HR: 1330h
AN: V12C-0600    [PDF]
TI: Mantle Melting in the Plagioclase-Spinel Transition Zone; Reconciling Experiments and Thermodynamic Models
AU: * Smith, P M
EM: psmith@gps.caltech.edu
AF: California Institute of Technology, MC 170-25, Pasadena, CA 91125 United States
AU: Asimow, P D
EM: asimow@gps.caltech.edu
AF: California Institute of Technology, MC 170-25, Pasadena, CA 91125 United States
AB: The slope of the solidus of plagioclase and spinel bearing lherzolite is important as it controls mantle melting behaviour during isentropic decompression. The invariant point in $CaO-MgO-Al_{2}O_{3}-SiO_{2}$ (CMAS), where the reaction of plagioclase lherzolite to spinel lherzolite intersects the solidus, expands to form a divariant surface in $CaO-MgO-Al_{2}O_{3}-SiO_{2}-Na_{2}O-FeO$ (CMASNF). Experimental results in two five component systems, CMASN and CMASF, suggest that the temperature of the surface increases slightly with pressure, so that the plagioclase-spinel transition interval could be the site of enhanced melting consistent with seismic observations (Presnall et al., 2000 and references therein). In contrast, calculations using the MELTS (Ghiorso \& Sack, 1995) and pMELTS (Ghiorso, 1998) algorithms predict a strong or weak temperature drop respectively as pressure is increased, giving a pronounced cusp in the solidus where melts would freeze (Asimow et al., 1995). We are currently using an integrated experimental and theoretical approach to explore the equilibrium between plagioclase, spinel, olivine, orthopyroxene, clinopyroxene and liquid to see if these results may be reconciled. Piston-cylinder experiments are being carried out in CMASNF and sub-systems. Using the parameterisation of Walter \& Presnall (1994) we have chosen a suitable bulk composition in CMASN that should intersect the univariant melting reaction over a range of pressures (11-15 kbar) whilst maximizing the modal proportion of each phase above and below the reaction. To account for interlaboratory differences in pressure-calibration we are redetermining the CMAS invariant point. We are also developing calibration software suitable for a new thermodynamic model of peridotite melting. The CMASNF system is outside the calibrated composition ranges of MELTS and pMELTS, which were constructed exclusively from natural system data. By using experiments from CMAS, CMASN and CMASF in our calibration procedure, together with suitable estimates of experimental uncertainty and the best available silicate liquid equations of state, we can test whether a thermodynamically consistent polybaric model can be generated that successfully reproduces the positive Clapeyron slope of the plagioclase-spinel lherzolite melting reaction observed by Presnall and co-workers.
DE: 3035 Midocean ridge processes
DE: 3210 Modeling
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting