Variable δ11B signatures reflect dynamic evolution of the Mariana serpentinite forearc

This study aims to uncover the evolving dynamics of element mobility in serpentinised ultramafic clasts within the Asùt Tesoru mud volcano in the Mariana forearc. By employing in situ analysis of trace elements and boron isotopes (δ¹¹B), our findings document a progressive ¹¹B depletion from lizardite- to antigorite-bearing serpentinites, accompanied by a reduction in the incompatible element inventory in some samples. This pattern aligns with either a chemical evolution linked to phase transitions along the slab interface of shallow forearc serpentinites dragged down to depth, or interaction with shallow vs. deep slab fluids. Our results support a scenario of complex fluid and mechanical mixing along the plate interface in the Mariana subduction system, with major implications for the B recycling in convergent margins.