Silicate Liquid Immiscibility in Tholeiitic Basalts

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Abstract
Immiscible silicate liquids, preserved as fresh glasses, constitute at least 32 per cent of a tholeiitic basalt in the Triassic-Jurassic basin of Southbury, Connecticut. This apparently uncommon abundance of immiscible liquids cannot be attributed to an unusual magma composition, for the rock is similar to many quartz-normative tholeiites associated with other basins of similar age in eastern North America. Nor can peculiar conditions of fractional crystallization be involved, for experiments carried out at one atmosphere total pressure reveal that two silicate liquids coexist over a wide temperature interval during crystallization, and that this interval is increased by raising the oxygen fugacity. Experiments on the standard diabase, W-1, which differs only slightly in composition from the Connecticut basalt, indicates the presence of immiscible liquids during the crystallization of this magma composition as well. It is concluded that liquid immiscibility is to be expected during the crystallization of many tholeiitic magmas, and at a sufficiently early stage of solidification to play a role in igneous differentiation.

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