Using titanium-in-quartz geothermometry and geospeedometry to recover temperatures in the aureole of the Ballachulish Igneous Complex, NW Scotland

Abstract

Abstract‘Titanium-in-Quartz’ geothermometry suggests quartzites could yield reliable temperature estimates. We here apply four calibrations of the titanium-in-quartz geothermometer to contact-metamorphosed quartzites surrounding the Ballachulish Igneous Complex, Scotland. Two agree broadly with thermal modelling and pre-existing geothermometry; two give temperatures consistently too low. As reported in earlier studies, the technique suffers from difficulties in analysing low titanium (Ti) levels with high spatial and analytical precision. However, this study finds that the critical problem is one of Ti heterogeneity, which poses difficulties in constraining the chemical activity of Ti during quartz growth under metamorphic conditions. Scanning electron microscope-cathodoluminescence (SEM-CL) textures support an interpretation of extensive Ti disequilibrium despite the presence of rutile, indicating dynamic interplay between grain boundary diffusion, fluid/melt percolation and grain growth. The strong zonation suggests a possible geothermometer based on apparent volume diffusion of Ti-in-quartz to derive grain growth histories. Analysis of rutile–quartz interaction implies peak contact temperatures of 645±12 °C, precise but reliant on external estimates of cooling rate from thermal models. Our conclusions support caution in applying Ti-in-quartz geothermometry in aureole settings. However, rutile–quartz juxtaposition prior to heating to >600 °C defines a Ti diffusion couple, employable as a thermometer if cooling rates are constrained by other means.