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The origins and concentrations of water, carbon, nitrogen and noble gas on Earth

Bernard Marty, EPSL Frontiers 2011

Earth and Planetary Science Letters, 2012, 313-314, 56-66

Abstract :

The isotopic compositions of terrestrial hydrogen and nitrogen are clearly different from those of the nebular gas from which the solar system formed, and also differ from most of cometary values. Terrestrial N and H isotopic compositions are in the range of values characterizing primitive meteorites, which suggests that water, nitrogen, and other volatile elements on Earth originated from a cosmochemical reservoir that also sourced the parent bodies of primitive meteorites. Remnants of the proto-solar nebula (PSN) are still present in the mantle, presumably signing the sequestration of PSN gas at an early stage of planetary growth. The contribution of cometary volatiles appears limited to a few percents at most of the total volatile inventory of the Earth. The isotope signatures of H, N, Ne and Ar can be explained by mixing between two endmembers of solar and chondritic compositions, respectively, and do not require isotopic fractionation during hydrodynamic escape of an early atmosphere. The terrestrial inventory of 40Ar (produced by the decay of 40K throughout the Earth’s history) suggests that a significant fraction of radiogenic argonmay be still trapped in the silicate Earth. By normalizing other volatile element abundances to this isotope, it is proposed that the Earth is not as volatile-poor as previously thought. Our planet may indeed contain up to 3000 ppm water (preferred range : 1000–3000 ppm), and up to 500 ppm C, both largely sequestrated in the solid Earth. This volatile content is equivalent to an 2 (±1) % contribution of carbonaceous chondrite (CI-CM)material to a dry proto-Earth,which is higher than the contribution of chondritic material advocated to account for the platinum group element budget of the mantle. Such a (relatively) high contribution of volatile-rich matter is consistent with the accretion of a few wet planetesimals during Earth accretion, as proposed by recent dynamical models. The abundance pattern ofmajor volatile elements and of noble gases is also chondritic, with two notable exceptions. Nitrogen is depleted by one order of magnitude relative to water, carbon and most noble gases, which is consistent with either N retention in a mantle phase during magma generation, or trapping of N in the core. Xenon is also depleted by one order of magnitude, and enriched in heavy isotopes relative to chondritic or solar Xe (the so-called “xenon paradox”). This depletion and isotope fractionation might have taken place due to preferential ionization of xenon by UV light fromthe early Sun, either before Earth’s formation on parent material, or during irradiation of the ancient atmosphere. The second possibility is consistent with a recent report of chondritic-like Xe in Archean sedimentary rocks that suggests that this process was still ongoing during the Archean eon (Pujol et al., 2011). If the depletion of Xe in the atmosphere was a long-term process that took place after the Earth-building events, then the amounts of atmospheric 129Xe and 131–136Xe, produced by the short-lived radioactivities of 129I (T1/2=16Ma) and 244Pu (T1/2=82Ma), respectively, need to be corrected for subsequent loss. Doing so, the I–Pu–Xe age of the Earth becomes ≤50 Ma after start of solar system formation, instead of 120 Ma as computed with the present-day atmospheric Xe inventory.

Co-variations of D/H and 15N/14N ratios among solar system reservoirs and objects. Meteorite, solar and Jupiter data are from refs in Marty et al., (2011), cometary data are from Hartogh et al. (2011), Jehin et al. (2009), Manfroid et al. (2009) Meech et al. (2011), Mars data are from Deloule (2002), Leshin et al. (1996), Murty and Mohapatra (1997), Owen et al. (1977). CI ; CM, CO, CV, CR and CB-CK refer to the different carbona- ceous chondrite groups. As their name implies, carbonaceous are rich in carbon and other volatile elements compared to ordinary chondrites. CI and CM are presumably the most primitive carbonaceous chondrite groups. The composition of the Martian atmo- sphere is attributed to isotopic fractionation of N and H during escape. However, the fact that the Martian atmosphere composition is within the field of cometary values could in- dicate a cometary contribution that could be more prominent on Mars than on Earth due to the small size of the Martian atmosphere. The newly determined D/H value of comet 103P/Hartley 2 is terrestrial-like reviving the possibility of a cometary origin for oceans, but its 15N/14N ratio measured in CN, is above the terrestrial value.

Volatile composition of the bulk Earth, obtained by adding the number of atoms in the surface and the bulk mantle inventories, normalized to the mass of the Earth.

Proposed evolution for the Xe isotopic fractionation, expressed as a δ deviation, rel- ative to modern atmospheric Xe, in per mil per mass unit, as a function of time (adapted from Pujol et al., 2011). The 3.5 Ga-old barite samples are from North Pole, Australia (Pujol et al., 2009 ; Srinivasan, 1976). The quartz sample data are from fluid inclusions trapped in hydrothermal quartz in 3.5-Ga old komatiite lava vacuoles. The Ar–Ar age of the fluid in- clusions is 3.0 Ga. The younger, 170 Ma-old barite sample is from the Belorechenskoe de- posit, Northern Caucasus, Russia (Meshik et al., 2001). The data fit an exponential decay curve, suggesting that the Xe isotopic fractionation was progressive with time.

Voir en ligne : http://dx.doi.org/doi:10.1016/j.eps...




publié mercredi 15 février 2012