The origin of hydrous, high-delta~18O voluminous volcanism: diverse oxygen isotope values and high magmatic water contents within the volcanic record of Klyuchevskoy volcano, Kamchatka, Russia

摘要

Klyuchevskoy volcano, in Kamchatka's sub-duction zone, is one of the most active arc volcanoes in the world and contains some of the highest delta~18O 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 ~14C- and tephro-chronologically-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 percent MgO) and high-Al (17-19 wt percent A1_2O_3, 3-6.5 wt percent MgO) basalt and basaltic andesite erupted coevally from the central vent and flank cones. Individual and bulk olivine delta~18O range from normal MORB values of 5.1 per thousand to values as high as 7.6 per thousand. Likewise, tephra and lava matrix glass have high-delta~18O values of 5.8-8.l per thousand. High-Al basalts dominate volumetrically in Klyuchevskoy's volcanic record and are mostly high in delta~18O. High-delta~18O olivines and more normal-delta~18O 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 DELTA~18O_olivine-glass values are out of equilibrium by up to 1.5 per thousand. 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 percent. Melt inclusion CO_2 concentrations vary from below detection (<50 ppm) to 1,900 ppm. These values indicate depths of crystallization up to approx 17 km (5 kbar). The variable H_2O and CO_2 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 delta~18O re-equilibration processes were incomplete. Major and trace elements in the variably high-delta~18O olivines suggest a peridotite source for the parental magmas. Voluminous, highest in the world with respect to delta~18O, 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-delta~18O litho-spheric 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-delta~18O lithospheric mantle has produced mafic parental magmas with a spectrum of delta~18O values. Differentiation of the higher delta~18O parental magmas has created the volumetrically dominant high-Al basalt series. Both basalt types incessantly rise and mix between themselves and with variable in delta~18O cumulates within dynamic Klyuchevskoy magma plumbing system, causing biannual eruptions and heterogeneous magma products.