Alteration Of Basaltic Glass Research Articles

Authigenic mineralization in Surtsey basaltic tuff deposits at 50 years after eruption

Alteration of basaltic glass and in situ mineral growth are fundamental processes that influence the chemical and material properties of Earth’s oceanic crust. These processes have evolved at the basaltic island of Surtsey (SW Iceland) since eruptions terminated in 1967. Here, subaerial and submarine lapilli tuff samples from a 192 m-deep borehole drilled in 2017 (SE-02b) are characterized through petrographic studies, X-ray powder diffraction analyses, and SEM–EDS imaging and chemical analyses. The integrated results reveal (i) multi-stage palagonitization processes in basaltic glass and precipitation of secondary minerals from matrix pore fluids, (ii) multi-stage crystallization of secondary phillipsite, analcime and Al-tobermorite in the vesicles of basaltic pyroclasts and (iii) variations in palagonitization processes as a function of thermal and hydrological domains. Although temperature appears to be an important factor in controlling rates of secondary mineralization, the chemistry of original basaltic components and interstitial fluids also influences reaction pathways in the young pyroclastic deposits. The integration of systematic mineralogical analyses of the 50-year-old tuff from one of the most carefully monitored volcanic sites on Earth, together with temperature monitoring in boreholes since 1980, provide a reference framework for evaluating mineralogical evolution in other Surtseyan-type volcanoes worldwide.

Characterization and Origin of Basalt-Derived Carnelian in the Mesozoic Newark Basin, New Jersey, USA

Carnelian occurs locally in New Jersey in the Newark basin as medium- to coarse-size pebbles in fluvial gravel and alluvium and colluvium formed from erosion of Lower Jurassic Preakness Basalt. Vesicles and molds of glauberite are preserved on lower surfaces and botryoidal textures on the upper surfaces of some pieces. The microstructure consists of length-fast chalcedony characterized by parallel fibrous bundles overlain by repetitive, wavy extinction bands. Only peaks of ɑ-quartz and minor moganite are recognized in X-ray diffraction patterns. Carnelian contains 97–98 wt.% SiO2, ~1.0 wt.% Fe2O3, and 1.0–1.4 wt.% LOI; other major elements are <0.1 wt.%. Trace element abundances are low except for Y, Nb, Ta, W, Th, and U. Rare earth element (REE) patterns display heavy REE enrichment and large negative Eu anomalies. Most trace elements were mobilized from Proterozoic sources, whereas Si was likely derived from the alteration of basaltic glass in the Preakness. Carnelian δ18OVSMOW values are high and range from +18.3 to +31.2‰, comparable to global occurrences of volcanic rock-derived chalcedony. We propose that carnelian precipitated in the first Preakness flow from the mixing of hydrothermal fluid with meteoric water under conditions of low temperature (20–80 °C) and neutral to slightly alkaline pH.

Phyllosilicate formation on early Mars via open-system acid alteration of basaltic glass

