Variation with depth of the stress tensor anisotropy inferred from microfault analysis

In order to study the Pyrenean tectonic phase, a quantitative method of stress analysis using microfault measurements is used on a calcareous plateau located in southern France. The method developed here allows the determination of several tectonic events and the evaluation of (with σ σ₂ σ₃).The Pyrenean compression is seen to occur in two stages, confirming previous geological studies.A recent canyon allows the study of the variation with depth of the R ratio on a vertical cross-section (300 m). With a simple model of gravity and tectonic stresses, the vertical variation of R can be used to estimate quantitatively the Pyrenean tectonic stresses. For realistic values of the parameters in this model, the horizontal tectonic stresses are obtained in the following range: 50–200 bar for the maximum horizontal principal stress, 10–25 bar for the other horizontal stress.These results seem to be consistent with in situ stress measurements, but they are much lower than those predicted by experimental rock mechanics.     

Constraining stress magnitudes using petroleum exploration data in the Cooper–Eromanga Basins, Australia

The magnitude of the in situ stresses in the Cooper–Eromanga Basins have been determined using an extensive petroleum exploration database from over 40 years of drilling. The magnitude of the vertical stress (S_v) was calculated based on density and velocity checkshot data in 24 wells. Upper and lower bound values of the vertical stress magnitude are approximated by S_v = (14.39 × Z)^1.12 and S_v = (11.67 × Z)^1.15 functions respectively (where Z is depth in km and S_v is in MPa). Leak-off test data from the two basins constrain the lower bound estimate for the minimum horizontal stress (S_hmin) magnitude to 15.5 MPa/km. Closure pressures from a large number of minifrac tests indicate considerable scatter in the minimum horizontal stress magnitude, with values approaching the magnitude of the vertical stress in some areas. The magnitude of the maximum horizontal stress (S_Hmax) was constrained by the frictional limits to stress beyond which faulting occurs and by the presence of drilling-induced tensile fractures in some wells. The maximum horizontal stress magnitude can only be loosely constrained regionally using frictional limits, due to the variability of both the minimum horizontal stress and vertical stress estimates. However, the maximum horizontal stress and thus the full stress tensor can be better constrained at individual well locations, as demonstrated in Bulyeroo-1 and Dullingari North-8, where the necessary data (i.e. image logs, minifrac tests and density logs) are available. The stress magnitudes determined indicate a predominantly strike-slip fault stress regime (S_Hmax > S_v > S_hmin) at a depth of between 1 and 3 km in the Cooper–Eromanga Basins. However, some areas of the basin are transitional between strike-slip and reverse fault stress regimes (S_Hmax > S_v ≈ S_hmin). Large differential stresses in the Cooper–Eromanga Basins indicate a high upper crustal strength for the region, consistent with other intraplate regions. We propose that the in situ stress field in the Cooper–Eromanga Basins is a direct result of the complex interaction of tectonic stresses from the convergent plate boundaries surrounding the Indo-Australian plate that are transmitted into the center of the plate through a high-strength upper crust.     

The importance of the effective intermediate principal stress (σ′2) to fault slip patterns

In normal faulting regimes, the magnitudes and orientations of the maximum and minimum principal compressive stresses may be known with some confidence. However, the magnitude of the intermediate principal compressive stress is generally much more difficult to constrain and is often not considered to be an important factor. In this paper, we show that the slip characteristics of faults and fractures with complex or nonoptimal geometry are highly sensitive to variation or uncertainty in the ambient effective intermediate principal stress (σ′₂). Optimally oriented faults and fractures may be less sensitive to such variations or uncertainties. Slip tendency (T_s) analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses and in any stress regime, thereby focusing efforts on the most important components of the system. We also show, for a normal faulting stress regime, that the proportion of potential surfaces experiencing high slip tendency (e.g., T_s ≥ 0.6) decreases from a maximum of about 38% where σ′₂ = σ′₃, to a minimum of approximately 14% where σ′₂ is halfway between σ′₃ and σ′₁, and increases to another high of approximately 29% where σ′₂ = σ′₁. This analysis illustrates the influence of the magnitude of σ′₂ on rock mass strength, an observation previously documented by experimental rock deformation studies. Because of the link between fault and fracture slip characteristics and transmissivity in critically stressed rock, this analysis can provide new insights into stress-controlled fault transmissivity.     

