SEARCH · Search USGS
Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Photographs of a Late Cretaceous species of Aquilapollenites illustrate the usefulness of the scanning electron microscope (SEM) for the study of fossil pollen . The SEM has the advantages of great depth of focus and wide range of magnification; using present techniques, its use for the study of unsectioned specimens is limited to the observation of surface detail.
This work describes dust deposits on the Spirit Rover over 2000 sols through examination of Pancam and Microscopic Imager observations of specific locations on the rover body, including portions of the solar array, Pancam and Mini-TES calibration targets, and the magnets. This data set reveals the three "cleaning events" experienced by Spirit to date, the spectral properties of dust, and the tendency of dust particles to form aggregates 100 um and larger.
People have been fascinated by the variety of life in a drop of water since microscopes were invented.
Numerous porphyry copper-molybdenum-gold and epithermal deposits define a belt that extends from Eastern Alaska to western Yukon, Canada. An orientation study conducted near the Taurus porphyry deposit was designed to test methods that require minimal sample collection, preparation, and analytical time to determine the viability of indicator mineral studies as a reconnaissance exploration method. Bulk stream sediments and altered and mineralized rocks were sieved to the 0.105−0.25 millimeter fraction (+140, −60 mesh) and passed over a shaking table to create a moderate to heavy mineral separate that was mounted in epoxy and subsequently analyzed using automated scanning electron microscope (SEM) techniques. Seven polished thin sections of core were also analyzed. Among the advantages of automated SEM techniques compared to visual mineral identification are that thousands of grains can be rapidly identified in each sample (about 1 hour per sample) and small quantities of indicator minerals that may be missed during traditional visual analyses can be detected. Automated SEM analyses of stream sediment and rock samples show that specific minerals (chalcopyrite, bornite, and jarosite) are indicators of potential mineralized areas. Svanbergite, an aluminum sulfate phosphate mineral, was identified in mineralized rocks and in nearly all stream sediment samples (up to 9 kilometers) downstream from the Taurus and other porphyry occurrences but not epithermal occurrences. It was not identified in areas with no known mineralization and thus it is possibly one of the best indicator minerals for porphyry copper (+/- molybdenum, gold) occurrences.
The refractory nature of zircon to temperature and pressure allows even a single zircon grain to preserve a rich history of magmatic, metamorphic, and hydrothermal processes. Isotopic dating of micro-domains exposed in cross-sections of zircon grains allows us to interrogate this history. Unfortunately, our ability to select the zircon grains in a heavy mineral concentrate that records the most geochronologic information is limited by our inability to predict internal zonation from observations of whole grains. Here we document the use of a petrographic microscope to observe and image the photoluminescence (PL) response of whole zircon grains excited under ultraviolet (UV) light, and the utility of this PL response in selecting grains for geochronology. While zircon fluorescence has long been known, there is limited documentation of its utility for and application to geochronologic studies. Our observations of zircon from an un-metamorphosed igneous rock, two meta-igneous rocks, and a placer deposit show that variations in the PL color are readily observable in real-time, both among grains in a population of zircons and within single grains. Analyses of cross-sections of the same grains demonstrate that the changes in PL correlate with zoning in backscattered electron (BSE) and cathodoluminescence (CL) images as well as with changes in U + Th concentration and spectroscopic proxies for radiation damage. In other words, the whole grain PL provides a low-resolution preview of the U + Th zoning expected in a cross-sectioned grain. We demonstrate the usefulness of this “preview” in identifying and selecting the subset of zircon grains in a heavy mineral separate that has metamorphic rims of sufficient width to date by secondary ionization mass spectrometry (SIMS). The data are also used to place preliminary constraints on the age and U + Th concentrations at which a yellow PL response is observed in natural samples. The PL response of zircon is well-known among spectroscopists, and these simple applications demonstrate several ways in which the response might be more effectively used by geochronologists.
The presence of disseminated gas hydrate was inferred based on pore fluid geochemistry and downhole logging data, but was rarely observed at Ocean Drilling Program (ODP) Leg 164 (Blake Ridge), and Leg 170 (Middle America Trench, offshore from Costa Rica) drilling sites. Gas hydrate nucleation is likely to occur first in larger voids rather than in constricted pore space, where capillary forces depress the temperature-pressure stability field for gas hydrate formation. Traditional macroscopic descriptions of sediment fail to detect the microscopic character of primary and secondary porosity in sediment hosting disseminated gas hydrate. Light transmission and scanning electron microscopy of sediments within and below the depth of gas hydrate occurrences reveal at least four general types of primary and secondary porosity: (1) microfossils (diatoms, foraminifera, and spicules) void of infilling sediment, but commonly containing small masses of pyrite framboids; (2) infauna burrows filled with unconsolidated sand and or microfossil debris; (3) irregularly shaped pods of nonconsolidated framboidial pyrite; and (4) nonlithified volcanic ash.