Smectites are widespread on Mars, but the water-rich, neutral-to-alkaline pH conditions favorable for smectite formation would be expected to have also produced abundant carbonates on early Mars, which are not observed. Smectite formation from basaltic glass on Mars could occur in acidic environments unfavorable for carbonate formation. Acidic smectite formation has been previously demonstrated in batch experiments (closed hydrologic systems), however, the mechanisms and octahedral composition of smectite forming in acidic flow-through (open hydrologic systems) environments are still not fully understood. We conducted hydrothermal (190°C) alteration experiments on Stapafell basaltic glass at 0.01 and 0.25 mLmin−1 flow rates corresponding to low and high water to rock ratio (W/R) flow conditions, and initial pH (pH0) values of 2, 3, 4 and 6. A batch low W/R experiment was conducted at pH0 2 for comparison to the open system experiment. Kaolinite, montmorillonite and chlorite formed at pH0 2 at low W/R; no phyllosilicates formed at pH0 2 at high W/R; and lizardite formed at pH0 ≥ 3 at both W/R ratios. Lizardite, kaolinite, and montmorillonite in these experiments formed by precipitation from solution and chlorite likely formed through alteration of montmorillonite and/or basalt. Saponite formed at pH0 2 in batch conditions by alteration of basaltic glass. Comparison of experimental data with martian observations of phyllosilicate assemblages indicated that smectite formation on Mars likely occurred under water-limited environmental conditions. Al-rich smectite could form in low W/R open system subsurface environments under a very narrow range of pH (pH < 3) while saponite could form in closed low W/R systems under acidic to alkaline conditions. The combination of open and closed hydrological regimes could be responsible for development of clay mineral stratigraphies observed on Mars. The acidic conditions required for formation of Al-rich smectite montmorillonite were unfavorable for carbonate precipitation, but carbonate precipitation could occur together with Fe/Mg-smectite saponite in closed systems at pH > 4. The lack of widespread carbonates on Mars could not therefore be explained solely by acidic conditions. Phyllosilicate formation under acidic conditions on Mars may affect biosignature stability in martian regolith as preservation capacity is lower in phyllosilicates that formed in or experienced acidic pH.

Alteration of basaltic glass within the Surtsey hydrothermal system, Iceland – Implication to oceanic crust seawater interaction

Low-temperature hydrothermal alteration of basaltic glass on the seafloor has important implications on the chemical evolution of the oceanic crust and seawater composition. However, mass fluxes resulting from seawater-glass interaction in this type of environment remain poorly understood. This study presents new results on element mobilities for the palagonitization of basaltic glass and bulk rock hydrothermal alteration at Surtsey volcano, Iceland over a time period of ~50-years. Assessments of element mobilities were based on 1) immobile trace element isocon mass balance for palagonitization, 2) the assumption of large scale TiO2 immobility during bulk rock alteration and 3) chemical analyses of pore fluids and authigenic minerals.Element mobilities of glass alteration vary between initial palagonitization and subsequent palagonite maturation: Initially, palagonitization of sideromelane leaches SiO2, Al2O3, MgO, CaO, Na2O, K2O from the glass. Following this, the maturation of the palagonitized glass causes re-uptake of all of the abovementioned elements except for CaO, which shows either no change or slight uptake, and Na2O, which continues to be lost from the palagonitized glass. Among major elements TiO2 and FeO can be considered immobile during palagonitization, but not during the subsequent aging process of palagonitized glass. Among trace elements Hf, Ta, Zr, Nb, Y and REE were found to be immobile at all stages of the alteration. On the bulk rock scale, comprising both glass alteration and the formation of authigenic phases, early-stage alteration at Surtsey releases SiO2, Al, Mg and Ca to the fluid, whereas late-stage alteration poses a sink of SiO2, Al, Mg, Ca and Na. Net element budgets during early- and late-stage alteration indicate that chemical exchange between basaltic tuffs and seawater derived fluids at Surtsey acts as a long-term source of dissolved Ca, Al and SiO2 for seawater as well as a long-term sink for seawater Mg and Na.

Element mobility and oxygen isotope systematics during submarine alteration of basaltic glass

Abstract Studies of the submarine alteration of basaltic glass may improve the understanding of crust-seawater interactions and the oceanic elemental cycle. Natural alteration processes such as the palagonitization of basaltic glass are complex, and their elucidation is crucial to the understanding of fluid-rock interactions. This study involves major and trace element and isotope mapping of a submarine-altered basaltic glass grain and reveals sequential development of alteration textures toward the grain core through the transition between unaltered glass and palagonite. The conversion from basaltic glass to palagonite in a low-temperature submarine environment results in enrichment in B, Rb, K, Li, H2O, U, Nb, Th, Ti, Cu, Ta, Zr, Hf, Ni, Sc, Fe, Cr, Pb, and Zn; and depletion in Si, Mg, Al, Sr, Na, Co, rare-earth elements, Ca, P, and V (in order of decreasing distribution coefficient). The glass-palagonite interface region has the highest high field strength element (HFSE) and Fe-Ti contents but the lowest Mg content, indicating that the Fe-Ti phases that host HFSE were precipitated first during initial palagonitization. At the same time, an inferred exchange of oxygen occurred, based on variation in δ18O, with values increasing from basaltic glass to palagonite. However, initial palagonite compositions were affected by subsequent precipitation and/or incorporation/adsorption of additional compounds such as Mg(OH)2 scavenged from pore water.

