Oxygen isotopes in single micrometer-sized quartz grains: tracing the source of Saharan dust over long-distance atmospheric transport

Oxygen isotope compositions were measured by ion microprobe in individual micrometer-sized quartz grains extracted from one aerosol sample collected on the Cape Verde Islands and from four surface samples (three soils and one sediment) representing potential source regions of aerosols in Western and Central Africa (Morocco, Algeria, Niger, and Chad). A large range of δ¹⁸O values, from +6.2‰ to +39.3‰ is present within the aerosol quartz grains. The different size fractions of the quartz grains from the surface samples overlap nearly entirely this range but show significant differences in their δ¹⁸O distributions for the different size fractions of the grains (i.e., different modes, different proportions of grains with low or high δ¹⁸O, ...). These differences in δ¹⁸O distributions can be related to different geological formations (i.e., mantle-derived magmatic rocks, crustal magmatic rocks, or sedimentary rocks) outcropping in each region, thus giving a fingerprint of the source region. Quartz grains with unusually high δ¹⁸O values between +30‰ and +40‰ were attributed to lacustrine cherts formed in evaporitic environments (Chad basin).The existence of distinct δ¹⁸O distributions for the surface samples, which reflect regional geology but indicate some transport of the grains, enables the characterization of mixing processes during dust emission in the atmosphere. Particles are mixed at a regional scale in the dust reservoir, but injection of fine particles into the high troposphere occurs as a discrete and localized event with no mixing during the subsequent long-range transport by the easterlies. The comparison of the δ¹⁸O distribution of the quartz from the aerosol sample with the equivalent size fractions in surface samples shows that the Niger area is the more probable source region for the aerosol although the Moroccan source cannot be excluded. This method gives a valuable tool to trace the source region of dust into the atmosphere or into sediment samples, allowing the reconstruction of air mass circulations.     

Relation between U-series nuclide migration and microstructural properties of sedimentary rocks

Natural radionuclide migration in Miocene sedimentary rocks has been studied at the Tono U deposit in Japan, which is a potentially useful analogue of radioactive waste isolation in geological environments.Uranium series disequilibrium studies have revealed that natural radionuclides have been redistributed during the past 3.5 × 10⁵ a in the U-mineralized zone. Permeability tests using core samples of the U-mineralized zone have shown that the microfabrics of sedimentary rocks, such as the connectivity of pores which control the groundwater movement, directly influence the redistribution of nuclides.Detailed observations using cathodoluminescence (CL) and electron probe microanalysis (EPMA) were carried out after a dye-impregnation test in order to improve the understanding of the importance of microfabrics in relation to nuclide migration. Results of the observations show that U has migrated within detrital grains, such as biotite and quartz, as well as between these grains. Uranium-series disequilibrium studies of each detrital quartz and biotite grain have been shown that these detrital minerals behave differently in respect of the migration of U, owing to their different textural properties. The detrital biotite flakes, especially along the (001)-cleavage planes, appear to have fixed U for a long period of time, whereas the U has remained mobile within the microfractures in detrital quartz grains.     

超临界流体的快速泄出:来自霓辉石英脉的证据

河南外方山地区熊耳群火山岩中新发现了一种霓辉石英脉群,脉表现出独特的矿物组合及晶体特征,如霓辉石英脉具有复杂的矿物组成,表现出强烈的不平衡特点;霓辉石英脉具明显的对称分带特征,各分带中矿物组成及特征显著不同;存在具熔蚀边结构的钾长石-霓辉石集合体团块;霓辉石具在明显核幔结构,内核化学成分与幔边部存在较大差异;钾长石晶体元素分布存在似"文象"结构,表现出Fe元素的熔体分离特征;脉中矿物普遍发育环带结构;霓辉石英脉交代致密火山岩围岩形成蚀变带,新生大量榍石、霓辉石等矿物且含量随着与脉壁距离的增加急剧减少,显示出强烈的流体/气体扩散交代作用.霓辉石英脉独特的矿物组合及晶体特征反映了其成因的复杂性以及生长环境的变化.根据霓辉石英脉矿物的特点,推测其源于流体、熔体和晶体的混合物,并经历了快速侵位、快速冷却的过程,因而能够保存流体演化各阶段的结晶信息,保留不同成因的晶体群.外方山霓辉石英脉矿物可划分为捕虏晶亚群、熔体晶体群、超临界流体亚群、热液晶体亚群和凝聚晶体亚群等.外方山霓辉石英脉矿物晶体群的深入研究可能有助于揭示岩浆熔体向流体转换的机制及侵位机制,进而可揭示岩浆-流体成矿作用.     

