Metamorphic incongruent solution,diffusion and pressure solution stripes

The formation of pressure solution stripes is discussed on the basis of previosly published observations and of local chemical equilibria between solution and solid phases. Chemically driven mineral reactions can be localized at early sites of nucleation and be enhanced by stresses to create patterns of cleavage stripes. These reactions cause transport of material over distances greater than single grains. They provide suitable sites and suitable chemistry for later incongruent pressure solution, potentially a much faster process than congruent pressure solution discussed in previous literature. Neither reaction nor straightforward incongruent pressure solution explains the solution of quartz, commonly observed at cleavage stripes. Quartz pressure solution is explicable on certain assumptions about diffusivities and the effects of stresses. This involves interaction between quartz and the established incongruent solution of other minerals, an overall process termed ‘interdependent pressure solution’. Electrical potential differences will be greatest during early mineral reactions, a feature which may provide a means to corroborate the chemical role in initiating ‘pressure solution stripes’.     

Role of electrochemical reactions in pressure solution

Using a Surface Forces Apparatus we have measured changes in the electrical potential difference between quartz and mica surfaces that correlate with the changing quartz dissolution rate when surfaces are pressed together at relatively low pressures (2-3 atm) in aqueous electrolyte solutions of 30 mM CaCl₂ at 25 °C. No detectable dissolution or voltage potential difference is measured in symmetrical systems (e.g. mica-mica or quartz-quartz) or between dry surfaces subjected to similar pressures, indicating that the dissolution can not be attributed to a simple pressure effect, slow aging (creep), or plastic deformation of the quartz surface. In quartz-mica systems brought together under pressure or to close proximity in electrolyte solution, the onset of quartz dissolution is marked by a sudden, rapid decrease in the quartz thickness at initial rates in the range from 1 to 4 nm/min, which after several hours settles into a constant rate of approximately 0.01 nm/min (∼5 μm/yr). Concomitantly, the potential drops to a constant value once the dissolution rate has stabilized. The decrease in the decay rate is interpreted as being due to saturation of the confined aqueous film and/or to the buildup of a Stern layer on the quartz surface, and the constant rate as being due to the steady-state chemical dissolution and diffusion of the dissolving silica into the surrounding reservoir. The dissolution is ‘non-uniform’: the surfaces become rough as dissolution proceeds, with the appearance of pits in a manner analogous to corrosion. On occasions, the process of rapid dissolution followed by a gradual transition to steady dissolution repeats itself, suggesting that the pit structure and Stern layer are fragile and subject to collapse and/or expulsion from the gap. Preliminary experiments on the dissolution of multi-faceted milled quartz particles (∼1.0 μm diameter) compressed between two muscovite surfaces suggest an asymmetry in the dissolution rates at different crystallographic planes. The origin of the electrical potential is interpreted as arising from the overlapping of the electric double-layers of two dissimilar surfaces when they are forced into close proximity. This electrical potential difference, for as yet unknown reasons, appears to be the driving force for the dissolution, rather than pressure.     

石灰岩裂隙摩擦强度愈合的压力溶解和应力腐蚀模型研究

为模拟试验中观察到的石灰岩裂隙摩擦强度愈合现象,提出用压力溶解和应力腐蚀相结合的综合模型来模拟裂隙摩擦强度的愈合。综合模型可同时考虑压力溶解和应力腐蚀对裂隙接触面的改造作用,通过裂隙接触面积的变化表征摩擦强度的愈合。模拟结果表明：压力溶解模型低估了短控制时间内摩擦强度的愈合量,通过进一步的参数敏感性分析发现,随着控制时间的增长,温度的升高显著地提高了压力溶解模型的模拟值,而有效应力的增加对模拟值的影响相对较小;应力腐蚀模型则主要反映应力压缩引起的裂隙接触面积增长;压力溶解模型和应力腐蚀模型结合则能较好地模拟裂隙摩擦强度的愈合。因此,用压力溶解模型模拟裂隙摩擦强度愈合时,特别是在模拟短控制时间内和低温条件下力学占主导作用的愈合时,应考虑应力腐蚀引起的裂隙接触面积变化。     

