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Spinel Lherzolite Xenoliths from Lorena Butte, Simcoe Mountains, Southern Washington (USA)

David S. Draper
The Journal of Geology
Vol. 100, No. 6 (Nov., 1992), pp. 766-775
Stable URL: http://www.jstor.org/stable/30065045
Page Count: 10
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Spinel Lherzolite Xenoliths from Lorena Butte, Simcoe Mountains, Southern Washington (USA)
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Abstract

Spinel lherzolite xenoliths from Lorena Butte, an alkali basalt cinder cone on the flanks of the Simcoe shield volcano in south-central Washington, are the first mantle xenoliths found in the Pacific Northwest. Several of the xenoliths contain unusually alkaline silicate glass in veins, in fractures, and as a thin film between grain faces. The samples equilibrated at temperatures of 930 to 980°C, and at least one, and possibly two, higher-temperature thermal events are recorded in coexisting ortho- and clinopyroxenes in two samples. The samples reflect oxidizing conditions, from 2.3 to 3.9 log units above the QFM oxygen buffer, near the upper $fO_{2}$ limit for spinel lherzolite mantle xenoliths. The Lorena Butte xenoliths afford many opportunities to refine petrologic hypotheses regarding mantle-derived and subduction-related Pacific Northwest magmatism.

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