E N V I R O N M E N T , E A R T H A N D P L A N E T A R Y S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

1 3 0 H I G H L I G H T S 2 0 2 3 I

X-rays open atmospheric windows to the Earth s evolving mantle

Micro X-ray absorption near-edge structure spectroscopy was used in apatite inclusions encapsulated in 2.4 to 2.1 billion-year-old zircon crystals to determine their relative abundances of sulfur speciation (S6+, S4+ and S2−). The results demonstrate that zircon-bearing magmas became more oxidised over time as a result of Earth s atmospheric oxygenation.

Modern plate tectonics exerts a first order control on Earth s climate, ocean levels and metallogenetic processes within the continental crust. Atmospherically altered surface materials are recycled to the mantle via subduction, while volatiles from the mantle are liberated to the atmosphere via volcanism. However, the interplay between Earth s atmospheric changes and the geochemical evolution of mantle-derived magmas has remained obscure for the ancient geological history.

The Great Oxidation Event (GOE) marks a period between ~2.45 and 2.20 billion years ago of major oxygen increase in the Earth s atmosphere [1]. During this time window of 250 million years, atmospheric oxygen levels increased by five orders of magnitude, causing a permanent and dramatic change in Earth s surface chemistry, particularly between 2.30 and 2.20 billion years ago [2]. In this study, magmas formed in sub-arc regions akin to modern subduction zones were investigated as potential tracers of the interaction between Earth s atmosphere and the mantle.

This work aimed to assess the mantle oxygen fugacity (fO2) across the GOE using magmas that share a common mantle source but that crystallised more than 200 million years apart (at 2.35 and 2.13 billion years ago [3]), therefore allowing comparative changes in fO2. Nonetheless, ancient rocks are susceptible to overprinting and modification of their initial geochemical signatures. Thus, the biggest challenge consisted of assuring that primary fugacity signatures could be obtained. Zircons are extremely robust and refractory and can further be analysed by other isotope systems, which provide temporal and geochemical context for the measured inclusions. In turn, apatites are very useful for determining fugacity because sulfur incorporation is intrinsically related to the redox state of host crystallising magmas [4].

Micro X-ray absorption near-edge structure (μ-XANES) spectroscopy was used at beamline ID21 to measure the sulfur K-edge in phosphate-mineral apatite inclusions shielded in silicate-mineral zircon hosts (Figure 105). The sulfur speciation (S2−, S4+ and S6+ and their proportions) was measured in both apatite grains from the matrix rocks and apatite inclusions in zircon. XANES spectra were acquired in the range of 2.46 to 2.53 keV, in continuous mode, with steps of 0.2 eV and 0.1 s per point, taking approximately 1 minute per point. The beam was then used for the construction of 2D micro X-ray fluorescence (μXRF) elemental maps and μ-XANES spectra for chemical speciation. Zircon host grains were analysed a few times to assess possible interferences. To separate the XRF K lines from S from the Zr L2 and L3 emissions lines, XRF spectra were batch fitted using the PyMca software. The S K-edge XANES spectra inorganic database was used to identify the different peak energy positions for natural minerals with distinct sulfur speciation. To identify S6+(~2,482 eV), S4+ (~2,478 eV) and S2−(~2,470 eV), gypsum, pyrrhotite and pyrite were used.

Apatite inclusions presented a remarkable feature of increased sulfur concentration towards the centre of the inclusion (Figure 105). This feature is taken to represent the incorporation and retention of sulfur upon crystallisation and, as such, a primary signature of the inclusions investigated. Apatite inclusions in zircon hosts of 2.35 billion-year-old magmas show a dominant S2− peak, whereas inclusions from the 2.13 billion-year-old magmas show a predominant S6+ peak. The contrasting sulfur speciation is readily interpreted as a change in fO2 across the GOE. This change is thought to be caused by recycling into the mantle of sediments that had been geochemically altered at the surface by the increase in atmospheric oxygen levels. This study opens a wide window of opportunities for the time-integrated investigation of the interaction between atmosphere and oceans with the evolving terrestrial mantle.