Petrography and olivine and glass chemistry of lavas from the Hawaii Scientific Drilling Project

Abstract

Many of the lavas from the Hawaii Scientific Drilling Project (HSDP) are olivine‐rich (>10 vol %) and weakly altered. The Mauna Loa lavas from the upper part of the HSDP hole are more olivine‐rich and generally have olivines with higher forsterite contents than the underlying Mauna Kea lavas. Olivine‐rich lavas from these volcanoes contain both euhedral, undeformed phenocrysts and kink‐banded xenocrysts of olivines, unlike what was assumed for typical subaerial Hawaiian tholeiites. The forsterite content of both types of olivine ranges widely (80–90%). Many 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. The olivine xenocrysts in these lavas are inferred to be derived from disaggregation 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 core based on whole rock compositions. The moderately evolved compositions of the HSDP glasses indicate quenching temperatures similar to those measured during the current Kilauea eruption.