Igneous Evolution of the Core and Mantle in the Parent Body of Group IVA Iron and Stony-Iron Meteorites

Details Igneous Evolution of the Core and Mantle in the Parent Body of Group IVA Iron and Stony-Iron Meteorites Igneous Evolution of the Core and Mantle in the Parent Body of Group IVA Iron and Stony-Iron Meteorites

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.287s&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 287

Mathematics

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Scientific paper

Group IVA is comprised of 52 irons lacking silicates, two with trace amounts of silica (Gibeon and Bishop Canyon) and two stony irons (Steinbach and Sao Joao Nepomuceno), which have high but varied proportions of a pyroxene-tridymite intergrowth (Prinz et al., 1984). Despite their remarkable composition, these stony irons are not geological freaks lacking cosmochemical significance but important clues to the complexity of asteroidal processes. Metal: Our crystallization models for Fe-Ni-S magmas using distribution coefficients from Jones and Malvin (1990) with minor modifications from Haack and Scott (1992) show that the IVA irons formed by fractional crystallization of a melt with about 1-3 wt% S. Unlike previous authors we are able to model the Ir, Au, Ga, Ge, and P vs Ni trends in IVA concurrently. We include the formation of a second S-rich immiscible liquid during crystallization and find that our models can match IVA trends as well as those of group IIIAB, which has a higher S content. Siderophile concentrations in Steinbach and Sao Joao metal are, surprisingly, entirely appropriate for IVA irons with 9.1 and 8.0% Ni, showing that they formed in two separate places in the IVA body by mixing of silicates with metal that was fractionally crystallizing. Silicates: Steinbach and Sao Joao contain 10-60 vol% of SiO2 and ortho- and clinobronzite. In the sponge-like silicate- metal intergrowths, typical pore sizes are 2-6 mm though metal crystals are larger. Textures suggest co-crystallization of silicates from a liquid. Slight compositional differences between the two pyroxenes exist in both meteorites, with Steinbach pyroxenes being more FeO-rich. Formation of all the pyroxene by reaction between olivine and SiO2 (Prinz et al., 1984) fails to account for the minor elemental abundances in pyroxene, e.g. 0.25 wt% Al2O3. But the occurrence of SiO2 without pyroxene suggests that the SiO2-pyroxene intergrowths did not form entirely from SiO2-pyroxene eutectic or peritectic liquids. Discussion: From the observed size and homogeneity of the Gibeon shower and our fractional crystallization modeling, we can infer that the parental liquid pool was at least many meters in size. Pools this big quickly sink through silicate (unless very reduced), so IVA metal very probably comes from a core. The wide range of metallographic cooling rates that is correlated with Ni concentration in IVA irons must therefore be an artefact. Liquids of pyroxene-silica compositions could be formed in the mantle (Prinz et al., 1984) but trapping them in the core at different stages of core crystallization seems very difficult. We prefer an origin for IVA stony irons by mixing olivine-pyroxene mantle material into the core during core solidification by processes like those that mixed olivine mantle into Fe,Ni cores to make pallasites, followed by addition of silica formed by oxidation of Si from the metal. Pieces of olivine-pyroxene, possibly in the form of a Brenham-like sponge, could be mixed into a crystallizing Fe,Ni core with about 2-4% S at temperatures around 1400 C. Pyroxene might be abundant in the mantle because small body size caused inefficient removal of trapped silicate liquid from an olivine cumulate. Alternatively, temperatures were never high enough to melt the mantle entirely. References: Haack H. and Scott E.R.D. (1992) Geochim. Cosmochim. Acta, submitted. Jones J.H. and Malvin (1990) Metall. Trans., 21B, 697-706. Prinz M., Nehru C.E., Delaney J.S., Fredriksson K., and Palme H. (1984) Meteoritics (abstract) 19, 291-292.

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