January 2010

Dr. Don Francis from McGill University
Full Professor and Dawson Chair
Monday,15 February 2010, 6:00 P.M.
McGill University
Otto Maass Building, Room OM-10

 

We ride across the universe on a ball of rock tethered to a star.   The surface of our rocky space craft is a thin crust that is distinctly bimodal in nature; young oceanic crust composed of basaltic rock versus old continental crust composed of granitic rock.  The bulk composition of the Earth’s convecting mantle and solid-liquid phase equilibria control the compositions of both the oceanic and continental crusts.  Although oceanic crust is actively being formed and destroyed today, significant volumes of continental crust are not, and the timing and processes responsible for the continental crust are the subject of considerable controversy. Surveys of crustal ages indicate that new continental crust formed only sporadically during the Earth’s history, possibly reflecting large scale reorganization of mantle convection.  Until recently, our knowledge of the first 600 Ma of Earth’s history has been limited by the oldest available samples with ages of 3.8 to 4.0 Ga.  However, detrital zircons in ~ 2.7 Ga rocks from Australia have recently been shown to indicate the existence of granitic crust and water on the surface of the Earth as early as 4.4 Ga.  Most recently, our research group has used a short-lived isotope system (146Sm – 142Nd) to identify ~4.3 Ga basaltic rocks and associated Fe-formation along the eastern shore of Hudson Bay.   These remnants of the earliest Earth indicate the existence of both continental and oceanic crusts, liquid water, and perhaps even life, on the surface of the Earth within a few hundred million years of its formation, a time during which it was experiencing intense meteorite bombardment.

 

 

 

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