Geochemical Perspectives Letters is a new internationally peer-reviewed journal of the European Association of Geochemistry:
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Solar wind implantation supplied light volatiles during the first stage of Earth accretion

Abstract:
The isotopic and elemental compositions of noble gases constitute a powerful tool to study volatile origin and evolution, due to their inertness, and can thus provide crucial information about the early stage of planetary formation. Two models are proposed to explain the light noble gas origin on Earth: the solar wind implantation model and the solar nebula gas dissolution model. However, noble gas measurements often show addition of air to the mantle-derived gas, which complicates the determination of mantle isotopic ratios. We analysed the noble gas isotopic compositions of single vesicles in samples from the Galápagos hotspot with laser ablation, in order to understand and remove this atmospheric component, as well as discriminate between the two scenarios. Based on the new high precision results and a new statistical approach, we show that the solar wind implantation model is more likely to explain the terrestrial He, Ne and Ar composition. This scenario could bring important constraints on the solar system environment during the early stage of planetary formation.


S. Péron, M. Moreira, B. Putlitz, M.D. Kurz
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Geochem. Persp. Let. (2017) 3, 151-159 | doi: 10.7185/geochemlet.1718 | Published 13 March 2017

Distribution of Zn isotopes during Alzheimer’s disease

Abstract:
Alzheimer’s disease is associated with abnormal homeostasis of Zn, because of deposits of Zn-rich amyloid-β fibrils. This connection between Zn and brain aging provides a possibility to use changes in Zn homeostasis to study the evolution of the disease. Here, we studied the evolution of the Zn isotopic composition of brain, serum and red blood cells from APPswe/PSEN1dE9 transgenic mice, which develop Alzheimer’s-like disease (including Aβ deposition starting after 6 months), in comparison to wild type controls. We found that wild type brains become progressively enriched in the lighter isotopes of Zn between 6 and 12 months. We interpret this enrichment in terms of changes in Zn speciation in the cytoplasm, where isotopically heavy unbound Zn2+ is bound to glutamate released by presynaptic vesicles of neurons. Brains from APPswe/PSEN1dE9 mice were enriched in the heavier isotopes of Zn when compared to wild type suggesting an increase in Zn content of the brain associated with Aβ plaques where Zn binds preferentially to isotopically heavy amino acids such as histidine and glutamate.


F. Moynier, J. Foriel, A.S. Shaw, M. Le Borgne
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Geochem. Persp. Let. (2017) 3, 142-150 | doi: 10.7185/geochemlet.1717 | Published 15 March 2017

Comment on “A cometary origin for atmospheric martian methane” by Fries et al., 2016

Comment on “A cometary origin for atmospheric martian methane” by Fries et al., 2016:
Reports of transient plumes of martian atmospheric methane (Mumma et al., 2009; Webster et al., 2015) have led to suggestions of biologic or abiotic surface sources. Schuerger et al. (2012) examined the production of methane near the surface from interplanetary dust particles. They found this mechanism was capable of yielding the background value of methane, but could not reproduce plume densities by bolide, airburst or other meteor impact process. Fries et al. (2016) draw on the work of Schuerger et al. (2012) and propose that the methane plumes are sourced instead from intense meteor showers with conversion at high altitudes.


M.M.J. Crismani, N.M. Schneider, J.M.C. Plane
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Geochem. Persp. Let. (2017) 3 | doi: 10.7185/geochemlet.1715 | Published 18 February 2017

Reply to Comment on “A cometary origin for martian atmospheric methane”
by Crismani et al., 2017

Reply to Comment on “A cometary origin for martian atmospheric methane”
by Crismani et al., 2017:

First, I would like to extend thanks to Crismani et al. for their commentary, which highlights an important uncertainty acknowledged in Fries et al. (2016), namely whether the total mass of infall material required to produce the observed methane is on par with that available from meteor showers. I would like to disagree on two points presented in the letter, and then discuss the implications of their findings.


M. Fries,
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Geochem. Persp. Let. (2017) 3 | doi: 10.7185/geochemlet.1716 | Published 18 February 2017

Repulsion between calcite crystals and grain detachment during water–rock interaction

Abstract:
Weathering in carbonate rocks is often thought to be governed by chemical dissolution. However, recent studies have shown that mechanical detachment of tiny grains contributes significantly to the overall surface retreat. Whether this detachment is caused by shear forces acting at the surface, or repulsive forces acting between grains, was not known. In this study, we used atomic force microscopy to examine the mechanism of grain detachment and we demonstrate that it occurs even in the absence of shearing fluid flow. This suggests that the removal of grains from rock surfaces can be caused by repulsive forces between calcite grains. Although these repulsive forces are expected to be sensitive to the ionic strength of the solution, we did not find enough evidence to demonstrate a correlation between salinity and the frequency of grain detachment. Importantly, our findings suggest that grain detachment occurs during water-rock interaction under low flow conditions over a range of salinities, with potential impacts on geological carbon sequestration and enhanced oil recovery in carbonate formations.


Y. Levenson, S. Emmanuel
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Geochem. Persp. Let. (2017) 3, 133-141 | doi: 10.7185/geochemlet.1714 | Published 31 January 2017

Evidence of sub-arc mantle oxidation by sulphur and carbon

Abstract:
The oxygen fugacity (ƒO2) of the Earth’s mantle at subduction zones exerts a primary control on the genesis of mineral deposits in the overlying magmatic arcs and on speciation of volcanic gases emitted into the atmosphere. However, the processes governing mantle ƒO2 such as the introduction of oxidised material by subduction are still unresolved. Here, we present evidence for the reduction of oxidised fluid-borne sulphur and carbon during alteration of depleted mantle by slab fluids at ultra-high pressure in the Bardane peridotite (Western Gneiss Region, Norway). Elevated ferric iron in metasomatic garnet, determined using synchrotron X-ray absorption near edge structure (XANES) spectroscopy, indicates that this process drove oxidation of the silicate assemblage. Our finding indicates that subduction oxidises the Earth’s mantle by cycling of sulphur and carbon.


A. Rielli, A.G. Tomkins, O. Nebel, J. Brugger, B. Etschmann, R. Zhong, G.M. Yaxley, D. Paterson
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Geochem. Persp. Let. (2017) 3, 124-132 | doi: 10.7185/geochemlet.1713 | Published 27 January 2017

Water in alkali feldspar: The effect of rhyolite generation on the lunar hydrogen budget

Abstract:
Recent detection of indigenous hydrogen in a diversity of lunar materials, including volcanic glass (Saal et al., 2008), melt inclusions (Hauri et al., 2011), apatite (Boyce et al., 2010; McCubbin et al., 2010), and plagioclase (Hui et al., 2013) suggests water played a role in the chemical differentiation of the Moon. Water contents measured in plagioclase feldspar, a dominant mineral in the ancient crustal lunar highlands have been used to predict that 320 ppm water initially existed in the lunar magma ocean (Hui et al., 2013) whereas measurements in apatite, found as a minor mineral in lunar rocks, representing younger potassium-enriched melt predict a bulk Moon with <100 ppm water. Here we show that water in alkali feldspar, a common mineral in potassium-enriched rocks, can have ~20 ppm water, which implies magmatic water contents of ~1 wt. % in chemically evolved rhyolitic magmas. The source for these wet, potassium-rich magmas probably contained ~1000 ppm H2O. Thus, lunar granites with ages from 4.3–3.9 Ga (Meyer et al., 1996) likely crystallised from relatively wet melts that degassed upon crystallisation. Geochemical surveys by the Lunar Prospector (Jolliff et al., 2011) and Diviner Lunar Radiometer Experiment (Glotch et al., 2010; Jolliff et al., 2011) indicating the global significance of evolved igneous rocks suggest that the formation of these granites removed water from some mantle source regions, helping to explain the existence of mare basalts with <10 ppm water, but must have left regions of the interior relatively wet as seen by the water content in volcanic glass and melt inclusions. Although these early-formed evolved melts were water-rich, their petrogenesis supports the conclusion that the Moon’s mantle had <100 ppm water for most of its history.


R.D. Mills, J.I. Simon, C.M.O’D. Alexander, J. Wang, E.H. Hauri
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Geochem. Persp. Let. (2017) 3, 115-123 | doi: 10.7185/geochemlet.1712 | Published 28 November 2016

Scandium speciation in a world-class lateritic deposit

Abstract:
Scandium (Sc) has unique properties, highly valued for many applications. Future supply is expected to rely on unusually high-grade (up to 1000 ppm) lateritic Sc ores discovered in Eastern Australia. To understand the origin of such exceptional concentrations, we investigated Sc speciation in one of these deposits. The major factors are unusually high concentrations in the parent rock together with lateritic weathering over long time scales in a stable tectonic context. At microscopic and atomic scales, by combining X-ray absorption near-edge structure spectroscopy, X-ray diffraction and microscopic and chemical analyses, we show that Sc-rich volumes are associated with iron oxides. In particular, Sc adsorbed on goethite accounts for ca. 80 % of the Sc budget in our samples. The remaining Sc is incorporated in the crystal structure of haematite, substituting for Fe3+. Scandium grades reflect the high capacity of goethite to adsorb this element. In contrast, the influence of haematite is limited by the low levels of Sc that its structure can incorporate. These crystal-chemical controls play a major role in lateritic Sc deposits developed over ultramafic–mafic rocks.


