Primary magmas of the high-magnesia basalts of Mauna Kea Volcano, Hawaii: An experimental study

The primary magma crystallization of the tholeiitic and alkaline series of Mauna Kea Volcano were experimentally studied. The experiments were conducted at pressures of 30-50 kbar and temperatures of 1450-1650degreesC on a belt-type apparatus. The character of the near-liquidus association depended on pressure and melt composition. Crystallization of olivine and clinopyroxene at 30 kbar and clinopyroxene and garnet at 40-50 kbar was observed in the tholeiitic system. Similarly, only clinopyroxene and olivine were stable near the liquidus of the alkaline melt at 30 kbar. However, olivine occurred also in experiments at 40-50 kbar coexisting with clinopyroxene and garnet. Orthopyroxene appeared only at rather high degrees of crystallization. The partition coefficients of Na and Ti between clinopyroxene and the melt change as pressure increases from 30 to 50 kbar: Na2OCpx/(NaOLiq)-O-2 increases from 0.4-0.5 to 0.8, while TiO2Cpx/TiO2Liq decreases from 0.3 to 0.15-0.2. The ratio TiO2/Na2O of melt can be used as an indicator of the pressure of magma generation. The high TiO2/Na2O values of the Hawaiian magmas suggest melt generation at pressures higher than 50 kbar and low degrees of melting. Under such conditions, the melt composition must be strongly undersaturated and similar to the primary magmas of the alkaline series. The formation of tholeiitic primary magmas is related to interaction of an initial alkaline melt with depleted harzburgites at pressures lower than 30 kbar.