MINOR AND TRACE-ELEMENT CONCENTRATIONS IN CARBONATES OF CARBONACEOUS CHONDRITES, AND IMPLICATIONS FOR THE COMPOSITIONS OF COEXISTING FLUIDS

Abundances of Fe, Mg, Mn, Sr, Na, B, and Ba in meteoritic calcites and dolomites have been determined using the ion microprobe. The compositions of these carbonates are consistent with their precipitation from, or recrystallization in equilibrium with, aqueous solutions at low temperatures. Coexisting calcites and dolomites in CM chondrites are not in equilibrium and presumably formed in distinct events. Calculated molar element/Ca ratios for fluids in equilibrium with these carbonates suggest that dolomites in CM and Cl chondrites formed from compositionally similar brines. Calcite solutions were also saline but lower in Mg, Fe, and Mn; calculations suggest that these solutions became more like dolomite solutions with increasing alteration of the host CM meteorite, reflecting progressive evolution of solution compositions. Asteroid brines may have formed by membrane filtration after the formation of compacted phyllosilicates lowered permeability, by addition of components to fluids during phyllosilicate-forming reactions, or through low-pressure boiling. Occurrences of vein-filling calcite, followed by dolomite and later by sulfates, in carbonaceous chondrites represent the final stages of a protracted aqueous alteration sequence.