In-situ lithium isotope geochemistry for a veined jadeitite from the New Idria serpentinite body, California: Constraints on slab-derived fluid and fluid-rock interaction

Lithium behavior in slab-derived fluids has been constrained using isotope geochemistry of subduction-related metamorphic and volcanic rocks. We investigated a veined jadeitite from the New Idria serpentinite diapir, California, which is regarded as direct precipitates from slab-derived fluids and therefore records slab-derived fluid signatures. We applied an in-situ measurement of Li concentration and δ⁷Li composition using an ablation volume correction (AVC) LA-MC-ICPMS. This method enabled determinations of millimeter- to submillimeter-scale isotopic variations in the jadeitite veins and host rocks (jadeite matrix), allowing immediate observations on fluid–rock interactions. Multiple-stage jadeite veins and their host rocks showed a wide range of Li concentrations from 4 to 68 μg/g and δ⁷Li compositions from −11.7 to +6.7‰, with a curvilinear correlation between them. Individual veins formed in different generations also showed wide isotopic variations as large as ~14‰. Those isotopic and compositional variations within/among veins can be readily explained by variable mixing between the matrix and infiltrated fluids. The initial infiltrated fluid compositions were estimated to be between +6.7 and +12.3‰, based on the δ⁷Li values of jadeites in the veins that were supposed to be unmodified by interactions with their matrices. The estimated fluid composition is mostly consistent with those inferred for slab-derived fluids proposed by previous studies. The New Idria jadeitite provides a line of evidence for the presence of high δ⁷Li fluids in the mantle wedge at the forearc depth. Our study also demonstrates that the high δ⁷Li composition of slab-derived fluids can be easily modified by interactions with surrounding rocks along their pathways.