 | 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. |
 | Core-mantle chemical interaction via convection within thermochemical piles Abstract: Dense thermochemical piles, widely believed to manifest themselves as large low-velocity shear provinces observed above the core-mantle boundary, are gravitationally stable against the ambient mantle, but convection can still happen within the piles. This “inter-pile convection” has two competing effects on core-mantle chemical interaction: one is to reduce the residence time of mantle material at the core-mantle boundary, and the other is to enhance the transfer of core-affected material. Based on convection diagnostics from the numerical simulations of thermochemical piles, we discuss how to constrain the likely formation mechanism of thermochemical piles using mantle geochemistry. |
 | Pore water chemical constraints on petrophysical shifts following biosilica diagenesis Abstract: Silica diagenesis commonly induces marked petrophysical changes to biosiliceous sediments across the sub-seafloor opal-A to opal-CT transition zone (TZA/CT). Integration of pore water thermodynamics with textural and mineralogical data reveals that sediments in the TZA/CT drilled at ODP Site 794 are sharply compacted via opal-A dissolution and matrix collapse. The porosity decline is linked to silica solubility through pore water saturation states with opal-A and opal-CT. A dissolved silicon (DSi) drop near the depth of the petrophysical shift implies that fluid expulsion removes the DSi produced by opal-A dissolution. Production of opal-CT from the pore fluid that is thermodynamically equilibrated with solubility of this silica mineral phase however slightly impacts the anomalous compaction at the TZA/CT. |
 | Additive impact on early-stage magnesium carbonate mineralisation Abstract: Carbon capture and utilisation has attracted significant interest due to increasing concerns about global warming. Mineral trapping via MgCO3 precipitation is a promising strategy, though restricted by the slow rate of magnesite (MgCO3) formation and high temperatures needed to avoid the formation of hydrated minerals. Amorphous magnesium carbonate (AMC) is a transient phase, determining the characteristics of the final crystalline MgCO3 phase(s). Research has focused on accelerating MgCO3 formation using additives, but their modus operandi is not completely understood. Here, AMC titration experiments were conducted at constant pH, monitoring solution transmittance, conductivity, and species size evolution to clarify the effect of citrate on the initial steps of MgCO3 precipitation. We demonstrate that citrate, similar to more complex additives, alters the hydration of free ions relative to ion associates, thereby destabilising prenucleation ion associates and delaying AMC nucleation. The system is thus forced to go through liquid–liquid separation before the formation of the solid, resulting in amorphous and crystalline phases with lower water content, which are more stable and efficient for C storage, having a positive impact on the cost of CO2 mineralisation. |
 | Halogen enrichment on the continental surface: a perspective from loess Abstract: Halogen (F, Cl, Br, and I) concentrations for 129 loess samples from worldwide localities yield geometric means of 517 ± 53 μg/g F, 150 ± 20 μg/g Cl, 1.58 ± 0.16 μg/g Br, 1.16 ± 0.11 μg/g I (2 standard errors). These concentrations, notably for Br and I, are substantially higher than previous estimates for the average upper continental crystalline bedrocks, with enrichment factors of 1.3−0.4+0.7 (F), 1.8−0.8+2.4 (Cl), 3.8−1.0+1.3 (Br), and 39−16+71 (I) (95 % confidence), documenting enrichment of halogens on the continental surface. These surface halogens are likely sourced from the oceans and may be influenced by climate fluctuations. Halogen ratios (Br/Cl, I/Cl, and Br/I) in loess are similar to those of organic-rich soils/sediments from both terrigenous and marine settings, suggesting that terrigenous and marine organic matter have indistinguishable halogen ratios. The Br/I ratios differ from those in the fine grained matrix of glacial diamictites, indicating that another process (beyond biological influence) is responsible for fractionating halogens in the upper continental crust. Using a mixing model, we calculate that over 80–90 % of loess originates from crystalline bedrocks, while the remainder (<10–20 %) derives from the halogen- and organic-rich sedimentary cover or other sources (e.g., marine aerosols). |
 | Dating recent aqueous activity on Mars Abstract: Amazonian-age Martian meteorites contain products of indigenous aqueous alteration; yet, establishing when this alteration occurred, and therefore when liquid water was available in the planet’s crust, has proven challenging. New 40Ar/39Ar dates for iddingsite within the Martian meteorite Lafayette show these minerals precipitated from liquid water at 742 ± 15 Ma (2σ). This age is the most precise constraint to date on water–rock interaction on Mars, and postdates formation of the host igneous rock by ∼580 Myr. We infer that magmatic activity most likely induced melting of local permafrost and led to alteration of the nakhlites, suggesting that activation of localised hydrological cycles on Amazonian Mars by magmatism was infrequent and transient, but not unusual. |
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