The weathering of stones due to dissolution

We hypothesize that the weathering of building stones can be attributed to surface dissolution processes. We assume that chemical interactions occur on grain boundaries and/or microcracks and that diffusion is the controlling process. A dissolution layer (rind) develops adjacent to the weathering surface. We quantify the extent of dissolution by introducing a damage variable f; f=0 for pristine rock, and when f=1 the rock disintegrates. We assume that the variations of the damage variable are given by the diffusion equation. We solve two problems. The first is for the structure of the transient dissolution boundary layer prior to surface disintegration. We find an incubation time t_i, before active weathering (disintegration) begins. The second is the solution for steady-state weathering with a constant weathering velocity v_w. Our results are entirely consistent with weathering studies on Carrara marble gravestones in the United Kingdom. Typical incubation times are t_i=20–30 years, and typical steady-state weathering velocities are v_w=5–50 m year^–1.     

浙江玄武岩风化壳红土磁性特征及其对成土过程的响应

以位于浙江磐安县的玄武岩风化壳红土（XZ剖面）为研究对象,对其进行系统环境磁学分析,结合常量元素、漫反射光谱、有机质等分析手段,研究玄武岩风化壳红土的磁性特征,并探讨磁性特征与风化成土过程之间的关系。结果表明：在风化成土初期,玄武岩风化壳红土磁性较强,具有较高的磁化率（χ）和饱和等温剩磁（SIRM）,磁性矿物主要为原生多畴（MD）颗粒亚铁磁性矿物（磁铁矿、钛磁铁矿）,磁性特征继承了母岩特性;随着风化成土作用的增强,土壤中的原生亚铁磁性矿物逐渐转化为次生不完全反铁磁性矿物（赤铁矿、针铁矿等）,导致土壤磁性降低,但细颗粒磁性矿物逐渐增加;强风化成土阶段,原生亚铁磁性矿物进一步减少,反铁磁性矿物仍表现出一定高值,同时大量成土成因的细颗粒次生强磁性矿物（SP颗粒磁铁矿、磁赤铁矿）逐渐生成,导致土壤χ持续升高。本研究显示玄武岩风化壳红土的磁学特征与风化成土过程具有紧密的关系,磁性参数χ_fd%可作为亚热带地区玄武岩风化壳红土成土过程和成土强度的指示指标。

     

The impact of pyrite variability, dispersive transport and precipitation of secondary phases on the sulphate release due to pyrite weathering

The objective of this study was to investigate the impact of flow, transport and geochemical parameters in the unsaturated and saturated zones on the release of SO₄ from overburden lignite spoil piles into the adjacent lake. A vertical one-dimensional model was set up using the reactive transport simulator SULFIDOX in order to account for the unsaturated zone. The SULFIDOX model was calibrated for effective diffusion using measured O₂ in the gas phase and SO₄ concentrations in the liquid phase from the unsaturated zone of the heap. The results show high sensitivity to O₂ supply and initially present gypsum, but the inclusion of secondary mineral precipitation in equilibrium is of minor importance for the results. To account for the transport of released SO₄ from the saturated zone into the surface water, scenarios were performed by using SULFIDOX results as input concentration for a two-dimensional vertical model set up with PROCESSING MODFLOW and MT3D. These scenarios indicate a rising discharge of SO₄ into the adjacent lake due to continued pyrite weathering for 80 a. Results are highly sensitive to dispersivity, whereas the spatial variability of pyrite distribution did not show any influence on the results. The consideration of initially present gypsum shows a major effect on the modelled SO₄ release.     

