GRANULITE THERMOMETRY, RETROGRADE CATION DIFFUSION, AND WHICH GARNET TO CHOSE?

The granulite uncertainty principal (Frost and Chacko 1989) outlined the challenges of granulite thermometry due to closure temperatures of many Fe-Mg exchange reactions being significantly below typical granulite facies temperatures. Retrograde cation diffusion between granulite peak temperatures and these closure temperatures cause significant underestimation of granulite peak metamorphic temperatures. Cores of larger garnets should preserve evidence of higher temperatures, however, here it is illustrated that many larger garnets also have concentrations of Fe-Mg bearing inclusions that reduce the retrograde cation diffusion domain size causing peak metamorphic temperatures to be underestimated (here by 250°C).

One medium-sized garnet from the Bondy Gneiss Complex, a granulite terrane in the western Grenville Province of Quebec, contains evidence of peak metamorphic temperatures at least as high as 950°C; 150 to 200°C higher than previously estimated for this region. This garnet has an inclusion-free core that was protected from the influence of retrograde cation diffusion by an inclusion-rich rim. The core of this garnet grew during peak metamorphic conditions at 1.2 Ga, while the spinel inclusion-containing rim of this garnet grew towards the termination of nearly isothermal decompression at 1.17 Ga. The presence of isothermal decompression is supported by the presence of spinel inclusions in the inclusion-rich rim armored from contact with quartz by either garnet or sillimanite, a dramatic drop in XGrs (10 to 3%) accompanied by an increase in XAlm (52 to 50%) in the transition between garnet core and garnet rim, and results from the GASP and spinel-garnet barometers (isothermal decompression from 10 kbars at 950°C to 7 kbars at 950°C).

This study emphasizes the importance of carefully selecting appropriate garnets for granulite geothermometry. In this case medium garnets (diameter between 5 and 10mm) with inclusion-free cores armored by Fe-Mg inclusion-rich rims should be the target.