HR: 11:35h
AN: V12A-06    [Abstracts]
TI: Estimating Temperatures and Dissolved H$_{2}$O Contents of Arc Andesite and Basalt Magmas
AU: * Grove, T L
EM: tlgrove@mit.edu
AF: Mass. Inst. of Technology, Dept. EAPS, 54-1218, Cambridge, MA 02139 United States
AU: Parman, S W
EM: parman@mit.edu
AF: Mass. Inst. of Technology, Dept. EAPS, 54-1218, Cambridge, MA 02139 United States
AU: Plank, T
EM: tplank@bu.edu
AF: Boston University, Dept. of Earth Sciences, Boston, MA 02215
AU: Kelley, K A
EM: kelley@dtm.ciw.edu
AF: Dept. of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington, DC 20015
AU: Sisson, T W
EM: tsisson@usgs.gov
AF: USGS, MS-910, Menlo Park, CA 94025
AB: New experimental data from water-saturated melting experiments on Mariana - Izu arc basalts along with recent published experiments allow refinement of the temperature - H$_{2}$O content estimation technique first developed by Sisson and Grove (1993, Contrib Min Pet 133: 167-184). The model (SG93) used H$_{2}$O -saturated experimental data on basaltic liquids saturated with olivine + plagioclase +augite. SG93 estimated pre-eruptive dissolved H$_{2}$O contents and temperatures using 3 equilibria: olivine - liquid, plagioclase - liquid and an empirical expression for multiply saturated liquids. The Mariana -Izu data and other new experiments provide 32 new equilibria for a total of 149 statements of equilibrium. In addition, Moore et al. (1998, Am Min 83: 36-42) provide an expression for estimating the H$_{2}$O contents of the vapor-saturated experiments and this has been used in the model. The changes improve the model's ability to estimate temperature and H$_{2}$O contents of andesite liquids. The revised expression for SG93 for multiply saturated liquids is: T $\deg$C = 1125 - 26.7$\times$H$_{2}$O +0.089$\times$ (P-1) +272$\times$Al\# - 104.2$\times$NaK\# +213$\times$Mg\# (r$^{2}$ = 0.97 and average error is \pm 14 $\deg$C) This revised expression provides a substantial improvement by extending model coverage over a broader range of SiO$_{2}$ contents ( 49 - 63 wt. %). Along with updated olivine - melt and plagioclase - melt equilibria the new calibration can provide a useful tool for estimating pre-eruptive H$_{2}$O contents and temperatures. The hygrometer permits temperature - H$_{2}$O estimates when coexisting mineral and melt compositions are known or can be estimated. The model also allows estimation of dissolved H$_{2}$O contents when melt inclusions data are not available. Calculations using the updated SG93 indicate that basalts and andesites from the Mariana - Izu, Central America and the Aleutian arcs contain up to 5 to 6 wt. % H$_{2}$O, consistent with melt inclusion data from those areas. Determining pre-eruptive H$_{2}$O contents for arc magmas remains a challenge, but these petrologic tools provide improved quantitative estimates that are essential for understanding the flux of volatiles through the mantle wedge above subduction zones.
DE: 8439 Physics and chemistry of magma bodies
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting