VOLCANIC ARC OF KAMCHATKA: A PROVINCE WITH HIGH-δ18O MAGMA SOURCES AND LARGE-SCALE 18O/16O DEPLETION OF THE UPPER CRUST

Abstract

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 δ18O values of phenocrysts, in combination with numerical crystallization modeling (MELTS) and experimental fractionation factors, are used to derive best estimates of primary values for δ18O(magma). Magmatic δ18O values span 3.5‰ 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-δ18O basaltic magmas (sampling mantle to lower crustal high-δ18O sources), and low-δ18O silicic volcanism (sampling low-δ18O upper crust). Among one hundred Holocene and Late Pleistocene eruptive units from 23 volcanic centers, one half represents low-δ18O magmas (+4 to 5‰). Most low-δ18O magmas are voluminous silicic ignimbrites related to large >10 km3 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-δ18O magmas are not found among the less voluminous products of stratovolcano eruptions and these volcanoes do not show drastic changes in δ18O during their evolution. Additionally, high-δ18O(magma) of +6.0 to 7.5‰ are found among basalts and basaltic andesites of Bezymianny, Shiveluch, Avachinsky, and Koryaksky volcanoes, and dacites and rhyolites of Opala and Khangar volcanoes (7.1–8.0‰). Phenocrysts in volcanic rocks from the adjacent Kurile Islands (ignimbrites and lavas) define normal-δ18O magmas. The widespread and volumetric abundance of low-δ18O 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-δ18O, predominantly silicic, volcanic rocks show negative correlation with δ18O, similar to the trend in Iceland. This indicates that low-δ18O volcanic rocks are largely produced by remelting of older, more radiogenic, hydrothermally altered crust that suffered δ18O-depletion during >2 My-long Pleistocene glaciation. The regionally-distributed high-δ18O values for basic volcanism (ca. + 6 to +7.5‰) in Kamchatka cannot be solely explained by high-δ18O 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. 1300°C), wedge-derived normal-δ18O, low-87Sr/86Sr basalts and from dehydration melting of lower crustal metabasalts, variably high in δ18O and 87Sr/86Sr.