Showing papers on "Petrography published in 1994"

Geofluids : origin, migration and evolution of fluids in sedimentary basins

Abstract: Geological fluids are a central theme linking the petrography and chemistry of all rock types, deformation processes on the microscopic to the continental scale, and the concentration of economic resources. The fundamental importance of fluid migration and evolution to rock composition and structure is reflected in a growing interest in fluid processes, including a series of successful conferences on water-rock interaction.

Near infrared spectra of muscovite, Tschermak substitution, and metamorphic reaction progress: Implications for remote sensing

Abstract: Near infrared (NIR) spectra of Precambrian metagraywacke in the Black Hills, South Dakota, demonstrate that reflectance spectroscopy can be used to monitor progressive changes in mineral chemistry as a function of metamorphic grade. The wavelength of a combination Al-O-H absorption band in muscovite, measured using both laboratory and field-portable NIR spectrometers, shifts from 2217 nm in the biotite zone to 2199 nm in the sillimanite + K-feldspar zone. The band shift corresponds to an increase in the Al vi content of muscovite, determined by electron microprobe, and is thus a monitor of Al 2 Si -1 (Fe,Mg) -1 (Tschermak) exchange. Spectroscopic measurements such as these are useful in the case of aluminum-deficient rocks, which lack metamorphic index minerals or appropriate assemblages for thermobarometric studies, and in low-grade rocks (subgarnet zone), which lack quantitative indicators of metamorphic grade and are too fine grained for petrographic or microprobe studies. More important, spectroscopic detection of mineral-chemical variations in metamorphic rocks provides petrologists with a tool to recover information on metamorphic reaction histories from high-spectral-resolution aircraft or satellite remote sensing data.

Origin of the Torlesse Terrane and Coeval Rocks, North Island, New Zealand

Abstract: On the basis of differing areal extent, age, petrographic modes, and bulk chemical composition, the sandstones of the northern quarter of the Torlesse terrane are subdivided into four new petrofaci...

Elemental mobility in sandstones during burial; whole-rock chemical and isotopic data, Frio Formation, South Texas

Abstract: The deepest (> 3.5 km) and most altered Oligocene Frio sandstones from south Texas typically contain authigenic minerals that exceed 30 volume percent of the rock. Elemental and isotopic analyses of whole rocks confirm conclusions based on petrography that authigenesis is accompanied by substantial material transfer involving both import to and export from the sandstone. From petrographic observations, Zr is apparently less mobile than other "immobile" elements such as Al, Ti, and REE, but nonuniform initial distribution of Zr due to sorting limits its usefulness for normalizing elemental variations. If Zr is assumed to be strictly immobile, and Zr was once uniformly distributed, then volume loss of approximately 38% from the silicate fraction of the deepest sandstones is suggested. Over the range of burial depths for samples used in this study (0.9-4.3 km), elemental trends controlled by feldspar reactions are the most readily documented. Dissolution and replacement of detrital K-feldspar is accompanied by loss of 2-3 weight percent of K2O from the whole rock. Loss of more than half of the initial Rb, Sr, and Pb also correlate with alteration of detrital feldspars. 87Sr/86Sr ratios for whole-rock and silicate fractions increase with depth as loss of nonradiogenic strontium characteristic of the volcanic-rich detrital fraction is overwhelmed by more radiogenic strontium derived from deeper in the basin. Relatively constant Na is compatible with feldspar alteration in which dissolution exceeds albitization. Changes in Nd concentrat ons and Nd also closely track the progress of alteration of detrital feldspar. Magnesium gain accompanies precipitation of authigenic chlorite and very minor ferroan dolomite. Whole-rock calcium values are dominated by calcite that includes both detrital and authigenic components. Significant dissolution of detrital carbonate grains does not occur, and calcium in authigenic carbonate cement exceeds by about 5 times the calcium released by dissolution of detrital silicates, thus requiring considerable import of calcium and CO2. Material transfer on the scale observed demonstrates that burial diagenesis is an open-system process for several major elements, and introduces a major obstacle to use of elemental data for provenance interpretation. Defining the sources of materials transported into sandstones, and the fate of materials exported, is key to documenting the "scale of system closure" in sedimentary basins and requires a better understanding of diagenetic processes in mudrocks.

