Jump conditions and dynamic surface tension at permeable interfaces such as the inner core boundary

We consider a fluid crossing a zone of rapid density change, so thin that it can be considered as a density jump interface. In this case, the normal velocity undergoes a jump. For a Newtonian viscous fluid with low Reynolds number (creeping flow) that keeps its rheological properties within the interface, we show that this implies that the traction cannot be continuous across the density jump because the tangential stress is singular. The appropriate jump conditions are established by using the calculus of distributions, taking into account the curvature of the interface as well as the density and viscosity changes. Independently of any intrinsic surface tension, a dynamic surface tension appears and turns out to be proportional to the mass transfer across the interface and to a coefficient related to the variations of density and viscosity within the interface. Explicit solutions are exhibited to illustrate the importance of these new jump conditions. The example of the Earth's inner core crystallisation is questioned.     

http://www.sciencedirect.com/science/article/pii/S1674987114000395

The study of fluid inclusions in high-grade rocks is especially challenging as the host minerals have been normally subjected to deformation, recrystallization and fluid-rock interaction so that primary in- clusions, formed at the peak of metamorphism are rare. The larger part of the fluid inclusions found in metamorphic minerals is typically modified during uplift. These late processes may strongly disguise the characteristics of the ＂original＂ peak metamorphic fluid. A detailed microstructural analysis of the host minerals, notably quartz, is therefore indispensable for a proper interpretation of fluid inclusions. Cathodoluminescence （CL） techniques combined with trace element analysis of quartz （EPMA, LA- [CPMS） have shown to be very helpful in deciphering the rock-fluid evolution. Whereas high-grade metamorphic quartz may have relatively high contents of trace elements like Ti and A1, low- temperature re-equilibrated quartz typically shows reduced trace element concentrations. The result- ing microstructures in CL can be basically distinguished in diffusion patterns （along microfractures and grain boundaries）, and secondary quartz formed by dissolution-reprecipitation. Most of these textures are formed during retrograde fluid-controlled processes between ca. 220 and 500 ℃, i.e. the range of semi-brittle deformation （greenschist-facies） and can be correlated with the fluid inclusions. In this way modified and re-trapped fluids can be identified, even when there are no optical features observed under the microscope.     

Complex vein systems as a data source in tectonics: An example from the Ugab Valley,NW Namibia

Neoproterozoic metaturbidites in the Lower Ugab Domain, Namibia, contain a complex network of four sets of quartz-calcite veins, overprinted by km-scale folds associated with four regional foliations. The veins formed by fluid overpressure predating the main deformation. Deformation structures developed at the junction of two mobile belts during the assembly of Gondwana, the NS Kaoko Belt, and the EW trending Damara Belt. Km-scale NS trending folds were initiated during EW constriction in the Kaoko Belt, while their further development and all subsequent events are related to constriction in the EW-Damara Belt, with coeval sinistral strike slip in the Kaoko Belt. Deformation of the veins, and development of four orthogonal foliations are due to gradual changes in the bulk tectonic framework rather than separate orogenic events. The veins are deformed in a complex manner allowing a full 3D reconstruction of regional sequence of events. The local complex tectonics could be reconstructed because of the perfect local exposure and the multitude of veins: it illustrates the potential complexity of tectonic events and structural evolution in apparently simple slate belts.     

