Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone John W. Shervais ⇑ , Marlon M. Jean Department of Geology, Utah State University, UT, USA Received 3 August 2011; accepted in revised form 9 July 2012; available online 21 July 2012 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 sub- ducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the com- position 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 lithol- ogies 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 concentra- tion of fluid mobile elements added to the source: C wr;add ¼½C cpx-obs =½½D cpx =ðD bulk  PFÞ  ½1 ðPF=D bulk Þ ð1=P Þ   ½C 0;wr  where C wr,add = concentration of FME added to mantle wedge during a given melt increment, C cpx-obs = concentration of ob- served pyroxene, D cpx and D bulk = mineral and bulk partition coefficients, P = melt proportion, and F = melt fraction re- quired 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The question of the geochemical flux through the mantle wedge during subduction is critical to our understanding of arc volcanism, and forms an important aspect of the global geochemical flux (Peacock, 1990; Plank and Langmuir, 1998; Tatsumi and Kogiso, 2003). These processes may be inferred indirectly in active subduction systems by measur- ing inputs and outputs, but this approach does not permit direct observation of the dynamic processes within the man- tle wedge source of arc magmas. Direct observation of the mantle wedge is possible, however, by studying outcrops of mantle peridotite that underlie ophiolites formed in fore-arc settings (Pearce et al., 1984; Shervais, 2001; Pearce, 2003; Metcalf and Shervais, 2008). This mantle reflects the pro- 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.07.006 ⇑ Corresponding author. E-mail address: john.shervais@usu.edu (J.W. Shervais). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 95 (2012) 270–285