Density of MORB eclogite in the upper mantle

In order to study the role of eclogite (subducted oceanic crust) in geodynamics, we carried out high-pressure experiments on natural mid-oceanic ridge basalt (MORB) using a multi-anvil apparatus. We determined phase relations of MORB as a function of pressure and temperature in the temperature range of 1200–1700 °C and pressures from 5 to 14 GPa. The present results differ from a previous high-pressure study of MORB ([Earth Planet. Sci. Lett. 77 (1986) 245]; conducted at 1200 °C) as follows: (1) coesite (Coe) or stishovite (St) appears throughout the studied pressure range; and (2) the clinopyroxene/garnet (Cpx/Gar) ratio is smaller, particularly in the low-pressure range (<12 GPa). Our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 GPa due to the coesite–stishovite phase transition. The density is considerably higher than estimates based on the previous experiments at pressures less than 12 GPa. Thus, the role of eclogite in driving subduction should be more important than previously thought. On the other hand, the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases, a result of the increase in clinopyroxene/(garnet+SiO2) ratio in eclogite with increasing temperature. As eclogite is still denser than peridotite even under subsolidus conditions, recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes.