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High-Mg Magmatism Through Time: Implications for Plate Tectonics

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. Summary.

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High-Mg Magmatism Through Time: Implications for Plate Tectonics

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  1. High-Mg Magmatism Through Time: Implications for Plate Tectonics C.M. LesherMineral Exploration Research CentreDepartment of Earth SciencesLaurentian University, Sudbury, Ontario mlesher@laurentian.ca

  2. 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

  3. High-MgO Magmas vs. Time

  4. Archean Komatiites Herzberg et al. submittedG3

  5. Archean vs. Younger “Plumes” adapted from Herzberg et al. submittedG3

  6. 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. 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?

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