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Northwest Africa 7533 (and Northwest Africa 7034 pairing group) is an exceptional martian meteorite as it is a polymict regolith breccia, revealing unprecedented insights into the martian crust [1]. Along with in situ observations by MSL Curiosity [2] and reconstruction models of the primitive crust [3], this meteorite has completely changed our view of the martian crust. It provides evidence of magma differentiation processes leading to a primitive felsic crust starting before 4.2-4.4 Gy, with insights into local thermal history, marked by a possible metamorphic event resetting U-Pb systems at 1.6-1.7 Gy [4,5]. This regolith breccia exhibits an impressive petrological diversity, with clasts ranging from basaltic, to noritic and monzonitic composition, with other impact-related objects and even sedimentary fragments. Accessory minerals, like phosphates and zircons, are present in the matrix and in the different clasts. Characterizing and dating these minerals has been critical for deciphering the history of NWA 7533, from the formation of its clasts to their dislocation and compaction. In this study, we used different analytical techniques in order to explore in detail the structural and chemical characteristics of phosphates and zircon. First, Raman and luminescence spectroscopy, including hyperspectral mapping, were used to obtain a detailed overview of their structure (Raman) and Rare-Earth Element (REE) content (luminescence). SEM and cathodoluminescence imagery, as well as TEM on FIB sections and SIMS analysis on key zircons were subsequently performed on several polished sections of NWA 7533. Zircons: In-depth Raman investigations show that NWA7533 zircons display wide textural variability, including heterogeneities at the grain scale. Perfectly crystallized areas coexist with highly degraded domains, caused either by disruption of the crystalline lattice and/or sub-micrometer mineral inclusions. Using TEM on FIB sections, the nanoscale texture of these zircons and the nature of the inclusions will be carefully discussed, considering preliminary results showing zonation of Si- or Fe-S-rich phases and magnetite. The textural characteristics can be related to the zircon petrological settings, as well as U-Pb ages and U content. We thus identified three main populations of zircons. Those analyzed in microbasaltic clasts show high homogeneity in shape, size and a high-level of crystallinity, with ages around 4.4-4.2 Gy and low U-content (?10 ppm). Zircons scattered in the matrix generally display a great variability in shapes and textures, and Raman analysis shows that they have a larger structural heterogeneity ranging from well crystallized to nearly amorphous. These degraded zircons range in age around 1.5 Gy and have a high U content (?100-300 ppm). Finally, zircons from feldspathic shards show intense textural variations at the grain scale, with alternation of preserved and degraded domains. In addition, REE luminescence spectra are different between these three populations of zircons, further supporting that these three zircon populations were not crystallized at the same magmatic stage and/or from the same magma. Phosphates: These minerals are important reservoirs of trace elements and water making them good targets for dating as well as excellent tracers of magmatic evolution and alteration processes. Using Raman spectroscopy, merrillite and apatite (mainly chlorapatites with some fluorapatites) are detected and appear structurally homogeneous throughout the samples although a spectacular diversity of microtexture is observed. On the other hand, luminescence spectra exhibit a more complex pattern with strong trivalent REE signatures (from Er3+, Sm3+, Eu3+ and Dy3+) in merrillite and associated overgrown apatite mantles (in a noritic clast) while the REE luminescence disappears in almost all apatite grains not associated with merrillite. Together with microtextural observations, this suggests the possible existence of at least two generations of apatites: one coming from the retrograde metamorphism of merrillite, the other being directly crystallized from a REE-depleted magma. Concluding remarks: These data open new questions on the thermal and geochronological history of NWA7533. Raman trends from amorphous zircons seems to follow data from terrestrial metamict zircons [6], which is further supported by high U-content and lower ages. But metamictization cannot fully explain the presence of degraded domains, apparent sub-micrometer inclusions still open the possibility of fluid alteration processes, which remains to be discussed, as they are not observed in the rest of the breccia. In addition, variability in luminescence patterns suggests two different generations of apatites with different formation mechanisms. [ABSTRACT FROM AUTHOR]

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ 15 N values found in well-preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much-needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ 15 N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non-quantitative oxidation of ammonium via the Feammox pathway, a metabolic co-cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ 15 N signatures in Archean sediments., (© 2023 The Authors. Geobiology published by John Wiley & Sons Ltd.)

Secondary Ion Mass Spectrometry (SIMS) extracts chemical, elemental, or isotopic information about a localized area of a solid target by performing mass spectrometry on secondary ions sputtered from its surface by the impact of a beam of charged particles. This primary beam sputters ionized atoms and small molecules (as well as many neutral particles) from the upper few nanometers of the sample surface. The physical basis of SIMS has been applied to a large range of applications utilizing instruments optimized with different types of mass analyzer, either dynamic SIMS with a double focusing mass spectrometer or static SIMS with a Time of Flight (TOF) analyzer. Here, we present a short review of the principles and major applications of three different SIMS instruments located in Switzerland., (Copyright 2022 Johanna Marin Carbonne, Andras Kiss, Anne-Sophie Bouvier, Anders Meibom, Lukas Baumgartner, Thomas Bovay, Florent Plane, Stephane Escrig, Daniela Rubatto. License: This work is licensed under a Creative Commons Attribution 4.0 International License.)

