Weathering rates over 40,000 years based on changes in rock properties of porous rhyolite

Weathering rates based on temporal changes in a variety of rock properties were examined for four dated lava domes made of porous rhyolite. The lava domes were erupted at 1.1, 2.6, 20 and 40 ka BP. The time from each eruptive event to the present provide elapsed weathering times. The lava domes have many cooling joints formed immediately after eruption. Joint-bounded blocks have a high porosity, resulting in deep weathering. The blocks have almost uniform characteristics from top to bottom in one outcrop. The mineralogical, chemical, physical and mechanical properties of the four rhyolites were analysed. These results show that the chemical properties and physical properties, including specific surface area, change slowly in the early stage of weathering (0–20,000 years) and quickly in the later stage (20,000–40,000 years), while mechanical strength and bulk density or porosity of the rhyolites change rapidly in the early stage and slowly in the later stage. Mechanical strength reduces by 70–90% relative to fresh rock in 40,000 years, although chemical and physical properties change by only 1023̄0 %. It is concluded that different weathering properties exhibit different rates and patterns of change over 40,000 years, and that mechanical strength is the property most susceptible to weathering.     

The dissolution kinetics of a granite and its minerals—Implications for comparison between laboratory and field dissolution rates

The present study compares the dissolution rates of plagioclase, microcline and biotite/chlorite from a bulk granite to the dissolution rates of the same minerals in mineral-rich fractions that were separated from the granite sample. The dissolution rate of plagioclase is enhanced with time as a result of exposure of its surface sites due to the removal of an iron oxide coating. Removal of the iron coating was slower in the experiment with the bulk granite than in the mineral-rich fractions due to a higher Fe concentration from biotite dissolution. As a result, the increase in plagioclase dissolution rate was initially slower in the experiment with the bulk granite. The measured steady state dissolution rates of both plagioclase (6.2 ± 1.2 × 10⁻¹¹ mol g⁻¹ s⁻¹) and microcline (1.6 ± 0.3 × 10⁻¹¹ mol g⁻¹ s⁻¹) were the same in experiments conducted with the plagioclase-rich fraction, the alkali feldspar-rich fraction and the bulk granite.Based on the observed release rates of the major elements, we suggest that the biotite/chlorite-rich fraction dissolved non-congruently under near-equilibrium conditions. In contrast, the biotite and chlorite within the bulk granite sample dissolved congruently under far from equilibrium conditions. These differences result from variations in the degree of saturation of the solutions with respect to both the dissolving biotite/chlorite and to nontronite, which probably was precipitating during dissolution of the biotite and chlorite-rich fraction. Following drying of the bulk granite, the dissolution rate of biotite was significantly enhanced, whereas the dissolution rate of plagioclase decreased.The presence of coatings, wetting and drying cycles and near equilibrium conditions all significantly affect mineral dissolution rates in the field in comparison to the dissolution rate of fully wetted clean minerals under far from equilibrium laboratory conditions. To bridge the gap between the field and the laboratory mineral dissolution rates, these effects on dissolution rate should be further studied.     

Thermal conductivity of soft Bangkok clay from laboratory and field measurements

This paper presents the results of a study on the thermal conductivity of a soft saturated clay (Bangkok clay) carried out in relation to an investigation into using thermal treatment to enhance the consolidation process of soft soils. The thermal conductivity of clay specimens was measured in the laboratory using a steady state method (divided bar test) and a transient state method (needle probe test). In general, the laboratory test results show that the thermal conductivity increased with the increase in soil density. However, the needle probe test was found to yield greater thermal conductivities than those derived from the divided bar test. Furthermore, to assess the validity of the laboratory test results, the heat transfer results obtained from a full-scale embankment test that employed prefabricated vertical thermo-drains (PVTD) were simulated numerically using the laboratory determined thermal conductivity values. The numerical analysis indicates that the field thermal conductivity was close to the value obtained from the needle probe test. However, it was also found that the changes in thermal conductivity values obtained from the two laboratory methods did not impact significantly on heat flow behaviour, suggesting that the two methods are acceptable for characterizing the thermal conductivity of soils.     

