Coupled alkali feldspar dissolution and secondary mineral precipitation in batch systems – 2: New experiments with supercritical CO2 and implications for carbon sequestration

In order to evaluate the extent of CO₂–water–rock interactions in geological formations for C sequestration, three batch experiments were conducted on alkali feldspars–CO₂–brine interactions at 150–200 °C and 300 bars. The elevated temperatures were necessary to accelerate the reactions to facilitate attainable laboratory measurements. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. SEM, TEM and XRD analysis of reaction products showed extensive dissolution features (etch pits, channels, kinks and steps) on feldspars and precipitation of secondary minerals (boehmite, kaolinite, muscovite and paragonite) on feldspar surfaces. Therefore, these experiments have generated both solution chemistry and secondary mineral identity. The experimental results show that partial equilibrium was not attained between secondary minerals and aqueous solutions for the feldspar hydrolysis batch systems. Evidence came from both solution chemistry (supersaturation of the secondary minerals during the entire experimental duration) and metastable co-existence of secondary minerals. The slow precipitation of secondary minerals results in a negative feedback in the dissolution–precipitation loop, reducing the overall feldspar dissolution rates by orders of magnitude. Furthermore, the experimental data indicate the form of rate laws greatly influence the steady state rates under which feldspar dissolution took place. Negligence of both the mitigating effects of secondary mineral precipitation and the sigmoidal shape of rate–ΔG_r relationship can overestimate the extent of feldspar dissolution during CO₂ storage. Finally, the literature on feldspar dissolution in CO₂-charged systems has been reviewed. The data available are insufficient and new experiments are urgently needed to establish a database on feldspar dissolution mechanism, rates and rate laws, as well as secondary mineral information at CO₂ storage conditions.     

Dissolution kinetics of soil clays in sulfuric acid solutions: Ionic strength and temperature effects

Significant amounts of sulfuric acid (H₂SO₄) rich saline water can be produced by the oxidation of sulfide minerals contained in inland acid sulfate soils (IASS). In the absence of carbonate minerals, the dissolution of phyllosilicate minerals is one of very few processes that can provide long-term acid neutralisation. It is therefore important to understand the acid dissolution behavior of naturally occurring clay minerals from IASS under saline–acidic solutions. The objective of this study was to investigate the dissolution of a natural clay-rich sample under saline–acidic conditions (pH 1–4; ionic strengths = 0.01 and 0.25 M; 25 °C) and over a range of temperatures (25–45 °C; pH 1 and pH 4). The clay-rich sample referred to as Bottle Bend clay (BB clay) used was from an IASS (Bottle Bend lagoon) in south-western New South Wales (Australia) and contained smectite (40%), illite (27%), kaolinite (26%) and quartz (6%). Acid dissolution of the BB clay was initially rapid, as indicated by the fast release of cations (Si, Al, K, Fe, Mg). Relatively higher Al (pH 4) and K (pH 2–4) release was obtained from BB clay dissolution in higher ionic strength solutions compared to the lower ionic strength solutions. The steady state dissolution rate (as determined from Si, Al and Fe release rates; R_Si, R_Al, R_Fe) increased with decreasing solution pH and increasing temperature. For example, the highest log R_Si value was obtained at pH 1 and 45 °C (−9.07 mol g⁻¹ s⁻¹), while the lowest log R_Si value was obtained at pH 4 and 25 °C (−11.20 mol g⁻¹ s⁻¹). A comparison of these results with pure mineral dissolution rates from the literature suggests that the BB clay dissolved at a much faster rate compared to the pure mineral samples. Apparent activation energies calculated for the clay sample varied over the range 76.6 kJ mol⁻¹ (pH 1) to 37.7 kJ mol⁻¹ (pH 4) which compare very well with the activation energy values for acidic dissolution of monomineralic samples e.g. montmorillonite from previous studies. The acid neutralisation capacity (ANC) of the clay sample was calculated from the release of all structural cations except Si (i.e. Al, Fe, K, Mg). According to these calculations an ANC of 1.11 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 (I = 0.25 M, 25 °C) compared to an ANC of 0.21 kg H₂SO₄/tonne clay/day at pH 4 (I = 0.25 M, 25 °C). The highest ANC of 6.91 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 and at 45 °C (I = 0.25 M), which is more than three times higher than the ANC provided under the similar solution conditions at 25 °C. In wetlands with little solid phase buffering available apart from clay minerals, it is imperative to consider the potential ANC provided by the dissolution of abundantly occurring phyllosilicate minerals in devising rehabilitation schemes.     

Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar

Here we report on an experimental investigation of the relation between the dissolution rate of albite feldspar and the Gibbs free energy of reaction, ΔG_r. The experiments were carried out in a continuously stirred flow-through reactor at 150 °C and pH_(150 °C) 9.2. The dissolution rates R are based on steady-state Si and Al concentrations and sample mass loss. The overall relation between ΔG_r and R was determined over a free energy range of −150 < ΔG_r < −15.6 kJ mol⁻¹. The data define a continuous and highly non-linear, sigmoidal relation between R and ΔG_r that is characterized by three distinct free energy regions. The region furthest from equilibrium, delimited by −150 < ΔG_r < −70 kJ mol⁻¹, represents an extensive dissolution rate plateau with an average rate . In this free energy range the rates of dissolution are constant and independent of ΔG_r, as well as [Si] and [Al]. The free energy range delimited by −70 ? ΔG_r ? −25 kJ mol⁻¹, referred to as the ‘transition equilibrium’ region, is characterized by a sharp decrease in dissolution rates with increasing ΔG_r, indicating a very strong inverse dependence of the rates on free energy. Dissolution nearest equilibrium, defined by ΔG_r > −25 kJ mol⁻¹, represents the ‘near equilibrium’ region where the rates decrease as chemical equilibrium is approached, but with a much weaker dependence on ΔG_r. The lowest rate measured in this study, R = 6.2 × 10⁻¹¹ mol m⁻² s⁻¹ at ΔG_r = −16.3 kJ mol⁻¹, is more than two orders of magnitude slower than the plateau rate. The data have been fitted to a rate equation (adapted from Burch et al. [Burch, T. E., Nagy, K. L., Lasaga, A. C., 1993. Free energy dependence of albite dissolution kinetics at 80 °C and pH 8.8. Chem. Geol.105, 137-162]) that represents the sum of two parallel reactions

R=k₁[1-exp(-ng^m₁)]+k₂[1-exp(-g)]^m₂,     

Global calibration of a novel,branched GDGT-based soil pH proxy

Recently, 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) were separated from 5-methyl brGDGTs, which are used in brGDGT-based proxies. Here we analyzed brGDGTs in 27 soil samples along the 400 mm isoline of mean annual precipitation in China by using tandem 2D liquid chromatography. The fractional abundance of 6-methyl brGDGTs showed a positive correlation with soil pH, while that of 5-methyl brGDGTs decreased with increasing soil pH. The abundance ratio of 6-/5-methyl brGDGTs, namely the isomerization of branched tetraethers (IBT), was calculated. The correlation of IBT with pH (pH = 6.33 − 1.28 × IBT; R² 0.89; root mean squared error, RMSE, 0.24) was much stronger than that of the traditionally used cyclization index of branched tetraethers (CBT) with pH (R² 0.52; RMSE 0.49) and comparable with that of CBT′ with pH (R² 0.88; RMSE 0.25). Compiling all available data from 319 soil samples resulted in a global calibration: pH = 6.53 − 1.55 × IBT (R² 0.72; RMSE 0.65), which has a better correlation than the CBT_5ME-pH proxy (R² 0.63; RMSE 0.78), but a weaker correlation than the CBT′-pH proxy (R² 0.85; RMSE 0.52). Our result suggests that the IBT is a promising indicator for soil pH, particularly in cases when some compounds in the CBT′ index cannot be determined.     

Dissolution kinetics of Devonian carbonates at circum-neutral pH, 50 bar pCO2, 105 °C,and 0.4 M: The importance of complex brine chemistry on reaction rates

The dissolution kinetics of carbonate rocks sampled from the Keg River Formation in Northeast British Columbia were measured at 50 bar pCO₂ and 105 °C, in both natural and synthetic brines of 0.4 M ionic strength. Natural brines yielded reaction rates of −12.16 ± 0.11 mol cm⁻² s⁻¹ for Log R_Ca, and −12.64 ± 0.05 for Log R_Mg. Synthetic brine yielded faster rates of reaction than natural brines. Experiments performed on synthetic brines, spiked with 10 mmol of either Sr or Zn, suggest that enhanced reaction rates observed in synthetic brines are due to a lack of trace ion interaction with mineral surfaces. Results were interpreted within the surface complexation model framework, allowing for the discrimination of reactive surface sites, most importantly the hydration of the >MgOH surface site. Dissolution rates extrapolated from experiments predict that CO₂ injected into the Keg River Formation will dissolve a very minor portion of rock in contact with affected formation waters.     

