Geochemical Perspectives Letters is an internationally peer-reviewed journal of the European Association of Geochemistry, produced by and for the geochemical community:
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The xenon isotopic signature of the mantle beneath Massif Central

The origin of the Central European Volcanic Province, which includes the Massif Central and the Eifel regions, is currently debated. Several different causes have been proposed to account for the volcanism observed in the area. Namely, both the presence of one or more mantle plumes under Europe, and the upwelling and melting of upper mantle related to the formation of the Alps, have been suggested as possible drivers of volcanism. In order to distinguish between these possibilities, we have analysed noble gases in the Lignat Spring to constrain the nature of the mantle source below the Massif Central. The gas has a 3He/4He ratio of 5.51 Ra, whereas its neon isotopic signature is identical to that of MORB source. The gas has an 40Ar/36Ar ratio of 1113 ± 3, far in excess of the atmospheric ratio. The xenon isotopic pattern is explained by 95 % atmospheric contamination of a MORB-like gas. The noble gases clearly show that the mantle beneath Massif Central has a geochemical signature similar to MORB source mantle, with the exception of helium, which more closely corresponds to SCLM signatures, and thus removes the need for the presence of a mantle plume in the region.

M. Moreira, V. Rouchon, E. Muller, S. Noirez

Geochem. Persp. Let. (2017) 6, 28–32 | doi: 10.7185/geochemlet.1805 | Published 22 February 2018

Atmospheric helium isotopic ratio from 1910 to 2016 recorded in stainless steel containers

The atmospheric helium isotope composition (RA= 3He/4Heair = 1.39 × 10-6) could have varied over recent times due to anthropogenic activities. In order to check this possibility, we conducted high-precision helium isotope measurements of air trapped in various stainless steel containers from France (pétanque balls, a float carburettor; 1910–2016) and Cape Grim, Tasmania (archived air tanks; 1978, 1988). We used a double collector mass spectrometer at the Centre de Recherches Pétrographiques et Géochimiques (CRPG, Nancy, France). We found a similar composition between the French and Cape Grim air samples. The temporal variation estimated from all samples including data previously published is not significant, with a trend of +0.002 ± 0.024 ‰/yr over 106 years (2σ). We suspect that the release of radiogenic 4He by fossil fuel exploitation could have been at least partly offset by the production of 3He (via the decay of 3H) from nuclear tests. This study supports the suitability of atmospheric helium as an inter-laboratory isotope standard.

C. Boucher, B. Marty, L. Zimmermann, R. Langenfelds

Geochem. Persp. Let. (2017) 6, 23–27 | doi: 10.7185/geochemlet.1804 | Published 19 February 2018

Th/U and U series systematics of saprolite: importance for the oceanic 234U excess

The presence of excess 234U in seawater is a compelling argument for active delivery of solutes from the continents to the oceans. Previous studies found, however, that the complementary 234U deficit on the continents is surprisingly modest, which would require protracted U loss from a large continental weathering pool. Our new compilation and statistical analysis of the published data, coupled with a mass balance calculation demonstrates that the apparent small 234U deficit in the continental weathering pool implied by previous studies is insufficient to balance the observed oceanic excess. Our new data for a saprolite weathering profile developed on Deccan basalt reveal a very strong overall loss of U (elevated Th/U) with a strong 234U deficit attributable to chemical weathering. The U and 234U deficits reported here from a geologically recent saprolite confirm the importance of the early stages of chemical weathering at the weathering front in the supply of nutrients to the oceans. Thus, as much as half the oceanic 234U inventory is likely sourced from a thin active saprolite zone.

N. Suhr, M. Widdowson, F. McDermott, B.S. Kamber

Geochem. Persp. Let. (2017) 6, 17–22 | doi: 10.7185/geochemlet.1803 | Published 14 February 2018

Noble gases and nitrogen in Tissint reveal the composition of the Mars atmosphere

Comparative planetology is crucial to unravel the origin and evolution of volatile elements on terrestrial planets. We report precise measurements of the elemental and isotopic composition of nitrogen and noble gases in the Martian meteorite Tissint. Ar-N2 correlations confirm discrepancies between results from Viking and Martian meteorites and those from the Mars Science Laboratory (MSL) mission. The Martian atmospheric 40Ar/36Ar ratio is estimated to be 1714 ± 170 (1σ), lower than the value determined by Viking but in agreement with, and with higher precision than, data from MSL. We confirm a solar wind-like origin for Martian Kr and Xe. Excesses on light Kr isotopes are lower than those measured by MSL. Cosmogenic excesses in the Xe isotopic spectrum could have been produced in space during exposure of the Tissint parent body to cosmic rays.

