Aluminous enstatites of lunar meteorites and deep-seated lunar rocks |
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Authors: | M A Nazarov L Ya Aranovich S I Demidova T Ntaflos F Brandstätter |
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Institution: | 1.Vernadsky Institute of Geochemistry and Analytical Chemistry,Russian Academy of Sciences,Moscow,Russia;2.Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM),Russian Academy of Sciences,Moscow,Russia;3.Departament für Lithosph?renforschung,Universit?t Wien,Wien,?sterreich;4.Naturhistorisches Museum,Wien,?sterreich |
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Abstract: | Fragments of aluminous enstatite from lunar meteorites of highland origin were investigated. It was found that such fragments
usually occur in impact breccias of troctolitic composition. The aluminous enstatite contains up to 12 wt % Al2O3 and shows low CaO (<1 wt %) and almost constant high Mg/(Mg + Fe) ratio (89.5 ± 1.4 at %) identical to that of the Earth’s
mantle. With respect to these parameters, the aluminous enstatites are distinctly different from common orthopyroxene of lunar
rocks. The aluminous enstatite associates with spinel (pleonaste), olivine, anorthite (clinopyroxene was never found), and
accessory minerals: rutile, Ti-Zr oxides, troilite, and Fe-Ni metal. The same assemblage was described in rare fragments of
spinel cataclasites from the samples of the Apollo missions. Thermobarometry and the analysis of phase equilibria showed that
the rocks hosting aluminous enstatite are of deep origin and occurred at depths from 25 km to 130–200 km at T from 800 to 1300°C, i.e., at least in the lower crust and, possibly, in the upper mantle of the Moon. These rocks could form
individual plutons or dominate the composition of the lower crust. The most probable source of aluminous enstatite is troctolitic
magnesian rocks and, especially, spinel troctolites with low Ca/Al and Ca/Si ratios. The decompression of such rocks must
produce cordierite-bearing assemblages. The almost complete absence of such assemblages in the surficial rocks of lunar highlands
implies that vertical tectonic movements were practically absent in the lunar crust. The transport of deep-seated materials
to the lunar surface was probably related to impact events during the intense meteorite bombardments >3.9 Ga ago. |
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