The Ongoing and the Early Differentiation of the Earth: the Role of Volatiles Rajdeep Dasgupta June 26, 2008COMPRES

Volatiles and Solid Earth Science

Short-Term Carbon Cycle

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 + CO 2 ) 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+CO 2 controls loci of carbon storage and release

Experiments with Peridotite+CO GPa, 1250 °C6.6 GPa, 1300 °C Dasgupta and Hirschmann (2006) - Nature

Solidus of Peridotite with trace CO 2 Dasgupta and Hirschmann (2006) - Nature Falloon and Green (1989) Deep Melting of Carbonated peridotite in the Earth’s upper mantle and generation of carbonatitic melt (with ~40 wt.% CO 2 ) For ppm C, this melting generates wt.% carbonatitic melt eclogite + CO 2

Geochemical Consequence of Deep Melting Dasgupta & Hirschmann (2006) - Nature 1  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 °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 Fe 3 C and Fe 7 C 3 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 D C (silicate-metal) from solubility Dasgupta and Walker (2008) - GCA

Constraint on D C (silicate-metal) from mantle carbon content Carbon content of the mantle? ￭ Measurement of CO 2 /Incompatible species ratio in glasses, fluids, gases and independent estimate of mantle He or Nb etc. CO 2 / 3 He, CO 2 / 4 He (e.g., Trull et al., 1993; Marty and Tolstikhin, 1998; Shaw et al., 2003; Resing et al., 2004) CO 2 /Ar (e.g., Tingle, 1998; Cartigny et al., 2001) CO 2 /Nb (e.g., Saal et al., 2002; Cartigny et al., 2008) CO 2 /Cl (e.g., Saal et al., 2002) Direct measurement of CO 2 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 CO 2 in mantle-derived fluids (trapped gas bubbles in basalts, hydrothermal vent fluids, plumes) and gases (e.g., Aubaud et al., 2005)

Constraint on D C (silicate-metal) from mantle carbon content Dasgupta and Walker (2008) - GCA Batch segregation of core melt

Constraint on D C (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 g C in the core

Summary

Interested in the role of volatiles…? The Road Ahead