The Ongoing and the Early Differentiation of the Earth: the Role of Volatiles Rajdeep Dasgupta June 26, 2008 COMPRES
Jan 05, 2016
The Ongoing and the Early Differentiation of the Earth:
the Role of Volatiles
Rajdeep DasguptaJune 26, 2008 COMPRES
Volatiles and Solid Earth Science
Short-Term Carbon Cycle
http://earthobservatory.nasa.gov/Library/CarbonCycle
Long-Term Carbon Cycle
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)
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 ?
Deep Cycling of Carbon
Dasgupta et al. (2004) - EPSL
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
If carbonated eclogite melts very deep, solidus of peridotite+CO2 controls loci ofcarbon storage and release
Experiments with Peridotite+CO2
6.6 GPa, 1250 °C 6.6 GPa, 1300 °C
Dasgupta and Hirschmann (2006) - Nature
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
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.
Carbon in the Core?
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?
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
Solubility of Carbon in core melts?
Texture of quenched Fe±Ni-C melt
Dasgupta and Walker (2008) - GCA
Carbon solubility in core melts
Dasgupta and Walker (2008) - GCA
Constraint on DC (silicate-metal) from solubility
Dasgupta and Walker (2008) - GCA
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)
Constraint on DC (silicate-metal) from mantle carbon content
Dasgupta and Walker (2008) - GCA
Batch segregation of core melt
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
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
Summary