Influence of carbonate facies on fault zone architecture

Normal faults on Malta were studied to analyse fault propagation and evolution in different carbonate facies. Deformation of carbonate facies is controlled by strength, particle size and pore structure. Different deformation styles influence the damage characteristics surrounding faults, and therefore the fault zone architecture. The carbonates were divided into grain- and micrite-dominated carbonate lithofacies. Stronger grain-dominated carbonates show localised deformation, whereas weaker micrite-dominated carbonates show distributed deformation. The weaker micrite-dominated carbonates overlie stronger grain-dominated carbonates, creating a mechanical stratigraphy. A different architecture of damage, the ‘Fracture Splay Zone’ (FSZ), is produced within micrite-dominated carbonates due to this mechanical stratigraphy. Strain accumulates at the point of juxtaposition between the stronger grain-dominated carbonates in the footwall block and the weaker micrite-dominated carbonates in the hanging wall block. New slip surfaces nucleate and grow from these points, developing an asymmetric fault damage zone segment. The development of more slip surfaces within a single fault zone forms a zone of intense deformation, bound between two slip surfaces within the micrite-dominated carbonate lithofacies (i.e., the FSZ). Rather than localisation onto a single slip surface, allowing formation of a continuous fault core, the deformation will be dispersed along several slip surfaces. The dispersed deformation can create a highly permeable zone, rather than a baffle/seal, in the micrite-dominated carbonate lithofacies. The formation of a Fracture Splay Zone will therefore affect the sealing potential of the fault zone. The FSZ, by contrast, is not observed in the majority of the grain-dominated carbonates.     

Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines,Italy)

A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting.The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization.Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.     

Effect of macropore tortuosity and morphology on preferential flow through saturated soil: A Lattice Boltzmann study

Unlike micropores where water moves upward or downward based on hydraulic gradient, in macropores, water flows predominantly downward due to the gravity. Therefore, models based on capillary flow are not capable of simulating macropore flow. There are attempts to model the macropore flow using two domains, one for capillary flow and another one for macropores. These models use Richard’s equation for capillary flow and Poiseuille’s law for macropores in which the macropore is approximated to be cylindrical or planar. This study quantifies the magnitudes of the errors induced by this assumption. Influence of macropore shapes and tortuosity was quantified by using a 3D Lattice Boltzmann model, which is capable of simulating fluid flow in micropores as well as macropores of cracked clays. Artificial macropores of constant sectional area and volume, but different shapes were generated in 3D and the influence of macropore shapes, shape related parameters, and tortuosity were systematically investigated. Macropore flow rate decreases with different shapes compared to cylindrical macropores and increase in aspect ratio of sectional shape leads to decrease in macropore flow rate. The maximum effect of bends/turnings along the tortuous macropore was about 25% on overall decrease of flow rate due to tortuosity. However, more detailed study is required on the influence of bends on macropore flow rate. The macropore flow rate reduces by about 70% for tortuosity of 1.41. A prediction equation is verified to predict the flow rate of different shapes and tortuous macropores based on straight cylindrical macropore using aspect ratio and tortuosity factor.     

西藏丁青县扎格拉金矿床地质特征、流体包裹体特征及成因

扎格拉金矿位于班公湖-怒江结合带东段及冈底斯-念青唐古拉板片北缘的腾冲-班戈岩浆弧带上;受被动陆缘深海复理石建造、 脆-韧性剪切带和深成岩浆活动控制。流体包裹体类型以单相和A型两相盐水溶液包裹体为主;次为B型两相盐水溶液包裹体、 富CO2两相包裹体和含CO2三相包裹体;成矿流体属中低温和中低盐度H2O-NaCl-CO2体系。氢、 氧同位素和相对富含K+、 Na+、Ca2+、 Mg2+、Cl-、SO2-4 的包裹体液相成分;显示成矿流体主要来自长期封存于沉积岩的深循环热（卤）水中;矿床成因类型为浅成中低温地下热（卤）水溶滤型金矿床。其成矿特征和流体性质与穆龙套型金矿床具有诸多相似性;研究该区域对西藏“沙丁板岩系”分布区金铜多金属找矿具有重要意义。     

The Moreira Gomes deposit of the Cuiú-Cuiú goldfield: Fluid inclusions and stable isotope constraints and implications for the genesis of granite-hosted gold mineralization in the Tapajós Gold Province,Brazil

