REVISITING FUCHSITE INCLUSIONS IN JACK HILLS ZIRCON AND THEIR PLATE TECTONIC IMPLICATIONS

The terrestrial environment conditions during the first 500 Ma of Earth's history are controversial. Although the remains of the ancient Earth are minimal, the study of these remains is crucial not only for our understanding of the evolution of plate tectonics but also for forming a habitable environment and the origin of life. Several studies hypothesize that the mineral inclusions (ex. muscovite) in the oldest dated zircon, the Jack Hills zircon, are primary and, therefore, can be used to constrain the source magma composition (Bell et al., 2015; Hopkins et al., 2008; Hopkins et al., 2012; Hopkins et al., 2010). However, others have questioned the primary nature of these mineral inclusions (Cavosie et al., 2011; Cavosie et al., 2019; Cavosie et al., 2005; Rasmussen et al., 2011; Rasmussen et al., 2012). Using novel geochemical analysis of muscovite inclusions in Jack Hills zircon, we test whether the mineral inclusions are primary and therefore relate to the zircon's petrogenesis. Several lines of evidence indicate that at least some of the muscovite inclusions are secondary. The highly variable Cr2O3 content of muscovite inclusions in zircons similar to matrix muscovite suggests that the inclusion assemblages are more likely representing the rock matrix. The change of crack positions exposed through polishing raises doubts that the detrital zircon may be permeable to post-depositional fluids. These results question the use of the mineral inclusions the Jack Hills zircon contain to infer initial magma chemistry.