Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.259m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 259-259
Mathematics
Logic
Scientific paper
Subtle but systematic changes in the compositions and relative abundances of olivine, pyroxene, and metal with increasing petrologic type in equilibrated (types 4-6) H, L, and LL chondrites suggest that metallic Fe in these meteorites was oxidized during metamorphism. Observed changes include increases in the mean Fe contents of olivine and pyroxene and in the mean Ni and Co contents of bulk metal, as well as increases in the olivine:low-Ca pyroxene ratio with decreasing metal abundance. This evidence for oxidation is at variance with the commonly accepted idea that chondritic Fe was reduced by graphite during metamorphism (Ringwood, 1965; Williams, 1971; Brett and Sato, 1984; Rubin et al., 1988). We suggest that graphite activity was lowered by its dissolution in taenite at metamorphic temperatures, so that redox state was determined largely by equilibrium between ferromagnesian silicates and metal. Oxygen fugacities calculated from chondrite mineral equilibria are 2-3 log units below intrinsic fO(sub)2 measurements (Brett and Sato, 1984). The mineralogies of type 3 chondrites do not conform to the oxidation sequence seen in types 4-6 chondrites, and there is some evidence to suggest that Fe in unequilibrated chondrites was actually reduced during mild heating. Apparently, redox conditions in the surficial layers of parent bodies were reducing, but were oxidizing in the hotter interiors. Much of the current confusion over oxidation versus reduction is attributable to comparing unequilibrated and equilibrated chondrites. Progressive oxidation of Fe during metamorphism implies reaction with an oxidizing agent no longer present in the meteorites. We suggest that this oxidant was an aqueous vapor, derived from heating small amounts of ices originally accreted into the parent asteroids. The condensation of this vapor in cooler, outer layers of asteroids could account for aqueous alteration phases documented in some type 3 chondrites (Alexander et al., 1989). Assuming that the vapor was pure H2O, the water:rock weight ratio required to account for the observed Fe oxidation was very modest, less than 3o/oo. However, the presence of even minor amounts of a fluid attending metamorphism has important implications for the interpretation of chemical and isotopic patterns in ordinary chondrites. For example, this vapor may have acted as a carrier gas to facilitate redistribution of volatile trace elements during metamorphism (Sugiura et al., 1984). Open-system metamorphism may also explain oxygen isotopic patterns seen in the data of Clayton et al. (1991). This pattern could have resulted from varying exchange with an isotopically light vapor at different temperatures, or with a vapor whose isotopic composition evolved as it permeated through the parent bodies. References: Alexander C.M. et al. (1989) Geochim. Cosmochim. Acta 53, 3045-3057. Brett R. and Sato M. (1984) Geochim. Cosmochim. Acta 48, 111-120. Clayton R.N. et al. (1991) Geochim. Cosmochim. Acta 55, 2317-2337. Ringwood A.E. (1965) Nature 207, 701-704. Rubin A.E. et al. (1988) In Meteorites and the Early Solar System (eds. J.F. Kerridge and M.S. Matthews), pp. 488-511. Univ. Arizona Press, Tucson, AZ. Sugiura N. et al. (1984) J. Geophys. Res. 89, B641-644. Williams R.J. (1971) Geochim. Cosmochim. Acta 35, 407-411.
Labotka Theodore C.
McSween Harry Y. Jr.
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