[S-IT19] Deep Earth Sciences

[SIT19-P12] Elasticity of high-pressure clinoenstatite under mantle conditions: implications for the origin of the X-discontinuity and partial melting of eclogite

*Jian Song¹, Wangsheng Qian¹, Shangqin Hao¹, Wenzhong Wang^1,2,3, Daoyuan Sun¹, Zhongqing Wu¹ (1.University of Science and Technology of China, 2.Department of Earth Sciences, University College London, 3.Earth and Planets Laboratory, Carnegie Institution for Science)

Keywords:the X-discontinuity, high-pressure clinoenstatite, the OPX-HPCPX transition, partial melting of eclogite, first-principle calculations, elasticity

The X-discontinuities, with 2~8% impedance contrasts between 250 km and 350 km depth, have been detected beneath various tectonic settings such as stable continents, hotspots, and near subduction zones. However, the genesis of the X-discontinuity remains under debate. The coesite-stishovite transition in eclogite is a popular mechanism due to its considerable wave impedance contrasts. But the sole coesite-stishovite transition is insufficient to explain the indistinguishable seismological Clapeyron slope of the X-discontinuity (Deuss and Woodhouse, 2004) since the transition has a large positive Clapeyron slope. On the contrary, the transition from orthopyroxene (OPX) to high-pressure clinopyroxene (HPCPX) is another candidate mechanism but is often ignored based on its relatively small impedance contrasts reported by Woodland and Angel (1997), which was assumed from Birch’s law without direct wave speed measurements.
In the study, we performed first-principles calculations to obtain the elasticity of high-pressure clinoenstatite (HPCEN, MgSiO₃), the Mg endmember of HPCPX, at high pressure and temperature. we find that the wave impedance contrasts of the OPX-HPCPX transition are ~6%, almost double the previous estimations. The OPX-HPCPX transition can explain the X-discontinuity as long as OPX content exceeds 30%. Although OPX in the pyrolite model is only 10%, OPX can be enriched by the melt/rock reaction: olivine + SiO₂ (melt₁) → OPX (+ melt₂). Since eclogite-derived silica-rich melts not only promote the enrichment of OPX, but also decrease the content of solid-state coesite, partial melting of eclogite or not determines which mechanism is dominant. Although partial melting of eclogite is merely considered before, we find eclogite is indeed subject to melting in hot or wet regions. Considering the high possibility of eclogite melting, we propose for the mechanism of the X-discontinuity that the coesite-stishovite transition dominates where eclogite undergoes no melting while the OPX-HPCPX transition dominates where partial melting of eclogite occurs. The hypothesis can interpret seismological observations in an unprecedented way, including the indistinguishable seismological Clapeyron slope of the X-discontinuity, the disappearance of 410 beneath Hawaii, the double X-discontinuities, and the anomalous shallow X-discontinuities beneath hotspots, indicating the wide distribution of the partial melting of eclogite. The discrimination of the origin of the X-discontinuity provides a key way to illuminate the melting situation of eclogite in the deep earth.