Current Issue

Volume 25

About the cover:  Radial blade-like morphology of biogenic vivianite [Fe3(PO4)2 • 8H2O] formed by Geobacter sulfurreducens, imaged by scanning electron microscopy. The study presented by Bronner et al. in Letter 2301 shows how microbially mediated coreduction of Fe(III) and elemental sulfur leads to the formation of vivianite under ferruginous conditions and its dissolution under sulfidic conditions, with the concurrent formation of Fe-S minerals (mackinawite, greigite). These processes regulate the availability of phosphate, a key nutrient that affects primary productivity and Earth’s biogeochemical evolution.
Image credit: Jeremiah Shuster. Download high-resolution cover.

Planetary accretion and core formation inferred from Ni isotopes in enstatite meteorites
Nickel is a siderophile and near-refractory element, making its isotopes a potential tool for tracing planetary accretion and differentiation. However, the origin of the Ni stable isotope difference between bulk silicate Earth (BSE) and chondrites remains enigmatic. To address this problem, we report high precision Ni stable isotope data of enstatite chondrites and achondrites that possess similar mass independent O and Ni isotope compositions like the Earth-Moon system. Bulk enstatite chondrites have δ60/58Ni values of 0.24 ± 0.08 ‰ (2 s.d., n = 13). Enstatite achondrites, including main-group aubrites, Shallowater and Itqiy, show relatively large δ60/58Ni variations, ranging from 0.03 ± 0.02 ‰ to 0.57 ± 0.04 ‰. This could reflect fractionations between sulfide and metal phases, as is evidenced by correlation between their S/Ni ratios and δ60/58Ni values. In enstatite achondrites, Ni is mainly hosted in metal and to a lesser extent in sulfides, so δ60/58Ni values in enstatite achondrites may represent the Ni isotopic values of the cores of their parent bodies. The overlapping δ60/58Ni values between bulk enstatite achondrites and enstatite chondrites indicate limited Ni stable isotope fractionation during core formation processes on reduced, sulfur-rich parent bodies. The Ni stable isotope gap between chondrites and the BSE could be possibly explained by chondrule-rich accretion model.

K. Zhu, H. Becker, J.-M. Zhu, H.-P. Xu, Q.-R. Man


Geochem. Persp. Let. (2023) 25, 1–7 | | Published 23 February 2023

Interaction between clay minerals and organics in asteroid Ryugu
The Hayabusa 2 spacecraft brought back to Earth grains of the carbonaceous asteroid Ryugu. Such grains represent the pristine state of CI chondritic materials and have been preserved from exposure to Earth’s atmosphere. Here, we show evidence of the presence of organics trapped within the interlayer space of smectite layers in Ryugu grains. No such organics are found in the Orgueil CI meteorite. We put forward that the organics within the interlayer space of smectite in Orgueil CI meteorite were lost during their oxidation on Earth. Also, we propose that the presence of organics within the interlayers space of smectite might be responsible for the possible NH infrared signature observed in Ryugu particles and potentially to a few large C-type asteroids including Ceres.

J.-C. Viennet, M. Roskosz, T. Nakamura, P. Beck, B. Baptiste, B. Lavina, E.E. Alp, M.Y. Hu, J. Zhao, M. Gounelle, R. Brunetto, H. Yurimoto, T. Noguchi, R. Okazaki, H. Yabuta, H. Naraoka, K. Sakamoto, S. Tachibana, T. Yada, M. Nishimura, A. Nakato, A. Miyazaki, K. Yogata, M. Abe, T. Okada, T. Usui, M. Yoshikawa, T. Saiki, S. Tanaka, F. Terui, S. Nakazawa, S.-I. Watanabe, Y. Tsuda


Geochem. Persp. Let. (2023) 25, 8–12 | | Published 9 March 2023

An isotopically light nitrogen reservoir in the ocean: evidence from ferromanganese crusts
Ferromanganese (FeMn) oxide crusts and nodules in the deep ocean have been studied extensively in the context of critical metals and metal isotope mass balances; however, their role in the marine nitrogen cycle has been unexplored. Here we investigated a suite of hydrogenetic and diagenetic marine FeMn crusts and nodules from the Pacific to determine their isotopic signature and contribution as another N sink from the modern ocean. Our results reveal unusually low δ15N values down to −12 ‰ in some hydrogenetic crusts, paired with low δ13C values in carbonate associated with these crusts and nodules. This pattern is most parsimoniously explained by partial oxidation of ammonium (nitrification) derived from benthic biomass. Nitrification generates isotopically light nitrite, which may adhere to FeMn oxides by adsorption. In contrast, the diagenetic and hydrogenetic nodules are enriched in 15N/14N to up to +12 ‰, likely due to retention of ammonium in phyllosilicate minerals. Overall, we conclude that FeMn oxide crusts and nodules are a novel archive of microbial activity that may be preserved in the sedimentary record on Earth and possibly Mars.

E.E. Stüeken, M. Bau


Geochem. Persp. Let. (2023) 25, 13–17 | | Published 15 March 2023

MnO/MgO ratios of arc basalts highlight the role of early garnet fractionation
Multiple lines of evidence suggest that garnet may play an important role in the generation of arc magmas, either as a residual phase during mantle melting or subsequently during crystallisation differentiation. Moreover, garnet stability is strongly pressure sensitive, and therefore garnet fractionation can serve as an indirect indicator of fractionation pressure. Here, we introduce MnO/MgO ratios as a compositional proxy uniquely sensitive to garnet fractionation. We show that primary mantle melts have nearly invariant MnO/MgO ratios that are in equilibrium with mantle olivine. Therefore, the subsequent evolution of this ratio is only a function of magmatic differentiation. Further, based on compiled experimental studies, garnet is the only phase that crystallises from basaltic magmas and preferentially partitions MnO relative to MgO. Thus, limited increases in MnO/MgO ratios during magmatic differentiation, as we observe in most continental arcs, are strong evidence for early garnet fractionation and require that crystallisation differentiation begins at or below the Moho of many arcs.

B.Z. Klein, O. Müntener


Geochem. Persp. Let. (2023) 25, 18–24 | | Published 16 March 2023