A slab origin for low-δ18O arc magmas

Plate tectonics is the primary method for cycling of material between the mantle, crust, and surface reservoirs of our planet. Oxygen isotopes (18O/16O, δ18O) in zircon have been shown to track source components through subduction, primarily by detecting the presence of isotopically heavy supracrustal material. Isotopically light signatures are relatively rare, suggesting recycling of high-temperature hydrothermal sources is negligible. Here, we report light δ18O data from magmatic-arc rocks of the 511–500 Ma Stavely Belt in western Victoria, Australia. These rocks demonstrate a two-stage mixing history: (1) constant, highly radiogenic εHf with decreasing δ18O, indicating sub-mantle δ18O initial compositions, interpreted to represent a sub-arc mantle contaminated with low-δ18O slab melts and/or fluids; and (2) decreasing εHf with increasing δ18O, implying crustal contamination with country-rock turbidites. These new data suggest that high-temperature hydrothermal sources can be recycled through subduction zones and alter the composition of the sub-arc mantle. We demonstrate how slab tearing could have driven this process, its connection to the architecture of the Delamerian Orogen, and implications for circum-supercontinent margins. Citation: Mole, D.R., Lewis, C.J., Bodorkos, S., Crowley, J.L., Schofield, A., Curtis, S.O. and Evans, N.J., 2025. A slab origin for low-δ18O arc magmas. Geology, 53(4), pp.311–316. https://doi.org/10.1130/G52782.1