Probing the conditions of mantle melting with iron isotopes

Abstract

While extraterrestrial basalts have near-chondritic iron isotopic compositions (~0 ‰), mid-ocean ridge and ocean island basalts have heavy δ^(56)Fe (~+0.1 ‰ relative to IRMM- 014) [e.g., 1]. This was interpreted to reflect kinetic isotope fractionation associated with vaporization during the Moon-forming impact [2] or equilibrium fractionation at core-mantle boundary conditions [3]. A more mundane interpretation is that Fe isotopes are fractionated during partial melting [4]. Indeed, fertile mantle peridotites define a δ^(56)Fe value of 0.02±0.03., indistinguishable from chondrites [4]. In addition, measurable isotopic fractionation between olivine and melt was recently documented in a natural example of magmatic differentiation [5]. Whether Eoarchean magmas had δ^(56)Fe similar to phanerozoic MORBs-OIBs is presently unknown. Dauphas et al. [6] suggested that such magmas could have had δ^(56)Fe closer to chondritic but analytical precision was insufficient to reach a definitive conclusion. We report high precision isotopic analyses of Fe [7] in chondrites and mafic igneous rocks. Several Eoarchean island arc basalts and boninites have Fe isotopic compositions similar to mantle peridotites. A quantitative model is presented that can explain Fe isotopic variations measured in mantle-derived magmas. While Fe isotopes may not be very useful for tracing processes of planetary formation, they provide invaluable information on the conditions of magma genesis through time.

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