Rationale: Iron isotopic signatures in pyrites are considered as a good proxy for reconstructing paleoenvironmental and local redox conditions. However, the investigation of micro-pyrites less than 20 μm in size has been limited by the available analytical techniques. The development of a new brighter radio-frequency plasma ion source (Hyperion-II source) enhances the spatial resolution by increasing the beam density 10 times compared with the Duoplasmatron source., Methods: Here we present high-spatial-resolution measurements of iron isotopes in pyrites using a 3 nA-3 μm primary 16 O - beam on two Cameca IMS 1280-HR2 ion microprobe instruments equipped with Hyperion sources at CRPG-IPNT (France) and at SwissSIMS (Switzerland). We tested analytical effects, such as topography and crystal orientation, that could induce analytical biases perceptible through variations of the instrumental mass fractionation (IMF)., Results: The δ 56 Fe reproducibility for the Balmat pyrite standard is ±0.25‰ (2 standard deviations) and the typical individual internal error is ±0.10‰ (2 standard errors). The sensitivity on 56 Fe + was 1.2 × 10 7  cps/nA/ppm or better. Tests on Balmat pyrites revealed that neither the crystal orientation nor channeling effects seem to significantly influence the IMF. Different pyrite standards (Balmat and SpainCR) were used to test the accuracy of the measurements. Indium mounts must be carefully prepared with a sample topography less than 2 μm, which was checked using an interferometric microscope. Such a topography is negligible for introducing change in the IMF. This new source increases the spatial resolution while maintaining the high precision of analyses and the overall stability of the measurements compared with the previous Duoplasmatron source., Conclusions: A reliable method was developed for performing accurate and high-resolution measurements of micrometric pyrites. The investigation of sedimentary micro-pyrites will improve our understanding of the processes and environmental conditions during pyrite precipitation, including the contribution of primary (microbial activities or abiotic reactions) and secondary (diagenesis and/or hydrothermal fluid circulation) signatures., (© 2020 John Wiley & Sons Ltd.)

One of the most conserved traits in the evolution of biomineralizing organisms is the taxon-specific selection of skeletal minerals. All modern scleractinian corals are thought to produce skeletons exclusively of the calcium-carbonate polymorph aragonite. Despite strong fluctuations in ocean chemistry (notably the Mg/Ca ratio), this feature is believed to be conserved throughout the coral fossil record, spanning more than 240 million years. Only one example, the Cretaceous scleractinian coral Coelosmilia (ca. 70 to 65 Ma), is thought to have produced a calcitic skeleton. Here, we report that the modern asymbiotic scleractinian coral Paraconotrochus antarcticus living in the Southern Ocean forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. P. antarcticus and Cretaceous Coelosmilia skeletons share a unique microstructure indicating a close phylogenetic relationship, consistent with the early divergence of P. antarcticus within the Vacatina (i.e., Robusta) clade, estimated to have occurred in the Mesozoic (ca. 116 Mya). Scleractinian corals thus join the group of marine organisms capable of forming bimineralic structures, which requires a highly controlled biomineralization mechanism; this capability dates back at least 100 My. Due to its relatively prolonged isolation, the Southern Ocean stands out as a repository for extant marine organisms with ancient traits., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO 4 ). The origin of this barite remains controversially debated. The mass-independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen-poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S-isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S-isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long-term increase in Δ 33 S sulfate values through the Paleoarchean., (© 2020 John Wiley & Sons Ltd.)

Stable isotope signatures of elements related to life such as carbon and nitrogen can be powerful biomarkers that provide key information on the biological origin of organic remains and their paleoenvironments. Marked advances have been achieved in the last decade in our understanding of the coupled evolution of biological carbon and nitrogen cycling and the chemical evolution of the early Earth thanks, in part, to isotopic signatures preserved in fossilized microbial mats and organic matter of marine origin. However, the geologic record of the early continental biosphere, as well as its evolution and biosignatures, is still poorly constrained. Following a recent report of direct fossil evidence of life on land at 3.22 Ga, we compare here the carbon and nitrogen isotopic signals of this continental Archean biosphere with biosignatures of cyanobacteria biological soil crusts (cyanoBSCs) colonizing modern arid environments. We report the first extended δ 13 C and δ 15 N data set from modern cyanoBSCs and show that these modern communities harbor specific isotopic biosignatures that compare well with continental Archean organic remains. We therefore suggest that cyanoBSCs are likely relevant analogs for the earliest continental ecosystems. As such, they can provide key information on the timing, extent, and possibly mechanism of colonization of the early Earth's emergent landmasses.

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28- to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ 56 Fe values of -1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ 33 S values > +3‰, while the other groups display more variable and closer to 0‰ Δ 33 S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ 33 S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass-independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro-organisms closely related to the last common ancestor had the ability to reduce Fe(III)., (© 2020 The Authors. Geobiology published by John Wiley & Sons Ltd.)

This corrects the article DOI: 10.1038/ncomms11977.

The significant degradation that fossilized biomolecules may experience during burial makes it challenging to assess the biogenicity of organic microstructures in ancient rocks. Here we investigate the molecular signatures of 1.88 Ga Gunflint organic microfossils as a function of their diagenetic history. Synchrotron-based XANES data collected in situ on individual microfossils, at the submicrometre scale, are compared with data collected on modern microorganisms. Despite diagenetic temperatures of ∼150-170 °C deduced from Raman data, the molecular signatures of some Gunflint organic microfossils have been exceptionally well preserved. Remarkably, amide groups derived from protein compounds can still be detected. We also demonstrate that an additional increase of diagenetic temperature of only 50 °C and the nanoscale association with carbonate minerals have significantly altered the molecular signatures of Gunflint organic microfossils from other localities. Altogether, the present study provides key insights for eventually decoding the earliest fossil record.