Iron isotopes may reveal the redox conditions of mantle melting from Archean to Present

Abstract

High-precision Fe isotopic data for 104 samples, including modern and ancient (≥ 3.7 Ga) subduction-related magmas and mantle peridotites, are presented. These data demonstrate that mid-ocean ridge and oceanic-island basalts (MORBs and OIBs) have on average small, but distinctly (~+ 0.06‰) higher 56Fe/ 54Fe ratios than both modern and Eoarchean boninites and many island arc basalts (IABs) that are interpreted to form by large degrees of flux melting of depleted mantle sources. Additionally boninites and many IABs have iron isotopic compositions similar to chondrites, fertile mantle peridotites, Eoarchean mantle peridotites, and basalts from Mars and Vesta. The observed variations are best explained by the bulk silicate Earth having a near-chondritic iron isotopic composition, with ~ + 0.3‰ equilibrium isotope fractionation between Fe 3+ and Fe 2+ and preferential extraction of isotopically heavier, incompatible Fe 3+ during mantle melting to form oceanic crust (as represented by MORBs and OIBs). A quantitative model that relates the iron isotopic composition of basaltic magmas to the degree of partial melting, Fe 3+/Fe 2+ ratio, and buffering capacity of the mantle is presented. The concept that redox conditions may influence iron isotopic fractionation during melting provides a new approach for understanding the redox conditions of magma genesis on early Earth and Mars. Experimental and theoretical work is required to establish iron isotopic fractionation as an oxybarometer of mantle melting.