ИСТИНА

The duration and rates of magmatic processes preceding eruption are yet to be understood. Complex core-rim zonation 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 olivines was measured by EMP for elements with different diffusivities and profile orientations were controlled by EBSD. Pressure determined from Cpx/Cpx-melt compositions range from 6 to 10 kbar and Al-in-Ol thermometry gave temperatures of 1130 to 1260 °C. These results and the mafic Ol composition suggest Ol-formation in primitive magmas in the Earth’s mantle. Ol-crystals show different types of Fo and Ni zonation: normal, reverse, and repetitive, indicating complex growth histories. We apply diffusion modeling to extract the times that these crystals spent in different magmas. Ol-cores were 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. Ol-crystals where the diffusion process did not reach the inner cores indicate diffusion times ranging from 400 to 1800 days. Some other inner cores were affected by diffusion with diffusion times ranging from 100 to 2000 days. Some Ol-crystals even completely equilibrated to low forsterite contents with this evolved melt. The next event in the Ol history was magma mixing with a high temperature mafic melt and as result the rims of Ol-cores were partly dissolved. Subsequent cooling results in new high-Mg and high-Ni Ol growth on the resorption interface. This growth stage formed the mantle of the crystals in which forsterite smoothly decreases towards the rims of crystals. By contrast, slow-diffusing elements (Cr, Al, P) 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 modeling 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.