EAR-PF: Quartz-in-zircon: An elastic model for quantifying depth and time scales of crystallization and exhumation of Hadean zircon

Gonzalez, Joseph P, Syracuse NY

Investigators

Abstract

Dr. Joseph Gonzalez has been awarded an NSF EAR postdoctoral fellowship to work at the University of Pavia developing an elastic model for quartz inclusions contained in zircon to better understand Hadean (>4 Ga) earth conditions. Zircon is common in crustal rocks and is the most frequently used mineral to infer ages and crystallization temperatures attained by rocks. Despite the extensive use of zircon for time and temperature determinations, there are no methods for determining the depth of zircon crystallization. The objective of this project is to develop a method to determine depth of zircon formation by investigating quartz crystals trapped as inclusions at various stages of growth in >4 Ga old Hadean zircon. Dr. Gonzalez will achieve these goals by modeling the elastic interactions between quartz inclusions and their zircon host and integrating model results with existing zircon geochronologic and thermometric determinations. The quartz-in-zircon elastic model applied to Hadean zircons from the Jack Hills province of Australia will give insights into the timing and evolution of the onset of Earth’s plate tectonics. During his tenure as an NSF Postdoctoral Fellow, Dr. Gonzalez will organize and participate in educational events hosted by the University of Pavia to explain ongoing research to the general public and prospective students. Furthermore, he will co-mentor undergraduate and graduate students and organize a dedicated website about the project, where plain language descriptions of ongoing results will be accessible to anyone interested. Elastic models of host-inclusion mineral pairs have become frequently used tools for investigating the thermobarometric conditions of inclusion entrapment during host mineral growth. However, elastic models have only been developed for elastically anisotropic inclusions (e.g. quartz) contained in elastically isotropic host minerals (e.g. garnet). In order to understand the elastic interactions between elastically anisotropic host and inclusion minerals (e.g. quartz inclusions in zircon), the reciprocal crystallographic orientation (CORs) between the host and inclusion must be incorporated. At the University of Pavia, a series of finite element models will be completed to quantify the effect of crystallographic orientation on the strain and stress between the zircon host and quartz inclusion minerals. Once the effect of orientation is known, model results can be applied to quartz inclusions contained within Jack Hills zircons to infer the crystallization depth. Single crystal X-ray diffraction will be used to obtain the crystallographic orientation and unit cell parameters of the zircon host and quartz inclusion, permitting calculation of the remnant pressure within the inclusion. Remnant pressures and crystallographic orientations can be used in finite element models and applied to 3-D X-ray tomographic models of targeted inclusions to determine the possible entrapment conditions of the quartz inclusion within the zircon host. Quartz-in-zircon results from Jack Hills zircons will be used to evaluate hypotheses that modern style plate subduction processes began under relatively cool thermal regimes in the Hadean Eon. More broadly, the quartz-in-zircon elastic model will be widely applicable to crustal rocks and can be combined with other petrologic methods to increase understanding of Earth processes. Broader impacts of this project include mentoring undergraduates and graduate students and general public outreach activities. 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.