Elasticity of high-pressure clinoenstatite under mantle conditions: Implications for the origin of the X-discontinuity

• Published in: Science China. Earth sciences Vol. 66; no. 4; pp. 718 - 729
• Main Authors: Song, Jian, Qian, Wangsheng, Hao, Shangqin, Wang, Wenzhong, Sun, Daoyuan, Wu, Zhongqing
• Format: Journal Article
• Language: English
• Published: Beijing Science China Press 01.04.2023; Springer Nature B.V
• Subjects: Acoustic velocity; Coesite; Discontinuity; Earth and Environmental Science; Earth mantle; Earth science; Earth Sciences; Eclogite; Elasticity; First principles; High pressure; Impedance; Laboratories; Melting; P waves; Phase transitions; Pressure; Research Paper; S waves; Seismology; Silica; Slopes; Sound velocity; Stishovite; X-discontinuity; Partial melting of eclogite; OPX-HPCPX transition; First-principles calculations; High-pressure clinoenstatite
• ISSN: 1674-7313; 1869-1897
• DOI: 10.1007/s11430-022-1024-5

The X-discontinuity is characterized by 2–8% wave impedance contrasts and indistinguishable seismological Clapeyron slopes. Its origin is not yet entirely understood and attributed to a variety of plausible causes, among which the coesite-stishovite transition is a popular mechanism due to its large impedance contrasts. However, the sole coesite-stishovite transition is insufficient to explain indistinguishable seismological Clapeyron slopes of the X-discontinuity. The orthopyroxene (OPX) to high-pressure clinopyroxene (HPCPX) transition has been excluded as a candidate mechanism in recent seismic studies because it can only cause small impedance contrasts based on the first-order estimate from the Birch’s law without direct sound velocity measurements. In this study, we performed first-principles calculations to obtain the elasticity of high-pressure clinoenstatite at high pressure and temperature. Our results show that the impedance contrast caused by the OPX-HPCPX transition is ∼5.7% for P wave and ∼6.9% for S wave, which are much larger than the previous empirical estimation and hence cannot be ignored. Given that eclogite is subject to partial melting in hot or wet regions, which will promote the enrichment of orthopyroxene by consuming silica, we suggested that both the coesite-stishovite transition and the OPX-HPCPX transition may be dominant mechanisms for the X-discontinuity, with the former dominating where eclogite is hard to melt and the latter dominating where partial melting of eclogite occurs. The model is consistent with seismological observations, indicating the important role of the OPX-HPCPX transition in the X-discontinuity and extensive occurrence of partial melting of eclogite. The proposed origin of the X-discontinuity provides a plausible way to illuminate the melting situation of eclogite in the deep earth.