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 ~1/3rd
Depleted mantle
50‐200 ppm CO2
Enriched mantle
up to 1000 ppm CO2
~2/3rd
Dasgupta and Hirschmann (2010) 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)
● Measurement of CO2/Incompatible species ratio in glasses, fluids, gases and independent estimate of mantle He or Nb etc.
3 CO2/ He (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) Mantle derived Carbonatites and Kimberlites on Continents
Belton (2004)
Oldonyo Lengai, Tanzania - Active Carbonatite Volcano Kjarsgaard (2005) Primary magma –38‐45 wt.% CO2 Primary magma –25‐15 wt.% CO2 Subduction Zones loci of continent formation
Does the present‐day subduction processes lead to efficient release of CO2 to exogenic system?
Did the CO2 fluxes (in‐ and out‐) in subduction zones remain the same throughout the Earth’s history? sediment basalt peridotite
Approaches
● Constraints on slab input and arc ouput
● Thermodynamic modeling (Gibbs free energy minimization) of sediment metamorphic devolatilization crust mantle Perple_X, Thermo‐Calc
● Laboratory experiments constraining devolatilization and melting Map from Plank and Langmuir (1998) Carbon in Altered Oceanic Crust
Near-isochemical addition
of sea-water CO2
2-5 wt.% CO2 in the top 200-500 meter of basaltic (Alt & Teagle, 1999; Alt, 2004; Kelley et al., 2005) crust Carbon carriers to Subduction Zones
(Sciutto and Ottonello, 1995; Kerrick & Connolly, 2001; Sleep and Zahnle, 2001; Jarrard, 2003; Alt, 2004)
Stability of Carbon‐bearing Phases in Subduction Zones?
Basalts, Sediments, Mantle Flux of during Metamorphic Dehydration
Metamorphic Decarbonation of AOC?
Carbonates remain as refractory phase in the residue as crust dehydrates
See also Yaxley & Green (1994), Kerrick & Connolly (2001); Connolly, 2005; Goran et al., 2006) Figure from Molina & Poli (2000) Release of Subducted Carbon – how, where ?
Partial melting of carbonate-bearing eclogite and metapelite is likely to control the depth of release of crustal carbon in the mantle Fate of Carbonate-bearing Basalts/Eclogite
Figure from Dasgupta (2013) ‐ RiMG Fate of Carbonate-bearing Sediments
Figure from Dasgupta (2013) ‐ RiMG Depth (km) RiMG ‐ (2013)
Dasgupta
from
Figure Fate of Carbonate-bearing Lithospheric Mantle 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) How do we get carbon out of the slab in modern subduction zones?
Behn et al. (2011)
Crustal Diapirs?
Currie et al. (2007) See also Gerya and Yuen (2003), Castro and Gerya (2008) Ancient Subduction of Carbonate‐bearing Basaltic Crust?
Figure from Dasgupta (2013) ‐ RiMG Can we estimate the possible change in temperature of subducting crust with time ?
Enhanced CO2 release at arcs >1.3‐1.5 Ga ?
Figure from Dasgupta (2013) ‐ RiMG Continental Lithospheric Mantle root of continent stability, longevity, modification
Fischer et al. (2010)
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? The effect of trace carbon on peridotite melting (C4+)
Mantle carbonation reactions
2Mg2SiO4 + CaMgSi2O6 + 2CO2 = 4MgSiO3 + CaMg(CO3)2
Mg2SiO4 + CO2 = MgSiO3 + MgCO3
2MgSiO3 + CaMg(CO3)2 = CaMgSi2O6 + 2MgCO3
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 Falloon and Green (1989)
See also: Newton and Sharp (1975); Wyllie and Huang (1976); Eggler (1978); Brey et al. (1983); Wyllie (1987); Falloon and Green (1989) Decompression melting in the upwelling mantle beneath ridges may commence ≥300 km
Carbonatitic melt of ~0.03 wt.% (for 100
ppm source CO2)
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) Dasgupta et al. (2013) The Combined Effect of H2O and CO2
Concept of freezing point depression
Hirschmann (2006)
H2O in melt T T carb silicate melting(F ) T peridotite fusion melt (1 (R /SXperidotite ) ln(1 OH- )) The Combined Effect of H2O and CO2
● DH2O (peridotite‐melt) is between DCe and DLa (peridotite‐melt)
● DCe and DLa (peridotite‐ carbonated melt) are known at high pressures
(Keshav et al., 2005; Dasgupta et al., 2009)
The carbonated silicate partial melt with ~25 wt.%
CO2 is estimated to have 6 wt.% H2O for a mantle with 200 ppm H2O
Modified after O’Leary et al. (2010) Beneath Continents…
Modified after Dasgupta (2013) – RiMG xenolith data + geotherms from Lee et al. (2011) –Ann. Rev. EPS Melting beneath Continents…
100 ppm CO2; dry 100 ppm CO2; 200 ppm H2O
Dasgupta – CIDER 2013 What if we throw oxygen fugacity into the mix?
How reduced can the mantle be to have carbonate or CO2‐bearing melt stable?
EMOD fO2 buffer – MgSiO3 + MgCO3 = Mg2SiO4 + C + O2
CO2 in the melt diluted
Figure from Stagno and Frost (2010) Oxygen fugacity (fO2)… reduction of an isochemical mantle with depth pressure
Garnet peridotite
courtesy Frost 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? 100
150
200 (km) 250
300 Depth 350
400 Shear wave velocity (km s‐1) Romanowicz (2009)
Foley (2008)