Petrographic Approach Research Articles

Geochemical and petrographic approaches to chert tool provenance studies: evidence from two western USA Holocene archaeological sites

Abstract Chert (flint, jasper and agate) from a late Holocene site in Elko, northern Nevada, USA, and a Mid-Holocene site east of Seattle, Washington, USA, represents a wide range of geological source environments for microcrystalline polymorphs of quartz. Potential chert source material and lithic artefacts from these two archaeological sites are utilized to develop laboratory approaches to provenance studies. Chert paragenesis determined by petrographic analysis and geochemical cluster fingerprinting using inductively coupled plasma (ICP) and ICP-mass spectrometry analyses accompanied by scanning electron microscope-energy dispersive X-ray fluorescence analysis provides compelling source to artefact correlations.

Abstract: Provenance of the Lower Paleozoic Wajid Sandstone, Southeastern Margin of the Arabian Shield: A Geochemical and Petrographic Approach

Abstract: Provenance of the Lower Paleozoic Wajid Sandstone, Southeastern Margin of the Arabian Shield: A Geochemical and Petrographic Approach

Abstract: Stratigraphic Prediction and Correlation in Deep Marine Reservoirs Using an Integrated Trace Element Geochemical, Petrographic and Heavy Mineral Approach: Examples from Modern and Ancient Analogues and U.K.C.S. Reservoirs

Abstract: Stratigraphic Prediction and Correlation in Deep Marine Reservoirs Using an Integrated Trace Element Geochemical, Petrographic and Heavy Mineral Approach: Examples from Modern and Ancient Analogues and U.K.C.S. Reservoirs

A petrographic approach for evaluating trace-element mobility in a black shale

ABSTRACT At the time of deposition, black shales record geochemical information related to the provenance and conditions of deposition for that rock. These initial geochemical conditions are susceptible to alteration and overprinting by the aggressive processes of black-shale diagenesis. In this study, we evaluate a sequence of Ordovician black shales from the southern Welsh Basin for the presence of such a diagenetic overprint. Through use of cathodoluminescence, backscattered electron imaging, reflected-light microscopy, fission tracks, and microprobe elemental mapping, we can discern textural relationships with in the Welsh black shales. The relationships observed constrain a paragenetic sequence and overall diagenetic history including the early diagenetic formation of pyrite, apatite (rare earth element (REE)-rich) and carbonate and later formation of diagenetic monazite (REE-, U-rich) and late-stage carbonate. The chemical reactions necessary to produce this mineral sequence require postdepositional redistribution of REE (including changing cerium anomalies (Ce/Ce*)) and uranium. As a result, REE, U, and their associated isotopic systems (Sm-Nd and U-Pb) can reflect a diagenetic overprint rather than the source area and depositional conditions. The paragenetic sequence described in this study records several postdepositional changes in the whole-rock mineralogy. Among these changes are the formation of REE-rich apatite and REE- and U-rich monazite. The formation of these phases requires that REE and U are being redistributed on at least a mineral scale and that the initial depositional and provenance information recorded by the whole rock has been disturbed. Therefore, the approach used in this study to carefully characterize the paragenetic sequence from the Llandeilo-Caradoc black shales should be considered necessary in order to fully evaluate the usefulness of trace-element proxies susceptible to diagenetic redistribution.

97/01645 Relating coal oxidation and hydrophobicity: a petrographic approach

97/01645 Relating coal oxidation and hydrophobicity: a petrographic approach

Relating coal oxidation and hydrophobicity: a petrographic approach

Long-wave fluorescence relative intensity measurements and a vitrinite microelasticity index were used to determine oxidation levels in a suite of artificially oxidized coals. These petrographic parameters were correlated with the agglomeration responses of the coals. Very definite relations were found between the petrographic indicators of oxidation and the agglomeration rates of the coals.

