Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts

• Published in: Earth and Planetary Science Letters
• Main Authors: Portnyagin, Maxim, Mironov, Nikita, Botcharnikov, Roman, Gurenko, Andrey, Almeev, Renat R., Luft, Cornelia, Holtz, Francois
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2019
• Subjects: Kamchatka
• Online Access: https://oceanrep.geomar.de/id/eprint/46550/; https://oceanrep.geomar.de/id/eprint/46550/1/Portnyagin-2019-EPSL.pdf; https://oceanrep.geomar.de/id/eprint/46550/2/Portnyagin-2019-EPSL-Supplementary%20tables.zip; https://doi.org/10.1016/j.epsl.2019.04.021

Highlights • New experiments with melt inclusions in olivine at 1200 °C and 300 MPa. • Coupled behavior H2O and SiO2 in inclusions during re-hydration and dehydration. • SiO2 mobility results from formation/destruction of metal vacant olivine. • SiO2-undersaturated arc melt inclusions may originate by dehydration. • New method to assess initial H2O in dehydrated inclusions. Abstract Primary subduction-related magmas build up modern continental crust and counterbalance massive recycling of crustal material into the deep mantle occurring at this tectonic setting. Melt inclusions in Mg-rich olivine are believed to be the best probes of primary subduction-related melts. However, unexpectedly, most of such inclusions are SiO2-undersaturated, in contrast to predominantly SiO2-saturated island-arc rocks. The origin of these melts has been explained by melting of amphibole-bearing pyroxenites in the lower crust or upper mantle. The current models fail, however, to explain the high abundance of SiO2-undersaturated melts as well as their compositional difference with host rocks for the major elements but not for incompatible trace elements. Here we report results from the investigation of rocks and melt inclusions in olivine from Klyuchevskoy volcano in Kamchatka. We show that experimental re-hydration of SiO2-undersaturated melt inclusions in olivine Fo85−90 at 300 MPa pressure and 1200 °C causes a concomitant enrichment of melt in H2O and SiO2 so that re-hydrated inclusions (4–5 wt% H2O) become as silica-saturated as primitive Klyuchevskoy rocks. An experimental dehydration of previously re-hydrated inclusions also resulted in coupled depletion of melt in H2O and SiO2. The estimated stoichiometry of SiO2 and H2O gain/loss is consistent with dissolution/crystallization of metal-defect olivine on inclusion walls. The migration mechanism of water is controlled by hydrogen diffusion in the octahedral metal (Mg, Fe) vacancies through olivine structure as confirmed by FTIR spectroscopy. We conclude that the previously ...