Showing papers on "Petrography published in 1999"

Quantitative compositional analysis using thermal emission spectroscopy: Application to igneous and metamorphic rocks

Abstract: The mineral composition of a suite of igneous and metamorphic rocks was determined using the thermal infrared emission spectra of these rocks in a linear spectral deconvolution algorithm. This algorithm assumes that the infrared spectrum of each rock is a linear mixture of the component mineral spectra weighted by volume abundance. A diverse suite of 36 common rock-forming and accessory minerals was used in the deconvolution. The model was tested by comparing the mineralogy derived from the infrared spectrum with petrographically estimated abundances for 45 igneous and 51 metamorphic rock samples. The mineral abundances derived from these two techniques agree to within 67-15% for the primary minerals feldspar, pyroxene, quartz, and calcite/ dolomite and 69 -17% for secondary minerals such as micas and amphiboles. These differences are comparable to the error for traditional thin section mode estimates, which are 65-15% for major minerals and #5% for minor minerals. The detection limit for the primary and secondary minerals found in the rocks analyzed ranged from 5 to 10%. Each major rock type studied here was easily distinguished by its spectral characteristics. The best results, in both the qualitative determination of the rock type and dominant minerals and the quantitative reproduction of absorption features and mineral composition, were obtained for igneous rock samples. For metamorphic rocks, pelite and quartzo-feldspathic samples gave slightly better results than calcareous or mafic samples. A controlled analysis, in which the end-member suite was reduced based on an initial estimate of the rock type, only improved the results by several percent for most primary and secondary minerals. The quality of the obtained results demonstrates that a linear deconvolution of infrared emission spectra provides an accurate, rapid technique for determining the quantitative mineral composition of rock samples in a laboratory and has application to future in situ measurements.

Petrographic Characteristics and Metamorphic Evolution of Ultrahigh-Pressure Eclogites in Plate-Collision Belts

Abstract: Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field. This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification of ultrahigh-pressure eclogites. In addition, we discuss the quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the do...

Characterization of weathered volcanic rocks in Hong Kong

Abstract: The mineralogical and fabric characteristics of weathered volcanic rocks, which are widespread in Hong Kong, are poorly documented. Consequently, the significant variations commonly observed, even at the same locality, in engineering properties and in situ behaviour of these rocks and the resulting soils cannot be adequately related to their basic mineralogical and fabric characteristics. A model for weathering of volcanic rocks, particularly those of pyroclastic origin, is presented in this paper. Modifications, similar to those recently proposed for granites by Irfan, are proposed to the material and mass weathering schemes commonly adopted in Hong Kong for the characterization of weathered volcanic rocks in engineering use. The appropriateness of various proposed petrographic and chemical weathering indices to generally fine-grained volcanic rocks is examined. A number of modifications are proposed to the standard soil preparation and testing methods for the soils formed from tropical weathering of volcanic rocks in order to obtain meaningful and more repeatable results.

Samarium‐neodymium and rubidium‐strontium systematics of nakhlite Governador Valadares

Abstract: — The Sm-Nd systematics of whole-rock and mineral separate samples from nakhlite Governador Valadares define a good 147Sm-143Nd mineral isochron age of 1.37 ± 0.02 Ga. This age is in excellent agreement with the 39Ar-40Ar and Rb-Sr ages obtained previously for this meteorite. However, the Rb-Sr isotopic data for our sample show that the isotopic system is disturbed. The lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The whole-rock and acid-washed mineral data yield a Rb-Sr age of 1.20 ± 0.05 Ga, which probably represents a lower limit to the crystallization age of the rock. The petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill (McSween, 1994). Thus, the Sm-Nd isotopic age probably represents the age of such a magmatic event. The initial e143Nd value determined for the rock at 1.37 Ga is +17 ± 1, indicating that the parent magma of the rock came from a light-rare-earth-element-depleted source of 147Sm/144Nd = ∼0.237 based on a simple two-stage evolution model. Results of the same model calculation for the initial 87Sr/86Sr ratio of the rock suggest that its source material was depleted in 87Rb/86Sr by ∼50% relative to the estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr values support a clinopyroxene-rich cumulate source for the genesis of the nakhlite Governador Valadares. Furthermore, our Sm-Nd age and e143Nd data and the previously published e142Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent planet, formation of cumulate sources ∼4.56 Ga ago, and late melting of the sources and formation of the rock ∼1.37 Ga ago. The good agreement of isotopic ages and petrographic features among Governador Valadares, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to ∼1.37 Ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history.

