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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Ultrahigh pressure structural changes in a 60 mol. % Al2O3-40 mol. % SiO2 glass

Abstract:
Structure of an Al-containing silicate glass (60 mol. % Al2O3–40 mol. % SiO2, A40S) is investigated up to 131 GPa, a pressure close to that of the Earth’s core-mantle boundary, by using our recently developed double stage large volume cell. The first peak (r1) of the pair distribution function, which corresponds to T–O distance (T = Al, Si), rapidly increases below 16 GPa, indicating an increase of average coordination number (CN) of T–O from ~4 to 6. The r1 linearly decreases in the pressure range of 25–110 GPa, but it displays a slope change and becomes nearly constant above 110 GPa. The slope change may imply a structural change in the A40S glass around 110 GPa, and may be explained by the change in Al–O distance associated with the Al–O CN increase from 6 to >6 as predicted by molecular dynamics simulations (Ghosh and Karki, 2018). Our observations suggest an important role for aluminum in densification of aluminosilicate at the deep lower mantle, which might imply a dense aluminosilicate magma with negative buoyancy.


I. Ohira, Y. Kono, Y. Shibazaki, C. Kenney-Benson, A. Masuno, G. Shen
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Geochem. Persp. Let. (2019) 10, 41–45 | doi: 10.7185/geochemlet.1913 | Published 3 May 2019

Calcium stable isotopes place Devonian conodonts as first level consumers

Abstract:
Conodont animals are an extinct group of jawless vertebrates whose hard parts, also known as conodont elements, represent the earliest evidence of a mineralised skeleton in the vertebrate lineage. Conodont elements are interpreted as parts of a feeding apparatus, which together with the presence of eyes and microwear patterns, support the controversial hypothesis that conodont animals were macrophagous predators and/or scavengers. Here, we explore the trophic position of five conodont genera (Palmatolepis, Polygnathus, Ancyrodella, Ancyrognathus and Icriodus) from five contemporary Late Devonian sites distributed worldwide (France, Morocco, Vietnam and Australia) by means of calcium (Ca) stable isotope compositions. The seawater Ca isotope composition was calibrated using contemporary Late Devonian brachiopod isotopic values. By comparison with extant marine trophic chain composed of cartilaginous fish, conodont Ca isotope compositions are indicative of a zooplanktivore - primary piscivore niche, with no genus-specific trophic pattern. The question of active predation or scavenging cannot be resolved definitively but our results strongly suggest that Late Devonian conodonts were first level consumers.


V. Balter, J.E. Martin, T. Tacail, G. Suan, S. Renaud, C. Girard
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Geochem. Persp. Let. (2019) 10, 36–39 | doi: 10.7185/geochemlet.1912 | Published 26 April 2019

Sea level rise produces abundant organobromines in salt-affected coastal wetlands

Abstract:
Global sea level rise exposes terrestrially derived natural organic matter to elevated salinities, which may alter the complex biogeochemical cycling of halogens in coastal wetland sediments. Here we show that sea level rise increases the natural production of organobromines in submerged soils and wetland sediments. We compared the concentrations and speciation of sedimentary chlorine and bromine along a salinity gradient in low-lying coastal forested wetlands in Winyah Bay (South Carolina, United States). Sorption differences between chloride and bromide were not observed, but up to 80 % of total retained bromine is organically bound, with the highest fraction of organically bound bromine found in formerly freshwater wetlands inundated by seawater. Wet/dry cycling of soils and the abundance of aromatic-rich natural organic matter in these salt-affected dieback forested wetlands promote bromination of organic matter, as demonstrated by laboratory simulations. Bromination of soil organic matter caused by continued sea level rise thus may be a major source of organobromines in coastal environments and possibly volatile halomethanes.


