Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..347f&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 347
Mathematics
Logic
3
Achondrites, Acapulco-Like, Geochemistry, Lodran-Like, Petrology, Primitive, Lodran
Scientific paper
Primitive achondrites are meteorites that have mineral and bulk compositions similar to chondrites, but have non-chondritic textures. These achondrites were metamorphosed at high temperatures, perhaps up to that of the Fe-FeS eutectic or chondrite silicate solidus [1]. We have initiated geochemical and petrologic study of several Acapulco- and Lodran-like achondrites in order to test petrogenetic models based largely on petrologic arguments. We have studied the following meteorites: ALHA81187 and ALHA81261 (Acapulco-like), LEW88280 and MAC88177 (Lodran-like) and EET84302 (transitional) [1]. Of our Acapulco-like achondrites, we have finished petrologic characterization only on ALHA81187. Our thin section is distinct from that studied by [1] in that orthopyroxene is the dominant silicate, and we found no plagioclase. The low plagioclase content is like that of Lodran-like achondrites, but our INM data (below) suggest that the thin section is unrepresentative. LEW88280 and MAC88177 are medium-grained, granular rocks with metal and troilite occurring as inclusions in silicates, as thin veins cutting silicates and as discrete grains of intergrown kamacite and troilite. Our thin section of EET84302 is metal, sulfide, and chromite. Clinopyroxene and plagioclase occur in minor amounts. Orthopyroxene grains contain abundant metal inclusions in linear trains. The texture is similar to cumulate sulfide textures found in some terrestrial igneous rocks. Acapulco-like achondrites have been suggested to be high grade metamorphic rocks in which partial melting of Fe-FeS and phosphates occurred [2], although the melts may not have left the parent rock. Our and literature [3-6] INAA data generally agree with this interpretation: Sm/Sc ratios of Acapulco-like achondrites are between 0.8-2 times H chondrites, and Na/Sc ratios are between ~0.7-1.2 times H chondrites indicating that neither silicate nor "phosphate" partial melts were lost from the rocks. Siderophile and chalcophile elements are fractionated. Y-74063 has high Se/Co and low Ir/Ni ratios [6], while ALHA81187 has low Se/Co and high Ir/Ni ratios. This variation is consistent with either fractionation by partial melting in the Fe-Ni-S system, or with heterogeneous distribution of metal and troilite in these achondrites. This can be tested through additional analyses of the meteorites. If the samples of Y-74063 and ALHA81187 are representative of these achondrites, then the results suggest that mobilization of Fe-FeS eutectic melts occurred. The Lodran-like achondrites are believed to be partial melting residues [7]. The trace lithophile element data on Lodran [8], LEW88280, and MAC 88177 are compatible with this interpretation. Highly incompatible elements are depleted relative to more compatible elements such as Sc: Sm/Sc ratios are ~0.1-0.5 times, Na/Sc ratios are 0.05-0.1 time, and Eu/Sc ratios are 0.05-0.4 times H chondrites. EET84302 is classified as a Lodran-like achondrite, but is recognized as being transitional to the Acapulco-like achondrites [1]. Our INAA data show that EET84302 has not lost a silicate partial melt: ratios of Sm/Sc and Na/Sc are ~1 time H chondrites, and Eu/Sc is about 1.8 times H chondrites. In lithophile trace element contents, EET84302 is identical to the Acapulco-like achondrites. Our sample of EET84302 was metal-rich (~40% metal) and chromite-rich (~3% based on INAA and EMPA data), and in this regard is distinct from Acapulco-like achondrites. These modal differences will have no effect on lithophile element ratios, however. References: [1] McCoy et al. (1993) LPS XXIV, 945. [2] McCoy et al. (1992) Meteoritics, 27, 361. [3] Palme et al. (1981) GCA, 45, 727. [4] Schultz et al. (1982) EPSL, 61, 23. [5] Kallemeyn and Wasson (1985) GCA, 49, 261. [6] Kimura et al. (1992) Proc. NIPR Sym. Ant. Met., 5, 165. [7] Bild and Wasson (1976) Min. Mag., 40, 721. [8] Fukuoka et al. (1978) LPS IX, 356.
Field Stephen W.
Lindström Markus
Mittlefehldt Dave W.
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