Isotopic constraints on slab-derived fluids and mantle wedge serpentinization

Mantle wedge serpentinization is driven by fluids released from subducting slab and represents a key mechanism of crust-mantle chemical exchange in subduction zones. However, the relative contributions of different slab lithologies to forearc serpentinization remain poorly constrained, despite the consequences for mineralogy, redox state, and the volatile budget in the mantle wedge. Here, we present radiogenic (⁸⁷Sr/⁸⁶Sr) and stable strontium isotope (d⁸⁸Sr) data for pore fluids, mantle wedge serpentinites, and subducted lithologies (metasediments, metabasalts), recovered from the Mariana Forearc during IODP Expedition 366.

Across all sample types, ⁸⁷Sr/⁸⁶Sr values cluster narrowly between 0.705 and 0.706, indicating strong buffering by a dominant Sr reservoir. These values overlap those of altered oceanic crust, with only a minor sedimentary contribution, implying extensive fluid-rock interaction and redistribution of Sr along the decollement zone. In contrast, d⁸⁸Sr values vary substantially and reflect both source and process-dependent modification of Sr during fluid transport. Compared to incoming altered oceanic crust (ODP801), metabasalts show increased Sr concentrations accompanied by lower d⁸⁸Sr, consistent with carbonate-controlled Sr addition during interaction with deeper slab-derived fluids along the plate interface. Metasediments largely retain their incoming d⁸⁸Sr composition, with one sample recording anomalously heavy values (0.63‰).

Pole fluids and serpentinites from the shallowest sites exhibit the heaviest d⁸⁸Sr values (∼0.6‰), which decrease systematically at greater depths (∼0.3‰ and ∼0.2‰). We interpret these trends as reflecting a depth-dependent transition in Sr-hosting phases and fluid sources, with sulphate precipitation in subducted sediments producing isotopically heavy fluids at shallower levels, and carbonate dissolution-precipitation reactions dominating Sr budgets and d⁸⁸Sr signatures at greater depths. Together, these results demonstrate that while radiogenic Sr isotopes are homogenized by forearc fluid-rock interaction, stable Sr isotopes provide a sensitive tracer of slab-fluid provenance and reaction pathways during mantle wedge serpentinization.