Stable isotopes in Allan Hills 84001: An ion microprobe study

Abstract

The temperature of formation of carbonates in ALH 84001 provide a test of the hypothesis that they contain features produced by ancient martian life [1], and contributes to our understanding of the global budgets of martian volatiles. We have determined the micrometer-scale distribution of O and C isotope ratios in carbonate concretions, enstatite, and SiO_2 in ALH 84001 by ion microprobe as a means of constraining the range of possible temperatures of fluid-rock interaction [2]. The principal results are: (1) Carbonate δ^(18)O is 9.5-20.6‰, SMOW, significantly expanding the range from bulk analysis. The magnesite rim is higher in δ^(18)O than Ca-rich (0.07 ≤ X_(ca) ≤ 0.13) cores. (2) Variations in δ^(18)O occur over length scales as small as 50 µm. (3) Interiors of fractured orthopyroxene that hosts carbonate are homogeneous to within ±1‰. (4) Secondary SiO_2 has a δ^(18)O of 20.4‰. These data are inconsistent with mutual O isotope equilibrium among carbonate and silicate minerals and indicate either fine-scale mineral-mineral equilibrium at a range of temperatures ≤ 300°C or a failure to attain mineral-mineral equilibrium. Mineralogical evidence for disequilibrium supports the latter interpretation. Our δ^(18)O values for carbonates have been confirmed by two studies [3], and extended to compositions of carbonate not found in our sample.