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.

Geochem. Persp. Let. (2019) 10, 26–30 | doi: 10.7185/geochemlet.1910 | Published 15 April 2019

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.

Geochem. Persp. Let. (2019) 10, 20–25 | doi: 10.7185/geochemlet.1909 | Published 29 March 2019

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.

Geochem. Persp. Let. (2019) 10, 8–13 | doi: 10.7185/geochemlet.1907 | Published 26 March 2019

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.

Geochem. Persp. Let. (2019) 10, 14–19 | doi: 10.7185/geochemlet.1908 | Published 26 March 2019

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 (δ¹³C_charcoal) 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 δ¹³C_charcoal 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.

Geochem. Persp. Let. (2019) 10, 1–6 | doi: 10.7185/geochemlet.1906 | Published 1 March 2019

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.

Geochem. Persp. Let. (2019) 9, 49–53 | doi: 10.7185/geochemlet.1905 | Published 19 February 2019

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.

Geochem. Persp. Let. (2019) 9, 38–42 | doi: 10.7185/geochemlet.1903 | Published 11 February 2019

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.

Geochem. Persp. Let. (2019) 9, 43–48 | doi: 10.7185/geochemlet.1904 | Published 6 February 2019

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 L_2,3-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.

Geochem. Persp. Let. (2019) 9, 32–37 | doi: 10.7185/geochemlet.1902 | Published 6 February 2019

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.

Geochem. Persp. Let. (2019) 9, 26–31 | doi: 10.7185/geochemlet.1901 | Published 21 January 2019

Abstract:
Noble gases serve as unique tracers of the origin and evolution of Earth’s volatile reservoirs owing to their inert nature and contribution from extinct and extant radioactivities. However, noble gases are low in abundance relative to many other elements, particularly in the Earth’s mantle. Additionally, mantle-derived samples show large post-eruptive atmospheric contamination, rendering the determination of the primary mantle composition challenging. The sources of mantle krypton and xenon remain debated due to their partially resolvable excess, if any, relative to the atmosphere. Atmospheric noble gases also appear to be recycled into the mantle via subduction, progressively overprinting the initial mantle signature. Here we develop a new protocol to accumulate non-contaminated mantle-derived xenon, in particular the low abundant ^124-126-128Xe. The results show the highest excesses in ^124-126-128Xe ever measured in the mantle relative to the atmosphere and point toward a chondritic origin for mantle xenon. The fissiogenic isotopes ^{131-132-134-136}Xe allow the onset of efficient xenon recycling in the mantle to be constrained at around 3 Gyr ago, implying that volatile recycling before 3 Ga would have been negligible.

Geochem. Persp. Let. (2018) 9, 21–25 | doi: 10.7185/geochemlet.1833 | Published 20 December 2018

Abstract:
Scandium is the subject of increasing interest and is even becoming critical for a number of institutions in Europe and the U.S.A. among others because of its economic value and supply risk. Despite this growing awareness, little is known about its geochemical behaviour. Recent studies have shown that Sc can be strongly associated with iron oxides including goethite but the mode of interaction is not yet fully understood. We investigated the incorporation of Sc in goethite structure in controlled batch systems. For the first time, we show the formation of a solid αFeOOH-αScOOH solution consistent with an ideal solution despite significant differences in atomic radii between Sc³⁺ and Fe³⁺. We believe that beyond this fundamental knowledge concerning the affinity of Sc for goethite, these results provide important insights into Sc speciation in primary or secondary sources, including Sc-rich lateritic deposits and bauxite residues after alumina extraction, for the future design of extraction processes with a low environmental footprint, as an alternative to current energy-intensive technologies.

Geochem. Persp. Let. (2018) 9, 16–20 | doi: 10.7185/geochemlet.1832 | Published 19 December 2018

Abstract:
The mechanisms driving the chemical complementarity between depleted MORB mantle (DMM) and continental crust (with an average 'andesitic' composition) remain unclear. By investigating Archean komatiites, and modern enriched (E) and normal (N) MORB samples, we demonstrate that partial melting of the mantle does not fractionate Ti isotopes, whereas intracrustal differentiation causes significant Ti isotopic fractionation between melts and minerals, specifically Fe-Ti oxides. Thus, Ti isotope ratios are tracers of these two magmatic regimes. N-MORB and late Archean (2.9-2.7 Ga) komatiites are depleted in the heavier Ti isotopes compared to E-MORB and middle Archean (3.5-3.3 Ga) komatiites. We show that the depletion in the heavier Ti isotopes of the DMM is due to mantle recycling of the isotopically light residues from the generation of felsic continental crust over 3.5-2.7 Ga. This process must have reached a steady state by ≈ 2.5 Ga, based on the uniform Ti isotopic composition of contemporary N-MORBs and late Archean komatiites. This change is likely due to a decrease in the mantle potential temperature related to the emergence of plate tectonics.

Geochem. Persp. Let. (2018) 9, 11–15 | doi: 10.7185/geochemlet.1831 | Published 13 December 2018