Diamondiferous Eclogites from the Udachnaya Kimberlite Pipe, Yakutia

Abstract: Xenoliths from the upper mantle have undergone a wide variety of processes at varied temperatures and pressures, as recorded by mineral compositions and textures. Eclogite xenoliths in kimberlites are a unique source from which to obtain information about such processes. Furthermore, eclogites that contain accessory diamond yield important compositional constraints on the deeper upper mantle. Whether eclogites have mantle or crustal origins is still a subject of controversy. Mineralogy and petrography of 29 eclogite xenoliths from the Udachnaya kimberlite in Yakutia, Siberia are presented and combined with previous studies of these eclogites by our group (Jerde et al., 1993, 1994; Snyder et al., 1993a; Sobolev et al., 1994). Five different petrographic groups are defined, based on texture, mineral color, and degree of alteration. Chemical compositions of eclogitic minerals span a complete range from high to low-jadeite content in clinopyroxenes and from pyrope-almandine to grossular in garnets. Eclogites ...

Petrography and stable isotope aspects of cold-vent activity imprinted on Miocene-age “calcari aLucina” from Tuscan and Romagna Apennines, Italy

Abstract: Over 20 occurrences of discontinuous limestone blocks, locally called “calcari aLucina,” were mapped in the Tuscan—Romagna region of the northern Italian Apennines. The limestones, consisting of a variable mixture of authigenic carbonates (calcite, dolomite, and aragonite), sulfides (primarily pyrite), and allogenic silicates, occur in association with turbidite and hemipelagite units that were deposited in foredeep basins during early to late Miocene times. The limestone blocks are interpreted to represent relicts of carbonate buildups formed around methane-rich fluid vents on the basis of their (1) striking petrographic similarities to carbonates from cold vents in the modern oceans; (2) unique chemosynthetic-like fauna, and (3) anomalously negativeδ13C values (δ13C = − 16‰ to − 58‰ PDB). The contemporaneous tectonism of the Apennine orogeny is likely to be the primary cause for the expulsion of the methane-rich fluids to the seabed in a manner analogous to the fluid-flow processes occurring at modern accretionary prisms.

Petrophysical study of faults in sandstone using petrographic image analysis and X-ray computerized tomography

Abstract: Petrographic image analysis (PIA) and X-ray computerized tomography (CT) provide local determinations of porosity in sandstone. We have investigated small faults called deformation bands in porous sandstones using these techniques. Because the petrophysical properties of the fault rock vary at a small scale (mm scale), the ability of PIA and CT to determine porosity in small volumes of rock and to map porosity distribution in two and three dimensions is crucial. This information is used to recognize the processes involved in fault development and the different kinds of microstructures associated with dilatancy and compaction. The petrophysical study of fault rock in sandstone permits one to make predictions of the hydraulic properties of a fault and thereby evaluate the sealing or fluid transmitting characteristics of faulted reservoirs and aquifers. The results of this study indicate that faulting in sandstone alters the original porosity and permeability of the host rock: the porosity is reduced by an order of magnitude and the permeability is reduced by one to more than seven orders of magnitude for faults associated with compaction.

Petrographic, Geochemical, and Isotopic Constraints on the Provenance of the Early Proterozoic Chelmsford Formation, Sudbury Basin, Ontario

