the ongoing and the early differentiation of the earth: the role of volatiles
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The Ongoing and the Early Differentiation of the Earth: the Role of Volatiles. Rajdeep Dasgupta. June 26, 2008. COMPRES. Volatiles and Solid Earth Science. Short-Term Carbon Cycle. http://earthobservatory.nasa.gov/Library/CarbonCycle. Long-Term Carbon Cycle. - PowerPoint PPT PresentationTRANSCRIPT
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The Ongoing and the Early Differentiation of the Earth:
the Role of Volatiles
Rajdeep DasguptaJune 26, 2008 COMPRES
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Volatiles and Solid Earth Science
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Short-Term Carbon Cycle
http://earthobservatory.nasa.gov/Library/CarbonCycle
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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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Partial melting of carbonated eclogite (eclogite + CO2) is likely to control the depth of release of crustal carbon in the mantle
Release of Subducted Carbon – how, where ?
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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 Earth’s upper
mantle and generation of
carbonatitic melt
Dasgupta et al. (2004) - EPSL
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If carbonated eclogite melts very deep, solidus of peridotite+CO2 controls loci ofcarbon storage and release
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Experiments with Peridotite+CO2
6.6 GPa, 1250 °C 6.6 GPa, 1300 °C
Dasgupta and Hirschmann (2006) - Nature
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Solidus of Peridotite with trace CO2
Dasgupta and Hirschmann (2006) - NatureFalloon and Green (1989)
Deep Melting of Carbonated peridotite in the Earth’s 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 + CO
2
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Geochemical Consequence of Deep Melting
Dasgupta & Hirschmann (2006) - Nature
1 1018 g of mantle/ yr
25% of the mantle/ Ga
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.
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Carbon in the Core?
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Behavior of C during early Earth differentiation (metal-silicate equilibration)?
C
Partitioning of C during core-mantle equilibration
Behavior of Carbon during magma ocean differentiation?
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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 Fe±Ni-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?
■ 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)
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)
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Constraint on DC (silicate-metal) from mantle carbon content
Dasgupta and Walker (2008) - GCA
Batch segregation of core melt
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Constraint on DC (silicate-metal) from mantle carbon content + metal solubility
Dasgupta and Walker (2008) - GCA
Batch segregation of core melt Fractional segregation of core melt
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Carbon in the Core?
Dasgupta and Walker (2008) - GCA
0.25 ± 0.15 wt.% C
Core is likely to be the most enriched deep Earth reservoir of carbon
~48 x 1024 g C in the core
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Summary