CONSTRAINING THE DEPTHS AT WHICH AN AZEOTROPE CONTROLS THE COMPOSITIONAL EVOLUTION OF CALCIC PLAGIOCLASE

Retention of the pseudo-azeotrope found in the pure plagioclase system only at high pressures to pressures below 1 GPa upon the addition of olivine melt components (Nekvasil et al., 2015 GRL) has important implications for the formation of anorthite in the lunar magma ocean, mid-ocean ridge basalts, and subduction zone magmas. Isobaric dynamic cooling experiments have been conducted in the olivine-plagioclase system in order to use plagioclase zoning trends to assess the pressure range over which the pseudoazeotrope exists and any change in its position in plagioclase compositional space within this pressure range. For these experiments, bulk compositions on the Ab side of the pseudo-azeotrope show normal zoning, while those on the An side show mild reverse zoning. Experimental results thus far suggest that at 0.35 GPa there is no evidence of the pseudoazeotrope at a bulk An/An+Ab ratio of 0.95 or below. However, at 0.5 GPa a pseudoazeotrope exists and lies at a bulk An/An+Ab ratio of 0.95. At 0.7 GPa, the pseudoazeotrope shifts to a bulk An/An+Ab ratio of 0.93 and at 1.0 GPa to a bulk An/An+Ab ratio of 0.90. This pressure dependence implies that anorthitic plagioclase in the lunar magma ocean crystallizing at 0.7 GPa should not show Ab-enrichment, yet anorthitic lunar magma that crystallized at shallow regions within the lunar crust would be expected to show normal Na-enrichment. These results additionally suggest that anorthite megacrysts in MORB and subduction zone magma that show mild reverse zoning(Lundstrom and Tepley, 2006 JVGR; Amma-Miyasaka and Nakagawa, 2002 JVGR) formed at depths >10km.