The petrographic microscope, designed to observe and measure the optical properties of minerals as a means of identifying them, has provided a foundation for mineralogical and petrological research for more than 120 years. Much of what is known today in these fields is attributable to this instrument, the development of which paralleled an evolution of fundamental optical theory and its correlation with mineral structure and composition. This instrument and its related accessories have evolved through a range of models and designs, which are in themselves distinctive for their scientific function and elegant construction, and are today prized by collectors of scientific instruments.
Sarcocystis sp. obtained from wild populations of grackles, Quiscalus quiscula (Linn.), were examined to clarify the effect of the parasite on the host. Electron micrographs are presented to show areas of muscle destruction adjacent to the parasite which appear to be mechanically produced by the parasite. The microtubules within the villus-like projections of the cyst suggest that their possible function is absorptive and/or conductive with regard to the production of a toxin or the conveyance of nutritive material to the developing cells. The proposed function of submembranous filaments and their relation to the conoid is discussed. Similarities in the ultrastructure to Toxoplasma and other protozoa tend to negate the relegation of Sarcocystis to the fungi and further emphasize its protozoan nature.
This paper describes the evaluation of petrographic textures representing the structural organization of the organic matter derived from coal and petroleum and their interaction phenomena in the making of steel electrodes, anodes and cathode blocks. This work represents the results of the Microscopy of Carbon Materials Working Group in Commission III of the International Committee for Coal and Organic Petrology between the years 2009 and 2013. The round robin exercises were run on photomicrograph samples. For textural characterization of carbon materials the existing ASTM classification system for metallurgical coke was applied. These round robin exercises involved 15 active participants from 12 laboratories who were asked to assess the coal and petroleum based carbons and to identify the morphological differences, as optical texture (isotropic/anisotropic), optical type (punctiform, mosaic, fibre, ribbon, domain), and size. Four sets of digital black and white microphotographs comprising 151 photos containing 372 fields of different types of organic matter were examined. Based on the unique ability of carbon to form a wide range of textures, the results showed an increased number of carbon occurrences which have crucial role in the chosen industrial applications. The statistical method used to evaluate the results was based on the “raw agreement indices”. It gave a new and original view on the analysts' opinion by not only counting the correct answers, but also all of the knowledge and experience of the participants. Comparative analyses of the average values of the level of overall agreement performed by each analyst in the exercises during 2009–2013 showed a great homogeneity in the results, the mean value being 90.36%, with a minimum value of 83% and a maximum value of 95%.
Research examining organic-matter hosted porosity has significantly increased during the last decade due to greater focus on understanding hydrocarbon migration and storage in source-rock reservoirs, and technological advances in scanning electron microscopy (SEM) capabilities. The examination of nanometer-scale organic-matter hosted porosity by SEM requires the preparation of exceptionally flat geologic samples beyond the abilities of traditional mechanical polishing, which can deform or otherwise obscure organic surfaces. To meet this demand, broad ion beam (BIB) milling was introduced as a sample preparation technique for SEM petrographic analysis of geologic samples. As with any sample preparation technique, there can be unintended consequences. In this study, we examined the development of nanometer-scale sputter-induced voids caused by BIB milling of thermoset plastic binder material [poly(methyl methacrylate), PMMA] used for the mounting of geologic samples, and artifact sputter-induced voids in the organic matter of Green River Formation and Kimmeridge Clay Formation source rocks. Development of artifact sputter-induced voids was evaluated in relation to variations in the slope of the sample examination surface (0.0–4.9% slope), effectively varying the angle of ion incidence. The results indicate that only minor variations in the angle of ion incidence can generate sputter-induced voids in both PMMA (10.0–386.2 nm diameter sputter-induced voids) and sample sedimentary organic matter (solid bitumen; 12.2–103.6 nm diameter sputter-induced voids). Overall, average artifact pore diameters increased with increasing ion incidence angle within PMMA. Although sputter-induced voids within solid bitumen in the Green River Formation sample were only found in limited extent, the size of these void artifacts falls within the same size range as naturally occurring meso-macroporosity. That is, the pore-like artifacts could easily be misconstrued as naturally developed organic porosity, which is a major concern for SEM-based porosity evaluation methods. This study describes the factors that contribute to the creation of artifact sputter-induced voids, their distinguishing characteristics, and best practices for avoiding the creation of ion-induced artifacts.
For more than a century, studies of sedimentology and sediment transport have measured bed-sediment grain size by collecting samples and transporting them back to the laboratory for grain-size analysis. This process is slow and expensive. Moreover, most sampling systems are not selective enough to sample only the surficial grains that interact with the flow; samples typically include sediment from at least a few centimeters beneath the bed surface. New hardware and software are available for in situ measurement of grain size. The new technology permits rapid measurement of surficial bed sediment. Here we describe several systems we have deployed by boat, by hand, and by tripod in rivers, oceans, and on beaches.
