 | Molecular biosignatures in planetary analogue salts: implications for transport of organics in sulfate-rich brines beyond Earth Abstract: Salts formed during evaporation or freezing of brines can potentially incorporate organic matter that can inform about past biological activity. We analysed the lipid fraction preserved within the contemporary Lost Hammer salt deposit (Canadian High Arctic) - an analogue to extraterrestrial salt systems - and paired this with space mission-relevant evolved gas analysis. Our findings show microbial organic matter (fatty acids and n-alkanes) is incorporated into Lost Hammer salts, which comprise polyhydrated sulfates and chlorides. We find a difference in the relative abundance of fatty acids vs. n-alkanes indicating how these biosignatures evolve across active and non-active parts of the spring. We also find differences between pristine salt-organic mixtures and deposits that may have been remobilised by subsequent dissolution and recrystallisation. In this system, n-alkanes have the highest preservation potential, surviving the likely dissolution and recrystallisation of hydrated salt phases. This is important for considering the fate of organic matter on icy moons such as Europa, where salts emplaced on the surface by briny extrusions may have undergone fractional crystallisation, or where subsurface salts are remobilised by localised melting. It is also relevant for once active brine systems on Mars, where cycles of groundwater recharge and/or deliquescence led to dissolution and re-precipitation of evaporitic salts. |
 | Refining Hf crust formation ages in Precambrian terranes Abstract: The mechanisms and timing of long term chemical differentiation of the Earth are fundamental questions in the geosciences. We present detrital zircon U-Pb, O and Hf isotope data from Fennoscandia to assess how crustal growth can be reconciled with its known >1.5 billion year geological history. A broadly linear evolution (176Lu/177Hf = 0.0403), from chondritic mantle at the age of the oldest identified Fennoscandian crust, to present day MORB values (ɛHf(0 Ma) ≈ +16), provides a good fit with the most radiogenic zircon and whole rock Hf isotope data from the region. This mantle reference gives crustal growth peaks that correlate with known regional orogenic events. In contrast, a conventional 4.5 Ga strongly depleted mantle generates growth peaks outside of known geologic activity. Applying the same approach to the East Pilbara Terrane and SW Greenland yields model age peaks that also align with known magmatic activity. We propose that more geologically relevant crust formation ages are obtained via referencing a mantle source defined by the most radiogenic zircons/samples in the studied region. |
 | Silicate and iron phosphate melt immiscibility promotes REE enrichment Abstract: A surging rare earth element (REE) demand calls for finding new REE resources. Iron oxide-apatite (IOA) deposits have substantial REE potential, but their REE enrichment mechanisms remain uncertain, hindering REE exploration. The dominant process of IOA deposit formation is also hotly debated. Here, we use novel layered piston-cylinder experiments to address these questions. Seventeen magmatic FeP–Si immiscibility experiments, across 800–1150 °C, and at 0.4 and 0.8 GPa, reproduced many natural textural (e.g., dendritic magnetite) and geochemical (e.g., DLFeP–LSiTi/Fe < 1) features of IOA deposits. Magmatic-hydrothermal fluid bubbles and iron oxide-bubble pairs formed as well. The results strongly support FeP–Si immiscibility as a controlling factor in IOA deposits, although not mutually exclusive with other models. Light REE partition into FeP liquids, preferentially to heavy REE, explaining the light REE enrichment of IOA deposits. Some DLFeP–LSiREE values reach above 100, much higher than previously reported. Hence, any FeP rich rock that experienced magmatic Fe-Si immiscibility (e.g., IOA, nelsonites) is expected to be light REE enriched and should be considered as a REE exploration target. |
 | Basalts record a limited extent of mantle depletion: cause and chemical geodynamic implications Abstract: Radiogenic isotope ratios in basalts from mid-ocean ridges are commonly thought to represent the time-integrated extent of incompatible element depletion of the sub-ridge, peridotitic mantle. Earth’s peridotitic mantle, however, is variably incompatible element depleted, and inherently heterogeneous as a consequence of prior melting. After aging for several 107–109 years in the mantle before remelting today, the heterogeneous peridotites are characterised by a much larger range of radiogenic isotope ratios than ridge basalts. The simple reason why ridge, but also ocean island basalts, reflect only a limited range of this enormous isotopic spectrum of peridotites is that mixing of melts from Earth’s heterogeneous mantle moderates peridotite heterogeneity. Variable peridotite compositions may nevertheless be responsible for isotopic differences between ridge and ocean island basalts, and contribute significantly to the thermochemical buoyancy of mantle plumes, and density-driven mantle flow in general. Variable peridotite depletion therefore connects geochemical and geophysical observables, and is a critical parameter for advancing our understanding of basalt generation, plume formation, and chemical geodynamic models of mantle convection. |
 | Abiotic syntheses of pyrite: clues to assess the biogenicity of pyrite spherules Abstract: Life proliferates almost everywhere on Earth but determining whether or not hyperthermophile microorganisms have colonised a given hydrothermal environment, such as black smoker chimneys, remains challenging. Some mineral phases like pyrite spherules have been proposed to possibly serve as biosignatures. Yet, little is known about the specificities of pyrites produced via abiotic processes under hydrothermal conditions, making these pyrite spherules only potential biosignatures at best. Here, we report results of abiotic syntheses of pyrites under conditions reproducing those existing in the chimneys of black smokers, in the presence or in the absence of various organic compounds. We experimentally show that no pyrite is produced in the absence of organic material, whereas the chemical nature of the organic compounds controls the shape and crystallinity of the pyrite produced. The presence of complex organic matter, here compounds derived from lysed cells, appears necessary for the production of pyrite spherules previously described as biogenic, suggesting that the pyrite spherules detected in natural black smokers may not be considered as biogenic stricto sensu, but rather as proxies of the presence of microorganisms. |
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