Showing papers on "Petrography published in 2011"

Formation of high δ18O fayalite-bearing A-type granite by high-temperature melting of granulitic metasedimentary rocks, southern China

Abstract: The genesis of A-type granites has been controversial. Fayalite granite is a member of the most reduced A-type granites, commonly thought to have been primarily sourced from tholeiitic rocks. In this paper we report petrography, whole-rock geochemistry, Sr-Nd isotope, and in situ zircon Hf-O isotope results for a fayalite-bearing A-type granite suite at Jiuyishan in southern China. High zircon δ 18 O (8.0‰–9.8‰), negative zircon eHf(t) (–6.2 to –2.3), and evolved whole-rock Sr-Nd isotopes ( I Sr = 0.7151–0.7181; eNd(t) = –7.4 to –6.6) indicate the reworking of old supracrustal rocks. Isotopic and geochemical results, particularly downward inflections of Zr and Ba at ∼70 wt% SiO 2 , point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite. Integrated petrological, geochemical, and isotopic studies present the first convincing case that reduced A-type granites can also be generated by high-temperature (>960 °C) melting of granulitic metasedimentary rocks, likely related to the upwelling of the asthenosphere and/or underplating and intrusion of basaltic melts. We emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity ( f O 2 ), low f H 2 O , and high temperature.

Use of Clumped-Isotope Thermometry to Constrain the Crystallization Temperature of Diagenetic Calcite

Abstract: We describe an approach to estimating the crystallization temperatures of diagenetic calcites using clumped-isotope thermometry, a paleothermometer based on the ^(13)C–^(18)O-bond enrichment in carbonates. Application of this thermometer to calcified gastropod shells and calcite cements in an early Eocene limestone from the Colorado Plateau reveals a record of calcite precipitation and replacement at temperatures varying from 14 to 123°C. The early Eocene host sediments were never deeply buried, but they experienced a significant thermal pulse associated with the emplacement of a late Miocene basalt flow. The combination of independent constraints on thermal history with clumped-isotope thermometry, petrographic (including cathodoluminescence) observations, and oxygen isotopic data provides an improved basis for estimation of the temperature and timing of diagenetic events and fluid sources. The petrography and calcite δ^(18)O values, taken alone, suggest that the aragonite-to-calcite transformation of gastropod shell material occurred simultaneously with early formation of cements and lithification of the matrix in the same sample. However, addition of clumped-isotope thermometry demonstrates that this phase transformation of shell material occurred at temperatures of 94–123°C in a highly rock-buffered microenvironment (i.e., with the isotopic composition of fluid buffered by coexisting carbonate), millions of years after lithification of the matrix and formation of initial low-temperature (14–19°C) calcite cements within shell body cavities. Clumped-isotope temperatures in excess of reasonable Earth-surface conditions recorded by later-formed cements demand that cement growth occurred in association with the lava emplacement. Our results illustrate the potential for clumped-isotope thermometry to constrain conditions of diagenesis and guide interpretations that would not be possible on the basis of conventional stable-isotopic and petrographic data alone, and demonstrate how petrographic characterization of clumped-isotope thermometry samples can benefit paleoclimate studies.

Prediction of lithofacies and reservoir quality using well logs, Late Cretaceous Williams Fork Formation, Mamm Creek field, Piceance Basin, Colorado

Abstract: Understanding the controls and distribution of reservoir quality is important for the economic success of tight-gas reservoirs in which diagenesis interacts with primary depositional variations in environment and texture to exert a strong control on pore networks, rock mechanical properties, and natural fractures. In the Upper Cretaceous Williams Fork Formation of the Piceance Basin, framework grain composition is a major control on compaction and the occurrence of authigenic phases. Alteration of volcanic grains in the upper Williams Fork led to grain-coating clay precipitation. Ferroan dolomite cement is found only in the deeper marine-influenced intervals in which dolostone fragments are present. This study shows that careful petrographic assessment of lithofacies heterogeneity can be upscaled by correlation with log properties to yield tools for field-scale reservoir quality prediction. Twelve lithofacies are identified based on sand-grain populations, cement types, and clay matrix content. Sandstones of the highest reservoir quality are those with grain-coating clays that inhibit quartz cementation; these sandstones can be identified based on high-density porosity log values. Sandstones with the poorest reservoir qualities are tightly cemented with carbonate and quartz cement or are rich in clay matrix. Carbonate-cemented intervals are identified by low-density porosity. Clay matrix–rich samples have high gamma-ray and low-density porosity values. The presence of abundant potassium feldspar in the upper intervals results in high gamma-ray readings even in the clean (clay matrix–free) sandstone.

Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth

Abstract: The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25–3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga. Monazite-xenotime thermometry of intergrowths in the inclusions and the quartz-muscovite rock matrix constrain temperatures to between 420–475 °C, corresponding with conditions during peak regional metamorphism. Petrography and U-Pb geochronology of zircon inclusions from other localities show that the replacement of primary inclusions may commence in the igneous host rock and continue through weathering, sedimentation, and diagenesis. With increasing metamorphic grade, the inclusion assemblage increasingly reflects the composition of the rock matrix. In Jack Hills, most of the inclusions have the same composition and abundances as the metamorphic matrix, consistent with their formation during metamorphism. The titanium content of quartz inclusions indicates formation temperatures of 350–490 °C, supporting a metamorphic origin. Several lines of evidence indicate that at least some of the muscovite inclusions are also secondary. The lack of apatite inclusions in zircons from Jack Hills, relative to zircon in common granitic rocks, suggests that secondary minerals may have replaced primary apatite. Thus, detrital zircon may not be impermeable to post-depositional fluids, raising doubts about the use of the mineral inclusions they contain to infer initial magma chemistry. These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust.

Correlation and Provenance of Upper Cretaceous (Campanian) Fluvial Strata, Utah, U.S.A., from Zircon U-Pb Geochronology and Petrography

Abstract: Detrital zircon and petrographic provenance analysis of Upper Cretaceous (Campanian) strata corroborate correlation of the fluvial Kaiparowits Formation in southern Utah with post-Castlegate strata of the Mesaverde Group in eastcentral Utah. Sandstone samples of the Kaiparowits Formation in the Kaiparowits Plateau and Mesaverde Group (Neslen, Bluecastle Tongue of Castlegate, Farrer, and Tuscher formations) in Gray Canyon north of Green River, Utah, contain six U-Pb detrital zircon age populations ranging in age from Archean to Late Cretaceous. Thrust-belt-derived sublitharenite and quartzarenite of the lower part of the Mesaverde Group (Neslen and Bluecastle) lack or have only rare Mesozoic grains, but contain abundant Archean, Proterozoic, and Paleozoic grains that record recycling of Proterozoic, Paleozoic, and Jurassic sandstones exposed in uplifted thrust sheets to the west. South- and southwest-derived feldspathic litharenite of the Kaiparowits and Farrer formations contains Proterozoic grains and a diverse population of Mesozoic grain ages. Archean grains are rare to absent. Maximum depositional ages interpreted from young zircon grains in two Kaiparowits samples statistically overlap 40Ar/39Ar ages (76–74 Ma) from interbedded bentonitic tuffs and weighted mean ages of young grains from the Farrer Formation. Kolmogorov-Smirnoff statistics, in conjunction with young grain ages and detrital age spectra, corroborate petrographic evidence, which indicates that the Kaiparowits rivers were connected northward with a river system that deposited the Farrer Formation. The young grain ages, although imprecise, indicate that the uppermost Mesaverde Group strata of the Tuscher Formation are younger than the Kaiparowits Formation and record partitioning of the former foreland basin by Laramide uplifts.

The lunar rock and mineral characterization consortium: Deconstruction and integrated mineralogical, petrologic, and spectroscopic analyses of mare basalts

Abstract: – The lunar rock and mineral characterization consortium (LRMCC) has conducted coordinated mineralogy/petrography/spectroscopy analyses of a suite of pristine lunar basalts. Four basalt slabs (two low-Ti, two high-Ti) and paired thin sections were analyzed. Thin sections were analyzed for mineralogy/petrography, while the slabs were used to prepare particulate separates of major mineral phases and bulk samples. Mineral separates and particulate bulk samples were crushed to controlled grain sizes and their reflectance spectra measured in the NASA RELAB at Brown University. The resulting data set provides an essential foundation for spectral mixing models, offers valuable endmember constraints for space weathering analyses, and represents critical new ground truth results for lunar science and exploration efforts.

Sandstone petrology and mudstone geochemistry of the Peruc–Korycany Formation (Bohemian Cretaceous Basin, Czech Republic)

Abstract: We have studied the petrography and the bulk-rock geochemistry of arenites and mudstones of the Cenomanian Peruc–Korycany Formation to characterize their provenance and sedimentary history, as well as the influence of weathering, hydraulic sorting, and recycling of the source rocks. The Peruc–Korycany Formation contains sedimentary facies reflecting both meandering- and braided-river systems and shallow-marine systems. Differences in the three depositional settings did not cause distinctly different modifications of the framework compositions of the arenites. The sand from the fluvial systems is very mature (Qm98F0Lt2). These fluvial arenites were subsequently modified by shallow-marine processes; reworking produced very slight decreases in the abundance of lithic fragments and polycrystalline quartz grains. The Cenomanian strata of the Bohemian Cretaceous Basin were derived dominantly from metasedimentary and crystalline rocks of the Palaeozoic Tepla-Barrandian and Cadomian Moldanubian units, respectivel...

