G. Lang Farmer

Investigating the provenance of “Grenville” age detrital zircon and the evolution of southwestern Laurentia

During the past year, the Farmer Group continued studying the provenance of “Grenville” age detrital zircon in western North America and the Neoproterozoic-to-early-Paleozoic evolution of southwestern Laurentia. This project involves the combined use of hafnium (Hf) and uranium-lead (U-Pb) isotopic data from detrital (sedimentary) zircon crystals to assess the sources and distribution of sediments transported across the North American continent during the Early Cambrian some 500 million years ago. The project was the focus of the master’s of science thesis completed by CIRES graduate student Amanda Howard, who demonstrated that the U-Pb ages and Hf isotopic compositions of approximately 1.1 giga-annum (Ga) zircon deposited along the western margin of North America during the Cambrian were not derived from distal sources in the present day Appalachians, as previously suggested, but instead were likely derived from more proximal areas in Texas and New Mexico. To assess an alternative possibility that the approximately 1.1 Ga zircon were derived from northern Mexico, new CIRES master’s of science student Aaron Hantsche began a study of the zircon age and chemical characteristics of previously unstudied approximately 1.1 Ga anorthosites and granites in northern Sonora. This work is part of a long-term collaboration among researchers at CIRES and at the University of Sonora, Hermosillo. Hantsche is currently setting up, in the CIRES clean-lab facility, the procedures required for the separation of hafnium from single dissolved zircon, in preparation for new high-precision Hf isotopic analyses of individual, approximately 1.1 Ga zircon.