Showing papers on "Petrography published in 2002"

Using Magnetic Resonance Imaging and Petrographic Techniques to Understand the Textural Attributes and Porosity Distribution in Macaronichnus-Burrowed Sandstone

Abstract: Magnetic resonance images are paired with petrographic data to evaluate the textural characteristics of rocks dominated by Macaronichnus segregatis, a trace fossil that is commonly associated with rocks deposited in shallow, marginal marine sedimentary environments. MRI techniques used revealed the three-dimensional geometry of the trace fossil. Burrows are typically horizontal and in plan view range between straight, sinuous, meandering, and spiral geometries. Changes in burrow morphology may be related to population density and patchy resource distribution. The pairing of MRI and petrographic data helped map the distribution of porosity in the burrowed rock. Because MRI images represent complex composites of nuclear spin density and MR relaxation times, each of which is related to pore size, stronger MR signals must be calibrated to known porous zones by integrating petrographic data with MR data. The complex distribution of porosity and its relationship to the matrix show that this fabric represents a dual porosity-permeability system and may affect the resource (reservoir or aquifer) quality of similarly burrowed sedimentary rocks. Future research should elaborate upon the porosity-permeability model for this and similar fabrics.

Karst-controlled diagenesis and reservoir development: Example from the Ordovician main-reservoir carbonate rocks on the eastern margin of the Ordos basin, China

Abstract: The Ordovician Majiagou Formation contains the main reservoir of the Ordos Central gas field in the Ordos basin. The producing zone at the top of member 5 consists of carbonate rocks modified during a long period of subaerial exposure and karstification from the Late Ordovician to the middle Carboniferous. On the eastern margin of the Ordos basin, the porosity of the exposed carbonate rocks of this unit ranges from 0.5 to 15.1%, and permeability ranges from 8%, and permeability ranges from <0.1 to 5 md. The main reservoir porosity is a dissolution-enhanced vuggy porosity, associated with dolomite. The carbonate rocks show great heterogeneity, reflecting the varying effects of karstification in creating and modifying porosity. Petrographic and geochemical analyses of various components in these carbonates provided evidence for depositional and diagenetic processes. The reservoir carbonates were deposited in shallow and restricted hypersaline environments and were later modified by karstification and burial diagenesis. Dolomitization appears to have resulted from mixing of marine and meteoric waters and probably occurred in both shallow and deep burial settings. Cementation by calcite also occurred in both shallow and deep environments, under different hydrodynamic conditions. Both depositional settings and diagenetic processes, such as leaching by meteoric water, paleokarstification, dolomitization, and cementation, controlled reservoir development. The outcrop and subsurface samples show similar petrographic features, porosity types, and geochemical characteristics, but the exposed section of the formation shows evidence of more alteration by meteoric water.

Tectonic history of a Jurassic backarc-basin sequence (the Gran Canon Formation, Cedros Island, Mexico), based on compositional modes of tuffaceous deposits

Abstract: The Jurassic Gran Canon Formation (Cedros Island, Baja California, Mexico) constitutes an unusually well preserved and exposed example of ancient backarc-basin fill. Petrofacies analysis conducted on tuff- aceous sandstone and tuff samples from this formation complement and reinforce prior lithofacies interpretations, but with some modification. When temporal and spatial trends in petrographic data (detrital modes) are analyzed and compared to mod- els based on data collected from Deep Sea Drilling Project and Ocean Drilling Pro- gram cores, the trends indicate a second, heretofore unrecognized, phase of backarc rifting. Basalt lavas interstratified with da- citic pyroclastic rocks of the primary vol- canic lithofacies, previously interpreted to record the eruption of differentiated mag- mas at the climax of growth of the Gran Canon island arc, are now as a result of this study considered to be the product of arc extension and rifting. Our method of modal analysis uniquely combines the quantification of textural at- tributes of pyroclastic and epiclastic debris that reflect eruption style and magma com- position, as well as the effects of reworking and mixing in marine settings. This study demonstrates that detailed petrographic analysis is useful in the interpretation of ancient volcaniclastic deposits suspected of having formed in backarc-basin settings.

