PETROGRAPHY AND OLIVINE AND GLASS CHEMISTRY OF LAVAS FROM THE HAWAII SCIENTIFIC DRILLING PROJECT

Many of the lavas from the Hawaii Scientific Drilling Project (HSDP) a re olivine-rich (>10 vol%) and weakly altered. The Mauna Loa lavas fro m the upper part of the HSDP hole are more olivine-rich and generally have olivines with higher forsterite contents than the underlying Maun a Kea lavas. Olivine-rich lavas from these volcanoes contain both euhe dral, undeformed phenocrysts and kink-banded xenocrysts of olivines, u nlike what was assumed for typical subaerial Hawaiian tholeiites. The forsterite content of both ty;pes of olivine ranges widely (80-90%). M any of the HSDP lavas have olivines with forsterite contents of 89-90% , indicating that they grew in magmas with at least 15 wt % MgO. Most of these lavas contain even higher MgO contents (18 to 28 wt %), which are a result of accumulation of olivine phenocrysts and xenocrysts. T he olivine xenocrysts in these lavas are inferred to be derived from d isaggregation of deformed dunite cumulates, which are present in many of these lavas. Glasses from pahoehoe crusts on some of the HSDP flows have major element compositions that confirm the subdivision of the c ore based on whole rock compositions. The moderately evolved compositi ons of the HSDP glasses indicate quenching temperatures similar to tho se measured during the current Kilauea eruption.