Local pressure lows as possible sinks of fluids in geologic structures

Local pressure lows in layered sections perturbed by anticlinal structures are studied analytically using equations for pressure change across interfaces. They are simple equations of pressure difference for low-angle structures and boundary integral equations for steeply dipping anticlines. Pressure may decrease locally near the crests of anticlines, as well as away from them at distances commensurate to the anticline height. Predicting stress patterns, which are specific for different groups of geologic structures, is a difficult task. However, some components of the stress field, such as low-pressure zones which may act as sinks for fluids, are relatively easy to constrain. Stress in these zones depends on the dip of anticlines and their curvature at each surface point. Negative curvature causes additional lateral extension and promotes further decrease of overburden pressure around the crests.     

Pegmatite fluids of different origins and their implications for mineralization

Characteristics of ore-forming fluids as inferred from detailed studies of inclusions indicate that pegmatites may have different origins.For example,the granitic pegmatite at Mufushan is originated from magma differentiation at 1100-200℃,while the No.3 Pegmatite Vein in Xinjiang may owe its origin to a pegmatitic magma produced via metamorphic anatexis at 1140-200℃.Pegmatite fluids of the above two types are a melt-liquid system and may evolve into a solid-melt or solid-liquid system that would have a critical bearing on metasomatism and intergranular solutions.The Minxi pegmatite,on the oter hand,resulted from metamorphic differentiation,with its fluid(formed at 400-180℃）exhibiting many features of metamorphism.Pegmatites of different origins are distributed in tectonic units of different characters and are different bot in the nature of ore-foming fluid and in the source of ore-metals.This concept of polygene launches a challenge to the traditional belief that pegmatite is exclusively originated from magma differentiation and may be helphfou for the establishment of a new theory of pegmatite genesis.     

Experimental knife-edge pressure solution of halite

Pressure solution experiments were carried out, using a quartz knife-edge 0.26 mm wide on halite single crystals in halite saturated solutions, to observe the detailed development of pressure solution contacts and the rates of pressure solution. A rate of about 3 μm/day was observed for initial knife-edge stresses ranging from 4.5 to 15 MPa. Close examination of the contact leads to the conclusion that the mechanism of pressure solution is a combination of plastic deformation at the contact and free surface pressure dissolution near its periphery. Free surface pressure dissolution increases the contact stress to about 18 MPa, high enough to cause plastic deformation, by changing the area of contact. This mechanism differs from a water film diffusion mechanism, previously suggested by many authors, but is similar in some ways to the undercutting hypothesis of Bathurst (1958). We infer a steady state plastic deformation instead of catastrophic grain crushing at the contact. Free surface dissolution plus the plastic deformation mechanism may be primarily responsible for pressure solution in relatively porous rocks.     

The role of speciation in alkaline igneous fluids during fenite metasomatism

Fenites associated with alkaline igneous rocks show a progression from a high temperature assemblage consisting of sodium-rich alkali feldspar + a sodium-ironrich mafic mineral, to an extreme end member assemblage consisting of pure potassium feldspar + iron oxide. The latter assemblage is only found in association with low temperature carbonatites. Segments of this distribution trend can be found in the contact aureole of single intrusive centers.In the east African Kisingiri nephelinite volcano, ijolite intruded a granodioritic basement, producing a fenitized contact aureole. During metasomatism of granodiorite, according to the mass balance model of Rubie (1982), feldspar only participated in an alkali exchange reaction, while quartz was replaced by sodic pyroxene. Outward from the intrusive contact, with decreasing temperature, feldspar became progressively K-enriched, while pyroxene was enriched in the acmite component. It is predicted that alkaliexchange reactions were controlled by NaCl⁰-KCl⁰ aqueous complexes close to the intrusive contact, while further out in the aureole Na⁺-K⁺ ions dominated at the lower temperatures and enhanced the level of potassium metasomatism of feldspar. With decreasing temperature in the aureole, the K/Na ratio of the fluid decreased and consequently the activity of acmite increased.Around carbonatites, where the level of CO₂ in the escaping fluid can be expected to be high, Na₂CO ₃ ⁰ -K₂CO ₃ ⁰ complexes may dominate. Alkali exchange between feldspar and these aqueous species enhances, even further, the stabilization of pure potassium feldspar. Boiling may also play an important role in potassium metasomatism as carbonatites are frequently associated with pyroclastic rocks. Formation of hematite instead of sodic pyroxene may be attributed to low , high and a CO₂-rich fluid.Important variables which determine the products of alkali metasomatism are shown to be temperature, pressure and CO₂ content of the fluid, as well as the K/Na ratio of the fluid.     

