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Journal of Petrology Advance Access published online on February 13, 2006
Journal of Petrology, doi:10.1093/petrology/egi099
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received April 20, 2005
Accepted December 20, 2005

Article

Cotectic Proportions of Olivine and Spinel in Olivine-Tholeiitic Basalt and Evaluation of Pre-Eruptive Processes

PETER ROEDER 1 *, EMMA GOFTON 2, and CARL THORNBER 3

1 DEPARTMENT OF GEOLOGICAL SCIENCES, QUEEN'S UNIVERSITY, KINGSTON, ONT., K7L 3N6, CANADA
2 DEPARTMENT OF EARTH AND OCEAN SCIENCES, THE UNIVERSITY OF BRITISH COLUMBIA, VANCOUVER, B.C., V6T 1Z4, CANADA
3 US GEOLOGICAL SURVEY, CASCADES VOLCANO OBSERVATORY, VANCOUVER, WA 98683, USA

* To whom correspondence should be addressed.
PETER ROEDER, E-mail: roeder{at}geol.queensu.ca


  Abstract

The volume %, distribution, texture and composition of coexisting olivine, Cr-spinel and glass has been determined in quenched lava samples from Hawaii, Iceland and mid-oceanic ridges. The volume ratio of olivine to spinel varies from 60 to 2800 and samples with >0·02% spinel have a volume ratio of olivine to spinel of approximately 100. A plot of wt % MgO vs ppm Cr for natural and experimental basaltic glasses suggests that the general trend of the glasses can be explained by the crystallization of a cotectic ratio of olivine to spinel of about 100. One group of samples has an olivine to spinel ratio of approximately 100, with skeletal olivine phenocrysts and small (<50 µm) spinel crystals that tend to be spatially associated with the olivine phenocrysts. The large number of spinel crystals included within olivine phenocrysts is thought to be due to skeletal olivine phenocrysts coming into physical contact with spinel by synneusis during the chaotic conditions of ascent and extrusion. A second group of samples tend to have large olivine phenocrysts relatively free of included spinel, a few large (>100 µm) spinel crystals that show evidence of two stages of growth, and a volume ratio of olivine to spinel of 100 to well over 1000. The olivine and spinel in this group have crystallized more slowly with little physical interaction, and show evidence that they have accumulated in a magma chamber.

Keywords: olivine; spinel; basalt glass; volume %; cotectic.
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