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

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 fO₂ 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.