Secular change of permeability in the fracture zone near the Nojima fault estimated using strain changes due to water injection experiments

Water injection experiments were performed in 1997, 2000 and 2003 at the 1800 m borehole near the fracture zone of the 1995 Hyogo-ken Nanbu earthquake. During these experiments, a contraction of about 10^− 8–10^− 7 was observed with three-component strainmeters at a bottom of the 800 m borehole, 70 m southwest of the 1800 m borehole. We estimated hydraulic properties of the fracture zone near the Nojima fault by using the strain data to investigate a healing of the fault during the postseismic stage. We calculated pore pressure changes due to the water injection using Darcy's equation and obtained strain changes due to the pore pressure changes as elastic deformations of the crust. The calculated strain changes have a nearly agreement with the observed strain changes. Hydraulic conductivity in 1997, 2000 and 2003 was determined to be 0.9 ± 0.2 × 10^− 6, 0.8 ± 0.2 × 10^− 6 and 0.4 ± 0.1 × 10^− 6 m/s, respectively. The reduced hydraulic conductivities in 2000 and 2003 suggest that the fractures had been healing.     

Use of Sr isotopes as a tool to decipher the soil weathering processes in a tropical river catchment,southwestern India

River water composition (major ion and ⁸⁷Sr/⁸⁶Sr ratio) was monitored on a monthly basis over a period of three years from a mountainous river (Nethravati River) of southwestern India. The total dissolved solid (TDS) concentration is relatively low (46 mg L⁻¹) with silica being the dominant contributor. The basin is characterised by lower dissolved Sr concentration (avg. 150 nmol L⁻¹), with radiogenic ⁸⁷Sr/⁸⁶Sr isotopic ratios (avg. 0.72041 at outlet). The composition of Sr and ⁸⁷Sr/⁸⁶Sr and their correlation with silicate derived cations in the river basin reveal that their dominant source is from the radiogenic silicate rock minerals. Their composition in the stream is controlled by a combination of physical and chemical weathering occurring in the basin. The molar ratio of SiO₂/Ca and ⁸⁷Sr/⁸⁶Sr isotopic ratio show strong seasonal variation in the river water, i.e., low SiO₂/Ca ratio with radiogenic isotopes during non-monsoon and higher SiO₂/Ca with less radiogenic isotopes during monsoon season. Whereas, the seasonal variation of Rb/Sr ratio in the stream water is not significant suggesting that change in the mineral phase being involved in the weathering reaction could be unlikely for the observed molar SiO₂/Ca and ⁸⁷Sr/⁸⁶Sr isotope variation in river water. Therefore, the shift in the stream water chemical composition could be attributed to contribution of ground water which is in contact with the bedrock (weathering front) during non-monsoon and weathering of secondary soil minerals in the regolith layer during monsoon. The secondary soil mineral weathering leads to limited silicate cation and enhanced silica fluxes in the Nethravati river basin.     

The relationship between regional stress field,fracture orientation and depth of weathering and implications for groundwater prospecting in crystalline rocks

In a uniform granite gneiss study area in central Zimbabwe, lineaments oriented parallel to the maximum regional compressive stress orientation exhibit the thickest regolith development, while lineaments oriented perpendicular to the maximum compressive stress show the shallowest development of weathered regolith. The principal fracture set orientations were mapped using aerial imagery. The regional stress field, estimated from global stress maps, was used to determine the stresses acting on each principal lineament orientation. Multi-electrode resistivity profiling was carried out across fractures with different orientations to determine their subsurface regolith conditions. The results indicate that the 360 and 060° lineaments, which are sub-parallel to the principal compressive stress orientation (σ₁) exhibit maximum development of the regolith, while 130° lineaments perpendicular to σ₁ do not exhibit significant regolith development. Since regolith thickness has been positively correlated with groundwater resources, it is suggested that fractures with orientations sub-parallel to the principal compressive stress direction constitute favourable groundwater targets. Knowledge of the regional stress field and fracture set orientations can be used as an effective low cost tool for locating potentially higher yielding boreholes in crystalline rock terrains.     