Comparative scanning electron microscopy study of oriented till blocks, glacial grains and Devonian sands in Estonia and Latvia

Samples of Middle Devonian (Eifelian age; 387–380 Ma) indurated and non‐cemented sandstone were compared with Pleistocene basal tills in Estonia and Latvia to test a hypothesis that glacial SEM (scanning electron microscopy) microtextures are distinctly different from those produced in a fluvial depositional environment. The deposits of Middle Devonian Aruküla Stage were emplaced in a continental water basin close to sea level and well away from any glacial source. Therefore, the SEM microtextures on quartz grains from the Aruküla Stage should show mainly the effect of stream transport. The basal tills are of Late Weichselian age deposited as ground moraine directly over the sandstone. Additional glaciofluvial and glaciolacustrine samples were included with the tills to determine whether glacial and fluvial‐lacustrine transport could be differentiated by the SEM microtextures. Samples of oriented blocks of till from a limited number of sites were studied without pretreatment to determine whether sand clast orientation could provide a method for determining glacial flow vectors. While there are some microtextural similarities between grains from glacial and glaciofluvial‐lacustrine depositional environments, the vast majority of grains from till deposits (50%–60%) are faceted, sharp edged, angular to subangular, and comprised of numerous and distinct microfeatures including abraded surfaces over microfractures, deep linear and curved troughs (striations), step features, and a preponderance of conchoidal and linear microfractures. Glaciofluvial and lacustrine grains contain abundant abrasion features and v‐shaped percussion cracks that make them very distinct from glacial grains. Fluvial transport produces primarily rounded grains, well abraded, with v‐shaped percussion scars dominating. Thus, it is possible to use microtextural differences between the three sample suites to identify particular depositional environments. Oriented till blocks provide information on sand clast orientation. Although carbonate coatings often obscure sand clasts in untreated blocks, it is possible to determine some microfabric information that can be useful in determining flow direction of the ice.     

Grain-scale melt distribution in two contact aureole rocks: implications for controls on melt localization and deformation

The grain‐scale spatial arrangement of melt in layer‐parallel leucosomes in two anatectic rocks from two different contact aureoles located in central Maine, USA, is documented and used to constrain the controls on grain‐scale melt localization. The spatial distribution of grain‐scale melt is inferred from microstructural criteria for recognition of mineral pseudomorphs after melt and mineral grains of the solid matrix that hosted the melt. In both rocks, feldspar mimics the grain‐scale distribution of melt, and quartz is the major constituent of the solid matrix. The feldspar pockets consist of individual feldspar grains or aggregates of feldspar grains that show cuspate outlines. They have low average width/length ratios (0.54 and 0.55, respectively), and are interstitial between more rounded and equant (width/length ratios 0.65 for both samples) quartz grains. In two dimensions, the feldspar pockets extend over distances equivalent to multiple quartz grain diameters, possibly forming a connected three‐dimensional intergranular network. Both samples show similar mesoscopic structural elements and in both samples the feldspar pockets have a shape‐preferred orientation. In one sample, feldspar inferred to replace melt is aligned subparallel to the shape‐preferred orientation of quartz, indicating that pre‐ or syn‐anatectic strain controlled the grain‐scale distribution of melt. In the other sample, the preferred orientation of feldspar inferred to replace melt is different from the orientations of all other mesoscopic or microscopic structures in the rock, indicating that differential stress controlled grain‐scale melt localization. This is probably facilitated by conditions of higher differential stress, which may have promoted microfracturing. Grain‐scale melt distribution and inferred melt localization controls give insight into possible grain‐scale deformation mechanisms in melt‐bearing rocks. Application of these results to the interpretation of deep crustal anatectic rocks suggests that grain‐scale melt distribution should be controlled primarily by pre‐ or syn‐anatectic deformation. Feedback relations between melt localization and deformation are to be expected, with important implications for deformation and tectonic evolution of melt‐bearing rocks.     