U-Pb isotopic constraints on diachronous metamorphism in the northern Monashee complex, southern Canadian Cordillera

Abstract: U–Pb isotopic data from the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera, indicate that the age of metamorphism varies according to structural position in a 6 km thick section. This metamorphism resulted in an unusual sequence in which rocks with the lowest-grade mineral assemblage (kyanite–sillimanite–staurolite–muscovite) are underlain and overlain by higher-grade rocks. Xenotime and monazite U–Pb dates vary progressively from 64 Ma in the structurally highest rocks to 49 Ma in the deepest rocks. Discordant U–Pb ages from Proterozoic and Cretaceous monazite and titanite are used to interpret the thermal significance of the early Tertiary dates. The discordant analyses define linear arrays with lower intercepts that broadly overlap with early Tertiary, and the amount of discordance varies with structural level; it is least in the deeper rocks and greatest in higher rocks. Electron microprobe work showed that the monazite discordance in the deeper rocks resulted from Tertiary mineral overgrowth and recrystallization rather than Pb diffusion. We use previous studies of Pb diffusion and the fact that Proterozoic monazite and titanite suffered only negligible to moderate amounts of diffusive Pb loss to contend that elevated temperatures (c. 600–650 °C are inferred from pelitic mineral assemblages) existed in the deeper rocks for a short duration, perhaps a few million years. The downwards younging 64–49 Ma U–Pb dates are interpreted as closely reflecting xenotime and monazite growth ages rather than cooling ages or substantially reset ages based on the lack of Pb diffusion in monazite and the previously obtained 40Ar/39Ar data which suggest that rapid cooling occurred immediately after the U–Pb dates. In addition, growth ages are interpreted as thermal peak ages based on U–Pb dates from coeval kyanite-bearing leucosomes, the consistent nature of the U–Pb dates throughout the study area, and petrographic relationships which suggest that monazite grew before or during development of the syn-metamorphic foliation. These interpretations lead us to conclude that metamorphism was diachronous according to structural level, with higher rocks attaining peak temperatures and cooling rapidly while deeper rocks were heating towards a thermal peak that was attained a few million years later. This thermal scenario requires that higher rocks cannot have been the heat source for the deeper metamorphism, as was previously proposed.

Mineralogy, petrography, geochemistry and genesis of the Paleoproterozoic Birimian manganese-formation of Nsuta/Ghana

Abstract: The manganese deposit of Nsuta, in the Ashanti Belt of Southern Ghana, is sandwiched between Birimian metasedimentary rocks. The metasedimentary rocks contain interbedded carbonate-rich layers, which exhibit a characteristic banded appearance near the contact with the orebody. The orebody is a carbonate-type manganese-formation and in terms of origin is considered here as a Mn-analogue of the volcanogenic-exhalative Algoma type iron-formation. The protolith of the orebody (chemical sediment including Fe-bearing rhodochrosite and alabandite) is envisioned to have been formed in a marine basin with relatively high CO2 activity and Eh-pH conditions were extremely low (Eh 1 to −0.6 Volt and pH 8 to 11) during Birimian times (2170–2180 Ma). These conditions occurred immediately below the shelf break in a shallow-marine environment. Subsequent submarine weathering (halmyrolysis) followed later by metamorphism of Eburnian age (2100 Ma) led to the formation of Mg-Ca-Fe-bearing rhodochrosite, the dominant mineral in the orebody. Other minerals of the orebody are: sulfides (e.g. two generations of alabandite sphalerite, pyrite, millerite, niccolite, gersdorffite, and molybdenite), oxides and hydroxides (vanadium-bearing jacobsite, galaxite; brucite, Mn2+-todorokite), Mn-silicates and an unknown boron mineral. Pyrochroite, possibly preceded by manganosite, occurs as a retrograde mineral. This mineral assemblage forms the protore of the Nsuta deposit. Opaque Mn4+-todorokite replacing Mn2+-todorokite, manganite, manganomelane, pyrolusite and nsutite which formed at the expense of rhodochrosite, are of supergene origin and represent the economic part of the deposit. The orebody is interleaved between the associated pelitic-psammitic metasedimentary rocks suggesting that its protoliths was deposited over a time interval during the sedimentation of the latter. Both units underwent subsequent processes (submarine weathering and metamorphism) together. The compositional differences between the orebody with high Mn and CO2 and low Si and Al contents relative to the metasedimentary rocks are explained by a model involving the continuous sedimentation of continent-derived materials (protolith of the metasedimentary rocks). During this time a pulsatory phase of submarine volcanism and consequent precipitation of materials of essentially volcanogenic-exhalative origin occurred (protolith of the orebody). From the exhalations, the carbonate minerals in both the manganese-rich sediments and the metasedimentary host-rocks (in the latter in the form of layers and disseminations leading to relatively high concentrations of Mn, Ca and CO2) were precipitated.