96/05662 FT-IR spectroscopy of Western Canadian coals and macerals

Coal fires have received increasing attention due to their environmental, economic, and social impacts. Their significant influence on coal properties is widely documented by geophysical and geochemical methods.The present paper demonstrates, by means of coal petrological methods, significant changes of microstructure, optical appearance, and optical properties of medium-rank (bituminous) coals affected by an underground coal fire. The studied changes are suggested to result from distinct oxidative and thermal alterations induced by a high-temperature regime (temperatures over 800 °C) encountered in the coal fire zone No. 8, Wuda Coalfield. The investigated coal seam No. 9 represents a high volatile A bituminous coal and is dominated by vitrinite, followed by inertinite with minor amounts of liptinite. The microstructure of the coal-fire affected coals facilitate a broad spectrum of characteristic features. The microscopic analyses revealed development of (1) non-tectonic fissures and non-tectonic fractures, (2) distinct micropores, (3) dark oxidations rims, and (4) bright spots interpreted as incipient mesophases. Optical properties were characterised for megasporinite and vitrinite. In the examined megasporinite a noticeable anisotropy and distinct granular appearance was observed. In the investigated coals both a dominant increase as well as a minor decrease in both random and maximum vitrinite reflectances were documented for (1) coal grains and (2) dark-rimmed vitrinite at non-tectonic fissures and non-tectonic fractures. In addition, also a characteristic bimodal vitrinite population was measured. The increase in bireflectance was clearly documented for two sub-zones within the coal fire zone No. 8. The applied Kilby's cross-plots suggested a large overlapping of the data sets of R'max and R'min, indicating marked changes in vitrinite structure. The applied coal petrological approach revealed distinct changes to the coal's microstructure, appearance, as well as to optical properties induced by the underground coal fire. The presented coal petrographic approach may help determine the extent of coal fires in coal mines and the direction of propagation of coal fire front.

Structure and origin of limestones

Abstract Sorby’s Presidential Address of 1879 on the structure and origin of limestones was essentially an interim report of research in progress. His petrographic approach to limestones, stemming from three decades of research, laid the foundations for a wide range of research lines, some of which were not fully exploited for almost a century, such as fluid inclusion studies in diagenesis. The significance of many of his discoveries (e.g. that some Jurassic ooids and Palaeozoic corals were originally calcitic) had geochemical implications that have only recently achieved research prominence (e.g. CO 2 and the ‘greenhouse’ Earth). In addition, Sorby’s legacy was the example of his peerless approach to research, applying ruthless empiricism to the problem at hand. In the case of limestones, this involved an application of meticulous descriptive petrography and innovative experimentation. His approach to understand more fully the complex problems posed by carbonates remains unsurpassed, involving a thorough integration of detailed observation, imaginative thinking and judicious use of analytical techniques.

Structure and origin of limestones

Sorby’s Presidential Address of 1897 on the structure and origin of limestones was essentially an interim report of research in progress. His petrographic approach to limestones, stemming from three decades of research, laid the foundations for a wide range of research lines, some of which were not fully exploited for almost a century, such as fluid inclusion studies in diagenesis. The significance of many of his discoveries (e.g. that some Jurassic ooids and Palaeozoic corals were originally calcitic) had geochemical implications that have only recently achieved research prominence (e.g. CO 2 and the ‘greenhouse’ Earth). In addition, Sorby’s legacy was the example of his peerless approach to research, applying ruthless empiricism to the problem at hand. In the case of limestones, this involved an application of meticulous descriptive petrography and innovative experimentation. His approach to understand more fully the complex problems posed by carbonates remains unsurpassed, involving a thorough integration of detailed observation, imaginative thinking and judicious use of analytical techniques.