Petrography and stable isotope geochemistry of the cretaceous El Abra Limestones (Actopan), Mexico: Implication on diagenesis

Abstract: Petrography and stable isotopes (carbon and oxygen) geochemistry of limestones from the El Abra Formation, Actopan, were studied to identify their digenetic environments. The major petrographic types identified are mudstone, wackestone, grainstone, and boundstone. Most of the studied samples show positive δ 13 C values, except two samples (2 and 28), which are slightly negative values (-0.27‰ and -0.02‰). The organic remains identified in foraminiferal wackestone type can be responsible for the negative δ 13 C values. The δ 18 O values range from -12.41‰ to -4.02‰ and indicate meteoric diagenesis.

A new methodological approach for the chemical characterization of black crusts on building stones: a case study from the Catania city centre (Sicily, Italy)

Abstract: Mineralogical, petrographic and chemical analyses were carried out on black crusts covering the stone surface of monuments and buildings of the historical city centre of Catania, one of the most beautiful Baroque places in eastern Sicily. Black crusts were studied through the careful and synergic employment of traditional techniques, including polarizing optical microscopy (POM), scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS), and infrared spectroscopic techniques (FT-IR), in combination with an innovative technique, i.e.laser ablation inductively coupled mass spectrometry (LA-ICP-MS). The validity of such an integrated approach to study black crusts has been recently demonstrated by the authors. The main goal of this study was to develop and test the reliability of the LA-ICP-MS method on black crusts in order to evaluate the degree of chemical contamination of examined stones and to determine the role of the different sources of pollution in Catania, both anthropogenic (domestic heating, industrial combustion, vehicular traffic) and natural (emissions by Mt Etna) in the formation of crusts. Results obtained demonstrated that this innovative approach has a double potential in the study of black crusts, since it allows the analysis of alteration and degradation processes induced by migration of specific chemical elements from the crust to substrate, and, at the same time, it represents a reliable indicator of the environmental pollution.

Petrogenesis of bimodal volcanic rocks from Maoershan Formation in Zhangguangcai Range:evidence from geochronology and geochemistry

Abstract: LA-ICP-MS zircon U-Pb dating and geochemical data of the volcanic rocks from Mesozoic Maoershan Formation in Zhangguangcai Range were obtained for to constraining their formation time and the regional tectonic background.Zircons from two representative volcanic rocks are euhedral-subhedral in shape and display striped absorption or oscillatory zoning in CL images with high Th/U ratios(0.40～2.08),implying their magmatic origin.The dating results indicate that the volcanic rocks from Maoershan Formation were formed in the Early Jurassic(179～184 Ma) which is older than the previously believed Late Jurassic.The volcanic rocks from the Maoershan Formation display a bimodal volcanic rock association based on their petrographic and geochemical data.The trachyte and trachyandesite are characterized by high alkali(especially K2O),enrichment in large ion lithophile elements(LILE) and light rare earth elements(LREE)in contents,whereas the rhyolites are chemically similar to the A-type rhyolite.The bimodal volcanic rock association in Lesser Xing'an Range and Zhangguangcai Range in the studied areas implies an intense extensional environment.Combined with the spacial compositional variation of the Early Jurassic igneous rocks,it is suggested that the volcanic rocks from Maoershan Formation were formed under an extensional environment similar to back-arc basin which could be related to subduction of the Paleo-Pacific plate(Izanagi) beneath the Eurasian continent.

Changing the fabric of life in post-Roman and early medieval Cornwall : an investigation into social change through petrographic analysis

Abstract: ...................................................................................3 Acknowledgments ......................................................................4

Evaluation of provenance, tectonic setting, and paleoredox conditions of the Mesoproterozoic-Neoproterozoic basins of the Bastar craton, Central Indian Shield: Using petrography of sandstones and geochemistry of shales

