2.12 Diffusion Chronometry
graph of diffusion model
graph of diffusion model
At equilibrium, the chemical composition of each phase in a rock should be uniform, unzoned. If there is compositional zoning in a mineral, the chemical diffusion that would have homogenized the mineral has not occurred or at least has not been completed. Using laboratory measurements of diffusion coefficients, it is possible to place constraints on the time-temperature history of that crystal.

For example, Figure 22 shows a gradient in the chemical composition of an olivine crystal from an andesite lava, with more Mg in the center of the crystal and less Mg at its surface. The petrologists who collected the rock and the chemical data postulated that the Mg-zoning of the olivine was caused by diffusion when the composition of the host magma was changed by magma mixing. They used diffusion modeling of the Mg-zonation to estimate how much time passed between the magma mixing and the eruption of the magma.

You can use the olivine diffusion data shown in Figure 23 and the sphere diffusion model of Figure 21 to make a rough estimate of their result. Assume that the olivine crystal is a sphere with the uniform composition of approximately Fo(81)Fa(19) at the time of mixing and that the resulting andesite had a temperature of 950°C. Assume further that there is no crystal growth or dissolution. How long would it take the olivine to completely homogenize by diffusion to the the new equilibrium composition of Fo(71)Fa(29)?

Q1. How long (in years) would it take to homogenize the olivine crystal?   Type a number in the box and press "Enter."