Title

Generation of Forsteritic Olivine (Fo99.8) by Subsolidus Oxidation in Basaltic Flows

Abstract

We identify olivine grains with compositions up to Fo99·8, which are found in multiple primitive basaltic lava flows from a monogenetic volcano in the Big Pine Volcanic Field, California, USA. In this study, we show that the forsterite in these basalts formed by subsolidus recrystallization in a high-fO2 environment. Olivine compositions are bimodal, with flows having either all normal compositions (Fo74·9–94·4) or highly forsteritic (Fo97·2–99·8) compositions. In many grains, the subhedral forsteritic olivine has a hematite and clinopyroxene rim, and internal parallel-oriented planes of hematite, clinopyroxene and orthopyroxene. Results of isotopic, chemical, crystallographic, petrographic and mineralogical analyses show that the forsterite formed through subsolidus oxidation of olivine phenocrysts. The forsteritic olivines generally occur in the thinner flows. We infer that a rapidly emplaced sequence of thin, vesicular, spatter-fed flows allowed the original olivine phenocrysts to become repeatedly reheated while exposed to air. Our study required sampling each flow, so it was difficult to avoid the altered portions of the thinner flows. Other studies would tend to avoid such flows, which may account for why such forsteritic olivines have not been more widely recognized.

Publisher

Oxford University Press

Publication Date

2-28-2012

Publication Title

Journal of Petrology

Department

Geology

Document Type

Article

DOI

10.1093/petrology/egs006

Keywords

Forsterite, Olivine, Subsolidus oxidation, symplectite, Vesicular basalt

Language

English

Format

text

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