Phase relations of basaltic crust determined by simultaneous pressure-temperature generation techniques to 52 GPa and 2000 K using a multi-anvil press with tungsten carbide anvils

Abstract:

Recent seismological studies demonstrated stagnation of subducted slabs between 660 and 1000 km depths and small scale (~10 km) scatters around 860-1800 km depths. These observations could be supported by recent geodynamic modelling that basaltic fragments from the top parts of subducted slabs are accumulated in the mid-mantle. Although the density of basalt is indispensable for such modelling, poorly known phase relations in basaltic systems prevent from evaluating the density. Laser-heated diamond-anvil cell (LHDAC) experiments have covered the mid-mantle conditions, but the laser heating resulted in phase disequilibrium due to inhomogeneous laser absorption among phases having different iron contents. Although large-volume multi-anvil press (MAP) experiments can achieve phase equilibrium because of their homogeneous P-T distributions, the pressure range had not covered the mid-mantle conditions so far. For these reasons, we have extended the pressure range of MAP experiments to 52 GPa, corresponding to a 1300-km depth in the Earth’s mantle. Using this technology, phase relations in the MORB system were determined at pressures of 27-52 GPa and a temperature of 2000 K in this study. The starting material was a fine-grained oxide mixture with a bulk composition of normal MORB. It was loaded together with a pre-synthesized aluminous akimotoite as a pressure calibrant: the pressures of individual runs were determined by the Al₂O₃ content in resultant bridgmanite coexisting with corundum. Phase identification and compositional analysis of run products were conducted using a TEM with an EDX system in combination with a FIB micro-sampling technique. We have identified the mineral assemblage of bridgmanite, stishovite, CaSiO₃ perovskite, Al-rich calcium-ferrite phase, and a new aluminous (hexagonal) phase at all the investigated pressures. We found a significant compositional change in bridgmanite at pressures of 40-45 GPa, which was not shown by the previous LHDAC studies. We will discuss the mineral chemistry of the MORB system and the fate of subducted basaltic crust in the mid-mantle.