Abstract

The McKinney Basalt, a composite pahoehoe and pillow lava unit in the Snake River Plain, has been studied by petrographic, chemical, strontium-isotopic, and experimental methods in an attempt to define its origin and evolutionary history. Differences in the compositions of olivine and plagioclase phenocryst cores and in the whole-rock composition of different samples are interpreted as evidence that McKinney Basalt erupted as sequential surges of lava that represent different degrees of fractional crystallization of an olivine tholeiite parental magma. Olivine and plagioclase geothermometry and experimental studies indicate that McKinney pillow lava erupted at a temperature of about 1190° to 1200°C. Compositions of iron-titanium oxides and plagioclase/glass europium partition coefficients indicate that as McKinney lava cooled, oxygen fugacity was buffered near the quartz-fayalite-magnetite buffer.

Experimental studies and chemical compositions of McKinney samples suggest that the parental magma underwent significant crystallization only at relatively low pressures (<8 kb). Theoretical calculations involving silica and alumina activities suggest that McKinney parental magma was formed likely by partial fusion of spinel peridotite or aluminous pyroxene peridotite mantle at depths of about 50 to 60 km. Under anhydrous conditions, melting would have occurred at this depth at a minimum temperature of about 1300°C. Under water-undersaturated conditions, melting would have occurred at a significantly lower temperature, whose value would depend on the water content of the magma.

Strontium isotopic data suggest that McKinney Basalt, like all other Snake River olivine tholeiites, was derived from a mantle region that is more radiogenic than typical suboceanic mantle.

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