Abstract: We carried out petrographic analyses of the sandstones and geochemical analyses of the shales from the Mesoproterozoic–Neoproterozoic Chhattisgarh and Indravati Basins to determine their tectonic setting, provenance, and paleoredox conditions. Petrographic study shows that the sandstone samples have high amounts of quartz but are depleted in feldspar and lithic fragments. The shales have been classified into the calcareous and noncalcareous shales. The noncalcareous shales have higher concentrations of most of the major elements and trace elements, including the rare earth elements (REEs), in comparison to the calcareous shales. However, this difference in elemental concentrations between the calcareous and noncalcareous shales proved to be significant only for SiO 2 , TiO 2 , Al 2 O 3 , MnO, CaO, K 2 O, loss on ignition, Rb, Sr, Nb, Ce, Pr, Sm, Gd, Hf, and Ta using the Student9s t -test at better than 95% confidence level. Upper continental crust (UCC)–normalized elemental ratios of the calcareous and noncalcareous shales suggest evolved sources similar to UCC. The sandstone petrology and Ni versus Cr diagram, chondrite-normalized REE patterns, and negative Eu/Eu* values of the calcareous and noncalcareous shale samples reveal that the sediments have been derived from felsic rocks (granites and gneisses) of the Bastar craton. The SiO 2 versus K 2 O/Na 2 O and SiO 2 /Al 2 O 3 versus K 2 O/Na 2 O tectonic-setting discrimination diagrams of the shales and the petrology of the sandstones indicate a passive-margin tectonic setting for Chhattisgarh and Indravati Basins. Geochemical parameters such as Ce/Ce* and Mn* suggest that the calcareous shales were deposited in a suboxic environment, compared to the oxic environment of the noncalcareous shales.

Genesis of the Jurassic Carbonate-Hosted Pb–Zn Deposits of Jebel Ressas (North-Eastern Tunisia): Evidence from Mineralogy, Petrography and Trace Metal Contents and Isotope (O, C, S, Pb) Geochemistry

Abstract: The Jebel Ressas Pb–Zn deposits in North-Eastern Tunisia occur mainly as open-space fillings (lodes, tectonic breccia cements) in bioclastic limestones of the Upper Jurassic Ressas Formation and along the contact of this formation with Triassic rocks. The galena–sphalerite association and their alteration products (cerussite, hemimorphite, hydrozincite) are set within a calcite gangue. The Triassic rocks exhibit enrichments in trace metals, namely Pb, Co and Cd enrichment in clays and Pb, Zn, Cd, Co and Cr enrichment in carbonates, suggesting that the Triassic rocks have interacted with the ore-bearing fluids associated with the Jebel Ressas Pb–Zn deposits. The δ¹⁸O content of calcite associated with the Pb–Zn mineralization suggests that it is likely to have precipitated from a fluid that was in equilibrium with the Triassic dolostones. The δ³⁴S values in galenas from the Pb–Zn deposits range from -1.5 to +11.4‰, with an average of 5.9‰ and standard deviation of 3.9‰. These data imply mixing of thermochemically-reduced heavy sulfur carried in geothermal- and fault-stress driven deep-seated source fluid with bacterially-reduced light sulfur carried in topography-driven meteoric fluid. Lead isotope ratios in galenas from the Pb–Zn deposits are homogenous and indicate a single upper crustal source of base-metals for these deposits. Synthesis of the geochemical data with geological data suggests that the base-metal mineralization at Jebel Ressas was formed during the Serravallian–Tortonian (or Middle–Late Miocene) Alpine compressional tectonics.

Occurrence and geochemistry of metamafic rocks from the Forquilha Eclogite Zone, central Ceara (NE Brazil): geodynamic implications

Abstract: In the northern region of the Borborema Province, a 30 km-long mafic/ultramafic belt of high pressure (HP) rocks called the Forquilha Eclogite Zone has been recently discovered in the Central Domain of Ceara State. The belt comprises three groups of rocks: (1) garnet amphibolites, (2) retrograded eclogites and (3) clinopyroxene–garnet amphibolites, all of them being hosted in garnet ± kyanite + sillimanite schists and orthogneisses which are often migmatized. The geochemical analyses of 30 representative samples confirm differences among the three groups that were initially classified by petrography. The garnet amphibolites are the most depleted rocks, with relatively flat rare earth element (REE)PM pattern and (SPIDER)PM pattern with negative Rb, Th, Nb, Sr and Zr anomalies. It is the most fractionated group, ranging from picrobasalts to andesites. Fractional crystallization and mineral accumulation are indicated by Eu and Sr anomalies. The data show geochemical affinities with island-arc basalts. The retrograded eclogites display flat (REE)PM, but without Eu anomalies. Nb/La ratios are low and variable, as for slightly enriched MORB from ocean floor or intra-oceanic back-arc environments. However, a genetic link with the Group 1 rocks cannot be completely discarded. The clinopyroxene–garnet amphibolites are the most enriched group. (REE)PM and (SPIDER)PM patterns show many features of alkaline basalts. In the Nb/Yb versus Th/Yb diagram the data cluster near the E-MORB standard. Several geochemical aspects of these rocks fit well those from the HP/UHP Pan-African/Brasiliano Suture Zone in the Dahomeyides/Hoggar regions of West Africa. However, differences in their extent and lack of alignment preclude a direct correlation among these zones. Copyright © 2010 John Wiley & Sons, Ltd.

A 3-D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X-ray tomographic microscopy-Method and applications

Abstract: – A method is described for imaging in 3-D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X-ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long-term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment-dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non-destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two-thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth.