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SHERGOTTY METEORITE: MINERALOGY, PETROGRAPHY AND MINOR ELEMENTS
Author(s) -
Smith J.V.,
Hervig R.L.
Publication year - 1979
Publication title -
meteoritics
Language(s) - English
Resource type - Journals
eISSN - 1945-5100
pISSN - 0026-1114
DOI - 10.1111/j.1945-5100.1979.tb00486.x
Subject(s) - pigeonite , whitlockite , fayalite , geology , meteorite , geochemistry , mineral redox buffer , achondrite , augite , chondrule , allende meteorite , ilmenite , mineralogy , chondrite , plagioclase , olivine , astrobiology , mantle (geology) , paleontology , quartz , physics , apatite
The unusual achondrite Shergotty resembles terrestrial diabases, and textural and chemical evidence indicates pre‐settling and post‐settling crystallization of zoned augite (En 48 Fs 19 Wo 33 ‐En 25 Fs 47 Wo 28 ) and pigeonite (En 61 Fs 26 Wo 13 ‐En 21 Fs 61 Wo 18 ) coupled with late crystallization of plagioclase (Ab 43 An 56 /Or 1 ‐Ab 56 An 41 Or 3 : now shocked to maskelynite), titanomagnetite‐ilmenite composite grains, mesostasis (normative Qz 34 Ab 21 An 5 Or 38 Fs 2 , assuming Fe as ferrous), whitlockite, pyrrhotite (Fe 0.94 S), fayalite (Fo 10 ), baddeleyite and chlorapatite. The oxide compositions (Usp 62 Mt 38 , Al 2 O 3 2.4, Cr 2 O 3 0.8 wt %; Ilm 95 Hm 5 ) indicate ∼ 850 °C and log oxygen fugacity − 14, while the occurrence of fayalite rims on mesostasis next to ilmenite indicates 890 °C. Bearing in mind experimental uncertainties, these data are consistent with late‐stage crystallization under relatively high oxygen fugacity, as indicated by coexistence of fayalite, Ti‐magnetite and a silica glass. The high alkali content of the maskelynite and mesostasis, coupled with the redox state, indicates that the Shergotty meteorite resembles terrestrial basalts more than any other meteorites. Nevertheless the absence of H 2 O, as shown by the occurrence of phosphorus in whitlockite rather than in hydroxylapatite, distinguish the Shergotty achondrite from typical terrestrial diabases. Whereas the FeO/MnO ratios of pyroxenes from the Moon, Earth and several differentiated meteorites are independent of FeO, the ratio for Shergotty pyroxenes changes from 30 to 40 with increasing FeO, and the linear trend extrapolates to 0.2 MnO for zero iron. Hence caution is needed in using FeO/MnO as a planetary indicator. For pyroxenes, Na is almost independent of Fe/Mg while Ti increases and Cr decreases with increasing Fe/Mg. Maskelynite contains 0.5–0.25 wt % K 2 O, 0.6 wt % FeO, 0.04 TiO 2 , 0.04–0.07 MgO, ∼ 0.01 BaO and 0.02–0.03 P 2 O 5 . A bulk analysis calculated from the mode and compositions of the minerals matches quite well with two bulk chemical analyses but not with a third.