Carbonate Formation Timescales Vary between Cm1 Chondrites Alh84051 and Alh84034

Introduction: Carbonaceous chondrites ALH84051 and ALH84034 rank amongst the most aqueously altered meteorites within the CM group and have been classified as petrologic type 1s. Nearly all primary anhydrous minerals have been replaced by secondary alteration phases, yet these meteorites still possess primitive whole-rock compositions [1,2]. Although elemental transport does not appear to have occurred on the large-scale during the alteration process, local mass transport was clearly an important process. In particular, the formation of carbonates, which are believed to have precipitated from aqueous fluids, occur extensively throughout CM1 chondrites. These carbonate phases, primarily dolomite and calcite, are important secondary minerals that preserve a record of the conditions and timing of their formation. Here we report new data on the petrography and short-lived radionuclide Mn-Cr systematics of carbonates obtained by nanoSIMS in the CM1 chondrite ALH84051 in a continued effort to constrain the alteration processes on the CM1 parent body. We also compare these data with our previous study of CM1 ALH84034. Petrography: Carbonates in both sections were primarily dolomite, with rarer calcite. Carbonates in both samples are morphologically similar with small angular to subrounded 20-50 μm individual grains interspersed throughout the meteorite and rare large (up to 300 μm diameter) carbonate agglomerates. ALH84034 carbonates were distributed randomly, though in some areas higher abundances of carbonate grains occur. The distribution of carbonates in ALH84051 is generally similar to ALH84034. However, the thin section of ALH84051 is particularly notable because it contains distinct veins of granular dolomite crystals which crosscut ~3/4 of the width of the sample. We have not observed a feature of this kind previously in any other CM chondrite. The veins, at least in the scope of a two-dimensional thin section, are not contiguous and were interrupted by phyllosilicate matrix grains of varying size and composition. Previous studies have shown that carbonate grains in ALH84034 exhibit complex zoning when imaged using cathodluminescence (CL) [3]. We examined zoning in multiple carbonate grains on JEOL 5800 SEM in using an Oxford Instruments CL detector. Following CL imaging, compositional zoning in individual carbonate grains was measured via electron probe analysis (EPMA) for Ca, Mg, Mn, Fe, and Sr composition. ALH84051 dolomites possessed similar zoning, with Mn ranging from 0.6 to 1.4 wt% and Fe 2.1-7.2 wt%.