Arterial faults and their role in mineralizing systems

In quartzo-feldspathic continental crust with moderate-to-high heat flow,seismic activity extends to depths of 10-20 km,bounded by isotherms in the 350-450 C range.Fluid overpressuring above hydrostatic in seismogenic crust,is heterogeneous but tends to develop in the lower seismogenic zone(basal seismogenic zone reservoir=b.s.z.reservoir) where the transition between hydrostatically pressured and overpressured crust is likely an irregular,time-dependent.3-D interface with overpressuring concentrated around active faults and their ductile shear zone roots.The term Arterial Fault is applied to fault structures that root in portions of the crust where pore fluids are overpressured(i.e.at hydrostatic pressure) and serve as feeders for such fluids and their contained solutes into overlying parts of the crust.While arterial flow may occur on any type of fault,it is most likely to be associated with reverse faults in areas of horizontal compression where fluid overpressuring is most easily sustained.Frictional stability and flow permeability of faults are both affected by the state of stress on the fault(shear stress,τ;normal stress,σ_n),the level of pore-fluid pressure,P_f,and episodes of fault slip,allowing for a complex interplay between fault movement and fluid flow.For seismically active faults the time dependence of permeability is critical,leading to fault-valve behaviour whereby overpressures accumulate at depth during interseismic intervals with fluid discharged along enhanced fault-fracture permeability following each rupture event.Patterns of mineralization also suggest that flow along faults is non-uniform,concentrating along tortuous pathways within the fault surface.Equivalent hydrostatic head above ground level for near-lithostatic overpressures at depth(1.65×depth of zone) provides a measure of arterial potential.Settings for arterial faults include fault systems developed in compacting sedimentary basins,faults penetrating zones of active plutonic intrusion that encounter overpressured fluids exsolved from magma,together with those derived from contact metamorphism of fluid-rich wallrocks,and/or from regional devolatilisation accompanying prograde metamorphism.Specially significant are active faults within accretionary prisms rooted into overpressured subduction interfaces,and steep reverse faults activated by high overpressures from b.s.z.reservoirs during compressional inversion.     

土压力公式的统一解

传统的土压力公式不能反映土体材料强度的中间主应力效应,但中间主应力效应是存在的且不容忽视的。基于俞茂宏统一强度理论,考虑中间主应力效应,推导了土压力公式的统一解,给出了具有广泛意义的土压力公式,可以充分发挥材料的强度潜能,Rankine土压力公式为其特例。根据不固结不排水强度指标与固结不排水强度指标,按有效应力法与总应力法对土压力的计算公式进行了推导和分析,得出了两种强度指标下的土压力公式。另外,通过已有文献的实例进行了对比分析。     

The role of fluids in the formation and subsequent development of early continental crust

A petrogenetic model is developed to explain the evolution and geochemical character of granitic rocks in early Archean (pre 3.6 Gyr) continental crust taking into account the following important geological constraints, viz.:

1. High geothermal gradients (probably in excess of 90 ° C/km) and resulting widespread granulite facies metamorphism even at relatively shallow depths
2. The fractionation of certain major and trace elements under granulite facies conditions
3. The composition and geochemical behaviour of fluids which emanate from or pass through terrains undergoing granulite facies metamorphism viz. carbonic fluids containing significant amounts of SO₂ and halogens.