Dolomite dissolution: An alternative diagenetic pathway for the formation of palygorskite clay

Palygorskite is a fibrous, magnesium‐bearing clay mineral commonly associated with Late Mesozoic and Early Cenozoic dolomites. The presence of palygorskite is thought to be indicative of warm, alkaline fluids rich in Si, Al and Mg. Palygorskite has been interpreted to form in peritidal diagenetic environments, either as a replacement of detrital smectite clay during a dissolution–precipitation reaction or solid‐state transformation, or as a direct precipitate from solution. Despite a lack of evidence, most diagenetic studies involving these two minerals posit that dolomite and palygorskite form concurrently. Here, petrological evidence is presented from the Umm er Radhuma Formation (Palaeocene–Eocene) in the subsurface of central Qatar for an alternative pathway for palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal carbonate cycles that are commonly capped by centimetre‐scale beds rich in palygorskite. Thin section, scanning electron microscopy and elemental analyses demonstrate that palygorskite fibres formed on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to completely dissolved dolomite crystal cores. These observations suggest that dolomite and palygorskite formed sequentially, and support a model by which the release of Mg²⁺ ions and the buffering of solution pH during dolomite dissolution promote the formation of palygorskite. This new diagenetic model explains the co‐occurrence of palygorskite and dolomite in the rock record, and provides valuable insight into the specific diagenetic conditions under which these minerals may form.     

上海典型黏土小应变剪切模量现场和室内试验研究

土体小应变剪切模量在基坑、隧道等地下工程的计算分析中有着重要作用,越来越受到人们的关注。虽然上海地区地下工程项目的建设规模不断扩大,但关于上海典型黏土层的小应变剪切模量的研究相对较少。分别采用现场波速测试和室内弯曲元试验对上海典型黏土层的初始剪切模量进行了研究。试验结果表明：在减小土样扰动、保持土样应力状态与原位土体等效的基础上,弯曲元试验得到的初始剪切模量与现场波速测试相一致;考虑了土体应力状态和孔隙比的经验公式能够合理地描述上海典型黏土层初始剪切模量的变化规律,并给出了各土层的经验参数。试验结果可为上海地区地下工程计算分析中小应变剪切模量的估算提供参考。     

An experimental study of the dissolution mechanism and rates of muscovite

Steady-state muscovite dissolution rates have been measured at temperatures from 60 to 201 °C and 1 ? pH ? 10.3 as a function of reactive solution K, Si, and Al concentration. The pegmatitic muscovite used in these experiments has a composition consistent with (Na_0.09, K_0.86)Fe_0.05Al_2.92Si_3.05O₁₀(OH_1.95, F_0.06). All experiments were performed in titanium mixed-flow reactors. All experiments were performed at far-from-equilibrium conditions with respect to muscovite. All reactive solutions were undersaturated with respect to secondary product phases other than for some experiments which were supersaturated with respect to bohemite and diaspore; steady-state dissolution was stoichiometric for all experiments that were undersaturated with respect to these phases.The variation of rates with reactive solution composition depends on the solution pH. At pH ? 7 rates were found to decrease significantly with increasing reactive fluid Al activity but be independent of aqueous SiO₂ activity. pH < 7 rates measured in the present study from 60 to 175 °C are consistent with

     

The effect of oxalate on the dissolution rates of oligoclase and tremolite

The effect of oxalate, a strong chelator for Al and other cations, on the dissolution rates of oligoclase feldspar and tremolite amphibole was investigated in a flow-through reactor at 22°C. Oxalate at concentrations of 0.5 and 1 mM has essentially no effect on the dissolution rate of tremolite, nor on the steady-state rate of release of Si from oligoclase. The fact that oxalate has no effect on dissolution rate suggests that detachment of Si rather than Al or Mg is the rate-limiting step. At pH 4 and 9, oxalate has no effect on the steady-state rate of release of Al, and dissolution is congruent. At pH 5 and 7, oligoclase dissolution is congruent in the presence of oxalate, but in the absence of oxalate Al is preferentially retained in the solid relative to Si.Large transient “spikes” of Al or Si are observed when oxalate is added to or removed from the system. The cause of the spikes is unknown; we suggest adsorption on feldspar surfaces away from sites of active dissolution as a possibility. Solutions in the reactors are undersaturated with respect to both gibbsite and kaolinite, so neither the spikes nor the incongruent dissolution can be explained by formation of a secondary precipitate.The rate of dissolution of tremolite is independent of pH over the pH range 2–5, and decreases at higher pH. The rate of dissolution of oligoclase in our experiments was independent of pH over the pH range 4–9. Since the dissolution rate of these minerals is independent of pH and organic ligand concentration, the effect of acid deposition from the atmosphere on the rate of supply of cations from weathering of granitic rocks should be minor.     

