I18-Microfocus Spectroscopy
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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The d-spacing values most likely represent a collapsed saponite or vermiculite, showing that in some places olivine has been replaced by crystalline clay.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.
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Apr 2020
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I18-Microfocus Spectroscopy
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Paul A.
Wallace
,
Sarah
Henton De Angelis
,
Adrian J.
Hornby
,
Jackie E.
Kendrick
,
Stephen
Clesham
,
Felix W.
Von Aulock
,
Amy
Hughes
,
James
E. P. Utley
,
Takehiro
Hirose
,
Donald B.
Dingwell
,
Yan
Lavallee
Diamond Proposal Number(s):
[9220]
Abstract: Volcanic environments often represent structurally active settings where strain localisation can promote faulting, frictional deformation, and subsequent melting along fault planes. Such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating. The formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion. A comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanism, as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt. Here, we experimentally investigate the impact of host-rock mineralogy on the compositional and textural evolution of a frictional melt during slip. High-velocity rotary shear (HVR) experiments were performed under controlled, volcanically relevant, coseismic conditions (1 m s−1 slip rate and 1 MPa normal stress) using three intermediate dome lavas with contrasting mineral assemblages, sampled from volcanic systems where fault friction is evident: (1) an amphibole-bearing andesite (Soufrière Hills Volcano, Montserrat); (2) an amphibole-poor dacite (Santiaguito dome complex, Guatemala); and (3) an amphibole-free andesite (Volcán de Colima, Mexico). For each sample, five HVR experiments were terminated at different stages of frictional melt evolution, namely: (1) at the onset of melting and (2) formation of a steady-state melt layer; and (3) after 5 m, (4) 10 m, and (5) 15 m of slip at steady-state conditions. Progressive mixing and homogenisation of selective, single-phase melts within the frictional melt layer through double-diffusion convection demonstrates the dependence of melt composition on slip behaviour. Amphiboles melted preferentially, leading to lower shear stress (∼1 MPa) and pronounced shear weakening during the frictional melting of amphibole-bearing lavas. The results highlight the implications of mineral assemblage on volcanic conduit flow processes, which may influence the explosivity of eruptions, and run-out distances of rapid granular flows.
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Apr 2019
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[9703]
Open Access
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.
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Nov 2018
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B22-Multimode InfraRed imaging And Microspectroscopy
I18-Microfocus Spectroscopy
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J. L.
Macarthur
,
J. C.
Bridges
,
L. J.
Hicks
,
R.
Burgess
,
K. H.
Joy
,
M. J.
Branney
,
G. M.
Hansford
,
S. H.
Baker
,
S. P.
Schwenzer
,
S. J.
Gurman
,
N. R.
Stephen
,
E. D.
Steer
,
J. D.
Piercy
,
T. R.
Ireland
Diamond Proposal Number(s):
[10328, 12761, 13690, 16688, 19641]
Abstract: Martian meteorite Northwest Africa (NWA) 8114 – a paired stone to NWA 7034 – provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/ΣFe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 °C for at least 7 days in an oxidising regolith environment.
We report an approximate 40Ar-39Ar maximum age of 1.13 Ga to 1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/ΣFe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket.
High temperatures, above 700 °C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days.
Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite.
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Nov 2018
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B18-Core EXAFS
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Diamond Proposal Number(s):
[13037, 13580]
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.
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Jul 2018
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[12005]
Abstract: The processes that control chemical weathering of bedrock in the deep critical zone at a mm-scale are still poorly understood, but may produce 100s of meters of regolith and substantial fluxes of silicate weathering products and thus may be important for modeling long-term, global CO2. Weathering controls are also difficult to ascertain, as laboratory determined dissolution rates tend to be 2-5 orders of magnitude faster than field determined dissolution rates. This study aims to establish (i) the incipient processes that control the chemical weathering of the Bisley bedrock and (ii) why weathering rates calculated for the watershed may differ from laboratory rates (iii) why rates may differ across different scales of measurement. We analyzed mineralogy, elemental chemistry, and porosity in thin sections of rock obtained from drilled boreholes using Scanning Electron Microscopy (SEM) with energy dispersive spectrometry, electron probe microanalysis, and synchrotron-based Micro X-ray Fluorescence (µXRF) and X-ray Absorption Near Edge Structure (XANES). Weathering ages were determined from U-series isotope analysis. Mineral specific dissolution rates were calculated from solid-state mineralogical gradients and weathering ages. Mineralogical and elemental transects across thin sections and SEM images indicate that trace pyrite is the first mineral to dissolve. Micro-XRF mapping at 2 µm resolution revealed sulfate in pore space adjacent to dissolving pyrite, indicating that the incipient reaction is oxidative. The oxidative dissolution of pyrite produces a low pH microenvironment that aids the dissolution of pyroxene and chlorite. The rate-limiting step of weathering advance, and therefore the creation of the critical zone in the Bisley watershed, is pyrite oxidation, despite the low abundance (∼0.5 vol %) of pyrite in the parent rock. The naturally determined dissolution rates presented here either approach, converge with, or in some cases exceed, rates from the literature that have been experimentally determined. The U-series weathering age data on the mm-scale integrates the weathering advance rate over the ∼4.2 ± 0.3 kyrs that the weathering rind took to form. The weathering advance rate calculated at a watershed scale (from stream chemistry data) represents a contemporary weathering advance rate, which compares well with that calculated for the weathering rind, suggesting that the Bisley watershed has been weathering at steady-state for the last ∼4 kyrs.