Far from equilibrium basaltic glass alteration: The influence of Fe redox state and thermal history on element mobilization

Elemental release from basaltic glasses at far from equilibrium conditions was investigated as a function of the Fe redox state (Fe(II)/Fetot = 0.35 and 0.80) and thermal history (quenched ↔ annealed). A flow-through column setup was used to ensure disequilibrium of basaltic glass and solution during the entire runtime. Percolation experiments were performed at 25 °C for up to 500 h with intermediate sample collection. Two different pH values were adjusted, and the effect of organic matter was tested by adding oxalic acid. Element concentrations in the percolate were measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).After an initial high release of elements from fresh glass surface, steady state was achieved after 240–430 h for Si at pH 5–7 and after 170–240 h for Al at pH 2. At near neutral conditions (pH 5–7) mobilization of Si is relatively high, while Fe and possibly Al are retained in precipitates. Under more acidic conditions (pH 2), the Si concentration of the solutions was very low compared to the other main constituents of the glass. Large amounts of Si-rich residues are formed after glass dissolution at pH 2. On the other hand, Fe (and Mn) is very mobile under acidic conditions, favored by complex formation with oxalic acid and chlorine. In the initial phase of the pH 2 experiments, the element release from reduced glasses is higher than from oxidized glasses. However, this trend is reversed when approaching steady state. Higher dissolution rates for oxidized glasses are predicted due to the progressive replacement of strong Si-O-Si with weaker Fe(III)-O-Si. At pH 5–7 the concentrations of elements in the percolate are too low to establish a systematic difference between oxidized and reduced glasses. Looking at the total amount of mobilized elements, the thermal history of the glasses has no significant effect in the case of oxalate-free solutions, but a noticeable increase of element release in the case of rapidly quenched glasses was observed when using 1 mM oxalic acid solution. The strong effect of oxalate on dissolution of quenched glasses is probably related to the more open glass network structure in quenched glasses.

Rapid determination of the original boron isotopic composition from altered basaltic glass by in situ secondary ion mass spectrometry

Rapid determination of the original boron composition from altered basaltic glass by SIMS boron isotopic profile analysis from the core to rim.

A nanoscale study of the formation of Fe-(hydr)oxides in a volcanic regolith: Implications for the understanding of soil forming processes on Earth and Mars

