Showing papers on "Petrography published in 2007"

Microsampling and Isotopic Analysis of Igneous Rocks: Implications for the Study of Magmatic Systems

Abstract: Isotopic fingerprinting has long been used to trace magmatic processes and the components that contribute to magmas. Recent technological improvements have provided an opportunity to analyze isotopic compositions on the scale of individual crystals, and consequently to integrate isotopic and geochemical tracing with textural and petrographic observations. It has now become clear that mineral phases are commonly not in isotopic equilibrium with their host glass/groundmass. Isotopic ratios recorded from core to rim of a mineral grain reflect the progressive changes in the magma composition from which the mineral crystallized. The sense of these changes and the relationship between isotopic composition and petrographic features, such as dissolution surfaces, can be used to constrain magma evolution pathways involving open system processes such as magma mixing, contamination and recharge.

Carbonate Assimilation at Merapi Volcano, Java, Indonesia: Insights from Crystal Isotope Stratigraphy

Abstract: Recent basaltic andesite lavas from Merapi volcano contain abundant, complexly zoned, plagioclase phenocrysts, analysed here for their petrographic textures, major element composition and Sr isotope composition. Anorthite (An) content in individual crystals can vary by as much as 55 mol% (An

19. petrology and geochemistry of basalts and related rocks from sites 214, 215, 216, dsdp leg 22, indian ocean

Abstract: Aspects of the chemistry, petrography, and genesis of eruptive rocks from Sites 214 and 216 on the Ninetyeast Ridge and from Site 215 from the central Indian Ocean basin are discussed. In conjunction with the major element data (Hekinian, Chapter 17) we present additional data for the trace elements B, Ba, Co, Cr, Cu, Ga, Li, Mo, Ni, Se, Sr, U, V, Y, Zr and the lanthanide elements (REE). Petrographic study of the samples confirms the details outlined by Hekinian. Features which are believed to be especially relevant to the interpretation of the geochemical data and to the understanding the origin of the Ninetyeast Ridge are emphasized in this chapter. Observations and interpretations of the chemical data lead to the suggestion that the volcanic rocks of Sites 214 and 216 on the Ninetyeast Ridge differ in composition, texture, and mineral paragenesis from typical mid-ocean ridge basalts. The petrography and chemistry are consistent with the hypothesis that the Ninetyeast Ridge samples are fractionated lavas which have cooled and crystallized in shallow magma chambers before extrusion. The basalts from Site 215 in the Central Indian Ocean Basin show textures and mineral paragenesis characteristic of deep-sea pillow basalts. However, they are generally more alkaline in character than either the Ninetyeast Ridge basalts or those from the mid-Indian Ocean Ridge.

The petrography of weathering processes: facts and outlooks

Abstract: Rock weathering has been investigated from atomic to global scales through the different but complementary approaches of mineralogy, petrography, geomorphology and geochemistry. The sequences of mineral reactions involved in the alteration process are now well known. They explain the global trend of weathering phenomena but do not account for the actual rock transformation dynamics. In particular, they ignore the intimate relation of the mineral reaction progress with the increase in connected porosity. At the hand specimen scale, heterogeneity is the rule: mineral reactions are controlled by local physicochemical conditions. Alteration processes depend largely on the rock microstructure properties. They proceed through nearly-closed, semi- and completely open microsystems which are interconnected by fractures or pores. Before being leached out by the solutions which flow in the large fractures (flux), the soluble elements migrate inside the connected porosity through chemical diffusion. The dissolution of the primary minerals is mediated through local gradients of chemical potential. With increasing alteration, the rock porosity increases, as does the length of the fluid passageways and their constrictivity and tortuosity. Consequently, the apparent diffusion coefficient for the most soluble elements decreases. The amplitude of the chemical potential gradients for the most soluble elements is reduced by the progressive coating of the reactive surfaces by clays and Fe oxyhydroxides. The residence time of these elements inside the weathered rock increases as alteration progresses; an effect enhanced by their temporary adsorption on the exchangeable sites of clays and Fe oxyhydroxides. Consequently, the weathering rate decreases with time. A possible new way to calculate weathering rates could be to measure the residence time of soluble elements inside the different microsystems during their migration towards the diluted solution which occurs in the large fractures.