In this model tonalitic and trondhjemitic intrusives are regarded as being derived dominantly by partial melting of mafic granulite. The ubiquitous potassic granites, which typical post-date sodic plutonic activity are interpreted to be anatectic melts generated under granulite or amphibolite facies conditions from the previously formed ‘plagiogranites’. The presence of a postulated granulite facies source area for Archean tonalitic rocks, and the geochemical character of fluids which accompany metamorphism under such conditions explains the HREE geochemistry of these suites and casts doubt on the validity of applying currently used trace element fractional melting or crystallization models to these terrains. Similarly it suggests that petrogenetic interpretations based on Sr and Pb isotopic systems must be reevaluated because of the extreme mobility of both parent and daughter elements under granulite facies conditions.     

厄立特里亚Koka金矿床成矿流体特征及其地质意义

厄立特里亚Koka金矿床产于努比亚地盾新元古代浅变质岩系中,矿体主要赋存于Koka微晶花岗岩内,受剪切构造控制,是在该国发现的大型造山型金矿床。矿床含金石英脉中石英中赋存的原生流体包裹体分为富CO_2包裹体、CO_2-H_2O包裹体和H_2O包裹体共3种类型,以大量发育富CO_2包裹体与CO_2-H_2O包裹体为特征。成矿流体具有富CO_2、中低温(210～360℃)、中低盐度(w(NaCl_(eq))=2.24%～8.51%)的特征。流体中阳离子主要为Na~+与少量K~+,阴离子为Cl~-与少量SO_4~(2-),气相成分主要为CO_2与H_2O,基本不含其他气体组分,流体属于NaCl-H_2O-CO_2体系。成矿流体密度变化范围较大(0.597～0.969 g/cm~3),其中高密度的富CO_2包裹体捕获的最小P-T条件为260～360℃、100～270 MPa,形成于区域变质作用时期。成矿流体的δD_(V-SMOW)范围为-57‰～-50.1‰,δ~(18)O_水范围为1.4‰～3.2‰,表明Koka金矿床成矿流体主要来源于变质热液,并伴有大气降水的混入。成矿流体中CO_2-H_2O包裹体气相分数变化范围很大(15%～80%),与之共生的H_2O包裹体具有相似的盐度以及较低的均一温度,表明初始的CO_2-H_2O型流体发生了不混溶作用,导致相分离,产生的大量富CO_2流体,并使金大量沉淀。     

Confined groundwater system in Tokyo

The Musashino Terrace and the Shitamachi Lowland, the main urban areas in the Tokyo metropolis, are in the eastern part of Tokyo and are underlain by thick Quaternary sediments. The geologic structrue of these sediments has a general strike of WNW—ESE, with a gentle dip to the east in the western part of the Musashino Terrace, and strikes generally to the E—W with a gentle dip to the north in the eastern part of the terrace and strikes WNW—ESE to N—S with a very gentle dip to the east in the Shitamachi Lowland. The basal sediments in this area consist chiefly of massive thick silty layers, whereas the sediments overlying them consist of alternating layers of silt, sand, and gravel, which act as confined aquifers throughout the Musashino Terrace and the Shitamachi Lowland. Confined groundwater systems are being developed in the Tama District, the western part of the terrace, and are mainly recharged with river water from the Tama River. The unconfined groundwater system is being developed in the surface layers such as terrace sand and gravel formations and the Kanto Loam Formation. These confined aquifers that are being developed throughout the terrace and the lowland have been exploited for household water supplies and industrial and air-conditioning use for nearly seven decades. The heavy utilization of the confined groundwater, however, has caused a major lowering of groundwater levels. As a result of the lowering of groundwater levels, land subsidence has occurred all over the Lowland and the terrace. At this time the national government and the Tokyo Metropolitan Government restricted groundwater withdrawals for industrial, air-conditioning use and the others except for household supply. Owing to these restrictions, not only have the water levels recovered, but land subsidence has rapidly declined since around 1973.     

