Parameterization of subgrid heterogeneities for hydrologic ...
Dejan Milidragovic and Don Francis Department of Earth and Planetary Sciences McGill University...
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Transcript of Dejan Milidragovic and Don Francis Department of Earth and Planetary Sciences McGill University...
Dejan Milidragovic and Don Francis Department of Earth and Planetary Sciences
McGill University
Picrite evidence for widespread Fe-rich
heterogeneities in the Archean mantle
Magmas and their sources: A tribute to the distinguished career of Fred Frey
1. Background
• Terrestrial basalts have a limited range of FeO contents (<13 wt. %)• Consistent with primary magmas in equilibrium with “pyrolitic” (Mg-
number = 0.88-0.92), or slightly more Fe-enriched (Mg-number ~0.85) peridotites
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• Archean mafic/ultramafic rocks of the Superior Province rocks are predominantly low-FeO (<13 wt. %)
• However, > 10 % of rocks have unusually high FeO, inconsistent with pyrolitic sources
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FeOTOT > 13 wt. %
1. Background
2. Neoarchean Fe-rich magmatism
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Neoarchean ferropicrites in NE and S/W Superior, Slave, Kaapvaal, Yilgarn, Karelia
FeTOT (cat)FeTOT (cat)Milidragovic and Francis (in preparation)
Main Archean (low-Fe) array
2. Neoarchean Fe-rich magmatism
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Neoarchean ferropicrites in NE and S/W Superior, Slave, Kaapvaal, Yilgarn, Karelia
Neoarchean ferropicrites in NE and S/W Superior, Slave, Kaapvaal, Yilgarn, Karelia
Milidragovic and Francis (in preparation)
• Globally, ca. 2.7 Ga represents a period of intense magmatism and continental crust growth.
2700 Ma
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2. Neoarchean Fe-rich magmatism
Proterozoic
Permian
E. Cretaceous
Jurassic
Ferropicrites since the ArcheanFerropicrites in post-Archean are very rare
Age (Ga)
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3. Geochemistry of ferropicrites• Ferropicrite characteristics
– Low Al2O3 and low Al2O3/TiO2 ratios often (mis)classifed as Aluminum-depleted komatiites (ADK)
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Milidragovic and Francis (in preparation)
3. Geochemistry of ferropicrites
Alkaline ferropicrites Subalkaline ferropicrites
Crustally contaminated ferropicrites from NESP
2 kinds of Neoarchean ferropicrites Alkaline – high Nb/Y ratios. Nd isotopic studies (e.g. Stone et al., 1995; Francis et al.,
1999; Goldstein & Francis, 2008) indicate short-lived trace element enrichment (≤ 3Ga).
Subalkaline – low Nb/Y ratios
Milidragovic and Francis (in preparation)9/18
Q-suiteolivine
2 kinds of Neoarchean ferropicrites Alkaline – high Ni contents. Similar to olivine tholeiites from Hawaii Subalkaline – low Ni contents
3. Geochemistry of ferropicrites
Alkaline ferropicrites Subalkaline ferropicrites
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4. Insights into Neoarchean mantle
Modified from Tuff et al. (2005)
Tem
pera
ture
(°C
)
Pressure (GPa)0 1 2 3 4 5 6 7
1700
1600
1500
1400
1300
1200
1100
1000
2 kinds of Neoarchean ferropicrites Alkaline – secondary garnet pyroxenite sources? Subalkaline – garnet-free peridotite sources?
Peridotite(low Ni)
Garnet-pyroxenite(high Ni)
melt
solidus
garnet
olivine
cpx
plag.
Ferropicrite phase relations
11/18Milidragovic and Francis (in preparation)
4. Insights into Neoarchean mantle
Alkaline ferropicrites
Subalkaline ferropicrites
MORB (n = 1726)
Iceland(n = 719)
Hawaii – tholeiiticn = 552
Hawaii – alkaline(n = 249)
2 types of Neoarchean ferropicrites Alkaline – low Sc/Fe ratios Subalkaline – high Sc/Fe ratios
• Sc is compatible in garnet but strongly incompatible in olivine garnet pyroxenite vs. peridotite
4. Insights into Neoarchean mantle
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Alkaline ferropicrite source:
• Metasomatism of “pyrolite” by trace-element and Fe-rich melts/fluids
• Metasomatism of Fe-rich peridotite by trace-element rich melts/fluids
Milidragovic and Francis (in preparation)
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Subalkaline ferropicrite source:
• Melting of peridotite with Mg-number 0.81-0.79
• Approaches Fe/Mg of ordinary chondrite or Martian mantle
Milidragovic and Francis (in preparation)
5. Fe-rich domains in the Archean mantle
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Cpx cumulates
6. The origin of Fe-enrichment
SNC meteorites
HED meteorites
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5. The origin of Fe-enrichment
Modified from Drake and Righter (2002)
6. Conclusions
• Fe-rich magmas were ubiquitous during the Neoarchean
• 2 types of Neoarchean ferropicrites– Garnet-pyroxenite and peridotite sources
• Archean ferropicrites show similarities to differentiated meteorites
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Thank you!
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