Solid-phase transfer into the forearc mantle wedge: Rutile and zircon xenocrysts fingerprint subducting sources

Voluminous flux of a hydrous component from subducting oceanic lithosphere into the forearc mantle and its contribution to arc magmas have long been recognised but the transfer of solid-phase materials has received less consideration. Although xenocrystic zircons are known from some arc magmas and ophiolitic chromitites their origins remain enigmatic. How and when such materials are transferred into the overlying wedge of mantle lithosphere and the length of their residency therein are unclear. Using zircons and rutiles recovered from ophiolitic rocks in New Caledonia, we demonstrate unambiguous association with subducting sediments and hence evidence for inter-plate transfer. The ‘fingerprints’ of age spectra for both minerals can be matched to lithospheric slab components subducted during Eocene time. Their occurrence in forearc harzburgite suggests that they were incorporated into their mantle wedge host at subduction depths beyond the onset of serpentinisation at ca. 300 °C but before the ca. 600 °C destabilisation temperature of the rutile U-Pb system. Relocation from slab to forearc mantle is therefore likely to have occurred at ca. 50-60 km depth within the subduction channel. Fluid-assisted conveyance of high field strength elements within solid-state accessory mineral phases into supra-subduction mantle wedges may represent a little-recognised but significant phenomenon that is both predictable and pervasive at convergent margins globally.