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

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Science China Earth Sciences 2023年第4期66卷 718-729页

作者： Jian SONG Wangsheng QIAN Shangqin HAO Wenzhong WANG Daoyuan SUN Zhongqing WU Laboratory of Seismology and Physics of Earth’s Interior School of Earth and Space SciencesUniversity of Science and Technology of ChinaHefei 230026China Department of Earth Sciences University College LondonLondon WC1E 6BTUnited Kingdom Earth and Planets Laboratory Carnegie Institution for ScienceWashington DC 20015USA CAS Center for Excellence in Comparative Planetology University of Science and Technology of ChinaHefei 230026China National Geophysical Observatory at Mengcheng Mengcheng 233527China

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

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The Alpe Arami Story: Triumph of Data over Prejudice

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Journal of Earth Science 2010年第5期21卷 731-743页

作者： Harry W Green II Larissa F Dobrzhinetskaya Krassimir N Bozhilov Institute of Geophysics and Planetary Physics Department of Earth Sciences University of California at Riverside Riverside CA92521 USA Department of Earth Sciences University of California at Riverside Riverside CA92521 USA

The Alpe Arami garnet peridotite of the Southern Swiss Alps is associated with eclogites and included within quartzofeldspathic gneisses. Controversy has swirled around the depth of origin of this massif since the 1970s when application of the newly-developed technique of thermobarometry suggested a depth of last equilibration of greater than 120 kin. Such controversy accelerated in 1996 when we reported microstructural evidence of extensive precipitation of ilmenite and spinel from olivine and proposed a much greater depth of origin. Subsequent experiments showed that it was possible to dissolve the observed amount of TiO2 in olivine, but only at depths in excess of 300 kin, agreeing with the earlier proposal. In 1999 we added new, independent, evidence concerning exsolution of high-pressure clinoenstatite from diopside that in-and-of-itself required a depth of origin in excess of 250 km. Subsequently, we also added evidence from the surrounding eclogites of very high pressures and experimental evidence that the pyroxenes included in the amoeboid garnets of this rock had exsolved from a majoritic parent at perhaps even greater pressures. In refutation of the first two of these observations, suggestions were made that （i） we had made a serious error in our estimate of how much ilmenite was present in olivine （and therefore how much TiO2 had been dissolved in olivine）; （ii） the ilmenite had not been exsolved from olivine but former titanian clinohumite had been present and broke down to yield the ilmenite; （iii） the pyroxene exsolved from diopside had been high-temperature clinoenstatite. In all three of these cases, the alternatives offered were claimed to be accommodated at low pressures. Here we review the essence of this controversy and show that the only scenario that can explain all of the data is the one that we originally proposed; indeed, the more recent data have strongly supported that interpretation and pushed the minimum origin of the massif to depths ap

关键词： Alpe Arami TiO2 in olivine high-pressure clinoenstatite deep exhumation.

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