The genesis of the silica sands of Black River, St Elizabeth, Jamaica

The only reported occurrence of quartz-rich sands in Jamaica is on the plains of Black River, St Elizabeth, where they outcrop in isolated pockets. The sands overlie limestones of the White Limestone Supergroup and the Coastal Group, and partly underlie Holocene peat deposits. Monocrystalline quartz, containing fluid and mineral inclusions, is the dominant mineral of the sands, with the heavy minerals. magnetite, haematite, epidote, rutile, anatase, zircon and tourmaline, occurring in minor amounts. Grain surface texture studies of the quartz grains have identified a series of mechanical and chemical features. Microtextures depicting marine, aeolian and pedological environments have been retained on the grains and represent the last three stages in the history of the grains. The petrography of the sands indicates an igneous provenance, with the major source of the detrital minerals being the Cretaceous granitoids that outcrop in the eastern half of the island. These rocks were unroofed during the uplift of Jamaica in the Late Tertiary and were subjected to extensive chemical weathering, which accounted for the breakdown of the majority of their primary silicate minerals. Fluvial transportation and deposition of detritus onto the shelf of the south coast commenced during this period and continued into the Pleistocene, providing terrigenous sand that was modified along the coast to quartz arenite by moderate to high energy marine conditions. Tidal currents, east-west longshore currents and weathering contributed to the maturity of the sand by the time it had been transported along the shelf to the vicinity of Black River. During low sea level stands in the Pleistocene, some of this sediment that became trapped on the shelf was blown inland by south-east winds.     

The parent material of Sydney laterites

The petrography of laterites in the Sydney district shows that source rocks range from shales—in which quartz grains are not common and average about 0.03 mm in diameter—to sandstones, with generally abundant quartz grains which may average up to 0.5 mm in diameter. Ferricretes or iron‐cemented duricrusts occur in rocks with coarser quartz grains, such as the Tertiary river gravels of the Maroota area. The porosity and permeability of these rocks have considerably influenced the nature and profiles of the laterites. Two possible sources of sesquioxides in the laterites appear to be the Wianamatta Shale and, perhaps more importantly, basic igneous rocks.     

Interpreting fracture development from diagenetic mineralogy and thermoelastic contraction modeling

Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 °C. The second set of microfractures (F2) is cemented by bituminite–pyrite, formed at temperatures between 60 and 100 °C, and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. The third set of microfractures (F3) is fully cemented by either quartz cement or calcite cement. The former has fluid inclusion homogenization temperatures between 149 and 175 °C. These temperatures are mostly higher than maximum burial temperatures (160 °C), suggesting that upward flow, caused by a pressure gradient, transported silica vertically which crystallized into the fractures. Upward decompression may have also caused a P_CO2 drop, which, at constant temperature, allowed simultaneous carbonate precipitation into the third microfracture set. The fourth set of thin-section scale microfractures (F4) is open or partially cemented by siderite–hematite and other iron oxides. The presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the Misoa formation. In order to time and place constraints on the depth of formation of the fourth set of microfractures, we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if, in fact, they could be associated with the uplift of the formation. Our results suggest that thermoelastic contraction, caused by the formation's uplift, erosion, and consequent cooling is a feasible mechanism for the origin of the last fracture set. Hence, we infer that meteoric water invasion into the fractures, at the end of the uplift, cause the precipitation of oxides and the transformation of siderite to hematite.     