C. Joe-Wong, D.R. Schlesinger, A.T. Chow, S.C.B. Myneni
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Geochem. Persp. Let. (2019) 10, 31–35 | doi: 10.7185/geochemlet.1911 | Published 23 April 2019

Exchange catalysis during anaerobic methanotrophy revealed by 12CH2D2 and 13CH3D in methane

Abstract:
The anaerobic oxidation of methane (AOM) is a crucial component of the methane cycle, but quantifying its role in situ under dynamic environmental conditions remains challenging. We use sediment samples collected during IODP Expedition 347 to the Baltic Sea to show that relative abundances of 12CH2D2 and 13CH3D in methane remaining after microbial oxidation are in internal, thermodynamic isotopic equilibrium, and we attribute this phenomenon to the reversibility of the initial step of AOM. These data suggest that 12CH2D2 and 13CH3D together can identify the influence of anaerobic methanotrophy in environments where conventional bulk isotope ratios are ambiguous, and these findings may lead to new insights regarding the global significance of enzymatic back reaction in the methane cycle.


J.L. Ash, M. Egger, T. Treude, I. Kohl, B. Cragg, R.J. Parkes, C.P. Slomp, B. Sherwood Lollar, E.D. Young
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Geochem. Persp. Let. (2019) 10, 26–30 | doi: 10.7185/geochemlet.1910 | Published 15 April 2019

Thermodynamic controls on redox-driven kinetic stable isotope fractionation

Abstract:
Stable isotope fractionation arising from redox reactions has the potential to illuminate the oxygenation of Earth’s interior, oceans, and atmosphere. However, reconstruction of past and present redox conditions from stable isotope signatures is complicated by variable fractionations associated with different reduction pathways. Here we demonstrate a linear relationship between redox-driven kinetic fractionation and the standard free energy of reaction for aqueous chromium(VI) reduction by iron(II) species. We also show that the intrinsic kinetic fractionation factor is log-linearly correlated with the rate constant of reaction, which is in turn a function of the free energy of reaction. The linear free energy relationship for kinetic fractionation describes both our experimental results and previous observations of chromium isotope fractionation and allows the magnitude of fractionation to be directly linked to environmental conditions such as pH and oxygen levels. By demonstrating that the magnitude of kinetic fractionation can be thermodynamically controlled, this study systematically explains the large variability in chromium(VI) isotope fractionation and provides a conceptual framework that is likely applicable to other isotope systems.


C. Joe-Wong, K.L. Weaver, S.T. Brown, K. Maher
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Geochem. Persp. Let. (2019) 10, 20–25 | doi: 10.7185/geochemlet.1909 | Published 29 March 2019

Onset of new, progressive crustal growth in the central Slave craton at 3.55 Ga

Abstract:
Ancient rock samples are limited, hindering the investigation of the processes operative on the Earth early in its history.  Here we present a detailed study of well-exposed crustal remnants in the central Slave craton that formed over a 1 billion year magmatic history. The tonalitic-granodioritic gneisses analysed here are broadly comparable to common suites of rocks found in Archean cratons globally. Zircon Hf isotope data allow us to identify a major change in the way continental crust was formed in this area, with a shift to distinctly positive εHf starting at ~3.55 Ga. The crust production processes and spatial distribution of isotopic compositions imply variable interaction with older crust, similar to the relationships seen in modern tectonic settings; specifically, long-lived plate margins. A majority of the Slave craton might have been formed by a similar mechanism.


J.R. Reimink, D.G. Pearson, S.B. Shirey, R.W. Carlson, J.W.F. Ketchum
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Geochem. Persp. Let. (2019) 10, 8–13 | doi: 10.7185/geochemlet.1907 | Published 26 March 2019

A lunar hygrometer based on plagioclase-melt partitioning of water

Abstract:
The Moon was initially covered by a magma ocean. Hydrogen detected in plagioclase of ferroan anorthosites, the only available samples directly crystallised from the lunar magma ocean (LMO), can be used to quantify LMO hydrogen content. We performed experiments to determine plagioclase-melt partition coefficients of water under LMO conditions with water contents of co-existing plagioclase and melt quantified using Fourier-Transform Infrared Spectroscopy. Results indicate lunar plagioclase can incorporate approximately one order of magnitude more water than previously assumed. Using measured water contents of lunar plagioclase, this suggests that ~100 μg/g H2O equivalent was present in the residual magma when 95 % of the initial LMO had crystallised. Our results constrain initial LMO water contents to ~ 5 μg/g H2O equivalent if water was conserved throughout LMO evolution. If on the other hand the initial LMO contained >1000 μg/g water as suggested by experiments on LMO crystallisation, >99 % hydrogen degassing occurred during the evolution of the LMO.