Petrography and Geochemistry of Terrigenous Sedimentary Rocks in the Neoproterozoic Rabanpalli formation, Bhima Basin, Southern India: Implications for Paleoweathering Conditions, Provenance and Source Rock Composition

Abstract: Petrographic, major, trace, and rare earth element compositions of quartz arenites, arkoses, and siltstones of Neoproterozoic Rabanpalli Forrnation of Bhima Basin have been investigated to understand the provenance. The quartz arenites, arkoses, and siltstones have large variations in major element concentrations. For example, quartz arenites and arkoses contain the hlgher Si0 2 (average with one standard deviation being 97±1,73±2,respectively) and lower Al 2 O 3 (0 95±0 4, 9 6±O 9, respectlvely) concentrations than siltstones (SiO 2 =64±4, A1 2 0 3 =14±1), which is mainly due to the presence of quartz and absence of other Al-bearing minerals in relation with rock types. This is also supported by our petrography, since quartz arenites and arkoses contain significant amount of quartz relative to feldspar and lithic fragments. The observed low CIA values and A-CN-K diagram suggest that the sedimentary rocks of Rabanpalli Formation have undergone K-metasomatism. The Co, Ni, Cr, Ba, Zr, Hf, and Th values are higher in siltstones than quartz arenites and arkoses. The EU/EU * , (La/LU) en , La/Sc, Th/Sc, Th/Co, Th/Cr, Cr/Th ratios, and Cr, Ni, V, and Sc values strongly suggest that these sediments were mainly derived from the felsic source rocks. This interpretation is also supported by the Th/Sc versus Sc bivariate and La-Th-Sc triangular plots. The rare earth element (REE) patterns of these rocks also support their derivation from felsic source rocks. Further more, these rocks exhibit higher LREE/HREE ratio (8±4) and a significant negative Eu anomaly (0 77±0 16), which indicate the felsic igneous rocks as a possible source rocks.

Exsolution lamellae in a clinopyroxene megacryst aggregate from Cenozoic basalt, Leizhou Peninsula, South China: petrography and chemical evolution

Abstract: Petrographic investigations and electron microprobe analyses have been performed on a rare aggregate of clinopyroxene megacrysts collected from Cenozoic basalts in Yinfengling, Leizhou Peninsula of South China. The aggregate, composed of several clinopyroxene megacrysts, shows abundant exsolution lamellae of garnet (Grt) and orthopyroxene (Opx), and granular texture. Cr- and Ti-poor spinels are also present in this sample. They occur predominantly as Sp–Opx–Grt clusters (Cr# = 0.025–0.034) at the interspace between different megacrysts, and subordinately as bleb-shaped (Cr# = 0.025–0.034) or thin-lamella crystals (Cr# = 0.006–0.021) in clinopyroxene. Three different assemblages of exsolution are identified, namely (1) Sp (high Cr/Al) and Opx; (2) Grt–Opx; (3) Sp (low Cr/Al) and Opx. In addition, some garnets were likely developed as response to breakdown of the high-Cr/Al Sp. The homogeneous compositions in all constituent minerals and the good agreement between calculated Cpx/Grt partition coefficients (K d’s) for trace elements and reference data strongly suggest a chemical equilibrium among coexisting minerals, probably attained by diffusion after the exsolution. Thermobarometric calculation based on exsolved assemblage yields a temperature of 900 ± 30°C and a pressure of 12 ± 2.2 kbar, corresponding to the present-day thermal gradient in the region. Much higher P–T estimates (T = 1,210 ± 30°C, P = 16.2 ± 3.5 kbar) are obtained for the reconstructed composition of cpx prior to exsolution. The contrast in thermal state before and after the exsolution might reflect the thermal evolution of the lithosphere beneath South China during the Cenozoic.

Field geometry, petrography and geochemistry of a dolomitization front (Late Jurassic, central Lebanon)

Abstract: This contribution describes the field geometry, petrography and geochemistry of a well-exposed dolomitization front in Upper Jurassic carbonates, and attempts to highlight the sedimentological, structural and relative sea-level controls on multiphase dolomitization and related diagenetic events. The data presented reflect the superposition of various diagenetic phases which have resulted in a single dolostone body, whose dimensions are well defined in the field. Local microbial intraclastic dolomites of Late Tithonian age accumulated in a hypersaline lagoon during relative sea-level fall. These pre-date beige hydrothermal dolostones (51 to 55 mol% CaCO3; δ18O: −9·3 to −4·0‰ V-PDB; δ13C: −1·5 to +2·1‰ V-PDB; 87Sr/86Sr: 0·70742; matrix porosity: ≈6%; Klinkenberg permeability: ≈0·5 mD), whose dolomitizing fluid circulated along faults and invaded the nearby facies. First, the burrows were dolomitized, then the bulk rocks, resulting in the investigated 'tongue'-shaped dolomite body. Upon Late Jurassic–Early Cretaceous uplift, near-surface water percolated through – and altered – the underlying beige dolostones. This event was followed by a ferroan dolomite cement phase, which occurred during further burial. This contribution, featuring a well-defined geometric pattern of a dolomitization front with a large petrographic and geochemical data set, may also serve as a case study illustrating the complexity of superimposed diagenetic processes which have to be taken into account in modelling exercises of multiphase hydrothermal dolomitization.

