Volcanic arc of Kamchatka:: a province with high-δ18O magma sources and large-scale 18O/16O depletion of the upper crust

We present the results of a regional study of oxygen and Sr-Nd-Pb isotopes of Pleistocene to Recent arc volcanism in the Kamchatka Peninsula and the Kuriles, with emphasis on the largest caldera-forming centers. The delta(18)O values of phenocrysts, in combination with numerical crystallization modeling (MELTS) and experimental fractionation factors, are used to derive best estimates of primary values for delta(18)O(magma). Magmatic delta(18)O values span 3.5parts per thousand and are correlated with whole-rock Sr-Nd-Pb isotopes and major elements. Our data show that Kamchatka is a region of isotopic diversity with high-delta(18)O basaltic magmas (sampling mantle to lower crustal high-delta(18)O sources), and low-delta(18)O silicic volcanism (sampling low-delta(18)O upper crust). Among one hundred Holocene and Late Pleistocene eruptive units from 23 volcanic centers, one half represents low-delta(18)O magmas (+4 to 5parts per thousand). Most low-delta(18)O magmas are voluminous silicic ignimbrites related to large > 10 km(3) caldera-forming eruptions and subsequent intracaldera lavas and domes: Holocene multi-caldera Ksudach volcano, Karymsky and Kurile Lake-Iliinsky calderas, and Late Pleistocene Maly Semyachik, Akademy Nauk, and Uzon calderas. Low-delta(18)O magmas are not found among the less voluminous products of stratovolcano eruptions and these volcanoes do not show drastic changes in delta(18)O during their evolution. Additionally, high-delta(18)O(magma) of +6.0 to 7.5parts per thousand are found among basalts and basaltic andesites of Bezymianny, Shiveluch, Avachinsky, and Koryaksky volcanoes, and dacites and rhyolites of Opala and Khangar volcanoes (7.1-8.0parts per thousand). Phenocrysts in volcanic rocks from the adjacent Kurile Islands (ignimbrites and lavas) define normal-delta(18)O magmas. The widespread and volumetric abundance of low-delta(18)O magmas in the large landmass of Kamchatka is possibly related to a combination of near-surface volcanic processes, the effects of the last glaciation on high-latitude meteoric waters, and extensive geyser and hydrothermal systems that are matched only by Iceland. Sr and Pb isotopic compositions of normal and low-delta(18)O, predominantly silicic, volcanic rocks show negative correlation with delta(18)O, similar to the trend in Iceland. This indicates that low-delta(18)O volcanic rocks are largely produced by remelting of older, more radiogenic, hydrothermally altered crust that suffered delta(18)O-depletion during >2 My-long Pleistocene glaciation. The regionally-distributed high-delta(18)O values for basic volcanism (ca. + 6 to +7.5%o) in Kamchatka cannot be solely explained by high-delta(18)O slab fluid or melt ( sediment) addition in the mantle, or local subduction of hydrated OIB-type crust of the Hawaii-Emperor chain. Overall, Nd-Pb isotope systematics are MORB-like. Voluminous basic volcanism (in the Central Kamchatka Depression in particular) requires regional, though perhaps patchy, remobilization of thick (30-45 km) Mesozoic-Miocene arc roots, possibly resulting from interaction with hot (ca. 1300degreesC), wedge-derived normal-delta(18)O, low-Sr-87/Sr-86 basalts and from dehydration melting of lower crustal metabasalts, variably high in delta(18)O and Sr-87/Sr-86. Copyright (C) 2004 Elsevier Ltd