Petrologic Analysis of Basalts from the Hawaii Scientific Drilling Project

Phase 2 of Ibe Hawaii Scientific Drilling Project (HSDP-2) collecled approximately 3098 m of continuous samples that represent the most complete eruptive history of the Mauna Kea (MK) hot spot volcano.

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The goal of the HSDP is to use the temporal and geochemical evolution of MK to better understand the spatial geochemical structure of the underlying mantle plume. The purpose of this study is to determine if MK basalts from HSDP-2 define an olivine control line and if the whole-rock compositions represent the liquid from which the lavas crystallized.

35 tholeiitic and alkali basalts from the MK volcano were examined petrographically to quantify olivine phenocrysts abundance. Olivine phenocrysts range in size from 0.4 – 4.6 mm and vary in abundance from 5.3-50.3 vol%. HSDP-2 samples display a wide range of textures; crystals within each sample vary in size, shape, abundance, and degree of alteration.

When compared to bulk MgO, olivine phenocrysts define an olivine control line with and R2 value of 0.85. The magma crystallization model MELTS {Ghiorso et al., /995} was used to determine whether the composition of the olivine measured in selected HSDP-2 samples represents the equilibrium olivine expected for a given bulk composition (MgO). HSDP-2 samples with M#≥75 and forsterite (Fo) content (measured < modeled) contain accumulated olivine.

Thus, not all HSDP-2 lavas are representative of mantle compositions; select samples must be corrected for accumulated olivine. Samples with Mg#<75 whose olivine composition falls below the equilibrium Fo range probably reflect the lack of measuring the most Fo-rich phenocryst.

Although the compositional data set for HSDP-2 is limited, this study calculated similar accumulated olivine constraints (Mg#≤75) as a previous study on HSDP-I samples (Mg#≤76) {Baker et al.. 1996}. Using MELTS, modal and modeled estimates differ by less than 8.9 vol. %, giving confidence to our results.

Shelley D. Miller, University of Tennessee – Knoxville.

Terms: Basalt

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