Unusual ultra-depleted dunite from Sibuyan Island (the Philippines): a residue for ultra-depleted MORB?

摘要

Ultra-depleted dunites from the Sibuyan Island, Romblon (Central Philippines) are primarily composed of oliv-ine, orthopyroxene (<5%), trace clinopyroxene and chromian spinel and are totally free of hydrous minerals and plagioclase. Mineral chemistry shows very refractory compositions (olivine Fo = 92-94; spinel Cr# > 0.75). Rare clinopyroxene preserved in the ultra-depleted dunites is anhedral and interstitial to olivine grains. They are neither subsolidus exsolution products from orthopyroxene nor metasomatic in origin. Clinopyroxene has high Mg# = 0.94-0.97 and low Al_2O_3 contents (<0.85 wt%). They are also very depleted in trace elements with only selected heavy rare earth elements detected during the LA-ICP-MS analysis. Light to middle REEs and LILEs were hardly detected, which is indicative of the very depleted character of the melt that precipitated the clinopyroxene. The calculated melts in equilibrium with the clinopyroxene have heavy REE contents similar to olivine-hosted ultra-depleted melts of MORB affinity from the East Pacific Rise and the Mid-Atlantic Ridge. The formation of the Sibuyan ultra-depleted dunites is being attributed to extremely high degree of dry melting below a mid-oceanic ridge.