 | Enhanced petrogenic organic carbon oxidation during the Paleocene-Eocene thermal maximum Abstract: The Paleocene-Eocene thermal maximum (PETM; ∼56 Ma) is a hyperthermal event associated with the rapid input of carbon into the ocean-atmosphere system. The oxidation of petrogenic organic carbon (OCpetro) may have released additional carbon dioxide (CO2), thereby prolonging the PETM. However, proxy-based estimates of OCpetro oxidation are unavailable due to the lack of suitable techniques. Raman spectroscopy is used to evaluate OCpetro oxidation in modern settings. For the first time, we explore whether Raman spectroscopy can evaluate OCpetro oxidation during the PETM. In the mid-Atlantic Coastal Plain, there is a shift from disordered to graphitised carbon. This is consistent with enhanced oxidation of disordered OCpetro and intensified physical erosion. In the Arctic Ocean, the distribution of graphitised carbon vs. disordered carbon does not change, suggesting limited variability in weathering intensity. Overall, this study provides the first evidence of increased OCpetro oxidation during the PETM, although it was likely not globally uniform. Our work also highlights the utility of Raman spectroscopy as a novel tool to reconstruct OCpetro oxidation in the past. |
 | The extent of liquid immiscibility in planetesimal cores Abstract: We report results of experiments in the system Fe0.9Ni0.1, S, P, C, O, which constrain the extent of liquid immiscibility in planetesimal cores. Immiscibility results in segregation of Fe-rich (P-rich, C-rich) and FeS-rich (O-rich) liquids, with the extent of immiscibility dependent on volatile/light element content. Parental liquids to iron meteorites are volatile-poor, and based on our results, mostly represent miscible core-forming liquids. However, as these parental liquids were variably modified during/after planetesimal disruption, they are unlikely to fully represent original compositions of planetesimal cores. To better constrain planetesimal core compositions, we use data from chondrite meteorites to provide upper bounds on core volatile element content. Modelled ‘chondrite cores’ are mostly immiscible liquids. The extent of immiscibility in planetesimal cores was sensitive to the degree of volatile loss during core formation, which was likely variable across planetary bodies, and dependent on thermal history and planetary differentiation processes. As such, evidence for immiscibility in core-forming liquids is useful in constraining the extent of degassing during differentiation. |
 | Cd isotope evidence for elevated productivity in the Middle Triassic Ordos Basin Abstract: The Middle Triassic witnessed the ecosystem recovery from the End-Permian Mass Extinction. The Ordos Basin is the globally earliest recovered lacustrine ecosystem and preserves a suit of hydrocarbon source rocks. However, dominant mechanisms for the abnormal accumulation of organic matter and ecosystem recovery remain highly debated. We present Cd isotope and redox sensitive trace element data of the Middle Triassic black shales from two wells in the Ordos Basin. The southern well is characterised by extremely high TOC contents (7.6–32.6 wt. %), enhanced Mo-U enrichments, and higher δ114Cdauth values (0.37 ± 0.26 ‰), while the northern well has moderate TOC contents (3.4–10.2 wt. %), slight Mo-U enrichments, and lower δ114Cdauth values (0.06 ± 0.29 ‰). The results suggest that an increase in primary productivity likely driven by active volcanism should play a first order role in the accumulation of organic matter, which was further promoted by local anoxia. The elevated primary productivity and massive burial of organic matter likely contributed to the ecosystem recovery in the Ordos Basin. |
