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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Chemical nature of the 3.4 Ga Strelley Pool microfossils

The biogenicity of putative traces of life found in early-Archean rocks is strongly debated. To date, only equivocal lines of evidence have been reported, which has prevented a full consensus from emerging. Here we report elemental and molecular data from individual organic microfossils preserved within the 3.4 billion-year-old cherts of the Strelley Pool Formation, Western Australia. The present results support the growing body of evidence advocating their biogenicity, promoting them as the oldest known authentic organic microfossils. These microfossils consist of nitrogen- and oxygen- rich organic molecules that have been only slightly degraded despite experiencing temperatures of ~300 °C. Such molecular preservation emphasises the palaeobiological potential of the Earth’s oldest geological record, whilst providing a promising window into the early biosphere.

J. Alleon, S. Bernard, C. Le Guillou, O. Beyssac, K. Sugitani, F. Robert

Geochem. Persp. Let. (2018) 7, 37–42 | doi: 10.7185/geochemlet.1817 | Published 16 August 2018

Nitrogen isotope signatures of microfossils suggest aerobic metabolism 3.0 Gyr ago

There is compelling evidence for early oxygenation of mid-Archean oceans. However, the biological use of molecular oxygen is still not ascertained. Here we report the nitrogen isotope composition measured in isolated microfossils (δ15Nµm) from the 3.0 billion years old Farrel Quartzite metasediments. We show that the quasi-null bulk δ15N values of Farrel Quartzite organic matter encompass a large 15N isotopic heterogeneity at the scale of isolated microfossils (-21.6 ‰ < δ15Nµm < +30.7 ‰). Rayleigh fractionation is required to yield such large δ15N variations. Based on these data, we propose a model in which negative δ15Nµm values determined on film-like and on spheroidal microfossils are explained by ammonia assimilation in the anoxic deeper levels of the water column, whereas positive δ15Nµm values determined on lenticular microfossils were driven by both ammonia assimilation and aerobic oxidation close to the sea surface. Since ammonium aerobic oxidation requires the presence of free molecular O2 within the water column, we further suggest that positive δ15Nµm values reflect an ocean redox stratification tightly related to O2 production by oxygenic photosynthesisers in a mid-Archean ocean 3.0 Gyr ago.

F. Delarue, F. Robert, K. Sugitani, R. Tartèse, R. Duhamel, S. Derenne

Geochem. Persp. Let. (2018) 7, 32–36 | doi: 10.7185/geochemlet.1816 | Published 24 July 2018

Mass-dependent triple oxygen isotope variations in terrestrial materials

High precision triple oxygen isotope analyses of terrestrial materials show distinct fields and trends in Δ'17O - δ'18O space that can be explained by well understood fractionation processes. The Δ'17O - δ'18O field for meteoric waters has almost no overlap with that of rocks. Globally, meteoric water defines a λ value of ~0.528, although a better fit to waters with δ18O values >-20 ‰ is δ'17O = 0.52654 (±0.00036) δ'18O + 0.014 (±0.003). Low temperature marine sediments define a unique and narrow band in Δ'17O - δ'18O space with high δ'18O and low Δ'17O values explained by equilibrium fractionation. Hydrothermal alteration shifts the rock composition to lower δ'18O values at low fluid/rock ratios, and finally higher Δ'17O when F/R ratios are greater than 1. In order to make the triple isotope data tractable to the entire geological community, consensus on a reporting scheme for Δ'17O is desirable. Adoption of λRL= 0.528 (λRL = slope of δ'17O - δ'18O reference line, the ‘Terrestrial Fractionation Line’ or TFL) would bring the ‘rock’ community in line with well established hydrological reporting conventions.

Z.D. Sharp, J.A.G. Wostbrock, A. Pack

Geochem. Persp. Let. (2018) 7, 27–31 | doi: 10.7185/geochemlet.1815 | Published 1 June 2018

Toxic anthropogenic pollutants reach the deepest ocean on Earth

Persistent organic pollutants (PCBs and PBDEs) were analysed in sediment core samples (0-2 cm) from the southern Mariana Trench at water depths of 7000-11000 m. ∑PCBs concentrations ranged from 931 to 4195 pg/g, far higher than those recorded before in marine sediments from shallower depths. Toxic Equivalence (TEQ) of dl-PCBs ranged from 0.650 – 14.9 pg/g, which is higher than most marine surficial sediments at <500-2500 m ocean depth, recovered from semi-industrial to industrial areas. However, ∑8PBDEs values (averaging ~136 pg/g) were lower than those in surficial sediments from shelf areas recorded in past studies. Evidently, anthropogenic pollutants have reached the deepest realm on Earth, and the Mariana Trench acts as a repository for POPs amplification. The high concentration of PCBs is an eye-opener, which is directly affecting our deep sea ecosystems, considering their pervasiveness and persistence in trench sediments.