CO2 and sulfuric acid controls of weathering and river water composition

Reactions of CO₂ with carbonate and silicate minerals in continental sediments and upper part of the crystalline crust produce HCO₃⁻ in river and ground waters. H₂SO₄ formed by the oxidation of pyrite and reacting with carbonates may produce CO₂ or HCO₃⁻. The ratio, ψ, of atmospheric or soil CO₂ consumed in weathering to HCO₃⁻ produced depends on the mix of CO₂ and H₂SO₄, and the proportions of the carbonates and silicates in the source rock. An average sediment has a CO₂ uptake potential of ψ = 0.61. The potential increases by inclusion of the crystalline crust in the weathering source rock. A mineral dissolution model for an average river gives ψ = 0.68 to 0.72 that is within the range of ψ = 0.63 to 0.75, reported by other investigators using other methods. These results translate into the CO₂ weathering flux of 20 to 24 × 10¹²mol/yr.     

3-D velocity structure of the 2003 Bam earthquake area (SE Iran): Existence of a low-Poisson's ratio layer and its relation to heavy damage

To understand the generation mechanism of the Bam earthquake (Mw 6.6), we studied three-dimensional V_P, V_S and Poisson's ratio (σ) structures in the Bam area by using the seismic tomography method. We inverted accurate arrival times of 19490 P waves and 19015 S waves from 2396 aftershocks recorded by a temporal high-sensitivity seismic network. The 3-D velocity structure of the seismogenic region was well resolved to a depth of 14 km with significant velocity variations of up to 5%. The general pattern of aftershock distribution was relocated by using the 3-D structure to delineate a source fault for a length of approximately 20 km along a line 4.5 km west of the known geological Bam fault; this source fault dips steeply westward and strikes a nearly north–south line. The main shallow cluster of aftershocks south of the city of Bam is distributed just under the minor surface ruptures in the desert. The 3-D velocity structure shows a thick layer of high V_S and low σ (minimum: 0.20) at a depth range of 2–6 km. The deeper layer, with a thickness of about 2 km, appears to have a low V_S and high σ (maximum: 0.28) from 6 km depth beneath Bam to a depth of 9 km south of the city. The inferred increase of Poisson's ratio from 2 to 10 km in depth may be associated with a change from rigid and SiO₂-rich rock to more mafic rock, including the probable existence of fluids. The main seismic gap of aftershock distribution at the depth range of 2 to 7 km coincides well with the large slip zone in the shallow thick layer of high V_S and low σ. The large slip propagating mainly in the shallow rigid layer may be one of the main reasons why the Bam area suffered heavy damage.     

