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Aqueous processes on parent bodies of chondritic meteorites Cosmochemical and mineralogical observations show that parent bodies of a common meteorite group, chondrites, have been subjected to pervasive aqueous alteration. Chondrites are clastic rocks formed from agglomeration of solar nebula particles and then altered in parent bodies (asteroids). Mineralogical, chemical, and isotopic investigations of most types of chondrites reveal signs of aqueous processes. Many newly-formed minerals, including oxidized phases (magnetite, Fe-rich olivine, serpentines, clay minerals, carbonates, sulfates, and phosphates), together with concentrations and oxidation states of trace elements in minerals, oxygen isotopic composition, and isotopic dating (I-Xe, Mn-Cr, U-Pb), indicate that these processes occurred in water-rich environments within ~10 million years after formation of the parent asteroids. It is widely accepted that water condensed as ice in the solar nebula was incorporated into the composition of parent bodies of chondrites, and then caused chemical alteration of chondritic components. Radioactive decay of short-lived radionuclides provided energy to melt ice shortly after formation of asteroids. Subsequent water-rock type reactions led to dissolution of minerals, formation of new solid phases, and separation of newly-formed gases into water-saturated pore space. In GEOPIG, major research interests are focused on oxidation-reduction processes driven by interaction of water with minerals and organic species, as well as by escape of gases into space. Our major method is theoretical thermodynamic analysis, which include calculation of equilibrium speciation and reaction-path modeling in water-rock-gas type multicomponent systems, and development of phase diagrams. Results of our modeling are compared with chemical, mineralogical and isotopic patters in chondrites. As a result, we can evaluate physical-chemical parameters that could have led to formation of mineral assemblages and organic compounds observed in meteorites. Ongoing research projects: 1. Generation of hydrogen during aqueous alteration on parent bodies of chondrites. We use thermodynamic calculations and published data on reaction kinetics to evaluate major pathways of hydrogen generation on parent bodies of chondrites. 2. Oxidation of parent bodies of chondrites driven by hydrogen escape. 3. Aqueous reduction processes on parent bodies of chondrites. 4. Transformation and synthesis of organic compounds during aqueous alteration on asteroids and outer solar system bodies. This work include thermodynamic modeling of stability of organic compounds at the parameters of aqueous alteration [Shock and Schulte 1990; Shock and McKinnon 1993; Schulte 1997; Schulte and Shock 2004]. Publications: Shock, E. L., and W. B. McKinnon (1993) Hydrothermal processing of cometary volatiles: Applications to Triton. Icarus 106, 464-477. Shock, E. L., and M. D. Schulte (1990) Amino-acid synthesis in carbonaceous meteorites by aqueous alteration of polycyclic aromatic hydrocarbons. Nature 343, 728-731. |