Decomposition boundary from high-pressure clinoenstatite to wadsleyite + stishovite in MgSiO3

Abstract

The reaction boundary between high-pressure clinoenstatite and wadsleyite + stishovite in MgSiO 3 was investigated using a multi-anvil high-pressure apparatus and synchrotron X-ray diffraction. Experimental pressures were monitored via in situ powdered X-ray diffraction of gold, which was also put into the sample chamber. Stable phases at each pressure and temperature were confirmed by powdered X-ray diffraction data and observing the recovered samples. The reaction boundary was found to occur at P (GPa) = 16.1 + 0.0064 × (T-1250) (K), where the pressure dependence of the slope of the reaction boundary, dP/dT, is similar to that of the phase boundary between wadsleyite and ringwoodite. Thus, the stability field of wadsleyite and stishovite expands to a low-temperature region, reconciling an inconsistency recorded between previous studies regarding the phase relation in MgSiO 3 .

Phase Transition of Enstatite‐Ferrosilite Solid Solutions at High Pressure and High Temperature: Constraints on Metastable Orthopyroxene in Cold Subduction

“…Alternatively, the phase transition path within a downgoing slab depends on the thermal structure of the slab, and the phase transition of a mineral could be different in different temperature regions. The phase relations of orthopyroxene at high pressure and high temperature (the high‐temperature region of a cold subducting slab) have been investigated by several authors (e.g., Akashi et al, 2009; Fei et al, 1990; Ito & Navrotsky, 1985; Ono et al, 2018). As shown in Figure 4a, the orthopyroxene→high‐pressure clinopyroxene transition occurs before the decomposition to wadsleyite plus stishovite.…”

“…As shown in Figure 4a, the orthopyroxene→high‐pressure clinopyroxene transition occurs before the decomposition to wadsleyite plus stishovite. Previous experimental studies suggested that the orthopyroxene → high‐pressure clinopyroxene transition occurs at temperatures around 1,000 K (Akashi et al, 2009; Woodland, 1998) and the high‐pressure clinopyroxene → wadsleyite + stishovite reaction occurs at T > 1,000 K (e.g., Hogrefe et al, 1994; Ono et al, 2018). However, the phase transition of orthopyroxene in the coldest part of a subducting slab could be different.…”

Phase Transitions of Fe‐, Al‐ and Ca‐Bearing Orthopyroxenes at High Pressure and High Temperature: Implications for Metastable Orthopyroxenes in Stagnant Slabs

“…This phase transition requires high temperatures of ∼1000 K (Akashi et al., 2009; Woodland, 1998). With increasing pressure, hpcpx either transforms into majorite at very high T (>1800 K) or decomposes into wadsleyite and stishovite at relatively low T (<1800 K; Fei et al., 1990; Ito & Navrotsky, 1985; Ono et al., 2018).…”

“…Previous studies suggested that the pressure‐induced phase transition of orthopyroxene depends on the temperature. Orthopyroxene transforms into high‐ P clinopyroxene (hpcpx) at ∼7 GPa, and the pressure of this transition increases with increasing temperature (Akashi et al., 2009; Fei et al., 1990; Ito & Navrotsky, 1985; Ono et al., 2018). This phase transition requires high temperatures of ∼1000 K (Akashi et al., 2009; Woodland, 1998).…”

“…Figure 4. (a)A schematic diagram of a cold subducting slab, showing the metastable phase transitions of orthopyroxene (α-opx → β-opx → γ-opx) in the cold regions of the slab, in comparison with the α-opx → hpcpx → wad + stv → rwd + stv → akm transition (hpcpx = high-pressure clinopyroxene; wad = wadsleyite; stv = stishovite; rwd = ringwoodite; akm = akimotoite) under higher temperature conditions(Ono et al, 2018). The variations in Fe and Al contents do not significantly influence the pressure of the α-opx → β-opx transition but significantly shift the boundary between the β-opx and γ-opx in the opposite direction; the addition of Ca increases the pressure of the α-opx → β-opx transition but decreases the pressure of the β-opx → γ-opx transition.…”