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Volume 22

About the cover:  An artist’s rendition of the interior structure of Mars, with the core in the centre surrounded by the silicate mantle and crust. The core is overlain by a FE-SEM quantitative chemical map (Fe in grey, S in yellow, O in blue) of an experimental iron alloy equilibrated under conditions similar to those of the Martian core. In Letter 2216, Gendre et al. show that large amounts of oxygen should dissolve into a sulfur-rich Martian core using high-pressure, high-temperature experiments. Their multi-stage core formation simulations indicate that up to 3.5 wt. % oxygen could be present in the Martian core, a substantial amount that is comparable to the most O-rich models of Earth’s core.
Image credit: Communication IPGP. Download high-resolution cover.

Melting experiments on Fe-C-O to 200 GPa; liquidus phase constraints on core composition

Abstract:
Recent theoretical calculations suggested that carbon and oxygen are important light elements in the Earth’s inner and outer core, respectively. We performed melting experiments on the Fe-C-O system and obtained ternary liquidus phase relations at ∼50, ∼136, and ∼200 GPa based on textural and compositional characterisations of recovered samples. Considering the previously reported Fe-C binary eutectic liquid composition, these results are extrapolated to 330 GPa, which constrains C and O concentrations in the liquid outer core that crystallises Fe at the inner core. Theory has predicted a possible range of the solid inner core composition in Fe-C-S-Si that explains seismological observations. The compositions of liquids Fe-C-O-S-Si in equilibrium with such solid Fe-C-S-Si alloys are calculated with the solid-liquid partition coefficient of C obtained in this study along with those of S and Si in the literature. These liquid compositions, however, do not satisfy constraints from both outer core observations and the liquidus phase relations examined in this study, suggesting that the inner core is not Fe-C-S-Si alloy but may include H as an important impurity element.

F. Sakai, K. Hirose, K. Umemoto

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Geochem. Persp. Let. (2022) 22, 1–4 | https://doi.org/10.7185/geochemlet.2218 | Published 17 May 2022

Sulfur solubility in a deep magma ocean and implications for the deep sulfur cycle

Abstract:
The Earth could have experienced sulfide segregation during its differentiation due to sulfur (S) saturation within a magma ocean. The relative timing of sulfide saturation during magma ocean crystallisation is strongly dependent on the solubility of S at sulfide saturation (SCSS). Here, we present SCSS data directly relevant for a deep terrestrial magma ocean obtained from laser heated diamond anvil cell experiments. Our new data, along with existing SCSS data obtained for similar compositions, was parameterised to derive a new predictive equation. Our parameterisation predicts that existing models strongly underestimate the SCSS over the P-T range of a deep magma ocean. Our SCSS models provide the S abundances required at any given stage of terrestrial accretion, and imply that sulfide saturation is much less efficient at stripping the Earth’s mantle of S during accretion than previously predicted. Applying our results to the most recent mantle S evolution models shows that the SCSS would be far too high to achieve sulfide saturation, until only perhaps the final stages of magma ocean crystallisation. To satisfy highly siderophile element systematics, either the S content of the magma ocean was considerably higher than currently assumed, or highly siderophile element abundances were affected by other processes, such as iron disproportionation.

E.S. Steenstra, O.T. Lord, S. Vitale, E.S. Bullock, S. Klemme, M. Walter

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Geochem. Persp. Let. (2022) 22, 5–9 | https://doi.org/10.7185/geochemlet.2219 | Published 20 May 2022

