Scaling and nature of melting processes in the mantle wedge: a record from the Josephine Peridotite (Klamath Mountains, USA)

The selected peridotite samples come from the Josephine ophiolite, emplaced 157 My ago and now part of the western Jurassic belt of the Klamath Mountains in Northern California and Southwest Oregon [1]. The ophiolite is partly dismembered but presents a complete ophiolitic section from mantle peridotites in the lower part (our samples) to pillow lavas at the top. The Josephine peridotite extends over >800km and has most likely originated in a fore-arc or back-arc setting [1]. It is mostly composed of massive harzburgites, believed to be the residue of partial melting processes, crosscut by several generations of dykes and veins of variable lithologies [2]. Previous studies have mainly focused on the dunite bodies because of their importance as melt extraction channels in the upper mantle. They are believed to have formed through meltrock reactions between depleted melts and the mantle wedge [3-6]. However, large areas of the Josephine peridotite have yet to be investigated. Here we propose to focus on the compositional variability of harzburgites and lherzolites, which likely represent the protolith of mantle wedge before the diking and veining events that later affected the massif. Our goal is to constrain the scale of melting processes in the upper mantle in subduction environments and better assess the role and distribution of subduction-derived fluids in the melting process. In order to achieve this, our study is based on the observation of geochemical gradients (major, minor and trace elements in minerals and whole-rocks) throughout the selected harzburgites and lherzolites. Based on major elements and most minor and trace element trends (e.g. HREE, Ti, Ni versus MgO wt%) we find that harzburgites and lherzolites are genetically related by a partial melting event, as suggested in previous studies [2]. However the concentrations of the most incompatible elements (e.g. LREE) and fluid-mobile elements (Ba, Rb, Sr) cannot result from a partial melting process but require the later addition of fluids and/or melts superimposed to the partial melting features recorded by the Josephine peridotites. Mapping out the melting and melt percolation processes that took place in the Josephine Peridotite should shed a new light on the extent and nature (dry or hydrous) of melting in the upper mantle from subduction environments.