G. Avice, D.V. Bekaert, H. Chennaoui Aoudjehane, B. Marty

Geochem. Persp. Let. (2017) 6, 11–16 | doi: 10.7185/geochemlet.1802 | Published 9 February 2018

Large oxygen excess in the primitive mantle could be the source of the Great Oxygenation Event

Before the Archean to Proterozoic Transition (APT) the tectonic regime was dominated by microplates floating on a low viscosity mantle. Such a regime restricted chemical exchange between the shallow and deeper mantle reservoirs. After the APT, a more global convection regime led to deep subduction of slabs. We propose that the improved vertical mixing of the mantle favoured the release to the Earth’s surface of an oxygen excess initially trapped in the deep mantle. This excess built up when the primordial lower mantle was left with a high Fe3+/(Fe2++Fe3+) ratio (#Fe3+), after metallic iron segregated down into the core. Our synchrotron-based in situ experiments suggest a primordial Fe3+excess of ~20 % for the mantle iron. By comparison with the #Fe3+ of the present mantle, this Fe3+excess would correspond to 500–1000 times the O2 content in the Earth’s atmosphere. The tectonic transition greatly facilitated the ascent of oxidised lower mantle material towards the Earth’s surface, inducing a continuous arrival of O2 at the Earth’s surface and into the atmosphere.

D. Andrault, M. Muñoz, G. Pesce, V. Cerantola, A. Chumakov, I. Kantor, S. Pascarelli, R. Rüffer, L. Hennet

Geochem. Persp. Let. (2017) 6, 5–10 | doi: 10.7185/geochemlet.1801 | Published 18 January 2018

Comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Levenson and Emmanuel, 2017

Comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Levenson and Emmanuel, 2017:
Levenson and Emmanuel suggested recently that the mechanism of carbonate rock weathering in fluids is not limited to nanoscale processes but that chemico-mechanical processes also take place at the micrometre scale, such as grain detachment from the material surface. This phenomenon was first observed in flowing liquids (Levenson and Emmanuel, 2016). In this case, the removal of the grain was understood to be a consequence both of mineral dissolution at grain boundaries and shear stress imposed by the fluid on the grain. Unexpectedly, this grain removal process has been subsequently observed in quiescent liquids. From these experiments, Levenson and Emmanuel (2017) showed atomic force microscopy (AFM) pictures where grains unambiguously disappeared from the surface, even when the rock was left in a solution at rest. The expulsion of the grains was interpreted to result from dissolution and from repulsive forces between the grain surface and the underlying surface. Based on AFM measurements, such repulsion is believed to be caused by interactions between the Debye layers, as well as hydration of the strongly hydrophilic calcite surfaces (Røyne et al., 2015).

M. Le Merrer, J. Colombani

Geochem. Persp. Let. (2017) 6, 1-2 | doi: 10.7185/geochemlet.1747 | Published 27 December 2017

Reply to comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Le Merrer and Colombani, 2017

Reply to comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Le Merrer and Colombani, 2017:
The comment by Le Merrer and Colombani (2017) focuses on the mechanisms that could account for our experiments, in which we observed the detachment of micrometre scale calcite grains from the surface of micritic limestone during contact with a reactive fluid (Levenson and Emmanuel, 2017). They discuss some of the forces acting on the grains and imply that our observations are likely to be artefacts of the experimental method. Furthermore, they suggest that because our measured calcite dissolution rates do not match exactly the dependence on ionic strength predicted by Colombani (2016), an “unidentified phenomenon” could be at play in our experiments. While Le Merrer and Colombani (2017) raise some valid points, we think that an alternative interpretation is possible. We are pleased to be able to discuss these aspects of our paper in greater detail.