In Situ Stress Measurements in the Lhasa Terrane, Tibetan Plateau, China

Tectonic activities are frequent in the Lhasa terrane because of the ongoing collision between the India and Eurasia plates. Knowledge of the stress state is critical to evaluate the crustal stability and the design of underground excavations. Because of the limitations imposed by natural conditions,little research has been performed on the present crustal in situ stress in the Tibetan Plateau,and further study is imperative. In this study,hydraulic fracturing measurements were conducted in Nyching County(LZX) and Lang County(LX),Lhasa terrane to characterize the shallow crustal stress state. The results indicate that the stress state in the LZX borehole is markedly different from that in the LX borehole,in both magnitude and orientation. At the same measurement depths,the magnitudes of horizontal principal stresses in the LX borehole are 1.5–3.0 times larger than those in the LZX borehole. The stress regime in the LX borehole favors reverse faulting characterized by SHShSv,where SH,Sh,and Sv are maximum horizontal,minimum horizontal,and vertical principal stresses,respectively. The SH and Sh values are approximately three and two times greater than Sv. Fracture impression results reveal that SH in the LX borehole are predominantly N–S,while in the LZX borehole the maximum horizontal principal stress is mainly in the NNE-direction. The heterogeneity of the regional stress state might be a result of the population and distribution of local structures and seismic activities. The stress state in the LX borehole has exceeded the critical state of failure equilibrium,and there is an optimally orientated pre-existing fault near the borehole. It can be concluded that the optimally orientated fault is likely to be active when the stress has built up sufficiently to destroy the frictional equilibrium; it is suggested that research focus should be placed on this in future. The stress states in boreholes LZX and LX indicate uniformity of the regional stress field and diversity of the local stress fields resulting from the interactions among regional dynamic forces,tectonic stress field,and geological structures.     

Quantification of Aperture and Relations Between Aperture,Normal Stress and Fluid Flow for Natural Single Rock Fractures

Accurate quantification of rock fracture aperture is important in investigating hydro-mechanical properties of rock fractures. Liquefied wood’s metal was used successfully to determine the spatial distribution of aperture with normal stress for natural single rock fractures. A modified 3D box counting method is developed and applied to quantify the spatial variation of rock fracture aperture with normal stress. New functional relations are developed for the following list: (a) Aperture fractal dimension versus effective normal stress; (b) Aperture fractal dimension versus mean aperture; (c) Fluid flow rate per unit hydraulic gradient per unit width versus mean aperture; (d) Fluid flow rate per unit hydraulic gradient per unit width versus aperture fractal dimension. The aperture fractal dimension was found to be a better parameter than mean aperture to correlate to fluid flow rate of natural single rock fractures. A highly refined variogram technique is used to investigate possible existence of aperture anisotropy. It was observed that the scale dependent fractal parameter, K _v, plays a more prominent role than the fractal dimension, D _a1d, on determining the anisotropy pattern of aperture data. A combined factor that represents both D _a1d and K _v, D _a1d × K _v, is suggested to capture the aperture anisotropy.     

雪峰山深孔水压致裂地应力测量及其意义

利用最新研制的深孔水压致裂地应力测量设备在雪峰山2000 m科钻先导孔内开展了原地应力测量,在孔深170~2021 m范围内获得了16个测段的有效地应力测量数据,是国内首次利用水压致裂法获得的孔深超过2000 m深度的原地应力测量成果。测量结果表明,地应力随孔深增加而逐渐加大,对实测数据进行线性回归,得到最大和最小水平主应力随深度变化的关系分别为:S_H=0.03328H+5.25408,S_h=0.0203H+4.5662,在孔深2021 m深度,其实测值分别为66.31 MPa和43.33 MPa。基于实测数据,结合钻孔成像测试和井温测试结果,对测点应力状态进行了综合分析。在170~800 m深度范围,三向主应力关系为S_H > S_h > S_v,有利于逆断层活动;孔深1000~2021 m表现为S_H > S_v > S_h,表明该区域深部应力结构属于走滑型。最大水平主应力方向为北西-北西西方向。基于实测地应力数据及莫尔-库伦破裂准则,对测区附近断层活动性进行了分析讨论,认为该区域断层处于稳定状态。      

Triaxial stress state deep in orogenic belts: an example from Turkey

A rare metachert pebble containing amphibole grains with microboudin structures in a wide range of orientations provides an opportunity to perform stress analysis in two orthogonal orientations on the foliation surface. The sample was analysed by the microboudin method to infer the triaxial stress state during microboudinage. Stress parameters proportional to the far-field differential stress were determined for sodic amphibole grains in the two orientations. The ratio of the stresses in the two orthogonal orientations (σ₁−σ₂)/(σ₁−σ₃) was calculated to be 0.64, indicating that σ₂ lies closer to the midpoint between σ₁ and σ₃ than to σ₃.     

