Preliminary Nanosims Analysis of Carbon Isotope of Carbonates in Calcium-aluminum-

Introduction: Carbonate minerals observed in primitive meteorites are products of either terrestrial weathering or aqueous alteration in the early solar system. Most of the carbonate minerals in carbonaceous chondrites occur primarily as isolated grains in matrix, as crosscutting veins, or as replacement minerals in chondrules [e.g., 1, 2]. A few calcium-aluminum-rich inclusions (CAIs) have been reported containing carbonate minerals as well [2, 3]. The C and O isotopes of carbonates in carbonaceous chon-drites, mostly measured by stepwise extraction of bulk samples with phosphoric acid [4-7], are largely distinctive from those of terrestrial carbonates, whereas textural and petrographic evidence indicates that some carbonates in primitive meteorites are terrestrial in origin [2]. This study attempts to investigate from the aspect of C isotope the origin of rare carbonate minerals in some CAIs. If of extraterrestrial origin, carbonates in CAIs can provide important information and constraints on the ubiquitous aqueous alteration process in the early solar system. Samples and Analytical Techniques: We have selected car-bonate-bearing CAIs from Murchison (CM) and Leoville (CV) for this study. One of the CAIs from Murchison is the extensively studied Blue Angel – an altered hibonite inclusion containing large amount calcite (10-70 µm) [3]. The other Murchison CAI is a 100x50 µm calcite fragment enclosing small grains of spinel, melilite, fassite and perovskite. The calcite in the type B Leoville 3537-2 CAI occurs as narrow veins (less than ~ 10 µm) at the center of the inclusion. Carbon isotope was measured with the Caltech NanoSIMS 50L ion microprobe. A rastering (3x3 µm) primary beam of ~ 10 pA was used to sputter the sample and generate secondary ions. Both 12 C and 13 C were simultaneously collected with EMs. Carbonate standards (calcite, dolomite, mag-nesite, and siderite) were used to check possible matrix effect and instrumental mass fractionation (IMF). Typical analytical errors under such conditions are ~1-2‰ (1σ) for δ 13 C.