Fluorescence Spectral and Geochemical Analyses of Kerogen, Organic Extracts and Crude Oils: A Petrographic Approach to Determination of the Onset of Oil Generation

Fluorescence Spectral and Geochemical Analyses of Kerogen, Organic Extracts and Crude Oils: A Petrographic Approach to Determination of the Onset of Oil Generation

The petrographic approach to coal liquefaction

The procedure for quantitative estimation of a maceral liquefaction reactivity is outlined in terms of the regression analysis which takes into account petrographic composition and coal rank. The procedure is based on an additive model for maceral liquefaction and includes a mathematical/statistical analysis of conversion and/or kinetic data for coals characterized by simplified petrographic composition, i.e. by contents of liptinite (exinite), vitrinite and inertinite. The procedure was applied to long contact time liquefaction data for low sulphur high volatile bituminous coals of different rank and to kinetic analysis of short contact time liquefaction data for subbituminous and bituminous coals. On average, inertinite reacts with tetralin three times slower than vitrinite which itself is only half as reactive as liptinite. This is the reason for a decreasing apparent rate constant with extent of conversion in liquefaction of petrographically inhomogeneous coals. The activation energies in liquefaction with tetralin of subbituminous and bituminous coals are respectively 11.4 and 125 kJ mol −1 for vitrinite, and 63.3 and 98 kJ mol −1 for inertinite. Shortcomings and possible modifications of the model are discussed.

Petrographic aspects of coal structure: reactivity of macerals in laboratory and natural environments

Petrographic aspects of coal reactivity are most satisfactorily observed with the microscope, unless the alteration is severe and large scale, for example in some industrial and natural cokes. Although qualitative microscopical observations are the most common way of following reactive changes, measurements, principally through reflectance photometry and fluorometry, can add a valuable quantitative dimension to the scale of reactivity. As with other aspects of coal reactivity, degree of reactivity is related to a range of experimental and/or environmental conditions and their severity. Reactivity is also type- and rank-dependent, with the maceral groups showing markedly different responses to the same set of conditions. This paper briefly emphasizes the simplicity and effectiveness of the coal petrographic approach in studying maceral reactivity and illustrates the economic benefits achievable in industrial applications through both qualitative and quantitative microscopical methods. The principal intention of the paper is to focus on the reactivity of coals and their constituents in the natural environment, concentrating especially on their responses to different influences during coalification and the ways in which coalification anomalies arise. Particular reference is made to the way in which the behaviour of the dominant petrographic optical parameter, vitrinite reflectance, is modified by such variables as rise of temperature, variation of heating rate, oxidation, pressure and stress, as well as by the time factor, the influence of other intimately associated organic matter and the physical properties of contiguous sediments. The results of laboratory heating experiments, with and without the introduction of pressure and stress, are described to help to explain some of the coal and maceral reactions in the natural environment.

Petrographic approach to beneficiation of Australian oil shales

Australia has five types of oil shales, cannel coal, torbanite, lamosite, tasmanite and marinite, each of which has a different organic composition. Cannel coal is composed of liptinite (mostly resinite, cutinite and sporinite) derived from terrestrial plants, vitrinite and inertinite, with only a small amount of mineral matter. Torbanite is composed of large telalginite, derived from Botryococcus and related algae, in a groundmass of vitrinite and inertinite. It is difficult to crush and petrographic analysis indicates that beneficiation would be difficult. Lamosite contains abundant lamalginite, derived from freshwater, planktonic algae, with minor vitrinite and telalginite; petrographic analysis confirms that comminution produces alginite-poor fines and a higher grade concentrate. Marinite is a relatively low grade oil shale composed of lamalginite and bituminite, both of which are unlikely to be easily separated from the mineral groundmass. Tasmanite is composed of telalginite, derived from marine precursors, which is easily separated from the mineral matter when the oil shale is crushed.

