Collaborative Research: Ion Microprobe Analysis of O and C Isotope Ratios in ALH84001

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9713978 Valley 9714028 Eiler, Stolper Abstract This award supports a collaborative study of the micron-scale distribution of oxygen isotopes (18O/16O) and carbon isotopes (13C/12C) in carbonates, silicates, oxides and possibly reduced carbon-bearing phases in the "Martian" meteorite (SNC class of meteorites), ALH84001, using the ion microprobe. It is supported by the Office of Polar Programs, the Chemistry Division, and the MPS Office of Multidisciplinary Activities. This work will provide constraints on the thermal and fluid-infiltration history experienced by this rock and the possible biogenic origin of ALH84001 carbonate. This work will build on a previously completed pilot study comprised of over 500 analyses of 12 well characterized carbonate standards, 11 oxygen isotope analyses in ALH84001 silicates, and 9 oxygen isotope analyses coupled with 4 carbon isotope analyses in ALH84001 carbonate concretions. This pilot study demonstrated the presence of gradients in the oxygen isotope composition of more than 10 parts in 10,000 in ALH84001 carbonates over 10's of microns, showing that there is sufficient `signal' in the fine- scale stable isotope variations of these minerals for us to extract meaningful information from our proposed analyses. Further characterization of the oxygen isotope zonation and search for low carbon-13 phases will be critical to interpreting the environment of formation of these carbonate concretions. The heterogeneity already documented in this sample indicates that much of this work will only be possible using ion microprobe techniques. The research work to be completed under this award includes a large number of analyses of 10-30 micron spots (about 5-10 microns deep) in a variety of carbonate, silicate and iron bearing minerals to establish compositional profiles for oxygen and carbon isotopes. These new analyses will allow the following: (1) test concretions for core-to-rim oxygen zonation; (2) test and refine previo us estimates of the temperatures of formation of the various carbonate mineral phases; (3) investigate the fluid-infiltration history recorded by non-carbonate phases, and (4) verify initial observations that the outer portions of carbonate concretions may contain low carbon-13 compositions, which may be related to purported `biofilms' at concretion margins that have been inferred in meteorite ALH84001. These analyses will evaluate published reports that the fractionation of carbon isotopes between carbonate and associated reduced carbon is consistent with biological processes. These constraints are central to the interpretation of the origin of carbonates in ALH84001, both with regard to the possibility of their biogenic origin and the evidence they provide of the nature of volatiles on early Mars.