 | Origin and significance of hydrocarbons in CO2-rich gases from Central Italy seismic areas Abstract: Tectonically active areas of Central Italy are marked by intense CO2 degassing, whose origin and role in earthquake processes are fundamental questions in geoscience. This study investigates the origin and geological controls on the geochemistry of light hydrocarbons from CO2-dominated gas emissions located in the inner sector of the Umbria-Marche Apennines (Central Italy), aiming to better understand the sources and migration pathways of geogenic fluids in the region. Our findings indicate that light hydrocarbons are predominantly thermogenic, with negligible abiotic contributions. We demonstrate that Mesozoic carbonate rocks are the primary source across the study area, though conditions of hydrocarbon formation and migration vary. Specifically, higher temperatures and open-system conditions prevail in the southern regions, likely due to thermal stress associated with Quaternary magmatism. We propose that light hydrocarbons form at crustal depths (≤5–6 km) and are transported to the surface by ascending CO2 from deeper sources. Finally, this work highlights that hydrocarbon geochemistry, combined with helium isotopes, can provide insights for reconstructing the circulation and origin of fluids in crustal reservoirs and assessing the thermal regime in tectonically active areas. |
 | Production of highly silicic 3.9 to 4.27 Ga crust on the Moon Abstract: The preserved remnants of a planetary body’s early crust represent a unique window into chemical and physical processes that shaped its beginning. While iron- and magnesium-rich magmas with low silica (SiO2) are common throughout a rocky body’s active lifetime, the timing of silica-rich crust production during the first 500 Myr of evolution remain uncertain. Silica-rich crust is relatively buoyant, influences the topography of rocky worlds, and concentrates heat-producing radioactive elements. However, identifying early silica-rich crust is challenging due to sample limitation and an inability to directly date silica-rich material like quartz and high-silica glass. Here, we show that the atomic environment around trace amounts of titanium in the mineral zircon is sensitive to the silica content of the magma it crystallised from, providing a new tool to recognise ancient silica-rich crust. By applying this method to 4.27 to 3.93 billion year old lunar zircons, we present evidence for early production of silica-rich crust on the Moon, suggesting this may be a common feature of planetary evolution. |
 | 238U/235U in deep-sea corals reflects limited expansion of seafloor anoxia in last ice age Abstract: Although much evidence suggests a decrease in deep ocean oxygen levels during the last glacial period, a quantitative global constraint on anoxic versus suboxic seafloor area is still lacking. Establishing such a constraint is challenging, because while changes in the biological pump are thought to drive deep ocean oxygen depletion during glacial time, concurrent changes in sea level and ocean circulation may have variable effects on the prevalence of anoxia on continental shelves. Here we use the uranium isotope redox proxy in cold-water corals to constrain anoxic seafloor area over the last 220 kyr. All samples show modern-like δ238U values within tight bounds (<0.05 ‰), allowing very little change in anoxic seafloor over this interval. This contrasts with coeval carbonate sediments that show large δ238U variations due to diagenetic alteration. It also contrasts with other redox proxy records, including authigenic uranium enrichments and sedimentary thallium isotopes, that show evidence of glacial oxygen depletion. We conclude that while the glacial ocean experienced an expansion of deep water suboxia, global seafloor anoxia remained roughly constant. |
 | Metal differentiation on asteroids recorded in Zn and Fe isotopic signatures of ureilites Abstract: Ureilites are meteorites that represent mantle restites of a planetesimal likely disrupted before the magma ocean stage and then reaccreted. Historically, it was speculated that evaporation shifts the Zn isotope ratios in ureilites toward heavier compositions. The fact that the ureilite parent body (UPB) is depleted in some moderately volatile elements (MVEs) makes ureilites an appealing target to study isotopic fractionation by evaporation in the early Solar System. Here, we show that Fe and Zn isotope ratios of bulk ureilites and their metal and silicate components rather record metal melting and extraction of Fe-FeS melts in the UPB, which also resulted in isotopic disequilibrium between the silicate and metal parts. This finding underlines that the isotopic evolution of MVEs in the early Solar System is not only affected by evaporation, but also by planetary differentiation processes due to the chalcophile and/or siderophile behaviour of many MVEs. It shows that to avoid interpretational bias due to undersampling of planetesimal reservoirs in meteorite collections, and to distinguish planetary differentiation from evaporation, isotopic compositions of MVEs should be combined with common geochemical proxies. |