Isotopic and Petrographic Record of Phreatic Diagenesis: Lower Cretaceous Sligo and Cupido Formations

ABSTRACT Equant calcite results from a complex spectrum of processes in shallow to burial phreatic environments. Variations in temperature, composition of pore fluids, and redox potential of the environment may give rise to successive generations of equant spar which are not easily characterized through conventional petrographic approaches. Because petrographic criteria alone are not conclusive as diagenetic indicators, it is essential to combine petrographic and geochemical techniques to obtain a more complete understanding of the environmental setting of phreatic diagenesis. Integration of petrographic and stable isotopic analyses of equant cements within reefal facies of the Lower Cretaceous Sligo and Cupido formations, south Texas and northeastern Mexico, indicate multiple phases of cementation in phreatic environments and attest to the chemical evolution of the precipitational environment through time. Two isotopically distinct early generations of clear equant calcite, stages I and 2, with variable luminescence, occur immediately below intervals of wholesale dissolution and exhibit oxygen isotopic signatures approximately 1 and 3 lighter than the estimated original isotopic composition of Cretaceous marine carbonate. Moreover, both stages of these c ments yield lighter carbon isotopic signatures, relative to marine carbonate compositions, which reflect the addition of vadose-derived bicarbonate during recharge to the phreatic lens. On the basis of combined petrographic and isotopic evidence, these two early stages of cementation are interpreted to have formed in a shallow, meteoric, phreatic environment. A third generation of brightly luminescent equant calcite, stage 3, exhibits significantly lighter oxygen and invariant carbon isotopic compositions and occurs in fractures which postdate earlier stages of diagenesis. Two-phase fluid inclusions and the lighter oxygen isotopic composition indicate precipitation at elevated temperatures. The fourth cement, stage 4, is a coarsely crystalline calcite of variable luminescence which occur in fractures, moldic pores, and stylolitic seams, postdates the third stage of cementation, and contains abundant two-phase fluid inclusions and inclusions of bitumen. These petrographic criteria indicate precipitation at greater depths and elevated temperatures, and, as such, stage 4 cements should exhibit lighter oxygen isotopic signatures. What is observed, however, is that the isotopic compositions of this calcite are approximately 6 heavier in 18O than those of stage 3. Such isotopic values are in marked contrast to the progressive trend towards lighter carbon and oxygen exhibited by the first three generations of cement and must therefore reflect precipitation from fluids enriched in 18O, which counteracts t e effect of elevated temperatures. By combining isotopic and petrographic data we have succeeded in identifying four distinct episodes of phreatic cementation. The isotopic trends and the corresponding petrographic characteristics indicate that pore fluids evolved from a meteoric to a connate composition throughout the diagenetic history of the Sligo-Cupido sediments. Furthermore, based on the trends exhibited at the four locations analyzed, it appears that the marginal facies of these formations underwent comparable subsidence and diagenetic histories.

The influence of tectonics on stratigraphy: A petrographic, geochronological and biostratigraphic approach on early Neogene sediments from north-central Italy

The influence of tectonics on stratigraphy: A petrographic, geochronological and biostratigraphic approach on early Neogene sediments from north-central Italy

The detection of weathering in coal by petrographic, rheologic and chemical methods

This study examines petrographic, rheologic, proximate and ultimate analyses techniques, to investigate the effects of natural weathering on coal. The applicability of such tests to both the detection of weathering and to the prediction of the properties of a fresh coal, based upon analyses of a weathered sample, is discussed. Rheologic parameters and a petrographic staining technique showed the most sensitive response to weathering. The latter test is considered the most suitable of these for the detection of weathering, as it is independent of prior knowledge of the properties of the fresh coal and of the petrographic composition. A petrographic approach is also considered most suitable for estimation of fresh coal characteristics from weathered samples. Rank may be closely approximated and petrographic type determined microscopically, even on highly weathered samples, and these two parameters may be used to estimate properties such as volatile matter, calorific value and dilatation. Minor modification should be made to vitrinite reflectance measurements, according to the degree of weathering as revealed by selective staining.

