HR: 1400h
AN: V53B-14    [Abstracts]
TI: P-T- H₂O Phase Relations of an Aleutian High-MgO Basalt: Comparison of Hydrous Experiments with Thermodynamic Models
AU: * Weaver, S L
EM: sweaver@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: Okmok Volcano in the Central Aleutians has erupted chemically heterogeneous lavas, ranging from voluminous, fractionated high-alumina basalts (HAB) to rarer, more primitive high-MgO basalts (HMB). These HMBs (9-12 wt.% MgO) represent near-primary magmas that may be unmodified products of mantle melting beneath the arc. Previous anhydrous studies of HMBs have constrained the P-T conditions at which the melts last equilibrated with a dry mantle peridotite assemblage. However studies have shown that the presence of a hydrous fluid within the mantle wedge has an important effect on mantle melting, causing melting to occur at higher pressures and lower temperatures than under anhydrous conditions. Previous anhydrous experimental studies on a natural HMB (ID-16) have shown that it is multiply saturated with five phases (olivine, spinel, clinopyroxene, orthopyroxene, and plagioclase) at 12 kbar and ~ 1300°C. However, it is unlikely that ID-16 equilibrated under anhydrous conditions given geophysical constraints. The goal of our experiments is to infer the P-T- H₂O conditions (if any) at which ID-16 is in equilibrium with a hydrated mantle peridotite using hydrous, piston-cylinder experiments. In addition, this data combined with existing anhydrous data can be used to test the validity of thermodynamic modeling techniques (pMELTS). Hydrous experiments have been performed at water-undersaturated conditions (3-10 wt.% H₂O) at temperatures ranging from 1050°C-1350°C and pressures ranging from 10-20 kbar. Preliminary results show that the liquidus at 5 wt.% H₂O is at ~1300°C, ~1250°C, and ~1160°C at 20, 15, and 10 kbar, respectively. The 5 wt.% H₂O liquidus is displaced down- temperature by ~150°C at 20 kbar, ~125°C at 15 kbar, and ~140°C at 10 kbar relative to the anhydrous liquidus. These results are consistent with those found using the thermodynamic modeling program pMELTS. Although we have not yet analyzed experiments at 3 and 10 wt.% H₂O, pMELTS modeling as well as a second model developed by Wood (2004) predict a likely point of multiple saturation at ~14 kbar and 1200-1300°C, and ~3-5 wt.% H₂O, approximately 2 kbar higher and 100°C lower than the anhydrous multiple saturation point. Data will be presented for experiments at 3 and 10 wt.% H₂O as well as mineral phases and compositions.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly