Phase equilibrium constraints on angrite petrogenesis

Publication Type  Journal Article
Year of Publication  1999
Authors  Longhi, J.
Journal Title  Geochimica Et Cosmochimica Acta
Volume  63
Issue  3-4
Pages  573-585
Journal Date  Feb
ISBN Number  0016-7037
Accession Number  ISI:000080964900020
Key Words  rare-earth elements; liquidus equilibria; low-pressure; lewis cliff-86010; solar nebula; system cmas; chondrites; crystallization; inclusions; meteorite

Parameterizations of liquidus boundaries and solid solution in the CMAS + Fe system (Shi, 1992) have been employed to depict the liquidus equilibria relevant to the petrogenesis of angrites, Angrites are basaltic achondrites characterized by highly aluminous augite (fassaite), intermediate Mg-Fe olivine, and late-stage CaFe-olivine (kirschsteinite). Two important features of the equilibria on the olivine liquidus surface relevant to angrite petrogenesis are: 1) the presence of a thermal divide on the of + aug + plag + liq boundary curve, which separates the compositions of source materials that produce low-silica angritic melts that crystallize highly aluminous augite from those that produce higher silica melts with tholeiitic to eucritic crystallization patterns; and 2) the change in the pseudo-invariant point on the low-silica side of the thermal divide from a plagioclase-peritectic involving spinel (ol + aug + plag + sp + liq) at high to intermediate Mg' (Mg/[Mg + Fe]) to two pseudo-eutectics involving kirschsteinite (ol + aug + plag + kir + liq and ol + kir + plag + sp + liq) at low Mg'.


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