HR: 18:05h
AN: V34A-08    [Abstracts]
TI: Experimental Studies of the P-T-H₂O Near-Liquidus Phase Relations of Basaltic Andesite From North Sister Volcano, High Oregon Cascades: Constraints on Lower-Crustal Mineral Assemblages
AU: * Mercer, C N
EM: cmercer@uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States
AU: Johnston, A D
EM: adjohn@uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States
AB: North Sister Volcano, in the central Oregon High Cascades, is a large, mafic stratovolcano that has erupted a remarkably compositionally limited range of basaltic andesite (53-55 wt.% SiO₂) for over 300 ka. Other workers have suggested that these "monotonous" lavas are generated by mantle melts that undergo deep crustal magmatic processes within "deep crustal hot zones" (DCHZ), where the parental melt composition is modified and buffered by reaction with lower-crustal rocks. However, the nature of the lower-arc crust is not well known and a better understanding of the pressure, temperature, composition, and water content of the lower-arc crust is necessary to help constrain these processes. The goal of our study is to elucidate the deep crustal lithology that is involved in the production of mafic arc magmas. We have used hydrous, high pressure experiments to provide P-T-H₂O and compositional constraints on the possible lower-crustal mineral assemblages beneath North Sister Volcano. Inverse approach high pressure experiments were conducted using a basaltic andesite from North Sister Volcano, in order to map the water-undersaturated liquidus surface from 5-20 kbar, 1000-1350°C, and 0-15 wt.% H₂O. These experiments identify permissible crustal mineral assemblages and compositions with which the North Sister basaltic andesites may have equilibrated in the deep crust before ascent as homogeneous magmas. The liquidus is saturated with plagioclase from 5-15 kbar and 0-5 wt.% H₂O, augite from 10-20 kbar at all water contents, garnet above 18 kbar at all water contents, spinel from 5-10 kbar with 10-15 wt.% H₂O, and amphibole from 10-20 kbar with 10-15 wt.% H₂O. Combining our experimental results with examples of tectonically exposed lower arc crust, trace element geochemistry, geophysical constraints, and melt inclusion volatile contents, we have determined that a mafic granulite at ~12 kbar (~40 km) and 1175°C is the most probable lithology involved in deep crustal processes beneath North Sister Volcano, and the resulting melts contain ~3.5 wt.% dissolved H₂O. Although our inverse approach experiments cannot constrain the mode of the plagioclase + augite in the granulite, using measured distribution coefficients from our experimental run products, we conclude that the assemblage with which North Sister basaltic andesites equilibrated in the DCHZ was greatly dominated by augite.
DE: 1020 Composition of the continental crust
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, and Petrology [V]
MN: 2007 Joint Assembly