Petrogenesis of Artifact-Bearing Fossil- Spring Tufa Deposits from Kharga Oasis, Egypt

Abstract: Petrographic and geochemical study of artifact-bearing fossil-spring tufas in stratigraphic contexts associated with Upper Acheulean and Middle Stone Age-to-historic artifacts provides a basis for characterizing and comparing Quaternary tufa deposition and diagenesis across the Kharga Oasis region of south-central Egypt. Analysis of tufa deposits at Refuf Pass, Midauwara Pass, A‘in ‘Amur, and Umm el Dabadib suggest that the low Mg-carbonates were precipitated by similar inorganic and biogeochemical processes operating within freshwater spring-fed alkaline stream environments throughout Quaternary time. Detailed petrographic studies suggest that the tufas are relatively pristine, with the original rock textures well preserved with minimal postdepositional alteration. Microstratigraphic details indicate that a variety of interformational facies were present within the former stream environments; understanding these contexts is valuable for interpreting prehistoric human activities. © 1999 John Wiley & Sons, Inc.

New petrographic and trace element data on thermally metamorphosed carbonaceous chondrites

Abstract: Mineral grains and matrix of heated chondrites Yamato (Y)-82162, Belgica (B)-7904, Y-86720, and Asuka (A)-881655 were examined for major elements and, where appropriate, phyllosilicate and matrix samples were characterized by TEM. CM chondrites A-881655 and B-7904 were only partially aqueously altered before thermal metamorphism initiated dehydration and recrystallization. Tochilinite is absent in both A-881655 and B-7904 probably due to mild thermal metamorphism. Phyllosilicates in B-7904 are dehydrated but not completely recrystallized to olivine and pyroxene. Y-86720 experienced a history very different from other CM chondrites: its chondrules and other coarse-grained components were completely altered by aqueous fluids. Fine-grained olivine was subsequently replaced during an episode of thermal metamorphism producing compositions near Fo10 and complete destruction of serpentine occurred. Y-82162 comes from a CI parent and is characterized by complete destruction of preexisting anhydrous silicates during aque­ ous alteration. Subsequent heating at 600-700° C resulted in dehydration of phyllosilicates. The matrix contains abundant fine-grained olivine which would not likely survive the original aqueous alteration and therefore must have been formed by phyllosilicate metamorphism. Alternating episodes of oxidation and sulfidization following aqueous alteration are evident in all but Y-86720. Either Y-86720 escaped an oxidizing event or effects of such an event were obliterated by later sulfidization. Based on RNAA data for thermally mobile trace elements in them and in Murchison CM2 samples heated for one week at 500° , 600° , and 700° C under conditions reasonable for interiors of primitive parent bodies, the four thermally metamorphosed carbonaceous chondrites can be ordered by the severity of open-system heating as 500° ::::::A-88165S

Micromorphology and petrography of Late Weichselian glaciolacustrine varves in southeastern Sweden

Abstract: The purpose of this investigation is to describe and interpret the sedimentology and petrography of Late Weichselian varves in southeastern Sweden in order to determine their nature and origin. It is focused on the microscopic evidence for glaciolacustrine varve sedimentation in the Baltic Ice Lake and the possibilities of making a detailed facies classification of the sediments in an area with an established varve chronology. The material examined was cores taken from five representative localities in an area below the highest shoreline of the Baltic Ice Lake in the provinces of Skane and Blekinge, i.e., below an altitude of 55–65 m. The investigations included chemical analyses, XRD, microscopy (polarized) and SEM-EDX investigations of the summer and winter layers of the varves. The results of the chemical, clay mineralogy (XRD) and microscopic examinations of the bedrock and mineral fragments indicate that the material in the different facies of the varved clay is mainly produced by moderate alteration and erosion from the local bedrock of predominantly granitoid rocks and of residual kaoline deposits. It is possible to distinguish microscopic evidence of four varve types differing in texture depending on the water depth and how close to the ice the different facies were deposited, i.e., if they were deposited proximally, distally or extramarginally in relation to the ice margin. It is also possible to relate these differences to changes in the palaeoclimate during the deglaciation of the area in the Bolling and Older Dryas chronozones.

