| Summary: | We present petrologic and Secondary Ion Mass Spectrometry (SIMS) oxygen isotope analyses of Ca-carbonate within a group of paired Antarctic CM2 chondrites. The carbonates can be grouped into two isotopically and morphologically distinct populations. Type 1 grains (small matrix grains) possess average δ^(18)O of 33.7 ± 2.3‰ (1σ) and average Δ^(17)O of 0.81‰ ± 0.90‰ (1σ). Type 2 grains (calcite aggregates) possess distinct oxygen isotopic compositions, average δ^(18)O of 19.4‰ ± 1.5‰(1σ) and average Δ^(17)O of 1.98 ± 0.9‰(1σ). These differences are interpreted to indicate that the two populations of calcite formed under different conditions at different times. The carbonates have textural features that suggest an extraterrestrial origin. The data presented here fall within error of a previously measured array for carbonates from CM falls (Benedix et al., 2003). The presence of two generations of carbonate suggests carbonate formation in two discrete events on the parent body of these meteorites. The oxygen isotopic data presented here deviate from prior bulk carbonate measurements undertaken for these meteorites. Most likely, this deviation is because bulk carbonate analyses included vein carbonate which formed during terrestrial weathering. © 2011 Elsevier Ltd. Received 8 November 2010; accepted in revised form 21 September 2011; available online 10 October 2011. We thank A.N. Krot (AE), L. Bonal, and one anonymous reviewer for their careful edits. Phillip M. Piccoli, Richard J. Walker, Penny King, Paul Niles, Adrian Brearley, and Alan J. Kaufman all contributed ideas, expertise, and/or their red pen to this work. This work was supported by NASA Cosmochemistry Grants NAG5- 11979 and NNX09AF72G, NSF Grant EAR01-15488, a University of Maryland GRB award, and the New Mexico Space Grant.
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