Earth’s Deep Carbon Cycle with an emphasis on subduction zones and continental lithospheric mantles

Rajdeep Dasgupta

CIDER 2013 July 16, 2013

http://earthobservatory.nasa.gov/Library/CarbonCycle

Depleted mantle

50‐200 ppm CO2

Enriched mantle

up to 1000 ppm CO2

Dasgupta and Hirschmann (2010)

~1/3rd

~2/3rd

● Measurement of CO2/Incompatible species ratio in glasses, fluids, gases and independent estimate of mantle He or Nb etc.

CO2/3He (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)

Deep Carbon – Estimating Flux and Concentration

• Direct measurement of CO2 in mantle derived melts/ glasses (MORBs, OIBs, 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)

Mantle derived Carbonatites and Kimberlites on Continents

Oldonyo Lengai, Tanzania -Active Carbonatite Volcano

Belton (2004)

Kjarsgaard (2005)

Primary magma – 25‐15 wt.% CO2Primary magma – 38‐45 wt.% CO2

Subduction Zonesloci of continent formation

Does the present‐day subduction processes lead to efficient release of CO2 to exogenicsystem?

Did the CO2 fluxes (in‐ and out‐) in subduction zones remain the same throughout the Earth’s history?

sediment basalt peridotite

sedimentcrust

mantle

Approaches

● Constraints on slab input and arc ouput

● Thermodynamic modeling(Gibbs free energy minimization) of metamorphic devolatilization

Perple_X, Thermo‐Calc

● Laboratory experiments constraining devolatilization and melting

Map from Plank and Langmuir (1998)

Carbon in Altered Oceanic Crust

(Alt & Teagle, 1999; Alt, 2004; Kelley et al., 2005)

Near-isochemical addition of sea-water CO2

2-5 wt.% CO2 in the top 200-500 meter of basaltic

crust

(Sciutto and Ottonello, 1995; Kerrick & Connolly, 2001; Sleep and Zahnle, 2001; Jarrard, 2003; Alt, 2004)

Carbon carriers to Subduction Zones

Stability of Carbon‐bearing Phases in Subduction Zones?

Basalts, Sediments, Mantle

See also Yaxley & Green (1994),Kerrick & Connolly (2001);

Connolly, 2005; Goran et al., 2006)

Carbonates remain as refractory phase 

in the residue as crust dehydrates

Figure from Molina & Poli (2000)

Metamorphic Decarbonation of AOC?

Flux of during Metamorphic Dehydration

Partial melting of carbonate-bearing eclogite and metapelite is likely to control the depth of release of crustal carbon in the mantle

Release of Subducted Carbon – how, where ?

Figure from Dasgupta (2013) ‐ RiMG

Fate of Carbonate-bearing Basalts/Eclogite

Fate of Carbonate-bearing Sediments

Figure from Dasgupta (2013) ‐ RiMG

Depth (km

)

Fate of Carbonate-bearing Lithospheric Mantle

Figure from Dasgupta (2013) ‐ RiMG

How do we get carbon out of the slab in modern subduction zones?

● Fluid infiltration induced decarbonation of basalts and sediments

(Molina and Poli, 2000; Connolly, 2005; Gormann et al., 2006; Poli et al., 2009)

Figure from Gormann et al. (2006)

Behn et al. (2011)

Currie et al. (2007)See also Gerya and Yuen (2003), Castro and Gerya (2008)

Crustal Diapirs?

How do we get carbon out of the slab in modern subduction zones?

Figure from Dasgupta (2013) ‐ RiMG

Ancient Subduction of Carbonate‐bearing Basaltic Crust?

Enhanced CO2 release at arcs >1.3‐1.5 Ga ?

Can we estimate the possible change in temperature of subducting crust with time ?

Figure from Dasgupta (2013) ‐ RiMG

Continental Lithospheric Mantleroot of continent stability, longevity, modification

How do we explain eruption of carbonatite, kimberlite, and other strongly alkalic magmas on continents? Why are these magmas rare in oceanic provinces?

Can carbon‐induced melting take place in continental lithospheric mantle?

What is the role of partial melting in explaining the geophysical properties of the mantle beneath continents?

Fischer et al. (2010)

The effect of trace carbon on peridotite melting (C4+)

2Mg2SiO4 + CaMgSi2O6 + 2CO2= 4MgSiO3 + CaMg(CO3)2

Mg2SiO4 + CO2 = MgSiO3 + MgCO3

2MgSiO3 + CaMg(CO3)2= CaMgSi2O6 + 2MgCO3

Mantle carbonation reactions

See also: Newton and Sharp (1975); Wyllie and Huang (1976); Eggler (1978); Brey et al. (1983); Wyllie (1987); Falloon and Green (1989)

Figure modified from Falloon and Green (1989)

Carbonated peridotite has solidus T 300-600 °C lower

Near solidus melt is a carbonate melt (40-45 wt.% CO2; <10 wt.% SiO2)

Figure modified from Dasgupta (2013) - RiMG

Falloon and Green (1989); Dasgupta and Hirschmann (2006, 2007a,b); Ghosh et al. (2009); Litasov and Ohtani (2010); Rohrbach and Schmidt (2011)

Decompression melting in the upwelling mantle beneath ridges may commence ≥300 km

Carbonatitic melt of ~0.03 wt.% (for 100 ppm source CO2)

Dasgupta et al. (2013)

The Combined Effect of H2O and CO2

Concept of freezing point depression

carb silicate melting( )peridotite

fusion meltperidotite OH(1 (R / ) ln(1 ))-

FTT

S X

T

H2O in melt

Hirschmann (2006)

The Combined Effect of H2O and CO2

Modified after O’Leary et al. (2010)

The carbonated silicate partial melt with ~25 wt.% CO2 is estimated to have 6 wt.% H2O for a mantle with 200 ppm H2O

● DH2O (peridotite‐melt) is between DCeand DLa (peridotite‐melt)

● DCe and DLa (peridotite‐ carbonated melt) are known at high pressures 

(Keshav et al., 2005; Dasgupta et al., 2009)

Beneath Continents…

Modified after Dasgupta (2013) – RiMGxenolith data + geotherms from Lee et al. (2011) – Ann. Rev. EPS

100 ppm CO2; dry 100 ppm CO2; 200 ppm H2O

Melting beneath Continents…

Dasgupta – CIDER 2013

Figure from Stagno and Frost (2010)

EMOD fO2 buffer – MgSiO3 + MgCO3 = Mg2SiO4 + C + O2

How reduced can the mantle be to have carbonate or CO2‐bearing melt stable?

CO2 in the melt diluted

What if we throw oxygen fugacity into the mix?

Oxygen fugacity (fO2)…reduction of an isochemical mantle with depth

Garnet peridotite

pressure

courtesyFrost and McCammon (2008)

Diamond to carbonated silicate melt transition beneath continents

Stagno et al. (2013)

Storage in the form of reduced carbon and mantle solidus

Solidus of diamond bearing mantle

Reduction/ increasing depth?

Foley (2008)

Romanowicz (2009)Shear wave velocity (km s‐1)

100

400

150

200

250

300

350

Depth (km)