F, Cl and S input via serpentinite in subduction zones: implications for the nature of the fluid released at depth Baptiste Debret, 1,2,3 Kenneth T. Koga, 1,2,3 Christian Nicollet, 1,2,3 Muriel Andreani 4 and St ephane Schwartz 5 1 Clermont Universit e, Universit e Blaise Pascal, Laboratoire Magmas et Volcans, Clermont-Ferrand 63038, France; 2 CNRS, UMR6524, LMV, Clermont-Ferrand 63038, France; 3 IRD, R163, LMV, Clermont-Ferrand 63038, France; 4 Laboratoire de G eologie de Lyon, ENS - Universit e Lyon 1, Villeurbanne France; 5 Institut des Sciences de la Terre, Universit e Grenoble I, Grenoble France ABSTRACT The abundances of F, Cl and S in arc magmas are systemati- cally higher than in other mantle-derived magmas, suggesting that these elements are added from the slab along with H 2 O. We present ion probe microanalyses of F, Cl and S in serpen- tine minerals that represent the P–T evolution of the oceanic lithosphere, from its serpentinization at the ridge, to its dehy- dration at around 100 km depth during subduction. F, Cl and S are incorporated early into serpentine during its formation at mid-ocean ridges, and serpentinized lithosphere then car- ries these elements to subduction zones. More than 50% of the F, Cl and S are removed from serpentine during the pro- grade metamorphic lizardite/antigorite transition. Due to the low solubility of F in water, and to the low amount of water released during this phase transition, the fluids mobilizing these elements must be dominated by SO X rather than H 2 O. Terra Nova, 0, 1–6, 2013 Introduction Serpentinites are a common constitu- ent (up to 60%) of oceanic litho- sphere (Cannat et al., 2010). Because they contain up to 13 wt% water, and are thought to be stable down to a depth of 150 km in a typical sub- duction zone (e.g. Wunder and Schre- yer, 1997), serpentinite dehydration at depth may modify the composition of the overlying mantle wedge, as well as the composition of the arc magmas derived from the wedge. Primitive melt inclusions of arc magmas are enriched in F, Cl and S relative to MORB magmas (M etrich et al., 1999; Straub and Layne, 2003). This suggests that these ele- ments are released during slab dehy- dration or melting, and can potentially act as tracers of fluid phases in arc magma sources. Accu- rate assessments of the F, Cl and S budgets in subducting slabs are needed to determine the amounts of elements transferred from the slab to the sub-arc mantle and to the deep mantle. At slow and ultra-slow spreading ridges, the mantle peridotites in the upper 3–6 km of the lithosphere are highly serpentinized to lizardite and chrysotile as a result of seawater cir- culation (Cannat et al., 2010). This process causes an enrichment in trace elements (Kodolanyi et al., 2012), Cl, S (Alt and Shanks, 2003) and F (Or- berger et al., 1999) in the serpenti- nites relative to unaltered peridotite. During subduction, the serpentinized oceanic lithosphere releases fluids via the transformation of chrysotile and lizardite to antigorite (at ~300 °C), and subsequently, at higher tempera- ture (>500 °C), via the breakdown of antigorite to secondary olivine (Evans, 2004). Information on the nature and chemical composition of the fluid released during these two reactions is essential for a better understanding of arc magma compo- sition and production at depth. Previous studies have shown that Cl is transferred by fluids from the slab to arc magmas during serpentine phase changes (Kendrick et al., 2011). Alt et al. (2012) suggested that the last stage of antigorite break- down is also accompanied by S release, based on bulk-rock analyses. The behaviour of F, however, is poorly constrained in subduction zones: while arc magma studies sug- gest that F, Cl and S are transferred from the subducting slab (e.g. Le Voyer et al., 2010), bulk-rock ser- pentinite data do not show any evi- dence for F loss during serpentine phase transitions (John et al., 2011). The aim of this study is to high- light the role of serpentinites in recy- cling F, Cl and S in subduction zones and to increase our under- standing of the nature and composi- tion of the released fluids. We report in situ measurements of halogen (F, Cl) and volatile element (S) concen- trations in serpentines collected in a present-day oceanic setting and in the Western Alps ophiolites. These ophiolites represent highly hydrated and serpentinized fragments of the Jurassic Ligurian Ocean, recording metamorphic grades from greenschist to eclogite facies (Lagabrielle and Cannat, 1990). Geological settings and petrographic study Alpine ophiolites, resulting from the exhumation of fragments of Tethyan oceanic lithosphere during subduc- tion and collision, are an analogue of the Atlantic Ocean lithosphere (Lagabrielle and Cannat, 1990) in which serpentinites are a major com- ponent (Cannat et al., 2010). We sampled a serpentinite suite from Alpine ophiolites, which records dif- ferent metamorphic conditions repre- senting a subduction geothermal gradient from 10 to ~100 km depth (Fig. 1A, B). We compare them with a ‘reference’ oceanic serpentinite from the Mid-Atlantic Ridge (ODP sample). The different varieties of Correspondence: Mr. Baptiste Debret, Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont-Ferrand 63038, France. Tel.: 00 33 673 91 1019; e-mail: b.de- bret@opgc.univ-bpclermont.fr © 2013 John Wiley & Sons Ltd 1 doi: 10.1111/ter.12074