M. Chassé, W.L. Griffin, S.Y. O'Reilly, G. Calas
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Geochem. Persp. Let. (2017) 3, 105-114 | doi: 10.7185/geochemlet.1711 | Published 23 November 2016

Late accretion history of the terrestrial planets inferred from platinum stable isotopes

Abstract:
Late accretion of chondritic material to differentiated planetary bodies is thought to have been common in the early solar system. However, the timing and scale of admixing this material to terrestrial planets are poorly constrained. Using platinum (Pt) stable isotope data in a range of solar system bodies, we show that Earth’s post-Archean mantle has chondritic 198Pt/194Pt, consistent with addition of a chondritic late-veneer after core formation. Conversely, terrestrial Archean samples record non-chondritic, heavy, 198Pt/194Pt, indicating preservation of early mantle components that escaped complete mixing with the late-veneer. These data suggest admixing of ≤50 % of the eventual full late-veneer inventory. Such effective mixing within Earth’s mantle by 3.85 Ga is most consistent with modern-style plate tectonics.  


J.B. Creech, J.A. Baker, M.R. Handler, J.-P. Lorand, M. Storey, A.N. Wainwright, A. Luguet, F. Moynier, M. Bizzarro
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Geochem. Persp. Let. (2017) 3, 94-104 | doi: 10.7185/geochemlet.1710 | Published 8 November 2016

Direct sensing of total alkalinity profile in a stratified lake

Abstract:
We demonstrate the direct detection of a total alkalinity depth profile through the use of an integrated thin layer electrochemical modulation instrument which acts as an alkalinity sensor. The technique uses a chemically selective proton pump that alters the concentration of hydrogen ions in the thin layer sample. As the proton pump releases hydrogen ions the resulting pH is recorded at the pH probe placed directly opposite the thin sample gap. This results in an acid-base titration that takes place directly in the thin layer sample and therefore obviates the need for traditional sample manipulation. Collected samples from a stratified lake were assessed with this alkalinity probe to record a total alkalinity profile, indicating a substantial increase from 2.59 to 4.11 mM with depth. Results of the new method were in excellent agreement with titration alkalinity data, and offer the potential for autonomous on site measurements of this key parameter.  


M. Ghahraman Afshar, M. Tercier-Waeber, B. Wehrli, E. Bakker
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Geochem. Persp. Let. (2017) 3, 85-93 | doi: 10.7185/geochemlet.1709 | Published 10 November 2016

Stable vanadium isotopes as a redox proxy in magmatic systems?

Abstract:
Recycling pathways of multivalent elements, that impact our understanding of diverse geological processes from ore formation to the rise of atmospheric oxygen, depend critically on the spatial and temporal variation of oxygen fugacity (fO2) in the Earth’s interior. Despite its importance, there is currently no consensus on the relative fO2 of the mantle source of mid-ocean ridge basalts compared to the sub-arc mantle, regions central to the mediation of crust-mantle mass balances. Here we present the first stable vanadium isotope measurements of arc lavas, complemented by non-arc lavas and two co-genetic suites of fractionating magmas, to explore the potential of V isotopes as a redox proxy. Vanadium isotopic compositions of arc and non-arc magmas with similar MgO overlap with one another. However, V isotopes display strikingly large, systematic variations of ~2 ‰ during magmatic differentiation in both arc and non-arc settings. Calculated bulk V Rayleigh fractionation factors (1000 lnαmin-melt of -0.4 to -0.5 ‰) are similar regardless of the oxidation state of the evolving magmatic system, which implies that V isotope fractionation is most influenced by differences in bonding environment between minerals and melt rather than changes in redox conditions. Thus, although subtle fO2 effects may be present, V isotopes are not a direct proxy for oxygen fugacity in magmatic systems.  


J. Prytulak, P.A. Sossi, A.N. Halliday, T. Plank, P.S. Savage, J.D. Woodhead
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Geochem. Persp. Let. (2017) 3, 75-84 | doi: 10.7185/geochemlet.1708 | Published 26 October 2016