Biomarkers record environmental changes along an altitudinal transect in the wettest place on Earth

The combined application of plant wax δD (δD_wax) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) has been suggested as proxy for paleo-elevation. In some of the altitudinal transects studied so far, nonlinear precipitation gradients, large variations in seasonality, soil and vegetation types introduced substantial amounts of scatter in the relationship between these proxies and altitude. To further evaluate the principal functioning of the proxies, δD_wax and brGDGTs were analysed in surface soils along an altitude gradient (from 28 m up to 1865 m a.s.l.) in Meghalaya, India. The transect experiences limited seasonal temperature variation and receives very high monsoonal precipitation along the whole elevation gradient. The δD_wax show a significant relation with altitude (r² = 0.72). The additional fits with stream water δD (r² = 0.72) and modelled precipitation δD (r² = 0.72) indicate that δD_wax tracks the altitude effect on precipitation. Also the brGDGT distribution shows a correlation with altitude, reflecting the decrease in temperature with higher elevation (r² = 0.65, or r² = 0.66 using the original and recalibrated methylation of branched tetraethers-cyclisation of branched tetraethers (MBT–CBT) proxy). Application of the original MBT–CBT calibration generates calculated air temperatures that overestimate measured air temperature by ∼6 °C, whereas temperatures similar to measured are obtained with the revised calibration. These results indicate that δD_wax and brGDGTs may faithfully and accurately track environmental changes with altitude for transects where seasonal and diurnal temperature variability is relatively limited. Furthermore, proxy application to locations that experience high rainfall increases the suitability as climatic indicators, as it excludes soil moisture availability as a limiting factor.     

Characterization of the CO2 fluid adsorption in coal as a function of pressure using neutron scattering techniques (SANS and USANS)

Small angle neutron scattering techniques have been applied to investigate the phase behavior of CO₂ injected into coal and possible changes in the coal pore structure that may result from this injection. Three coals were selected for this study: the Seelyville coal from the Illinois Basin (R_o = 0.53%), Baralaba coal from the Bowen Basin (R_o = 0.67%), and Bulli 4 coal from the Sydney Basin (R_o = 1.42%). The coals were selected from different depths to represent the range of the underground CO₂ conditions (from subcritical to supercritical) which may be realized in the deep subsurface environment. The experiments were conducted in a high pressure cell and CO₂ was injected under a range of pressure conditions, including those corresponding to in-situ hydrostatic subsurface conditions for each coal. Our experiments indicate that the porous matrix of all coals remains essentially unchanged after exposure to CO₂ at pressures up to 200 bar (1 bar = 10⁵ Pa). Each coal responds differently to the CO₂ exposure and this response appears to be different in pores of various sizes within the same coal. For the Seelyville coal at reservoir conditions (16 °C, 50 bar), CO₂ condenses from a gas into liquid, which leads to increased average fluid density in the pores (ρ_pore) with sizes (r) 1 × 10⁵ ≥ r ≥ 1 × 10⁴ Å (ρ_pore ≈ 0.489 g/cm³) as well as in small pores with size between 30 and 300 Å (ρ_pore ≈ 0.671 g/cm³). These values are by a factor of three to four higher than the density of bulk CO₂ (ρ_CO2) under similar thermodynamic conditions (ρ_CO2 ≈ 0.15 g/cm³). At the same time, in the intermediate size pores with r ≈ 1000 Å the average fluid density is similar to the density of bulk fluid, which indicates that adsorption does not occur in these pores. At in situ conditions for the Baralaba coal (35 ^OC, 100 bar), the average fluid density of CO₂ in all pores is lower than that of the bulk fluid (ρ_pore / ρ_CO2 ≈ 0.6). Neutron scattering from the Bulli 4 coal did not show any significant variation with pressure, a phenomenon which we assign to the extremely small amount of porosity of this coal in the pore size range between 35 and 100,000 Å.     

Carbonate dissolution in Mesozoic sand- and claystones as a response to CO2 exposure at 70 °C and 20 MPa

