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Volume 24

About the cover:  An artist’s rendition of the JAXA Hayabusa2 spaceship sampling the surface of the asteroid 162173 Ryugu.
In December 2020, this mission returned the first samples collected from a Cb-type asteroid (Ryugu) to Earth. In
Letter 2238, Moynier et al. report the first Ca stable isotopic composition of two samples from Ryugu and show that it falls within the range defined by the CI-chondrites, the most primitive meteorite characterised by near solar composition. Illustration by Akihiro Ikeshita (©Akihiro Ikeshita). Download high-resolution cover.

The Solar System calcium isotopic composition inferred from Ryugu samples
Abstract:
The Hayabusa2 spacecraft has returned samples from the Cb-type asteroid (162173) Ryugu to Earth. Previous petrological and chemical analyses support a close link between Ryugu and CI chondrites that are presumed to be chemically the most primitive meteorites with a solar-like composition. However, Ryugu samples are highly enriched in Ca compared to typical CI chondrites. To identify the cause of this discrepancy, here we report stable Ca isotopic data (expressed as δ44/40CaSRM915a) for returned Ryugu samples collected from two sites. We found that samples from both sites have similar δ44/40CaSRM915a (0.58 ± 0.03 ‰ and 0.55 ± 0.08 ‰, 2 s.d.) that fall within the range defined by CIs. This isotopic similarity suggests that the Ca budget of CIs and Ryugu samples is dominated by carbonates, and the variably higher Ca contents in Ryugu samples are due to the abundant carbonates. Precipitation of carbonates on Ryugu likely coincided with a major episode of aqueous activity dated to have occurred ∼5 Myr after Solar System formation. Based on the pristine Ryugu samples, the average δ44/40CaSRM915a of the Solar System is defined to be 0.57 ± 0.04 ‰ (2 s.d.).

F. Moynier, W. Dai, T. Yokoyama, Y. Hu, M. Paquet, Y. Abe, J. Aléon, C.M.O’D. Alexander, S. Amari, Y. Amelin, K.-I. Bajo, et al.

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Geochem. Persp. Let. (2022) 24, 1–6 | https://doi.org/10.7185/geochemlet.2238 | Published 19 October 2022

Quantifying seawater exchange rates in the Eocene Arctic Basin using osmium isotopes
Abstract:
The closure of seaways that connected the Arctic Ocean to the global ocean during the early Paleogene led to severe hydrographic restriction. We present new osmium isotope data from organic-rich sediments deposited in the central Arctic Ocean during the Early–Middle Eocene. The new data show that the long term isotopic composition of osmium in Arctic seawater began to diverge from that of the global ocean at ∼54 Ma, after the Eocene Thermal Maximum 2 hyperthermal event. This divergence was probably caused by the gradual closure of seaways connecting the Arctic Ocean to the global ocean. The Os data are used to calculate water exchange rates between the Arctic and surrounding oceans and to calculate Arctic Ocean salinity during the Early Eocene. The results show that the development of severe, long term Arctic Basin restriction after ∼54 Ma occurred as open ocean seawater input decreased below ∼0.01 Sv, resulting in a mean basin salinity between 8–16 PSU, depending on model assumptions.

A.J. Dickson, M. Davies, M.-L. Bagard, A.S. Cohen

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Geochem. Persp. Let. (2022) 24, 7–11 | https://doi.org/10.7185/geochemlet.2239 | Published 26 October 2022

Secular variability in zircon phosphorus concentrations prevents simple petrogenetic classification
Abstract:
Phosphorus (P) concentrations in zircon have been used to discriminate their derivation from metaluminous versus strongly peraluminous granites (SPGs) based on the empirical observation of lower P concentrations in zircon from Phanerozoic metaluminous versus peraluminous granites. Higher P concentrations in zircon from Phanerozoic SPGs reflect enhanced apatite solubility in peraluminous melts and overall higher P concentrations in peraluminous granites. However, SPGs derived from partial melting of Precambrian sedimentary rocks have lower P concentrations compared to Phanerozoic metaluminous granites, reflecting lower P concentrations in Precambrian versus Phanerozoic sedimentary sources. We demonstrate that zircons from Precambrian SPGs also have lower P concentrations compared to Phanerozoic counterparts, likely reflecting lower P concentrations in their parental melts. Applying the P-in-zircon proxy to the detrital zircon record does not effectively discriminate between metaluminous and peraluminous sources and underestimates contributions from peraluminous granites. Although detrital zircons are an important early Earth archive, a uniformitarian perspective cannot always be applied when using trace element proxies developed on Phanerozoic samples.

C.E. Bucholz, J. Liebmann, C.J. Spencer

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Geochem. Persp. Let. (2022) 24, 12–16 | https://doi.org/10.7185/geochemlet.2240 | Published 11 November 2022