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Abstract:
Redox reactions often occur and significantly affect many geological processes. To simulate redox reactions in low temperature (T < 400 °C) hydrothermal experiments, fused silica was used as a hydrogen membrane to impose an externally fixed H₂ pressure (P_H2) on a fused silica capillary capsule (FSCC; 150 μm inner diameter, 375 μm outer diameter and ∼6 mm long) to define the redox state of the sample in the FSCC. At 300 °C, it required less than 7 hours to reach osmotic equilibrium. In this study, a constant P_H2 was imposed on an FSCC, which originally contained a 0.5 m (mole/kg H₂O) SnCl₄ + 0.5 m HCl aqueous solution, at 300 °C and vapour saturation pressure. In situ Raman spectra of the sample solution collected at 300 °C show that the reduction rate of Sn^IV to Sn^II species increased substantially with an increase of 1.1 bar of P_H2. We characterised precipitation and dissolution of cassiterite under various P-T-pH-P_H2 conditions and greatly increased our capabilities for performing rigorous hydrothermal experiments at temperatures below 400 °C, in which redox control is difficult to ensure without in situ approaches.

I.-M. Chou, R. Wang, J. Fang

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Geochem. Persp. Let. (2021) 20, 1–5 | doi: 10.7185/geochemlet.2135 | Published 2 December 2021

Article views: 2,773

Abstract:
Earth’s history has been characterised by complex interactions between life and the environment, which are often difficult to resolve. Here, we propose a new carbon cycle turbulence index (CT_index), based on the carbonate-carbon isotope (δ¹³C_carb) record, to measure the extent of environmental perturbation over the last billion years. The CT_index trend is closely linked to Phanerozoic biotic extinction rates (ERs), as calculated from a palaeobiology database, supporting a strong environmental control on biotic ERs. We use the empirical CT_index—ER relationship to compare the extent of environmental perturbation due to greenhouse gas emissions with that during the Permian-Triassic (PTr) transition (∼252 Ma), representing the most severe mass extinction of the Phanerozoic. At the current peak of fossil fuel emissions, the CT_index indicates a moderate future environmental perturbation. However, if fossil fuel emissions increase into the next century, a pronounced CT_index peak greater than that which occurred during the PTr transition is indicated, which suggests the potential for a severe “sixth mass extinction” in the future.

Z.-H. Li, Z. Guo, Z.-Q. Chen, S.W. Poulton, Y. Bao, L. Zhao, F.-F. Zhang

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Geochem. Persp. Let. (2021) 20, 11–15 | doi: 10.7185/geochemlet.2137 | Published 30 December 2021

Article views: 2,834

A. Luguet, G.M. Nowell, E. Pushkarev, C. Ballhaus

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Geochem. Persp. Let. (2022) 20, 19–21 | doi: 10.7185/geochemlet.2202 | Published 13 January 2022

Article views: 2,971

Abstract:
The evolution of the nitrogen concentration and isotopic composition during the degassing of Earth’s magma ocean, and thus in the primitive atmosphere, is key to understanding how habitable conditions developed on Earth. To constrain nitrogen degassing from the magma ocean, we determined the variations of the N content at N₂ gas saturation, N speciation, and N isotopic composition of a magma ocean analogue (basaltic komatiite) at oxygen fugacities (fO₂) from IW−4.2 to IW (where IW is the logarithmic difference between experimental fO₂ and that at Fe-FeO equilibrium). We performed a series of N degassing experiments in a piston cylinder at 1.5 GPa and 1550 °C in pure forsterite capsules. N concentrations in the mafic silicate melts decreased from 13,481 ± 735 ppm under the most reducing conditions to 798 ± 4 ppm at IW, controlled by N speciation (as determined by Raman spectroscopy), which changed from nitride (±N-H complexes) to molecular N₂ with increasing fO₂. Nitrogen occurs solely as N₂ in the degassed gas, regardless of fO₂. Nitrogen isotopic compositions (as determined by secondary ion mass spectroscopy) became significantly lighter in the degassed melt (quenched glass), down to −41 ± 13 ‰ relative to the initial composition (measured in an undegassed sample), following open system degassing trends (variable with fO₂ conditions), indicative of Rayleigh fractionation. These findings imply that an atmosphere in equilibrium with a reduced magma ocean would be N-depleted, whereas with increasing magma ocean fO₂ conditions, the primitive atmosphere would have become more enriched in N₂ gas.

C. Dalou, C. Deligny, E. Füri

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Geochem. Persp. Let. (2022) 20, 27–31 | doi: 10.7185/geochemlet.2204 | Published 19 January 2022

Article views: 2,791

Abstract:
Oxygen fugacity (fO₂) is poorly constrained in mantle reservoirs, especially those sampled by ocean island basalts (OIBs). This is partly due to complications from secondary processes acting on fO₂ in OIB parental magmas. To investigate these issues in situ trace element data of olivine in lavas from the island of La Réunion are reported. La Réunion lavas are useful for examining post-melting modification due to their limited range in isotopic signatures, indicating a largely homogenous mantle source. Vanadium abundances in olivine were used to constrain the lava fO₂ to between +0.2 and +1.5 ΔFMQ, intermediate between that of mid-ocean ridge basalts (MORBs) and other OIBs. The variable fO₂ of Réunion lavas results from assimilation processes within the volcanic edifice, and not source heterogeneity. The Réunion mantle plume likely has fO₂ similar to that of MORB, despite isotopic evidence for the plume sampling an early-formed isolated mantle reservoir. Assimilation processes can have a pronounced effect on lava fO₂ and should always be accounted for when constraining mantle redox.

R.W. Nicklas, R.K.M. Hahn, J.M.D. Day

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Geochem. Persp. Let. (2022) 20, 32–36 | doi: 10.7185/geochemlet.2205 | Published 21 January 2022

Article views: 2,909