Namurian bentonites in the Pennine Basin, UK – origin and magmatic affinities

Abstract: Nine Namurian clay bands retrieved from boreholes in the northern part of the Pennine Basin are, on the basis of their petrography, mineralogy and geochemistry, shown to be volcanic in origin and are therefore bentonites. The bentonites, which have a fragmental texture, are normally graded and show rare preservation of shard textures, representing vitric tuff deposits that have been altered subsequently to clay-dominated horizons. Crystals are a minor component of the bentonites, but biotite, in particular, is concentrated at the base of the beds. A clay mineral assemblage of mixed-layer illite–smectite with subordinate kaolinite identifies most of the samples as K-bentonites, but kaolinite dominates two samples that can be classed as tonsteins. Temporal variation of salinity within the depositional basin is suggested to explain these different clay assemblages. The major element geochemistry of the bentonites reflects their clay mineralogy and the compositions of diagenetic minerals present, the latter including pyrite, carbonates and hydroxyapatite. Enrichment of the bentonites in some trace elements (including Ba, Sr, Pb, Cu and Ni) can be related to the presence of the diagenetic minerals, but the extent to which the elements are added from external sources as opposed to being redistributed within the ash is unclear. Immobile trace element systematics suggest a rhyodacite/dacite composition for the original ash and derivation from the collision of plates, this being supported by evidence provided by the rare earth elements (REE) in one group of samples. However, in another group of samples, variations in REE concentrations may be caused by mobility of these elements during alteration. The chemistry of the Namurian bentonites contrasts markedly with that of the local Carboniferous volcanics but is comparable, in some respects, with one group of Westphalian tonsteins, although the latter are more rhyolitic in character. It is suggested that the Namurian bentonites and the Westphalian tonsteins of acid affinities originated from volcanic activity associated with a destructive plate margin in the Variscan externides and that the observed compositional trend may reflect magma evolution possibly related to the progressive east–west closure.

Sandstone Diagenesis, Reservoir Potential, and Sequence Stratigraphy of the Eocene Tyee Basin, Oregon

Abstract: Sandstone petrography and diagenetic analysis within a sequence stratigraphic framework provides a better understanding of the reservoir characteristics in the Eocene Tyee basin, an accretionary and forearc sequence, southern Oregon Coast Range. Detailed comparison of relative abundance of major detrital framework grains documents a marked difference of sandstone composition in each depositional sequence. Such a difference is mainly due to an abrupt change in provenance from a local Klamath Mountains metasedimentary source to a more distant extrabasinal Idaho Batholith-Clarno volcanic arc source. Furthermore, the composition of framework grains varies systematically from the lowstand systems tract to the highstand systems tract within a depositional sequence. This suggests that the patterns of sedimentation and sandstone composition can be affected by relative changes in sea level and tectonic uplift in the source area. In addition, the Eocene Tyee basin sandstones exhibit a down-section distribution of authigenic minerals, consisting of early-formed zeolites and late-stage quartz as well as a change in the abundance of smectite to mixed-layer chlorite/smectite with increasing burial depth. The down-section distribution of authigenic minerals is also causally linked to the compositional variation of detrital framework grains in each depositional sequence with increasing burial temperature. Much primary porosity has been filled with these authigenic minerals, thus diminishing the permeability of potential reservoir rocks. Secondary porosities and permeabilities of reservoir quality (averaging 10.80%; 2.76 md), however, are present locally in some highstand delta-front sandstone facies in the southern part of the basin as well as in lowstand turbidite sandstones in the deeper part of the basin to the north. The development of these reservoir-quality sandstones within the Eocene Tyee basin sequence is due to a complex burial diagenesis, which is directly related to temporal and spatial variations in original detrital mineralogy, in sedimentation pattern, and in burial temperature in the basin.

Petrography and Geochemistry of Holocene Sands in the Western Gulf of Mexico: Implications for Provenance and Tectonic Setting

Abstract: Petrographic and geochemical analysis of recent fluvial, beach, and dune sediments derived from the same source terrane was carried out in the western Gulf of Mexico to show the usefulness of the these three environments in determining the tectonic setting of the source. Petrographic analysis showed that dunes concentrate the less heavy minerals and quartz grains by means of the selectiveness of the wind as a transport agent. In contrast, the heavier grains, such as the rock fragments remain in the beach and fluvial environments because of the high transport energy. Beach and dune sands do not define a particular tectonic setting because of the selective wind action of the backshore of the beach and the dune environments and the less intense chemical weathering. The fluvial sands are the most representative in terms of interpretation of tectonic setting because they reflect the volcanic domain of the source rocks of the Trans-Mexican Volcanic Belt. The geochemical results of all three sedimentary environments reflect the calcalkaline character of the igneous source rocks of the Trans-Mexican Volcanic Belt. The samples fall in a continental island-arc-margin field with basaltic and andesitic source rocks. A correlation and factor analysis indicates an alkali-basalt source that contributes to the composition of the beach, dune, and fluvial sediments. The geochemical analysis of the sediments more accurately reflects the tectonic setting regardless of the depositional setting of the sediments.