Geological and Geochemical Characteristics of Gold Mineralization in the Salu Bulo Prospect,Sulawesi, Indonesia

The Salu Bulo prospect is one of the gold prospects in the Awak Mas project in the central part of the western province, Sulawesi, Indonesia. The gold mineralization is hosted by the meta‐sedimentary rocks intercalated with the meta‐volcanic and volcaniclastic rocks of the Latimojong Metamorphic Complex. The ores are approximately three meters thick, consisting of veins, stockwork, and breccias. The veins can be classified into three stages, namely, early, main, and late stages, and gold mineralization is related to the main stage. The mineral assemblage of the matrix of breccia and the veins are both composed of quartz, carbonate (mainly ankerite), and albite. High‐grade gold ores in the Salu Bulo prospect are accompanied by intense alteration, such as carbonatization, albitization, silicification, and sulfidation along the main stage veins and breccia. Alteration mineral assemblage includes ankerite ± calcite, quartz, albite, and pyrite along with minor sericite. Pyrite is the most abundant sulfide mineral that is spatially related to native gold and electrum (<2–42 μm in size). It is more abundant as dissemination in the altered host rocks than those in veins. This suggests that water–rock interaction played a role to precipitate pyrite and Au in the Salu Bulo prospect. The Au contents of intensely altered host rocks and ores have positive correlations with Ag, Ni, Mo, and Na. Fluid inclusions in the veins of the main stage and the matrix of breccia are mainly two‐phase liquid‐rich inclusions with minor two‐phase, vapor‐rich, and single‐phase liquid or vapor inclusions. CO₂ and N₂ gases are detected in the fluid inclusions by Laser Raman microspectrometry. Fluid boiling probably occurred when the fluid was trapped at approximately 120–190 m below the paleo water table. δ¹⁸O_SMOW values of fluid, +5.8 and +7.6‰, calculated from δ¹⁸O_SMOW of quartz from the main stage vein indicate oxygen isotopic exchange with wall rocks during deep circulation. δ³⁴S_CDT of pyrite narrowly ranges from ?2.0 to +3.4‰, suggesting a single source of sulfur. Gold mineralization in the Salu Bulo prospect occurred in an epithermal condition, after the metamorphism of the host rocks. It formed at a relatively shallow depth from fluids with low to moderate salinity (3.0–8.5 wt% NaCl equiv.). The temperature and pressure of ore formation range from 190 to 210°C and 1.2 to 1.9 MPa, respectively.     

Composition of Monocrystalline Detrital and Authigenic Minerals,Metamorphic Grade,and Provenance of Torlesse and Waipapa Graywacke,Central North Island,New Zealand

Sandstones of the juxtaposed and partially coeval quartzofeldspathic Torlesse terrane and volcanogenic Waipapa terrane of North Island, New Zealand, are generally described as having been derived from silicic continental arc and evolved intermediate volcano-plutonic arc sources, respectively. Modal and chemical compositions of the two terranes differ slightly as a result. From textural considerations, their single-grain (unitary) detrital mineral populations are inferred to have been derived largely from the plutonic components in their sources. Intensive microscopic and electron microprobe study of two representative samples shows that the unitary detrital mineral assemblages in the two terranes are virtually identical, comprising quartz, plagioclase, K-feldspar, white mica, epidote, titanite, pumpellyite, ilmenite, rutile, tourmaline, zircon, and apatite. Detrital chlorite, garnet, and graphite also occur in the Torlesse sample, whereas amphibole, clinopyroxene, and prehnite occur in the Waipapa sample. Authigenic mineral assemblages are also similar, consisting of quartz, albite, chlorite, phengitic mica, epidote, titanite, pumpellyite, pyrite, and calcite. Stilpnomelane and pyrrhotite also occur in the Torlesse sample, and prehnite in the Waipapa specimen. These assemblages define upper prehnite-pumpellyite to lower pumpellyite-actinolite facies conditions (Torlesse) and lower prehnite-pumpellyite facies metamorphism (Waipapa). By comparison with published compositional data for minerals from plutonic, metamorphic, and volcanic rocks, electron microprobe analyses of individual minerals confirm that the unitary detrital grains in both terranes were largely derived from calc-alkaline S-type granitoid plutonic rocks. Contrasts in mineral compositions between the two terranes show that the Torlesse unitary mineral detritus was derived almost entirely from granodiorite, whereas the Waipapa grains originated from a mixed diorite, monzonite, and granodiorite plutonic component in their source. In neither terrane was detritus derived from granite in the strict sense. Although the plutonic components in their sources are lithologically similar, the compositional contrasts seen indicate that they were not coeval or spatial components of the same terrane. Detailed electron microprobe analysis of unitary detrital phases in low-grade metasedimentary rocks thus enables identification of specific source terrane lithotypes, and hence is a valuable complement to existing petrographic, modal, and chemical approaches that define more generalized provenances.     