Abstract Iron(hydr)oxides are one of the most important constituents of regoliths and soils derived from volcanic rocks both on Earth and Mars, often giving them their characteristic red color. This study deciphers for the first time an underlying mechanism for the formation of Fe-(hydr)oxides in a regolith which can occur during the weathering of basaltic glass; Fe-(hydr)oxides are prominent alteration products of volcanic regolths. An excellent example of these conditions is the early stage of basaltic glass weathering in the Martian regolith simulant JSC MARS-1A. In this study, a combination of focused ion beam technology and analytic transmission electron microscopy is employed in order to characterize basaltic glass weathering down to the nanometer level. Our results show that the formation of Fe-(hydr)oxide phases such as ferrihydrite, magnetite/maghemite and hematite during alteration of basaltic glass is based on complex and formerly unknown sequences of dissolution-precipitation reactions and pressure induced coalescence, segregation, aggregation, densification and growth processes. The weathering of the glass starts with its dissolution and subsequent precipitation of hydrous amorphous silica-bearing pockets rimmed by nano-size domains of ferrihydrite. An increase in molar volume during this process leads to an overall volume expansion, which promotes (a) growth of the hydrous silica through coalescence of individual pockets, (b) agglomeration of ferrihydrite domains to larger and denser aggregates in between layers or along the surfaces of plagioclase, hydrous amorphous silica and amorphous Al-(hydr)oxides, (c) formation of hematite within dense aggregates of ferrihydrite or as larger nanoparticles within an hydrous amorphous Si-Al-rich phase and (d) the break-up of plagioclase crystals and the replacement of these fragments by an hydrous amorphous Fe-Al-Si-bearing phase. At a later weathering stage, ferrihydrite nano-domains can also transform into magnetite/maghemite nanoparticles, which occur as layers around and on the surface of larger plagioclase crystals. This study also indicates the presence of past nano-environments in close proximity to each other, as for example layers of imogolite and ferrihydrite/hydrous amorphous silica occur only nanometers apart from each other on the opposite sides of unaltered glass. In accord with previous mineralogical studies of JSC MARS-1, the observed bulk and nano-mineralogical composition indicate that early alteration processes of basaltic glass under dry and cold conditions are mainly controlled by the formation of Fe-(hydr)oxides and minor imogolite and kaolinite. Recent mineralogical studies indicate that alteration processes at these conditions may have been the dominant weathering processes over long time periods on the Martian surface.

Experimental Constraints on Abiotic Formation of Tubules and Other Proposed Biological Structures in Subsurface Volcanic Glass.

Formation of microtubules in volcanic glass from subsurface environments has been widely attributed to in situ activity of micro-organisms, but evidence directly linking those structures to biological processes remains lacking. Investigations into the alternative possibility of abiotic tubule formation have been limited. A laboratory experiment was conducted to examine whether moderate-temperature hydrothermal alteration of basaltic glass by seawater would produce structures similar to those ascribed to biological processes. Shards of glass were reacted with artificial seawater at 150°C for 48 days. Following reaction, the shards were uniformly covered with a brick-red alteration rind 10-30 μm thick composed primarily of phyllosilicates. Inspection of the margins of reacted shards with light microscopy did not reveal any tubule structures. However, the alteration products did include features containing micron-sized spheroidal structures that resemble granular alteration textures, which some investigators have attributed to biological activity. This result suggests that the granular textures may be at least partially abiotic, and that biological activity may make a smaller contribution to alteration of the oceanic crust than has been previously proposed. Also, while the experimental results do not exclude the possibility that tubules form abiotically, they do place limitations on the conditions under which this may occur.

Experimental hydrothermal alteration of basaltic glass with relevance to Mars

AbstractPhyllosilicates, carbonates, zeolites, and sulfates on Mars give clues about the planet's past environmental conditions, but little is known about the specific conditions in which these minerals formed within the crust and at the surface. The aim of the present study was to gain increased understanding on the formation of secondary phases by hydrothermal alteration of basaltic glass. The reaction processes were studied under varying conditions (temperature, pCO2, water:rock ratio, and fluid composition) with relevance to aqueous hydrothermal alteration in fully and partly saturated Martian basalt deposits. Analyses made on reaction products using X‐ray diffraction (XRD) and scanning electron microscope (SEM) were compared with near infrared spectroscopy (NIR) to establish relative detectability and spectral signatures. This study demonstrates that comparable alteration minerals (phyllosilicates, carbonates, zeolites) form from vapor condensing on mineral surfaces in unsaturated sediments and not only in fully water‐saturated sediments. In certain environments where water vapor might be present, it can alter the basaltic bedrock to a suite of authigenic phases similar to those observed on the Martian surface. For the detection of the secondary phases, XRD and SEM‐EDS were found to be superior to NIR for detecting and characterizing zeolites. The discrepancy in detectability of zeolites between NIR and XRD/SEM‐EDS might indicate that zeolites on Mars are more abundant than previously thought.

