PETROGENESIS OF ULTRAMAFIC XENOLITHS FROM THE 1800 KAUPULEHU FLOW, HUALALAI VOLCANO, HAWAII

The 1800 Kaupulehu flow on Hualalai Volcano, Hawaii, contains abundant xenoliths of dunite, wehrlite, and olivine clinopyroxenite with minor gabbro, troctolite, anorthosite, and websterite. The petrography and mineral compositions of 41 dunite, wehrlite, and olivine clinopyroxenite xenoliths have been studied, and clinopyroxene separates from eight of these have been analyzed for Ba, K, Rb, Sr, rare earth elements, Sr-87/Sr-86, and Nd-143/Nd-144. Temperatures of equilibration obtained by olivine-spinel and pyroxene geothermometry range from 1000 to 1200-degrees-C. Mineralogical data combined with published fluid inclusion data indicate depths of origin in the range of 8-30 km. The rarity of orthopyroxene, the presence of Fe-rich olivine (Fo81-89) and clinopyroxene (Fs5-12), and the occurrence of high TiO2 in spinel (0.9-2.8 wt.%) and clinopyroxene (0.35-1.33 wt.%) all indicate that the xenoliths are cumulates, not residues from partial fusion. The separated clinopyroxenes have Sr-87/Sr-86 (0.70348-0.70367) and Nd-143/Nd-144 (0.51293-0.51299) values that are different from Sr and Nd isotope ratios of Pacific abysall basalts (< 0.7032 and > 0.5130, respectively). Also, clinopyroxenes and spinels in the xenoliths have generally higher TiO2 contents (> 0.35 and > 0.91 wt.%, respectively) than their counterparts in abyssal cumulates (< 0.40 and < 0.70 wt.%, respectively). These differences indicate that the xenoliths are not a normal component of oceanic crust. Because the xenoliths and alkalic to transitional Hualalai lavas have similar values for Cr/(Cr + Al) and Cr/(Cr + Al + Fe3+) of spinels, Sr-87/Sr-86 of clinopyroxenes, and whole-rock He-3/He-4, we conclude that the xenoliths are cumulates from such magmas. Multiple parental magmas for the xenoliths are indicated by slightly heterogeneous Sr-87/Sr-86 of clinopyroxene separates. Depths of formation of the xenoliths are estimated to be approximately 8-30 km. Extensive crystallization of olivine in the absence of pyroxenes and plagioclase is a characteristic and prominent feature of Hawaiian tholeiitic magmatism. Dunite xenoliths crystallized from alkalic magmas have previously been reported from Mauna Kea Volcano (Atwill & Garcia, 1985) and Loihi Seamount (Clague, 1988). Our finding of an alkalic signature for dunite xenoliths from a third Hawaiian volcano, Hualalai, shows that early olivine crystallization should be considered a characteristic not just of Hawaiian tholeiitic magmatism but also of Hawaiian alkalic magmatism.