The Time-Dependent Reduction of Sliding Cohesion due to Rock Bridges Along Discontinuities: A Fracture Mechanics Approach

Summary In this paper, a fracture mechanics model is developed to illustrate the importance of time-dependence for brittle fractured rock. In particular a model is developed for the time-dependent degradation of rock joint cohesion. Degradation of joint cohesion is modeled as the time-dependent breaking of intact patches or rock bridges along the joint surface. A fracture mechanics model is developed utilizing subcritical crack growth, which results in a closed-form solution for joint cohesion as a function of time. As an example, a rock block containing rock bridges subjected to plane sliding is analyzed. The cohesion is found to continually decrease, at first slowly and then more rapidly. At a particular value of time the cohesion reduces to value that results in slope instability. A second example is given where variations in some of the material parameters are assumed. A probabilistic slope analysis is conducted, and the probability of failure as a function of time is predicted. The probability of failure is found to increase with time, from an initial value of 5% to a value at 100 years of over 40%. These examples show the importance of being able to predict the time-dependent behavior of a rock mass containing discontinuities, even for relatively short-term rock structures. Received November 5, 2001; accepted July 24, 2002; Published online November 19, 2002     

Experimental study on permeability response in fractured rock to the effect of hydro-mechanical coupling,fracture geometry,and component content

Natural Hazards - The conductivity of fractured rock mass caused by the hydro-mechanical behavior and attributes of the rock is critical for fluid seepage and solute diffusion. To study the...     

青藏高原与大陆动力学——地体拼合、碰撞造山及高原隆升的深部驱动力

运用地体和地体活动论观点,提出青藏高原结构划分的新方案;强调青藏高原的形成经历了新元古代以来长期活动的过程,青藏高原是一个“非原地”诸多地体会聚、拼合以及经历复合碰撞造山的“造山的高原”;大型走滑断裂在青藏高原形成中起着地体相对位移、侧向挤出、移置及使高原几何形态扭曲的作用。提出青藏高原隆升的“南缘超深俯冲(>600km)、北缘陆内俯冲、腹地深部热结构及岩石圈范围内的向NE右旋隆升”的多元驱动力机制。     

Facies pattern of the middle Permian Barren Measures Formation, Jharia basin, India: The sedimentary response to basin tectonics

In the Lower Gondwana succession of the Jharia basin of eastern India, the Barren Measures Formation is characterized by the cyclic disposition of fine-grained lacustrine deposits and relatively coarse-grained fluvial deposits. The cyclic variation in the rate of coarse clastic input is attributed to the sedimentary response to basin tectonics. The sandstone-shale alternations of the Barren Measures succession can be correlated with the tectonic cyclothems developed on the hangingwall dip-slope and adjoining trough in a continental half-graben setting. Enhancement of the gradient of the hangingwall dip-slope during reactivation of the basin margin faults led to progradation of the existing fluvial system towards the half-graben trough and deposition of the coarser clastics on the fine-grained lacustrine deposits of the trough. Peneplanation of the hangingwall slope and slow increase in the lake level caused lacustrine transgression and retrogration of the fluvial system on the hangingwall block. The fluvial sediments were onlapped by the fine-grained lacustrine deposits. Episodic rejuvenation of the basin margin faults thus caused development of tectonic cyclothem on the hangingwall block. The paleocurrent pattern indicates that a persistent northward paleoslope was maintained during Barren Measures sedimentation. The inferred depositional settings were much more extensive than the present limit of the outcrop. The faults, presently defining the northern limit of the Barren Measures Formation, were possibly emplaced after Barren Measures sedimentation. The final movement along these fault planes caused preservation of the downthrown hangingwall block and the Barren Measures sediments on the footwall block were eroded during subsequent denudation. The Southern Boundary Fault came into existence after the deposition of the Barren Measures sediments.     

The coupled release of REE and Pb to the soil labile pool with time by weathering of accessory phases, Wind River Mountains, WY

