Fast Seismic Anomalies Under Continents Explained by the Delaminated Lower Continental Crust-Implications From High Pressure-Temperature Elasticity of Jadeite

Seismic tomography has shown that the shear wave velocities (Vs) under continents, especially under cratons, are extremely fast at 100-200 km depth, which is difficult to explain by low temperatures or high Mg#. Alternatively, delaminated eclogitic lower continental crust has been proposed to account for these fast seismic anomalies. However, the thermoelastic properties of jadeite which constitutes up to 60-80 mol% of clinopyroxene in the potentially delaminated lower continental crust are not well constrained. In this study, we measured the single-crystal elasticity of jadeite by Brillouin spectroscopy under simultaneous high pressure and temperature conditions for the first time. We found that the temperature dependence of Vs of jadeite is extremely small if not negligible. The seismic velocities of the potentially delaminated lower continental crusts were subsequently modeled and found to match the widely observed fast seismic anomalies under cratons between 100 and 200 km depth. The seismic wave velocity variation images show the potential composition and temperature heterogeneities inside the Earth. Fast shear wave velocities (similar to 7% higher than the global average) have been observed under continents at 100-200 km depths. A candidate explanation of this fast shear wave velocity anomaly is the existence of delaminated eclogitic lower continental crust. However, due to the lack of knowledge of the thermoelastic properties of clinopyroxene, which is the dominant mineral phase (up to 60 vol%) in delaminated eclogitic lower continental crust, evaluation of this hypothesis is difficult. Clinopyroxene in the potentially delaminated lower continental crust is jadeite-rich (up to 60-80 mol%) due to its high Na content (2.5-3.5 wt%). In this study, we report single-crystal elasticity of jadeite at high pressure-temperature conditions. We found the Vs of jadeite is much higher than all the other major upper mantle minerals under upper mantle conditions. The calculated seismic velocities of the potentially delaminated lower continental crusts could easily account for the fast shear wave anomalies observed under cratons. High pressure-temperature single-crystal elasticity measurements of jadeite are conducted by Brillouin spectroscopy Jadeite is among the seismically fastest phases in the Earth's upper mantle Delaminated lower crust can help explain the fast seismic anomalies under cratons