S. Dasgupta, X. Peng, S. Chen, J. Li, M. Du, Y.-H. Zhou, G. Zhong, H. Xu, K. Ta

Geochem. Persp. Let. (2018) 7, 22–26 | doi: 10.7185/geochemlet.1814 | Published 14 May 2018

Geochemistry and metallogeny of Neoproterozoic pyrite in oxic and anoxic sediments

The Neoproterozoic Dalradian Supergroup contains widespread diagenetic sulphides present as pyrite. The sulphides occur in both carbonaceous shales and glacial diamictites, that were deposited in relatively reducing and oxidising conditions respectively. The trace element compositions of the pyrite, and consequently the whole rock compositions, contrast between the two lithologies. The highest concentrations of selenium, tellurium and gold are all found in diamictite-hosted pyrite. The data suggest that increased mobility of these elements in oxidising conditions led to greater uptake when pyrite was precipitated. As one model for the formation of orogenic gold ore deposits assumes a sulphide-rich protolith, pyrite ultimately formed during relatively oxidising conditions could make a contribution, including the widespread pyrite precipitated during the Neoproterozoic ‘Snowball Earth’ glaciations.

J. Parnell, M. Perez, J. Armstrong, L. Bullock, J. Feldmann, A.J. Boyce

Geochem. Persp. Let. (2018) 7, 12–16 | doi: 10.7185/geochemlet.1812 | Published 2 May 2018

Properties of molten CaCO3 at high pressure

We report here the structure of molten CaCO3 studied by in situ X-ray diffraction using a Paris-Edinburgh press up to 8.7 GPa. Variations are observed in the medium range order with shrinkage of the intermolecular contributions, reflecting higher packing efficiency of the carbonate molecules. Density of the melt is obtained from the radial distribution functions, assuming a constant coordination number of 3 for the C-O contribution. Bulk modulus values increase by a factor of 2 over the experimental pressure-temperature range, reaching a value similar to that of underlying crystalline phases at the highest pressure investigated. These are the first direct density measurements of compressed CaCO3 melt; they agree well with recent ab initio predictions (Li et al., 2017), which implies that the reported flattening of the melting curve followed by a slightly negative curve above 8 GPa cannot be due to crystal-melt density inversion. Instead, the enthalpy of fusion is likely responsible, and we note the peculiar high diffusivity of oxygen in crystalline calcite-V in this regard, with further implications for the mobility of carbonate melts at depth.

J. Hudspeth, C. Sanloup, Y. Kono

Geochem. Persp. Let. (2018) 7, 17–21 | doi: 10.7185/geochemlet.1813 | Published 30 April 2018

Colloidal origin of microbands in banded iron formations

Precambrian banded iron formations record the composition of Earth’s atmosphere and hydrosphere during the global rise of oxygen. It has been suggested that the banded texture of these rocks points to fluctuations in ocean chemistry although this remains a subject of debate. Here we show, by petrographic and electron microscopy of Palaeoproterozoic banded iron formations from the Hamersley Province, NW Australia, that not all iron oxide microbands represent primary sedimentary layers. Some iron oxide laminae are derived from abundant hematite particles that were originally encapsulated in chert layers and subsequently liberated by removal of quartz during post-depositional deformation by dissolution–precipitation creep. The liberated hematite particles progressively accumulated in layer-parallel aggregates forming microbands, with new hematite crystals forming via non-classical crystallisation pathways during diagenesis and metamorphism. Therefore, microbands do not necessarily correspond to fluctuations in the depositional environment.

M.S. Egglseder, A.R. Cruden, A.G. Tomkins, S.A. Wilson, A.D. Langendam

Geochem. Persp. Let. (2018) 6, 43–49 | doi: 10.7185/geochemlet.1808 | Published 11 April 2018

Silicon and oxygen isotopes unravel quartz formation processes in the Icelandic crust

Quartz formation processes in the Icelandic crust were assessed using coupled δ18O and δ30Si systematics of silica deposits formed over a wide temperature range (<150 to >550 °C). Magmatic quartz reveals δ18O (-5.6 to +6.6 ‰) and δ30Si (-0.4 ± 0.2 ‰) values representative of mantle- and crustally-derived melts in Iceland. Hydrothermal quartz and silica polymorphs display a larger range of δ18O (-9.3 to +30.1 ‰) and δ30Si (-4.6 to +0.7 ‰) values. Isotope modelling reveals that such large variations are consistent with variable water sources and equilibrium isotope fractionation between fluids and quartz associated with secondary processes occurring in the crust, including fluid-rock interaction, boiling and cooling. In context of published δ18O and δ30Si data on hydrothermal silica deposits, we demonstrate that large ranges in δ30Si values coupled to insignificant δ18O variations may result from silica precipitation in a hydrothermal fluid conduit associated with near-surface cooling. While equilibrium isotope fractionation between fluids and quartz seems to prevail at high temperatures, kinetic fractionation likely influences isotope systematics at low temperatures.