Abstract: The early Proterozoic Chelmsford Formation, a coarse-grained turbidite unit in the Sudbury Basin, was deposited near the margin of the Superior Province craton at about the time of Penokean orogenesis. Petrography, geochemistry, and Nd and Pb isotopes are consistent with derivation of the Chelmsford Formation from late Archean Superior Province rocks. The framework mineralogy of medium sandstones is dominated by quartz, plagioclase, and minor K-feldspar, consistent with derivation from plutonic rocks. The abundance of matrix (up to 40%), however, allows additional sources. The behavior of Al2O3 relative to Na2O, CaO, and K2O constrains the source to have had, on average, a minor weathering history. Slightly elevated SiO2 and low Z , relative to average plutonic rocks, suggest that there was no more than a minor recycled sedimentary component in the source. High abundances and sympathetic variations in ferromagnesians indicate a mafic component. REE and whole-rock Nd and Pb isotopes constrain the Chelmsford Formation to have been derived from late Archean, LREE,-enriched rocks without significant Eu anomalies, and allow neither a significant component of Penokean mantle-derived material nor early Archean crust. Pb isotopes in feldspar populations require a Superior Province source. Although the age, location, and facies of the Chelmsford Formation are consistent with deposition in response to early Proterozoic active margin processes, the integrated provenance data require the sediment source to have been dominated by relatively fresh Superior Province rocks associated with basement uplift.

Petrography and petrology of the ALH-77005 shergottite

Abstract: The ALH-77005 shergottite is a cumulate gabbroic rock consist­ ing of brown olivine, lowand high-Ca pyroxene, plagioclase glass, Ti-poor and -rich chromite, ilmenite, whitlockite, and sulfides. Chromites in ALH-77005 show four types of chemical zoning, and these types suggest multiple magma mixing. Some plagioclase glass has plagioclase rims, and the rims were produced from plagioclase melts by rapid crystallization. Ubiquitous occurrences of plagioclase glass and shock-melt pockets that were produced by in-situ melting indicate that ALH-77005 has experienced shock pressures ranging from 50 to 80 GPa.

Post-metamorphic low delta 13 C calcite in the Cycladic Complex (Greece) and their implications for modeling fluid infiltration processes using carbon isotope compositions

Abstract: Calcite veins and overgrowths with δ 13 C values of -12 ‰ to -8 ‰ and δ 18 O values of 24 ‰ to 27 ‰ are shown to be characteristic features of the Cyclades metamorphic complex of Greece. Field and petrographic characteristics and the marked carbon and oxygen isotope disequilibrium that the veins and overgrowths show with host metamorphic rocks indicate that they developed during post-metamorphic evolution of the complex. The low δ 13 C values are interpreted to be either the result of the oxidation of organic carbon or solution of soil CO 2 by surface waters moving downwards along extensional fractures generated during the late stages of exhumation. Carbon and oxygen isotopic compositions reported for the carbonate fraction of many pelitic schists on Naxos overlap with the range of these late carbonates and may in part reflect a contribution of this late calcite. Uncertainties regarding the origin of low δ 13 C values in the carbonates of metamorphic rocks shows that they must be used with caution as tracers of fluid-rock exchange

The primitive lavas of Roccamonfina volcano, Roman region, Italy: new constraints on melting processes and source mineralogy

Abstract: Within a large collection of lavas from the Roccamonfina volcano are rocks which represent the most mafic samples yet recorded from Roccamonfina and which are amongst the least differentiated lavas found in the Roman co-magmatic region as a whole. These rocks extend both high-K and low-K series to more primitive values. However, petrographic and geochemical considerations rule out a primary origin, and even these mafic samples appear to record the effects of repeated episodes of fractional crystallization and hybridization. Relatively potassic samples from the low-K series are apparently transitional between low-K and high-K series, as previously delineated. However, these intermediate-K samples are not transitional in their Sr isotopic composition, suggesting that there is no continuum between low-K and high-K magma source regions. Rather, the compositional range within the low-K series appears predominantly to reflect variation in the degree of melting of a common mantle source. Analysis of the low-K series data, using an inverse method suggests a source containing amphibole and garnet, and indicates that these phases were consumed during the melting processes responsible for the low-K series magmas. The role of amphibole is further indicated by the association of low K2O with elevated Rb concentration and, for example, higher Ce/Yb. Such variations are taken to reflect the consumption of high K/Rb amphibole during the initial phase of partial melting.

