Thermoelastic Properties of Eclogitic Garnets and Omphacites: Implications for Deep Subduction of Oceanic Crust and Density Anomalies in the Upper Mantle

Synchrotron-based high-pressure/high-temperature single-crystal X-ray diffraction experiments to 24GPa and 700K were conducted on eclogitic garnets (low-Fe: Prp(28)Alm(38)Grs(33)Sps(1) and high-Fe: Prp(14)Alm(62)Grs(19)Adr(3)Sps(2)) and omphacites (low-Fe: Quad(57)Jd(42)Ae(1) and high-Fe: Quad(53)Jd(27)Ae(20)), using an externally heated diamond anvil cell. Fitting the pressure-volume-temperature data to a third-order Birch-Murnaghan equation of state yields the thermoelastic parameters including bulk modulus (K-T0), its pressure derivative (K-T0), temperature derivative ((K-T/T)(P)), and thermal expansion coefficient ((T)). The densities of the high-Fe and low-Fe eclogites were then modeled along typical geotherms of the normal mantle and the subducted oceanic crust to the transition zone depth (550km). The metastable low-Fe eclogite could be a reason for the stagnant slabs within the upper range of the transition zone. Eclogite would be responsible for density anomalies within 100-200km in the upper mantle of Asia. Plain Language Summary Eclogite mainly consists of pyrope-almandine-grossular garnet and sodium-rich pyroxene (omphacite) and is a key component of the Earth's upper mantle and oceanic crust. It plays an important role in the mantle convection. The lack of thermoelastic parameters of eclogitic garnets and omphacites hampers accurate modeling of eclogite density at deep-Earth pressure-temperature conditions. In this study, we obtained the thermoelastic parameters of natural eclogitic garnets and omphacites and then modeled the densities of high-Fe and low-Fe eclogites in the subducted oceanic crust and the normal upper mantle. In the upper mantle, eclogite enhances the slab subduction into the transition zone; however, the presence of the metastable low-Fe eclogite would promote the slab stagnation within the upper range of the transition zone. Additionally, eclogite can explain positive density anomalies at depths of 100-200km of the upper mantle of Asia identified by seismic observations.