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Abstract: New mineralogical studies of Lafayette reveal that it contains a notably variable abundance of martian carbonate. Four percent was identified in mesostasis (3.2%) and olivine-hosted (0.8%) fractures in one polished section, but only 0.2% of both textural types in another. The Lafayette carbonates are Mg0.0-2.0Cc13.2-38.6Sd17.7-81.9Rh3.1-42.9. They have undergone variable but extensive amounts of dissolution and replacement as the nakhlite secondary fluid evolved, associated with the precipitation of ferric saponite in olivine fractures and a serpentine-like phyllosilicate in the mesostasis. The mesostasis carbonate has undergone the highest degree of corrosion and replacement. TEM analysis has shown the presence of Fe-(hydr)oxide (likely ferrihydrite) nanoparticles on olivine-hosted carbonates which can be linked to the cessation of more extensive carbonate dissolution at those sites. The mesostasis serpentine-like mineral has been described here on the basis of WDS and EDX analyses, HRTEM and Fe-K XANES, as odinite, a ferric, 0.7 nm d001-spacings phyllosilicate mineral with a characteristic 1:1 serpentine-like structure. The carbonate dissolution stage and then formation of Fe-(hydr)oxide nanoparticles occurred under circumneutral-alkaline conditions 7 < pH < 10. This range of pH is also where the general dissolution mechanism switched from a proton-promoted, to a water hydrolysis reaction associated with a reduction in the dissolution rates. As dissolution rates were reduced and the fluid had cooled to ≤50°C, the precipitation of the ferric saponite and odinite, a phyllosilicate associated with temperatures of ∼25°C, dominated over the carbonate dissolution. The extensive dissolution of such crustal carbonate across the upper martian crust, producing bicarbonate and carbon dioxide, and the coupled formation of ferric phyllosilicates, would lead to the formation of CH4 in substantial amounts via a Fischer-Tropsch type reaction. The results of our study illustrate a process to explain the relatively low abundance of detected carbonate on Mars and a likely source for some of the methane on Mars.

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Abstract: The CM carbonaceous chondrite meteorites provide a record of low temperature (<150 °C) aqueous reactions in the early solar system. A number of CM chondrites also experienced short-lived, post-hydration thermal metamorphism at temperatures of ∼200 °C to >750 °C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300–500 °C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (>500 °C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from ∼13 wt% to ∼3 wt% and a subsequent reduction in the intensity of the 3 μm feature in IR spectra. We estimate that the heated CM chondrites have lost ∼15 - >65% of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of ∼20–50 GPa. However, not all heated CM chondrites retain shock features suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu.

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Abstract: We report on the alteration history of the olivine-phyric, highly depleted (HD) shergottite, Northwest Africa (NWA) 10416, paying particular attention to the origin of the aqueous alteration seen affecting the meteorite’s olivine megacrysts. The rock’s interior displays 1 mm, zoned, altered olivine megacrysts set in a groundmass of clinopyroxene, unzoned olivine, and interstitial plagioclase and maskelynite. Synchrotron micro X-ray diffraction (µ-XRD) and transmission electron microscopy (TEM) show that plagioclase and maskelynite have been partially replaced by kaolinite. The relict olivine megacryst cores display a unique concentric colouration for Martian meteorites, having central amber-coloured zones surrounded by a brown mantle zone, with the rims remaining clear and unaltered. This colouration is a result of fluid alteration and partial replacement, with hydration. TEM analysis revealed the ∼200 nm scale banded and largely amorphous nature of the alteration, but with some (∼ 20%) relict crystalline olivine patches. Although the coloured olivine zones show cation and anion site vacancies compared to stoichiometric olivine, a relict igneous compositional trend is preserved in the megacrysts, from Mg-rich altered cores (Mg# = 76) to unaltered stoichiometric rims (Fo53). Synchrotron Fe-K X-ray absorption near-edge structure (XANES) analysis revealed that the coloured zones of the megacryst have different Fe oxidation values. High ferric contents are present in the brown mantle zones (Fe3+/ΣFe ≤ 0.92) and the amber zones (Fe3+/ΣFe ≤ 0.30), whereas the clear rims are ferrous. This suggests alteration occurred in an oxidising environment and that the sharp contrast in colour of the megacryst (brown to clear) is a record of a relict fluid reaction front. In order to test the terrestrial or extraterrestrial origin of the alteration, olivine material from a shock-melt vein was analysed by TEM. The analysis revealed 0.952 nm curved d-spacing’s from clay alteration undisturbed by any shock effects, strongly suggesting a terrestrial origin. The d-spacing values most likely represent a collapsed saponite or vermiculite, showing that in some places olivine has been replaced by crystalline clay. Oxygen isotope analysis of bulk (Δ17O = 0.309 ± 0.009 (2σ) ‰) and amber-coloured megacryst material (= 0.271 ± 0.002 (2σ) ‰), are also consistent with terrestrial alteration. We propose a model in which, during the meteorite’s time in Northwest Africa, low-temperature, likely acidic, groundwater exploited fractures. The fluid altered the olivine megacrysts in a way that was controlled by the pre-existing, igneous compositional zonation, with Mg-rich olivine being more susceptible to alteration in this fluid environment. The plagioclase and maskelynite were also altered to a high degree. After the alteration event it is likely that NWA 10416 had a significant residence time in Northwest Africa, accounting for terrestrial calcite and the dehydration of some clay phases.

