DIFFUSION MODELING OF CORONAS AROUND OLIVINE IN AN OPEN SYSTEM

An open-system modification is introduced to Joesten's theory for steady-state diffusion-controlled growth of layered structures. The modified theory produces a model which is fully consistent with data for coronas between olivine and plagioclase from NE Scotland, having the layer sequence Pl|Hbl + Spl|Hbl|Opx|Ol. Relative immobility of Al and Si, inferred from the two-mineral symplectite, leads to the useful approximation of closure to Al and Si. The resulting model is consistent with Al, Si immobility, because it requires an Onsager diffusion coefficient for Al (LAlAl) lower by an order of magnitude than that for Mg, while LAlAl LSiSi≳1. Iron is less constrained than Mg to be mobile, with Ca and Na unconstrained, because their fluxes are relatively small. The results are insensitive to small errors in the data, except in Al and Si contents. Low Al Si in plagioclase, as an example, requires lower LAlAl LSiSi in the model. Variation of metasomatic inputs can explain the layer sequence Pl|Hbl + Spl|Opx + Spl|Opx|Ol, reported in the literature but not accounted for by the more restricted version of the theory previously available. The modified theory remains simple, and is likely to be applicable to other coronas, though complex examples may reveal its limitations. © 1990.