HR: 16:15h
AN: V52F-02    [PDF]
TI: High-Pressure Experiments on Harzburgite Melting With and Without H$_{2}$O
AU: * Parman, S W
EM: parman@mit.edu
AF: Massachusetts Inst. of Technology, 54-1212 400 Main St., Cambridge, MA 02139 United States
AU: Grove, T L
EM: tlgrove@mit.edu
AF: Massachusetts Inst. of Technology, 54-1212 400 Main St., Cambridge, MA 02139 United States
AB: The effect of H$_{2}$O on the composition of harzburgite-saturated melts has been quantified with a series of hydrous and anhydrous melting experiments using a piston-cylinder device. Experimental conditions were 1.2-2.2 GPa and $1175-1500\deg$C. Melt H$_{2}$O contents range from 0-10 wt.%. The observed effect of temperature on SiO$_{2}$ activity coefficient ($\gamma$) in the experimentally produced melts closely matches that predicted from thermodynamic properties. Pressure produces a small but discernable decrease in $\gamma$ SiO$_{2}$. The primary effect of H$_{2}$O on $\gamma$ SiO$_{2}$ is dilution. In addition, H$_{2}$O lowers equilibrium temperatures, and so the hydrous experiments tend to produce lower (less ideal) $\gamma$ SiO$_{2}$, but the trend between anhydrous and hydrous activity coefficients is continuous. Thus, there is no discernable effect of H$_{2}$O on non-ideal mixing terms, and so hydrous melts are not intrinsically more SiO$_{2}$ rich than anhydrous melts. If this is so, then why are hydrous arc magmas typically SiO$_{2}$-rich? It is proposed that the high SiO$_{2}$ contents (and high SiO$_{2}$/[MgO+FeO] ratios) are the result of the low temperatures at which hydrous melting occurs, relative to anhydrous melting. Partition coefficients for MgO and FeO increase at lower temperatures, while the partition coefficient for SiO$_{2}$ is buffered by olivine-orthopyroxene equilibria. Therefore, the SiO$_{2}$/(MgO+FeO) ratios of harzburgite saturated melts increase as temperature falls in both hydrous and anhydrous systems. In this model, the SiO$_{2}$-rich character of arc-lavas is primarily a thermal effect, albeit one induced by the temperature lowering effects of H$_{2}$O. The experimentally measured mineral/melt partition coefficients (this study and literature data) have been parameterized in terms of pressure, temperature and melt H$_{2}$O content. These expressions have been used to construct a Gibbs-Duhem based numerical model that predicts the compositions of hydrous and anhydrous ol-opx saturated melts. Comparisons with experimental data not included in the model indicate that it is the most accurate model available for predicting the compositions of high degree mantle melts, with or without H$_{2}$O.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting