Lithium abundance and isotope systematics of forearc serpentinites, Conical Seamount, Mariana forearc: Insights into the mechanics of slab‐mantle exchange during subduction

Abstract

Variable lithium contents and contrasting Li isotopic signatures are evident in the serpentininite muds and clasts extruded at Conical Seamount in the Mariana forearc. The muds have high Li contents compared to mantle values (3–7 ppm) and a mean δ 7 Li value of ∼+6‰. The clasts are generally lower in Li, and δ 7 Li varies from −6‰ to +10‰, with higher δ 7 Li correlating broadly to higher lithium contents. The diverse δ 7 Li in the serpentinite clasts suggests Li exchanges between mantle rocks and evolving slab‐derived fluids from different depths, producing a subarc mantle that is zoned in terms of Li abundances and δ 7 Li. Mariana forearc Li isotopic signatures reach values as high as those reported in Mariana arc lavas, but neither Mariana lavas nor lavas from other “mature” arcs show the variability observed in the serpentinites. Although elevated serpentinite Li contents indicate forearc releases of Li, large differences in δ 7 Li are not evident in the slab outputs at forearc versus arc depths in the Marianas, despite evidence for changes in other alkaline species (e.g., Ba, Sr, K). It is thus possible that the Li sampled by Mariana arc volcanoes is in large part Li which was released in the forearc and transported in the mantle via convection to arc depths, as has been proposed to explain B isotopic variations [ Straub and Layne , 2002 ]. The uniform δ 7 Li signatures observed in “mature” volcanic arcs [ Tomascak et al. , 2002 ] are consistent with higher overall fluxes of slab fluids, which generate larger melting anomalies, including both modified and nonmodified mantle wedge domains. High or low δ 7 Li in lavas from anomalous arcs (e.g., Panama [ Tomascak et al. , 2000 ]) may indicate that the volumes of mantle sampled in these magmatic events are smaller and thus more likely to reflect the signatures of hydrated “plums” [e.g., Allègre and Turcotte , 1986 ; Phipps Morgan and Morgan , 1999 ] with extremely high or low δ 7 Li.