The genesis of the stable isotope (O, H) record in arc magmas: the Kamtchatka's case

We have determined delta(18)O in 120 chemically well-documented samples of lavas characteristic of the major areas and types of volcanism, most of whi ch Quaternary in age, from the subduction zone volcanism in the Kamchatka p eninsula, far-eastern Russia. Hydrogen isotopic data have also been obtaine d on about half of the samples. This volcanism has been highly active from the Cretaceous to the present and results from high convergence rates betwe en the North Pacific plate and the Northeastern part of the Eurasian plate. The peninsula is the northernmost extension of the Japan-Hokkaido-Kurile I sland Are that runs continuously over about 2000 km. Hundreds of Quaternary are-related volcanic edifices occupy its two main areas: the Eastern Volca nic Front (EVF), and the Central Kamchatka Depression (CKD). In Kamchatka, the contributions of sediments and continental crust are low, which makes e asier the specific evaluation of the mantle wedge and the descending slab c ontributions. delta(18)O range from 5.3 to 8.5 parts per thousand, delta D from -70 to -212 parts per thousand and water contents from 0.01 to 0.9% in basic lavas. This high scatter is pristine and inherited from the mantle s ource of the magmas. Comparable ranges and scatter in evolved lavas (andesi tes to rhyolites) are mainly due also to parent magma variability. Combined delta(18)O, delta D values and H2O contents are explained in terms of a mi xing, differentiation and degassing model. All the lavas result from mixtur es of normal mantle magmas with a slab-derived, water-rich fluid/magma whic h is an essentially equimolecular mixture of water and plagioclase-like sil icates, together with Mg and Fe concentrations around 1 and 3%, respectivel y. The average initial water concentration in the resulting lavas is 5.5%. if that characteristic concentration is a general feature in subduction zon es, this would correspond to an upward volcanic and plutonic water flux of around 10(15) g/year. That 'leakage' flux equilibrates the flux subducted t hrough sediments and hydrated oceanic crust so that the remaining flux subd ucted into the mantle is of the order of the ridge flux. The water flux cou ld be a significant part of the water involved at mid crustal levels in ana texis phenomena and explain the convergence of granites' delta(18)O's towar ds the 8.5-10 parts per thousand range. (C) 1999 Elsevier Science B.V. All rights reserved.