Y.H. Lin, H. Hui, Y. Li, Y. Xu, W. van Westrenen
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Geochem. Persp. Let. (2019) 10, 14–19 | doi: 10.7185/geochemlet.1908 | Published 26 March 2019

Widespread and intense wildfires at the Paleocene-Eocene boundary

Abstract:
Discovery of impact spherules associated with the onset of the Carbon Isotope Excursion (CIE) that marks the Paleocene-Eocene (P-E) boundary (~56 Ma) indicates that the P-E transition was coincident with an extraterrestrial impact. Charcoal abundances increase >20 times background immediately above the P-E spherule layer at two Atlantic Coastal Plain palaeo-continental shelf localities located >200 km apart. Individual charcoal shards (~100 μm long; 58-83 wt. % carbon) show charred plant features. The carbon isotope ratio of charcoal (δ13Ccharcoal) through the peak shows that it originated from pre-impact vegetation that burned. We consider two scenarios to explain this widespread, synchronous increase in charcoal at the P-E boundary: 1) warming-induced, continental-scale drying; and 2) impact-induced wildfires. Differentiating between these two hypotheses depends critically on the observed sequence of events, which on the western North Atlantic margin is: the impact spherule horizon, followed by the peak in charcoal (derived from vegetation that grew before the CIE and impact), and finally the nadir of the CIE. Importantly, the pre-excursion δ13Ccharcoal remains constant through the CIE onset, requiring a dramatic increase in sedimentation. This work clarifies our understanding of the timing and sequence of events following an extraterrestrial impact at the P-E boundary.


M.K. Fung, M.F. Schaller, C.M. Hoff, M.E. Katz, J.D. Wright
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Geochem. Persp. Let. (2019) 10, 1–6 | doi: 10.7185/geochemlet.1906 | Published 1 March 2019

Zircon halogen geochemistry: Insights into Hadean-Archean fluids

Abstract:
Understanding the distribution of halogens in rocks can potentially trace ancient lithosphere, hydrosphere, and atmosphere interactions. Although no sedimentary rocks older than 3.8 Ga are known, insights into sediment-atmosphere exchange on early Earth could be obtained from knowledge of halogen contents in ancient zircons. Here we present the first study of halogen abundances in Jack Hills zircons together with younger zircons of known provenance to provide geologic context. The relatively low (ca. 0.1-0.6 μg/g) chlorine concentrations in most Hadean and Archean Jack Hills zircons are similar to the average concentration in younger igneous zircons. In contrast, significant Cl enrichments are found in a subset of ca. 3.9–3.8 Ga zircons ([Cl]average= 1.43 ± 0.27 μg/g) that appear to record halogen transport under hydrothermal conditions. Such Cl-bearing fluids in early Earth history may reflect extraction of halogens from the interior to near surface environments.


H. Tang, D. Trail, E.A. Bell, T.M. Harrison
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Geochem. Persp. Let. (2019) 9, 49–53 | doi: 10.7185/geochemlet.1905 | Published 19 February 2019

Oxidised micrometeorites as evidence for low atmospheric pressure on the early Earth

Abstract:
Reconstructing a record of the partial pressure of molecular oxygen in Earth’s atmosphere is key for understanding macroevolutionary and environmental change over geological history. Recently, the oxidation state of iron in micrometeorites has been taken to imply the presence of modern Earth concentrations of oxygen in the upper atmosphere at 2.7 Ga, and therefore a highly chemically stratified atmosphere (Tomkins et al., 2016). We here explore the possibility that the mixing ratio of oxygen in Earth’s upper atmosphere, that probed by micrometeorites, may instead be sensitive to the surface atmospheric pressure. We find that the concentrations of oxygen in the upper atmosphere required for micrometeorite oxidation are achieved for a 0.3 bar atmosphere. In this case, significant water vapour reaches high up in the atmosphere and is photodissociated, leading to the formation of molecular oxygen. The presence of oxidised iron in micrometeorites at 2.7 Ga may therefore be further evidence that the atmospheric pressure at the surface of the early Earth was substantially lower than it is today.


P.B. Rimmer, O. Shorttle, S. Rugheimer
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Geochem. Persp. Let. (2019) 9, 38–42 | doi: 10.7185/geochemlet.1903 | Published 11 February 2019

Radiogenic Ca isotopes confirm post-formation K depletion of lower crust

Abstract:
Heat flow studies suggest that the lower crust has low concentrations of heat-producing elements. This could be due to either (i) greater fractions of basaltic rock at depth or (ii) metamorphic depletion of radioactive elements from rocks with more evolved (andesitic to granodioritic) compositions. However, seismic data suggest that lower crust is not predominantly basaltic, and previous studies (using Pb and Sr isotopes) have shown that lower crustal rocks have experienced significant losses of U and Rb. This loss, however, is poorly constrained for K, which is inferred to be the most important source of radioactive heat in the earliest crust. Our high precision Ca isotope measurements on a suite of granulite facies rocks and minerals from several localities show that significant losses of K (~60 % to >95 %) are associated with high temperature metamorphism. These results support models whereby reduction of heat production from the lower crust, and consequent stabilisation of continental cratons in the Precambrian, are largely due to high temperature metamorphic processes. Relative changes in whole rock K/Ca suggest that 20-30 % minimum (granitic) melt removal can explain the K depletions.


M.A. Antonelli, D.J. DePaolo, T. Chacko, E.S. Grew, D. Rubatto
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Geochem. Persp. Let. (2019) 9, 43–48 | doi: 10.7185/geochemlet.1904 | Published 6 February 2019

Magma properties at deep Earth’s conditions from electronic structure of silica

Abstract:
SiO2 is the main component of silicate melts and thus controls their network structure and physical properties. The compressibility and viscosities of melts at depth are governed by their short range atomic and electronic structure. We measured the O K-edge and the Si L2,3-edge in silica up to 110 GPa using X-ray Raman scattering spectroscopy, and found a striking match to calculated spectra based on structures from molecular dynamic simulations. Between 20 and 27 GPa, [4]Si species are converted into a mixture of [5]Si and [6]Si species and between 60 and 70 GPa,  [6]Si becomes dominant at the expense of [5]Si with no further increase up to at least 110 GPa. Coordination higher than 6 is only reached beyond 140 GPa, corroborating results from Brillouin scattering. Network modifying elements in silicate melts may shift this change in coordination to lower pressures and thus magmas could be denser than residual solids at the depth of the core-mantle boundary.


S. Petitgirard, C.J. Sahle, C. Weis, K. Gilmore, G. Spiekermann, J.S. Tse, M. Wilke, C. Cavallari, V. Cerantola, C. Sternemann
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Geochem. Persp. Let. (2019) 9, 32–37 | doi: 10.7185/geochemlet.1902 | Published 6 February 2019

The primordial He budget of the Earth set by percolative core formation in planetesimals

Abstract:
The primordial He budget of the Earth’s interior is commonly thought to have been set by full liquid metal-silicate equilibration in a terrestrial magma ocean. However, incomplete metal-silicate equilibration during accretion will have a substantial effect on this budget. Here we present liquid-solid partitioning experiments indicating that He behaves as a moderately siderophile element during percolative core formation in planetesimals. Mass balance considerations show that even minor disequilibrium will allow the Earth’s early core to incorporate sufficient primordial He—and possibly other noble gases—to supply the lower mantle throughout Earth’s history. We conclude that the high 3He/4He ratios in basalts may well represent primarily the last vestiges of metal-silicate disequilibrium in a terrestrial magma ocean preserved from the time of Earth’s formation.


A.S.G. Roth, C. Liebske, C. Maden, K.W. Burton, M. Schönbächler, H. Busemann
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Geochem. Persp. Let. (2019) 9, 26–31 | doi: 10.7185/geochemlet.1901 | Published 21 January 2019