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Individual flows in the Grande Ronde Basalt are chemically homogeneous on a megascopic scale, but are heterogeneous on a microscopic scale. Small-scale heterogeneity is manifested by wide ranges in mineral and glass compositions. Compared to the composition of the bulk rock (mg# ≈ 0.43), the compositions of minerals (e.g., augite = En76 to En0) and glass (53 to 78 wt % SiO2) indicate differentiation as extensive as that in the Skaergaard intrusion. However, large-scale crystal segregations within the flow are not observed. It is interpreted that the small-scale heterogeneity resulted from post-eruptive micro-local differentiation during the course of crystallization.

This style of differentiation is referred to as quench fractionation because the erupted liquid was fractionated by the removal of quench crystals that form the groundmass. Although some heterogeneity in glass composition can be attributed to compositional regimes adjacent to crystal-liquid interfaces, quench fractionation differentiated much of the liquid in a pseudo-Rayleigh fashion that was rapid and slightly less efficient. Most glasses have compositions indicating the removal of minerals in mass proportions comparable to those of the groundmass. Glass compositions are also consistent with the liquid line of descent along the crest of the two-liquid solvus. Liquid immiscibility was an attendant, but subordinate, post-eruptive fractionation process involving liquids that locally encountered conditions of metastable or stable immiscibility.

A model for quench fractionation was developed using modified mixing calculations to determine the distribution of elements in the Cohassett basalt. The final distribution of Fe2+ constrains controls on the oxidation-reduction state of aqueous solution in basalt–ground water interactions. Results of this model indicate that 86 to 90 percent of the Fe2+ in the basalt resides in groundmass minerals (30 to 40 percent in Fe-Ti oxides, and >50 percent in groundmass pyroxene), and that glass in entablature and colonnade parts of the flow contains <14 and <4 percent of the Fe2+, respectively. It is indicated that reducing capacity of basalt during basalt–ground water interaction is initially dominated by reactions involving pyroxene and Fe-Ti oxides rather than glass. These reactions would be enhanced under conditions conducive to glass dissolution because removal of mesostasis glass would expose minerals in the part of the rock where Fe2+ in mafic minerals is most highly concentrated and where their surface areas (per mass) are largest.

The quench fractionation model explains unusual compositional features in Grande Ronde Basalt, many of which are also observed in tholeiitic basalts throughout the world and on the moon. Because silicate liquid compositions can be strongly affected by even small amounts of quench fractionation, the process may also be important in evaluating the origins of most types of basic lavas.

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