Experimental Studies of the P-T-H2O Near-Liquidus Phase Relations of Basaltic Andesite From North Sister Volcano, Central Oregon Cascades: Constraints on Mid- to Lower-Crustal Mineral Assemblages

HR: 0800h
AN: V51D-1525 [Abstracts]
TI: Experimental Studies of the P-T-H2O Near-Liquidus Phase Relations of Basaltic Andesite From North Sister Volcano, Central Oregon Cascades: Constraints on Mid- to Lower-Crustal Mineral Assemblages
AU: * Mercer, C N
EM: cmercer@uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AU: Johnston, A D
EM: adjohn@uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AB: The lavas of North Sister Volcano in the central High Cascades of Oregon are distinctive because of their extremely limited range in compositional variation. They are basaltic andesites with about 53 wt.% SiO₂, low incompatible element abundances, and gently negatively sloping REE patterns with La and Lu abundances of about 30X and 10X chondrites, respectively. These characteristics have led others to postulate that the uniformity of these lavas may result from a process whereby precursor lavas are ``buffered'' by reaction with mid- to lower-crustal rocks. The goal of our experiments was to constrain possible mineral assemblages that North Sister magmas may have equilibrated with within the crust beneath this continental arc volcano. Piston- cylinder experiments were run at 5, 10, 15, and 20 kbar with bulk water contents ranging from zero to 10 wt.% using Au₇₅Pd₂₅ capsules and CaF_{2} /MgO/graphite furnace assemblies. Bulk composition was
maintained without need to presaturate the capsules with Fe. The liquidus under anhydrous conditions extends from about
1225°C at 5 kbar to 1355 °C at 20 kbar and is displaced down-temperature from this reference by about 100° and
125 °C respectively with 3 and 5 wt.% H₂O. Under anhydrous conditions, plagioclase (An_ {~81}) is the liquidus phase between 5 and 15 kbar but is replaced by an assemblage of augite plus garnet at 20 kbar. With 3 wt.% H₂O, plagioclase persists on the liquidus to only 10 kbar, and is replaced by augite plus garnet at 15 kbar and garnet alone at 20 kbar. With 5 wt.% H₂O, plagioclase is no longer a liquidus phase. Instead, we observe olivine + augite + hercynite-rich spinel at 5 kbar, augite alone between 10 and 15 kbar, and augite + garnet at 20 kbar. Although we have not yet run the series of experiments containing 10 wt.% H₂O, we expect amphibole will join the near- liquidus assemblages at low to moderate pressure. Trace element calculations aimed at distinguishing between these possible ``buffering'' assemblages will be presented.
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005