Assessment of temperature gradients in multianvil assemblies using spinel layer growth kinetics

[1] We present an empirical equation to link the thickness of a MgAl2O4 spinel layer growing at the interface between MgO and Al2O3 multianvil high-pressure assembly pieces to pressure ( P), temperature (T), and time (t), extending the recent study of Watson et al. [ 2002] to the pressure range 6-16 GPa and temperature range 1673-2273 K. For the spinel thickness DeltaX we obtain [GRAPHICS] This equation can be used to assess the thermal gradient in any multianvil assembly where MgO and Al2O3 filler pieces are in contact at P-T conditions within the MgAl2O4 spinel stability field. As an illustration, we show that the central hot spot, in which temperatures can be considered constant, is close to 1 mm long in a common octahedral multianvil assembly with 8 mm edge length and rhenium furnace, while it extends to 3 mm long in an octahedron with 18 mm edge length using a straight graphite heating element. In addition, we present the results of a spinel growth experiment performed at 2273 K and 15 GPa with O-18 enriched MgO, which shows that oxygen is mobile at a length scale exceeding that of the spinel layer. This finding raises the possibility that under some circumstances growth of spinel ( and of reaction-product layers in other oxide systems) might be accomplished by concurrent fluxes of oxygen and cations. This "mobile oxygen'' model differs from the more conventional Mg-Al exchange model proposed by Watson and Price [2002] for the lower-pressure experiments and might explain the observed differences in systematics between the high- and low-pressure data sets.