Earth’s Deep Carbon CycleMarc Hirschmann

University of Minnesota

COMPRES, June, 2006

Thanks to:

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Raj Dasgupta

+ Neil Smith, Nikki Dellas(undergrad researchers)

Mantle20%

Oceans80%

Oceans20%

Mantle80%

88 ppm 1400 ppmH2O Concentration in Mantle

The Carbon CycleLong-Term Carbon Cycle

Carbon Flux in & out of the Mantle

(Alt & Teagle, 1999;Sleep & Zahnle, 2001; Jarrard, 2003)

(Total surface carbon=7 X 1022 g)

Flux of Subducted Carbon

(Kerrick & Connolly, 2001; Jarrard, 2003)

Flux of Subducted Carbon

(Yaxley & Green, 1994;Kerrick & Connolly, 2001;

Connolly, 2005; Goran et al., 2006)

Carbonates remain as

refractory phase in the residue as crust dehydrates

(Molina & Poli, 2000)

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 ?

Solidus Detection – Carbonated Eclogite

Subsolidus Supersolidus

Dasgupta et al. (2004) - EPSL

Solidus of Carbonated Eclogite (SLEC1)

Dasgupta et al. (2004) - EPSL

Deep Cycling of Carbon

(Yasuda et al., 1994;Hirschmann, 2000;

van Keken et al., 2002;Peacock et al., 2003)

Dasgupta et al. (2004) - EPSL

Fate of Carbonated Eclogite inUpwelling Mantle

Deep Melting of Carbonated

Eclogite

May account for seismic low-velocity

zone atop the 410 km discontinuity

(e.g., Song et al., 2004)Dasgupta et al. (2004) - EPSL

If carbonated eclogite melts very deep, solidus of peridotite + CO2 controls loci ofCarbon storage and release

Solidus of Carbonated Peridotite (PERC)

Dasgupta & Hirschmann (2006) - Nature

Peridotite Solidus – H2O versus CO2

Dasgupta & Hirschmann (in revision) - AmMin

Solidus of Mantle Peridotite with trace CO2

Dasgupta & Hirschmann (2006) - Nature

Seismic Evidence of Deep Melting ?

MELT Experiment beneath EPR

Hammond&Toomey (2003)

Gu et al. (2005)

Deep Melting Beneath Mid-Ocean Ridges

Dasgupta & Hirschmann (2006) - Nature

Deep Melting of Peridotite + CO2 – implication for mantle geochemistryDasgupta & Hirschmann (2006) - Nature

Res

idue

De p

l et io

n

Increasing Compatibility

Fraction of Mantle Undergoing Deep Melting

1 1018 g of mantle/ yr

3 1018 g of mantle/ yr

75% of the mantle/ Ga

25% of the mantle/ Ga

Dasgupta & Hirschmann (2006) - Nature

Solidus of Carbonated Peridotite (PERC)

Dasgupta & Hirschmann (2006) - Nature

Trace amounts of H2O enhance melting….

Hirth & Kohlstedt, 1996; Asimow & Langmuir, 2003

Origin of Chemical Lithosphere and Volatile induced Melting of PeridotiteHirth & Kohlstedt (1996)

Aubaud et al., 2004

Peridotite Dehydration is aided by Partial Melting

Deep Carbonated Silicate Melting Beneath Ridges

Transition from

carbonatite to carbonated

silicate melts at 3 GPa

Silicate Melting is

enhanced in presence of carbonates

Dasgupta & Hirschmann (submitted)

Deep Carbonated Silicate Melting & H2O Extraction Beneath Ridges

Oxygen fugacity of Transition Zone Is likely to be highly reducing

McCammon, Science, 2005

Redox Melting:

C + O2 +

Mg2SiO4

->

MgCO3 (melt) +

MgSiO3

Channels on Venus

Up to 7000 km long!Most assume these are lava channelsBasalts, komatiites or carbonatites?

Conclusions•The biosphere exists by permission of the mantle

•Carbonated silicate melting is likely in many or all loci hydrous silicate melting in the mantle, with possibly key effects

•The distribution of oxidized and reduced carbon in the mantle remains uncertain.

Bezos & Humler, 2005

Ferric/ferrous iron in MORB glasses

Effect of Variable CO2 on the Solidus of Mantle Peridotite

Dasgupta & Hirschmann (2006) - Nature

Solidus of Mantle Peridotite with 120-1200

ppm CO2 is likely to be ~50 °C less than PERC

Influence of Na on Carbonated Peridotite Solidus