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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)
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