In Situ Stresses in Borehole Blanche-1/South Australia Derived from Breakouts,Core Discing and Hydraulic Fracturing to 2 km Depth

The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of S _v ≤ S _h < S _H and a consistent East–West orientation of maximum horizontal compression in agreement with existing stress data for Australia. The minor horizontal stress S _h derived from the hydraulic fracturing closure pressure values is about equal to the overburden stress and may be regarded as most reliable.     

A method for determining both diffusion and sorption coefficients of rock medium within a few days by adopting a micro-reactor technique

Migration properties characterized by physico-chemical factors such as distribution coefficient (K_d) and diffusion coefficient (D_e) are of great concern in performance assessment of high-level radioactive waste disposal in a deep geologic environment. These coefficients are normally obtained with different sample geometries using conventional methods, i.e., crushed samples by the batch sorption method for K_d determination and block samples by the through-diffusion method for D_e. A size dependence on both K_d and D_e has been reported and an additional correction due to size difference is required to maintain consistency of the data set. A fast method was developed, hereafter referred to as the micro-channel method, to determine both the sorption coefficient (R_d) and D_e using non-crushed rock sample by adopting the micro-reactor technique. In this method, a radionuclide solution is injected into a micro-channel (20 mm length, 4 mm width, 160 μm depth), which is in contact with a plate-shaped rock sample. A part of the injected radionuclide can diffuse into the rock matrix and/or adsorb on the rock surface and this results in an inlet-outlet concentration difference. A breakthrough curve is easily obtained with a short observation period because the injection amount is extremely small and is comparable to that escaping by diffusion into the matrix. The breakthrough curve is analyzed by a two-dimensional diffusion-advection equation to evaluate R_d and D_e.In the present study, tritiated water (specific activity, 1.2 × 10⁴ Bq/mL; pH, 6) was injected into the micro-channel, and the breakthrough curve of ³H obtained. A series of experiments was carried out by changing the flow rate of the tritiated water (2.6 × 10⁻⁵–7.7 × 10⁻⁴ m/s). Rock samples were biotite granite from the Makabe area, Japan. The diffusion coefficient evaluated by least squares fitting to the numerical solutions (D_e = 1.5 × 10⁻¹¹ m²/s) agreed well with that obtained by the through-diffusion method (1.3 × 10⁻¹¹ m²/s). The breakthrough curve of Cs ([Cs] = 1.0 × 10⁻⁷ mol/L, pH 6) labeled with ¹³⁴Cs (specific activity adjusted to 4.9 × 10¹ Bq/mL) was also obtained. A nearly constant R_d value (5.5 × 10⁻² m³/kg) was found when the flow rate was less than 2.5 × 10⁻⁴ m/s. This implied that the sorption equilibrium is reached and K_d is obtained by the present method. This value was almost identical to K_d obtained by the batch sorption method (5.0 × 10⁻² m³/kg), but the testing period was very different; 1 day and 7 days, respectively. It is concluded that application of the micro-channel method provided advantages when compared with the conventional methods.     

Fault slip analysis of Quaternary faults in southeastern Korea

The Quaternary stress field has been reconstructed for southeast Korea using sets of fault data. The subhorizontal direction of the maximum principal stress (σ₁) trended ENE and the direction of the minimum principal stress (σ₃) was nearly vertical. The stress ratio (Φ = (σ₂ − σ₃) / (σ₁ − σ₃)) value was 0.65. Two possible interpretations for the stress field can be made in the framework of eastern Asian tectonics; (1) The σ_Hmax trajectory for southeast Korea fits well with the fan-shaped radial pattern of maximum principal stress induced by the India–Eurasia collision. Thus, we suggest that the main source for this recent stress field in southeast Korea is related to the remote India–Eurasia continental collision. (2) The stress field in Korea shows a pattern similar to that in southwestern Japan. The origin for the E–W trending σ_Hmax in Japan is known to be related to the mantle upwelling in the East China Sea. Thus, it is possible that Quaternary stress field in Korea has evolved synchronously with that in Japan. We suggest further studies (GPS and in situ stress measurement) to test these hypotheses.     

