High-precision relative thermobarometry: theory and a worked example

A number of sources of uncertainty are involved in thermobarometric calcula tions, the most important of which are associated with analytical precision , activity-composition (a-x) relationships, and thermodynamic data. Statist ical treatment of these uncertainties results in relatively large uncertain ties on the calculated values of pressure and temperature. Little can be do ne, at least in the short term, about the magnitude of such uncertainties, and any thermobarometric calculations in which they are not taken into acco unt should be treated with caution. Given that uncertainties associated wit h a-x models and thermodynamic data are systematic when applied to multiple samples with the same mineral assemblage, a solution to the problem of imp recise absolute thermobarometry can be obtained via a relative thermobarome tric technique referred to as the Delta PT approach. The Delta PT approach offers a major improvement in the precision of thermobarometry if the calcu lations can be presented in a Delta PT context.