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

Abstract

Synchrotron-based high-pressure/high-temperature single-crystal X-ray diffraction experiments to ~24 GPa and 700 K 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 (α 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 (550 km). 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 100–200 km in the upper mantle of Asia.