Microstructural evidence for magma confluence and reusage of magma pathways: implications for magma hybridization,Karakoram Shear Zone in NW India

In the Karakoram Shear Zone, Ladakh, NW India, Miocene leucogranitic dykes form an extensive, varied and complex network, linking an anatectic terrane exposed in the Pangong Range, with leucogranites of the Karakoram Batholith. Mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions, and microstructures suggest anatexis was contemporaneous with shearing. The network is characterized by continuous and interconnected dykes, with only rare cross‐cutting relationships, forming swarms and chaotic injection complexes where magmatic rocks cover up to 50% of the outcrop area. Despite this volume of magma, the system did not lose continuity, suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously. Leucogranites in this network, including leucosomes in migmatites, carry an isotopic signature intermediate between the two main anatectic rocks in the source, suggesting efficient homogenization of the magmatic products. Here, we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features. These include: (i) narrow, tortuous corridors of fine‐grained minerals cutting across or lining the boundaries of larger grains, interpreted to be remnants of magma‐filled cracks cutting across a pre‐existing magmatic rock; (ii) corrosion of early formed grains at the contact with fine‐grained material; (iii) compositional zoning of early formed plagioclase and K‐feldspar grains and quartz overgrowths documented by cathodoluminescence imaging; (iv) incipient development of rapakivi and anti‐rapakivi textures, and (iv) different crystallographic preferred orientation of early formed quartz and fine‐grained quartz. Mapping of the fine‐grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the S‐C fabric defined by pre‐existing magmatic grains. We conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches. New influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an S‐C fabric related to syn‐magmatic shearing. A mixed isotopic signature resulted not from the mixing of magmas, but from the micro‐scale interaction between new magma batches and previously crystallized magmatic rocks, through local equilibration.     

Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Submicroscopic opaque particles from highly shocked granite-gneisses close to the core of the Vredefort impact structure have been investigated by means of micro-analytical techniques with high spatial resolution such as electron diffraction, orientation contrast imagery and magnetic force microscopy. The opaque particles have been identified as nano- to micro-sized magnetite that occur in several distinct modes. In one sample magnetite occurs along relict planar deformation features (PDFs) in quartz, generally accepted as typical shock lamellae. The magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations. In most instances, the groups of magnetite within different quartz grains are systematically misorientated such that they share a subparallel <101> direction. The magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space. In a second sample, orientations of magnetite particles have been measured in microfractures (non-diagnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientations of magnetite particles are diverse, with only a minor portion having a preferred orientation. Scanning electron microscopy shows that magnetite along the relict PDFs is invariably associated with other microcrystalline phases such as quartz, K-feldspar and biotite. Petrographic observations suggest that these microcrystalline phases crystallized from locally formed micro-melts that intruded zones of weakness such as microfractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heat may have dissipated rapidly resulting in melts crystallizing relatively close to where they were generated. Magnetic force microscopy confirms the presence of magnetic particles along PDFs. The smallest particles, <5 μm with high aspect ratios 15:1 usually exhibit intense, uniform magnetic signals characteristic of single-domain magnetite. Consistent offsets between attractive and repulsive magnetic signals of individual single-domain particles suggest consistent directions of magnetization for a large proportion of particles. Received: 16 November 1998 / Accepted: 17 May 1999     

