Partial melting of MORB-like eclogite at high pressure and temperature

Olivine-poor mafic heterogeneities such as eclogite may be present in the uppermost mantle, and potentially may participate in formation of oceanic basalts. The presence of such assemblages can affect the volumes and compositions of basalts produced by such a hybrid peridotite-eclogite mantle source. To investigate and evaluate this potential contribution, melting experiments on an eclogite similar to subducted oceanic crust were performed in a piston-cylinder apparatus at conditions equivalent to the deeper regions of mid-oceanic ridges (2–3 GPa, 70–100 km depth). Experiments were principally at 3.0 GPa and resulting partial melts and residual minerals were analyzed by electron microbeam techniques. Modes of melt and minerals were determined by mass balance calculations from the composition data.; The 3.0 GPa solidus is near 1310°C, and the solidus phases are clinopyroxene, garnet, and minor rutile and quartz. Rutile and quartz are stable to 1335°C and 1365°C, respectively, and garnet reacts out above 1475°C, leaving clinopyroxene as the liquidus phase. The liquidus is located at 1500–1525°C. Overall garnet modes are low (<20%) and clinopyroxene is Al-rich and cation deficient, indicating the presence of a significant number of vacancies in the octahedral site. This unusual clinopyroxene is the major phase affecting trace element signatures during eclogite melting. Therefore, clinopyroxene/silicate liquid trace element partitioning at 3.0 GPa was investigated by growing clinopyroxene grains of suitable composition and size for analysis by secondary ionization mass spectrometry.; The melting data allow estimation of adiabatic melting rates for eclogite upwelling beneath oceanic ridges (average melting rate ∼59%/GPa and ∼13%/GPa near the eclogite solidus). Calculation of the resulting enhanced crustal thickness from the presence of eclogite correlates with expected geochemical signatures from the clinopyroxene/silicate liquid trace element partitioning data. Approximately 1–3% (by weight) of eclogite can be present in MORB source regions without exceeding the observed range of crustal thickness and Sm/Yb ratios, and melting of eclogite may also account for elevated (230Th)/(238U) activity ratios in oceanic basalts.