Formation conditions of allivalites, olivine-anorthite crystal enclaves, in the volcanics of the Kuril-Kamchatka arc

In this paper we address allivalites, coarse-and giant-textured olivine-anorthite rocks occurring as separate blocks in the eruption products of many volcanoes from the frontal part of the Kuril-Kamchatka arc. New data are reported on the petrography, mineralogy, and composition of melt inclusions in minerals from ten allivalite samples from Ksudach, Il’inskii, Zavaritskii, Kudryavyi, and Golovnin volcanoes. The crystallization temperatures of allivalite minerals were estimated as 970–1080°C at a melt water content of 3.0–3.5 wt % and oxygen fugacity NNO = 1–2. A genetic link was established between the compositions of melt inclusions and interstitial glasses in allivalites and volcano rocks. The cumulate nature of allivalites was demonstrated. Using mass balance calculations, the degree of fractionation of primary melts during the formation of cumulate layers was estimated for various volcanoes as 22–46%.