Provenance Analysis of Muddy Sandstones

Abstract: A new approach to determine the precursor grains of detrital and diagenetic clay in sandstones requires a reiterative use of optical microscopy and electron beam microanalysis. Excluding muddy sandstone successions from analysis of the siliciclastic fill of a basin may lead to incomplete if not erroneous provenance interpretation because of biased samping of the sediments. To minimize problems inherent in muddy sandstones, we have combined a two-tier optical modal analysis with energy dispersive X-ray analysis (EDXA) to identify precursors of clay. This is a new method of point counting and estimating the mineral compositions of muddy sandstones. Because we apply the principle of identifying the mineral under the cross hair, albeit on a micron scale, we call this the new or modified G zzi-Dickinson method. Calculations of mass-balance type using the results of this new method may be performed to reconstruct the original composition of the parental detrital material. We have used muddy siliciclastic rocks (outcrop and core samples of siltstones and sandstones) from the Oligocene, intra-arc Medicine Lodge Beds of southwestern Montana, USA for this study. Eocene volcanic rocks of the Challis region and thrusts that have transported pre-Cenozoic sedimentary and metasedimentary rocks in this region were the sources of the siliciclastic sediments. Results of modal analysis by the standard Gazzi-Dickinson method and by the new/modified Gazzi-Dickinson method, using both optical and EDXA on a micron scale, plot in different clusters in a QFL diagram. Our results show that the standard Gazzi-Dickinson method is not truly independent of grain size even in the context of provenance analysis. On the other hand, the modified version has the promise of achievi g truly mineralogical modal petrologic analysis of detrital rocks of any grain size.

Shocked lithologies at the Wanapitei impact structure, Ontario, Canada

Abstract: The ∼7.5 km diameter Wanapitei impact structure (46°45′N; 80°45′W) lies entirely within Lake Wanapitei in central Ontario, Canada. Impact lithologies are known only from glacial float at the southern end of the lake. Over 50% of the impact lithologies recovered from this float can be classified as suevite, <20% as highly shocked and partially melted arkosic metasediments of the target rock Mississagi Formation or, possibly, the Serpent Formation and <20% as glassy impact melt rocks. An additional <5% of the samples have similarities to the suevite but have up to 50% glass clasts and are tentatively interpreted as fall-back material. The glassy impact melt rocks fall into two textural and mineralogical types: a perlitically fractured, colorless glass matrix variant, with microlites of hypersthene with up to 11.5% Al2O3 and a “felted” matrix variant, with evidence of flow prior to the crystallization of tabular orthopyroxene. These melt glasses show chemical inhomogeneities on a microscopic scale, with areas of essentially SiO2, even when appearing optically homogeneous. They are similar in bulk composition for major elements, but the felted matrix variant is ∼5×more enriched in Ni, Co and Cr, the interelement ratios of which are indicative of an admixture of a chondritic projectile. Mixing models suggest that the glassy impact melt rocks can be made from the target rocks in the proportions: ∼55% Gowganda wacke, ∼42% Serpent arkose and ∼3% Nipissing intrusives. Geologic reconstructions suggest that this is a reasonable mixture of potential target rocks at the time of impact.

Controls on the mechanisms of fluid infiltration and front advection during regional metamorphism: a stable isotope and textural study of retrograde Dalradian rocks of the SW Scottish Highlands

Abstract: Vein-controlled retrograde infiltration of H2O-CO2 fluids into Dalradian epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite-rich marble bands to dolomite and spatially associated 18O enrichment of about 10%. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H2O-salt mixture with a low CO2 content, and that the temperature of the fluid was about 400d C. Detailed petrographic and textural (backscattered electron imaging) studies at one garnet-grade locality show that advection of fluid into marbles proceeded by a calcite-calcite grain edge flow mechanism, while alteration of non-carbonate wall-rock is associated with veinlets and microcracks. Stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall-rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4–28.6 m3/m2 have been computed from measured advection distances. Coincidence of isotope and reaction fronts is considered to result from reaction-enhanced kinetics of isotope exchange at the reaction front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection distance occurring in nearly pure calcite matrix. This relationship indicates that fluid flow in carbonates is only possible along fluid-calcite-calcite grain edges. However, experimental constraints on dihedral angles in calcite-fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite-calcite grain junctions rather than to an open calcite pore geometry. The regional extent of the retrograde infiltration event has been documented from the high δ18O of dolomite-ankerite carbonates from veins and host-rocks over an area of least 50 × 50 km in the SW Scottish Highlands. Isotopically exotic 18O-rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral reaction in wall-rocks only where lithology, pore geometry or mineral solubilities, pressure and temperature have been appropriate for pervasive infiltration to occur.

