Ruohan Gao

Anorthitic Plagioclase

High-An (100*Ca/Ca+Na) plagioclase xenocrysts and xenoliths have been observed at convergent and divergent margin settings. Calcic plagioclase is also the dominant mineral of the anorthositic lunar crust.

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The occurrence of high-An plagioclase in subduction zone lavas is commonly attributed to high melt H2O contents. In contrast, the origin of extremely high-An plagioclase at mid-ocean ridges and in lunar crust appears to be enigmatic, given that fresh MORB and lunar anorthosite have low H2O contents.

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I examine a suite of Pacific lower oceanic crust xenoliths that contain high-An plagioclase. The geochemical characteristics of these xenoliths provide constraints on the origin of high-An plagioclase in relatively anhydrous melts.

Gao, R., Lassiter, J. C., Clague, D. A. & Bohrson, W. A. (201X) “Origin of high-An plagioclase at mid-ocean ridges.” In preparation.