μXAFS and TEM studies of Fe(III) oxides precipitated on submarine basaltic glass from South Pacific Gyre

In the present study, we investigated secondary products formed on the surface of altered basaltic glass from the sediment-basalt interface by multiple characterizations with EPMA, μXANES, μEXAFS, TEM, and EELS. μXANES and EPMA analyses showed that secondary products with high Fe abundance were precipitated on the surface of basalt glass and that the Fe species in the secondary product was dominantly composed of Fe(III) oxides, hematite, which was also agreeable with EELS result. Characterization by TEM and μEXAFS suggested that the hematite was nanoparticulate with particle size of 10–20 nm. This is the first study showing the occurrence of nanosized hematite as secondary products in altered basaltic glass, although there are many studies on the formation of Fe(III) (hydr)oxides in basalt alteration.

Experimental Microbial Alteration and Fe Mobilization From Basaltic Rocks of the ICDP HSDP2 Drill Core, Hilo, Hawaii

The interaction of a single bacterial species (Burkholderia fungorum) with basaltic rocks from the ICDP HSDP2 drill core and synthetic basaltic glasses was investigated in batch laboratory experiments to better understand the role of microbial activity on rock alteration and Fe mobilization. Incubation experiments were performed with drill core basaltic rock samples to investigate differences in the solution chemistry during biotic and abiotic alteration. Additionally, colonization experiments with synthetic basaltic glasses of different Fe redox states and residual stresses were performed to evaluate their influence on microbial activity and surface attachment of cells. In biotic incubation experiments bacterial growth was observed and the release of Fe and other major elements from drill core basaltic rocks to solution exceeded that of abiotic controls only when the rock sample assay was nutrient depleted. The concentration of dissolved major elements in solution in biotic colonization experiments with synthetic basaltic glasses increased with increasing residual stress and Fe(II) content. Furthermore, the concentration of dissolved Fe and Al increased similarly in biotic colonization experiments indicating that their dissolution might be triggered by microbial activity. Surface morphology imaging by SEM revealed that cells on basaltic rocks in incubation experiments were most abundant on the glass and surfaces with high roughness and almost absent on minerals. In colonization experiments, basaltic glasses with residual stress and high Fe(II) content were intensely covered with a cellular biofilm. In contrast, glasses with high Fe(III) content and no residual stress were sparsely colonized. We therefore conclude that structurally bound Fe is most probably used by B. fungorum as a nutrient. Furthermore, we assume that microbial activity overall increased rock dissolution as soon as the environment becomes nutrient depleted. Our results show that besides compositional effects, other factors such as redox state and residual stress can control microbial alteration of basaltic glasses.

Nature and composition of interbedded marine basaltic pumice in the ∼52–50 Ma Vastan lignite sequence, western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels – one at ∼5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ∼52–50 Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated altered basaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO 2, Al 2 O 3, TiO 2, MgO, alkalies and REE, but very high in Fe 2 O 3, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO 2, Al 2 O 3, MgO and variable gain in Fe 2 O 3, TiO 2, Ni, V, Zr, Zn and REE. Bubble initiation to growth in the ascending basaltic magma (liquidus ∼1200–1250 ∘C) may have occured in ∼3 hr. Short-distance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.

