G. Lang Farmer
Ph.D. University of California, Los Angeles, 1983
Professor, Geological Sciences
Chair of Geological Sciences Department
Application of radiogenic isotope systematics to earth sciences.
Current Research: The provenance of "Grenville" age detrital zircon in western North America and the Neoproterozoic to early Palezoic evolution of southwestern Laurentia
My research group began a new project in 2013 involving the combined use of hafnium and uranium-lead 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 period some 500 million years ago. Zircon grains, which are naturally occurring zirconium silicate minerals, are common in continental rocks and are exceptionally resistant to weathering and erosion. As a result, these grains are preserved in continentally derived sediments. The hafnium and uranium-lead isotopic compositions of detrital zircons provide a measure of the original continental source of the zircons and their transport in past continental river systems. In this study, our goal is to assess the significance of changes in the age spread and relative peak height of specific detrital zircon age populations in sedimentary successions. We are targeting Mesoproterozoic to Cambrian sedimentary successions in the southwestern United States and Mesoproterozoic (1.0 to 1.3 giga-annum, Ga) granitic rocks from throughout North America, including the Pikes Peak granite in Colorado (see photo). Our data reveal that the hafnium isotopic compositions of detrital zircon derived from Mesoproterozoic source rocks vary over a huge range and demonstrate that circa 1.0 Ga detrital zircons in North America do not share the same provenance, as was previously assumed based on their uranium-lead ages alone. These results will require fundamental modifications to existing models of the Precambrian to early Paleozoic tectonic and paleogeographic evolution of North America.