GPT: An Excel spreadsheet for thermobarometric calculations in metapelitic rocks

GPT takes advantage of Excel spreadsheet utilities (e.g. the possibility of entering a large number of data) to calculate 72 thermometers, 59 barometers, two fO2-meters, thirteen fH2O-meters and fourteen XH2O-meters, as well as the equations for the calculation of the H2O fugacity coefficient corresponding to different mineral assemblages. Each calibration uses expressions by different authors. Because these calibrations are two-variable expressions, one of them needs to be assumed to obtain the other (e.g. a pressure needs to be assumed in a thermometer and a temperature in a barometer). GPT, however, allows the simultaneous solution of two two-variable expressions through iteration, provided the mineral assemblage studied contains any of the sub-assemblages used in the calibration. The P and T values thus obtained can easily be represented through spreadsheet graphics as a formatted P-T chart. In this way, for instance, probe sites of a garnet profile can be represented by symbols and linked by lines representing the P-T path followed by the sample, assuming that the right mineral compositions are used. In order to make GPT easier to use it has been divided into four different spreadsheets that cover the whole P-T geological range. The spreadsheet can be divided easily into smaller sheets adapted to the user's purposes. Copyright © 1996 Elsevier Science Ltd.