A data brief on the Smithsonian Abyssal Volcanic Glass Data File

Abstract: [1] It is well known that the compositions of glasses quenched by contact with seawater are a good guide to unmodified erupted liquid compositions We present here an updated version of the Smithsonian Abyssal Volcanic Glass Data File (AVGDF) of the major and minor element compositions of such glasses mainly from seafloor spreading centers determined by electron microprobe analyses These data help define the major and minor element compositions of the most abundant terrestrial volcanic rock The AVGDF contains 9050 individual glass analyses along with the sample donor, latitude, longitude, depth, tectonic setting, ship-cruise-dredge, and the major and minor elements SiO2, Al2O3, FeO*, MgO, CaO, Na2O, K2O, TiO2, P2O5, and MnO (for some samples) The samples are from 664 Atlantic, 28 Caribbean, 89 Indian, and 1304 Pacific localities These include dredge, submersible, DSDP and ODP sites The data serve to (1) map out and model processes associated with magma generation and fractionation in the asthenosphere and lithosphere beneath seafloor spreading centers and (2) as an educational aide for exercises in the use of such data at the graduate and undergraduate levels in geochemistry, geophysics, and igneous petrology The extensive polished sample collection is available on loan for additional petrographic (in reflected light) or microbeam analyses

Mineralogical and geochemical aspects of impact craters

Abstract: The importance of impact cratering on terrestrial planets is obvious from the abundance of craters on their surfaces. On Earth, active geological processes rapidly obliterate the cratering record. To date only about 170 impact structures have been recognized on the Earth’s surface. Mineralogical, petrographic, and geochemical criteria are used to identify the impact origin of such structures or related ejecta layers. The two most important criteria are the presence of shock metamorphic effects in mineral and rock inclusions in breccias and melt rocks, as well as the demonstration, by geochemical techniques, that these rocks contain a minor extraterrestrial component. There is a variety of macroscopic and microscopic shock metamorphic effects. The most important ones include the presence of planar deformation features in rock-forming minerals, high-pressure polymorphs (e.g. of coesite and stishovite from quartz, or diamond from graphite), diaplectic glass, and rock and mineral melts. These features have been studied by traditional methods involving the petrographic microscope, and more recently with a variety of instrumental techniques, including transmission electron microscopy, Raman spectroscopy, cathodoluminescence imaging and spectroscopy, and high-resolution X-ray computed tomography. Geochemical methods to detect an extraterrestrial component include measurements of the concentrations of siderophile elements, mainly of the platinum-group elements (PGEs), and, more recently, chromium and osmium isotopic studies. The latter two methods can provide confirmation that these elements are actually of meteoritic origin. The Cr isotopic method is also capable of providing information on the meteorite type. In impact studies there is now a trend towards the use of interdisciplinary and multi-technique approaches to solve open questions.

Massive recrystallization of low-Mg calcite at high temperatures in hydrocarbon source rocks: Implications for organic acids as factors in diagenesis

Abstract: We document massive recrystallization of low-Mg calcite lime mudstone source rocks at moderately high temperatures in the Smackover Formation of Mississippi. This process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons, secondary porosity generation in reservoirs, and global environmental change. This interpretation is based on contrasting the following petrographic and bulk-rock geochemical characteristics of organic-poor vs. organic-rich lime mudstones:

Paleomagnetic dating of the smectite‐to‐illite conversion: Testing the hypothesis in Jurassic sedimentary rocks, Skye, Scotland