Moreira Gomes is a recently discovered deposit (21.7 t Au) of the Cuiú-Cuiú goldfield, Tapajós Gold Province, Amazonian Craton. The mineralized zone is about 1200 m long, 30–50 m wide, and at least 400 m in depth. The zone is controlled by a subvertical, east–west-trending structure that is related to a left lateral strike-slip fault system. The host rocks are predominantly tonalites of the Creporizão Intrusive Suite (1997 ± 2 Ma) of uncertain tectonic setting (magmatic arc or post-collision). Hydrothermal alteration and mineralization are predominantly of the fissure-filling type and locally pervasive. Sericitization, chloritization, sulfidation, silicification, carbonatization and epidotization are the observed alteration types. Pyrite is the predominant sulfide mineral and bears inclusions of chalcopyrite, galena, sphalerite and minor hessite and bismuthinite. Gold occurs predominantly as inclusions in pyrite and subordinately in the free-milling state in quartz veins. Ag, Pb and Bi have been detected by semi-quantitative EDS analysis.Three types of fluid inclusions, hosted in quartz veins and veinlets, have been identified. (1) one- and two-phase CO₂ inclusions; (2) two- and three-phase H₂O–CO₂-salt inclusions, and (3) two-phase H₂O-salt inclusions. The CO₂-bearing types are interpreted as the product of phase separation of an immiscible fluid. This fluid presents low to moderate density, low to moderate salinity (1.6–11.8 wt.% NaCl equivalent) and was trapped at 280° to 350 °C. The chemical system of the aqueous inclusions may contain CaCl₂ and/or MgCl₂, salinity varies from zero to 10.1 wt.% NaCl equivalent. Only locally salinities up to 25% have been recorded. This fluid was trapped between 120° and 220 °C and is interpreted as resulting from mixing of a hotter and more saline aqueous fluid (in part derived from phase separation of the H₂O–CO₂ fluid) with a cooler and dilute aqueous fluid.The δ³⁴S values of pyrite (−0.3‰ to 3.9‰) are probably related to magmatic sulfur. The isotopic composition of inclusion fluids and of the fluid in equilibrium with hydrothermal minerals (quartz, chlorite, and calcite) show δ¹⁸O and δD values that range from +0.5 to +9.8‰, and from −49 to −8‰, respectively. Mineral pairs show equilibrium isotopic temperatures that are compatible with the fluid inclusion homogenization temperatures and with textural relationships of the hydrothermal minerals.Isotopic results combined with mineralogical and fluid inclusion data are interpreted to reflect a magmatic-hydrothermal system that evolved in at least three stages. (1) Exsolution of a CO₂-bearing magmatic fluid between 400 °C and 320–350 °C and up to 2.1 kbar (6 km in depth) followed by phase separation and main precipitation of the hydrothermal assemblage composed of chlorite–sericite–pyrite–quartz-gold. (2) Cooling and continuous exsolution of CO₂ produced a CO₂-depleted and slightly more saline aqueous fluid that was trapped mainly at 250°–280 °C. The predominant hydrothermal assemblage of stage 1 continued to form, but epidote is the main phase at this stage. (3) Mixing of the stage 2 aqueous fluid with a cooler and dilute aqueous fluid of meteoric origin, which was responsible for the main carbonatization phase. The mineralizing fluid was neutral to slightly alkaline and relatively reduced. H₂S (and/or HS^-) might have been the main sulfur species in the fluid and Au(HS)^2- was probably the gold transporting complex. Gold deposition occurred as a consequence of a combination of mechanisms, such as phase separation, mixing and fluid-rock interaction.The Moreira Gomes is a granite-hosted gold deposit that is interpreted to be a product of a magmatic-hydrothermal gold system. The age of ore formation (∼1.86 Ga) is consistent with the final stages of evolution of the widespread high-K, calc-alkaline Parauari Intrusive Suite, although the transitional to predominantly alkaline Maloquinha Intrusive Suite cannot be ruled out. Notwithstanding, the deposit does not show the classic features of (oxidized or reduced) intrusion-related gold deposits of Phanerozoic magmatic arcs.     