The response to CO₂ exposure of a variety of carbonate cemented rocks has been investigated using pressurised batch experiments conducted under simulated reservoir conditions, 70 °C and 20 MPa, and with a durations of up to14 months. Calcite, dolomite, ankerite and siderite cement were present in the unreacted reservoir rocks and caprocks. Core plugs of the reservoir rocks were used in order to investigate the alterations in situ. Crushing of the caprock samples was necessary to maximise reactions within the relatively short duration of the laboratory experiments. Synthetic brines were constructed for each batch experiment to match the specific formation water composition known from the reservoir and caprock formations in each well. Chemical matched synthetic brines proved crucial in order to avoid reactions due to non-equilibra of the fluids with the rock samples, for example observations of the dissolution of anhydrite, which were not associated with the CO₂ injection, but rather caused by mismatched brines.Carbonate dissolution as a response to CO₂ injection was confirmed in all batch experiments by both petrographical observations and geochemical changes in the brines. Increased Ca and Mg concentrations after 1 month reaction with CO₂ and crushed caprocks are ascribed to calcite and dolomite dissolution, respectively, though not verified petrographically. Ankerite and possible siderite dissolution in the sandstone plugs are observed petrographically after 7 months reaction with CO₂; and are accompanied by increased Fe and Mn contents in the reacted fluids. Clear evidence for calcite dissolution in sandstone plugs is observed petrographically after 14 months of reaction with CO₂, and is associated with increased amounts of Ca (and Mg) in the reacted fluid. Dolomite in sandstones shows only minor dissolution features, which are not clearly supported by increased Mg content in the reacted fluid.Silicate dissolution cannot be demonstrated, either by chemical changes in the fluids, as Si and Al concentrations remain below the analytical detection limits, nor by petrographical changes, as partly dissolved feldspar grains and authigenic analcime are present in the sediments prior to the experiments. It is noteworthy, that authigenic K-feldspar and authigenic albite in sandstones show no signs of dissolution and consequently seem to be stable under the experimental conditions.     

Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH,CO2, and oxalate

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO₂ (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m⁻² h⁻¹ at pH 2.78 in the batch experiments to 0.009 μmol Si m⁻² h⁻¹ at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H⁺ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO₂ demonstrated that dissolution rates were independent of CO₂ within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO₂ have been elevated over the last decade. This release of magmatic CO₂ has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO₂ may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO₂ can be excluded.     

Dissolution kinetics of diopside as a function of solution saturation state: Macroscopic measurements and implications for modeling of geological storage of CO2

Measurements of the dissolution rate of diopside (r) were carried out as a function of the Gibbs free energy of the dissolution reaction (ΔG_r) in a continuously stirred flow-through reactor at 90 °C and pH_90 °C = 5.05. The overall relation between r and ΔG_r was determined over a free energy range of −130.9 < ΔG_r < −47.0 kJ mo1⁻¹. The data define a highly non-linear, sigmoidal relation between r and ΔG_r. At far-from-equilibrium conditions (ΔG_r ? −76.2 kJ mo1⁻¹), a rate plateau is observed. In this free energy range, the rates of dissolution are constant, independent of [Ca], [Mg] and [Si] concentrations, and independent of ΔG_r. A sharp decrease of the dissolution rate (∼1 order of magnitude) occurs in the transition ΔG_r region defined by −76.2 < ΔG_r ? −61.5 kJ mo1⁻¹. Dissolution closer to equilibrium (ΔG_r > −61.5 kJ mo1⁻¹) is characterised by a much weaker inverse dependence of the rates on ΔG_r. Modeling the experimental r-ΔG_r data with a simple classical transition state theory (TST) law as implemented in most available geochemical codes is found inappropriate. An evaluation of the consequences of the use of geochemical codes where the r-ΔG_r relation is based on basic TST was carried out and applied to carbonation reactions of diopside, which, among other reactions with Ca- and Mg-bearing minerals, are considered as a promising process for the solid state sequestration of CO₂ over long time spans. In order to take into account the actual experimental r-ΔG_r relation in the geochemical code that we used, a new module has been developed. It reveals a dramatic overestimation of the carbonation rate when using a TST-based geochemical code. This points out that simulations of water-rock-CO₂ interactions performed with classical geochemical codes should be evaluated with great caution.     

Effects of citrate on the dissolution and transformation of biotite,analyzed by chemical and atomic force microscopy

The aim of this study was to investigate the dissolution and transformation characteristics of phyllosilicate under low molecular weight organic acids in the farmland environment (pH 4.0–8.0). Changes of dissolution and morphology of biotite were evaluated using chemical extraction experiments and in situ/ex situ atomic force microscopy (AFM) with fluids of citric acid (CA) solution at pH 4.0, 6.0, and 8.0. Results of extracting experiments show that CA solutions contributed to the release rate of potassium (K), silicon (Si), and aluminum (Al) from biotite relative to a control aqueous solution. In situ AFM observations indicate that the dissolution of biotite from the biotite (0 0 1) surface occurred on the terrace, segment, and fringe of pits, while new etch pits did not readily form on biotite (0 0 1) surfaces in aqueous solutions. However, dissolution rates of terraces can be greatly accelerated with the help of citrate. In pH 4.0 CA solution, 70 min dissolution reactions of biotite (0 0 1) surfaces result in more etch pits than in pH 6.0 and 8.0 solutions. In addition, the transformation of biotite occurred simultaneously with the dissolution process. Secondary coating was observed on the biotite (0 0 1) surface after 140 h of immersion in a weak acid environment. Thus, the protons have a dominant role in the dissolution process of biotite with organic (carboxyl) acting as a catalyst under acidic condition. Based on the theory of interactions on a water–mineral interface in a weak acid environment, dissolution of biotite starts from defect/kink sites on the surface, one layer by one layer, and develops along the [h k 0] direction. A secondary coating that forms on the biotite (0 0 1) surface may restrain the formation and growth of etch pits, whereas this process may have a positive role on the stability of soil structure during long-term soil management.     