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Feb 2018
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[15947]
Abstract: Two flow-through experiments were conducted to assess serpentinization of intact dunite cores. Permeability and fluid chemistry indicate significantly more reaction during the second experiment at 200°C than the first experiment at 150°C. Permeability decreased by a factor of 2.4 and 25 during the experiments at 150 and 200°C, respectively. Furthermore, hydrogen and methane concentrations exceeded 600 µmol/kg and 300 µmol/kg during the 200°C experiment, and were one and two orders of magnitude higher, respectively, than the 150°C experiment. Fe K-edge X-ray absorption near edge structure analyses of alteration minerals demonstrated Fe oxidation that occurred during the 200°C experiment. Vibrating sample magnetometer measurements on post-experimental cores indicated little to no magnetite production, suggesting that the hydrogen was largely generated by the oxidation of iron as olivine was converted to ferric iron (Fe(III)) serpentine and/or saponite. Scanning electron microscopy images suggested secondary mineralization on the post-experimental core from the 200°C experiment, portraying the formation of a secondary phase with a honeycomb-like texture as well as calcite and wollastonite. Scanning electron microscopy images also illustrated dissolution along linear bands through the interiors of olivine crystals, possibly along pathways with abundant fluid inclusions. Energy dispersive X-ray spectroscopy identified Cl uptake in serpentine, while Fourier transform-infrared spectroscopy suggested the formation of serpentine, saponite, and talc. However, no change was observed when comparing pre- and post-experimental X-ray computed tomography scans of the cores. Furthermore, (ultra) small angle neutron scattering datasets were collected to assess changes in porosity, surface area, and fractal characteristics of the samples over the ≈ 1 nm- to 10 µm-scale range. The results from the 200°C post-experimental core generally fell within the range of values for the two pristine samples and the 150°C post-experimental core that underwent negligible reaction, indicating that any change from reaction was smaller than the natural variability of the dunite. Even though there was little physical evidence of alteration, the initial stage of serpentinization at 200°C was sufficiently significant to have a dramatic effect on flow fields in the core. Furthermore, this experiment generated significant dissolved hydrogen concentrations, while simulating open system dynamics. Even though open systems prevent elevated hydrogen concentrations due to continual loss of hydrogen, we speculate that this process is responsible for stabilizing ferric Fe-rich serpentine in nature, while also oxidizing more ferrous Fe (Fe(II)) iron and cumulatively generating more hydrogen than would be possible in a closed system.
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Jul 2017
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I08-Scanning X-ray Microscopy beamline (SXM)
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Diamond Proposal Number(s):
[11282]
Abstract: Rivers are significant contributors of Fe to the ocean. However, the characteristics of chemically reactive Fe remain poorly constrained, especially in large Arctic rivers, which drain landscapes highly susceptible to climate change and carbon cycle alteration. The aim of this study was a detailed characterisation (size, mineralogy, and speciation) of riverine Fe-bearing particles (> 0.22 µm) and colloids (1 kDa – 0.22 µm) and their association with organic carbon (OC), in the Lena River and tributaries, which drain a catchment almost entirely underlain by permafrost. Samples from the main channel and tributaries representing watersheds that span a wide range in topography and lithology were taken after the spring flood in June 2013 and summer baseflow in July 2012. Fe-bearing particles were identified, using Transmission Electron Microscopy, as large (200 nm – 1 µm) aggregates of smaller (20 nm - 30 nm) spherical colloids of chemically-reactive ferrihydrite. In contrast, there were also large (500 nm – 1 µm) aggregates of clay (illite) particles and smaller (100 - 200 nm) iron oxide particles (dominantly hematite) that contain poorly reactive Fe. TEM imaging and Scanning Transmission X-ray microscopy (STXM) indicated that the ferrihydrite is present as discrete particles within networks of amorphous particulate organic carbon (POC) and attached to the surface of primary produced organic matter and clay particles. Together, these larger particles act as the main carriers of nanoscale ferrihydrite in the Lena River basin. The chemically reactive ferrihydrite accounts for on average 70 ± 15 % of the total suspended Fe in the Lena River and tributaries. These observations place important constraints on Fe and OC cycling in the Lena River catchment area and Fe-bearing particle transport to the Arctic Ocean.
