The Ongoing and

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The Ongoing and the Early Differentiation of the
Earth the Role of Volatiles
Rajdeep Dasgupta
June 26, 2008
COMPRES
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Volatiles and Solid Earth Science
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(No Transcript)
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Short-Term Carbon Cycle
http//earthobservatory.nasa.gov/Library/CarbonCyc
le
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Long-Term Carbon Cycle
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C Fluxes IN and OUT (How uncertain?)
(Marty and Tolstikhin, 1998 Plank and Langmuir,
1998 Sleep and Zahnle, 2001 Jarrard, 2003
Resing et al., 2004 Hayes and Waldbauer, 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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Deep Cycling of Carbon
Dasgupta et al. (2004) - EPSL
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Fate of Carbonated Eclogite in the Upwelling
Mantle
Deep Melting of Carbonated Eclogite in the
Earths upper mantle and generation of
carbonatitic melt
Dasgupta et al. (2004) - EPSL
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If carbonated eclogite melts very deep, solidus
of peridotiteCO2 controls loci of carbon storage
and release
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Experiments with PeridotiteCO2
6.6 GPa, 1250 C
Dasgupta and Hirschmann (2006) - Nature
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Solidus of Peridotite with trace CO2
Deep Melting of Carbonated peridotite in the
Earths upper mantle and generation of
carbonatitic melt (with 40 wt. CO2)
For 30-350 ppm C, this melting generates 0.03-0.3
wt. carbonatitic melt
eclogite CO2
Falloon and Green (1989)
Dasgupta and Hirschmann (2006) - Nature
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Geochemical Consequence of Deep Melting
25 of the mantle/ Ga
1 ? 1018 g of mantle/ yr

• Release of highly incompatible tracers and
  volatiles
• U-Th-Pb-He and K-Ar, Rb-Sr, Sm-Nd may be strongly
  fractionated in the depleted residual peridotite
  and in small-degree carbonatitic melt
• If stored for long, both carbonatite extracted
  residue and carbonatite implanted metasomatized
  mantle lithology may evolve as geochemical
  reservoirs with distinct isotopic signatures.

Dasgupta Hirschmann (2006) - Nature
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Carbon in the Core?
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Behavior of Carbon during magma ocean
differentiation?
Behavior of C during early Earth differentiation
(metal-silicate equilibration)?
Partitioning of C during core-mantle equilibration
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Solubility of Carbon in core melts?
Carbon solubility determinations at 2 GPa at
1300-2500 C
Starting mix Fe or Fe-5.2 wt. Ni in graphite
capsule
Electron microprobe analysis of carbon content
using LPC2 crystal and experimentally synthesized
Fe3C and Fe7C3 crystals as standards
Dasgupta and Walker (2008) - GCA
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Solubility of Carbon in core melts?
Texture of quenched FeNi-C melt
Dasgupta and Walker (2008) - GCA
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Carbon solubility in core melts
Dasgupta and Walker (2008) - GCA
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Constraint on DC (silicate-metal) from solubility
Dasgupta and Walker (2008) - GCA
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Constraint on DC (silicate-metal) from mantle
carbon content
Carbon content of the mantle?

• Direct measurement of CO2 in mantle derived
  melts/ glasses (MORB, OIB, Arc Lavas and melt
  inclusions)
• (e.g., Dixon et al., 1997 Bureau et al., 1998)
• Direct measurement of CO2 in mantle-derived
  fluids (trapped gas bubbles in basalts,
  hydrothermal vent fluids, plumes) and gases
• (e.g., Aubaud et al., 2005)

? Measurement of CO2/Incompatible species
ratio in glasses, fluids, gases and
independent estimate of mantle He or Nb etc.
CO2/3He, CO2/4He (e.g., Trull et al.,
1993 Marty and Tolstikhin,
1998 Shaw et al., 2003 Resing
et al., 2004) CO2/Ar (e.g., Tingle,
1998 Cartigny et al., 2001) CO2/Nb
(e.g., Saal et al., 2002 Cartigny et al.,
2008) CO2/Cl (e.g., Saal et al., 2002)
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Constraint on DC (silicate-metal) from mantle
carbon content
Batch segregation of core melt
Dasgupta and Walker (2008) - GCA
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Constraint on DC (silicate-metal) from mantle
carbon content metal solubility
Fractional segregation of core melt
Batch segregation of core melt
Dasgupta and Walker (2008) - GCA
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Carbon in the Core?
Core is likely to be the most enriched deep Earth
reservoir of carbon
48 x 1024 g C in the core
Dasgupta and Walker (2008) - GCA
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Summary
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The Road Ahead
Interested in the role of volatiles?
Rajdeep.Dasgupta_at_rice.edu