The origin of the Serpent Mound structure in south-central Ohio has been disputed for many years. Clearly, more evidence was needed to resolve the confusion concerning the origin of the Serpent Mound feature either by endogenic processes or by hypervelocity impact. A petrographic study of 21 samples taken from a core 903 m long drilled in the central uplift of the structure provides evidence of shock metamorphism in the form of multiple sets of planar deformation features in quartz grains, as well as the presence of clasts of altered impact-melt rock. Crystallographic orientations of the planar deformation features show maxima at the shock-characteristic planes of {101??3} and {101??2} and additional maxima at {101??1}, {213??1}, and {516??1}. Geochemical analyses of impact breccias show minor enrichments in the abundances of the siderophile elements Cr, Co, Ni, and Ir, indicating the presence of a minor meteoritic component.
Mercury (Hg) release from inoperative Hg mines in the California Coast Range has been documented, but little is known about the release and transport mechanisms. In this study, tailings from Hg mines located in different geologic settings-New Idria (NI), a Si-carbonate Hg deposit, and Sulphur Bank (SB), a hot-spring Hg deposit-were characterized, and particle release from these wastes was studied in column experiments to (1) investigate the mechanisms of Hg release from NI and SB mine wastes, (2) determine the speciation of particle-bound Hg released from the mine wastes, and (3) determine the effect of calcinations on Hg release processes. The physical and chemical properties of tailings and the colloids released from them were determined using chemical analyses, selective chemical extractions, XRD, SEM, TEM, and X-ray absorption spectroscopy techniques. The total Hg concentration in tailings increased with decreasing particle size in NI and SB calcines (roasted ore), but reached a maximum at an intermediate particle size in the SB waste rock (unroasted ore). Hg in the tailings exists predominantly as low-solubility HgS (cinnabar and metacinnabar), with NI calcines having >50% HgS, SB calcines having >89% HgS, and SB waste rock having ???100% HgS. Leaching experiments with a high-ionic-strength solution (0.1 M NaCl) resulted in a rapid but brief release of soluble and particulate Hg. Lowering the ionic strength of the leach solution (0.005 M NaCl) resulted in the release of colloidal Hg from two of the three mine wastes studied (NI calcines and SB waste rock). Colloid-associated Hg accounts for as much as 95% of the Hg released during episodic particle release. Colloids generated from the NI calcines are produced by a breakup and release mechanism and consist of hematite, jarosite/alunite, and Al-Si gel with particle sizes of 10-200 nm. ATEM and XAFS analyses indicate that the majority (???78%) of the mercury is present in the form of HgS. SB calcines also produced HgS colloids. The colloids generated from the SB waste rock were heterogeneous and varied in composition according to the column influent composition. ATEM and XAFS results indicate that Hg is entirely in the HgS form. Data from this study identify colloidal HgS as the dominant transported form of Hg from these mine waste materials.
Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of leftlateral transform motion between the African and Arabian plates since early Miocene (???20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/ Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the ??m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull-aparts along them. The damage zones of the individual faults are only 5-20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20-40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere. Copyright 2009 by the American Geophysical Union.
No abstract available.
No abstract available.
No abstract available.
Sublimates were sampled from high-temperature (up to 800°C) fumaroles at Merapi volcano, Indonesia in January 1984. Sampling is accomplished by inserting silica tubes into high-temperature vents. Volcanic gas flows through the tubes and sublimates precipitate on the inner walls in response to the temperature gradient. With decreasing temperature (800–500°C) in the tubes, there are five sublimate zones: 1) cristobalite, magnetite, and halite; 2) halite, sylvite, K-Ca sulfate (K 2 Ca[SO 4 ] 2 ), acmite, wollastonite, and pyrite; 3) halite, sylvite, galena, Pb-Bi sulfide, aphthitalite, sphalerite, and Cs-K sulfate; 4) halite, sylvite, galena, PbKCl 3 , aphthitalite, and Na-K-Fe sulfate ([Na, K] 2 Fe[SO 4 ]Cl 2 ); and 5) halite, sylvite, galena, PbKCl 3 , and various sulfates of Pb, Cu, and Zn. Texturally, the sublimate phases grade from large, well-formed crystals at their highest-temperature occurrence to more numerous, smaller crystals that are less perfect at lower temperatures. These changes imply that the crystal nucleation and growth rates increase and decrease, respectively, as temperature decreases. Several of the sublimate phases also exhibit highly anisotropic morphologies (whiskers, platelets, spheres), especially below their highest temperature of deposition. Overall, the textural data suggest that the gas is saturated or slightly supersaturated with the phases at their hottest occurrence, but that the gas becomes increasingly supersaturated with the phases at lower temperatures. The anisotropic morphologies probably form because the crystals grow toward more supersaturated conditions in the center of the tube. The gas fails to maintain equilibrium with the precipitating sublimates because the high velocity of the carrier gas prevents complete mass transfer from the gas stream to the tube walls.