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Volume 11

Loss of immiscible nitrogen from metallic melt explains Earth’s missing nitrogen

Abstract:
Nitrogen and carbon are essential elements for life, and their relative abundances in planetary bodies are important for understanding planetary evolution and habitability. The high C/N ratio in the bulk silicate Earth (BSE) relative to chondrites has been difficult to explain through partitioning during core formation and outgassing from molten silicate. Here we propose a new model that may have released nitrogen from the metallic cores of accreting bodies during impacts with the early Earth. Experimental observations of melting in the Fe-N-C system via synchrotron X-ray radiography of samples in a Paris-Edinburgh press reveal that above the liquidus, iron-rich melt and nitrogen-rich liquid coexist at pressures up to at least 6 GPa. The combined effects of N-rich supercritical fluid lost to Earth’s atmosphere and/or space as well as N-depleted alloy equilibrating with the magma ocean on its way to the core would increase the BSE C/N ratio to match current estimates.


J. Liu, S.M. Dorfman, M. Lv, J. Li, F. Zhu, Y. Kono
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Geochem. Persp. Let. (2019) 11, 18–22 | doi: 10.7185/geochemlet.1919 | Published 30 July 2019

Abiogenesis not required to explain the origin of volcanic-hydrothermal hydrocarbons

Abstract:
Abiotic formation of n-alkane hydrocarbons has been postulated to occur within Earth's crust. Apparent evidence was primarily based on uncommon carbon and hydrogen isotope distribution patterns that set methane and its higher chain homologues apart from biotic isotopic compositions associated with microbial production and closed system thermal degradation of organic matter. Here, we present the first global investigation of the carbon and hydrogen isotopic compositions of n-alkanes in volcanic-hydrothermal fluids hosted by basaltic, andesitic, trachytic and rhyolitic rocks. We show that the bulk isotopic compositions of these gases follow trends that are characteristic of high temperature, open system degradation of organic matter. In sediment-free systems, organic matter is supplied by surface waters (seawater, meteoric water) circulating through the reservoir rocks. Our data set strongly implies that thermal degradation of organic matter is able to satisfy isotopic criteria previously classified as being indicative of abiogenesis. Further considering the ubiquitous presence of surface waters in Earth’s crust, abiotic hydrocarbon occurrences might have been significantly overestimated.


J. Fiebig, A. Stefánsson, A. Ricci, F. Tassi, F. Viveiros, C. Silva, T.M. Lopez, C. Schreiber, S. Hofmann, B.W. Mountain
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Geochem. Persp. Let. (2019) 11, 23–27 | doi: 10.7185/geochemlet.1920 | Published 29 July 2019

Graphite floatation on a magma ocean and the fate of carbon during core formation

Abstract:
Carbon is a strongly siderophile element and current models assume that in a magma ocean, most of the carbon is sequestered into the core. Here we show that (i) for an initially highly reduced magma ocean, most of the carbon will be reduced to graphite, which is less dense than a peridotite melt; (ii) the graphite can be dynamically stable at the surface of a magma ocean; (iii) the equilibrium of the primordial atmosphere with graphite buffers CO and CO2 fugacity to such low values, that only traces of carbon dissolve in the magma ocean. Therefore, under very reducing conditions, most of the carbon may remain near the surface during core formation of a terrestrial planet and the extent of carbon sequestration into the core may be limited. We suggest that the ureilite meteorites may be the remnants of such a graphite-rich surface layer on a partially or completely molten planetesimal. A similar, graphite-enriched surface may also exist on Mercury.


H. Keppler, G. Golabek
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Geochem. Persp. Let. (2019) 11, 12–17 | doi: 10.7185/geochemlet.1918 | Published 9 July 2019

182W evidence for core-mantle interaction in the source of mantle plumes

Abstract:
Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth’s core during its segregationaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope 182Hf decayed entirely to 182W in the mantle after metal-silicate segregation. Therefore, the 182W isotopic composition of the Earth’s mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the 182W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.


H. Rizo, D. Andrault, N.R. Bennett, M. Humayun, A. Brandon, I. Vlastelic, B. Moine, A. Poirier, M.A. Bouhifd, D.T. Murphy
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Geochem. Persp. Let. (2019) 11, 6–11 | doi: 10.7185/geochemlet.1917 | Published 20 June 2019

Fluid discharge linked to bending of the incoming plate at the Mariana subduction zone

Abstract:
Tectonically induced bending of incoming plates at subduction zones can result in normal faulting in the upper ocean crust. Seismic surveys and numerical models indicate enhanced permeability and fluid circulation when this occurs. Yet, direct geological evidence of such effects on the seafloor is lacking. Here we report Human Occupied Vehicle (HOV) based observations of the existence of fluid discharge features on the seafloor of the incoming plate of the Mariana subduction zone. These features include fluid discharge points and associated pockmarks, which are striking, and occur in abundance in several depth related fields. The existence of Galatheid crabs, a typical seep related organism, also indicates fluid discharge from the seafloor. Alteration of the coexisting basaltic ocean crust is extensive, with iddingsite-rich muds within and overlapping the apparent fluid discharge zones. Our findings are significant in that they suggest that structural deformation of the incoming plate could substantially influence chemical exchange between the upper ocean crust and seawater in a new way. We further suggest that these fluid discharge points may represent previously unknown niches for H2-based chemolithotrophic life and microbial ecosystems at deep trenches. Observations reported here contrast both chemically and physically with serpentine mud volcano formation associated with the shallower Mariana forearc region.


M. Du, X. Peng, W.E. Seyfried Jr., K. Ta, Z. Guo, S. Chen, I.-M. Chou, J. Li, H. Xu
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Geochem. Persp. Let. (2019) 11, 1–5 | doi: 10.7185/geochemlet.1916 | Published 5 June 2019