Abstract: [1] A hypothesized connection between clay diagenesis and magnetite authigenesis is supported by the results of paleomagnetic, rock magnetic, geochemical, and petrographic studies on Jurassic sedimentary rock of Skye, Scotland. The results have implications for understanding remagnetization mechanisms and for the development of a paleomagnetic method to date clay diagenesis. Previous diagenetic studies indicate that rock in north Skye contains abundant detrital smectite, whereas the clays in the same age rock in south Skye have altered to illite because of Tertiary igneous activity. Geochemical (87Sr/86Sr, δ13C, δ18O) studies confirm that sedimentary rocks in south Skye are altered. The magnetization in the rocks in north Skye is weak and unstable. In contrast, the rocks in south Skye contain a multicomponent magnetization. At intermediate temperatures (225°–450°C) a magnetization with southerly declinations and negative inclinations or its antipodal equivalent is present. At higher temperatures (450°–580°C) a magnetization with northerly declinations and positive inclinations or its antipodal equivalent is removed. Both magnetizations have directions similar to Tertiary igneous rocks on Skye. Rock magnetic studies indicate the presence of pyrrhotite and magnetite. The intermediate-temperature component resides in pyrrhotite and/or magnetite and is interpreted as a thermoviscous remanent magnetization or a thermochemical remanent magnetization related to the heat and/or hydrothermal activity associated with the igneous activity. The high-temperature component resides in magnetite and is a chemical remanent magnetization (CRM). The presence-absence test and the timing of acquisition for this CRM suggest that magnetite authigenesis is related to the smectite-to-illite conversion and that clay diagenesis is a viable remagnetization mechanism.

Fluid inclusion record of the hypozonal orogenic Renco gold deposit (Zimbabwe) during the retrograde P-T evolution

Abstract: The hypozonal Renco gold deposit in Zimbabwe is hosted by a shear zone system cutting granulite-facies enderbites. The geologic history of the region testifies to a polyphase tectonometamorphic evolution. Along the exhumation path, three major events are notable during which fluid inclusions could have been formed. Detailed petrographic work and comparison of calculated isochores with the P–T evolution, allows correlation of inclusion formation with the tectonometamorphic evolution. Fluid inclusions in the mineralized lithologies of the Renco gold mine are dominated by carbonic inclusions of highly variable densities. The high density carbonic inclusions are related to the peak of metamorphism. Their lower density counterparts show a bimodal occurrence. Some are re-equilibrated equivalents of the inclusions trapped at peak metamorphic conditions whereas others clearly post-date this event and are related to gold mineralization. The latter type of carbonic inclusions occurs in distinct microstructural domains of quartz crystals (i.e. subgrain-, grain boundaries). Therefore, these inclusions are interpreted to have formed because of selective trapping of CO2 from a mixed H2O–CO2 low salinity ore fluid. All the aqueous inclusions observed at Renco can be attributed to a late, retrograde greenschist-facies overprint.

Late-stage crystallization history of the jurassic north mountain basalt, nova scotia, canada. i. textural and chemical evidence for pervasive development of silicate-liquid immiscibility

Abstract: The Jurassic (201 Ma) quartz-normative continental tholeiitic basalts of the North Mountain Basalt (NMB) Formation of southern Nova Scotia, Canada, record evidence of pervasive silicate-liquid immiscibility. The basalts, up to 400 m thick, typically with phenocrysts of plagioclase (An50–70) and clinopyroxene (Wo40 En40Fs20), are subdivided into lower, middle, and upper units on the basis of the nature of the flows and petrographic features. Petrographic observations combined with image analysis indicate that the middle and upper units are characterized by an abundance of mesostasis material (i.e., quenched residual melt) that contains skeletal clinopyroxene, acicular plagioclase, skeletal Fe–Ti oxides, Fe–Ti–P-rich globules, and andesitic to rhyolitic glass. Raster analysis of the mesostasis tracks an in situ fractionation that culminated in formation of an interstitial felsic glass (i.e., 74 wt.% SiO2) that contains skeletal apatite and Fe-rich clinopyroxene. The composition of the skeletal clinopyroxene, ca. Wo20En15Fs65, acicular plagioclase (An35–55, ≤2.5 wt.% FeO), Fe–Ti–P-rich globules, and intergranular glass of the mesostasis are consistent with formation from an interstitial melt that evolved in a disequilibrium environment promoted by rapid cooling of the lavas. The mineralogical and chemical features are consistent with the process of silicate-liquid immiscibility within the residual liquids of basaltic systems, in this case late-stage intergranular melts. The presence within the NMB of (1) Fe-rich clinopyroxene-bearing mafic pegmatite, (2) thin (i.e., 1–2 cm) seams of rhyolite associated with the mafic pegmatites, and (3) locally abundant segregation pipes of mixed mafic–felsic composition, is considered to reflect mobilization of the Fe- and silica-rich immiscible melts.