B.I. Kleine, A. Stefánsson, S.A. Halldórsson, M.J. Whitehouse, K. Jónasson

Geochem. Persp. Let. (2018) 7, 5–11 | doi: 10.7185/geochemlet.1811 | Published 3 April 2018

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

Ballhaus et al. (2017) use electric-discharge experiments to argue that lightning strikes could produce ultra-high pressure (UHP) and super-reduced (SuR) phases “identical to those found in ‘high-pressure’ ophiolites” and that thus there is “not sufficient evidence to challenge long-established models of ophiolite genesis”, specifically for the UHP processing of Tibetan ophiolites. However, the authors produced no evidence for UHP phases in their experiments. There are pertinent observations, relevant to the authors’ assertions, in the literature regarding the relationship between the UHP and SuR assemblages in the Tibetan peridotites. Their conclusions are not consistent with this evidence.

W.L. Griffin, D. Howell, J.M. Gonzalez-Jimenez Q. Xiong S.Y. O’Reilly

Geochem. Persp. Let. (2018) 7, 1-2 | doi: 10.7185/geochemlet.1809 | Published 9 March 2018

Reply to Comment on "Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes"

Griffin et al. (2018) discard our lightning experiments because we did not identify ultra-high pressure (UHP) phases. Our experiments (Ballhaus et al., 2017) provide the first rational explanation of many unusual findings in the so-called UHP ophiolites and hence undermine the foundations on which the resulting speculative geotectonic scenarios are based. Little room seems left to postulate that ultramafic rocks along the Jarlung-Zangbo suture zone have seen Transition Zone (TZ) pressures (McGowan et al., 2015; Griffin et al., 2016a); that chromite crystallised as high pressure polymorph in the calcium ferrite (CF) structure (Xiong et al., 2015); or that the upper mantle is super-reduced (Griffin et al., 2016b).

C. Ballhaus, R.O.C. Fonseca, A. Bragagni

Geochem. Persp. Let. (2018) 7, 3-4 | doi: 10.7185/geochemlet.1810 | Published 9 March 2018

Diffusive fractionation of Li isotopes in wet, highly silicic melts

The discovery of large lithium isotopic gradients in geologic media has motivated recent work examining the kinetic fractionation of Li isotopes in silicate materials. Here, piston-cylinder experiments were used to determine Li diffusivities in rhyolitic melts containing ~6 wt. % H2O at 1 GPa pressure and 790-875 ºC. Lithium transport in wet rhyolitic melt is almost an order of magnitude faster than diffusion in dry obsidian glass over the investigated temperature range. Li isotope profiles collected by ion microprobe show that the kinetic exponent β = 0.228 for diffusive fractionation of Li isotopes in wet rhyolite. This value is very close to β = 0.215 determined by Richter et al. (2003) for Li isotope diffusion in a dry basalt-rhyolite couple at 1350 °C. The similarity of the two values indicates little or no dependence of βLi in silicate melts on either temperature or melt composition. The new data confirm a very high potential for diffusive fractionation of 6Li from 7Li and can be confidently used to model deviations in δ7Li to determine the time-temperature histories of natural rhyolite samples.

M.E. Holycross, E.B. Watson, F.M. Richter, J. Villeneuve

Geochem. Persp. Let. (2018) 6, 39–42 | doi: 10.7185/geochemlet.1807 | Published 5 March 2018

Oxygen minimum zones in the early Cambrian ocean

The relationship between the evolution of early animal communities and oceanic oxygen levels remains unclear. In particular, uncertainty persists in reconstructions of redox conditions during the pivotal early Cambrian (541-510 million years ago, Ma), where conflicting datasets from deeper marine settings suggest either ocean anoxia or fully oxygenated conditions. By coupling geochemical palaeoredox proxies with a record of organic-walled fossils from exceptionally well-defined successions of the early Cambrian Baltic Basin, we provide evidence for the early establishment of modern-type oxygen minimum zones (OMZs). Both inner- and outer-shelf environments were pervasively oxygenated, whereas mid-depth settings were characterised by spatially oscillating anoxia. As such, conflicting redox signatures recovered from individual sites most likely derive from sampling bias, whereby anoxic conditions represent mid-shelf environments with higher productivity. This picture of a spatially restricted anoxic wedge contrasts with prevailing models of globally stratified oceans, offering a more nuanced and realistic account of the Proterozoic-Phanerozoic ocean transition.

R. Guilbaud, B.J. Slater, S.W. Poulton, T.H.P. Harvey, J.J. Brocks, B.J. Nettersheim, N.J. Butterfield

Geochem. Persp. Let. (2018) 6, 33–38 | doi: 10.7185/geochemlet.1806 | Published 1 March 2018

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