Y. Levenson, S. Emmanuel

Geochem. Persp. Let. (2017) 6, 3-4 | doi: 10.7185/geochemlet.1748 | Published 27 December 2017

Iodine proxy evidence for increased ocean oxygenation during the Bitter Springs Anomaly

The Neoproterozoic Bitter Springs Anomaly (BSA; 810–800 Ma) is characterised by an 8 ‰ negative δ13C excursion and is coeval with multiple indicators of increasing oxygenation of the ocean and atmosphere. Here, we use carbonate iodine contents to provide the first constraints on the evolution of local upper ocean redox conditions spanning the BSA. Iodine speciation in seawater is strongly redox sensitive, and carbonates precipitated proximal to O2-depleted water record low I/(Ca + Mg). Data from the Akademikerbreen Group of Svalbard show a major rise of I/(Ca + Mg) during the recovery phase of the BSA. Other relatively high I/(Ca + Mg) values are also associated with rising δ13C throughout the section. Combined with existing palaeoredox proxies (e.g., Cr and S isotopes), our new iodine data most likely reflect an oxygenation event.

W. Lu, S. Wörndle, G.P. Halverson, X. Zhou, A. Bekker, R.H. Rainbird, D.S. Hardisty, T.W. Lyons, Z. Lu

Geochem. Persp. Let. (2017) 5, 53–57 | doi: 10.7185/geochemlet.1746 | Published 18 December 2017

Hydrocarbons as ore fluids

Conventional wisdom holds that aqueous solutions are the only non-magmatic fluids capable of concentrating metals in the Earth’s crust. The role of hydrocarbons in metal concentration is relegated to providing geochemical barriers at which the metals are reduced and immobilised. Liquid hydrocarbons, however, are also known to be able to carry appreciable concentrations of metals, and travel considerable distances. Here we report the results of an experimental determination of bulk solubilities of Au, Zn, and U in a variety of crude oils at temperatures up to 300 °C and of the benchtop-scale transport experiments that simulate hydrocarbon-mediated re-deposition of Zn at 25–200 °C. It has been demonstrated that the metal concentrations obtained in solubility experiments are within the range of concentrations that are typically considered sufficient for aqueous fluids to form ore bodies. It has also been shown that Zn can be efficiently transported and re-deposited by hydrocarbons. These results provide direct evidence of the ability of natural crude oils to mobilise metals available in hydrocarbon-associated host rocks, and transport them in concentrations sufficient to contribute to ore-forming processes.

A.A. Migdisov, X. Guo, H. Xu, A.E. Williams-Jones, C.J. Sun, O. Vasyukova, I. Sugiyama, S. Fuchs, K. Pearce, R. Roback

Geochem. Persp. Let. (2017) 5, 47–52 | doi: 10.7185/geochemlet.1745 | Published 15 December 2017

Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes

Since ultra-high pressure (UHP) minerals have been discovered in ophiolites from Tibet and the Polar Urals, it is speculated that the mantle sections of ophiolites may originate deep within the mantle. The UHP minerals are frequently found together with ultra-reduced silicides, carbides, and nitrides. Consequently, it is argued that the deep mantle, or at least domains within it, must be highly reduced, so reduced that practically all transition elements at depth are present in the metallic state. We find it problematic to rewrite the history of ophiolite complexes based on these observations and suggest we should search for alternative and more realistic modes of origin. Electric discharge experiments at >6000 K reported here show that the UHP and highly reduced phase assemblages may precipitate from plasmas. We argue that the mineral assemblages may originate by lightning strikes. As such, they may not record the origin and emplacement history of the mantle lithologies within which they occur.