The mechanics of intersecting echelon veins and pressure solution seams in limestone

Many studies that describe the formation of echelon vein arrays relate the causative stresses implicitly to the deformation, reliant on simple shear kinematics, such that the vein-to-array angle and the array width are the primary physical quantities. In contrast, we identify twelve physical quantities to describe echelon veins in two dimensions, including coeval, vein-intersecting, pressure solution seams. A finite element method is used to reproduce vein shapes in linear elastic and elastic-perfectly plastic model limestone. Model vein geometries are designed using values within the range of geometries measured from echelon veins at Raplee Anticline and Comb Monocline, Utah.Four physical quantities are significant for describing echelon vein shapes: vein spacing, vein-array angle, limestone elastic stiffness, and closing of orthogonal pressure solution seams. Pressure solution seam closing influences the mechanical interaction between adjacent veins, and for a range of conditions, causes a nearly linear vein opening distribution (triangular shapes) and encourages straight vein propagation, both of which approximate field measurements. Model results show that small spacing of veins with seams and large vein-array angles promote straight vein traces in limestone with stiffness typical of laboratory measurements, given the physical geologic conditions inferred from the burial history of the limestone strata.     

A new mechanism for pressure solution in porous quartzose sandstone

The mechanism of pressure solution, a source of controversy for years, must be understood before we can evaluate the effectiveness of pressure solution during geological processes. The water film diffusion (WFD) mechanism proposed by Weyl (1959) and Rutter (1976, 1983) is believed by many to be the primary mechanism responsible for intergranular pressure solution (IPS) in non-porous metamorphic rocks as well as porous sedimentary rocks. Tada and Siever (1986), experimenting with halite single crystals, suggested the new plastic deformation plus free-face pressure solution (PD + FFPS) mechanism.The effectiveness of PD + FFPS as an IPS mechanism is theoretically evaluated for porous quartzose sandstone and compared with WFD. The result suggests that, though the driving force of the reaction (relative activity increase) is 4 to 5 orders of magnitude larger in WFD, the ease of diffusion (diffusion path width times the diffusion coefficient) is 7 to 9 orders of magnitude larger in PD + FFPS. Consequently. PD + FFPS yields diffusion rates 2 to 5 orders of magnitude faster than WFD.In WFD, diffusion is always the rate-controlling process, whereas either dissolution at IPS contacts or precipitation on free grain surfaces may be the rate-controlling process in PD + FFPS, when temperatures are low and/or grain sizes are small. The dissolution or precipitation rate of PD + FFPS is faster than the diffusion rate of WFD except when the total free grain surface area is very small. In final stages of compaction, when the total free grain surface area has become very small, WFD replaces PD + FFPS.     

Solubility behavior of alkali aluminosilicate components in aqueous fluids and silicate melts at high pressure and temperature

The solubility behavior of K₂O, Na₂O, Al₂O₃, and SiO₂ in silicate-saturated aqueous fluid and coexisting H₂O-saturated silicate melts in the systems K₂O-Al₂O₃-SiO₂-H₂O and Na₂O-Al₂O₃-SiO₂-H₂O has been examined in the 1- to 2-GPa pressure range at 1100°C. Glasses of Na- and K-tetrasilicate compositions with 0, 3, and 6 mol% Al₂O₃ were used as starting materials. In both systems, the oxides dissolve incongruently in aqueous fluid and silicate melt. When recalculated to an anhydrous basis, the aqueous fluids are enriched in alkalis and depleted in silica and alumina relative to their proportions in the starting materials. The extent of incongruency is more pronounced in the Na₂O-Al₂O₃-SiO₂-H₂O system than in the K₂O-Al₂O₃-SiO₂-H₂O system.The partition coefficients of the oxides, D_oxide^fluid/melt, are linear and positive functions of the oxide concentration in the fluid for each composition. There is a slight dependence of the partition coefficients on bulk composition. No effect of pressure could be discerned. For alkali metals, the fluid/melt partition coefficients range from 0.06 to 0.8. For Al₂O₃ this range is 0.01 to 0.2, and for SiO₂, it is 0.01 to 0.32. For all compositions, D_K2O^fluid/melt∼D_Na2O^fluid/melt>D_SiO2^fluid/melt>D_Al2O3^fluid/melt for the same oxide concentration in the fluid. D_K2O^fluid/melt, D_Na2O^fluid/melt, and D_SiO2^fluid/melt correlate negatively with the Al₂O₃ content of the systems. This correlation is consistent with a solubility model of alkalis that involve associated KOH°, NaOH°, silicate, and aluminate complexes.     