Fluid inclusions record thermal and fluid evolution in reservoir sandstones, Khatatba Formation, Western Desert, Egypt: A case for fluid injection

Abstract: A fluid inclusion and petrographic study, focused on quartz overgrowths, was performed in reservoir sandstones from the Jurassic Khatatba Formation (Salam oil field, Egypt's Western Desert). The combination of detailed fluid inclusion petrography and scanning electron microscope (SEM) cathodoluminescence imaging has allowed us to relate individual fluid inclusion assemblages, that is, the most finely discriminated groups of petrographically associated fluid inclusions, to specific growth zones of authigenic quartz, establishing the relative timing of entrapment of the inclusions. After entrapment, fluid inclusions in authigenic quartz have been preserved without reequilibration, as indicated by the narrow ranges of homogenization temperatures (<4-5 degreesC) in most fluid inclusion assemblages. Three main growth zones are distinguished under SEM cathodoluminescence in the quartz overgrowths and are termed Q1, Q2, and Q3 from the oldest to the youngest. Zone Q1 is further subdivided into three subzones and contains abundant primary aqueous inclusions. Their homogenization temperatures range from 162 to 130 degreesC, with the earliest assemblages having the highest temperatures and with some large temperature fluctuations indicated between successive assemblages. Most Q1 inclusions have salinities in the freshwater to seawater range, with a trend toward increasing salinity through time. Zone Q2 contains primary aqueous inclusions with homogenization temperatures (overall range 148-125 degreesC) also recording large temperature fluctuations and cooling events. The Q2 fluid inclusions have high salinities (~5-20 wt. % NaCl equivalent), with salinity increasing through time. Zone Q3 contains both aqueous and oil inclusions of primary origin. The Q3 aqueous inclusions have homogenization temperatures (overall range 134-112 degreesC) recording overall cooling and high salinities (21-24 wt. % NaCl equivalent). In early Q3 subzones, oil inclusions appear to be of medium gravity, undersaturated with respect to gas. In later Q3 subzones, oil inclusions are of gas-saturated lighter oil. Our results indicate that fluid flow, involving drastic changes in temperature and salinity, was responsible for the precipitation of some of the quartz cement. The earliest quartz (Q1) precipitated from freshwater and seawater at temperatures significantly higher than those expected from the burial history and thermal maturity of these rocks. This quartz is interpreted to have precipitated during cooling of injected fluids that originated as hot connate fluids from deeper parts of the basin. The Q2 precipitation is interpreted to have resulted from episodes of injection of hot saline brines from below. Late quartz cement (Q3) precipitated during oil charge, from progressively cooler and more saline brines interpreted to have refluxed from the surface; it preserves a record of increasing oil maturity and gas saturation through time.

Kimberlites from the Wawa area, Ontario

Abstract: Two newly discovered kimberlites from the Wawa area, Ontario, are Group-1 kimberlites in their petrographic, mineralogical, geochemical, and isotopic features. They contain mantle minerals and xeno...

Petrography, mineralogy and trace element chemistry of Yamato-86029, Yamato-793321 and Lewis Cliff 85332: Aqueous alteration and heating events

Abstract: We have examined mineral grains and matrix of new Antarctic carbonaceous chondrites Yamato (Y)-86029, Lewis Cliff (LEW) 85332 and Yamato (Y)-793321 for evidence of parent body aqueous alteration and heating events. Y86029 and LEW 85332 are unique carbonaceous chondrites with unusual textures. Y86029 is the second thermally metamorphosed CI chondrite recovered from Antarctica. It is characterized by destruction of preexisting anhydrous silicates during aqueous alteration accompanied by formation of carbonates, which together with the phyllosilicates were subsequently heated at metamorphic temperatures of at least 500600 0 C. The carbonates are now present as Fe-rich drusy aggregates and giant periclase clasts and globules that have rarely been encountered in carbonaceous chondrites. LEW 85332 is an unequilibrated and highly brecciated type-3.0-3.1 chondrite that shows no clear evidence of alteration of chondrules or matrix silicate grains. However, matrix phyllosilicates underwent extensive aqueous alteration and subsequent dehydration by moderate thermal metamorphism (at least 5oo-600 ° C). This suggests that the formation of hydrous phases and subsequent heating occurred prior to final accretion in the parent body. Y-793321 is a CM2 chondrite that contains a variety of clasts exhibiting varying degrees of aqueous alteration. Petrographic evidence of thermal metamorphism in Y-793321 is only limited to a few phyllosilicate clasts in the matrix. The presence of tochilinite, carbonates and partially dehydrated phyllosilicates suggests that it experienced very mild thermal metamorphism. RNAA data based on 15 thermally mobile trace elements confirm the severity of open-system heating as Y-793321<500 ° C

