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

Abstract

(Mg, Fe)SiO 3 orthopyroxene is an abundant mineral of oceanic subducting slabs. In-situ high-pressure and high-temperature single-crystal X-ray diffraction has been used to investigate the phase transition of orthopyroxene across the enstatite-ferrosilite (En-Fs) join (En 70 Fs 30 , En 55 Fs 45 , En 44 Fs 56 and Fs 100) up to 24.3 GPa and 800 K, simulating conditions within the coldest part of a subduction zone consisting of an old and rapidly subducting slab. Instead of the orthopyroxene → high-pressure clinopyroxene transition, the α-opx → β-opx and β-opx → γ-opx phase transition are observed at 7.2-15.3 and 11.6-21.1 GPa (depending on the Fs content), respectively. This study indicates that the pressure-induced phase transition of (Mg, Fe)SiO 3 orthopyroxene under relatively low temperature (<800 K) could be different than those occurring under relatively high temperature (>800 K). Additionally, the α-opx → β-opx → γ-opx phase transition could exist within the center of the extremely cold slabs (like Tonga), where such low temperature persists to~600-km depth. Plain Language Summary (Mg, Fe)SiO 3 is the most abundant chemical compound in Earth's mantle. Investigating its phase relations at high-pressure and high-temperature is of fundamental importance in constraining the composition and understanding the structure of the Earth's interior. Previous studies have well established the phase diagram of (Mg, Fe)SiO 3 at high pressure and high temperature. Orthopyroxene is a low-pressure phase; with increasing pressure and temperature, orthopyroxene transforms to the high-pressure clinopyroxene, then, it transforms to akimotoite or decomposes to wadsleyite + stishovite. These phase transitions require temperatures higher than 900 K, which are easily satisfied in the normal mantle or relatively warm subduction zone conditions. However, extremely low-temperature region exists in the center of some old and rapidly subducting slabs (e.g., Tonga), and such low-temperature region (~800 K) can extend to~600-km depth. In this study, we investigate the phase relations of (Mg, Fe)SiO 3 orthopyroxene with several different compositions to 24.3 GPa and 800 K. A different phase transition path is observed, that is, the α-opx → β-opx → γ-opx at pressures within the transition zone depths. Therefore, the result of this study adds one more piece of information to our understanding of phase relations of (Mg, Fe)SiO 3 orthopyroxene within the extremely cold subducted slabs.

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

“…The structure of this high-P phase was solved and refined with Pbca symmetry (Table S3a in the Supporting Information S1) using diffraction data collected at 28.1 GPa during the decompression. This structure is the same as that of the γ-opx phase previously observed in Fe-rich orthopyroxene at high P Xu et al, 2020).…”

“…High P-T experiments at temperatures lower than 800 K have shown metastable phase transitions of orthopyroxene. Several studies (Xu et al, 2018(Xu et al, , 2020Zhang et al, 2014) 1 and 2). In natural orthopyroxenes, Mg 2+ , Fe 2+, and Al 3+ are the most abundant cations other than Si 4+ , and Ca 2+ usually occurs as a very minor component, as in San Carlos En 79 Fs 9 MgTs 10 Wo 2 in this study.…”

Elastic Anomalies Across the α‐β Phase Transition in Orthopyroxene: Implication for the Metastable Wedge in the Cold Subduction Slab

“…The third-or fourth-order finite equations were used to fit all the C ij s for α-orthopyroxene. We considered a 2.3(1)% jump in density across the α-to β-phase using literature results (Figure S8 in Supporting Information S1) (Dera et al, 2013;Finkelstein et al, 2015;Xu et al, 2018Xu et al, , 2020Xu et al, , 2022Zhang & Bass, 2016;D. Zhang et al, 2013; J. S. Zhang et al, 2012).…”

“…to lack experimental constraints, here we considered the same temperature derivative of the elastic moduli for the β-phase as α-orthopyroxene (Table S4 in Supporting Information S1) (Jackson et al, 2003;Zhao et al, 1995). Literature results on the influence of temperature on the phase transition pressure of orthopyroxene were applied in our modeling (Table S5 in Supporting Information S1) (Xu et al, 2018(Xu et al, , 2020J. S. Zhang et al, 2014).…”

Delamination in Tibet: Deriving constraints from the density of eclogite

“…High-temperature conditions up to 700 K are provided by the heating resistor. Setup details for the employed thermocouples and heaters can be found in our previous articles (Xu et al, 2019(Xu et al, , 2020bYe et al, 2021).…”