Complex zoning in olivines of the Shiveluch volcano (Kamchatka)

Gordeychik, B.; Churikova, T.; Simakin, A.; Kronz, A.; Wörner, G.

Авторы: Gordeychik B., Churikova T., Simakin A., Kronz A., Wörner G.
Сборник: The System Earth and its Materials - from Seafloor to Summit. GeoBremen-2017. Joint Meeting of DGGV and DMG. Bremen, Germany, September 24-29, 2017
Тезисы
Год издания: 2017
Место издания: Universität Bremen Bremen, Germany
Первая страница: A-330
Аннотация: Complex zonation patterns in olivine from Shiveluch basalts were studied by electron microprobe. Olivine cores (Fo87-92) show bell-shaped or flat zonation. In the mantle forsterite first decrease, then abruptly falling with a steeper gradient towards the rim. Fo-Ni relations identify distinct core populations that together form a convex trend in Fo-Ni space. From these different cores, irregular Fo-Ni trajectories meet in a common mantle composition. From there, growth follows the common concave trend in the crystal mantle to the rims. We conclude that cores do not fall on a fractional crystallization trend; in mantles the composition became common and mantle with rim follow typical fractionation trend. We recognize four sequential growth and diffusion events: (1) “core” diffusion at an early stage; (2) “deep” diffusion resulting in variably homogenized cores and forming trend of cores; (3) diffusion between core and mantle; and (4) diffusion at the outermost rim. Diffusion in the cores at an early stage (1) is observed only in most magnesian rich cores and is described by the analytical solution. “Deep” diffusion in cores (2) is described using Newton's law of cooling. Due to mixing with another melt, there are prominent Ni-inflections between the cores and mantles. The width of the diffusion zone (3) at these boundaries is less for Ni than for Fo, reflecting different diffusion coefficients, allowing to estimate the time after mixing both over Ni and Fo. Zonation within the mantle towards the rims has smoother gradients for Ni and Fo compared to the gradients at the Ni inflections. This means that the rim gradients cannot be result of a diffusion process only, because the time of diffusion within the outer mantle cannot be longer than at the core-mantle boundary. Rim zonation thus is dominated by growth and this is confirmed by element maps that show narrow growth zones where the width of growth zones increase according to relative diffusivities (4) for P→Al→Cr→Ca→Ni→Fo. Minor element profiles can be extracted as cps-equivalent semi-quantitative values from these element maps. Our data show Fo and trace element zonation reveals a complex history with alternate growth, mixing and diffusion prior to eruption. Elemental maps are essential way to document these processes and form the basis for further diffusion modeling. This research was supported by RFBR-DFG grant # 16-55-12040.
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Complex zoning in olivines of the Shiveluch volcano (Kamchatka)тезисы доклада

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