Stress state analysis of a collapsed overhanging rock slab: A case study

This paper describes the geomechanical back-analysis of the flexural failure of an overhanging limestone slab (volume = 68 m³) that collapsed on 26/01/1999 on the abandoned Cellina Valley state road SS 251 (western Friuli, NE Italy). The survey carried out on site examining the contact surfaces between the block and the slope has ascertained that the rock slab was connected to the stable rock mass by means of a single great rock bridge. The detailed reconstruction of the slab geometry and of the actual restraint conditions acting before the rockfall permitted an estimation of the stress state by using a 3D finite element code (Strand7). The finite element numerical model, referred to the restrained cross-section measured on site (A_res = 2.82 m²), gives a maximum value of σ_t = 5.19 MPa for the tensile stress, which is in good agreement with the estimated value for the characteristic strength mobilized at rupture by the intact rock material (T₀ = 3.5–5.5 MPa). Considerably lower maximum tensile stress (σ_t = 2.25 MPa), smaller than one half, is obtained if the whole contact surface is assumed as resisting surface (A_res = 10.50 m²) and the rock bridge is neglected.     

Solid inclusions of magmatic halite and sylvite in felsic granitoids, Sierra Norte, Córdoba, Argentina

This study provides evidence for the existence of halite and sylvite solid inclusions in igneous quartz and feldspars, the first to be reported in intrusive rocks, and to partially constrain the physicochemical environment that lets halides crystallize under magmatic conditions.Halite and sylvite solid inclusions were found included in quartz and feldspars from a micrographic–granophyric assemblage in a miarolitic aplite and, rarer, in alkali-feldspar from a miarolitic monzogranite. Monzogranite and aplite represent I-type, K-enriched postcollisional rocks of the Late Cambrian–Early Ordovician Sierra Norte–Ambargasta batholith in the Eastern Sierras Pampeanas. Both granitoids fall among the most evolved felsic rocks of the batholith, with aplite approaching haplogranitic compositions. Halite is far more common than sylvite and the presence and distribution of one or both halides are erratic within the felsic intrusive bodies. Halides occur as small skeletal grains, commonly in cross-shaped aggregates of less than 50 μm. No K or Na was found at the detection limits of EDS in either halite or sylvite respectively. Textural relationships suggest that the alkali-chlorides separated from the melt near the minima along the quartz–feldspar cotectics of P_H2O > 160 < 200 MPa in a silica-, and potassium-rich magmatic system at approximately 750–700 °C, prior to the H₂O-vapor saturated miarole-forming stage.Computed ratios for the magmatic volatile phase (MVP) coexisting with melt at the early stage of aplite crystallization are: NaCl/HCl = 0.11–0.97 and KCl/HCl = 0.24–1.62, being the highest range of values (0.79–0.97 and 1.45–1.62, respectively) found in those alkali-chloride-bearing samples. Maximum HCl/ΣCl_(MVP) (0.28 to 0.31) indicates higher total Cl concentration in the MVP of alkali-chloride-bearing aplites, which is much higher in the halite-free aplite samples (HCl/ΣCl_(MVP) = 0.59 to 0.74). One miarolitic monzogranite sample, where halite solid inclusions are present, also yielded the highest ratios for NaCl/HCl_(MVP) (0.91) and KCl/HCl_(MVP) (1.46), and the HCl/ΣCl_(MVP) is 0.30. A high HCl concentration in the fluid phase is suggested by the log f(HF)/f(H₂O) = − 4.75 to − 4.95, log f(HCl)/f(H₂O) = − 3.73 to − 3.86, and log f(HF)/f(HCl) = − 0.88 to − 1.22, computed at 750 °C after biotite composition. The Cl concentrations at 800 °C, computed with a D^v/l_Cl = 0.84 + 26.6P (P at 200 MPa), yielded values within the range of  70 to 700 ppm Cl in the melt and  4000 to 40 000 ppm Cl in the coexisting MVP. The preferential partitioning of Cl in the vapor phase is controlled by the D^v/l_Cl; however, the low concentration of Cl in the melt suggests that high concentrations of Cl are not necessary to saturate the melt in NaCl or KCl.Cl-saturation of the melt and coexisting MVP might have been produced by a drop in Cl solubility due to the near-haplogranitic composition of the granitoids after extreme fractionation, probably enhanced by fluctuating reductions of the emplacement pressure in the brittle monzogranite host. Liquid immiscibility, based in the differential viscosity and density among alkali-chloride saturated hydrosaline melt, aluminosilicate felsic melt, and H₂O-rich volatiles is likely to have crystallized halite and sylvite from exsolved hydrosaline melt. High degrees of undercooling might have been important at the time of alkali-chloride exsolution. The effectiveness of alkali-chloride separation from the melt at magmatic temperatures is in line with the interpretation of “halite subtraction” as a necessary process to understand the origin of the “halite trend” in highly saline fluid inclusions from porphyry copper and other hydrothermal mineralizations, despite the absence of the latter in the Cerro Baritina aplites, where this process preceded the exsolution of halite-undersaturated fluids.Pervasive alteration of the monzogranite country rock as alkali-metasomatic mineral assemblages, the mineral chemistry of some species, and the association of weak molybdenite mineralization are compatible with the activity of alkaline hypersaline fluids, most likely exsolved during the earliest stages of aplite consolidation.     

