EAR-PF: Numerical Modeling Perspectives on Zircon Petrochronology

Andersen, Nathan Lee, Eugene OR

Investigators

Abstract

Dr. Nathan Andersen has been granted an NSF EAR Postdoctoral Fellowship to work at the University of Oregon to better understand magma evolution by modeling zircon dates in igneous rocks. The accumulation of magma reservoirs in the continental crust is a fundamental process that is responsible for the growth of the crust, the segregation of economically valuable ore deposits, and the generation of volcanic eruptions. Dating and compositional measurements of the mineral zircon are commonly employed to reconstruct the history of magma reservoir growth in the geologic record. Numerical models of magma emplacement and thermo-chemical evolution have promoted an increasingly sophisticated understanding of the processes by which magma is accumulated and stored. However, uniting the zircon and modeling perspectives is subject to the fundamental difficulty of relating chemical analyses made at the crystal-scale of nano and/or micro meters to processes at the magma reservoir scale of meters to tens of kilometers. Thus, these methods are poorly integrated. The objective of this project is to develop a numerical modeling framework that couples the growth rate of zircon crystals to the physical, chemical and thermal evolution of the host magma. This modeling approach will also serve as a basis for videos and animations illustrating magma reservoir processes that will be used to communicate the results of this research to the general public and for the production of publicly available materials for K-12 and undergraduate earth science education. The ubiquity of zircon in the crust and its physical and chemical robustness make it an indispensable tracer of magma reservoir longevity and chemical evolution. Yet, these characteristics also contribute to ambiguous geologic interpretations of the geologic record. While advances in the analysis of zircon over the last two decades have resulted in dramatic improvements in analytical precision and spatial resolution, these new capabilities have revealed previously unappreciated complexities in the zircon record that drive contemporary petrologic controversies. This project will integrate a magma dynamics model --including multi-phase flow, phase equilibrium, and heat flow -- with a model of Zr-diffusion-limited zircon growth that also incorporates a range of trace element partitioning behavior and the development of enriched boundary layers. This framework will address three principal and interrelated questions surrounding the interpretation of zircon data: i) How does the trade-off between the spatial resolution and analytical precision of the zircon date affect its interpretation? Particularly, do dates produced for whole crystals lead to the same conclusions as those produced by in situ techniques? How well do either capture the evolution of the host magma system?; ii) What criteria are most effective at distinguishing inherited zircons from those crystallized in situ?; iii) How do -micrometer to sub-micrometer - scale variations in zircon chemistry relate to the host melt evolution? Initial models will comprise a single magma reservoir in the crust. Guided by the results of these simulations, greater complexity will be implemented through the tracking of individual zircon crystals that move within the magma reservoir, the development of interconnected reservoirs that comprise a trans-crustal magma system, and calibrated simulations designed to reproduce well-studied natural systems. This approach will yield new insights into the timescales and conditions of magma storage, the relationship between voluminous ignimbrites and plutons, and the scaling of volcanic eruptions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.