A ROAD TO RHYOLITE: FRACTIONAL CRYSALLIZATION EXPERIMENTS ON A CONTINENTAL OLIVINE THOLEIITE

Intraplate magmatic provinces, ocean islands for example, exhibit a wide variety of sub-alkalic rock types from basalt to rhyolite. Extrusive rocks found on ocean islands such as Thingmuli, Iceland, and Pinzon, Galapagos show similar major-element trends on variation diagrams, which have been attributed to simple fractional crystallization of a “primitive” olivine tholeiite (Spulber & Rutherford J. Pet, 1983). Experiments have been conducted to indicate the conditions under which a continental olivine tholeiite could reproduce the ocean island tholeiitic trend. A natural olivine tholeiite from the Snake River Plain (10.7 wt% MgO) was crystallized in Au80Pd20 capsules within evacuated silica glass tubes, with 8 to 10% excess metallic iron to compensate for observed Fe loss to capsule walls. All powder was first melted above its liquidus (1280ºC) and subsequently cooled to the crystallization temperature of interest, where it remained for 24-72 hours. The composition of the residual liquid at ~75% crystallization was then synthesized from oxides and gels. This allowed for simulation of incremental fractionation. The high temperature assemblage is dominated by olivine, plagioclase and trace amounts of chromite. At lower temperatures, augite joins the assemblage and begins to control the liquid line-of-descent. There is an increase in silica, as the glass compositions evolve towards the rhyolite field. All major element trends closely match that of the natural rocks found in ocean islands. While these experiments reinforce the hypothesis that silica-oversaturated rocks found in ocean island regimes are a product of extensive fractionation of a basalt, they also demonstrate that a typical continental flood basalt could produce rocks typical of ocean islands. This suggests that the ocean island tholeiitic trend is not sensitive to geographic province, but primarily results from the shallow level at which fractionation takes place.