Soil pH governs production rate of calcium carbonate secreted by the earthworm Lumbricus terrestris

Lumbricus terrestris earthworms exposed to 11 soils of contrasting properties produced, on average, 0.8 ± 0.1 mg_CaCO3 earthworm^?1 day^?1 in the form of granules up to 2 mm in diameter. Production rate increased with soil pH (r² = 0.68, p < 0.01). Earthworms could be a significant source of calcite in soils.     

Kaolinite and smectite dissolution rate in high molar KOH solutions at 35° and 80°C

Experiments measuring kaolinite and smectite dissolution rates were carried out using batch reactors at 35° and 80°C. No potential catalysts or inhibitors were present in solution. Each reactor was charged with 1 g of clay of the ≤2 μm fraction and 80, 160 or 240 ml of 0.1–4 M KOH solution. An untreated but sized kaolinite from St. Austell and two treated industrial smectites were used in the experiments. One smectite is a nearly pure montmorillonite, while the second has a significant component of beidellitic charge (35%). The change in solution composition and mineralogy was monitored as a function of time. Initially, the 3 clays dissolved congruently. No new formed phases were observed by XRD and SEM during the pure dissolution stage. The kaolinite dissolution is characterized by a linear release of silica and Al as a function of the log of time. This relationship can be explained by a reaction affinity effect which is controlled by the octahedral layer dissolution. Far from equilibrium, dissolution rates are proportional to a^0.56±0.12_OH⁻ at 35°C and to a^0.81±0.12_OH⁻ at 80°C. The activation energy of kaolinite dissolution increases from 33±8 kJ/mol in 0.1 M KOH solutions to 51±8 kJ/mol in 3 M KOH solutions. In contrast to kaolinite, the smectites dissolve at much lower rates and independently of the aqueous silica or Al concentrations. The proportionality of the smectite dissolution rate constant at 35 and 80°C was a^0.15±0.06_OH⁻. The activation energy of dissolution appears to be independent of pH for smectite and is found to be 52±4 kJ/mol. The differences in behavior between the two kinds of minerals can be explained by structural differences. The hydrolysis of the tetrahedral and the octahedral layer appears as parallel reactions for kaolinite dissolution and as serial reactions for smectite dissolution. The rate limiting step is the dissolution of the octahedral layer in the case of kaolinite, and the tetrahedral layer in the case of smectite.     

Effect of organic-matter type and thermal maturity on methane adsorption in shale-gas systems

A series of methane (CH₄) adsorption experiments on bulk organic rich shales and their isolated kerogens were conducted at 35 °C, 50 °C and 65 °C and CH₄ pressure of up to 15 MPa under dry conditions. Samples from the Eocene Green River Formation, Devonian–Mississippian Woodford Shale and Upper Cretaceous Cameo coal were studied to examine how differences in organic matter type affect natural gas adsorption. Vitrinite reflectance values of these samples ranged from 0.56–0.58 %R_o. In addition, thermal maturity effects were determined on three Mississippian Barnett Shale samples with measured vitrinite reflectance values of 0.58, 0.81 and 2.01 %R_o.For all bulk and isolated kerogen samples, the total amount of methane adsorbed was directly proportional to the total organic carbon (TOC) content of the sample and the average maximum amount of gas sorption was 1.36 mmol of methane per gram of TOC. These results indicate that sorption on organic matter plays a critical role in shale-gas storage. Under the experimental conditions, differences in thermal maturity showed no significant effect on the total amount of gas sorbed. Experimental sorption isotherms could be fitted with good accuracy by the Langmuir function by adjusting the Langmuir pressure (P_L) and maximum sorption capacity (Γ_max). The lowest maturity sample (%R_o = 0.56) displayed a Langmuir pressure (P_L) of 5.15 MPa, significantly larger than the 2.33 MPa observed for the highest maturity (%R_o > 2.01) sample at 50 °C.The value of the Langmuir pressure (P_L) changes with kerogen type in the following sequence: type I > type II > type III. The thermodynamic parameters of CH₄ adsorption on organic rich shales were determined based on the experimental CH₄ isotherms. For the adsorption of CH₄ on organic rich shales and their isolated kerogen, the heat of adsorption (q) and the standard entropy (Δs⁰) range from 7.3–28.0 kJ/mol and from −36.2 to −92.2 J/mol/K, respectively.     

