Carbonaceous chondrites—II. Carbonaceous chondrite phyllosilicates and light element geochemistry as indicators of parent body processes and surface conditions |
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| Authors: | TE Bunch S Chang |
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| Institution: | Ames Research Center, NASA, Moffett Field, CA 94035, U.S.A. |
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| Abstract: | Petrographic observations and analyses of CM matrices are consistent with their origin as in situ low temperature (<400°K) aqueous alteration products in a parent body regolith. At least four different phyllosilicates were tentatively characterized in Murray and Murchison meteorites, in addition to Fe- and Mg-serpentines in Nogoya. In comparison with bulk meteorite compositions, all phyllosilicates and bulk matrices show enrichment of K relative to Na. Possible loss of Na and possibly some Cl, with addition of H2O and CO2 and water-soluble organic compounds during alteration, indicates a partially open system during alteration. Poorly characterized phases (PCP) are fine-grained (< 1 μm) admixtures of variable proportions of phyllosilicates, carbonaceous matter and opaque oxides of sulfur with high Fe, Ni and Cr contents. Calcite and some magnetite show paragenetic overlap with PCP and phyllosilicates. Carbonaceous matter is largely associated with PCP in altered CM matrices. In the unaltered CV Allende, carbonaceous matter is concentrated on olivine surfaces as a micromounded coating, particularly in the dark haloes that surround some chondrules and aggregates. Precursive alteration material may have been analogous to similarly coated olivine mixed with smaller amounts of metal and sulfides.Synthesis of the water soluble organic compounds found in CM matrices may have occurred prior to or in the same environment as did aqueous alteration of the precursive phases. Preservation or partial preservation of this organic matter may reflect the degree of overlap in episodes of synthesis and alteration.Nogoya is 95% altered and has a bulk carbon content of 5.2 wt%, which is higher than any meteorite. In addition, it has the lowest measured ratio of any other carbonaceous chondrite, except for Karoonda. |
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