Reaction mechanisms,physical conditions,and mass transfer during hydrothermal alteration of mica and feldspar in granitic rocks from south-central Maine,USA

Samples of granitic rock from south-central Maine contain primary igneous minerals altered by hydrothermal fluids. The reaction mechanisms (by which the over-all mineralogical change during the alteration was accomplished) involve several different mineral-fluid reactions at different reaction sites in the rock. The reactions involve both molecular and charged species in solution. The different reaction sites correspond to alteration of different primary igneous minerals. Biotite is partially converted to chlorite+sphene; microcline to muscovite; plagioclase to various combinations of muscovite, epidote, and calcite. The different reaction sites are linked by exchange of ions: some reaction sites produce ions consumed at other sites and vice versa. Physical conditions during the hydrothermal event are estimated from mineralogical and thermochemical data: P = 3,500 (±300) bars; T =425 ° (± 25 °)C. The fluid was characterized by X _{CO 2} = 0–0.13; ln([K⁺]/[H⁺ ]) = 10.0; ln([Ca²⁺]/[H⁺]²)=9.1; ln([Na⁺]/[H⁺]) = 10.5; Fe/(Fe+Mg) = 0.95. Amounts of secondary minerals in altered rock, when compared to the inferred mineral reactions that formed them, indicate that small but significant amounts (0.01–0.3mol/ 1,000cm³ altered rock) of CO₂, H₂O, H⁺, and K⁺ were added to the granites by fluids during the alteration, as well as lesser amounts (< 0.01–0.03 mol/1,000cm³ altered rock) of Mg²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Na⁺, and Ti⁴⁺. The sole element leached from the granitic rocks during alteration was Ca in amounts 0.1–0.3 mol/1,000 cm³ rock. By estimating the composition of the hydrothermal fluids before and after reaction with the granites and by measuring the amount of material added to or subtracted from the granites during the alteration, the amount and volume of hydrothermal fluid involved can be calculated. Two independent calculations require minimum volumes in the range 100–1,000 cm³ fluid/1,000cm³ altered rock to participate in the hydrothermal event.     

