FrontiersThe origins and concentrations of water, carbon, nitrogen and noble gases on Earth
Highlights
► Terrestrial volatiles are chondritic, solar and cometary contributions are minor. ► The volatile content of bulk Earth is estimated from potassium-argon systematics. ► The Earth's volatile budget is equivalent to ~ 2% carbonaceous chondrite material. ► Isotope fractionation of atmospheric xenon lasted for several Ga.
Introduction
The origins and abundances of volatile elements (here, the highly volatile, also called atmophile, elements: water, carbon, nitrogen and noble gases) are still debated. Since the pioneering work of William W. Rubey in the 50s which showed that the oceans could not be derived from the alteration of rocks at the Earth's surface (Rubey, 1951), many studies have investigated the origin of terrestrial volatiles, often using noble gases as physical tracers (Albarède, 2009, Allègre et al., 1983, Caffee et al., 1999, Dauphas, 2003, Fisher, 1982, Holland et al., 2009, Javoy et al., 1986, Marty, 1989, Ozima, 1975, Pepin, 1991, Phinney et al., 1978, Porcelli et al., 2001, Staudacher and Allègre, 1982, Tolstikhin and Marty, 1998, Turner, 1989). Relevant data to constrain models are from the geological record (present-day mantle and atmosphere, ancient sedimentary rocks) and from the analysis of the composition of cosmochemical reservoirs: primitive and differentiated meteorites sampling the asteroid belt and inner planetary bodies, planetary atmospheres analyzed by space probes, and samples returned by dedicated space missions. In the first part of this contribution, available cosmochemical data are used to assess the origin of terrestrial volatiles.
Contrary to the case of refractory elements, it is not possible to estimate the volatile element content of the mantle source from the analysis of mantle-derived lavas because, owing to their low solubility in silicate melts, these elements have been lost and fractionated during magma degassing. Only remnants are found in mantle-derived magmas emitted along mid ocean ridge and oceanic intraplate volcanoes. Thus indirect approaches are necessary, that are based on the calibration of a volatile element of interest to a tracer whose geochemical cycle is well documented, e.g., 3He (Marty and Jambon, 1987), or Nb (Saal et al., 2002). In this contribution, the volatile (H, C, N, noble gases) compositions of the mantle regions sampled by mantle-derived lavas are revisited. The best documented reservoir is the depleted mantle (DM) sampled by mid-ocean ridge volcanism. Documenting directly the bulk mantle (BM) volatile content is not possible because the size, the location and the composition of the deep reservoir(s), presumably sampled by mantle plumes, are not known. A mass balance approach is attempted here based on the terrestrial budget of radiogenic 40Ar. From estimates of the K content of the Earth (Arevalo et al., 2009, Jochum et al., 1983), the amount of 40Ar produced by the decay of 40K (T1/2 = 1.25 Ga) over the Earth's history can be computed. The amount of 40Ar still trapped in the silicate Earth is then obtained by subtracting the known amount of 40Ar present in the atmosphere (Allègre et al., 1996, Turner, 1989). The amount of other volatile species still trapped in the silicate Earth is then estimated based on their relations to 40Ar in mantle-derived lavas and gases.
Section snippets
The chondrite connection
The isotopic composition of hydrogen, expressed as the D/H ratio, shows large variations among cosmochemical reservoirs (Fig. 1). The solar system formed from a cloud of gas and dust, the protosolar nebula (PSN), the composition of which being best preserved in the Sun and in the atmospheres of the giant planets. The elemental composition of the Sun is known from wavelength measurements of the solar photosphere and by comparison with primitive meteorites for refractory elements. The D/H ratio
Reservoir inventories
In this section, three different terrestrial reservoirs are considered: the atmosphere sensu lato (air, oceans, sediments, crust), the “depleted mantle” (DM) sourcing basalts at mid-ocean ridges, and the bulk mantle (BM) that represents the silicate Earth (excluding the atmosphere). The amounts of volatile elements in the different terrestrial reservoirs are normalized to the carbonaceous chondrite composition for comparison purpose (Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8). Such normalization
The volatile content of the Earth
The bulk Earth volatile content is obtained by adding the ATM and BM inventories (Fig. 7). Overall, a chondritic abundance for volatile elements emerges, consistent with the evidence of a genetic link with chondrites from H and N isotopes. The bulk Earth abundance of noble gases is dominated by the atmospheric inventory, and Ne and Ar isotopic ratios are effectively within the chondritic values. The isotopic composition of atmospheric krypton is slightly different from the chondritic
Conclusions
The origin of terrestrial volatile elements has been re-evaluated in the light of the stable and noble gas isotopes. These tracers point to a chondritic, rather than solar or cometary, source. A small (≤ 10%) fraction of mantle volatiles might have been derived from the protosolar nebula during an early stage of the proto-Earth growth. Light noble gas compositions are consistent with mixing between chondritic and solar end-members, rather than due to fractionation of an originally solar
Acknowledgments
This work was supported by the European Research Council under the European Community's Seventh Framework Programme; (FP7/2007–2013 grant agreement no. 267255 to B.M.). I am grateful to Pete Burnard for having contributed the 4He/40Ar subsection and for helpful comments on the ms. Although the ideas presented here are on my own responsibility, this work benefitted from discussions with Francis Albarède, Nicolas Dauphas, Colin Goldblatt, Allessandro Morbidelli, Robert Pepin, Magali Pujol, Kevin
Bernard Marty is Professor of Geochemistry at the Institut Universitaire de France and the Ecole Nationale Supérieure de Géologie, Nancy France. His field is the cosmochemistry and geochemistry of volatile elements, notably using stable isotopes and noble gases. He is conducting his research at the Centre de Recherches Géochimiques et Pétrographiques (UPR CNRS 2300).
References (142)
Time-dependent models of U–Th–He and K–Ar evolution and the layering of mantle convection
Chem. Geol.
(1998)- et al.
Abundances of the elements: meteoritic and Solar
Geochim. Cosmochim. Acta
(1989) - et al.
The K/U ratio of the silicate Earth: insights into mantle composition, structure and thermal evolution
Earth Planet. Sci. Lett.
(2009) - et al.
Xe isotopic fractionation in a cathodeless glow discharge
Geochim. Cosmochim. Acta
(1986) Trapped neon-argon isotopic correlations in gas meteorites and carbonaceous chondrites
Geochim. Cosmochim. Acta
(1971)On the origin of trapped helium, neon and argon isotopic variations in meteorites-I. Gas-rich meteorites, lunar soil and breccia
Geochim. Cosmochim. Acta
(1972)- et al.
Volatile (C, N, Ar) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge
Earth Planet. Sci. Lett.
(2001) - et al.
Towards a consistent mantle carbon flux estimate: insights from volatile systematics (H2O/Ce, delta D, CO2/Nb) in the North Atlantic mantle (14 degrees N and 34 degrees N)
Earth Planet. Sci. Lett.
(2008) - et al.
Excess 3He in deep waters on the East Pacific Rise
Earth Planet. Sci. Lett.
(1975) The dual origin of the terrestrial atmosphere
Icarus
(2003)
The late asteroidal and cometary bombardment of Earth as recorded in water deuterium to protium ratio
Icarus
Hydrogen isotope heterogeneities in the mantle from ion probe analysis of amphiboles from ultramafic rocks
Earth Planet. Sci. Lett.
Iodine abundances in oceanic basalts — implications for Earth dynamics
Earth Planet. Sci. Lett.
Implications of terrestrial 40Ar/36Ar for atmospheric and mantle evolutionary models
Phys. Earth Planet. Inter.
Noble gas composition of the solar wind as collected by the Genesis mission
Geochim. Cosmochim. Acta
The H/C ratios of Earth's near-surface and deep reservoirs, and consequences for deep Earth volatile cycles
Chem. Geol.
