Microstructures of Metamorphic Rocks
Reconciling theory and observation
Critical overstep for andalusite growth
The diagram below shows a contoured plot of the values of parameter b (the slope of the size distribution’s right tail), derived from the computer model, over a range of heating rates and at various values of the critical overstep for nucleation, using reaction rate data applicable to the growth of andalusite. On it we have superimposed several sets of natural observations based on the measured b and modelled heating rate at different localities in the Bushveld aureole. The observations are consistent with each other and with a critical overstepping of about 5 to 10 degrees.
However, the result presumes that laboratory measurements of reaction rates, performed in the presence of excess aqueous fluid, are applicable to the natural metamorphic environment. If the natural reaction rates are slower than those in the laboratory, the critical overstep for andalusite nucleation woud be greater than 5 - 10°.
An alternative approach to estimating the critical overstepping in the Bushveld hornfelses, based on the evidence for metastable sequences of reaction, is described elsewhere on this site (Waters & Lovegrove 2002, J. Metamorphic Geol.).