The Dikili-Çandarlı volcanics, western Turkey: Magmatic interactions as recorded by petrographic and geochemical features

Abstract: Located in the northwestern part of the Aegean region, Dikili-Candarli volcanic suite contains products representative for the western Anatolian Miocene volcanism. They can be divided into two main groups: the Dikili and the Candarli groups. The Dikili group is Early–Middle Miocene in age and consists mainly of pyroclastic rocks, andesitic-dacitic lavas, lava breccia, lahar flows and associated sedimentary rocks. The lavas contain disequilibrium phenocrysts assemblages. The Candarli group consists of Upper Miocene–Pliocene lava and sediment associations.The volcanic rocks consist mainly of rhyolitic domes and basaltic trachyandesite-basaltic andesite lavas erupted along the NW–SE- and NE–SW-trending fault systems; the faults controlled the development of the Candarli depression. Major- and trace-element chemistry indicates that the lavas are dominantly high-K, calc-alkaline, intermediate to acidic in composition. Chemical and textural characteristics of the minerals reveal that mixing was a common process in the generation of this magma. In particular, petrography, textural evidence and crystal chemistry of the phenocrysts together with variations in rock compositions indicate that basaltic-basaltic andesitic magma intruded dacite magma and is partially hybridized with it. New petrographic and geochemical data of Dikili-Candarli volcanics are closely similar to those of the active continental margin volcanism which are interpreted as mantle-derived magmas contaminated by crustal materials.

Ion microprobe U-Pb zircon geochronology of a late tectonic granitic-gabbroic rock complex within the Hercynian Iberian belt

Abstract: Ion microprobe (SIMS) dating shows that three plutonic rock bodies representative of the major, ~ 15 000 km 2 , late tectonic (late-D 3 ), plutonic rock series in the Central Iberian Zone of the Hercynian belt in west-central Iberia have indistinguishable zircon crystallization ages. Ledrada biotite granite and Colmenar cordierite-bearing biotite granite show gradual transitions in field appearance, petrography and chemistry and have statistically indistinguishable weighted average ages of 306.8 ± 1.9(2σ) Ma and 306.5 ± 1.5(2σ) Ma, respectively, which indicates that they originated during a single event involving a heterogeneous magma which notably varied in Al-content. The third rock body, Navahermosa meta-gabbronorite, has a weighted average zircon crystallization age of 305.6 ± 1.4(2σ) Ma, statistically indistinguishable (variance analysis, F-test, α = 0.05) from the granites. Zircon crystals in the gabbronoritic rock are anhedral, skeletal, millimetre-sized and partake in main magmatic textures, whereas the zircon grains in the granites are of more common appearance, much smaller, usually euhedral and enclosed in main magmatic crystals. As gestation times of granitic zircon, the time between zircon crystallization and magmatic intrusion, may be up to 5 Myr, the crystallization age of the gabbronoritic zircon may be the best estimate of the time of emplacement of the magmatic complex. Our study indicates co-existence of basic and silicic magmas in the Hercynian crustal section at c. 306 Ma, suggesting common genetic control. The two granitic rocks carry inherited zircon ranging from c. 1300 to 330 Ma, indicating that pre-Hercynian basement rocks of Proterozoic to Palaeozoic age contributed to the granitic magma.

Relationships between mineralogical composition, water absorption and hydric dilatation in the "Macigno" sandstones from Lunigiana (Massa, Tuscany)

