EXCEPTIONALLY HIGH WATER AND OTHER VOLATILE CONCENTRATIONS IN OLIVINE-HOSTED MELT INCLUSIONS FROM THE YELLOWSTONE HOTSPOT AND COLUMBIA RIVER FLOOD BASALTS

The Yellowstone hotspot track, comprising the Snake River Plain (SRP) and Yellowstone Plateau volcanic field, is widely attributed to mantle melting in response to impingement of a deep seated mantle plume on the southwesterly migrating North American plate. Origin of the mid-Miocene Columbia River Basalts (CRB) and coeval basalts in Oregon and northern Nevada is also attributed by many to effects of the Yellowstone plume, although the lithosphere in that region consists of accreted Permian to Cretaceous oceanic terranes. Propagation of Basin & Range extension into this region raises the possibility that some of these basalts may be melts of lithospheric rather than plume mantle. Other complicating factors such as lateral variation in lithosphere age, composition, and thickness may also contribute. H₂O and other volatiles (S, F, Cl), as well as major and trace elements have been analyzed in over 150 olivine-hosted melt inclusions from 15 basalt samples taken throughout the CRB-SRP region and covering both lavas contemporaneous with passage of the hypothesized plume and later lavas probably unrelated to the plume. H₂O concentrations as high as 3.3 wt% have been observed in melt inclusions in a Gerritt Basalt flow at Mesa Falls, Idaho. Some melt inclusions from the Malheur Gorge area of East Central Oregon preserve H₂O concentrations as high as 4.2 wt%. The highest H₂O concentrations are in all cases found in the most primitive melt inclusions, and thus are interpreted as magmatic minima. These values significantly exceed the maximum H₂O concentrations observed in Hawaiian melt inclusions of 0.9 wt%. Maximum observed concentrations of other volatiles are 2854 ppm S in the Malheur Gorge, 2606 ppm F in Picture Gorge Basalt (within the CRB), and 1100 ppm F in a Gerritt Basalt flow. High Ba concentrations and Ba/Th in melt inclusions suggest that the H₂O may have a subduction zone origin.