Date of Award

5-2018

Document Type

Thesis campus only

First Advisor

Diane R. Smith

Abstract

Indian Heaven is the most voluminous field of Quaternary basalt in the Cascades north of central Oregon. More than 50 vents have issued dominantly basalt; andesites are minor and dacites are absent. Basalt types include low-K tholeiites (LKT) to intraplate basalts (IPB) and shoshonites (SHO) and MgO contents range from 5.8 to 8.7 wt%. Core compositions of olivine grains in 18 basalts and one andesite were analyzed to assess whether the olivines were in equilibrium with their host liquids. The total range in mole % forsterite for olivine cores is 66 to 89. Both Rhodes diagrams and textural observations are used as evidence for disequilibrium. Most samples contain at least some olivines that appear to be in equilibrium with their host magma, however several samples contain olivines that are clearly out of equilibrium. These characteristics are consistent with incorporation of antecrysts from more felsic magmas and/or older crystal “mushes.”

Temperatures and pressures were calculated for samples containing > 7.5 wt% MgO and no mineral chemistry and/or texture evidence for olivine disequilibria. Pre-eruptive conditions range from 1210 to 1248°C and depths of 27 ± 2.2 km, indicating pooling of magmas in the lower crust for all basalts types. Segregation T-Depth estimates range from 1258 to 1473°C and depths of ~32-80 km. The deepest, hottest conditions (1369-1473°C and 55-80 km) are observed for the LKTs, with progressively lower T-depth estimates for IPBs (1303-1380°C and 37-60 km) and the SHOs (1258-1263°C and 32-34 km). These high segregation temperatures imply anatexis was triggered by decompression of convecting mantle and not only by influx of slab fluids.

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