第三系红层中石膏溶蚀特性及其对工程的影响

第三系红层中石膏分布比较广泛,大量的实验证明,石膏发生溶蚀的主要是含Ca2 的物质,并且溶蚀速率和与水接触的方式以及水头压力大小有直接关系。结合现场资料,将渗透系数K=10-5cm s作为石膏在第三系红层泥岩封闭下发生溶蚀的判据。同时针对水利工程中石膏溶蚀以及对砼的腐蚀性提出了相应的防治措施。     

Decoupled crystallization and eruption histories of the rhyolite magmatic system at Tarawera volcano revealed by zircon ages and growth rates

We obtained U–Th disequilibrium age data on zircons from each of the four rhyolite eruptions that built Tarawera volcano in the last 22 ka within the Okataina Volcanic Center (OVC), caldera, New Zealand. Secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near-eruption age to ~100 ka prior to eruption. Age-depth profiling of crystals reveals long periods of continuous (~34 ka) and discontinuous growth (~90 ka). Growth hiatuses of up to ~40 ka duration occur, but do not all relate to obvious resorption surfaces. Age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts. These features are best explained by periodic, complete, or partial, sub-solidus storage and/or inclusion in larger crystal phases, followed by rapid liberation prior to eruption. This is supported by high abundances of U and Th (~500 − >2,000 ppm) in some zircons consistent with periods of high crystallinity (>70%) in the magmatic system, based on crystal/melt partitioning. Contemporaneous but contrasting rim-ward trends of these elements within crystals, even in the same lava hand sample, require synchronous growth in separate melt bodies and little connectivity within the system, but also significant crystal transport and mixing prior to eruption. Many crystals record continuity of growth through the preceding ~60 ka OVC caldera-collapse and subsequent eruptions from Tarawera. This demonstrates a decoupling between eruption triggers, such as shallow crustal extension and mafic intrusion, and the crystallization state of the OVC silicic magmatic system. The data highlights the need to distinguish between the time for accumulation of eruptible magma and the long-term magma residence time based on the age of crystals with high closure temperatures, when assessing the potential for catastrophic eruptions.     

Equations for the dissolution reaction rates of montmorillonite,illite, and chlorite

The available experimental data on the dissolution kinetics of montmorillonite, illite, and chlorite were analyzed. The reliability of the data was assessed and possible reasons for discrepancies were discussed. The dissolution rates of the minerals were described by quantitative and qualitative dependencies on various parameters. The quantitative relations were described by various equations, which were proposed in other studies or derived by the author on the basis of the mathematical processing of the raw data. General equations were obtained for the dissolution rate of each of the minerals as a function of temperature, pH, and the degree of saturation of the solution. For the dependencies that were not constrained by experiments, experimental data for structurally similar minerals or theoretical equations were invoked.     

Direct comparison of UK temperatures and Greenland snow accumulation rates, 15000—12000 yr ago

A palaeotemperature record based on fossil coleopteran evidence and dated by accelerator mass spectroscopy (AMS) ¹⁴C determinations on plant macrofossils has been obtained for the last glacial-interglacial transition from the site of Gransmoor in eastern England. Calibration of the radiocarbon measurements enabled a direct comparison to be made between this palaeotemperature curve and the snow accumulation record from the GISP-2 Greenland ice-core. The similarity between the two data sets suggests a degree of correspondence in the pattern and timing of climatic change in Greenland and the British Isles at the end of the last cold stage.     

Relation between the dissolution rates of single minerals and reservoir rocks in acidified pore waters

A series of kinetic experiments has been carried out to investigate the rates of dissolution (release of Al and Si) of common sandstone minerals in response to acidification of pore waters (pH = 3), using an experimental procedure designed to maximise the proportion of solid to fluid, and to minimise possible damage from agitation. The results have then been compared with those from experiments using disaggregated sandstones from two North Sea reservoirs. Experiments were carried out at 25 °C and 80 °C and in 0.01, 0.1 and 1 M NaCl solutions, with a pH of 3. Hydrochloric acid was used as the source of acidity and rate constants were determined based on both release of Al and Si. Mineral dissolution rates were closely comparable to literature values, despite the different experimental technique, except in the case of smectite where particle aggregation appears to have inhibited reaction. The dissolution rates calculated for reservoir sandstones based on their modal mineralogy and surface areas agree within a factor of 2 with the measured vales. Based on the reaction rates measured here, reservoir rocks rich in feldspar, illite and/or smectite are likely to react most rapidly with acidified pore waters.     

