Pressure-induced Pbca–P2₁/c phase transition of natural orthoenstatite: The effect of high temperature and its geophysical implications

Highlights

• High P–T Raman of natural orthoenstatite to 16.6 GPa and 673 K.

• No phases other than OEN (Pbca) or HPCEN2 (P2₁/c) were discovered.

• Clapeyron slope for the OEN (Pbca) → HPCEN2 (P2₁/c) transition is small.

Abstract

In-situ high-pressure (P) high-temperature (T) Raman spectroscopy has been used to investigate the effect of temperature on the high-pressure phase transition of Mg-rich orthoenstatite (OEN) to a newly-discovered P2₁/c phase (HPCEN2) up to 673 K and 18.2(10) GPa. Two natural orthoenstatite samples were used in this study: near end-member Mg orthoenstatite (Zabargad Island, Egypt), and Al + Fe-bearing orthoenstatite (San Carlos, Arizona). For San Carlos OEN (SC-OEN), the experiment was performed at room temperature, 373, 573 and 673 K; For Zabargad Island OEN (Zabg OEN), experiments were performed at 573 and 673 K. The three phases OEN, HPCEN2, and another high-pressure phase with space group C2/c (denoted by HPCEN) are readily distinguished by a characteristic doublet, triplet, or singlet, respectively, in the 660–680 cm⁻¹ range. Similarly, splitting of a peak near 1100 cm⁻¹ is indicative of an OEN → HPCEN2 transition. For both samples, no phase other than OEN and HPCEN2 was observed within the investigated P–T range. The recovered products after slow cooling for over 24 h from 673 K and 16.6(9) GPa were OEN. The Clapeyron slope (dP/dT) of this transition is bracketed between +0.020 to −0.0026 GPa/K for Zabg-OEN, and +0.0023 to −0.0049 GPa/K for SC-OEN. Our results suggest a possible stability field for HPCEN2 at the bottom of the upper mantle.

Introduction

Mg-rich Fe-bearing pyroxene with approximate formula (Mg,Fe)SiO₃, is widely considered to be one of the major minerals in Earth’s upper mantle. Four polymorphs of (Mg,Fe)SiO₃ are potentially stable under upper mantle conditions: orthoenstatite (OEN) with space group Pbca (Morimoto and Koto, 1969), low-pressure clinoenstatite (LPCEN) with space group P2₁/c (Morimoto et al., 1960), high-pressure clinoenstatite (HPCEN) with space group C2/c (Angel et al., 1992), and a newly discovered high-pressure monoclinic polymorph (HPCEN2), also with space group P2₁/c (Zhang et al., 2012).

Early studies suggested that OEN transforms into HPCEN between 6–9 GPa at high temperature, and that HPCEN transforms to LPCEN upon cooling and decompression (Pacalo and Gasparik, 1990, Kanzaki, 1991, Ulmer and Stadler, 2001, Angel et al., 1992, Shinmei et al., 1999, Kung et al., 2004, Akashi et al., 2009). These experiments lead to the hypothesis that LPCEN, OEN, and HPCEN are the stable polymorphs over the range of upper mantle conditions (Angel et al., 1992l; Woodland, 1998). However, the rarity of LPCEN in nature strongly argues against this view of LPCEN stability (Anthony et al.,, Coe and Kirby, 1975, Ito, 1975). Moreover, most previous experiments in the literature used either flux-grown synthetic crystals with impurities from the fluxes incorporated in the structure (e.g. Mo, V, etc.), or were performed under hydrous/flux-bearing/high-shear-stress environments (e.g. Grover, 1972, Ito, 1975). Enstatite has been shown to be very sensitive to those impurities and stress environments (e.g. Coe and Kirby, 1975), thus we must view the phase relations of OEN as a function of P, T, chemical composition (X) and stress state σ (not just P and T). In fact, the product recovered upon slow cooling from temperatures of over 1000 °C at room-pressure was found to be OEN instead of LPCEN (Jackson et al., 2004, Brenizer, 2006, Reynard et al., 2009). The only previous studies utilizing natural enstatite samples under nearly water/stress-free conditions by Zhang et al., 2012, Zhang et al., 2013, provided both X-ray and Raman evidences of a new high pressure phase transition (OEN → HPCEN2) in the enstatite system, and raised questions about the OEN equilibrium phase relations (e.g. the stability field of HPCEN (C2/c) and the newly discovered high pressure phase HPCEN2).

In order to examine the stability of HPCEN2, it is necessary to perform phase identification of Mg-rich Ca-poor pyroxene under in situ high P–T and water/stress-free conditions. As for the effect of composition on the high-pressure phase relations, it has been shown that natural impurities of several percent Fe and Al could change the onset of the OEN → HPCEN2 transition by up to 3 GPa at room temperature conditions; however, no other phases were observed during the experiments (Zhang et al., 2013). This indicates that natural compositional variations do not stabilize an additional phase of (Mg,Fe)SiO₃ at room temperature and high pressure. However, the combined effects of temperature and composition are still unknown. Thus, in this study, we have performed in situ high-pressure high-temperature Raman experiments with near end-member Mg orthoenstatite from Zabargad Island, Egypt, and Al, Fe-bearing orthoenstatite from San Carlos, Arizona, to address the effects of temperature and composition on the newly-discovered Pbca→P2₁/c phase transition.