It is apparent from the discussion by Engi and Evans that they misunderstood the intent of our publication on the olivine-spinel geothermometer, the principal purposes of which were twofold: (1) to point out that Jackson in 1969 may have made an unfortunate choice of the free energy value for FeCr204 in the formulation of the olivine-spinel geothermometer and this resulted in his conclusion that the distribution of iron and magnesium between olivine and spinel in the Stillwater intrusion occurred at magmatic temperatures; (2) to present experimental results which support the idea that there has been considerable subsolidus re-equilibration between olivine and chromite in some basic layered intrusions.
It is this second point which is the key to our paper and we believe that it raises a question of fundamental importance to the application of so-called igneous geothermometers. The implication of our work is that some geothermometers may yield subsolidus blocking temperatures and not magmatic temperatures when applied to plutonic igneous rocks. This conclusion is in agreement with the work of Duchesne (1972) on the Fe-Ti oxides of the Bjerkrem-Sogndal massif and with that of Buchanan (pers. comm.) on the Bushveld pyroxenes.
A number of points are raised by Engi and Evans which cloud the central issue of our paper and require clarification.