Thermocalc and experimental modelling of melting of pelite, Morton Pass, Wyoming

Abstract

Calculated results using thermocalc for melting of pelitic compositions are compared with the results of experimental melting of pelite from Morton Pass, Wyoming, USA. The experiments were carried out at 1, 2 and 3.5kbar, dominantly at 2kbar. For experimental charges with ('wet') and without ('dry') added H2O, the agreement is good. This is true for the compositions without added H2 O, in which liquid first appears with the production of orthopyroxene at ∼800 °C, and for the runs with added H2O, in which melting begins at ∼700 °C, and continues, with the appearance of orthopyroxene between 780 and 800 °C at 2kbar. The compositions of melts are also compared: the comparison is generally good, except in the modelling of wet runs below ∼780 °C, where the calculated values for FeO and MgO are about one-tenth of the already low analytical values, and in somewhat low calculated values of Al2O3 compared to the analytical data. A quantitative model to illustrate melting of pelite at Morton Pass is calculated, giving T-X (H2O) conditions for the observed sets of natural assemblages, along with the reactions at and near the beginning of melting. © 2009 Blackwell Publishing Ltd.