Solution mechanisms of silicate in aqueous fluid and H2O in coexisting silicate melts determined in-situ at high pressure and high temperature

The structure of H₂O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na₂O·4SiO₂–H₂O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H₂O at nominal densities, ρ_fluid, (from EOS of pure H₂O) of 0.90 and 0.78 g/cm³. With ρ_fluid = 0.78 g/cm³, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρ_fluid = 0.90 g/cm³, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na₂O·4SiO₂ starting material.Molecular H₂O (H₂O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H₂O)-ratio is positively correlated with temperature. The OH/(H₂O)° in melts is greater than in coexisting fluids. Structural units of Q³, Q², Q¹, and Q⁰ type are observed in all phases under all conditions. An expression of the form, 12Q³ + 13H₂O2Q² + 6Q¹ + 4Q⁰, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.     

Magnetic fabrics and rock magnetism of Archaean and Proterozoic dike swarms in the southern São Francisco Craton, Brazil

Magnetic fabric and rock magnetism studies were performed on three mafic dike swarms (total of 38 dikes) from the southernmost part of the São Francisco Craton (SFC) (Minas Gerais State, SE Brazil). They cut Archaean granite–gneiss–migmatite and paleoprototerozoic terranes. These swarms are classified as basic–noritic (Sm–Nd age  2.65 Ga), basic (Rb–Sr age  1.87 Ga) and metamorphic (Rb–Sr age  1.87 Ga) suites, in which the second is the most important. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). In most sites magnetic susceptibility is dominantly carried by ferromagnetic minerals, however, in some sites the paramagnetic contribution exceeds 70% of bulk susceptibility. Mainly coarse to fine-grained Ti-poor titanomagnetite up to pure magnetite carry the magnetic fabrics.Three primary AMS fabrics are recognized which are all coaxial with the AARM fabric. Normal AMS fabric is dominant in the basic suite (16 of 20 analyzed dikes) and occurs in 4 and 3 dikes from the basic–noritic and metamorphic suites, respectively. This fabric is interpreted as a result of magma flow in which the analysis of K_max inclination permitted to infer that the majority of dikes were fed by inclined flows (30° < K_max < 60°), although 44% of dikes from the basic suite were fed by horizontal or sub-horizontal flows (K_max < 30°). Intermediate AMS fabric was found in 50% of dikes from the basic–noritic and metamorphic suites, but in only 2 dikes from the basic suite. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. Inverse AMS fabric is a minority (2 dikes from each suite). The parallelism between AMS and AARM tensors for dikes with abnormal fabrics suggests a primary origin for them. Gyroremanent magnetization (GRM) effect was negligible for the majority of dikes, but it was found in two dikes from the basic suite with normal AMS fabric.Magnetic fabrics recognized for the three studied swarms do not depend on magnetic mineralogy, geochemical composition, dike strikes, nor the age of the swarms since the same magnetic minerals and magnetic fabric types are found in dikes from all suites. Inclined and horizontal flows allow us to infer the relative position of at least three magma sources (or magma chambers) from which the dikes were fed.     

Magnitude of σ1, σ2, and σ3 at mid-crustal levels in an orogenic belt: Microboudin method applied to an impure metachert from Turkey

We revised an equation for estimating palaeostress magnitude using the microboudin technique by incorporating the influence of time on the fracture strength of minerals. The equation was used to estimate triaxial palaeostresses from a rare sample of metachert from Turkey that contains microboudinaged, columnar tourmaline grains in a wide range of orientations within the foliation plane. The estimated principal palaeostresses are σ₁ = 605 MPa, σ₂ = 598 MPa, and σ₃ = 597 MPa. As the microboudinage is considered to have occurred immediately before the rock encountered the brittle-plastic transition during exhumation, these stress values correspond to conditions at approximately 18 km depth and 300 °C within a Cretaceous orogenic belt.     

