Influence of Basaltic Melt on the Creep of Polycrystalline Olivine under Hydrous Conditions

This chapter presents a study where constant displacement-rate and load-relaxation experiments were performed at high pressures and temperatures to investigate the rheological behavior of partially molten aggregates of fine-grained olivine with a small amount of included basaltic melt under hydrous conditions. Two-phase samples with melt fractions of either 0.9 or 8.6 vol% of basalt and grain sizes of 12 or 15 μm were fabricated by hydrostatically hot-pressing powders of San Carlos olivine plus synthetic basalt. Single-phase samples with a grain size of 12 μm were prepared by hot-pressing powders of San Carlos olivine. Infrared spectra revealed that the olivine in the sample without basalt and in the sample with 0.9 vol% of basalt was saturated with water-derived species, while the olivine in the sample with 8.6 vol% of basalt was undersaturated because the water partitioned largely into the melt phase. The difference in creep strength between the melt-free and melt-rich samples was a result of two competing effects. First, the presence of melt caused the partially molten sample to be weaker than the single-phase olivine sample. Second, the presence of water resulted in the hydrolytic weakening of both samples; however, because the partially molten sample was undersaturated with water, while the single-phase sample was fully saturated, the hydrolytic weakening effect was larger in the latter than in the former. If both samples had been fully saturated with water, the difference in creep strength might have been larger. Hence, the rheology of partially molten upper mantle rocks in an ascending mantle plume or a mid-oceanic ridge environment will depend critically on the partitioning of water between the melt and solid phases and on the permeability of the rock (that is, melt fraction).