Contrasting geochemical fingerprints of back-arc basins: In-situ evidence from the Sulu Sea and Celebes Sea basalts

The Southeast Asian subduction system, shaped by the convergence of multiple tectonic plates, hosts a series of Cenozoic marginal basins that have evolved through an intricate combination of geodynamic processes. The nature of the mantle sources of volcanism underlying these basins remains a subject of debate. Here, to clarify the characteristics of these mantle sources, we present in-situ major, trace element, and Sr isotopic data for clinopyroxene and plagioclase derived from volcanic rocks of the Sulu Sea and the Celebes Sea. Despite the fact that both basins occupy back-arc tectonic settings, they differ markedly in the extent of contributions from subduction-related fluids and sediments. Sulu Sea volcanism, primarily influenced by back-arc extension and intra-arc rifting, shows geochemical signatures indicative of fluid release from altered oceanic crust (AOC-fluid) associated with stagnant slabs in the mantle transition zone. In contrast, Celebes Sea volcanism records a stronger slab input, with contamination by recycled oceanic crust that underwent plagioclase fractionation. Integrated seismic tomography and in-situ mineral geochemistry collectively affirm the distinction between the mantle source features and evolution pathways of the Sulu Sea vs. Celebes Sea back-arc basin volcanism.