An experimental study of the kinetics of lherzolite reactive dissolution with applications to melt channel formation

The kinetics of lherzolite dissolution in an alkali basalt and a basaltic andesite was examined experimentally at 1,300°C and 1 GPa using the dissolution couple method. Dissolution of lherzolite in basaltic liquids produces either the melt-bearing dunite–harzburgite–lherzolite (DHL) sequence or the melt-bearing harzburgite–lherzolite sequence depending on whether the reacting melt is or close to olivine saturation (alkali basalt) or olivine + orthopyroxene saturation (basaltic andesite). The dunite in the DHL sequence is pyroxene-free and the harzburgites in both sequences are clinopyroxene-free. The melt fraction and olivine grain size in the dunite are larger than those in the harzburgite. The olivine grain size in the dunite and harzburgite in the DHL sequence also increases as a function experimental run time. Across the sharp dunite–harzburgite and harzburgite–lherzolite interfaces, systematic compositional variations are observed in the reacting melt, interstitial melt, olivine, and to a lesser extent, pyroxenes as functions of distance and time. The systematic variations in lithology, grain size, mineral chemistry, and melt compositions are broadly similar to those observed in the mantle sections of ophiolites. The processes of lherzolite dissolution in basaltic liquids involve dissolution, precipitation, reprecipitation, and diffusive transport in the interstitial melts and surrounding minerals. Preferential dissolution of olivine and clinopyroxene and precipitation of orthopyroxene in the basaltic andesite produces the melt-bearing harzburgite–lherzolite sequence. Preferential dissolution of clinopyroxene and orthopyroxene and precipitation of olivine results in the melt-bearing DHL sequence. Preferential mineral dissolution can also affect the composition of the through-going melt in a dunite channel or harzburgite matrix. Systematic variations in melt fraction and mineral grain size in the peridotite sequences are likely to play an important role in the development of channelized or diffuse porous melt flow in the mantle.An erratum to this article can be found at     

A comparison of the fabric and permeability anisotropy of consolidated and sheared silty clay

This paper reports the geotechnical aspects of an experimental programme investigating the permeability of shear zones in clay-rich sediments. Oedometric and ring shear permeameters were used to investigate and compare permeability anisotropy in consolidated and sheared silty clay, to simulate the behaviour of wall-rock sediments and shear zone sediments respectively. In line with other studies, consolidated silty clay was found to have no significant permeability anisotropy, although clay fabric anisotropy was well developed. However, sheared silty clay showed an increase in permeability anisotropy (r_k = 3−16) with decreasing void ratio (e = 0.8−0.4), corresponding to effective stresses of 100 kPa to 4 MPa. This level of anisotropy was retained during shearing along the unloading path, and no significant dilation or enhancement of permeability was observed.     

Searching for the perfect surface area normalizing term—a comparison of BET surface area-, geometric surface area- and mass-normalized dissolution rates of anorthite and biotite

Dissolution rates were calculated for a range of grain sizes of anorthite and biotite dissolved under far from equilibrium conditions at pH 3, T = 20 °C. Dissolution rates were normalized to initial and final BET surface area, geometric surface area, mass and (for biotite only) geometric edge surface area. Constant (within error) dissolution rates were only obtained by normalizing to initial BET surface area for biotite. The normalizing term that gave the smallest variation about the mean for anorthite was initial BET surface area. In field studies, only current (final) surface area is measurable. In this study, final geometric surface area gave the smallest variation for anorthite dissolution rates and final geometric edge surface area for biotite dissolution rates.     