Understanding the Mineralogical Composition of Ancient Greek Pottery through Raman Microprobe Spectroscopy

Abstract: Mineralogical studies using Raman microprobe spectroscopy and high-magnification optical spectroscopy were performed on sherds of pottery vessels that were used as cooking pots and water jars in Hellenistic Greece (third and second century BC). Of specific interest was the mineralogical identification of the so-called inclusions, which are phases that morphologically stand out from the fired clay matrix and result from the presence of inorganic temper in the original paste mixture. The sizes of mono- and polymineralic inclusions in the sherds range from 5 μm to 2 mm. The combined use of Raman microprobe spectroscopy and incident-light optical microscopy permits identification of specific minerals (such as quartz, albite, calcite, hematite, anhydrite, epidote, rutile, anatase, and apatite) in various different types of inclusions and then visual evaluation of the abundance of those minerals in each sample. The application of our nondestructive analytical approach is documented on a set of four sherds (each on the order of several cm2) from excavations of the ancient Agora of Athens. The information on the mineralogy obtained via Raman spectroscopy can shed light on the provenance of the raw materials and firing conditions of the pottery and thus aids archeologists with their historical interpretations.

Cathodoluminescence record of K-metasomatism in ash-flow tuffs: Grain-scale mechanisms and large-scale geochemical implications

Abstract: Near-surface, low-temperature, K-metasomatism of rhyolitic to dacitic ash-flow tuffs is a volumetrically and petrologically significant feature of the western United States and may be a primary alteration path in arid lands. Cathodoluminescence (CL) petrography of K-metasomatized ash-flow tuffs quickly yields information about the mineralogical changes and reaction mechanisms that take place during alteration for large sample suites and links these properties to geochemical changes. Tertiary volcanic rocks from Creede (Colorado), Socorro (New Mexico), and the Harcuvar Mountains (Arizona), were altered in different geologic settings, yet have very similar CL textures and chemistry. Original igneous feldspars were replaced by adularia primarily by dissolution and crystallization that occurred preferentially along preexisting fractures, surfaces, compositional zoning features, and cleavages. Fluid flow was primarily along grain boundaries, but was in some cases inhomogeneous at the thin-section scale.

Petrography and Major Element Geochemistry of Late Maastrichtian- Early Palaeocene Sediments of Tiruchirapalli, Tamil Nadu - Palaeoweathering and Provenance Implications

Abstract: Petrographic and geochemical studies of the Late Maastrichtian-Early Palaeocene sediments of Kallamedu and Niniyur Formations of Tiruchirapalli have brought out significant results on palaeoweathering and source area characteristics. In the bivariate plot of SiO 2 vs Al 2 0 3 +K 2 0+Na 2 0 3 all the clastic samples fall in the arid field indicating that the arid climatic condition prevailed in the source region during the deposition of these sediments. In the Al 2 0 3 vs Na 2 0 plot, the clastic samples fall between low grade Archaean shale and post-Archaean shale suggesting that the detrital components have been derived from low to moderately weathered terrain. The Chemical Index or Alteration of clastic sediments range from 40 to 69 with lower values for sandstones (40 to 57) than for silty shale and shale (61 to 69) and this indicates that the source rocks have undergone low to moderate chemical weathering. In QFL diagram, most of the sandstones fall in the field of cratonic interior basin which indicate that these sediments were deposited in the stable craton. SiO 2 /Al 2 0 3 ratios are high observed for sandstone, silty shale and shale, with a range starting from 3.15 to 8.75. The higher ratios of SiO 2 /Al 2 O 3 in clastic sediments indicate a largely felsic rock provenance. Major element data indicate an overall taphrogeosynclinal continental rift-related depositional basin.