Rapid oxygen diffusion during high temperature alteration of zircon

Abstract

The mineral zircon through its isotopic and elemental signatures comprises the greatest archive recording the evolution of Earth’s continental crust. Recognising primary from secondary zircon compositional signatures is thus important for the accurate interpretation of this archive. We report two examples of metasedimentary rocks from high-grade shear zones within the Southern Granulite Belt of India, where anomalously high and homogeneous oxygen isotope signatures indicate disturbance of this isotopic system. Utilising the combined U-Pb-Hf-O and trace element signatures from these zircon grains, we postulate that fluid-assisted alteration has led to complete resetting of the oxygen isotope signatures. This case study presents a rarely observed natural example of potentially fast diffusion of oxygen under hydrous conditions. Given the pervasive nature of fluid interaction within high-grade and highly deformed rocks, we expect that such isotopic disturbance might be more common to nature than is currently reported. A lack of correlation between isotopic disturbance with cathodoluminescence or Th/U values, suggests that these altered zircon grains would not clearly be classified as metamorphic, in which case they would be expected to yield primary compositions. Caution is therefore advised when using detrital δ¹⁸O zircon compilations without a high level of scrutiny for primary versus secondary compositions.