C. Ballhaus, R. Wirth, R.O.C. Fonseca, H. Blanchard, W. Pröll, A. Bragagni, T. Nagel, A. Schreiber, S. Dittrich, V. Thome, D.C. Hezel, R. Below, H. Cieszynski

Geochem. Persp. Let. (2017) 5, 42–46 | doi: 10.7185/geochemlet.1744 | Published 20 November 2017

Magnetotactic bacteria as a new model for P sequestration in the ferruginous Lake Pavin

The role of microorganisms in the geochemical cycle of P has received great interest in the context of enhanced biological phosphorus removal and phosphorite formation. Here, we combine scanning and transmission electron microscopies, confocal laser scanning microscopy and synchrotron-based x-ray microfluorescence to analyse the distribution of P at the oxic-anoxic interface in the water column of the ferruginous Lake Pavin. We show that magnetotactic bacteria of the Magnetococcaceae family strongly accumulate polyphosphates and appear as P hotspots in the particulate fraction at this depth. This high accumulation may be characteristic of this family and may also relate to the chemical conditions prevailing in the lake. As a result, these magnetotactic cocci can be considered as new models playing a potentially important role in the P geochemical cycle, similar to sulphide oxidising bacteria such as Thiomargarita and Beggiatoa but thriving in a ferruginous, poorly sulphidic environment.

S. Rivas-Lamelo, K. Benzerara, C.T. Lefèvre, C.L. Monteil, D. Jézéquel, N. Menguy, E. Viollier, F. Guyot, C. Férard, M. Poinsot, F. Skouri-Panet, N. Trcera, J. Miot, E. Duprat

Geochem. Persp. Let. (2017) 5, 35–41 | doi: 10.7185/geochemlet.1743 | Published 15 November 2017

Tin isotopic fractionation during igneous differentiation and Earth’s mantle composition

Tin exists both under the 2+ and 4+ oxidation states in igneous systems, and thus its geochemical behaviour changes as a function of oxygen fugacity. To characterise the redox state of Sn during magmatic differentiation and how this affects its isotope composition, we have measured Sn isotopic and elemental abundances in a suite of samples from the Kilauea Iki lava lake. Sn behaves as a highly incompatible element during fractional crystallisation. Lattice strain modelling shows that Sn2+ has mineral-melt partition coefficients (Dmin/melt) ≈1 in plagioclase and clinopyroxene, whereas it is highly incompatible in all phases save for ilmenite, attesting to the sole presence of Sn4+ in basaltic liquid at the Fayalite-Magnetite-Quartz (FMQ) buffer. Furthermore, Sn isotopes are unfractionated during crystallisation of silicates, but decrease to lighter values upon ilmenite precipitation. Isotopic fractionation is onset by the coordination change between Sn4+ in the melt (6- to 8-fold) and ilmenite (6-fold). The Sn isotope composition of komatiites, which are high degree, high temperature partial melts are used to estimate the Sn isotope composition of the bulk silicate Earth (BSE). Komatiites have δ122Sn within the range of the basalts (before ilmenite precipitation) and together provide the best estimate of the BSE of 0.49 ± 0.11 ‰ (2 s.d., n = 9).

N. Badullovich, F. Moynier, J. Creech, F.-Z. Teng, P.A. Sossi

Geochem. Persp. Let. (2017) 5, 24–28 | doi: 10.7185/geochemlet.1741 | Published 15 November 2017

A geochemical link between plume head and tail volcanism

Geodynamical models of mantle plumes often invoke initial, high volume plume ‘head’ magmatism, followed by lower volume plume ‘tails’. However, geochemical links between plume heads, represented by flood basalts such as the Deccan Traps, and plume tails, represented by ocean islands such as La Réunion, are ambiguous, challenging this classical view of mantle plume theory. Using Sr-Nd-Os isotope data, we demonstrate a geochemical link between archetypal plume head and tail volcanism in the Réunion hotspot. Similar plume head-tail relationships have not been definitively shown in previous geochemical studies for Réunion or other global hotspots. Such a link is enabled by use of compatible elements, such as Os, which can circumvent complexities introduced by magmatic assimilation of crust or lithosphere because these elements are scarce in crust compared to primary mantle melts. We calculate Sr-Nd-Os isotopic compositions for the Réunion primary magma and find these are identical to predictions for the Deccan primary magma. Our result provides geochemical evidence for a temporally stable mantle plume that samples a primitive reservoir associated with the African large low-shear-velocity province and with a heritage beginning at the Cretaceous-Palaeogene boundary.

B.J. Peters, J.M.D. Day

Geochem. Persp. Let. (2017) 5, 29–34 | doi: 10.7185/geochemlet.1742 | Published 3 November 2017