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Phenocrystic olivines from the eastern Azores

Published online by Cambridge University Press:  05 July 2018

Gary M. Boone  and
Louis A. Fernandez
Gary M. Boone
Affiliation:
Department of Geology, Syracuse University, Syracuse, New York 13210, U.S.A.
Louis A. Fernandez
Affiliation:
Department of Geology, Syracuse University, Syracuse, New York 13210, U.S.A.
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Summary

Phenocrystic and xenocrystic olivines and one olivine nodule sample, all from volcanics in the eastern Azores, range in composition from Fo87 to Fo69. Most are phenocrysts from the Nordeste alkali basaltic complex, eastern São Miguel Island, in which olivine-bearing rocks range from ankaramite to latite. CaO, MnO, and NiO trends are related to Mg-Fe contents, both for the concentrates as a whole, and also within analysed zoned crystals. It is shown that anomalous plots of Δ2ϑ(220LiF—062olivine) are related to pronounced Mg-Fe zonal gradients and, in samples from some flows, to mixtures of different generations of olivine. Regressions of Δ2ϑ vs. Fo (mol %) and FeO (wt %) for the least-zoned olivines are in close accord with those for Hawaiian olivines reported by Murata and others (1965).

MnO percentages increase linearly with FeO; this and the Hawaiian trend are indistinguishable. Limited data for the most magnesian olivines indicate that NiO increases with FeO up to ∼ 15 wt % of FeO; the trend then decreases with further iron enrichment. This trend is apparent from other olivine data, and its relationship to the onset of pyroxene crystallization is discussed. CaO in olivine phenocrysts is inversely related to normative anorthite percentages in host rocks; the trend appears to be controlled largely by co-precipitation and fractionation of calcic plagioclase.

Type
Research Article
Information
Mineralogical Magazine , Volume 38 , Issue 294 , June 1971 , pp. 165 - 178
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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Footnotes

1

Present address: Department of Geology and Geophysics, Yale University.

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