Lithogenic concentrations of trace metals in soils and saprolites over crystalline basement rocks: A case study from SW Nigeria

In this study, an assessment of the lithogenic concentrations of trace metals in soils and saprolite over basement rock units in Ibadan, SW-Nigeria is presented in respect of bedrock types and geochemical controls on the weathering-associated release of trace metals. Consequently, soil, weathered and fresh rock samples from the Precambrian Basement of SW Nigeria were collected from three different bedrock units within Ibadan metropolis and subjected to mineralogical and geochemical analyses. The analytical results revealed major proportions of oxides in the range of 18–20% Al₂O₃, 2–6% Na₂O and 1–6% K₂O for weathered profiles over granite-gneiss and pegmatite units, compared to 2–3% Al₂O₃, <0.5% Na₂O and <1.0% K₂O over schist-quartzite. For the trace elements, weathered profiles on granite-gneiss and schist-quartzite settings exhibit similar enrichment trends (enrichment factor, EF l) for most of the trace elements, unlike the pegmatite bedrock. However, enrichments are relatively greater in the top soil unit compared to the intermediate saprolite unit, especially for Pb, Ni, Zn, Cr, Co, Rb, Sr and Ba, a situation attributed to leaching and redistribution within the weathered profiles through pedogenetic process and percolating groundwater.Furthermore, the estimated weathering indices using Ruxton Ratio (RR = {SiO₂/Al₂O₃}) and Chemical Index of Alteration (CIA = 100{Al₂O₃/[Al₂O₃ + CaO + Na₂O + K₂O]}) revealed RR of 2.9–3.7 and CIA of 54–73% for granite-gneiss and pegmatite units, implying medium levels of weathering, compared to RR of 30.8–35.5 and CIA of >60% for schist-quartzite units, which suggest weak chemical weathering. Also, the estimated high percentage loss, especially for Pb, Rb, Sr, Ba relative to the bedrocks, shows that the trace elements can be mobilized within the weathering profiles even at a low degree of chemical weathering. Such weathering-induced release of trace metals is of environmental significance as natural lithogenic input sources and as background reference for future monitoring of possible human/anthropogenic impacts.     

Chemistry of fluids from a natural analogue for a geological CO2 storage site (Montmiral, France): Lessons for CO2–water–rock interaction assessment and monitoring

Chemical and isotope studies of natural CO₂ accumulations aid in assessing the chemical effects of CO₂ on rock and thus provide a potential for understanding the long-term geochemical processes involved in CO₂ geological storage. Several natural CO₂ accumulations were discovered during gas and oil exploration in France’s carbogaseous peri-Alpine province (south-eastern France) in the 1960s. One of these, the Montmiral accumulation at a depth of more than 2400 m, is currently being exploited. The chemical composition of the water collected at the wellhead has changed in time and the final salinity exceeds 75 g/L. These changes in time can be explained by assuming that the fraction of the reservoir brine in the recovered brine–CO₂–H₂O mixture varies, resulting in variable proportions of H₂O and brine in the sampled water. The proportions can be estimated in selected samples due to the availability of gas and water flowrate data. These data enabled the reconstruction of the chemical and isotope composition of the brine. The proportions of H₂O and brine can also be estimated from isotope (δ²H, δ¹⁸O) composition of collected water and δ¹⁸O of the sulfates or CO₂. The reconstituted brine has a salinity of more than 85 g/L and, according to its Br⁻ content and isotope (δ²H, δ¹⁸O, δ³⁴S) composition, originates from an evaporated Triassic seawater that underwent dilution by meteoric water. The reconstitution of the brine’s chemical composition enabled an evaluation of the CO₂–water–rock interactions based on: (1) mineral saturation indices; and (2) comparison with initial evaporated Triassic seawater. Dissolution of K- and SO₄-containing minerals such as K-feldspar and anhydrite, and precipitation of Ca and Mg containing minerals that are able to trap CO₂ (carbonates) are highlighted. The changes in concentration of these elements in the brine, which are attributed to CO₂ interactions, illustrate the relevance of monitoring the water quality at future industrial CO₂ storage sites.