Abstract: Despite its good technical properties, the “Macigno” sandstone undergoes a typical process of decay when used outdoor, consisting prevalently in a flaking of the stone surface with detachment of rock fragments. In order to understand the causes of this decay, the relationships between mineralogy and physical rock properties were investigated. Twenty-six rock samples were collected from quarries situated near the villages of Fivizzano and Pontremoli (the Lunigiana area of northwestern Tuscany, Italy) and examined for their chemical (XRF), mineralogical (XRPD) and petrographic (optical microscopy) characteristics, as well as physical properties such as real and bulk densities, water absorption capacity and linear dilatation coefficient of water-saturated samples. The phyllosilicate minerals of the sandstone were studied on oriented aggregates of the <4 μm fractions by X-ray powder diffraction, and their chemical composition determined by EDS microprobe analyses. The cationic exchange capacity (C.E.C.) was measured on fifteen selected samples. The studied sandstones are fine- to coarse-grained, arkoses and lithic arkoses whose colour ranges from grey to yellowish. The normative carbonate fraction consists essentially of calcite; its amount is generally low (<1–10%), with the exception of three samples collected in the Turlago-Labiano quarry, which contained calcite concentrations of up to 14, 15 and 38%, respectively. The normative non-carbonate fraction is made up of quartz (19–34%), K-feldspars (7–13%), plagioclase (14–33%), mica-like minerals (4–10%) and chlorite s.l. (5–26%). Mica-like minerals include muscovite (illite), minor amounts of biotite and sporadic illite/smectite mixed-layers (I/S). Chlorite s.l. includes chlorite s.s. and chlorite/smectite mixed-layer phases (Chl/S). The content of the sandstone smectitic component, predominantly associated to Chl/S, ranges from 1 to 7 (wt.%). C.E.C. varies from 2.4 to 11.1 meq/100g. Water absorption capacity (wt.% of dry samples) as well as hydric dilatation of the stone upon water saturation increase with increasing smectite content, indicating that this swelling component may play an important role in the “Macigno” sandstones decay.

Textural, Elemental, and Isotopic Characteristics of Pleistocene Phreatic Cave Deposits (Jabal Madar, Oman)

Abstract: Two main types of karst formation are commonly known: the surficial meteoric one and the subsurface (hypogenic) karst, which can be related to both carbonic (H2CO3) and sulfuric (H2S) acids. This paper documents evidence for a third, CO2-regime related, type of karst that is less commonly described. Petrographic and geochemical properties of exhumed Pleistocene phreatic cave deposits from the diapiric Jabal Madar dome in northern Oman are documented and discussed in a process-oriented context. These calcites form at the interface between two fundamentally different diagenetic and hydrogeological domains: the deep-seated, hydrothermal and the near-surficial, meteoric-vadose one. Four calcite phases are recognized: (i) acicular, (ii) blocky to stubby elongated, (iii) proto-palisade, and (iv) macro-columnar calcites. The macro-columnar calcites, forming the last stage of precipitation, are conspicuous due to their cyclical red zonation, and they form the main (geochemical) focus of this study. Fluid inclusion data point to fluid temperatures of between 30 to 50°C (monophase liquid inclusions) and elevated salinities (1.6 to 7.3 wt.% NaCl equivalent). Low carbon-isotope data (−8 to −9‰) are in agreement with the influx of soil-zone CO2 whereas decreasing δ18O (−15‰) values might point to mixing of saline hydrothermal and 18O depleted, meteoric freshwater, i.e., two fluid sources. Trace-element and stable-isotope data shift between the different cement phases and vary cyclically across the red zoning in macro-columnar calcites. With respect to the intra-crystal variability, these patterns are perhaps best explained in the context of redox potential. Two interpretations are presented; the one favored here suggests that the cyclical red zoning in macro-columnar calcites is controlled by Pleistocene monsoonal climate patterns.

Frontiers in Textural and Microgeochemical Analysis

Abstract: Igneous rocks display a large and varied range of textures and compositions, reflecting complex magma pathways, differentiation processes and cooling histories. Integrating geochemical analyses with petrographic and textural information allows us to gain valuable insights into the details of the magma system: the rock9s texture serves as a window into the crystallization history of the magma, while the compositions of the components (crystals and glass) document the conditions and pathways of evolution. In this issue of Elements we take a look at the frontiers in igneous petrology, focussing on the state of the art in textural and microgeochemical analysis and on how we can use some of the latest approaches to unravel the complexities of the magmatic system.

Petrographic classification of middle ordovician fossil meteorites from Sweden

Abstract: The maximum diameter of chromite (FeCr2O4) grains within L chondrites reflects the petrographic type of the sample. On the basis of our measurements of nine recent L chondrites, L3 chromite D-mac = 34-50 mu m, L4 = 87-150 mu m, L5 = 76-158 mu m, and L6 = 253-638 mu m. This variation reflects the crystallization of the chromite grains during parent body thermal metamorphism.We use this calibration to classify six fossil meteorites from the Middle Ordovician in Sweden as type 3 (or 4) to 6. The high flux of L chondrites at 470 Ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent L chondrite falls. The fossil meteorites have in places preserved recognizable chondrule textures, including porphyritic olivine, barred olivine, and radiating pyroxene. A large relict clast and fusion crust have also been tentatively identified in one fossil meteorite. Apart from chromite, all of the original meteorite minerals have been replaced by carbonate (and sheet silicate and sulfate) during diagenesis within the limestone host. The preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules. The range (0.4-0.6 mm) is consistent with measurements made in the same way on recent L chondrites. (Less)