A simple and robust method for calculating temperatures of granitoid magmas

Abstract

Calculating the temperatures of magmas from which granitoid rocks solidify 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. This 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, petrologists 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 reside in zircon crystals of varying origin (liquidus, captured or inherited crystals) with unknown modal abundance, ZrBULK-ROCK cannot be equated with ZrMELT that is unknown. Hence, the calculated magma temperatures TZr using ZrBULK-ROCK have no significance in both theory and practice. As an alternative, we propose to use the empirical equation T_{SiO_{2}} (°C) = -14.16 × SiO2 + 1723 for granitoid studies, not to rely on exact values for 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 ∝ 1/SiO2.