Abstract
Magmas in volcanic arcs have geochemical and isotopic signatures that can be related to mantle metasomatism due to fluids and melts released by the down-going oceanic crust and overlying sediments, which modify the chemistry and mineralogy of the mantle wedge. However, the effectiveness of subduction-related metasomatic processes is difficult to evaluate because the composition of arc magmas is often overprinted by interactions with crustal lithologies occurring during magma ascent and emplacement. Here, we show unequivocal evidence for recycling of continental crust components into the mantle. Veined peridotite xenoliths sampled from Tallante monogenetic volcanoes in the Betic Cordillera (southern Spain) provide insights for mantle domains that reacted with Si-rich melts derived by partial melting of subducted crustal material. Felsic veins crosscutting peridotite and the surrounding orthopyroxene-rich metasomatic aureoles show the highest 18O/16O ratios measured to date in upper mantle assemblages worldwide. The anomalously high oxygen isotope compositions, coupled with very high 87Sr/86Sr values, imply the continental crust origin of the injected melts. Isotopic anomalies are progressively attenuated in peridotite away from the veins, showing 18O isotope variations well correlated with the amount of newly formed orthopyroxene. Diffusion may also affect the isotope ratios of mantle rocks undergoing crustal metasomatism due to the relaxation of 18O isotope anomalies to normal mantle values through time. Overall, the data define an O isotope “benchmark” allowing discrimination between mantle sources that attained re-equilibration after metasomatism (>5 Myr) and those affected by more recent subduction-derived enrichment processes.
Highlights
Mantle wedges at destructive plate margins are significantly heterogeneous in chemical and isotopic compositions due to the reaction of the original mantle mineralogy with fluids and/or melts from the subducted slab[1,2,3,4,5,6,7]
Especially in continental settings, shallow-level crustal contamination can modify the composition of the erupted magmas, making it difficult to discriminate between processes occurring within the mantle source and those affecting the magmas en route to the surface[17,18,20]
The studied samples provide an almost unique opportunity to directly study the effect of mantle metasomatism without any possible influence of shallow-level crustal contamination
Summary
Luigi Dallai[1], Gianluca Bianchini 2, Riccardo Avanzinelli[3,4], Claudio Natali2,3 & Sandro Conticelli 3,4,5. Sub-continental lithospheric mantle interacts with percolating fluids or melts released by the subducting lithosphere[12,13,14,15,16,17,18] Evidence of these processes is found in the chemical and isotopic compositions of subduction-derived magmatic rocks worldwide[8,13,19]. The limited oxygen isotope fractionation at mantle temperatures and the narrow O isotope compositional variations in “typical” mantle rocks (δ18Ool = 5.18 ± 0.28‰; δ18Oopx = 5.69 ± 0.28‰; δ18Ocpx = 5.57 ± 0.36‰)[24] make oxygen isotopes a powerful tool for identifying recycled crustal material in the mantle[23,25] It is difficult, to define the specific (oceanic vs continental) nature of the subducted components, and evidence for recycling of continental crust is generally rare and elusive[26]. The studied samples provide an almost unique opportunity to directly study the effect of mantle metasomatism without any possible influence of shallow-level crustal contamination
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