1. Penrose Conference – June 2006 High-Mg Magmatism Through Time: Implications for Plate Tectonics C.M. Lesher Mineral Exploration Research Centre Department of Earth Sciences Laurentian University, Sudbury, Ontario mlesher@laurentian.ca

2. Penrose Conference – June 2006 Summary Global tectonics (plate, subduction, or otherwise) is ultimately driven by heat loss from the mantle Mantle potential temperature (T P ) is related (in one way or another) to the maximum eruption temperatures (T E ) of mantle plumes T E and therefore T P decreased relatively abruptly from ~1660 o C at 2.7 Ga to ~1500 o C at 2.0 Ga This would have had a profound effect on global tectonics and corresponds to many other fundamental changes in tectonic style, magmatism, metamorphism, ore deposits, etc. at the Archean-Proterozoic Boundary

3. Penrose Conference – June 2006 High-MgO Magmas vs. Time

4. Penrose Conference – June 2006 Archean Komatiites Herzberg et al. submitted G 3

5. Penrose Conference – June 2006 adapted from Herzberg et al. submitted G 3 Archean vs. Younger “Plumes”

6. Penrose Conference – June 2006 Explanations Accumulation of an insulating CBL above the core (Campbell & Griffiths 1992 J Geol)? Evolution toward compositionally lighter OIB- type plumes (Campbell & Griffiths 1992 J Geol)? Change from whole-mantle dominated to 2- layer dominated convection? Change from hotter to warmer plumes and ability to pass through and/or “interact” with 670 km discontinuity?

7. Penrose Conference – June 2006 Implications Max T of plumes (and therefore also ambient mantle…) was much higher in the Archean than in the Proterozoic-Phanerozoic Much hotter mantle in Archean means: lower viscosity mantle, faster plate motion more magnesian oceanic crust, greater hydration, less subductible? Much cooler mantle in Proterozoic-Phanerozoic means: higher viscosity mantle, slower plate motion less magnesian oceanic crust, less hydration, more subductible?