Iron isotopic systematics of oceanic basalts

The iron isotopic compositions of 93 well-characterized basalts from geochemically and geologically diverse mid-ocean ridge segments, oceanic islands and back arc basins were measured. Forty-three MORBs have homogeneous Fe isotopic composition, with delta Fe-56 ranging from +0.07 parts per thousand to +0.14 parts per thousand and an average of +0.105 +/- 0.006 parts per thousand (2SD/root n, n = 43, MSWD = 1.9). Three back arc basin basalts have similar delta Fe-56 to MORBs. By contrast, OIBs are slightly heterogeneous with delta Fe-56 ranging from +0.05 parts per thousand to +0.14 parts per thousand in samples from Koolau and Loihi, Hawaii, and from +0.09 parts per thousand to +0.18 parts per thousand in samples from the Society Islands and Cook-Austral chain, French Polynesia. Overall, oceanic basalts are isotopically heavier than mantle peridotite and pyroxenite xenoliths, reflecting Fe isotope fractionation during partial melting of the mantle. Iron isotopic variations in OIBs mainly reflect Fe isotope fractionation during fractional crystallization of olivine and pyroxene, enhanced by source heterogeneity in Koolau samples. (C) 2012 Elsevier Ltd. All rights reserved.