A simple and robust method for calculating temperatures of granitoid magmas

Abstract

Calculating the temperatures of magmas from which granitoid rocks cumulate is a key task of studying their petrogenesis, but few geothermometers are satisfactory. Zircon saturation thermometry has been the most widely used because it is conceptually simple and practically convenient, and because it is based on experimental calibrations with significant correlation of the calculated zircon saturation temperature (TZr) with zirconium (Zr) content in the granitic melt (i.e., TZr  ZrMELT). However, application of this thermometry to natural rocks can be misleading, resulting in the calculated TZr having no geological significance. The thermometry requires Zr content and a compound bulk compositional parameter M of the melt as input variables. As the Zr and M information of the melt is not available, users simply use bulk-rock Zr content (ZrBULK-ROCK) and M to calculate TZr. In the experimental calibration, TZr shows no correlation with M, thus the calculated TZr is only a function of ZrMELT. Because granitoid rocks represent cumulates or mixtures of melt with crystals before magma solidification and because significant amount Zr in the bulk-rock sample resides in zircon crystals of varying origin (liquidus, captured or inherited crystals) with unknown modal abundance, the ZrBULK-ROCK does not in any way represent ZrMELT that is unknown. Hence, ZrBULK-ROCK cannot be used to calculate TZr or to estimate magma temperatures. As an alternative, we suggest to use the empirical equation TSiO2/MgO (°C) = 1127.5  (SiO2/MgO)[-0.055] for granitoid studies, not to rely on the exact values of individual samples but focus on the similarities and differences between samples and sample suites for comparison. This simple and robust thermometry is based on experimentally determined phase equilibria with T  MgO and T  1/SiO2.