PETROLOGICAL AND GEOCHEMICAL ANALYSIS OF XENOLITHS FROM THE HUALALAI VOLCANO CRATER

Ultramafic xenoliths were entrained in alkali basalt of the 1800 Kaupulehu Flow of the Hualalai Volcano, Hawaii. Xenoliths range from 2-3 cm to >20cm in size and are very common in the crater. These xenoliths are dunite, wehrlite, and clinopyroxenite, and to a lesser extent gabbro, troctolite, and websterite. I collected 12 samples and chose 3 for further study on the basis of macroscopic differences in color, fissility, and minerology. Preceding studies by Chen et al. (1991) and Bohrson, Clague (1988) determined the petrogenesis of these xenoliths was cumulate amalgamation. My aim was to learn the methods for studying ultramafic rocks and compare the results with the aforementioned findings.

Samples in this study represent the more abundant established mineralogies of xenoliths in this locality. Polished thin sections were examined using optical microscope, scanning electron microscope, and detailed chemical analyses obtained using an electron probe microanalyzer. The results were then compared to the existing geochemical and petrological data. Two of the samples are wehrlites and the remaining lherzolite. The clinopyroxene in all three samples straddle the pyroxene diopside-endiopside-augite sub-group junction while the orthopyroxene is bronzite.

Mantle rock and its melt residue is distinguishable from depleted crustal rock by higher magnesium and lower chromium amounts. The magnesium number (Mg/Fe²⁺+Mg)% can be obtained by the ratio of the magnesium-bearing Forsterite and the iron-bearing Fayalite in the solid solution of olivine. In spinel, trivalent chromium readily replaces aluminum. The chromium number (Cr³⁺/Cr³⁺+Al)% in spinels is the best indicator of the degree of depletion of mantle sources as it increases rapidly with the degree of melting. The resulting magnesium number from my analysis was 72-79 for olivine, 70-75 for clinopyroxene, and 72 for orthopyroxene. Whole rock average was 71. Typical mantle xenoliths have a magnesium number ranging from 87-92; my analyzed samples are on average >10% less than conventional values for mantle-derived olivines. These results confirm that the Hualalai xenoliths are cumulates.