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Compositions of synthetic augite and hypersthene coexisting at 810°C: Application to pyroxenes from lunar highlands rocks

1974; American Geophysical Union; Volume: 1; Issue: 3 Linguagem: Inglês

10.1029/gl001i003p00134

1944-8007

D. H. Lindsley, H. E. King, A. C. Turnock,

Spacecraft and Cryogenic Technologies

Hydrothermal experiments on synthetic Ca‐Mg.‐Fe pyroxenes at 810°C and P fluid = P total = 15 kbar show that augites coexisting with orthopyroxene range in CaSiO 3 content from 43 ± 1 mole percent (Fe/Fe+Mg = 0.18) to 38±1 mole percent (Fe/Fe+Mg = 0.75). Experimentally determined distribution coefficients K D = (X Mg Hyp · X Fe Aug )/(X Fe Hyp · X Mg Aug ) are compatible with a constant value 0.690, but probably increase with increasing bulk Fe/Fe+Mg. If the CaSiO 3 content of augites is insensitive to total pressure, these data may be used to estimate temperatures of equilibration for coexisting natural pyroxenes that are low in “non‐quadrilateral” components such as Al, Ti, Mn, and Cr. In particular, many lunar metamorphic rocks appear to have equilibrated to temperatures of 800°C and lower. These results support the view that there has been profound thermal metamorphism of the lunar crust in the interval 4.6 to approximately 4.0 G.y.