Accurate determination of the melting relations of primitive peridotite under lower-mantle conditions up to 35 GPa using advanced multi-anvil techniques

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It is thought that the Earth’s mantle experienced extensive melting one or more times in its early history; this melting is referred to as the terrestrial magma ocean. Solidification of the terrestrial magma ocean may have produced compositional layering, which should provide the initial state for the evolution of the Earth’s mantle. Of the various relevant parameters, the melting relations of primitive peridotite under lower-mantle conditions are particularly important to understand the solidification process of the magma ocean; these relations will be determined in this project using multi-anvil presses.Even though there have been several studies conducted to determine the melting relations of primitive peridotite, these studies have encountered several limitations. (1) Steep temperature gradients in the sample chambers have caused segregation of the melt from the solids, resulting in misinterpretations of the solidification sequence. (2) The pressure range of conventional multi-anvil presses is limited to 25 GPa. The relevant pressure interval under lower-mantle conditions is only 2 GPa. (3) The temperature range of conventional multi-anvil presses is limited to 2800 K, which is insufficient to determine liquidus temperatures in the deep mantle. (4) Pressure estimations likely contain large errors because, in previous studies, pressure calibrations were conducted at much lower temperatures than those of the melting experiments, despite the decrease in the pressure with increasing temperature at a constant press load. (5) Solidus temperatures are difficult to determine due to the trace amounts of melt present.To overcome these problems, we will adopt the following advanced multi-anvil techniques. (1) Temperature gradients in the sample chambers will be suppressed to nearly zero to prevent melt segregation using a new furnace design developed by my research group. (2) The pressure range will be expanded to 35 GPa via our ultrahigh-pressure generation technique using a multi-anvil press with hard carbide anvils. (3) The temperature range will be expanded to 4000 K using a boron-doped diamond heater developed by a postdoc in my research group. (4) Sample pressures at the same temperature conditions and in the same high-pressure cell as the melting experiments will be calibrated by means of in situ X-ray diffraction. (5) The solidus temperatures will be determined by observing sudden grain growth by means of in situ X-ray diffraction.Using these technologies, we will determine the melting relations of nearly dry primitive peridotite “KLB-1” from above-liquidus to below-solidus temperatures at pressures of 25 GPa, 30 GPa, and 35 GPa. The water content in the samples will be estimated in a sub-solidus run.