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High-Mg Magmatism Through Time: Implications for Plate Tectonics C.M. Lesher Mineral Exploration Research Centre Department of Earth Sciences Laurentian University, Sudbury, Ontario [email protected] Penrose Conference – June 2006 Summary Global tectonics (plate, subduction, or otherwise) is ultimately driven by heat loss from the mantle Mantle potential temperature (TP) is related (in one way or another) to the maximum eruption temperatures (TE) of mantle plumes TE and therefore TP decreased relatively abruptly from ~1660oC at 2.7 Ga to ~1500oC 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 Penrose Conference – June 2006 High-MgO Magmas vs. Time Penrose Conference – June 2006 Archean Komatiites Penrose Conference – June 2006 Herzberg et al. submitted G3 Archean vs. Younger “Plumes” Penrose Conference – June 2006 adapted from Herzberg et al. submitted G3 Explanations Accumulation of an insulating CBL above the core (Campbell & Griffiths 1992 J Geol)? Evolution toward compositionally lighter OIBtype plumes (Campbell & Griffiths 1992 J Geol)? Change from whole-mantle dominated to 2layer dominated convection? Change from hotter to warmer plumes and ability to pass through and/or “interact” with 670 km discontinuity? 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? Penrose Conference – June 2006