with a clear zoning of Mn, Mg,and Fe. This zoning is well correlated with CL images. The heterogeneous
distribution of minor elements and CL intensity likely reflects the crystal
growth of these grains.
BEI

Gray-scale CLCFig. 5: d13C and d18O
variation of calcite inY791198. Colors as Fig. 3.
Also shown are data fromprevious studies [5-8].

Color CLBA

20 mm
Mg

BEIMn

Gray-scale CLFeColor CL

20 mm
Mg

Here we propose that the d13C values of dissolved carbon species inaqueous solutions were locally heterogeneous, and did not significantly
change during carbonate formation. Fujiya et al. suggested that the variable
d13C values of CM carbonates are attributed to the mixing between two
carbon reservoirs with different C isotope ratios, namely, 13C-rich CO2 ice and13C-poor organic matter, with variable proportions [8]. Our new data areconsistent with this argument.

References：MnFeFig. 2: Representative calcite grains in Y791198.

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J. M. (2007) GCA, 71, 5565-5575. [4] Alexander C. M. O’D. et al. (2015)
Meteoritics & Planet. Sci., 50, 810-833. [5] Fujiya W. et al. (2015) GCA, 161,101-117. [6] Vacher L. G. et al. (2017) GCA, 213, 271-290. [7] Vacher L. G.
et al. (2018) GCA, 239, 213-234. [8] Fujiya W. et al. (2018) LPS XLIX,Abstract #1377. [9] Tyra M. et al. (2016) GCA, 175, 195-207. [10] Clayton R.
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