Acid weathering of basalt and basaltic glass: 2. Effects of microscopic alteration textures on spectral properties

AbstractAcid alteration has long been proposed for the Martian surface, and so it is important to understand how the resulting alteration textures affect surface spectra. Two basaltic materials of varying crystallinity were altered in two different H2SO4 solutions (pH 1 and pH 3) for 220 days. The unaltered and altered samples were studied in the visible and near infrared (VNIR) and thermal infrared (TIR), and select samples were chosen for scanning electron microscopy analysis. Materials altered in pH 3 solutions showed little to no physical alteration, and their spectral signatures changed very little. In contrast, all materials altered in pH 1 acid displayed silica‐rich alteration textures, and the morphology differed based on starting material crystallinity. The more crystalline material displayed extensive alteration reaching into the sample interiors and had weaker silica spectral features. The glass sample developed alteration layers tens of microns thick, exhibiting amorphous silica‐rich spectral features that completely obscured the substrate. Thus, the strong absorption coefficient of silica effectively decreases the penetration depth of TIR spectral measurements, causing silica abundances to be grossly overestimated in remote sensing data. Additionally, glass samples with silica layers exhibited distinct concave up blue spectral slopes in the VNIR. Spectra from the northern lowland plains of Mars are modeled with high abundances of amorphous silica and exhibit concave up blue spectral slopes and are thus consistent with acid altered basaltic glass. Therefore, we conclude that large regions of the Martian surface may have formed through the interaction of basaltic glass with strongly acidic fluids.

Structural Iron (II) of Basaltic Glass as an Energy Source for Zetaproteobacteria in an Abyssal Plain Environment, Off the Mid Atlantic Ridge.

To explore the capability of basaltic glass to support the growth of chemosynthetic microorganisms, complementary in situ and in vitro colonization experiments were performed. Microbial colonizers containing synthetic tholeitic basaltic glasses, either enriched in reduced or oxidized iron, were deployed off-axis from the Mid Atlantic Ridge on surface sediments of the abyssal plain (35°N; 29°W). In situ microbial colonization was assessed by sequencing of the 16S rRNA gene and basaltic glass alteration was characterized using Scanning Electron Microscopy, micro-X-ray Absorption Near Edge Structure at the Fe-K-edge and Raman microspectroscopy. The colonized surface of the reduced basaltic glass was covered by a rind of alteration made of iron-oxides trapped in a palagonite-like structure with thicknesses up to 150 μm. The relative abundance of the associated microbial community was dominated (39% of all reads) by a single operational taxonomic unit (OTU) that shared 92% identity with the iron-oxidizer Mariprofundus ferrooxydans PV-1. Conversely, the oxidized basaltic glass showed the absence of iron-oxides enriched surface deposits and correspondingly there was a lack of known iron-oxidizing bacteria in the inventoried diversity. In vitro, a similar reduced basaltic glass was incubated in artificial seawater with a pure culture of the iron-oxidizing M. ferrooxydans DIS-1 for 2 weeks, without any additional nutrients or minerals. Confocal Laser Scanning Microscopy revealed that the glass surface was covered by twisted stalks characteristic of this iron-oxidizing Zetaproteobacteria. This result supported findings of the in situ experiments indicating that the Fe(II) present in the basalt was the energy source for the growth of representatives of Zetaproteobacteria in both the abyssal plain and the in vitro experiment. In accordance, the surface alteration rind observed on the reduced basaltic glass incubated in situ could at least partly result from their activity.

Silicon isotope systematics of acidic weathering of fresh basalts, Kilauea Volcano, Hawai’i