Petrographic thin sections of archaeologial sediments: A new method for paleobotanical studies

Abstract: The use of petrographic thin sections to examine archaeological sediments has centered on the study of detrital and chemical components of the material. There has been relatively little effort made to study botanical remains—particularly charcoal—with this technique. This paper illustrates that millimeter-size or even smaller fragments examined in undisturbed, artificially indurated samples, which normally would escape collection with standard flotation and sieving techniques, can furnish significant paleobotanical information about the types of vegetal matter found in a site. Samples from cave sediments in Israel and an open-air site in Canada are presented. © 1994 John Wiley & Sons, Inc.

Sill emplacement in wet sediments: fluid inclusion and cathodoluminescence studies at Grunehogna, western Dronning Maud Land, Antarctica

Abstract: The Grunehogna and Kullen sills of western Dronning Maud Land were emplaced into water-saturated, partially lithified sediments of the mid-Proterozoic Ritscherflya Supergroup. Fluid inclusion and optical and scanning electron microscope cathodoluminescence studies of quartz grains within and adjacent to the sills (the ‘sill-sediment interactive zone’, referred to as the ‘interactive zone’) showed that only a single generation of later fluid inclusions is present. The fluids are Na-K-Ca-Cl aqueous solutions with temperatures of homogenization between 95 and 105 °C. Individual quartz particles in the sedimentary rocks above the interactive zone have unique fluid inclusion populations. Petrographic and cathodoluminescence evidence indicated that there are three types of quartz grains in the interactive zone. These consist of (i) a xenocrystic type, (ii) a high-temperature quartz that crystallized either from the magma or as a result of anatectic melting of xenolithic (and/or xenocrystic) material, and (iii) euhedral quartz crystals that have grown in vugs in the peripheral zone. Cathodoluminescence of xenocrystic grains indicated the existence of healed microfractures along which fluid inclusions were concentrated and secondary and authigenic overgrowths on older generation quartz grains had developed. The presence of zonal luminescence patterns of detrital particles, coupled with evidence for equilibrium polygonization in the sedimentary rocks adjacent to the peripheral zone up to about 8 m above the upper contact of the Grunehogna sill suggested local thermal tempering or sintering and recrystallization. Luminescence of detrital quartz particles in the overlying Hogfonna Formation reflected variation in the characteristics of their source terrains, and showed limited evidence of heating by the magma. The interactive zone fluid inclusions are formation waters which penetrated microfractures during diagenesis. We show that no large-scale hydrothermal circulation system is likely to develop following intrusion into a water-saturated environment.

Plagiogranites and gabbroic rocks from the Mingora ophiolitic melange, Swat Valley, NW Frontier Province, Pakistan

Abstract: Major, trace element composition and Sr isotopic data were collected for gabbroic rocks, plagiogranites and albitites in the ophiolite assemblage from Swat Valley (NW Frontier Province, Pakistan). Petrographic study revealed that these rocks were subjected to important structural and mineralogical modifications due to greenschist-epidote-amphibolite facies sub-sea-floor metamorphism and to brecciation. On the other hand, the examination of whole rock chemical composition and of chemical trends showed that these rocks were affected by some chemical modifications, concerning especially Na20, K20 and Rb. The very low contents of HFS (high field strength) and RE elements found in gabbroic rocks and plagiogranites were considered to be a primary magmatic feature pointing in part to their cumulitic nature and in part to an origin from a refractory parental magma. The Sr isotopic data indicate that gabbroic rocks and plagiogranites were subjected to exchange with sea water. The particular chemical features shared by gabbroic rocks and plagiogranites suggested that fractional crystallization was a possible evolution process. In contrast, albitites are characterized by anomalously high contents in HFSE and LREE and by values of the 87Sr/S6Sr ratio very close to sea water. These features suggest a more complex origin with respect to gabbroic rocks and plagiogranites. KEVWORDS: plagiogranites, gabbroic rocks, albitites, ophiolites, Pakistan.

