READING BETWEEN THE LINES: MODELLING MINERAL ZONING VIA DISEQUILIBRIUM ACCESSORY MINERAL GROWTH

The growth rings of mineral crystals provide snapshots of geologic systems—but when were those photos taken? Analysing discrete zones of crystals provides great details of evolving crustal processes unavailable for direct observation. Therefore, understanding the dissolution and growth of these crystals is critical in their use as proxies for temperature-pressure-composition conditions in magmatic and metamorphic systems. Attempts at accessory mineral modelling using the saturation of essential structural components in the melt paired with phase equilibrium modelling has tried to provide context for these data. However, these models are calculated assuming equilibrium between the entire accessory mineral and the rest of the system—conditions in which trace element zoning would be absent.

We pair thermodynamic phase equilibrium modelling with trace element and accessory mineral modelling to simulate equilibrium and disequilibrium growth of zircon. Using metasedimentary and igneous compositions to model the growth of zircon during crustal anatexis and magma emplacement respectively and better predict the zoning observed in the rock record. Our models indicate disequilibrium growth is a critical aspect in the formation and preservation of the zoning in accessory mineral crystals. Additionally, the trace element concentrations and ratios (e.g. Ti, Th/U, Gd/Yb) of accessory mineral grains are transient during crystal growth and may not be diagnostic indicators for petrochronology and tectonic reconstructions. Further, the relationship between cross-sectional area and volume is another important aspect of decoding accessory mineral data as relative thin rims can represent significant crystal growth. Temporally linking crystal growth and trace element data will lead to a better understanding of how system conditions evolve, geologic timescales, and ultimately enable more precise reconstructions of the Earth’s processes.