Silicate Liquid Immiscibility in Tholeiitic Basalts

Immiscible silicate liquids, preserved as fresh glasses, constituteat least 32 per cent of a tholeiitic basalt in the Triassic-Jurassicbasin of Southbury, Connecticut. This apparently uncommon abundanceof immiscible liquids cannot be attributed to an unusual magmacomposition, for the rock is similar to many quartz-normativetholeiites associated with other basins of similar age in easternNorth America. Nor can peculiar conditions of fractional crystallizationbe involved, for experiments carried out at one atmosphere totalpressure reveal that two silicate liquids coexist over a widetemperature interval during crystallization, and that this intervalis increased by raising the oxygen fugacity. Experiments onthe standard diabase, W-1, which differs only slightly in compositionfrom the Connecticut basalt, indicates the presence of immiscibleliquids during the crystallization of this magma compositionas well. It is concluded that liquid immiscibility is to beexpected during the crystallization of many tholeiitic magmas,and at a sufficiently early stage of solidification to playa role in igneous differentiation.