Petrographic analyses of organo-mineral relationships: depositional conditions of the Oxford Clay Formation (Jurassic), UK

Abstract: Five samples representing the range of variability of palynofacies and organic geochemical parameters of the organic-rich facies of the Peterborough Member (formerly Lower Oxford Clay) and one sample from the Stewartby Member (Middle Oxford Clay) were studied using optical microscopy (transmitted light and fluorescence) and scanning electron microscopy (backscattered electron mode). The organic matter assemblage of the Oxford Clay Formation is dominated by amorphous organic matter of marine origin (75 to 95%). Whatever their organic content, the sediments display a highly microbioturbated texture, with edge-to-face disposition of clay minerals and numerous microburrows, signifying that the sea floor was never completely anoxic. The presence of abundant organic matter in the sediment has enhanced diagenetic transformations, and in particular is related to the formation of pyrite, calcite and authigenic quartz. The nature of diagenetic changes, particularly those involving silica species, provides information on the interstitial environment. The sediment displays, at the micrometric scale, numerous microenvironments with varying reducing conditions, resulting in different diagenetic products. Pyrite displays two habits, framboidal and euhedral. Euhedral pyrite, which characterizes more anoxic interstitial environments than framboids, is found in the most organic-rich sediments. The abundance of large coccolith-rich faecal pellets in the most organic-rich sediments is correlated with chemical evidence of intense trophic activity.

Mineralogic and petrographic records of sediment-fluid interaction in the sedimentary sequence at Middle Valley, Juan de Fuca Ridge, Leg 139

Abstract: Petrographic and mineralogic studies of the sedimentary cover drilled at Middle Valley on the Juan de Fuca Ridge allow characterization of the newly formed silicate and carbonate phases and a determination of their origin. At all drill sites, the sediments are composed of a mixture of detrital and authigenic minerals in various proportions. The unmodified sediments forming the surficial cover consist of detrital minerals, including quartz, feldspar, amphibole, mica, clays (smectite, chlorite), and biogenic remains. Quartz, feldspar, and mica are still present in hydrothermally modified sediments where they coexist with newly formed minerals, except in Hole 858B, which is located close to an active hydrothermal vent where pure hydrothermal layers consist solely of authigenic minerals. The main authigenic phases in the modified sediments are silicate minerals and calcite concretions with minor sulfides. The most common authigenic silicate in the deepest layers downhole is Mg-chlorite. At Site 858, a mineralogic sequence is clearly established from the seawater/sediment interface to the deeper parts of the hole. The shallowest hydrothermal unit from Hole 858B is composed of Mg-smectite and authigenic pyrite, which downhole sequentially overlies corrensite-rich layers which contain some talc and anhydrite. In the deepest unit, chlorite is the dominant phyllosilicate coexisting with authigenic quartz. All of these silicates seem to be the product of a direct precipitation from hydrothermal fluids. This sequence reflects a thermal gradient related to the vertical influx of hydrothermal fluids. Secondary precipitation of carbonate concretions was generated from Ca released by biogenic phases and hydrothermal fluids. The concretions show several stages of crystallization and seem to occur preferentially in oxidizing areas and in the shallowest deposits. The occurrence of the concretions is favored by high porosity in the sediments. The presence of pure hydrothermal layers locally interbedded with unmodified sediments, and the variable mineralogy of the carbonate nodules suggest that convective circulation of fluids acts at various scales in this hydrothermal system and that the sedimentary cover favors lateral fluid circulation.