FRACTIONAL CRYSTALLIZATION EXPERIMENTS ON CONTINENTAL AND OCEAN ISLAND THOLEIITIC SUITES: THE EFFECTS OF PRESSURE AND WATER CONTENT

Stepwise fractional crystallization experiments at 4.3, 6.8 and 9.3 kbar on virtually anhydrous (~0.13 wt% H2O) Snake River Plain (SRP) tholeiite and a Craters of the Moon (COM) ferrobasalt in graphite capsules all produce a liquid path characterized by silica depletion and strong Fe-Ti and P enrichment. Higher pressures produce the more extreme compositions typical of continental ferrobasalt and jotunite (ferrodiorite). Similar experiments on a dried Galapagos tholeiite, exemplifying ocean island tholeiites, produced a trend similar to that of the continental tholeiites with silica depletion and strong Fe-Ti-P enrichment of the evolving liquids. Under such dry conditions at elevated pressure, the melts do not appear to evolve towards the intermediate compositions found in the continental tholeiitic suites. This elevated pressure trend appears highly sensitive to water content and pressure. In the pressure range 4.3 –9.3 kbar the COM sample with a bulk water content of ~1 wt%, produced liquids trending towards SiO2 and K2O enrichment. Key experiments are being repeated in Au80Pd20 capsules in an attempt to separate the effect of elevated pressures from the chemical effect of graphite on the Fe-enrichment trend. Zero kbar experiments on both the SRP basalt and a Galapagos tholeiite using Au80Pd20 capsules in evacuated silica-glass tubes also produced liquids showing a silica-enrichment trend. This latter trend, however, is similar to the ocean island tholeiitic trend toward sodic rhyolite.