Abstract <p>The cinder cones of the Tolbachik volcanic fissure zone, characterized by olivine-bearing basaltic lavas, provide extensive material for studying zoning patterns in olivine phenocrysts by diffusion chronometry. High-precision microprobe analysis of composition profiles across olivine phenocrysts has made it possible to reveal distinct forsterite and nickel zoning localized both at the outer rims and within the cores of the phenocrysts. Diffusion chronometry models successfully approximate all observed zoning profiles for both forsterite and nickel. Two similarly looking types of zoning were identified; these may even coexist within a single phenocryst but differ in their behavior in forsterite–nickel variation diagrams. Zoning of the first type is attributed to phenocryst growth zones formed during fractional crystallization of the magmatic melt. Zoning of the second type is attributed to zones affected by Fe–Mg–Ni diffusion within the phenocryst. Diffusion chronometry models were applied exclusively to the zones affected by diffusion. Applying these models to growth zones, which may appear mathematically well-fitted, yields false time estimates and does not make it possible to asses the real duration of magmatic processes. Time estimates for the rapid lava rise from the Krasny cone of Tolbachik volcano have been constrained to 2.5–6 days. Longer time estimates have been ruled out based on the combined investigation of forsterite and nickel distribution in the olivine zoning profiles.</p>