the ongoing and the early differentiation of the earth: the role of volatiles

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

Interested in the role of volatiles…?

Rajdeep.Dasgupta@rice.edu

The Road Ahead