广西苍梧社垌石英脉型钨钼多金属矿床流体演化及来源示踪

广西苍梧社垌矿床是大瑶山隆起南侧新发现的一个大型斑岩-矽卡岩-石英脉型钨钼多金属矿床。本文重点对其中的石英脉型矿床进行了研究,依据脉体穿插关系及矿物共生组合将成矿过程划分为I石英-黄铁矿阶段、II石英-白钨矿-辉钼矿阶段、III石英-多金属硫化物阶段以及IV石英-方解石-萤石阶段,其中II和III阶段为主成矿阶段。从早到晚,均一温度逐渐下降(第一阶段550℃→370℃,第二阶段370℃→330℃,第三阶段330℃→210℃,第四阶段190℃→150℃),流体密度逐渐上升(0.61g/cm3→0.72g/cm3→0.82g/cm3→0.94g/cm3),盐度先升后降(第一阶段5.86%～8.55%NaCleqv,第二阶段4.49%～43%NaCleqv,第三阶段0.53%～46.37%NaCleqv,第四阶段0%～12.85%NaCleqv)。激光拉曼成分分析显示,社垌石英脉型矿床的成矿流体属于H2O-NaCl体系,但是该体系的流体成分在成矿前后发生了较大的变化,反映第I阶段以氧化环境为主,(II、III、)IV阶段则为还原环境。氢氧同位素研究显示成矿流体来自于岩浆水,后期大气降水的加入导致矿质发生沉淀。此外金属硫化物的δ34S组成(-3.8‰～+1.7‰)平均为-0.46‰,接近于零值,也表明为岩浆来源。引起矿质沉淀的主要原因是流体混合导致的温度下降等环境条件的改变。总体而言,社垌石英脉型钨钼多金属矿床的成矿流体主要来自岩浆热液,为中高温、中低盐度、低密度的NaCl-H2O流体体系,钨钼等多金属成矿与区内加里东期岩浆作用密切相关。     

四川省白玉县呷村-有热矿区成矿流体地球化学

四川省白玉县呷村银多金属矿床是我国著名的VMS型矿床之一,该矿床由西矿带热液流体补给通道相的脉状-网脉状矿化系统和东矿带的海底盆(洼)地卤水池喷气-化学沉积系统组成。有热矿床紧邻呷村矿床的南部,实质上是呷村矿带(体)的自然南延部分,具有相同的地质背景和成矿环境。本文分别对呷村西矿带、东矿带以及有热矿床进行了主成矿期石英的流体包裹体测试和氢、氧同位素分析以及硫化物的硫同位素分析。显微测温结果显示,呷村矿床从西矿带到东矿带,即由深部向浅部表现为成矿温度下降(258.0～209.8℃),流体的盐度略变小(4.42%～4.18%NaCleqv),而流体的密度增大(0.816～0.894g/cm3),并且有热矿床成矿流体特征(平均成矿温度为244.3℃;平均盐度为4.71%NaCleqv;平均密度为0.841g/cm3)与呷村西矿带流体特征更类似。显微激光拉曼光谱分析显示流体包裹体的液相成分主要为H2O,气相成分为H2O、CO2、N2以及CH4。氢、氧同位素研究显示,成矿流体为海水和岩浆水的混合流体。硫同位素分析结果表明,呷村西矿带(δ34S平均值为-3.65‰)与呷村东矿带的硫(δ34S平均值为-0.68‰)和有热矿床(δ34S平均值为-3.74‰)的硫都由深部岩浆提供,并且有热矿床与呷村西矿带的硫同位素特征更类似。成矿流体物理化学特征和同位素示踪结果表明,有热矿床目前已知矿体可与呷村西矿带对比,暗示可能存在尚未发现的类似呷村东矿带的富矿体。呷村-有热矿区的成矿机制为:在海水对流的成矿模式下,由岩浆水和海水混合而成的成矿流体,携带来自岩浆来源的成矿物质,自下而上向上运移和循环,在热液补给通道和海底发生淀积作用,形成脉状-网脉状矿体和块状矿体。