Synthesis and characterization of polycrystalline KAlSi3O8 hollandite [liebermannite]: Sound velocities vs. pressure to 13 GPa at room temperature

A polycrystalline specimen of liebermannite [KAlSi₃O₈ hollandite] was synthesized at 14.5 GPa and 1473 K using glass starting material in a uniaxial split-sphere apparatus. The recovered specimen is pure tetragonal hollandite [SG: I4/m] with bulk density of within 98% of the measured X-ray value. The specimen was also characterized by Raman spectroscopy and nuclear magnetic resonance spectroscopy. Sound velocities in this specimen were measured by ultrasonic interferometry to 13 GPa at room T in a uniaxial split-cylinder apparatus using Al₂O₃ as a pressure marker. Finite strain analysis of the ultrasonic data yielded K_S0 = 145(1) GPa, K₀′ = 4.9(2), G₀ = 92.3(3) GPa, G₀′ = 1.6(1) for the bulk and shear moduli and their pressure derivatives, corresponding to V_P0 = 8.4(1) km/s, V_S0 = 4.9(1) km/s for the sound wave velocities at room temperature. These elasticity data are compared to literature values obtained from static compression experiments and theoretical density functional calculations.     

Distribution of glycerol dialkyl glycerol tetraether lipids along an altitudinal transect on Mt. Xiangpi,NE Qinghai-Tibetan Plateau,China

The distribution of glycerol dialkyl glycerol tetraethers (GDGTs) can reflect continental environmental changes. Recently, the distribution of branched GDGTs (bGDGTs) has been proposed as a novel tool for paleoelevation reconstructions. Here we report the variation in TEX₈₆ (tetraether index of 86 carbon atoms) of isoprenoidal GDGTs (iGDGTs) and MBT (methylation of branched tetraether index) of bGDGTs along an altitudinal transect on Mt. Xiangpi, NE Qinghai-Tibetan Plateau. Both TEX₈₆ and MBT values of surface soils showed significant linear decreases with altitude (TEX₈₆: R² = 0.65; n = 50; MBT: R² = 0.69; n = 24). We suggest that the apparent relationships between the two indices and altitude may be related to temperature. Our preliminary investigation suggests that the TEX₈₆ index can potentially be applied as a paleoelevation indicator in addition to the MBT index on the Qinghai-Tibetan Plateau.     

Magmatic evolution of the Tuwu–Yandong porphyry Cu belt,NW China: Constraints from geochronology,geochemistry and Sr–Nd–Hf isotopes

The Tuwu–Yandong porphyry Cu belt is located in the Eastern Tianshan mountains in the eastern Central Asian Orogenic Belt. Petrochemical and geochronological data for intrusive and volcanic rocks from the Tuwu and Yandong deposits are combined with previous studies to provide constraints on their petrogenesis and tectonic affinity. New LA–ICP–MS zircon U–Pb ages of 348.3 ± 6.0 Ma, 339.3 ± 2.2 Ma, 323.6 ± 2.5 Ma and 324.1 ± 2.3 Ma have been attained from intrusive units associated with the deposits, including diorite, plagiogranite porphyry, quartz albite porphyry and quartz porphyry, respectively. The basalt and andesite, which host part of the Cu mineralization, are tholeiitic with high Al₂O₃, Cr, Ni and low TiO₂ contents, enriched LREEs and negative HFSE (Nb, Ta, Zr, Ti) anomalies consistent with arc magmas. Diorites are characterized by low SiO₂ content but high MgO, Cr and Ni contents, similar to those of high-Mg andesites. The parental magma of the basalt, andesite and diorite is interpreted to have been derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts. The ore-bearing plagiogranite porphyry is characterized by high Na₂O, Sr, Cr and Ni contents, low Y and Yb contents, low Na₂O/K₂O ratios and high Sr/Y ratios and high Mg#, suggesting an adakitic affinity. The high ε_Nd(t) (5.02–9.16), low I_Sr (0.703219–0.704281) and high ε_Hf(t) (8.55–12.99) of the plagiogranite porphyry suggest they were derived by a partial melting of the subducted oceanic crust followed by adakitic melt-mantle peridotite interaction. The quartz albite porphyry and quartz porphyry are characterized by similar Sr–Nd–Hf isotope but lower Mg# and whole-rock (La/Yb)_N ratios to the plagiogranite porphyry, suggesting they were derived from juvenile lower crust, and negative Eu anomalies suggest fractionation of plagioclase. We propose that a flat subduction that started ca. 340 Ma and resulted in formation of the adakitic plagiogranite porphyry after a period of “steady” subduction, and experienced slab rollback at around 323 Ma.     

