Abstract
Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earth’s lower mantle, and its melting behaviour under pressure is important to constrain rheological properties and melting behaviours of the lower mantle materials. Significant discrepancies exist between the melting temperatures of MgO determined by laser-heated diamond anvil cell (LHDAC) and those based on dynamic compressions and theoretical predictions. Here we show the melting temperatures in earlier LHDAC experiments are underestimated due to misjudgment of melting, based on micro-texture observations of the quenched samples. The high melting temperatures of MgO suggest that the subducted cold slabs should have higher viscosities than previously thought, suggesting that the inter-connecting textural feature of MgO would not play important roles for the slab stagnation in the lower mantle. The present results also predict that the ultra-deep magmas produced in the lower mantle are peridotitic, which are stabilized near the core–mantle boundary.
Highlights
Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earth’s lower mantle, and its melting behaviour under pressure is important to constrain rheological properties and melting behaviours of the lower mantle materials
We performed a series of laser-heated diamond anvil cell (LHDAC) experiments using single crystal MgO as the starting material, which was loaded in a sample chamber together with an argon pressure medium
Such a temperature plateau was observed in the previous melting experiment using LHDAC6 at high pressure and is thought to be usable for detecting melting of MgO at least at ambient pressure
Summary
Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earth’s lower mantle, and its melting behaviour under pressure is important to constrain rheological properties and melting behaviours of the lower mantle materials. Experiments using multianvil apparatus have provided detailed melting relations of model mantle materials at high pressure and temperature, which have been used to constrain the formation of the deep magma ocean and subsequent chemical differentiation of the deep mantle[2,3] These experiments have been limited to pressures up to B30 GPa, equivalent to the depths of the uppermost region of the lower mantle, due to the limitation in pressure and temperature generation in conventional multianvil technology using tungsten carbide as the second-stage anvils. Static high-pressure experiments using laser-heated diamond anvil cell (LHDAC)[5,6,7,8,9], as well as those by dynamic compression[10,11,12] and ab initio calculations[13,14,15,16,17], have been attempted to constrain the melting relations of peridotitic mantle rocks and of relevant major minerals, such as MgSiO3 bridgmanite and MgO, under the lower mantle conditions. The results of the earlier LHDAC study reporting a shallow melting curve for MgO, as well as those for some refractory metals[24,25], are likely attributed to miss-assignment of the former phenomenon to the melting of the sample, leading to a substantial underestimation of the melting temperatures under high pressure
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