A macro- to nano-scale interrogation of 4 Gyr of tectonism, metamorphism and alteration in the Acasta Gneiss Complex

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

The origin and growth of continental crust on Earth is an important and enduring question in the Earth sciences. The configuration and interaction of tectonic plates throughout Earth’s history is recognized to play a critical role in Earth’s crustal evolution. The Acasta Gneiss Complex (AGC) of the Northwest Territories, Canada, exposes the oldest known continental crustal rocks. This project will characterize the detailed geochemistry of an important mineral, zircon, in the AGC to evaluate the role of plate tectonic style (“stagnant-lid” vs. “mobile-lid”) in continent formation. Ratios of trace elements and isotopes in zircon will allow the PIs to track changes in AGC crustal compositions over the course of approximately 1 billion years of Earth’s history, starting 4 billion years ago. Because of their extreme age, AGC zircon grains accumulate damage to the crystal structure over geologic time. As a result, many (though not all), zircon grains in Acasta have had their age and chemistry altered. This work combines diverse methods of imaging, mass spectrometry and structural characterization of zircon, allowing the PIs to identify unaltered zircon chemistries and to evaluate the timing of alteration processes. Results of the characterization of zircon will also provide information about changes in continental crustal production and links between tectonic style and the formation of the oldest continents on Earth. This research will also support community outreach in collaboration with the University of Wisconsin Geology Museum to improve scientific literacy and expand the public’s understanding of geologic time. The 4 to 3 Ga Acasta Gneiss Complex (AGC), Canada, comprises over one billion years of magmatism, including the oldest known exposures of felsic terrestrial crust, and thereby is a critical archive of early terrestrial differentiation. The mineral zircon has been invaluable for tracking the chronology and sources of magmatism in the AGC; however, pervasive alteration of age, trace element, and isotope chemical information (often related to radiation-damage-induced loss of crystallinity and recrystallization) has impaired efforts to construct robust U-Pb geochronological, δ18O, and trace element records spanning the duration of AGC magmatism. This work will apply a novel and diverse set of characterization methods (plasma and secondary ion mass spectrometry, Raman spectroscopy, and atom probe tomography) to: 1) expand the primary U-Pb isotope, δ18O, and trace element record for AGC zircon grains in tandem with the whole-rock geochemical record, including tectonic provenance proxies Sc and Nb as well as alteration proxy OH in zircon; and 2) interrogate the grain-scale to nanoscale manifestations of repeated thermal events and fluid interactions within zircon grains by atom probe tomography. Ultimately, the work will clarify primary versus secondary zircon chemistry, the length scales of element mobility during a multistage thermal and metamorphic history, and the primary 4-3 Ga magmatic and tectonic history of the AGC. This research will also support community outreach in collaboration with the University of Wisconsin Geology Museum to improve scientific literacy and expand the public’s understanding of geologic time. 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.