Rare earth element (REE) distributions and Pb isotope compositions were explored in soils varying in age from ca. 0.4 to ?300 ka, developed on moraines in the Wind River Mountains, Wyoming. Soil extracts (0.6 M HCl) were used to examine the soil labile pool while the major element distribution in soil profiles was used to determine the extent of weathering at different soil depths. The results show that the chondrite-normalized REE patterns of the deepest bulk soil within each profile reflects the composition of the moraine till, except for the oldest soil. Up to ca. 12 ka, the soil extract fraction is enriched in light REE, indicating early release of light REE to the soil labile pool while that of the two oldest soils are relatively enriched in heavy REE. In the soil extracts the La/Sm ratio normalized to the deepest soil (La_D/Sm_D) decreases systematically with soil age. Similarly, the Eu-anomaly in the deepest soil from each profile (Eu_D/Eu_D*) decreases slightly with soil age in the three young soils; however, Eu_D/Eu_D* increases with soil age in the older soils. The systematic trends of these two ratios indicate the depletion of light REE in young soils and the enrichment of Eu and heavy REE in the older soils. Based on the Pb isotope ratios, the relative contribution of Pb to the soil labile pool via mineral weathering of U- or Th-rich phases was assessed for the different stages of weathering. The whole-soil profile ²⁰⁸Pb/²⁰⁴Pb ratio was found to decrease with soil age and with La_D/Sm_D, whereas it increased with the Eu_D/Eu_D* ratio. In each horizon, Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb) ratio generally decrease with soil age. In order to overcome possible effects from parent material heterogeneity, the amount of radiogenic Pb as compared to the whole-soil composition was calculated and this was found to decrease systematically with soil age.     

浙江南部近海沉积物柱状样中脂类生物标志物的组成及形态分布

湖泊沉积物中直链脂肪族化合物(MCCs)对气候变化响应敏感,环境条件的变化可促使MCCs在游离态和结合态间发生转化,是古气候重建的重要生物标志物。以往研究中对样品前处理通常采用混合溶剂提取法得到MCCs游离态组分,而以化学键合、物理吸附等形式赋存的结合态组分无法通过溶剂提取,不能获取结合态组分特征对气候环境变化的指示信息。本文将热裂解这一有机质高效分解技术与气相色谱-质谱(GC-MS)联用,实现了对内蒙古伊和沙日乌苏湖泊沉积物有机质组成的精细刻画,并识别、分析了共计3类、71项结合态MCCs化合物。结果表明：热解温度是影响热裂解-气相色谱-质谱(Py-GC/MS)分析沉积物样品中结合态MCCs分布特征的主要因素,450℃以下热解能量较低导致结合态MCCs解析不充分,550℃和650℃下各类结合态MCCs(含正构烷烃、正构烯烃及α-正构脂肪酮)充分解析,可经GC-MS实现在线分离、识别;随着热解温度升高,裂解产物中MCCs平均链长趋于降低,过高的裂解能量影响了产物中MCCs化合物链长特征对气候变化指示的敏感性。经研究各类MCCs分布特征发现,550℃下正构烷烃平均链长特征与传统气候替代指标游离态正构烷烃单体碳同位素δ¹³C_27~33具有较好的相关性(R=0.83)。结合态正构烷烃ACL_25~31指标响应了区域有效降水变化特征,识别了区域内5.6ka、3.9ka前后的干旱期,反映了东亚夏季风强度变化,对应了北半球中纬度有效降水变化。

     

测井技术在评价准噶尔盆地吉木萨尔凹陷烃源岩中的应用

准噶尔盆地东部吉木萨尔凹陷主要生油层系二叠系测井研究发现,平地泉组烃源岩泥页岩具有荧光显示,而正常泥岩却没有;烃源岩泥页岩的自然伽玛、自然电位和电阻率都有增大的现象,特别是地层真电阻率增大明显.△logR法表明正常砂泥岩的电阻与声波时差曲线基本重合,而烃源岩层砂泥岩的电阻率曲线与声波时差曲线则明显不重合.平地泉组是主力生油层,这一结果与前人的研究具有一致性,也与有机碳热解的化验结果一致.同时发现烃源岩电阻越大,烃源岩产生的原油成熟度越高.     

