ORIGIN OF ALKALIC ROCKS FROM HUALALAI VOLCANO, HAWAII

At least two different processes in the mantle can produce alkalic magmas. These are (1) low degrees of melting of peridotite source rocks in the mantle or (2) differentiation of tholeiitic magmas at high pressures. In order to test which of these processes are responsible for the alkalic rocks of the Hualalai Volcano in Hawaii, we analyzed ten samples with known ages ranging from less than 200 years to more than 12,950 years.

Recasting bulk compositions of these rocks into normative olivine, diopside, nepheline, albite, anorthite and TiAl2(Mg Si2)-1, Fe(Al)-1, Fe(Mg)-1, Mn(Mg)-1 and K(Na)-1 exchange components using the algorithm of Sack et al., (1987) and then projecting the bulk compositions of the Hualalai rocks onto the nepheline-olivine-diopside base from the plagioclase apex shows that all compositions fall between 8-30 Kb and 1 bar cotectics. All bulk compositions plot closer to high-pressure cotectics and away from the 1 bar cotectic suggesting that the rocks originated under high-pressure conditions.

To constrain the temperature of the magmas of these rocks, we have retrieved the temperature data from a normative "olivine/(albite+anorthite)"-temperature plot using Takahashi and Kushiro (1983) data. Using Sack et al., (1987) "nepheline/(nepheline+olivine+diopside)"-temperature plot, we derived an equation relating temperature to pressure for multiply saturated magmas. Finally, we use our calculated temperatures to solve for pressures.

When plotted on an experimentally determined solidus and liquidus P-T diagram for peridotite (Takahashi et al., 1993), our points show that the alkalic rocks from the Hualalai Volcano plot very close to the solidus suggesting that they represent low degrees of melting of upper mantle rocks.