HR: 10:40h
AN: V32C-02    [Abstracts]
TI: Shallow mantle melting beneath Newberry Volcano, central Oregon, USA
AU: * Barr, J A
EM: jaybarr@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, United States
AU: Till, C B
EM: ctill@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, United States
AU: Grove, T L
EM: tlgrove@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, United States
AB: Newberry Volcano lies ~60km behind the main Cascade arc in central Oregon and at the western terminus of a trend of volcanic centers that originate from the same location as the eastward trending volcanic series now traced to the Yellowstone hot spot. Alkali basalts of Luna Butte represent one of the primitive end members of the spectrum of lavas erupted from Newberry Volcano and possibly represent a primary mantle melt input into the Newberry system. Phase equilibrium experiments have been carried out on the Luna Butte composition from 0.8 to 1.2 GPa. We find a near-liquidus multiple-saturation with clinopyroxene, olivine, and plagioclase at 1.0 GPa and 1300°C. The liquids produced in these experiments are silica-undersaturated, but appear to be only slightly offset from the predicted multiple-saturation that would include orthopyroxene. Based on the major element composition of the lava, we expect that the Luna butte composition will be multiply saturated with a spinel lherzolite residual assemblage at 1.0 to 1.2 GPa. The lava's composition and its near aphyric character precludes the possibility of significant crustal level fractionation of olivine, and the high-pressure multiple saturation point at 1.0 GPa places the depth of the anhydrous multiple-saturation point right at the seismic Moho beneath Newberry. The phase equilibria and multiple saturation conditions are similar to those of a primitive lava from Jordan Valley volcanic center in eastern Oregon, along the trace of the purported split in the Yellowstone Hot Spot trend. High Sr and Ba contents of these two lavas suggest there may be a fluid component involved in the generation process, which would serve to increase the depth of the multiple-saturation point. This could allow the compositions to saturate with the spinel lherzolite assemblage predicted for the Luna Butte alkali-basalt. However, to produce the silica-rich lavas found elsewhere at Newberry volcano, e.g. high-silica rhyolites, there must be additional magmatic inputs into the sub-volcanic system that may not be present in the eastern High Lava Plains.
DE: 1065 Major and trace element geochemistry
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting