Volume 20

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.

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.

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.

Abstract:
Tourmaline is the most common boron-rich mineral in magmatic systems. In this study, we determined experimentally the fractionation of boron isotopes between granitic melt and tourmaline for the first time. Our crystallisation experiments were performed using a boron-rich granitic glass (B₂O₃ ≈ 8 wt. %) at 660−800 °C, 300 MPa, and a_H2O = 1, in which tourmaline occurs as the only boron-hosting mineral. Our experimental results at four different temperatures show a small and temperature-dependent boron isotope fractionation between granitic melt and tourmaline (Δ¹¹B_melt–Tur = +0.90 ± 0.05 ‰ at 660 °C and +0.23 ± 0.12 ‰ at 800 °C), and the temperature dependence can be defined as Δ¹¹B_melt–Tur = 4.51 × (1000/T [K]) − 3.94 (R² = 0.96). Using these boron isotope fractionation factors, tourmaline can serve as a tracer to quantitatively interpret boron isotopic ratios in evolved magmatic systems. Our observation that ¹¹B is enriched in granitic melt relative to tourmaline suggests that the δ¹¹B of late-magmatic tourmaline should be higher than tourmaline that crystallised at an early stage, if B isotope fractionation is not affected by other processes, such as fluid loss.