Crustal rhyolite melts at mantle depths

Melts with rhyolite compositions originate from partial melting of crustal rocks or extensive differentiation of basaltic melts, at temperatures in the range of 800 °C. Accordingly, they are confined to the shallow continental crust. Nevertheless, experimental studies have demonstrated that dacite-rhyolite melts can be generated at higher temperature (> 1000°c) and pressure (>2 GPa), by partial melting of continental crustal lithotypes, but direct evidence for their occurrence has never been found. This implies that rhyolite melts may be produced at mantle conditions either by subduction of sedimentary material or exhumation of subducted continental crust. Ephemeral rhyolite melt inclusions were found preserved in peridotite xenoliths from Tallante (Betic Cordillera, southern Spain) that are remnants of a supra-subduction mantle wedge. Here, the interaction of silica-rich melts with peridotite generated hybrid mantle domains, characterized by the occurrence of millimetre-sizes felsic veins with crust-like Sr-Nd-Pb-O- isotope compositions. The “Tallante” composite xenoliths were found among a wide population of peridotitic xenoliths, and display extreme compositional and isotopic heterogeneities both within the ambient peridotite and within the felsic veins. The latter consist of orthopyroxene, plagioclase, and quartz, and they are separated from the surrounding peridotite by an orthopyroxene-rich reaction zone. In their mineral phases, rhyolite glass inclusions and interstitial films associated to quartz crystals were observed. Petrological evidence and thermodynamic modelling indicate that rhyolite melts were originated by partial melting of near an-hydrous garnet-bearing metapelites at temperatures above 1000 °C. Partial melting was likely triggered by near-isothermal decompression during rapid exhumation of previously subducted crustal slivers. The melts reacted with the ambient lithospheric mantle at lower temperature (900 °C) and produced orthopyroxene, followed by plagioclase, quartz, and phlogopite. On the basis of chemical characteristics, it is hypothesized that potassic (HK-calc-alkalic to shoshonitic) and ultrapotassic magmas may originate from metasomatic mantle sources generated from the interaction of crustal rhyolitic melts with mantle peridotite.