Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

Abstract

Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This “subduction factory” is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones.The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source:Cwr,add=[Ccpx-obs/[[Dcpx/(Dbulk-PF)]∗[1-(PF/Dbulk)](1/P)]]-[C0,wr]where Cwr,add=concentration of FME added to mantle wedge during a given melt increment, Ccpx-obs=concentration of observed pyroxene, Dcpx and Dbulk=mineral and bulk partition coefficients, P=melt proportion, and F=melt fraction required to model the observed MREE–HREE concentrations. Application of this algorithm to CRO peridotites shows that fluid influx must be continuous with open system melting, which allows us to calculate FME concentrations for small melt increments. Addition of the calculated FME concentrations to depleted MORB mantle (DMM) asthenosphere or refractory arc mantle (RAM) results in pooled magmas that match primitive arc tholeiites and boninites.