Reactive melt ascent through the crust and the detrimental effects of coupled dissolution-precipitation on zircon and monazite geochronology

Institute of Geophysics and Tectonics Seminar by Dr Nathan Dazcko, Macquarie University

I summarise the different modes of melt transfer recognised in the lower crustal sections of the well-exposed Mesozoic magmatic arc of Fiordland, New Zealand and in the mid-crustal sections of the long-lived Palaeozoic intercontinental Alice Springs Orogeny, Central Australia. I focus on syntectonic, channelized porous melt flow. The melt-rock interaction in these systems involved coupled dissolution-precipitation and an overview of the key features of this process is given in the context of the role of fluid in metamorphism and crustal systems. Volume diffusion is contrasted with coupled dissolution-precipitation and examples of coupled dissolution-precipitation in common silicate minerals are presented. A overview of new monazite-granite melt reaction experiments run at T=750°C and P = 7 kbar demonstrates that new monazite formed by melt-mediated coupled dissolution-precipitation can: (1) incorporate common-Pb; (2) inherit the isotopic character and therefore age of parent monazite; (3) display ages that smear along Concordia; and/or (4) display reset ages. The theoretical background on the process of coupled dissolution-precipitation is combined with the new experimental results to reevaluate examples of complex zircon and monazite U-Pb data in the literature. Examples from Fiordland, East Antarctica, Western Australia, Central Australia and many other locations are used to highlight the pitfalls of interpreting complex zircon and monazite U-Pb data.