Monday, October 19, 2020 - 12:00pm
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EPS Virtual Colloquium

Jesse Reimink, Assistant Professor of Geosciences, Pennsylvania State University, will give a talk on tracer isotope geochemistry, and Ann Bauer, Assistant Professor, Dept. of Geoscience, University of Wisconsin, Madison, will present “Hafnium Isotopes in Zircons Document the Gradual Onset of Mobile-lid Tectonics.”

Jesse Reimink Abstract: The earliest era of geochronology produced data showing an age zonation of continental crust.  This observation, to a first order, suggested that continents may have grown progressively around an old core.  However, this observation does not consider the potential for intra-crustal recycling and reworking, or the removal of continental crust into the mantle.  Tracer isotope geochemistry can provide additional clues to the origin of continental crust as younger rocks will retain isotopic signatures of older, now recycled, crust, where it was present.  Here, I present new zircon Hf-isotope data from the well-preserved Slave craton to evaluate how large swaths of Mesoarchean continental crust was produced.  The isotopic signatures point to juvenile, mantle-derived crust formation occurring on the margins of older continental nuclei.  The isotopic information matches with the age progression of continental rocks, suggesting that continental crust in this region was formed by juvenile addition to a pre-existing crustal nucleus throughout the Mesoarchean and into the Proterozoic. 

Ann Bauer Abstract: Constraining the timing and mechanism of early continental crust growth on Earth is critical for the geosciences as continents exert a first-order control on many Earth processes. We have compiled and evaluated the lutetium-hafnium isotope compositions of globally distributed detrital zircons >3.2 Ga. This compilation reveals different trends in the Hf isotope data for zircons older than ~3.8 Ga relative to those younger than ~3.6 Ga. Notably, the trends in the global detrital zircon dataset are very similar to those observed in zircons recovered from well-characterized, metaigneous rocks of the 4.02 to 2.9 Ga Acasta Gneiss Complex of northern Canada. We used the whole-rock and zircon geochemical signatures of the Acasta rocks as a template to interpret the trends observed in the detrital zircon dataset and argue that the global zircon Hf isotope data record a transition from an early stagnant-lid tectonic regime, in which continental crust was formed by repeated re-melting of ancient mafic crust that, unlike in the present-day, persisted at the surface for long periods of time, to the widespread inception of mobile-lid tectonics and associated crust production processes ~3.8-3.6 Ga.

Contact Name: 

Katrina Blanch

Harvard University
Center for the Environment

Address: 26 Oxford Street, 4th Floor, Cambridge
Phone: (617) 495-0368

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