Neodymium isotopes trace marine provenance of Arctic sea ice

Abstract:
Radiogenic neodymium (Nd) isotopes (ɛNd) have the potential to serve as a geochemical tracer of the marine origin of Arctic sea ice. This capability results from pronounced ɛNd differences between the distinct marine and riverine sources, which feed the surface waters from which the ice forms. The first dissolved Nd isotope and rare earth element (REE) concentration data obtained from Arctic sea ice collected across the Fram Strait during RV Polarstern cruise PS85 in 2014 confirm the incorporation and preservation of the parental surface seawater ɛNd signatures despite efficient REE rejection. The large ɛNd variability between ice floes and within sea ice cores (−32 to −10) reflects changes in water mass distribution during ice growth and drift from the central Arctic Ocean to Fram Strait. In addition to the parental seawater composition, our new approach facilitates the reconstruction of the transfer of matter between the atmosphere, the sea ice and the ocean. In conjunction with satellite-derived drift trajectories, we enable a more accurate assessment of sea ice origin and spatiotemporal evolution, benefiting studies of sea ice biology, biodiversity, and biogeochemistry.

G. Laukert, I. Peeken, D. Bauch, T. Krumpen, E.C. Hathorne, K. Werner, M. Gutjahr, M. Frank

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Geochem. Persp. Let. (2022) 22, 10–15 | https://doi.org/10.7185/geochemlet.2220 | Published 10 June 2022

Constant iron isotope composition of the upper continental crust over the past 3 Gyr

Abstract:
The Fe isotopic composition of twenty four glacial diamictite composites with depositional ages ranging from the Mesoarchean to the Palaeozoic serve as proxies of the average upper continental crust (UCC) and can be used to track how δ56Fe may have changed in the continental crust through time. The diamictites have elevated chemical index of alteration (CIA) values and other characteristics of weathered regoliths (e.g., strong depletion in soluble elements such as Sr), which they inherited from their upper crustal source regions. The δ56Fe values in the diamictite composites range from −0.59 ‰ to +0.23 ‰. Excluding three samples impacted by the incorporation of materials from Fe formations, the diamictites have an average δ56Fe of 0.12 ± 0.13 ‰ (2σ), overlapping the recent estimated average δ56Fe of 0.09 ± 0.03 ‰ (2 s.d.) in the upper continental crust (Dauphas et al., 2017, and references therein). There is no obvious correlation between δ56Fe of the glacial diamictites and the CIA. Our data suggest that the Fe isotope composition of the upper continental crust has been relatively constant throughout Earth history and that chemical weathering is not important in producing Fe isotope variations in the upper continental crust. Pre-Great Oxidation Event (GOE) anoxic weathering, when iron was soluble in its divalent state, did not generate different Fe isotopic signatures from the post-GOE oxidative weathering environment in the upper continental crust. Therefore, the large Fe isotopic fractionations observed in various marine sedimentary records are likely due to processes occurring in the oceans (e.g., biological activity) rather than abiotic redox reactions on the continents.

X.-M. Liu, R.M. Gaschnig, R.L. Rudnick, R.M. Hazen, A. Shahar

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Geochem. Persp. Let. (2022) 22, 16–19 | https://doi.org/10.7185/geochemlet.2221 | Published 10 June 2022

How long for plastics to decompose in the deep sea?

Abstract:
The deep sea floor is recognised as one of the most important final destinations for plastic debris. It is not clear whether the plastic debris in the deep sea could be degraded. Likewise, little is known about how long plastics might last at the deep sea floor. A total of 103 plastic debris were recovered using the manned submersible “Shenhaiyongshi” on the deep sea floor (746–3997 m) of the South China Sea (SCS). We found that abundant corrosion structures were present on the surface of polyethylene (PE), which was the dominant type of plastic sample (80 %). The rod-like, filamentous and peanut-like morphologies of the corrosion structures are well in line with those of microorganisms, suggesting that they were derived from biodegradation. The calculation of volume loss of corroded PE showed that about 0.53–6.86 % PE were degraded. Assuming that the most degraded plastic reached the deep sea floor 40 years ago, these plastics will require about 292 years to be totally degraded. Our results provide unique insights into the fate of deep sea plastics and answer the unsolved question about how long plastics may persist in deep sea.

X. Zhang, X. Peng

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Geochem. Persp. Let. (2022) 22, 20–25 | https://doi.org/10.7185/geochemlet.2222 | Published 27 June 2022