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![Page 1: Geochemical Constraints on Archean Mantle Dynamics S.W. Parman Durham University Collaborators: Stan Hart, Mark Kurz, Tim Grove, Mike Walter, Graham Pearson,](https://reader036.fdocuments.us/reader036/viewer/2022062722/56649f2a5503460f94c448fa/html5/thumbnails/1.jpg)
Geochemical Constraints on Archean Mantle Dynamics
S.W. Parman
Durham University
Collaborators: Stan Hart, Mark Kurz, Tim Grove, Mike Walter, Graham Pearson, Geoff Nowell, Jeroen van Hunen
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Geochemistry and Geodynamics
• Historically has been focused on reservoirs (primitive mantle, EM1, EM2, HIMU,D”…)
• New data constrain ages and timescales of mantle processes
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Main points• Helium isotopes do not require a primitive
mantle or layered mantle convection
• Mantle evolution is punctuated by large melting events in the Archean/Proterozoic
• The source of plumes (ocean islands) has ancient (> 2 Ga) heterogeneities in it
• Mixing time in the convecting mantle is < 2 Ga old
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Pattern of Mantle Convection?
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Layered mantle convection
DHe<<DU+Th
Allegre, Kurz, Moreira, Porcelli…
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Zindler and Hart, 1986
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Graham, 2002
Graham, 2002
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Ar constraints
Allegre et al., 1996
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So what’s the problem?
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van der Hilst et al., 1997
lowermantle
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Graham, 2002
Graham, 2002
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Ellam and Stuart, 2004
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Layered mantle convection
DHe<<DU+Th
Allegre, Kurz, Moreira, Porcelli…
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If He more compatible than U+Th…
DHe>DU+Th
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Parman et al., Nature 2005
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If He more compatible than U+Th…
DHe>DU+Th
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Implications• Primitive mantle not required by He
isotopes
• Whole mantle convection is consistent with data
Davies, 2002
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He isotopes do not require the existence of primitive mantle
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Xie & Tackley, 2004
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If He more compatible than U+Th…
DHe>DU+Th
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Database kindly provided by
Atosa Abedini, USGS
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Iceland and Hawaii
Kemp et al., 2006
Condie, 1998
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Parman, Nature, 2007
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Davies, EPSL (1995)
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Implications• Mantle convection in
Archean was punctuated by large melting events
• These events appear to have produced large areas of the continental crust
Davies, EPSL (1995)
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Davies, 2002
Where is plume source?
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Burke & Torsvik, 2004
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Wen, EPSL 2006
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plumeconv.
conv.
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Pearson, Parman and Nowell, Nature, 2007
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Implications
• Plume source is ancient
• Convecting mantle (MORB source) is <2 Ga old
Ferrachat & Ricard, 1998
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The End