ИСТИНА

Complex core-rim zoning in high-Mg olivine crystals from different Kamchatka rocks were measured for elements of different diffusivity, from fast-diffused Mg-Fe-Ni through middle-diffused Ca and Cr. Slowly-diffused Al and P mainly show the crystal growth history. The most intriguing repetitive zoning patterns were found in rocks from tuff ring of Shiveluch volcano. This complex zoning enables reconstruction of the entire olivine crystallization history from mantle conditions to eruption, including episodes of growth, magma mixing, dissolution, and overgrowth. Three different sequential diffusion episodes changed the geochemistry of the olivine phenocrysts. Our data show that magma ascent can be extremely short during maar/tuff ring eruption: only 1–10 days, while residence times for the cores range from 100 to 2000 days. The actual problem of diffusion chronometry is the origin of edge zoning: if it forms due to crystal growth or due to diffusion? It was found that "Ni-enriched" olivine phenocrysts that fall off and above the Fo-Ni convex downward trends of crystallization have been observed in several occurrences, including subduction and non-subduction settings. Our models show that diffusion trend in the Fo-Ni diagram clearly contrasts with the crystallization trend. We demonstrate diffusion trends for olivines from Kamchatka recent arc basalts from Shiveluch, Klyuchevskoy, Zarechny, and Avachinsky volcanoes and from several mafic complexes, including kimberlites.