Volume 6

About the cover: Pétanque is a French game in which opposing teams attempt to throw metal balls as close as possible to a small wooden ball, the “cochonnet”. Pétanque balls are made of two welded stainless steel hemispheres, inside of which about 150 cm³ of air is trapped. They thus constitute adequate reservoirs for long-term storage of atmospheric helium, as helium will otherwise diffuse through glass or organic material. In Letter 1804 by Boucher et al., pétanque balls manufactured from the 1960s up to the present were used to investigate whether the isotopic composition of helium has varied during the last decades due to a possible contribution of helium isotopes from human activity.
Photo: Yves Marrocchi, CRPG. Download high-resolution cover.

	Colloidal origin of microbands in banded iron formations Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 43–49 \| doi: 10.7185/geochemlet.1808 \| Published 11 April 2018 Article views: 6,738
	Oxygen minimum zones in the early Cambrian ocean Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 33–38 \| doi: 10.7185/geochemlet.1806 \| Published 1 March 2018 Article views: 8,325
	The xenon isotopic signature of the mantle beneath Massif Central Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 28–32 \| doi: 10.7185/geochemlet.1805 \| Published 22 February 2018 Article views: 6,370
	Atmospheric helium isotopic ratio from 1910 to 2016 recorded in stainless steel containers Abstract: The atmospheric helium isotope composition (R_A= ³He/⁴He_air= 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 23–27 \| doi: 10.7185/geochemlet.1804 \| Published 19 February 2018 Article views: 6,297
	Th/U and U series systematics of saprolite: importance for the oceanic ²³⁴U excess Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 17–22 \| doi: 10.7185/geochemlet.1803 \| Published 14 February 2018 Article views: 8,473
	Noble gases and nitrogen in Tissint reveal the composition of the Mars atmosphere Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 11–16 \| doi: 10.7185/geochemlet.1802 \| Published 9 February 2018 Article views: 9,118
	Large oxygen excess in the primitive mantle could be the source of the Great Oxygenation Event Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 5–10 \| doi: 10.7185/geochemlet.1801 \| Published 18 January 2018 Article views: 12,291
	Reply to comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Le Merrer and Colombani, 2017 Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2017) 6, 3–4 \| doi: 10.7185/geochemlet.1748 \| Published 27 December 2017 Article views: 5,903
	Comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Levenson and Emmanuel, 2017 Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2017) 6, 1–2 \| doi: 10.7185/geochemlet.1747 \| Published 27 December 2017 Article views: 6,117

	Colloidal origin of microbands in banded iron formations Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 43–49 \| doi: 10.7185/geochemlet.1808 \| Published 11 April 2018 Article views: 6,738
	Oxygen minimum zones in the early Cambrian ocean Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 33–38 \| doi: 10.7185/geochemlet.1806 \| Published 1 March 2018 Article views: 8,325
	The xenon isotopic signature of the mantle beneath Massif Central Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 28–32 \| doi: 10.7185/geochemlet.1805 \| Published 22 February 2018 Article views: 6,370
	Atmospheric helium isotopic ratio from 1910 to 2016 recorded in stainless steel containers Abstract: The atmospheric helium isotope composition (R_A= ³He/⁴He_air= 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 23–27 \| doi: 10.7185/geochemlet.1804 \| Published 19 February 2018 Article views: 6,297
	Th/U and U series systematics of saprolite: importance for the oceanic ²³⁴U excess Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 17–22 \| doi: 10.7185/geochemlet.1803 \| Published 14 February 2018 Article views: 8,473
	Noble gases and nitrogen in Tissint reveal the composition of the Mars atmosphere Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 11–16 \| doi: 10.7185/geochemlet.1802 \| Published 9 February 2018 Article views: 9,118
	Large oxygen excess in the primitive mantle could be the source of the Great Oxygenation Event Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2018) 6, 5–10 \| doi: 10.7185/geochemlet.1801 \| Published 18 January 2018 Article views: 12,291
	Reply to comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Le Merrer and Colombani, 2017 Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2017) 6, 3–4 \| doi: 10.7185/geochemlet.1748 \| Published 27 December 2017 Article views: 5,903
	Comment on “Repulsion between calcite crystals and grain detachment during water-rock interaction” by Levenson and Emmanuel, 2017 Abstract: 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 HTML \| PDF \| PDF + SI Geochem. Persp. Let. (2017) 6, 1–2 \| doi: 10.7185/geochemlet.1747 \| Published 27 December 2017 Article views: 6,117