Carbonate recrystallisation and organic matter maturation in heat-affected sediments from the Shaban Deep,Red Sea

Parasound profiles across the Shaban Deep in the Red Sea indicate turbiditic transport of surface sediments from the topographic height (basalt ridge) into the interior of the deep. This is supported by petrographical and (isotope-) geochemical evidence in the East Basin of the Shaban Deep where the presence of variable mixtures of authochtonous and allochthonous sediment compounds had been found.The uppermost 170 cm of both sediment cores 17008-1 and 17009-3 reveal “normal” stable oxygen isotope values for the planktonic foraminifera Globigerinoides ruber near ?1‰ which is indicative for carbonate formation in Red Sea surface water around 27 °C. However, below 182 cm in core 17008-1 highly variable δ ¹⁸O values for G. ruber between 0.26 and ?10.68‰ occur which are not the result of temperature-controlled oxygen isotope fractionation between foraminiferal carbonate and Red Sea surface water. The lowest δ¹⁸O values of ?10.68‰ measured for highly-altered foraminifera shells suggests carbonate precipitation higher than 90 °C.Organic petrographical observations show a great diversity of marine-derived macerals and terrigenous organic particles. Based on petrographical investigations sediment core 17008-1 can be subdivided in intervals predominantly of authochtonous character (i.e. 1, 3, 5 corresponding to core depths 0–170 cm, 370–415 cm, 69–136 cm), and allochthonous/thermally altered character (e.g. 2 and 4 corresponding to core depths 189–353 cm and 515–671 cm). Allochthonous/thermally altered material displays a wide to an extremely wide range of maturities (0.38–1.42% R_r) and also natural coke particles were found.Similarly, the organic geochemical and pyrolysis data indicate the predominance of well-preserved, immature algal and bacterial remains with a minor contribution of land plant material. Sediments below 170 cm (core 17008-1) contain contributions of re-sedimented pre-heated material most likely from the area of the basaltic ridge. This is documented by individual coke particles reduced hydrogen indices and elevated T_max values up to 440 °C.An “oil-type” contribution (evidenced by mature biomarkers, hopene/hopane ratios, elevated background fluorescence, n-alkane distribution) is also present in the sediments which most likely originated at greater depth and impregnated the surface sediments.The heat source responsible for recrystallisation of foraminiferal carbonate and maturation of organic particles in Shaban Deep sediments most likely is attributed to modern basalt extrusions which now separate the Shaban Deep subbasins.     

Genesis of the Proterozoic Mangabeira tin–indium mineralization,Central Brazil: Evidence from geology,petrology, fluid inclusion and stable isotope data

