CAN FRACTIONATION OF AN OLIVINE THOLEIITE GIVE RISE TO POTASSIC RHYOLITES?

Experimental investigations carried out in a piston-cylinder apparatus, using graphite capsules to house the sample materials, have determined that under specific conditions of temperature, pressure, and bulk water content, fractional crystallization of a Hi-Al gabbro from the Laramie Anorthosite Complex (Rossier, in prep), and of a tholeiite (8 wt.% MgO) from the Snake River Plain (Litvin, Stony Brook M.S. Thesis, 2002) can produce the silica-saturated potassic alkalic series of rocks commonly found in anorthosite/potassic granite complexes and in volcanics from the Craters of the Moon. Fractionation of these starting materials with low bulk water contents (<0.10 wt.% H₂O) at 9.3 kbar reproduced the silica-depletion, Fe-enrichment segment of the trend exhibited in these natural rock suites; fractionation of these same materials at 4.3 kbar with relatively high bulk water content (0.8 wt.% H₂O) produced liquids of rhyolitic compositions consistent with the rhyolites associated with this trend. These experiments generated the two characteristic segments of the natural trend, but not the continuous path.

Ongoing experimental work is testing whether an even less evolved olivine tholeiite (10.7 wt.% MgO) from the Snake River Plain can also reproduce this distinctive trend under these same conditions of fractionation. Experiments have shown that fractionation of this starting material at 9.3 kbar, under nominally anhydrous (<0.10 wt.% H₂O) conditions, can reproduce the silica-depletion, Fe-enrichment segment of the silica-saturated potassic trend. We are now evaluating whether the rhyolites can be generated in two ways. First, by fractionation of this same olivine tholeiite with a high bulk water content (2 wt.% H₂O) at 4.3 kbar, as indicated by previous experiments. Second, by fractionation of a ferrobasaltic liquid with relatively high bulk water content (~1.75 wt.% H₂O) at 4.3 kbar. The ferrobasaltic composition used was itself generated from earlier fractionation of the olivine tholeiite at 9.3 kbar along the silica-depletion, Fe-enrichment segment of this trend, and may produce the continuous natural path from olivine tholeiite to rhyolite.