Experimental investigation of olivine and olivine-rich rocks at high pressure and high temperature
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Olivine is the most abundant mineral in Earth's upper mantle and is one of the major minerals discovered in extraterrestrial objects. Its physical properties govern the dynamics of the upper mantle. The most dynamic regions of the upper mantle are sites where melting and melt segregation occurs. These regions are also the most variable in terms of their oxygen fugacity. We therefore conducted piston cylinder experiments to determine the intergranular melt distribution, and explore a range of oxygen buffers. We annealed olivine aggregates in metallic and graphite capsules to determine the oxygen fugacities set by the capsule materials. These experiments show that oxygen fugacities are below their corresponding metal-oxide buffers. The oxygen fugacity in nickel80-iron20 and graphite capsules most closely represents the intrinsic oxygen fugacity of Fo90 olivine, while iron capsules are too reducing perhaps explaining the formation of "dusty" olivine in chondrites. We annealed olivine-basalt aggregates in order to determine the melt distribution. The results show that the length of olivine grain boundaries wetted by melt (grain boundary wetness) increases with increasing melt content to values well above those predicted by a simplified model which is commonly applied to this system. At fixed melt content the grain boundary wetness increases with increasing grain size. These observations emphasize that the dihedral angle of the simplified system is not adequate to characterize the melt distribution in partially molten rocks. Our observations indicate that at upper mantle grain sizes the shear viscosity of partially molten rocks is one order of magnitude lower than predicted by the simplified model. Naturally partially molten rocks exist in the form of olivine-rich troctolites or plagioclase dunites, but the conditions for their formation are not entirely clear. We therefore conducted step-cooling experiments that indicate that slow cooling of samples with a steady-state microstructure reproduces the interstitial geometry observed in natural samples. The grain boundary wetness determined from the interstitial phases is somewhat reduced during slow cooling relative to samples quenched from high temperature. The microstructural similarity of experimental and natural samples suggests that mush zones identified beneath mid-ocean ridges may have lower melt contents than previously envisioned.