CAN FRACTIONATION OF AN OLIVINE THOLEIITE GIVE RISE TO FERRODIORITES, FERROBASALTS, AND ANORTHOSITES?

Previous experimental investigations have determined that under specific conditions of temperature, pressure, and bulk water content, fractional crystallization can account for the development of the silica-saturated potassic alkalic series of rocks commonly found in anorthosite/potassic granite complexes and in volcanics from the Craters of the Moon. These previous studies have used parental materials that were least-evolved members of these suites (e.g. a Hi-Al gabbro from the Laramie Anorthosite Complex, and a tholeiite (8 wt.% MgO) from the Snake River Plain). Recent experiments have tested whether the Si-depletion, Fe-enrichment trend exhibited in this series of rocks could be replicated under the same conditions of temperature, pressure, and bulk water content, but using a less evolved parental liquid. This possibility has been experimentally explored using an olivine tholeiite (10.7 wt.% MgO) from the Snake River Plain as the possible parental material.

Piston-cylinder experiments were conducted at 4.3, 9.3, and 12.3 kbar in graphite capsules on virtually anhydrous (<0.10 wt.% H₂O) olivine tholeiite parent material. While the liquid trends at each of these pressures show enrichment in Fe, Ti and P, liquids produced at 4.3 and 12.3 kbar only show mild Si-depletion. Experiments carried out at 9.3 kbar, however, show a markedly greater Si-depletion trend, along with strong Fe-Ti-P enrichment. The high-aluminum gabbroic liquids generally believed to be parental to anorthosites are also generated along this crystallization trend. These experimental data indicate that the typical Si-depletion and Fe-Ti-P enrichment observed in the Snake River Plain volcanics and the mafic units commonly found in anorthosite complexes can be produced by fractionation of an olivine tholeiite parent material with low bulk water content at 9.3 kbar. Several key experiments are being repeated in Au₈₀Pd₂₀ capsules to separate the chemical effect of carbon (graphite capsules) from the mechanical effect of the elevated pressures on the Fe-enrichment trend. Experiments at these three pressures with varying bulk water contents are being performed to determine whether fractionation of this olivine tholeiite parental material can also give liquids of the potassic granitic compositions similar to those found in this series.