Oxygen isotope geochemistry of extrusive rocks of the Kurile-Kamchatka arc

Oxygen isotope composition was analyzed in more than 180 samples from 61 vo lcanoes of the Kurile-Kamchatka are. The range of delta(18)O in petrographi cally fresh rock varieties is from 2.4 to 9.3 parts per thousand SMOW and s uggests contributions from two types of crustal materials in the magmatic p rocesses of Kamchatka and the Kurile Islands: enriched in O-18 and depleted in O-18 relative to the mantle. Basalts from various segments of the are s how average values of delta(18)O = 5.8 +/- 0.5 parts per thousand except fo r volcanoes in the Central Kamchatka Depression (Klyuchevskoy Group and Shi veluch), which exhibit higher average values delta(18)O = 6.5 +/- 0.7 parts per thousand. The average values of delta(18)O in intermediate (6.6 +/- 0. 9 parts per thousand) and acid (7.0 +/- 1.4 parts per thousand) lavas of Ka mchatka are higher than in the similar rocks from the majority of the Kuril e Islands (6.2 +/- 0.4 parts per thousand), except for the volcanoes of Sim ushir I. and adjacent submarine volcanoes in the northwest, where higher de lta(18)O values were found. The lavas of Eastern Kamchatka (back zone) that are richer in potassium are higher by 0.5-1 parts per thousand in delta(18 )O compared to the low-potassium lavas of the front zone, whereas the low a nd moderately potassic rocks of the Kuriles do not differ in oxygen isotope composition. The differentiated series of Eastern Kamchatka display high s lopes of the delta(18)O-SiO2 dependence (1.0-1.1 parts per thousand delta(1 8)O per 10% SiO2), which is not consistent with their formation in a closed system and suggests magma contamination in chambers and conduits. The enri chment of the Kamchatka volcanics in the "crustal" isotope O-18 and Sr-87 c ompared to the extrusive rocks of the Kuriles is probably related primarily to the specific structural features of the Earth's crust and to a smaller degree to the character of subduction processes. Unusually low values of de lta(18)O (2.4-5.4 parts per thousand, average 4.6 +/- 1.0 parts per thousan d) are typical of the ignimbrites that were formed by greatest explosions i n the regions of the calderas of Uzon, Malyi Semyachik, Ksudach, Gorelyi, U ksichan, and Tekletunup. It is suggested that ignimbrite formation was acco mpanied by water penetration from enclosing and underlying rocks directly i nto magma chambers. This process is probably responsible for the anomalous isotopic signature and unusual physical properties of water-saturated ignim brite magmas.