The fluvial geochemistry, contributions of silicate, carbonate and saline-alkaline components to chemical weathering flux and controlling parameters: Narmada River (Deccan Traps), India

The Narmada River in India is the largest west-flowing river into the Arabian Sea, draining through the Deccan Traps, one of the largest flood basalt provinces in the world. The fluvial geochemical characteristics and chemical weathering rates (CWR) for the mainstream and its major tributaries were determined using a composite dataset, which includes four phases of seasonal field (spot) samples (during 2003 and 2004) and a decade-long (1990-2000) fortnight time series (multiannual) data. Here, we demonstrate the influence of minor lithologies (carbonates and saline-alkaline soils) on basaltic signature, as reflected in sudden increases of Ca²⁺-Mg²⁺ and Na⁺ contents at many locations along the mainstream and in tributaries. Both spot and multiannual data corrected for non-geological contributions were used to calculate the CWR. The CWR for spot samples (CWR_spot) vary between 25 and 63 ton km⁻² year⁻¹, showing a reasonable correspondence with the CWR estimated for multiannual data (CWR_multi) at most study locations. The weathering rates of silicate (SilWR), carbonate (CarbWR) and evaporite (Sal-AlkWR) have contributed ∼38-58, 28-45 and 8-23%, respectively to the CWR_spot at different locations. The estimated SilWR (11-36 ton km⁻² year⁻¹) for the Narmada basin indicates that the previous studies on the North Deccan Rivers (Narmada-Tapti-Godavari) overestimated the silicate weathering rates and associated CO₂ consumption rates. The average annual CO₂ drawdown via silicate weathering calculated for the Narmada basin is ∼0.032 × 10¹² moles year⁻¹, suggesting that chemical weathering of the entire Deccan Trap basalts consumes approximately 2% (∼0.24 × 10¹² moles) of the annual global CO₂ drawdown. The present study also evaluates the influence of meteorological parameters (runoff and temperature) and physical weathering rates (PWR) in controlling the CWR at annual scale across the basin. The CWR and the SilWR show significant correlation with runoff and PWR. On the basis of observed wide temporal variations in the CWR and their close association with runoff, temperature and physical erosion, we propose that the CWR in the Narmada basin strongly depend on meteorological variability. At most locations, the total denudation rates (TDR) are dominated by physical erosion, whereas chemical weathering constitutes only a small part (<10%). Thus, the CWR to PWR ratio for the Narmada basin can be compared with high relief small river watersheds of Taiwan and New Zealand (1-5%) and large Himalayan Rivers such as the Brahmaputra and the Ganges (8-9%).     

欧龙布鲁克地块西段达肯大坂岩群物质组成及变形特征研究

达肯大坂岩群是欧龙布鲁克地块结晶基底的重要组成部分,在形成过程中保存了丰富的物质记录和演化信息,是研究欧龙布鲁克地块乃至我国西部古老地壳及构造演化的窗口。通过对达肯大坂岩群详细的物质组成及构造剖面研究,根据岩石组合及变质—变形特征,将其进一步细化为混合片麻岩岩段、(混合岩化)流变片麻岩岩段、条带状片麻岩岩段、片岩岩段和大理岩岩段,野外调查表明,达肯大坂岩群中发育多期构造变形作用,根据野外变形特征及各期变形的叠加改造关系,初步建立了达肯大坂岩群相对变形序列,为区域构造过程的探讨与对比提供了新的基础资料。     

The Cretaceous sediment-hosted copper deposits of San Marcos (Coahuila,Northeastern Mexico): An approach to ore-forming processes

