Crystal-mush compaction in the Cohassett flood-basalt flow, Hanford, Washington

Abstract

The 40-m-thick central part of the Cohassett flow, which was probably emplaced by inflation of an earlier more Ti-rich flow, exhibits textural evidence of crystal-mush compaction during solidification. Although most of the crystallization started in the roof zone as water circulated through fractures in the crust, plumes of dense crystal mush must have dripped from the roof to the floor of the sheet, thus displacing upward the colonnade/entablature boundary. The mush initially consisted of loosely stacked plagioclase crystals and liquid surrounding rapidly grown pyroxene oikocrysts containing radial chadacrysts of plagioclase. Upon compaction in the floor zone, the liquid between the loosely stacked plagioclase crystals was expelled as the crystals packed tightly together to form sub-parallel grains wrapped around the pyroxene oikocrysts, which recrystallized into granular aggregates of augite and pigeonite. Stacks of sinking plagioclase crystals and rising liquid accumulated on the upper and lower sides, respectively, of horizontal plagioclase phenocrysts. This asymmetric texture clearly indicates the relative motion of solids and liquid. The degree of compaction, as determined from variations in the amount of interstitial liquid and quantitative measures of textural anisotropy, accounts for the chemical variation through the central part of the flow, with compatible elements being concentrated in the lower part of the flow and incompatible elements concentrated in the upper part. Modeling of the compaction process using the IRIDIUM melt infiltration and reaction program of [Boudreau, A., 2003. IRIDIUM: a program to model reaction of silicate liquid infiltrating a porous solid assemblage. Computers and Geosciences 29, 423–429] shows that the observed chemical and compaction/dilation profiles are to be expected in a 40-m-thick sheet of basaltic magma solidifying on the Earth's surface. Compaction of crystal mush should be even more likely in slowly cooled intrusive bodies.