Zoning patterns in orthopyroxene and garnet in granulites: implications for geothermometry

Abstract

ABSTRACTCompositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and Al in orthopyroxene had been quenched‐in. The extent of late Fe‐Mg exchange was controlled by neighbouring minerals, with negligible Fe‐Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe‐Mg minerals. Cores of grains in contact with exchangeable Fe‐Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high‐temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe‐Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe‐Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120d̀ C; (2) Fe‐Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperature‐sensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150d̀ C higher than maximum Fe‐Mg exchange temperatures, even in grains with flat Fe/(Fe ‐ Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than Fe‐Mg exchange thermometry.