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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