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Jul 2017
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[3897, 12301, 14575]
Abstract: Mineral authigenesis from their dissolved sea salt matrix is an emergent feature of sea ice brines, fuelled by dramatic equilibrium solubility changes in the large sub-zero temperature range of this cryospheric system on the surface of high latitude oceans. The multi-electrolyte composition of seawater results in the potential for several minerals to precipitate in sea ice, each affecting the in-situ geochemical properties of the sea ice brine system, the habitat of sympagic biota. The solubility of two of these minerals, gypsum (CaSO4·2CaSO4·2H2O) and hydrohalite (NaCl··2H2O), was investigated in high ionic strength multi-electrolyte solutions at below-zero temperatures to examine their dissolution–precipitation dynamics in the sea ice brine system. The gypsum dynamics in sea ice were found to be highly dependent on the solubilities of mirabilite and hydrohalite between 0.2 and View the MathML source-25.0°C. The hydrohalite solubility between -14.3-14.3 and View the MathML source-25.0°C exhibits a sharp change between undersaturated and supersaturated conditions, and, thus, distinct temperature fields of precipitation and dissolution in sea ice, with saturation occurring at View the MathML source-22.9°C. The sharp changes in hydrohalite solubility at temperatures View the MathML source⩽-22.9°C result from the formation of an ice–hydrohalite aggregate, which alters the structural properties of brine inclusions in cold sea ice. Favourable conditions for gypsum precipitation in sea ice were determined to occur in the region of hydrohalite precipitation below View the MathML source-22.9°C and in conditions of metastable mirabilite supersaturation above View the MathML source-22.9°C (investigated at -7.1-7.1 and View the MathML source-8.2°C here) but gypsum is unlikely to persist once mirabilite forms at these warmer (View the MathML source>-22.9°C) temperatures. The dynamics of hydrohalite in sea ice brines based on its experimental solubility were consistent with that derived from thermodynamic modelling (FREZCHEM code) but the gypsum dynamics derived from the code were inconsistent with that indicated by its experimental solubility in this system. Incorporation of hydrohalite solubility into a 1D thermodynamic model of the growth of first-year Arctic sea ice showed its precipitation to initiate once the incoming shortwave radiation dropped to 0 W m−2, and that it can reach concentrations of 9.9 g kg−1 within the upper and coldest layers of the ice pack. This suggests a limited effect of hydrohalite on the albedo of sea ice. The insights provided by the solubility measurements into the behaviour of gypsum and hydrohalite in the ice–brine system cannot be gleaned from field investigations at present.
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Jun 2017
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[10358]
Abstract: Cable bacteria have recently been identified in various sedimentary marine settings worldwide. These filamentous microbes mediate electrogenic sulphur oxidation (e-SOx) over centimetre-scale distances, leading to a distinct separation of oxygen- and sulphide-bearing sediment zones. Here we present results of a year-long monthly assessment of the impact of cable bacteria on sedimentary Fe and Mn dynamics at three sites located along a water depth gradient in a seasonally-hypoxic coastal marine lake (Grevelingen, The Netherlands). Fluorescence In Situ Hybridisation (FISH) shows the presence of cable bacteria at two sites in spring. Micro-sensor profiling (O2, pH, H2S) and pore water profiles of dissolved Mn, Fe2+, Ca2+ and SO42- reveal the geochemical signature of e-SOx at these sites, i.e. the development of a broad suboxic zone, characterised by a low pH and acidic dissolution of Ca/Mn carbonates and Fe sulphides. Cable bacteria activity, as reflected by dissolution of FeS in spring, was highest at the deepest and most hypoxic site. In spring, dissolved Mn and Fe2+ released at depth due to e-SOx diffused upwards and was sequestered as Mn- and Fe-(oxyhydr)oxides near the sediment surface, with Mn oxides acting as an oxidant for part of the upward diffusing Fe2+. Strikingly, the thickness of the Fe-(oxyhydr)oxide-bearing surface layer of the sediment was greatest at the most hypoxic site, emphasising the key role of cable bacteria in creating oxidised surface sediments. X-ray absorption fine structure analyses confirm the seasonality in Fe-(oxyhydr)oxide formation and reveal that the sediment Mn oxides were of biogenic (birnessite) and abiotic (hausmannite) origin. Upon the onset of hypoxia in early summer, the sediment Fe-(oxyhydr)oxides were mostly converted to Fe-sulphides but the Mn oxides dissolved and the Mn was lost to the overlying water. After summer hypoxia, Beggiatoaceae mats colonised the sediment with little further change in sediment geochemistry. Our results confirm that cable bacteria act as a key control on the coupled cycling of Fe and Mn in surface sediments of seasonally hypoxic basins.
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Jul 2016
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