Ultra-depleted melts from Kamchatkan ophiolites : evidence for the interaction of the Hawaiian plume with an oceanic spreading center in the Cretaceous?

We report new data on the major and trace element composition of melt inclusions in spinel phenocrysts (Mg# = 0.7-0.8, Cr/(Cr + Al) = 0.32-0.52, TiO2 = 0.06-0.60 wt.%) from Cretaceous MORB-like basalt (La/Yb = 0.94, Th/Nb = 0.055, Th/La = 0.041) in the Kamchatsky Mys ophiolites (Eastern Kamchatka). The melt inclusions preserved primitive melts (Mg# up to 0.72), which are remarkably depleted in incompatible trace elements compared to common MORBs. Numerous ultra-depleted inclusions from the studied sample have extraordinarily low Na2O (0.20-0.67 wt.%), TiO2 (0.16-0.5 wt.%), K (1.5-25 ppm), La (0.015-0.040 ppm), Zr (0.9-2 ppm), B (0.01-0.03 ppm), Ti/Zr = 300-1074, La/Yb = 0.008-0.075 and represent the most depleted melts known until now. The ultra-depleted melts from the Kamchatkan ophiolites are only comparable to a single melt inclusion from MORB of 9 degrees N Mid-Atlantic Ridge [Sobolev and Shimizu, Nature 363 (1993) 151-154] yet have higher FeO, CaO, heavy rare-earth element (Dy, Er, Yb) contents and lower Na2O and SiO2. These melts, possibly the last melt fractions produced in an upwelling mantle column, could represent the highest degrees (up to similar to 20%) of near-fractional melting of mantle with T-p >= 1400 degrees C, which started melting at similar to 75 km depth and continued to shallow depths of similar to 20 km. The presence of melts ranging in composition from ultra-depleted to compositions similar to Mauna Loa Volcano, Hawaii, high potential mantle temperature and association with rocks akin the Cretaceous Hawaiian tholeiites suggest that the trace element depleted melts preserved in spinel phenocrysts could have originated from extensive melting of a depleted component intrinsic to the Hawaiian plume or ambient upper mantle entrained and heated up at the plume margins. (C) 2009 Elsevier B.V. All rights reserved

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