MSA AWARD LECTURE: AN ACCESSORY MINERAL AND EXPERIMENTAL PERSPECTIVE ON THE EVOLUTION OF THE CRUST

Accessory minerals are common targets for geochronological studies because the lattices of these crystals structurally accommodate radioactive elements. The geochemistry of these minerals may provide a strong complement to age data, if this information can be translated effectively and accurately into physical constraints. This translation step has benefited greatly from the application of high temperature/pressure experiments, which has led to a new understanding of accessory mineral chemistry in diverse crustal settings. Perhaps no single example highlights this more than the application of new experimental results to zircon. Zircon is the only primary, terrestrial mineral confirmed from before 4 billion years, which makes the development of new tools particularly important to early Earth studies. For example, one key compositional boundary of silicate magmas is defined by whether the molar ratio of Al₂O₃/(CaO+Na₂O+K₂O) is larger or smaller than unity. Experiments and measurements of natural samples show that Al concentrations in zircon are distinctly different for peraluminous vs. metaluminous melts, a feature which might also be used to search for evidence of orogenic events within the detrital zircon record. Second, Si and O comprise ~75% of the atoms in the crust, where the isotope systematics of the former remains relatively unexplored at the igneous mineral scale. The complementarity of the Si- and O- isotopic systems has the potential to reveal new clues about crustal evolution and recycling. Toward this end, equilibrium fractionation studies between zircon and other igneous phases are currently underway, as are in situ Si isotope measurements. And finally, multivalent element partitioning in zircon, monazite, and apatite are promising tracers for the oxidation state of crystallization environments. Experiments have also revealed that solubility, stability, and the mechanism of dissolution of these minerals in fluids can vary as a function of oxygen fugacity. This presentation will focus on these advances and others, which couple experimental results and accessory mineral chemistry, to explore the evolution of the crust.