ИСТИНА

Complex core-rim zoning in high-Mg olivines (Ol) from tuff ring of Shiveluch volcano, Kamchatka, enables reconstruction of the Ol-crystallization history from source to eruption. Zonation in olivine was measured by EMP for major and trace elements with different diffusivities and profile orientations controlled by EBSD. Pressure from Cpx-melt compositions range from 6 to 10 kbar the Al-in-Ol thermometry gave temperatures of 1130 to 1260 °C. Ol-crystals show different types of Fo and Ni zonation: normal, reverse, and repetitive, indicating complex growth histories. Ol-cores formed from a high-Mg, high-Ni melt and initially had a uniform composition. These Ol-crystals then interchanged by diffusion to different extents with an evolved melt. Diffusion modelling allows extracting the times that these crystals spent in different magmas. Un-equilibrated inner cores indicate diffusion times ranging from 400 to 1800 days. Cores were affected by diffusion gave times ranging from 100 to 2000 days. Some Ol-crystals even completely equilibrated to low forsterite contents with evolved melt. Next, magma mixing with a high temperature mafic melt resulted in partial dissolution of Ol-cores. Subsequent cooling gave new high-Mg and high-Ni Ol growth on the resorption interface. Forsterite smoothly decreases in the new mantle of the crystals towards the rims of crystals where diffusion has partially smoothed out the zonation. By contrast, slow-diffusing elements (Cr, Al, P) still show concentric, oscillatory and correlated growth bands that are likely formed by kinetic effects during fast crystal growth. The duration of this last growth and diffusion stage is only 1-10 days as suggested by combined Ni-Fo diffusion modelling across the resorption interface. This suggests a very short time between the last recharge event and eruption. Our research shows that magmatic processes in mafic magmas that ascend from their mantle source to surface may be quite complex and time of ascent from Moho depths can be very fast – just a few days.