The Mangabeira deposit is the only known Brazilian tin mineralization with indium. It is hosted in the Paleo- to Mesoproterozoic Mangabeira within-plate granitic massif, which has geochemical characteristics of NYF fertile granites. The granitic massif is hosted in Archean to Paleoproterozoic metasedimentary rocks (Ticunzal formation), Paleoproterozoic peraluminous granites (Aurumina suite) and a granite–gneiss complex. The mineralized area comprises evolved Li-siderophyllite granite, topaz–albite granite, Li–F-rich mica greisens and a quartz–topaz rock, similar to topazite. Two types of greisens are recognized in the mineralized area: zinnwaldite greisen and Li-rich muscovite greisen, formed by metasomatism of topaz–albite granite and Li-siderophyllite granite, respectively. Cassiterite occurs in the quartz–topaz rock and in the greisens. Indium minerals, such as roquesite (CuInS₂), yanomamite (InAsO₄·2H₂O) and dzhalindite (In(OH₃)), and In-rich cassiterite, sphalerite, stannite group minerals and scorodite are more abundant in the quartz–topaz rock, and are also recognized in albitized biotite granite and in Li-rich muscovite greisen. The host rocks and mineralized zones were subsequently overprinted by the Brasiliano orogenic event.Primary widespread two-phase aqueous and rare coeval aqueous-carbonic fluid inclusions are preserved in quartz from the topaz–albite granite, in quartz and topaz from the quartz–topaz rock and in cassiterite from the Li-rich muscovite greisen. Eutectic temperatures are − 25 °C to − 23 °C, allowing modeling of the aqueous fluids in the system H₂O–NaCl(–KCl). Rare three-phase H₂O–NaCl fluid inclusions (45–50 wt.% NaCl equiv.) are restricted to the topaz–albite granite. Salinities and homogenization temperatures of the aqueous and aqueous-carbonic fluid inclusions decrease from the topaz–albite granite (15–20 wt.% NaCl equiv.; 400 °C–450 °C), to the quartz–topaz rock (10–15 wt.% NaCl equiv.; 250 °C–350 °C) and to the greisen (0–5 wt.% NaCl equiv.; 200 °C–250 °C). Secondary fluid inclusions have the same range of salinities as the primary fluid inclusions, and homogenize between 150 and 210 °C.The estimated equilibrium temperatures based on δ¹⁸O of quartz–mica pairs are 610–680 °C for the topaz–albite granite and 285–370 °C for the Li-rich muscovite greisens. These data are coherent with measured fluid inclusion homogenization temperatures. Temperatures estimated using arsenopyrite geothermometry yield crystallization temperatures of 490–530 °C for the quartz–topaz rock and 415–505 °C for the zinnwaldite greisens. The fluids in equilibrium with the topaz–albite granite have calculated δ¹⁸O and δD values of 5.6–7.5‰ and − 67 to − 58‰, respectively. Estimated δ¹⁸O and δD values are mainly 4.8–7.9‰ and − 60 to − 30‰, respectively, for the fluids in equilibrium with the quartz–topaz rock and zinnwaldite greisen; and 3.4–3.9‰ and − 25 to − 17‰, respectively, for the Li-rich muscovite greisen fluid. δ³⁴S data on arsenopyrite from the quartz–topaz rock vary from − 1.74 to − 0.74‰, consistent with a magmatic origin for the sulfur. The integration of fluid inclusion with oxygen isotopic data allows for estimation of the minimum crystallization pressure at ca. 770 bar for the host topaz–albite granite, which is consistent with its evolved signature.Based on petrological, fluid inclusion and isotope data it is proposed that the greisens and related Mangabeira Sn–In mineralization had a similar hydrothermal genesis, which involved exsolution of F-rich, Sn–In-bearing magmatic fluids from the topaz–albite granite, early formation of the quartz–topaz rock and zinnwaldite greisen, progressive cooling and Li-rich muscovite greisen formation due to interaction with meteoric water. The quartz–topaz rock is considered to have formed in the magmatic-hydrothermal transition. The mineralizing saline and CO₂-bearing fluids are interpreted to be of magmatic origin, based on the isotopic data and paragenesis, which has been documented as characteristic of the tin mineralization genetically related to Proterozoic within-plate granitic magmatism in the Goias Tin Province, Central Brazil.     

FTIR microspectroscopy of Ediacaran phosphatized microfossils from the Doushantuo Formation,Weng'an,South China

Phosphatized microfossils from ca. 580 Ma from the Doushantuo Formation in the Weng'an region of South China were analyzed by Fourier transform infrared (FTIR) microspectroscopy for their chemical characterization. Two morpho-types of phosphatized embryo-like fossils (Megasphaera and Megaclonophycus) were analyzed, together with algal fossils. Transmission IR spectra of the microfossils have absorption bands of around 2960 cm^− 1 and 2925 cm^− 1, indicating the presence of aliphatic hydrocarbon (anti-symmetric aliphatic CH₃ and aliphatic CH₂), and have an additional band of around 1595 cm^− 1, probably derived from aromatic moieties (aromatic CC). In addition, IR microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo-like fossils. The embryo-like fossils appear to show three types of CH₃/CH₂ peak height ratios (R_3/2) and aromatic CC/CH₂ peak height ratios (R_CC/2 values): (1) high-R_3/2/low-R_CC/2 type (R_3/2 = ~ 0.2–1.0 and R_CC/2 ~ 0–2), (2) low-R_3/2/medium-R_CC/2 type (R_3/2 = ~ 0.2–0.6 and R_CC/2 = ~ 1–4); and (3) low-R_3/2/high-R_CC/2 type (R_3/2 = ~ 0.2–0.6 and R_CC/2 ~ 1–8). All three types are contained in both Megasphaera and Megaclonophycus. Raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration, indicating that the organics were embedded in situ prior to thermal maturation. The IR spectroscopic differences might reflect differences in original organic compositions of microorganisms, and/or immediate post-mortem alteration. This suggests that the origins of phosphatized embryo-like fossils are more diverse than was previously recognized on the basis of their morphology. A comparison of R_3/2 and R_CC/2 values in the embryo-like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo-like fossils belong to animal embryo, others are algae, but none of them originate from bacteria.