Unveiling rodingite metasomatism using B and Sr isotopes: Insights from the Voltri Massif, Western Ligurian Alps, Italy

The recycling of volatiles and incompatible elements at subduction zones is a key driver of Earth’s geochemical evolution, with metasomatic reactions governing elemental fluxes. Rodingites, found at mafic–ultramafic interfaces, may transform into metarodingites during subduction, often accompanied by chlorite-rich blackwall zones. While these rocks are potential geochemical reservoirs, their role in boron (B) cycling remains unclear. This study presents new in-situ δ11B data from chlorite, along with trace element and Sr isotope analyses of clinopyroxene and clinozoisite, from metarodingite–blackwall assemblages in the Voltri Massif (Western Ligurian Alps, Italy). The metarodingites exhibit assemblages of clinopyroxene + garnet + chlorite ± epidote, consistent with rodingitization under oceanic conditions. Preserved mantle-like 87Sr/86Sr and REE signatures in igneous clinopyroxene cores suggest limited overprinting by seawater-derived fluids. Trace element redistribution in metamorphic minerals (e.g., HFSE loss, Eu anomalies) points to prograde subduction overprint, and Sr isotopes suggest seawater-derived recycling during burial. Chlorite δ11B values vary spatially in metarodingites, reflecting differences in water/rock ratios and interaction with B-rich fluids during oceanic metasomatism. In contrast, blackwall chlorite shows more uniform δ11B, possibly due to Mg-metasomatism at high water/rock ratios and equilibration at the mafic–ultramafic boundary. Whether these signatures are oceanic or subduction-related remains unresolved, pending geochronological constraints. These data suggest chlorite may serve as a previously unrecognized B reservoir, releasing 11B-enriched fluids during high-pressure breakdown, thereby influencing deep volatile cycling