Synchrotron‐based high‐pressure/high‐temperature single‐crystal X‐ray diffraction experiments to ~24 GPa and 700 K were conducted on eclogitic garnets (low‐Fe: Prp₂₈Alm₃₈Grs₃₃Sps₁ and high‐Fe: Prp₁₄Alm₆₂Grs₁₉Adr₃Sps₂) and omphacites (low‐Fe: Quad₅₇Jd₄₂Ae₁ and high‐Fe: Quad₅₃Jd₂₇Ae₂₀), using an externally heated diamond anvil cell. Fitting the pressure‐volume‐temperature data to a third‐order Birch‐Murnaghan equation of state yields the thermoelastic parameters including bulk modulus (K_T0), its pressure derivative (K′_T0), temperature derivative ((∂K_T/∂T)_P), and thermal expansion coefficient (α_T). The densities of the high‐Fe and low‐Fe eclogites were then modeled along typical geotherms of the normal mantle and the subducted oceanic crust to the transition zone depth (550 km). The metastable low‐Fe eclogite could be a reason for the stagnant slabs within the upper range of the transition zone. Eclogite would be responsible for density anomalies within 100–200 km in the upper mantle of Asia.

Synchrotron‐based high‐pressure and temperature single‐crystal X‐ray diffraction experiments were conducted on two hydrous orthoenstatite samples (oEn#1: Mg_1.004Si_0.996O₃, ~619 ppm water; oEn#2: Mg_0.947Ni_0.055Si_0.998O₃, ~696 ppm water) to ~34 GPa and 700 K, using resistively heated diamond anvil cells. The α‐opx (Pbca space group)→β‐opx (P2₁/c space group) phase transition of oEn#1 occurs at 12.90(2) GPa, and the β‐opx phase persists to 34.25(1) GPa. The α‐β transition of oEn#2 occurs at 13.50(1) GPa, and a new isosymmetric β‐opx→β‐opxII transition takes place at 29.80(4) GPa. The β‐opxII phase is preserved down to 24.53(3) GPa during decompression. The transition to the monoclinic β‐opxII phase is interpreted as a result of incorporation of Ni²⁺ into the orthoenstatite structure. Fitting the third‐order Birch‐Murnaghan thermal equation of state to the single‐crystal P‐V‐T data yields the thermoelastic parameters of the α‐ and β‐opx phases for both orthoenstatite samples. This study is the first attempt to determine the thermal equation of state of the β‐opx phase. Our results suggest that several hundred ppm of water has negligible effects on the bulk modulus of orthoenstatite but notably enhances the thermal expansion. The potential effects of metastable orthoenstatite on subduction zone dynamics are discussed, and the possible contributions of displacive phase transitions to enhancement of the transformational faulting mechanism of the deep‐focus earthquakes in subducted slabs are considered. The presence of metastable orthoenstatite within cold slabs could promote slab stagnation above the 660‐km discontinuity.

(Mg, Fe)SiO₃ 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 (En₇₀Fs₃₀, En₅₅Fs₄₅, En₄₄Fs₅₆ and Fs₁₀₀) 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)SiO₃ orthopyroxene under relatively low 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.