REACTION MECHANISMS IN SILICEOUS DOLOMITE CONTACT AUREOLES: GROWTH OF PROGRADE MINERALS DURING COOLING?

Calcite-dolomite thermometry in contact aureoles commonly yield a wide range of temperatures (e.g. Roselle 1997; Cook and Bowman 1994). Temperatures calculated from calcite analyses having the maximum Mg content from a specimen tend to show decreasing values of T away from the intrusion, with T values similar to those expected. This has led to the interpretation that these types of analyses are the most reliable to use for thermometry. However, the Mg contents of calcites from a single specimen typically display a Gauss-like distribution, yielding temperatures that lie well below the maximum T, even when samples with dolomite unmixing have been excluded. This variation can be interpreted as the retrograde re-equilibration of calcite produced at higher T, but it also is consistent with the interpretation that much of the calcite growth took place at times when the rock was substantially below the maximum T (Cook and Bowman, 1994).

The latter interpretation is supported by stable isotopic studies in contact aureoles, which show that calcite and dolomite are not equilibrated and that the calcite is in isotopic equilibrium with minerals formed by prograde reactions that produced calcite (Baumgartner and Valley, 2002). This suggests that the calcite compositions reflect temperatures at the time of calcite growth in prograde reactions rather than chemical re-equilibration with dolomite during cooling.

These data are consistent with the concept proposed by Foster and Dutrow (2000) that significant prograde mineral growth can occur after maximum T due to overstepped reactions. The mechanism should develop in carbonates when nucleation and growth of minerals such as tremolite and forsterite are not sufficiently rapid to establish equilibrium throughout a rock during rapid heating. This situation can allow substantial nucleation and growth of these minerals to occur via overstepped reactions that take place during cooling, after the maximum T was attained. The reactions continue until the rock cools below the temperatures for the reactions or until a reactant has been exhausted. The mechanism can explain calcite growth due to prograde reactions at temperatures well below the thermal maximum and also allows for growth of prograde minerals from metastable reactions such as the one proposed by Baumgartner et al. (2003).