Open Access

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Abstract: Water on the present day Martian surface is thought to exist in two thermally distinct sub-surface reservoirs: as ice in the cryosphere and as groundwater located deeper in the crust. These sub-surface environments are thought to contain saline, rather than pure, water and laboratory studies on whether or not clathrate hydrates can form in such environments are lacking. We fill this gap by performing synchrotron radiation X-ray powder diffraction to investigate the formation and evolution of clathrate hydrates in weak chloride solutions at CO2 pressures, and over temperature ranges, that are similar to those found in the Martian regolith. We have found that clathrate hydrates can form under conditions relevant to the Martian cryosphere, despite the presence of chloride salts. We find that the dissociation temperatures for CO2 clathrate hydrates formed in saline solutions are depressed by 10–20 K relative to those formed in pure water, depending on the nature of the salt and the CO2 pressure. We suggest that the inhibiting effect that salts such as MgCl2, CaCl2 and NaCl have on clathrate hydrate formation could also be related to the salts’ effect on the formation of the low temperature phase of ice. However, despite the inhibiting effect of the salts, we conclude that the presence of clathrate hydrates should still be possible under conditions likely to exist within the Martian cryosphere.

Open Access

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Abstract: Cobalt is a waste product in many industrial processes and its most common radioactive isotope – 60Co – is a by-product of nuclear reactors. To better understand the mobility and fate of Co in natural and contaminated environments we investigated Co sorption behaviour to the common soil and sediment constituents ferrihydrite, kaolinite, humic acid (HA), and ferrihydrite-HA and kaolinite-HA organo-mineral composites using sorption batch experiments, synchrotron X-ray absorption spectroscopy (EXAFS), and scanning transmission electron microscopy (STEM). We measured the sorption of Co to the end-member mineral and organic phases and the composites as a function of pH, ionic strength and Co concentration, and also for the composites as a function of organic carbon concentration, with composites made containing a wide range of organic carbon contents. We then determined the molecular mechanisms of Co sorption to the end-member phases and the composites, and used this information to develop molecularly constrained thermodynamic surface complexation models to quantify Co sorption. Sorption to the ferrihydrite-HA and kaolinite-HA organo-mineral composites was found to be intermediate to both of the end-member phases, displaying enhanced sorption respective to the mineral end-member phase at mid-low pH. EXAFS analysis shows that there is a universal sorption mechanism accounting for Co sorption to the end-member mineral and organic phases and the organo-mineral composites at mid-high pH, in which Co sorbs to these phases via inner-sphere bidentate binuclear surface complexes. At mid-low pH, sorption to all the phases except ferrihydrite is the result of outer-sphere complexation. Our new molecularly constrained thermodynamic surface complexation models for Co sorption to ferrihydrite, kaolinite, HA, and ferrihydrite-HA and kaolinite-HA organo-mineral composites, show that Co sorption to the composites cannot be modelled assuming linear additivity of Co sorption to the end-member phases.