Silicon stable isotopes are fractionated by a host of low-temperature aqueous processes, making them potentially useful as a weathering proxy. Here we characterize the silicon isotope signature of surficial chemical weathering of glassy basaltic lava flows at Kilauea Volcano, Hawaii. Fresh basalt flow surfaces (<40years old) frequently feature opaque amorphous silica surface coatings up to 80μm thick. These silica coatings and associated silica cements are enriched in the heavier isotopes of Si (δ30SiNBS-28=+0.92‰ to +1.36‰) relative to their basaltic substrate (δ30SiNBS-28=−0.3‰ to −0.2‰). Secondary clays and opals are typically depleted in 30Si relative to the dissolved reservoirs from which they precipitated, so this sense of isotopic fractionation is unusual. Mechanisms capable of producing isotopically heavy secondary minerals were explored by conducting batch alteration experiments on fresh basaltic glass. Batch acidic alteration of basalt glass in HCl, H2SO4, and HF produced silica-rich surface layers resembling the Hawaiian surface coatings. Differences in fluid chemical composition affected the direction and magnitude of Si isotope fractionation. Basalt leaching in HCl or H2SO4 produced 30Si-enriched fluids (1000 lnαprecip-Si(aq)≅−0.8‰) and 30Si-depleted secondary silica layers. In contrast, HF-bearing experiments produced highly 30Si-depleted fluid compositions (1000 lnαprecip-Si(aq) up to +8‰). Larger isotopic fractionations were observed in experiments with lower fluid–rock ratios. In Hawaii, where altering fluids contain H2SO4 and HCl but minimal HF, high δ30Si values for the silica coatings were likely achieved by Rayleigh fractionation. Aqueous 30Si-enriched silica was released during incongruent basalt dissolution then subsequently transported and deposited from an evaporating solution at the flow surface. Our results indicate that (1) altering fluid chemistry and fluid–rock ratio impact the Si isotope signature of chemical weathering and (2) δ30Si of solids produced by low temperature aqueous alteration may diverge sharply from watershed- or landscape-scale weathering trends.

Long-term alteration of basaltic glass: Mechanisms and rates

The long-term behavior study of archaeological artifacts and natural minerals and glasses revealed discrepancies between laboratory and field data. For a better understanding of the cause of these discrepancies and to reinforce the use of basaltic glass as an analog for nuclear waste glasses, this study focuses on the determination of alteration rates and processes of synthetic basaltic glass in residual rate regime. Laboratory batch experiments were performed at high surface-to-volume ratios at 90 and 30°C for more than 1000days. In all the experiments, the residual rate regime was reached after about 6months. The residual alteration rates at 30 and 90°C were 4.0±1.0×10−6 and 9.5±3.2×10−6g·m−2·d−1, respectively. At 90°C, this residual alteration rate is five orders of magnitude lower than the forward alteration rate (0.8g·m−2·d−1). Altered powders and monoliths were characterized by Transmission Electron Microscopy and Time-of-Flight Secondary Ion Mass Spectrometry. From glass core to solution, the altered materials are structured as follows: pristine glass, gel (corresponding to the palagonitic layer of natural glasses) and intergranular clays. To assess the passivating properties of this alteration film, we used solid characterization, an isotopically-tagged post-leaching experiment and the measurement of mobile species diffusion coefficients through the alteration film at different stages of reaction using various techniques (solution analysis and X-ray Reflectometry). These characterizations showed that the alteration film formed during residual rate alteration is passivating even without clogged porosity within the gel. Diffusion coefficients of water and alkali metals – respectively diffusing to and from the pristine glass – through the alteration film dropped from 10−20 to 10−19 m2·s−1 during the first alteration stages to 10−25 m2·s−1 in residual rate regime.

Mineralogical aspects of CO2 sequestration during hydrothermal basalt alteration — An experimental study at 75 to 250 °C and elevated pCO2

