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 oceanicisland basalts (MORBs and OIBs) have on average small, but distinctly (~+ 0.06‰) higher 56 Fe/ 54 Fe 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.

Iron isotope fractionation in planetary crusts

“…The composition of BCR-2 measured here is slightly lighter than that reported by Craddock and Dauphas (2011). Three modern island arc basalts (IABs) treated with different techniques (routine, long and UTEVA chemistries) also agree with the data in a previous study (Dauphas et al, 2009a).…”

“…Both theoretical calculations and experimental determinations show that Fe(III)-bearing phases tend to be enriched in the heavy isotopes of iron compared to Fe(II)-bearing phases (Polyakov and Mineev, 2000;Schauble et al, 2001;Schuessler et al, 2007;Shahar et al, 2008). Such equilibrium isotope fractionation between Fe(III) and Fe(II) may explain, at least in part, the heavy iron isotope composition of MORBs and OIBs relative to that of chondrites and other planetary basalts (Dauphas et al, 2009a). Indeed, terrestrial basalts are formed under more oxidizing conditions than martian meteorites or HEDs (McCammon, 2005;Wadhwa, 2008).…”

“…AC-E is a granite from Ailsa Craig Island, Scotland. In addition, three modern island arc basalts (IABs) from New Britain (NMNH 116852-1, 116852-3 and 116852-11) were also analyzed here to test the accuracy of the measurements (see Dauphas et al (2009a) for a detailed discussion about IABs). (Jarosewich, 1990); NWA1950 (Gillet et al, 2005); Los Angeles (Warren et al, 2000); EETA79001, ALHA77005, Nakhla, ALH84001 (Lodders, 1998); Mil03346 (Day et al, 2006); GRO95555, MET00436, MET00424 ; NWA1461 (Warren et al, 2009);Shalka (McCarthy et al, 1972); ALHA77256 (Sack et al, 1991); NWA1240, NWA049 (Barrat et al, 2003); Cachari (Barrat et al, 2000); Agoult, Dag945 (Yamaguchi et al, 2009); Nuevo Laredo, NWA4523 (Barrat et al, 2007b); D'Orbigny, Sahara99555 (Mittlefehldt et al, 2002);LEW86010 (McKay et al, 1988); NWA1670 (Jambon et al, 2008); NWA1296 (Jambon et al, 2005); NWA2999 (Gellissen et al, 2007).…”

“…In the past decade, the development of high-resolution Multi-Collector InductivelyCoupled-Plasma Mass-Spectrometers (MC-ICP-MS) has allowed measurements of iron isotope composition at high precision (Belshaw et al, 2000;Zhu et al, 2001;Weyer and Schwieters, 2003;Dauphas et al, 2009b;Millet et al, 2012). Following this improvement, small yet resolvable iron isotopic variations in igneous rocks have been discovered (Poitrasson et al, 2004;Weyer et al, 2005;Schoenberg and von Blanckenburg, 2006;Weyer and Ionov, 2007;Heimann et al, 2008;Teng et al, 2008Teng et al, , 2011Dauphas et al, 2009a;Weyer and Seitz, 2012).…”

“…(3) Crust formation: It has been widely observed that iron isotopes could be fractionated during various magmatic differentiation processes of the terrestrial crust, such as partial melting, mineral fractionation and fluids exsolution (Williams et al, 2004;Poitrasson and Freydier, 2005;Weyer and Ionov, 2007;Teng et al, 2008;Schuessler et al, 2009). Island arc basalts also show iron isotope fractionation that may be related to the degree of partial melting (Dauphas et al, 2009a). During partial melting, Fe(III) is more incompatible in olivine and pyroxene than Fe(II) is.…”