Handicrafts in Oman and their role in rural community development

Oman has a long tradition of practising handicrafts, though few are of high aesthetic value. Omani handicrafts have always been functional, made for a rural population which has valued the artefacts for what they do but much less for their appearance. Nevertheless, handicraft skills are very widespread throughout the rural population. In the past they have been used to process raw materials of agricultural origin, notably from the date palm and from livestock, and also from wild trees and minerals. In addition to local resources, Omani craftsmen have worked imported cotton yarn and imported precious and semi-precious metals. The craftsmen not only provided equipment for rural producers and finished goods for domestic use but they also performed the important community roles of linking primary producer with primary producer and consumer, and making both more aware of the raw materials of the environment in which they lived. In doing this, craftsmen, with their mutually respected skills, played a vital role in creating the complex network of interdependencies from which a community is built. However, the Omani and other regional oil industries put great pressure on all rural activities, and particularly on the highly labour-intensive crafts. People sought work in the urban centres and in the armed forces at the same time as a flood of cheap imports prevented craftsmen from raising their prices to keep abreast of inflation and of heightened personal material aspirations. The impact of oil was so rapid that the craftsmen had no time to adapt their techniques or their products to meet the new circumstances. Craft growth and change needed government support to see it through a difficult transitional period. Unfortunately, however, the government of Oman has only played a small and somewhat confused role in the crafts. Many people can no longer practise their inherited skills and the role that they played in maintaining the community has been largely destroyed. Concentrated effort needs to be made, craft by craft, initially at a village scale, to train and to modify techniques where necessary and to expand market openings. Success in a single village can then form the basis of expanded regional or national projects, drawing in other resources such as credit from the Agricultural Bank and using the marketing networks of the Central Marketing Authority. An approach which has been used to redevelop spinning and weaving and to introduce irrigation channel manufacture in a coastal village in northern Oman is providing valuable information that could be used as the basis of other craft projects in Oman and in countries with comparable craft development problems.     

高温高压下水流体pH值测量

高温高压下水流体的pH值是影响矿物-水流体相互作用平衡和动力学过程的一个重要参数。确定高温高压流体pH的方法主要包括电位势测量、化学分析与热力学计算及电导率方法。电位势测量在化学领域应用广泛,也被用来测量了洋中脊的热液流体的pH值。人们尝试了各种形式的电极,如氢电极、钯氢化物电极、金属-金属氧化物电极和钇稳定的氧化锆(YSZ)电极。玻璃电极只取得了有限的成果,通常用来测量常温及低温下的流体的pH值。对金属-金属氧化物电极在较宽的温度范围内进行了研究,但它需要利用合适的参比溶液来校正,与玻璃电极一样都表现出较大的偏移和误差以及不可回测性。陶瓷薄膜电极,如钇稳定的氧化锆(YSZ)电极,为测量相对较高温度下的pH值提供了一个可靠的方法。利用高温淬火水的化学分析和常温下的pH的测量,通过求解络合反应质量平衡与所有组分质量平衡的联立方程,来计算指定温度压力下pH值和水溶液种类的分布,是获得高温pH值的另一个重要而常用的方法,但热力学数据中的数值误差和不确定性以及分析误差能够影响计算出的高温溶液的pH值的精度。对高温高压超临界水流体的电导测量已经取得很大的成就,它被用来作为确定溶液内部离子反应平衡常数的一种最常用的方法,通过获得的这些反应的平衡常数,就可以得到在高温高压下流体的就位物质组成,这个方法可以测量更高温度压力下流体的pH值。