Iron isotopic composition of supra-subduction zone ophiolitic peridotite from northern Tibet

We present high-precision iron isotope data for harzburgites and constituting minerals from the Yushigou suprasubduction zone ophiolite in the North Qilian orogen of northern Tibet to provide insights into iron isotope behavior in subduction zones. The Yushigou harzburgites represent typical melting residues of silica-enriched forearc mantle wedge with significant melt extraction and minimal melt percolation. The delta Fe-56 ranges from -0.096% to 0.110% in olivine, from -0.111% to 0.105% in orthopyroxene and from -0.141% to 0.054% in spinel. Coexisting minerals fall on a line with a slope of 1 in the delta-delta plot, indicating iron isotope equilibrium between mineral pairs. Iron isotopic fractionation between olivine and orthopyroxene is minor, which is consistent with theoretical predictions and previous observations. Good iron isotopic equilibrium between olivine and orthopyroxene is a further indicator that silica enrichment in arc peridotites is inherited from their mantle source. In contrast, significantly negative iron isotopic fractionation between spinel and olivine contradicts with most previous constraints, which may be ascribed to Cr substitution in spinel. Bulk rock harzburgites have delta Fe-56 ranging from -0.097% to 0.076%, the average of which (0.002 +/- 0.020%) is similar to average delta Fe-56 determined for abyssal peridotites. Based on this inference, the lower delta Fe-56 of arc magmas than MORB should be due to smaller discrepancy in Fe3+/RFe between arc magmas and arc peridotites when compared with MORB and abyssal peridotites. Our modelling demonstrates that this can be achieved if arc magmas are produced by redox-buffered melting of silica-enriched mantle wedge while MORB are produced by redox-unbuffered melting of the depleted mantle. (C) 2019 Elsevier Ltd. All rights reserved.