The deep carbon cycle and melting in Earth's interior
TL;DR: Carbon geochemistry of mantle-derived samples suggests that the fluxes and reservoir sizes associated with deep cycle are in the order of 1012−13−g−C/yr and 1022−23−g C, respectively.
About: This article is published in Earth and Planetary Science Letters.The article was published on 2010-09-15. It has received 803 citations till now. The article focuses on the topics: Mantle convection & Transition zone.
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TL;DR: Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments.
Abstract: Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5–10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory.
484 citations
Cites background from "The deep carbon cycle and melting i..."
...carbonates in subducting sediments (25), we retain the lower-bound flux of 13 Mt C/y in subducting sediments (1), but revise the upper bound from 17 Mt C/y to 23 Mt C/y (also see ref....
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...(8); 20–80% in Dasgupta and Hirschmann (1); and 18–70% in Johnston et al....
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...Dasgupta R, Hirschmann MM, Smith ND (2007) Water follows carbon: CO2 incites deep silicate melting and dehydration beneath mid-ocean ridges....
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...Dasgupta R, Hirschmann MM (2010) The deep carbon cycle and melting in Earth’s interior....
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...Dasgupta R, Hirschmann MM (2006) Melting in the Earth’s deep upper mantle caused by carbon dioxide....
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TL;DR: This paper presented petrological evidence for a large amount of dense recycled oceanic crust in the head of the plume and developed a thermomechanical model that predicts no pre-magmatic uplift and requires no lithospheric extension.
Abstract: Large igneous provinces (LIPs) are known for their rapid production of enormous volumes of magma (up to several million cubic kilometres in less than a million years), for marked thinning of the lithosphere, often ending with a continental break-up, and for their links to global environmental catastrophes. Despite the importance of LIPs, controversy surrounds even the basic idea that they form through melting in the heads of thermal mantle plumes. The Permo-Triassic Siberian Traps--the type example and the largest continental LIP--is located on thick cratonic lithosphere and was synchronous with the largest known mass-extinction event. However, there is no evidence of pre-magmatic uplift or of a large lithospheric stretching, as predicted above a plume head. Moreover, estimates of magmatic CO(2) degassing from the Siberian Traps are considered insufficient to trigger climatic crises, leading to the hypothesis that the release of thermogenic gases from the sediment pile caused the mass extinction. Here we present petrological evidence for a large amount (15 wt%) of dense recycled oceanic crust in the head of the plume and develop a thermomechanical model that predicts no pre-magmatic uplift and requires no lithospheric extension. The model implies extensive plume melting and heterogeneous erosion of the thick cratonic lithosphere over the course of a few hundred thousand years. The model suggests that massive degassing of CO(2) and HCl, mostly from the recycled crust in the plume head, could alone trigger a mass extinction and predicts it happening before the main volcanic phase, in agreement with stratigraphic and geochronological data for the Siberian Traps and other LIPs.
483 citations
TL;DR: In this paper, the authors show that global MORBs yield average Fe{sup 3+}/{Sigma}Fe ratios of 0.16 {+-} 0.01 (n = 103), which trace back to primary MORB melts equilibrated at the conditions of the QFM buffer.
383 citations
Cites background from "The deep carbon cycle and melting i..."
...Likewise, carbon present in the mantle has the potential to buffer melting through the reaction C+O2↔CO2, and therefore define the fO2, but the stability of graphite is itself controlled by fO2 (Ballhaus, 1993; Blundy et al., 1991; Dasgupta and Hirschmann, 2010)....
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TL;DR: The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression, which implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe3+ in silicate minerals during upwelling.
Abstract: Determining the oxygen fugacity of Earth's silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet's formation. Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found. Here we report the results of experiments to test garnet oxythermobarometry equilibria under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe(3+) in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe(3+) to total iron in the bulk rock. This 'redox melting' relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.
378 citations
TL;DR: This work demonstrates that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth’s transition zone and lower mantle and shows experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres.
