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

Abstract

The phase transitions of natural San Carlos Fe‐, Al‐ and Ca‐bearing orthopyroxene and two synthetic orthopyroxenes containing Al (up to 12 wt. % Al2O3) and Fe (up to 15 wt. % FeO) were investigated at high pressures and temperatures up to ∼30 GPa and 700 K using synchrotron‐based single‐crystal X‐ray diffraction. These orthopyroxenes undergo α‐opx → β‐opx → γ‐opx phase transitions with increasing pressure and temperature. We quantitatively described how the composition influences the pressures of the phase transitions. The results indicated that the variations in Mg, Fe, and Al contents do not significantly influence the pressure of the α‐opx → β‐opx transition (Pα‐β). For the β‐opx → γ‐opx phase transition, increasing Fe content decreases the Pβ‐γ while increasing Al content increases the Pβ‐γ. The incorporation of Ca increases the Pα‐β and decreases the Pβ‐γ. The thermal equations of states of α‐opx and β‐opx for these three orthopyroxenes were determined using the unit‐cell volume collected at high pressures and temperatures, which have been used to establish a density model to interpret the cold stagnant slabs. Our experimental results have indicated that β‐opx and γ‐opx are possible metastable phases of natural orthopyroxenes within cold stagnant slabs in the mantle transition zone.

High-pressure phase transitions of Fe-bearing orthopyroxene revealed by Raman spectroscopy

“…Addition of Al leads to an increase in the α-to β-phase transition pressure. Orthopyroxene with 2-24 mol.% Al has a phase transition to occur at 14-16 GPa (Xu et al, 2022;Zhang et al, 2013b). In contrast, En 91 Fs 9 contained virtually no other trace elements, consistent with previous research results.…”

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).…”

“…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 ) (Dera et al., 2013; Finkelstein et al., 2015; Xu et al., 2018, 2020, 2022; Zhang & Bass, 2016; D. Zhang et al., 2013; J. S. Zhang et al., 2012). The density of the β ‐phase between 15 and 20 GPa was also derived following the self‐consistent procedure using the Brillouin data (Sata et al., 2002).…”

“…Elevating the temperature at ambient conditions was observed to cause a linear decrease in all elastic moduli (Hogrefe et al., 1994; Jackson et al., 2004, 2007; Reynard et al., 2010; Zhao et al., 1995). Along a slab geotherm that is 700–1000 K colder than the surrounding mantle, metastable α ‐orthopyroxene in the harzburgite layer of the sinking slab will transform into the monoclinic β ‐phase ( P2 1 / c space group) with a Clapeyron slope of −8.9 to 20 MPa/K which could be carried down to 500‐km depth (Lin, 2004; Xu et al., 2018, 2022; J. S. Zhang et al., 2012, 2014). However, experimental constraints on the elasticity of β ‐phase at high pressures are rather limited, and the influence of the α ‐ to β ‐phase transition on the elasticity of orthopyroxene is still under debate (Kung et al., 2004; Zhang & Bass, 2016; D. Zhang et al., 2013).…”

High‐Pressure and High‐Temperature Single‐Crystal Elasticity of Cr‐Pyrope: Implications for the Density and Seismic Velocity of Subcontinental Lithospheric Mantle

“…To examine whether the elastic properties of Cr‐pyrope at high P and high T can be calculated using the end‐member component elastic properties, we calculated the elastic properties ( ρ , K S , G , V P , and V S ) of Prp‐Cr#12 at high P and high T using the directly measured elastic parameters (Table S4 in Supporting Information ) in this study and then compared the results to those calculated using the elastic parameters interpolated from linear mixing of end‐member elastic parameters (end‐member model; Table S5 in Supporting Information ; Angel et al., 2018; Arimoto et al., 2015; Bass, 1986, 1989; Cameron et al., 1973; Chantel et al., 2016; Dymshits et al., 2014; Faccincani et al., 2021; Fan et al., 2020; Gréaux & Yamada, 2019; Gwanmesia et al., 2014, 2006; Hao et al., 2020; Hugh‐Jones, 1997; Isaak et al., 1992; Jackson et al., 2007; Jacobsen et al., 2010; Jeanloz & Thompson, 1983; Klemme et al., 2005; Kroll et al., 2012; Li & Neuville, 2010; Ma et al., 2009; Z. Mao et al., 2015; Milani et al., 2015, 2017; Ohashi, 1984; Sang & Bass, 2014; Sinogeikin & Bass, 2000, 2002; Soga, 1967; Tribaudino et al., 2008; Wang & Ji, 2001; Xu et al., 2018, 2020, 2022; Yang & Ghose, 1994; Zhang & Bass, 2016a, 2016b; Zhang et al., 1998; Zhao et al., 1997, 1998; Zou, Gréaux, et al., 2012). We calculated the elastic properties at pressures of 3–8 GPa at 1000 and 1600 K, which are the lower and upper bound of typical SCLM P ‐ T conditions where garnet‐peridotite stabilizes.…”