BP and Oasis impact structures, Libya, and their relation to Libyan Desert Glass

Abstract: We have conducted petrographic, geochemical, and isotopic studies on a suite of target rock samples from two impact structures, BP and Oasis, in southeastern Libya. Both structures occur in Lower Cretaceous sandstone of the Nubia Group and are deeply eroded. Earlier microscopic thin-section studies provided evidence for the impact origin of the structures by demonstrating the presence of shock-characteristic planar deformation features in quartz. No absolute ages have yet been determined for these structures. From the proximity of these structures to the occurrence of the enigmatic Libyan Desert Glass (LDG), previously interpreted as impact glasses formed from a mature sandstone, and from the absence of disturbed strata of the sandstone of the Nubia Group in the area of the occurrence of the LDG, a possible relation was suggested between BP and Oasis impact structures and the LDG. Most of the target rocks at both structures have somewhat lower SiO2 and higher contents of other major oxides than LDG, but this distinction disappears when the composition of the sandstones is recalculated to a water-free basis. Also, there is a good correlation between the major and trace element compositions of samples from the target rocks of BP and Oasis structures and samples of LDG, but the refractory trace element content of the LDG is generally somewhat higher than that of the target rocks of the two Libyan craters. Rare earth elements (REE) from the BP and Oasis impact structures and from LDG have similar abundances and display a similar chondrite-normalized pattern. The isotopic ratios of Nd and Sr for samples of the two structures and for the LDG are in a similar range. They are characterized by negative eNd values and positive eSr values, which are characteristic of upper continental crustal rocks. In an eNd vs. eSr diagram, LDG values plot within the field defined by the BP and Oasis rocks, and in a 1/Sr vs. eSr plot LDG is within the range defined by BP and Oasis target rocks. From the available petrographic, geochemical, and isotopic data, we conclude that the target rocks of the Libyan structures could represent the parent material for LDG. However, without further age information it is not possible to conclude unambiguously that the BP or Oasis structures are the source craters for the LDG. Abate, B., Koeberl, C., Kruger, F. J., and Underwood, J. R., Jr., 1999, BP and Oasis impact structures, Libya, and their relation to Libyan Desert Glass, in Dressler, B. O., and Sharpton, V. L., eds., Large Meteorite Impacts and Planetary Evolution II: Boulder, Colorado, Geological Society of America Special Paper 339. 177 * Present addresses: (Abate) Ethiopian Institute of Geological Surveys, P.O. Box 30389, Addis Ababa, Ethiopia; (Underwood) 9518 Topridge Dr. #3, Balcones Place, Austin, TX 78750-3500, USA.

Lithium and boron distributions in geological samples

Abstract: The concentration of elements in whole rocks and maps of their distribution in petrographic thin sections were determined using ion beam microprobe analysis. Lithium and B contents in rocks and minerals are measured using the 7Li(p,α)4He and 11B(p,α)8Be reactions. X-rays are simultaneously detected for elements heavier than Na, including Cl, to corroborate microscopic mineral identification. The ion beam analysis data are integrated with observations under the petrographic and scanning electron microscopes, as well as analysis using X-ray fluorescence and the electron microprobe. Lithium, B and Cl can be used to assess volcanic and hydrothermal processes. In a study of 62 samples, Li, B and Cl increase proportionally with increasing silica content in fresh volcanic rocks from the Taupo Volcanic Zone in New Zealand. The median values for Li, B and Cl in rhyolites (SiO2%=70–76%) are 35, 20 and 800 ppm (wt), respectively, and 19, 11 and 340 ppm (wt), respectively, for andesites (SiO2=56–62%). Boron and Cl preferentially partition into the glassy matrix of rhyolite and andesite. In rhyolites, Li occurs mainly in minerals such as hornblende and biotite but resides in the glassy matrix of andesites. During hydrothermal alteration of volcanic rocks, Cl always partitions into hydrothermal solutions while Li and B are preferentially redistributed in the rock. As hydrothermal alteration proceeds, Cl in the rock decreases and B and Li increase proportionally, depending on the type of mineralization present and the temperature of alteration.

Deterioration of Iowa Highway Concrete Pavements: A Petrographic Study

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