Abstract: Rohrbach and Schmidt present an experimental study that defines how some of the deepest melts in Earth's mantle are generated. They show that carbonatite melts reduce to immobile diamond when recycled at depths greater than about 250 kilometres. They infer that when such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with Fe3+, leading to carbonatite 'redox melting' at depths of around 660 kilometres and 250 kilometres, to form deep-seated melts in Earth's mantle. Very low seismic velocity anomalies in the Earth’s mantle1,2 may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth’s upper mantle is likely to be triggered by the presence of small amounts of carbonate3. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth’s transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase4,5,6. This ‘redox freezing’ process leads to diamond-enriched mantle domains in which the Fe0, resulting from Fe2+ disproportionation in perovskites and garnet, is consumed but the Fe3+ preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe3+ leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth’s mantle.
377 citations
References
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TL;DR: In this paper, the authors compared the relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites and found that the most consistent composition of the Earth's core is derived from the seismic profile and its interpretation, compared with primitive meteorites, and chemical and petrological models of peridotite-basalt melting relationships.
10,830 citations
TL;DR: This article evaluated subducting sediments on a global basis in order to better define their chemical systematics and to determine both regional and global average compositions, and then used these compositions to assess the importance of sediments to arc volcanism and crust-mantle recycling, and to re-evaluate the chemical composition of the continental crust.
2,973 citations
"The deep carbon cycle and melting i..." refers background in this paper
...If extremely enriched popping-rock data (Javoy and Pineau, 1991) are excluded, most of the recent data agree on estimates within a factor of five, ranging from 1.2 to 6.0×1013 g of C/yr....
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...Estimates range from 1.8×1013 g of C/yr to 3.7×1013 g of C/yr (Sano and Williams, 1996; Marty and Tolstikhin, 1998; Hilton et al., 2002), which is ~40–70% of the initial budget entering the trench; the balance could be introduced to the deep mantle....
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...This would imply a total flux of carbon via subduction is (6.1–11.4)×1013 g of C/yr....
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...For a subduction rate of 3 km2/yr (Reymer and Schubert, 1984), this amounts to subduction of 6.1×1013 g of C/yr via basaltic crust subduction (Fig....
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...The composition and carbonate fraction of subducting sediments of ocean-floor sediments vary significantly from one subduction zone to the other (Plank and Langmuir, 1998; Jarrard, 2003)....
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TL;DR: In this article, the authors reported that the resulting densities in the lower mantle are in good agreement with shock-wave measurements on rocks having FeO contents in the range 10 ± 2% by weight.
Abstract: RECENTLY, Birch1 reported data on the density and composition of the mantle and core. He wrote: “The resulting densities in the lower mantle are found to be in good agreement with shock-wave measurements on rocks having FeO contents in the range 10 ± 2% by weight … except for iron oxide, the chemical composition of the mantle is indeterminate. The density of the outer core is lower than that of iron by about 10%”.
2,659 citations
TL;DR: Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years.
Abstract: Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level. The rapid termination would have resulted in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally.
2,233 citations
TL;DR: In this article, an algorithm for the construction of phase diagram sections is formulated that is well suited for geodynamic problems in which it is necessary to assess the influence of phase transitions on rock properties or the evolution and migration of fluids.
1,780 citations
"The deep carbon cycle and melting i..." refers background in this paper
...Phase equilibria experiments (Yaxley and Green, 1994;Molina and Poli, 2000; Poli et al., 2009) and thermodynamic calculations (Kerrick and Connolly, 2001b; Connolly, 2005) suggest that carbon remains stable in residual crust as calcite during shallow dehydration andpossible hydrousmelting....
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..., 2009) and thermodynamic calculations (Kerrick and Connolly, 2001b; Connolly, 2005) suggest that carbon remains stable in residual crust as calcite during shallow dehydration andpossible hydrousmelting....
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