 | A pristine low-Ti cumulate source for Chang’e 5 basalts revealed by Sr-Nd-Hf isotopes Abstract: The Chinese Chang’e 5 mission has returned the youngest mare basalts (∼2.0 Ga) identified thus far. However, their mantle source remains highly debated. Here, we present an integrated study of the mineralogy, chemical composition, Pb–Pb age, and whole rock Sr–Nd–Hf isotopic compositions of one Chang’e 5 (CE5) basalt clast. The indistinguishable age (1.96 ± 0.09 Ga), similar mineral chemical compositions and Sr–Nd isotopic compositions to those of previously reported CE5 basalts indicate that they had a common mantle source. After correction for cosmic ray effects, the whole rock εNd-εHf values plot exactly on the regression line defined by Apollo low-Ti basalts (R2 = 0.95), revealing their source affinity from a depleted mantle cumulate, consistent with their depleted Sr isotopic features. Combined with its deep negative Eu anomaly (Eu/Eu* of 0.44), the modelling calculations imply that the CE5 basalts were derived from a moderate-stage olivine(±orthopyroxene)+pigeonite+plagioclase cumulate with removal of most plagioclase. Furthermore, combining the CE5 basalts and Apollo 12 and 15 low-Ti mare basalts could be best interpreted as having this common, depleted pristine low-Ti cumulate source with variable TIRL (trapped instantaneous residual liquid). Our observations emphasise a pristine low-Ti cumulate source formed by the lunar magma ocean (LMO) for the CE5 basalts, leaving the melting mechanism a mystery. |
 | Young oxygenation of the Archean Keonjhar Palaeosol, India, from 138La-138Ce chronometry Abstract: Though there is increasing evidence from different geochemical proxies for intermittent oxygenation prior to the Great Oxidation Event (GOE), it remains ambiguous whether post-depositional processes altered these proxy signals. In this regard, the ca. 3.0 Ga old Keonjhar Palaeosol in the Singhbhum Craton in India has received particular attention, as the presence of Ce anomalies in this Archean palaeosol has been interpreted to reflect oxic surface weathering conditions during the Mesoarchean. Trace element analyses from this study revealed only small or no Ce anomalies and depleted REE patterns relative to the protolith. Most importantly, 138La-138Ce chronometry constrains a 138La-138Ce age of <620 Ma. This result precludes previous claims for Archean atmospheric oxygenation preserved in the Keonjhar palaeosol. Corresponding 147Sm-143Nd data yielded an isochron age of 1765 ± 180 Ma and 176Lu-176Hf data do not define an isochron. Altogether, these results indicate post-Archean REE mobilisation. Our findings emphasise the importance of combining palaeo-redox-proxies with radiogenic isotope analysis to validate their significance. |
 | Modelling redox state via V-Sc-Ti-Yb partitioning in mantle derived melts Abstract: The ratios of multivalent V to homovalent elements (e.g., V/Sc, V/Ti, V/Yb) are pivotal proxies for assessing mantle oxygen fugacity (fO2), yet their geochemical behaviours during partial melting remain debatable. Here, we conducted petrological modelling to explore the responses of these ratios to fO2 variations in sub-arc and mid-ocean ridge basalt (MORB) mantle sources, considering diverse mineral assemblages and chemical compositions. Our results suggest that V/Sc is relatively unaffected by changes in mantle source characteristics and melting extent, positioning it as a more dependable indicator of mantle fO2 and as a tool for tracing the evolution of mantle fO2 in terrestrial planets. In contrast, V/Ti and V/Yb are both more sensitive to the degree of melting and V/Ti is notably affected by mantle chemical composition. These insights refine our understanding of redox conditions across various tectonic environments on Earth. For instance, applying our model to data sets of primitive basalts reveals that the contemporary sub-arc mantle is generally more oxidised than the MORB mantle, consistent with impacts of slab-derived fluids and sediments in arc regions. |
<< Previous issueNext issue >>
