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

作　　者：Jian SONG Wangsheng QIAN Shangqin HAO Wenzhong WANG Daoyuan SUN Zhongqing WU 

机构地区：[1]Laboratory of Seismology and Physics of Earth’s Interior,School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China [2]Department of Earth Sciences,University College London,London WC1E 6BT,United Kingdom [3]Earth and Planets Laboratory,Carnegie Institution for Science,Washington DC 20015,USA [4]CAS Center for Excellence in Comparative Planetology,University of Science and Technology of China,Hefei 230026,China [5]National Geophysical Observatory at Mengcheng,Mengcheng 233527,China

出　　处：《Science China Earth Sciences》2023年第4期718-729,共12页中国科学（地球科学英文版）

基　　金：supported by the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grant No.XDB41000000);the National Key R&D Program of China(Grant No.2018YFA0702703);the National Natural Science Foundation of China(Grant Nos.41925017,41721002);the Fundamental Research Funds for the Central Universities(Grant No.WK2080000144)。

摘　　要：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.

关 键 词：X-discontinuity OPX-HPCPX transition High-pressure clinoenstatite Partial melting of eclogite First-principles calculations 

分 类 号：P542.5[天文地球—构造地质学] P588.1[天文地球—地质学]