Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.341d&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 341
Other
2
Iron-Manganese, Ordinary Chondrites, Oxygen Isotopes, Redox, Volatile Elements
Scientific paper
If the Fe/Mn ratio of CI chondrites (~95) is an average for the solar system, then the variation of Fe/Mn ratios (50-150) among other chondritic meteorites reflects processing of the prechondritic material. The known, moderate volatility of Mn is often invoked to explain these variations [1], but CI normalized abundance data for O and C chondrites [2] reveals that although carbonaceous chondrites are indeed depleted in Mn, as reflected in their high Fe/Mn ratios, the ordinary chondrites are essentially undepleted. Relative to CI, however, the ordinary chondrites have variable Fe abundances and this is reflected in their bulk Fe/Mn (atom) ratios. The low Fe/Mn ratios of ordinary chondrites are, therefore, not a reflection of Mn enrichment relative to CI but of Fe depletion. In detail, ordinary chondrites also reveal evidence of processing prior to their assembly as O-chondrites. Figure 1 shows the variation of Fe, Mn, and Mg in ordinary chondrites relative to CI. The line shows the trace of iron loss or addition relative to Orgueil [2]. Both bulk and silicate fractions of OCs fall on or close to this line, confirming that they are related to the CI average by Fe loss. The order of increasing Fe loss (H-L-LL) reflects the general classification of these meteorites. However, the silicate-only fraction of the OC [3] shows the opposite trend (LL-L-H) with Fe loss, reflecting the metal/silicate fractionation present in the meteorites. The variation of bulk OC Fe/Mn reflects the removal of Fe metal from the OC precursors by a reduction reaction presumably of the type FeO(silicate) + CO(nebula) = Fe(metal) + C02 (gas). Clearly H-chondrites show little of this metal removal in bulk. L and LL chondrite precursors, however, have been progressively depleted in Fe resulting in lower bulk Fe/Mn ratios. In contrast, the metal rich H-chondrites have the most reduced silicates (lowest silicate Fe/Mn) although much of the metal fraction is still present in the meteorites. The LL chondrites contain the least evidence of in situ reduction, as their silicates have Fe/Mn ratios close to their bulk Fe/Mn. The reaction that reduced the FeO to metal was probably controlled by in situ carbon rather than nebular CO. Since the lowering of Fe/Mn in the bulk OCs reflects nebular reduction by CO, other evidence of that reduction should be sought. The progressive shift of the oxygen isotope ratios in sequence H-L-LL toward heavier oxygen probably reflects this reduction reaction with a gas reservoir enriched in heavy oxygen as suggested by [4]. The removal of Fe metal prior to the assembly of OC meteorites, therefore provides a link to the undifferentiated iron meteorites. The fractionation of the silicates in OCs relative to the bulk meteorites may reflect either in situ reaction of FeO with C in the chondritic host, or incomplete separation of metal formed by nebular reaction. Correlation of the Fe-Mn-Mg results for chondrules from UOCs with their oxygen isotope signature should provide a measure of these competing effects, although the effects of Mn volatility may obscure the data at the scale of a chondrule. References: [1] Ganapathy and Anders (1977) Proc. LPSC 8th. [2] Wasson and Kallemeyn (1988) Phil. Trans. R. Soc. A235. [3] Jarosewich (1990) Meteoritics, 25. [4] Clayton and Mayeda (1992) Ann. Rev. Earth Planet. Sci. Acknowledgment: NAG9-304
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