Controls on the iron isotopic composition of global arc magmas

• Published in: Earth and Planetary Science Letters
• Main Authors: Foden, J., Sossi, P., Nebel, O.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 2018
• Subjects: Iron isotopes; volcanic arcs; subduction; oxidation; thermal parameter; Pacific; Tonga; Sunda; Kamchatka
• Online Access: http://hdl.handle.net/2440/113823; https://doi.org/10.1016/j.epsl.2018.04.039

Available online 17 May 2018 We determined the iron isotope composition of 130 mafic lavas from 15 arcs worldwide with the hypothesis that the results would reflect the relatively high oxidation state of arc magmas. Although this expectation was not realized, this Fe isotope data set reveals important insights into the geodynamic controls and style of the melting regimes in the sub-arc mantle. Samples are from oceanic arcs from the circum-Pacific, the Indonesian Sunda–Banda islands, Scotia and the Lesser Antilles as well as from the eastern Pacific Cascades. Their mean δ57Fe value is +0.075 ± 0.05‰ significantly lighter than MORB (+0.15 ± 0.03‰). Western Pacific arcs extend to very light δ57Fe (Kamchatka = −0.11 ± 0.04‰). This is contrary to expectation, because Fe isotope fractionation factors (Sossi et al., 2016, 2012) and the incompatibility of ferric versus ferrous iron during mantle melting, predict that melts of more oxidized sources will be enriched in heavy Fe isotopes. Subducted oxidation capacity flux may correlate with hydrous fluid release from the slab. If so, a positive correlation between each arc's thermal parameter (ϕ) and δ57Fe is predicted. On the contrary, the sampled arcs mostly contribute to a negative array with the ϕ value. High ϕ arcs, largely in the western Pacific, have primary magmas with lower δ57Fe values than the low ϕ eastern Pacific arcs. Arcs with MORB-like Sr-, Nd- and Pb-isotopes, show a large range of δ57Fe from heavy MORB-like values (Scotia or the Cascades) to very light values (Kamchatka, Tonga). Although all basalts with light δ57Fe values have MORB-like Pb-, Nd- and Sr-isotope ratios some, particularly those from eastern Indonesia, have heavier δ57Fe and higher Pb- and Sr- and lower Nd-isotope ratios reflecting sediment contamination of the mantle wedge. Because basalts with MORB-like radiogenic isotopes range all the way from heavy to light δ57Fe values this trend is process-, not source composition-driven. Neither the slab-derived influx of fluids with light iron or ...