Meteorites: A Petrologic‐Chemical Synthesis

Meteoritics is an intricate multidimensional jigsaw puzzle where most of the pieces are missing and where recognizing which pieces fit requires powers of observation that are inadequately developed. In such a discipline, the role of the summarizer is to describe the current appearance of the puzzle in order to assist those who will continue the construction and to convince others that the matched pieces really fit.Dodd summarizes meteoritics up to about 1980 in a format suitable for use as a textbook in a university course. The viewpoint is largely petrographic, but he introduces many aspects of meteorite chemistry and chronology necessary to recognize the pieces that may, or may not, fit the puzzle. The petrographic approach is valid because meteoritics is still in its description and classification stage; however, he acknowledges that study of the cache of meteorites, discovered recently in Antarctica, may add to and partially modify the descriptive framework. He describes a current consensus, which considers carbonaceous chondrites and ordinary chondrites as separable groups (and subgroups), enstatite chondrites and achondrites as a related assemblage, iron meteorites and pallasites as possible relatives, the eucrites, howardites, diogenites, and mesosiderites as genetic relatives, plus a number of unclassified achondrites and metallic meteorites that may be unrelated to the others. Recent major advances in understanding the origin of meteorites and the relationship of the groups discussed include the characterization of calc‐aluminous inclusions (CAI) in carbonaceous chondrites and the discovery of isotopic anomalies that apparently reflect the pre‐solar and initial processes of solar system formation; the characterization of chondrules in unequilibrated ordinary chondrites that apparently formed in different environments than the CAI and may have been metamorphosed to form equilibrated chondrites; an understanding through isotopic studies that the formative period of chondrites was probably less than 100 million years after condensation began in the solar system, with the carbonaceous chondrites representing an earlier stage than the ordinary chondrites; that iron meteorites, pallasites, and enstatite meteorites are inadequately studied, but show more variety and sub‐classification than was possible in the past; and that the eucrite association represents basaltic volcanism shortly after the origin of the solar system. The current view is that most of the meteorites formed in smallish asteroidal bodies, although recently there have been speculations that the rare unassociated Shergottite achondrites may be from a Mars‐like planet. The range of possible meteorite formation scenarios is growing somewhat more limited; however, the complexity and diversity of parent bodies is more evident now than it was ten years ago, and a uniform model for solar system formation and early planetary history is still far off.

Petrographic Approach for Study of Ancient Potash Evaporites, Salado Formation (Permian), New Mexico: ABSTRACT

Petrographic analysis of a potash evaporite, the Salado Formation (Permian) of New Mexico and Texas, has allowed distinction of primary sedimentary features, from metamorphic alteration overprints. Primary sedimentary textures in laminated anhydrite, thin-bedded halite, and muddy halite include vertical growth structures (gypsum swallowtails and chevron halite), incorporative and displacive intrasediment growth of gypsum, halite, and glauberite, carbonate-gypsum and gypsum-halite couplets, and gypsum wave ripples. The Salado primary chemical and depositional environment, interpreted by comparison with similar features from modern evaporite environments, is a salt pan up to halite saturation alternating with a perennial brine body stage at gypsum and sometimes halite saturation. A complex diagenetic-metamorphic history has imparted a secondary alteration overprint on the Salado salts. Halite has recrystallized; gypsum has dehydrated to anhydrite, reacted with brine to form polyhalite, or dissolved, leaving a void now occupied by halite or sylvite. Formation of new minerals, the most important being sylvite, carnallite, langbeinite, and kieserite, has occurred as displacive or incorporative intrasediment growth (langbeinite), or as void filling cement (sylvite, carnallite). Further alteration is recorded as reaction of brine with langbeinite to form kieserite, kainite, leonite, and bloedite. From their observed distribution, texture, and mineralogy, and by comparison with experimental data, secondary features in the Salado are interpreted to have resulted from the subsurface migration of alien, non-seawater composition brines at elevated temperatures. End_of_Article - Last_Page 597------------