Diffusion modelling to constrain the duration of metamorphism

Abstract

In this study the application of the garnet-ilmenite system to the diffusion modelling technique was investigated. Experiments were undertaken to accurately determine the rates of geochemical processes that are associated with the garnet-ilmenite system. The diffusion parameters of elements within ilmenite have been quantified and it was found that the diffusivity is relatively fast compared to garnet. Therefore in a garnet-ilmenite diffusion model ilmenite diffusivity can be treated as infinite. Additionally, experiments on garnet with an ilmenite thin film as a diffusant source provided further constraints on the rate of diffusion in garnet. Evidence of diffusion profiles preserved in garnet surrounding ilmenite inclusions in natural samples was obtained. Samples from the Barrovian metamorphic sequence showed possible diffusional features in Mn concentration profiles adjacent to ilmenite inclusions in garnet from sillimanite zone samples. However, it was also observed that analytical artefacts affected Ti concentration profiles up to 30 micrometers from the boundary. Further evidence was obtained from samples from the Ivrea-Verbano Zone in Italy. These samples preserved Mn and Mg diffusion profiles adjacent to ilmenite inclusions in garnet. However, in these samples retrograde reactions caused the ilmenite to be enriched in Mn compared to garnet, therefore, partitioning between garnet and ilmenite was incongruent. Nonetheless, this result shows that the garnet-ilmenite system can provide important information on the tectonometamorphic history of a terrane. The garnet-ilmenite system was used to place constraints on the duration of metamorphism. This duration was obtained using a simple forward model of the diffusional processes. From this, the duration of the thermal event associated with the emplacement of magma in the Ivrea-Verbano Zone was determined to be between 100 000 years to 1 Ma. Additionally, constraints on the duration of Buchan metamorphism in Scotland were obtained from argon diffusion modelling. The results from modelling were compared to apparent age spectra from cordierite grade samples that preserved detrital ages. This comparison allowed a constraint to be placed on the maximum duration these rocks could have endured and still preserve pre-Grampian ages. The results indicate that the peak conditions of the cordierite zone of the Buchan metamorphic sequence could not have endured longer than 100 000 years. Diffusion modelling to obtain timescales for thermal events provides insight into the tectonometamorphic history of a terrane. The results of this study have added to the mounting evidence that the duration of regional metamorphism is short (e.g. O'Brien and Vrana, 1995; O'Brien, 1997; Dachs and Proyer, 2002; Faryad and Chakraborty, 2005; Viete et al., 2011b; Viete et al., 2011a; Ague and Baxter, 2007; Camacho et al., 2009). However, there are large uncertainties on the diffusion parameters utilised in diffusion models. Therefore for diffusion modelling to provide better constraints on the duration of regional metamorphism accurate rates of diffusion must be determined.