The origin of hydrous, high-dO voluminous volcanism: diverse oxygen isotope values and high magmatic water contents within the volcanic record of Klyuchevskoy volcano, Kamchatka, Russia

Klyuchevskoy volcano, in Kamchatka’s subduction zone, is one of the most active arc volcanoes in the world and contains some of the highest dO values for olivines and basalts. We present an oxygen isotope and melt inclusion study of olivine phenocrysts in conjunction with major and trace element analyses of Cand tephrochronologically-dated tephra layers and lavas spanning the eruptive history of Klyuchevskoy. Whole-rock and groundmass analyses of tephra layers and lava samples demonstrate that both high-Mg (7–12.5 wt% MgO) and high-Al (17–19 wt% Al2O3, 3–6.5 wt% MgO) basalt and basaltic andesite erupted coevally from the central vent and flank cones. Individual and bulk olivine dO range from normal MORB values of 5.1% to values as high as 7.6%. Likewise, tephra and lava matrix glass have highdO values of 5.8–8.1%. High-Al basalts dominate volumetrically in Klyuchevskoy’s volcanic record and are mostly high in dO. High-dO olivines and more normaldO olivines occur in both high-Mg and high-Al samples. Most olivines in either high-Al or high-Mg basalts are not in oxygen isotopic equilibrium with their host glasses, and DOolivine–glass values are out of equilibrium by up to 1.5%. Olivines are also out of Fe–Mg equilibrium with the host glasses, but to a lesser extent. Water concentrations in olivine-hosted melt inclusions from five tephra samples range from 0.4 to 7.1 wt%. Melt inclusion CO2 concentrations vary from below detection (\50 ppm) to 1,900 ppm. These values indicate depths of crystallization up to *17 km (5 kbar). The variable H2O and CO2 concentrations likely reflect crystallization of olivine and entrapment of inclusions in ascending and degassing magma. Oxygen isotope and Fe–Mg disequilibria together with melt inclusion data indicate that olivine was mixed and recycled between high-Al and high-Mg basaltic melts and cumulates, and Fe–Mg and dO re-equilibration processes were incomplete. Major and trace elements in the variably highdO olivines suggest a peridotite source for the parental magmas. Voluminous, highest in the world with respect to dO, and hydrous basic volcanism in Klyuchevskoy and other Central Kamchatka depression volcanoes is explained by a model in which the ascending primitive melts that resulted from the hydrous melt fluxing of mantle wedge peridotite, interacted with the shallow high-dO lithospheric mantle that had been extensively hydrated during earlier times when it was part of the Kamchatka forearc. Following accretion of the Eastern Peninsula terrains several million years ago, a trench jump eastward caused the old forearc mantle to be beneath the presently active arc. Variable interaction of ascending flux-melting-derived melts with this older, high-dO lithospheric mantle has produced mafic parental magmas with a spectrum of dO values. Differentiation of the higher dO parental magmas has created the volumetrically dominant high-Al basalt series. Communicated by T.L. Grove. Electronic supplementary material The online version of this article (doi:10.1007/s00410-008-0330-0) contains supplementary material, which is available to authorized users. S. Auer I. Bindeman (&) P. Wallace Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, USA e-mail: [email protected] V. Ponomareva Institute of Volcanology and Seismology, 9 Piip Boulevard, Petropavlovsk-Kamchatsky 683006, Russia M. Portnyagin IFM-GEOMAR Leibniz Institute for Marine Sciences, The University of Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany 123 Contrib Mineral Petrol DOI 10.1007/s00410-008-0330-0