东营凹陷富有机质烃源岩顺层微裂隙的发育与油气运移

根据岩心观察，在东营凹陷沙河街组三段下亚段和沙河街组四段上亚段烃源岩中，发现大量顺层面分布的微裂隙，且大部分为亮晶方解石所充填。裂隙发育段在平面上的展布较广，在纵向上始于2900～3000m左右，与烃源岩成熟阶段对应。岩石学和有机地化研究表明，裂隙一般限于高有机质丰度、纹层理发育的页岩中，而有机质丰度较低、具块状层理的泥岩一般不发育，表现出较强的非均质性。裂隙发育与有机质丰度以及成熟度的关系表明，烃类生成是水平裂隙产生的主要原因。进入成熟阶段的富有机质烃源岩大量生烃，造成孔隙流体压力迅速上升，并最终导致顺层裂隙的产生。由于沙河街组三段下亚段和沙河街组四段上亚段以泥、页岩互层为特征，因而裂隙发育的非均质性造成裂隙发育段与裂隙不发育段的纵向叠置，并决定了其排烃过程以近似于三角洲前缘砂泥岩互层的排烃方式，从而一定程度提高了烃源岩的排烃速率和排烃效率。     

The complexity of sediment recycling as revealed by common Pb isotopes in K-feldspar

Detrital zircon U-Pb geochronology has become the gold standard in evaluating source to sink relationships in sedimentary basins. However, the physical and chemical robustness of zircon, which make it such a useful mineral for provenance studies, is also a hindrance as zircon can be recycled through numerous sedimentary basins, thus obscuring the first cycle source to sink relationship. An elegant approach to addressing this potential issue is to compare the Pb isotope composition of detrital K-feldspar, a mineral which is unlikely to survive more than one erosion-transport-deposition cycle, with that of magmatic K-feldspar from potential basement source terranes. Here we present new in situ Pb isotope data on detrital K-feldspar from two Proterozoic arkosic sandstones from Western Australia, and magmatic K-feldspar grains from potential igneous source rocks, as inferred by the age and Hf isotope composition of detrital zircon grains. The data indicate that the detrital zircon and K-feldspar grains could not have been liberated from the same source rocks, and that the zircon has most likely been recycled through older sedimentary basins. These results provide a more complete understanding of apparently simple source to sink relationships in this part of Proterozoic Western Australia.     

Microfabric analyses applied to the Witwatersrand gold- and uranium-bearing conglomerates: constraints on the provenance and post-depositional modification of rock and ore components

Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle-ductile crystal-plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.     

Morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy

In pelitic schists composed mainly of quartz and albite grains, the morphology of intergranular pores, which were filled with water, was studied by transmission electron microscopy (TEM). Although some pores are defined by crystallographic planes (F-face), most of their form has an ideal shape determined by interface tensions between grains and fluid. High-resolution TEM observations demonstrate that pore-free regions at grain boundaries are tight even at the nanometer scale, showing that the wetting angle is larger than 0° in this rock. The pore distribution in two-grain junctions can be compared to a "necklace microstructure" developed by instability of a fluid film along the boundary induced by microcracking. Wetting angles for pores located at grain edges of quartz and albite decrease in the order albite/albite, quartz/quartz, and quartz/albite. The quartz/quartz wetting angle in a calcite-free sample is smaller than that in a calcite-containing sample. This angle also changes due to grain misorientation. Our results confirm that solid-solid and solid-fluid interfacial energies control the geometry of intergranular fluid in natural rocks.     