Noble-gases in submarine glasses from midoceanic ridges and Loihi Seamount — constraints on the early history of the Earth
Geochim. Cosmochim. Acta
Water in oceanic basalts — evidence for dehydration of recycled crust
Earth Planet. Sci. Lett.
Carbon and nitrogen isotopes in the mantle
Chem. Geol.
The hydrogen isotope composition of seawater and the global water cycle
Chem. Geol.
Hydrogen isotope geochemistry of SNC meteorites
Geochim. Cosmochim. Acta
Titanium isotopes and the radial heterogeneity of the solar system
Earth Planet. Sci. Lett.
Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity
Geochim. Cosmochim. Acta
Interlayer trapping of noble gases in insoluble organic matter of primitive meteorites
Earth Planet. Sci. Lett.
Adsorption of xenon ions onto defects in organic surfaces: implications for the origin and the nature of organics in primitive meteorites
Geochim. Cosmochim. Acta
Neon and xenon isotopes in MORB: implications for the earth–atmosphere evolution
Earth Planet. Sci. Lett.
The nitrogen record of crust–mantle interaction and mantle convection from Archean to present
Earth Planet. Sci. Lett.
C/3He in volatile fluxes from the solid Earth: implications for carbon geodynamics
Earth Planet. Sci. Lett.
Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: assessment of shallow-level fractionation and characterization of source composition
Geochim. Cosmochim. Acta
Gas geochemistry of geothermal fluids, the Hengill Area, Southwest Rift-Zone of Iceland
Chem. Geol.
Plume-derived rare gases in 380 Ma carbonatites from the Kola region (Russia) and the argon isotopic composition in the deep mantle
Earth Planet. Sci. Lett.
Nitrogen isotopes in the recent solar wind from the analysis of Genesis targets: evidence for large scale isotope heterogeneity in the early solar system
Geochim. Cosmochim. Acta.
Noble gases in carbonaceous chondrites
Geochim. Cosmochim. Acta
The composition of the Earth
Chem. Geol.
The concentration, behavior and storage of H2O in the suboceanic upper mantle — implications for mantle metasomatism
Geochim. Cosmochim. Acta
Solubilities of nitrogen and noble gases in silicate melts under various oxygen fugacities: implications for the origin and degassing history of nitrogen and noble gases in the earth
Geochim. Cosmochim. Acta
Nitrogen and heavy noble gases in ALH 84001: signatures of ancient Martian atmosphere
Geochim. Cosmochim. Acta
Carbon and helium isotope systematics of North Fiji Basin basalt glasses: carbon geochemical cycle in the subduction zone
Earth Planet. Sci. Lett.
Volatile element isotopic systematics of the Rodrigues Triple Junction Indian Ocean MORB: implications for mantle heterogeneity
Earth Planet. Sci. Lett.
Noble-gas-rich separates from the Allende meteorite
Geochim. Cosmochim. Acta
The survival of mantle geochemical heterogeneities
Volatile accretion history of the terrestrial planets and dynamic implications
Nature
Constraints on evolution of Earth's mantle from rare gas systematics
Nature
The argon constraints on mantle structure
Geophys. Res. Lett.
Anomalous nitrogen isotope ratio in comets
Science
Neon isotopes constrain convection and volatile origin in the Earth's mantle
Nature
Trapping of gases in water ice and consequences to comets and the atmospheres of the inner planets
The composition of cometary volatiles
Large excess of heavy nitrogen in both hydrogen cyanide and cyanogen from comet 17P/Holmes
Astrophys. J.
Noble gases and radionuclides in Lost City and other recently fallen meteorites
J. Geophys. Res.
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Bernard Marty is Professor of Geochemistry at the Institut Universitaire de France and the Ecole Nationale Supérieure de Géologie, Nancy France. His field is the cosmochemistry and geochemistry of volatile elements, notably using stable isotopes and noble gases. He is conducting his research at the Centre de Recherches Géochimiques et Pétrographiques (UPR CNRS 2300).