High Pressure Partial Melting of Eclogite CO2 and Peridotite CO2 Constraints on Deep Cycling of Carb

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Earths Deep Carbon Cycle
Marc Hirschmann University of Minnesota
COMPRES, June, 2006
Thanks to
Neil Smith, Nikki Dellas (undergrad researchers)
Raj Dasgupta
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88 ppm 1400 ppm H2O Concentration in Mantle

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The Carbon Cycle
Long-Term Carbon Cycle
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Carbon Flux in out of the Mantle
(Alt Teagle, 1999 Sleep Zahnle, 2001
Jarrard, 2003)
(Total surface carbon7 X 1022 g)
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Flux of Subducted Carbon
(Kerrick Connolly, 2001 Jarrard, 2003)
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Flux of Subducted Carbon
(Molina Poli, 2000)
Carbonates remain as refractory phase in the
residue as crust dehydrates
(Yaxley Green, 1994 Kerrick Connolly, 2001
Connolly, 2005 Goran et al., 2006)
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Release of Subducted Carbon how, where ?
Partial melting of carbonated eclogite (eclogite
CO2) is likely to control the depth of release
of crustal carbon in the mantle
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Solidus Detection Carbonated Eclogite
Subsolidus
Supersolidus
Dasgupta et al. (2004) - EPSL
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Solidus of Carbonated Eclogite (SLEC1)
Dasgupta et al. (2004) - EPSL
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Deep Cycling of Carbon
(Yasuda et al., 1994 Hirschmann, 2000 van Keken
et al., 2002 Peacock et al., 2003)
Dasgupta et al. (2004) - EPSL
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Fate of Carbonated Eclogite in Upwelling Mantle
Deep Melting of Carbonated Eclogite
May account for seismic low-velocity zone atop
the 410 km discontinuity (e.g., Song et al., 2004)
Dasgupta et al. (2004) - EPSL
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If carbonated eclogite melts very deep, solidus
of peridotite CO2 controls loci of Carbon
storage and release
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Solidus of Carbonated Peridotite (PERC)
Dasgupta Hirschmann (2006) - Nature
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Peridotite Solidus H2O versus CO2
Dasgupta Hirschmann (in revision) - AmMin
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Solidus of Mantle Peridotite with trace CO2
Dasgupta Hirschmann (2006) - Nature
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Seismic Evidence of Deep Melting ?
MELT Experiment beneath EPR
HammondToomey (2003)
Gu et al. (2005)
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Deep Melting Beneath Mid-Ocean Ridges
Dasgupta Hirschmann (2006) - Nature
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Deep Melting of Peridotite CO2 implication
for mantle geochemistry
Dasgupta Hirschmann (2006) - Nature
Residue Depletion
Increasing Compatibility
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Fraction of Mantle Undergoing Deep Melting
3 ? 1018 g of mantle/ yr
75 of the mantle/ Ga
25 of the mantle/ Ga
1 ? 1018 g of mantle/ yr
Dasgupta Hirschmann (2006) - Nature
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Solidus of Carbonated Peridotite (PERC)
Dasgupta Hirschmann (2006) - Nature
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Trace amounts of H2O enhance melting.
Hirth Kohlstedt, 1996 Asimow Langmuir, 2003
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Origin of Chemical Lithosphere and Volatile
induced Melting of Peridotite
Hirth Kohlstedt (1996) Aubaud et al., 2004
Peridotite Dehydration is aided by Partial Melting
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Deep Carbonated Silicate Melting Beneath Ridges
Transition from carbonatite to carbonated
silicate melts at 3 GPa
Silicate Melting is enhanced in presence of
carbonates
Dasgupta Hirschmann (submitted)
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Deep Carbonated Silicate Melting H2O
Extraction Beneath Ridges
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Oxygen fugacity of Transition Zone Is likely to
be highly reducing
McCammon, Science, 2005
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Redox Melting C O2 Mg2SiO4 -gt MgCO3
(melt) MgSiO3
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Channels on Venus
Up to 7000 km long! Most assume these are lava
channels Basalts, komatiites or carbonatites?
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Conclusions

• The biosphere exists by permission of the mantle
• Carbonated silicate melting is likely in many or
  all loci hydrous silicate melting in the mantle,
  with possibly key effects
• The distribution of oxidized and reduced carbon
  in the mantle remains uncertain.

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Ferric/ferrous iron in MORB glasses
Bezos Humler, 2005
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Effect of Variable CO2 on the Solidus of Mantle
Peridotite
Solidus of Mantle Peridotite with 120-1200 ppm
CO2 is likely to be 50 C less than PERC
Dasgupta Hirschmann (2006) - Nature
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Influence of Na on Carbonated Peridotite Solidus