Chromium isotope evidence for recycled anoxic sediments in the circum-Mediterranean mantle

Abstract

Subduction delivers variable sediment components to convergent margins and plays a critically important role in governing mantle oxygen fugacity (fO2), but the redox states of recycled sediments are difficult to track and remain poorly quantified. Here, we present stable chromium (Cr) isotope data for mafic potassic lavas from the circum-Mediterranean to evaluate sediment redox impacts on the mantle. These lavas display elevated δ53Cr values relative to mid-ocean ridge basalts, indicating mantle metasomatism by melts derived from organic-rich, anoxic sediments. Western Mediterranean lavas show systematically higher δ53Cr values than their eastern counterparts. Crucially, at a given MgO content, western lavas also feature higher Cr concentrations. Because Cr (II) is more incompatible than Cr (III) during mantle melting, these elevated Cr abundances and higher δ53Cr directly demonstrate a more reduced mantle source in the west, driven by a greater mass fraction of subducted anoxic sediments. This mantle fO2 contrast provides an insightful regional redox context for the sparse porphyry mineralization in the west. Chromium isotopes provide a new proxy for tracing sediment recycling and regional porphyry mineralization in convergent margins.