Operational principle,testing, and applications of the AWID-flat jack for absolute stress determinations using voltage measurements

Summary Stress measurements can be performed with the, measuring principle for a flat jack discussed in this paper without any material parameters of the flat jack being of influence. This means that no calibration measurements are required by this flat jack for absolute measurements and there is no dependence on temperature. It is called Absolut Widerstands Druckmesskissen or short AWID-Flat Jack.Basis of evaluation is a change in the electrical resistivity of the flat jack, which is caused by two metal sheets separating from each other when inflated with hydraulic oil as soon as the external pressure is reached.Besides theoretical considerations concerning the mode of operation of the flat jack, this paper presents laboratory measurements performed in an autoclave as well as in a tube filled with salt grit under a uniaxial press. Changes of stress can be measured if the flat jack is cemented into a borehole under initial stress. The absolute stress of the bedrock can be measured after a certain time of adjustment in rock capable of creep (salt, clay, etc.).The advantages of the AWID measuring system are confirmed by in situ-measurements in a salt pillar loadable with variable pressure.In a temperature experiment at the Asse salt mine (Federal Republic of Germany) where the salt was heated up to 200°C, the advantages of the AWID measurement system were confirmed.     

A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations

Systematic variations in mineralogy and chemical composition across dunite-harzburgite (DH) and dunite-harzburgite-lherzolite (DHL) sequences in the mantle sections of ophiolites have been widely observed. The compositional variations are due to melt-rock reactions as basaltic melts travel through mantle peridotite, and may be key attributes to understanding melting and melt transport processes in the mantle. In order to better understand melt-rock reactions in the mantle, we conducted laboratory dissolution experiments by juxtaposing a spinel lherzolite against an alkali basalt or a mid-ocean ridge basalt. The charges were run at 1 GPa and either 1,300°C or 1,320°C for 8–28 h. Afterward, the charges were slowly cooled to 1,200°C and 1 GPa, which was maintained for at least 24 h to promote in situ crystallization of interstitial melts. Cooling allowed for better characterization of the mineralogy and mineral compositional trends produced and observed from melt-rock reactions. Dissolution of lherzolite in basaltic melts with cooling results in a clinopyroxene-bearing DHL sequence, in contrast to sequences observed in previously reported isothermal-isobaric dissolution experiments, but similar to those observed in the mantle sections of ophiolites. Compositional variations in minerals in the experimental charges follow similar melt-rock trends suggested by the field observations, including traverses across DH and DHL sequences from mantle sections of ophiolites as well as clinopyroxene and olivine from clinopyroxenite, dunite, and wehrlite dikes and xenoliths. These chemical variations are controlled by the composition of reacting melt, mineralogy and composition of host peridotite, and grain-scale processes that occur at various stages of melt-peridotite reaction. We suggest that laboratory dissolution experiments are a robust model to natural melt-rock reaction processes and that clinopyroxene in replacive dunites in the mantle sections of ophiolites is genetically linked to clinopyroxene in cumulate dunite and pyroxenites through melt transport and melt-rock reaction processes in the mantle.     

Kinetics of dissolution of mechanically comminuted rock-forming oxides and silicates—II. Deformation and dissolution of oxides and silicates in the laboratory and at the Earth's surface

Comparison of the patterns of fracture under tensile stress, indentation, and scratching of periclase. quartz, and corundum indicates that the properties relevant to dissolution of rock-forming oxides and of rock-forming non-layer silicates should be changed by mechanical comminution in essentially the same way as those of quartz. The changes are accomplished by brittle fracture under the tensile component of the stress field, which does not generate subsurface damage, and by microplastic behavior under local stresses with high net compressive and shear components, which does.Mechanical comminution should therefore affect the apparent rates of dissolution (rates calculated with respect to the initial interface area) of rock-forming oxides and of rock-forming non-layer silicates in essentially the same way in which it affects the apparent rate of dissolution of quartz. This is supported by the available evidence on the effect of dry grinding on the kinetics of dissolution of feldspars, pyroxenes, and olivines in aqueous solutions.Different effects of mechanical comminution on solubilities and dissolution rate constants can be related to certain measured or calculated properties of the considered minerals. Notably, the effect of grain size on the dissolution rate constant can be rigorously related to the Kelvin effect.The available evidence on the mechanical comminution at the bases of dry-based glaciers in highgradient segments of streams, in certain high-energy coastal and epeiric environments, and in sandy deserts indicates that such mechanical comminution should significantly affect the simultaneous or subsequent dissolution of the comminuted material.