In the San Marcos ranges of Cuatrociénegas, NE Mexico, several sediment-hosted copper deposits occur within the boundary between the Coahuila Block, a basement high mostly granitic in composition and Late Paleozoic to Triassic in age, and the Mesozoic Sabinas rift basin. This boundary is outlined by the regional-scale synsedimentary San Marcos Fault. At the basin scale, the copper mineralization occurs at the top of a ～1000 m thick red-bed succession (San Marcos Formation, Berrisian), a few meters below a conformable, transitional contact with micritic limestones (Cupido Formation, Hauterivian to Aptian). It consists of successive decimeter-thick roughly stratiform copper-rich horizons placed just above the red-beds, in a transitional unit of carbonaceous grey-beds grading to micritic limestones. The host rocks are fine- to medium-grained arkoses, with poorly sorted and subangular to subrounded grains. The detrital grains are cemented by quartz and minor calcite; besides, late iron oxide grain-coating cement occurs at the footwall unmineralized red-beds. The source area of the sediments, indicated by their modal composition, is an uplifted basement. The contents of SiO₂ (40.70–87.50 wt.%), Al₂O₃ (5.91–22.00 wt.%), K₂O (3.68–12.50 wt.%), Na₂O (0.03–2.03 wt.%) and CaO (0.09–3.78 wt.%) are within the ranges expected for arkoses. Major oxide ratios indicate that the sedimentary-tectonic setting was a passive margin.The outcropping copper mineralization essentially consists in a supergene assemblage of chrysocolla, malachite and azurite. All that remains of the primary mineralization are micron-sized chalcocite grains shielded by quartz cement. In addition, pyrite subhedral grains occur scattered throughout the copper-mineralized horizons. In these weathered orebodies copper contents range between 4.24 and 7.72 wt.%, silver between 5 and 92 ppm, and cobalt from 8 to 91 ppm. Microthermometric measurements of fluid inclusions in quartz and calcite crystals from footwall barren veinlets gave temperatures of homogenization between 98 °C and 165 °C, and ice-melting temperatures between ?42.5 °C and ?26.1 °C.The primary copper mineralization formed during the early diagenesis, contemporary with the active life of the Sabinas Basin. The mineralizing fluids were dense, near neutral, moderately oxidized brines that originally formed from seawater that, driven by gravity, infiltrated to the deepest parts of the basin and dissolved evaporites. As a result, they became hydrothermal fluids of moderate temperature capable of leaching high amounts of copper. The source of this metal could be mafic detrital grains and iron oxides of the underlying Jurassic and Lower Cretaceous red-beds. Copper precipitation took place when the brines passed through the redox boundary marked by the transition from red- to grey-beds. The upward movement of the brines was promoted by a high heat flow that allowed their convective circulation and their ascent along the synsedimentary San Marcos Fault.     

鄂尔多斯盆地罕台庙地区中侏罗统直罗组下段下亚段还原性与铀成矿关系

通过对鄂尔多斯盆地罕台庙地区中侏罗统直罗组下段下亚段地层还原性的地质参数进行统计,发现该地层的还原性强弱与铀矿体的厚度、品位和平米铀量具有一定的关系。地层还原性较强或较弱,均不利于铀富集,仅在适当范围内,才有利于富铀矿体的形成,才能使铀矿体的厚度、品位和平米铀量达到最大值。因此,地层或砂体的还原性研究,对铀矿体的精确空间定位具有显著的指导性作用。     

山东文登金矿成矿条件及成矿过程——阴极发光技术在包裹体研究中的应用

使用阴极发光技术可取得一般岩石显微镜无法得到的地质信息,并可将包裹体的研究从原来的“群分析”统计规律研究水平提高到“单包体分析”水平,获得更小时间单位成矿流体的演化特征,因而能更准确地模拟和恢复地质作用过程。运用阴极发光对石英等矿物的研究,发现蚀变带中的石英按不同的形态特征和发光性可以分为三期,和成矿关系密切为第一、二期石英。根据阴极发光下显示的生长微形态,系统地测试三期石英不同生长时期的包裹体物相,从而获取了整个成矿过程流体的p-t-v-x参数,确定构造作用加热大气降水形成成矿流体,体系开放流体两次减压沸腾导致金矿沉淀的矿体定位机理和成矿动力学机制。     

The Miduk porphyry Cu deposit,Kerman, Iran: A geochemical analysis of the potassic zone including halogen element systematics related to Cu mineralization processes

Miduk hypogene and supergene porphyry Cu–Mo mineralization occurs within the Miocene porphyritic quartz–diorite and host Eocene plagioclase–hornblende phyric andesitic pyroclastic and flow sequence. Both the host rocks were extensively altered by hydrothermal fluids to dominantly potassic, phyllic, and argillic with interstitial to distal propylitic types.