EXPERIMENTAL DETERMINATION OF THE MELTING CURVE OF MAGNESIUM SILICATE PEROVSKITE AT LOWER MANTLE CONDITIONS, AND ITS GEOPHYSICAL IMPLICATIONS

We present the first static measurement of the melting curve of Mg(,0.9)Fe(,0.1)SiO(,3) (perovskite structure) at lower mantle conditions, demonstrating that it melts at 3000 (+OR-) 300 K independent of pressure over the range of 25-65 GPa. Samples of natural orthopyroxene were compressed in a Mao-Bell type diamond anvil cell and heated using a Nd-YAG laser operating at up to (DBLTURN)25 W in the TEM(,00) mode. The laser beam is focussed to (DBLTURN)35 microns at the sample generating temperature profiles in both the horizontal and vertical directions. To measure the temperature profile, a small sampling slit is scanned across the image of the heated spot. This gives line (strip) integrals across the intensity distribution of the heated spot. A set of line integrals collected at one wavelength can be inverted with an abel transform to obtain the intensity as a function of radial distance. Given the radial intensity distribution at two or more wavelengths, the vertically averaged radial distribution of temperature can be resolved with a spatial resolution of 2-3 microns. Temperature gradients are one the order of 100 K per micron when the sample is solid (T < 3000 K) and 400 K per micron when the sample is partially molten (3000 K < T < 6000 K). The melting point is determined by measuring the lowest peak temperature at which glass is formed in the sample.;The melting temperature of magnesium silicate perovskite places an upper bound on the geotherm of the lower mantle. This upper bound is consistent with either one- or two-layer mantle convection. Our data suggest that the homologous temperature T/T(,m) (DBLTURN) 0.70 or 0.85 for the lower mantle, assuming one or two layer convection, respectively. The flat Clapeyron slope for the fusion curve of perovskite (dT(,m)/dP (DBLTURN) 0 (+OR-) 4 K/GPa), combined with a small but systematic Fe enrichment of the melt (the difference in the mean atomic weight is 4+/-2%) is consistent with the idea that melts sink in the lower mantle. The Clapeyron slope also implies that T/T(,m )(DBLTURN) constant in the lower mantle, suggesting that the viscosity is relatively constant with depth through the lower mantle.