The alteration of basaltic glass was studied experimentally at elevated CO2 pressures (~10–25bar) and hydrothermal conditions (75–250°C) to determine the effects of temperature and extent of reaction (ξ) on the secondary mineral formation and compositional evolution of alteration assemblages resulting from CO2–water–basalt interaction. At <100°C, the alteration products consisted of concentric layers of ankerite and dolomite–ankerite solid solutions (Ca–Mg–Fe carbonates) and amorphous silica. At ≥150°C, mixed Ca–Mg–Fe smectites and chlorite, calcite, amorphous silica and zeolites formed instead of Ca–Mg–Fe carbonates. Competing reactions between carbonates and clays for major divalent cations (Ca, Mg and Fe) were affected by the extent of reaction with smectites formed initially and progressively being replaced by calcite and chlorite. The basaltic glass dissolution rate and mechanism were also affected by temperature and reaction time. At lower temperatures (≤150°C), a hydrated leached layer formed on the glass surface and the mass fluxes in the system were largely controlled by the dissolution rate and mechanism of the glass, whereas at higher temperature (250°C) the dissolution rate of the basaltic glass was fast with the reactions primarily driven by secondary mineral replacement and growth. We conclude that the driving force for carbonate mineralization in basaltic glass at elevated CO2 conditions is linked to the geochemical behavior and mobility of Ca, Mg and Fe together with the availability of Si for Al–Si-mineral formation. At <100°C, the divalent cations are available for Ca–Mg–Fe carbonate formation, whereas at ≥150°C abundant clays limit the availability of Mg and Fe resulting in calcite being the only carbonate formed. This implies a quantitatively more effective fixation of CO2 at <100°C associated with the formation of Ca–Mg–Fe carbonates.

A combined TEM and NanoSIMS study of endolithic microfossils in altered seafloor basalt

The incipient alteration of basaltic glass to palagonite in recent seafloor lavas from the arctic Mohns Ridge was studied by complimentary high-spatial-resolution geochemical techniques: TEM-EDS (transmission electron microscopy energy dispersive X-ray spectroscopy) and NanoSIMS (nano-scale secondary ion mass spectrometry). Rounded to elongated pores 0.5–2 μm across were found embedded in compact palagonite that have sizes and shapes comparable to microbial cells. In-situ elemental mapping revealed that the micropore rims are comparable in composition to the bulk palagonite and that some are enriched in manganese. Elevated concentrations of carbon and nitrogen were also found in some of the micropores. Hence these structures are interpreted as fossilised bacteriomorphs of endolithic microorganisms that inhabited fractures in the basaltic glass. The preferential accumulation of Mn in some of the cell encrustations suggests the mineralisation of Mn-oxidising bacteria. These data provide further evidence for the involvement of microorganisms in the colonisation and chemical alteration of recent seafloor volcanic glass and identify micro-scale Mn enrichments associated with micropores as a promising biosignature in such rocks.

Microtextures, microchemistry, and mineralogy of basaltic glass alteration, Jeju Island, Korea, with implications for elemental behavior

The meteoric alteration of basaltic glass in hyaloclastite (0.55 to 0.2 Ma) from Jeju Island, Korea, was examined using high-resolution electron microscopy, in situ sample preparation using focused ion beam, and microchemical analysis. The glass particles showed a sequential development of alteration textures toward the glass interior as follows: oscillatory zoning of smectite and amorphous bands/hemispheres, congruent dissolution of glass with smectite infilling in interparticle pore, and the replacement of nanogranular amorphous aggregate (NAA) for porous leached glass (LG) followed by a partial crystallization of smectite. Chemical compositions of LG widely varied from partial to complete leaching of almost all kinds of cations, to a preferred leaching of Na only. The order of leaching in LG followed Na + &gt;Mg 2+ &gt;Ca 2+ = Fe 2+ &gt;Al 3+ &gt;Ti 4+ . Leached cations were removed from the hyaloclastite-water system or incorporated into secondary phases. Sodium was all but removed from the system. Magnesium was largely lost with partial incorporation into smectite. Calcium was significantly retained in NAA, partly incorporated into the smectite interlayer, and partly lost. Iron and Al were partitioned between NAA and smectite. Titanium was mostly retained in NAA and Ti-rich silicates. The molar ratio of major cations to Si was reversed from Ca 2+ &gt;Mg 2+ &gt;Na + in basaltic glass to Na + &gt;Mg 2+ &gt;Ca 2+ in groundwater. The geochemical evolution of pore water was influenced by the microtextural and microchemical evolution of glass particles, and vice versa. Systematic microtextural, microchemical, and mineralogical analyses of natural glass alteration are critical to understanding the long-term behavior of glass alteration.