Distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry: examples from Tethyan ophiolites in the Western Alps

Large amounts of fluid, bound up in the hydrated upper layers of the ocean crust, are consumed at convergent margins and released in subduction zones through devolatilization. The liberated fluids may play an integral role in subduction zone processes, including the generation of arc‐magmas. However, exhumed subduction zone rocks often record little evidence of large‐scale fluid flow, especially at deeper levels within the subduction zone. Basaltic pillows from the high‐pressure Corsican and Zermatt‐Saas ophiolites show a range of δ 18 O values that overall reflect seafloor alteration prior to subduction. However, comparison between the δ 18 O values of the cores and rims of the pillows suggests that the δ 18 O values of the pillow rims at least have been modified during subduction and high‐pressure metamorphism. Pillows that have not undergone high‐pressure metamorphism generally have rims with higher δ 18 O values than their cores, whereas the converse is the case in pillows that have undergone high‐pressure metamorphism. This reversal in the core to rim oxygen isotope relationship between unmetamorphosed and metamorphosed pillows is strong evidence for fluid–rock interaction occurring during subduction and high‐pressure metamorphism. However, the preservation of different δ 18 O values in the cores and rims of individual pillows and within and between different pillows suggests that fluid flow within the subduction zone was strongly channelled. Resetting of the δ 18 O values in the pillow rims was probably due to fluid‐hosted diffusion that occurred over relatively short time‐scales (<1 Myr).