HIGH PRESSURE ORIGIN OF ALKALIC ROCKS FROM HALEAKALA VOLCANO, HAWAII

Nine alkalic rock samples from Haleakala Volcano, East Maui, Hawaii, ranging in composition from basalt to hawaiite to mugearite have been analyzed to test the hypothesis that alkalic rocks represent low degrees of melting of mantle rocks at high pressures. Four of these samples are from the post-erosional stage highly-alkaline historical flow (1790?) and five are from the older alkalic-stage Hana Volcanic Series (HVS). Bulk chemical analyses were determined using X-ray Fluorescence. These analyses were recast into normative minerals using the algorithm of Sack et al. (1987). When plotted on Sack et al.’s (1987) olivine-nepheline-high calcium pyroxene diagram, Haleakala rocks plot close to the high-pressure cotectic. This indicates that they have a deep-seated origin but additional calculations were made to confirm this finding.

Pressure-temperature (P-T) conditions of the Haleakala alkaline rocks were determined using the data of Takahashi et al. (1983) and Sack et al. (1987). Calculated P-T conditions for two of our historic flow samples yield 1417°C/ 21.7 kilobar and 1415°C/ 21.5 kilobar. The P-T conditions of the HVS rocks range from 1240°C/ 7.6 kilobar to 1353°C/ 14.7 kilobar. These results, as compared to the experimentally determined high-pressure mantle peridotite solidi (Kushiro, 1983; Takahashi et al., 1993), indicate that all of the alkalic rocks used in this study formed from less than 10 percent melting of mantle material and the alkalic rocks of the historic flow originate at a greater depth than the HVS rocks.