Cathodoluminescence imaging and titanium thermometry in metamorphic quartz

Cathodoluminescence (CL) of quartz from metamorphic rocks representing a range of conditions from the garnet grade to the migmatite grade reveals a variety of textures, that is, a function of metamorphic grade and deformation history. Ti concentrations, determined by electron microprobe and ion microprobe, generally correlate with CL intensity (blue wavelengths), and application of the Ti‐in‐quartz thermometer (TitaniQ) reflects the temperature of quartz growth or recrystallization, and, in some settings, modification by diffusion. Quartz from garnet grade samples is not visibly zoned, records temperatures of 425–475 °C, and is interpreted to have recrystallized during fabric formation. Quartz grains from staurolite grade samples are zoned in CL with markedly darker cores and brighter rims, some of which are interpreted to have been produced by the dominant staurolite‐producing reaction, whereas others are interpreted as having formed by diffusion of Ti into quartz rims. Quartz from the matrix of kyanite and sillimanite grade samples are generally unzoned, although locally displays slightly brighter rims (higher Ti); quartz inclusions within garnet and staurolite have distinctly brighter rims, which are interpreted as having been produced by diffusive exchange with the host mineral. Quartz from migmatite grade samples displays highly variable CL intensity, which is dependent on the location of the grain. Matrix grains in melanosomes are largely unzoned or rarely zoned with darker cores. Leucosome quartz is strongly zoned with bright cores and dark rims and is interpreted as having formed during crystallization of the melt. Locally within the leucosome is observed oscillatory‐zoned quartz, which is interpreted as a subsolidus recrystallization to achieve strain relaxation. Quartz inclusions within garnet or plagioclase crystals often show bright domains separated by zones of dark CL. These enigmatic textures possibly reflect local melting fluxed by fluid inclusions. Temperatures calculated from the Ti–in–quartz thermometer are a function of the metamorphic grade of the sample, the textural setting of the quartz, the reaction history and the deformation history of the rock. The TitaniQ temperatures can be used to constrain the conditions at which various metamorphic processes have occurred.     

Facies controlled diagenetic evolution of the Delhi Group Sandstones,Bayana Basin,Rajasthan

The Bayana Basin forms the eastern most limit of north Delhi fold belt covering parts of northeastern Rajasthan. The deposition of sediments took place during middle Proterozoic in fluvial and shallow marine environments. The rocks are mainly clastics and include conglomerate, sandstone and shale. During mechanical compaction rearrangement of grains took place and point and long contacts were formed. The sandstones are cemented by iron oxide, silica, carbonate and barite. The porosity has developed due to dissolution of iron, carbonate cement and feldspar grains. Dissolution of quartz along grain boundaries and silica rich compaction pore water seems to be the main source of silica. These observations suggest progressive compaction which initiated at the sediment water interface and continued till deep burial diagenesis in a rapidly subsiding basin.     

Oxide and sulphide isograds along a Late Archean, deep-crustal profile in Tamil Nadu, south India

Oxide–sulphide–Fe–Mg–silicate and titanite–ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high‐grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo‐ilmenite in the high‐grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe–Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite–pyrite micro‐veins along silicate grain boundaries formed over a wide range of post‐peak metamorphic temperatures and pressures ranging from high‐grade SO₂ to low‐grade H₂S‐dominated conditions. Oxygen fugacities estimated from the orthopyroxene–magnetite–quartz, orthopyroxene–hematite–quartz, and magnetite–hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO₄ during high‐grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase.     

Scanning electron microscopy of Pleistocene tills in Estonia

Tills from four Pleistocene glaciations were recovered from drill cores in Estonia and subjected to particle size and microtexture analyses by Scanning Electron Microscope (SEM). All tills were deposited by thick continental ice-sheets following the transport of, at most, several hundred kilometers during four Fennoscandian glaciations. The main problem is to determine if the type and range of microtextures present on the grain surfaces are diagnostic of transport in continental ice. The frequency of occurrence of microtextures including fractures, abrasion, and relief features are used to test the ability of continental ice to damage quartz particles emplaced as till. The range of quartz dissolution and presence of coatings on grains are also used to reconstruct the paleoenvironment that existed prior to transport as well as to estimate diagenetic effects that occurred following emplacement. The available data indicate a high degree of reworking of quartz grains from one glaciation to another. While the shapes and microtextures of grains from source rocks are not known, the great range of fracture and abrasion microfeatures, and high frequency of occurrence on grains in all tills, indicate that glaciers are effective crushing agents. An increase in the prevalence of chemically etched grains from older to younger tills suggests that some grains ( c . 50%) escape crushing, either because of preservation in the ice and lack of grain-to-grain contact, or as a result of massive reworking of weathered grains following interglaciations.