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Showing papers on "Ankerite published in 1992"


Journal Article
TL;DR: In this article, structural parameters for a stoichiometric dolomite single crystal at room pressure and at 1.50, 2.90, 3.70, and 4.0 GPa were refined using X-ray intensity data.
Abstract: Structural parameters have been refined using X-ray intensity data for a stoichiometric dolomite single crystal at room pressure and at 1.50, 2.90,3.70, and 4.69 GPa; and for a single crystal of a ferroan dolomite with approximately 70 molo/o CaFe(COr)' at room pressure and at 1.90,2.97, and 4.0 GPa. The principal structural change with increasing pressure is compression of the CaOu and (Mg,Fe)Ou octahedra. The CO, group remains essentially invariant throughout the pressure range studied. The effect of the polyhedral compression is reflected in the anisotropic compression of the unit-cell parameters. In both dolomite and ankerite, c is approximately three times as compressible as a. In addition to displaying similar axial compressibilities, dolomite and ankerite display similar bond compressibilities and bulk moduli. The isothermal bulk modulus of dolomite determined from cell volume compression data, assuming the pressure derivative is 4, is 94 GPa, and that of ankerite is 9l GPa. The polyhedral bulk moduli of CaO. and (Mg,Fe)Ou are also similar, with CaOu being slightly more compressible than (Mg,Fe)Ou' The latter has a greater compressibility than generally observed in other oxides and silicates. The distortion of the octahedra, though already small, decreases slightly with pressure. No phase change was observed in either compound throughout the pressure range studied.

123 citations


Journal ArticleDOI
TL;DR: The Kuparuk Formation (Neocomian) consists of glauconitic sandstones and mudrocks of marine origin, with a mixture of mixtures of relatively early-formed Fe-rich siderite and relatively late-formed Mg-rich Siderite, with mean compositions of (Fe77.5Mg13.8Ca7.8Mn0.53(CO3)2) as mentioned in this paper.
Abstract: The Kuparuk Formation (Neocomian) consists of glauconitic sandstones and mudrocks of marine origin. Authigenic siderite is common throughout the formation, either concentrated in distinct layers or as isolated rhombs. Examination of siderite-rich samples with back-scattered electron imaging reveals complex compositional zonation in the siderite. The samples are principally composed of mixtures of relatively early-formed Fe-rich siderite and relatively late-formed Mg-rich siderite, with mean compositions of (Fe77.5Mg13.8Ca7.9Mn0.7 )CO3 and (Fe55.8Mg32.8Ca10.8Mn0.6 )CO3, respectively. Fe-rich siderites are most abundant in glauconitic sandstone intervals, whereas Mg-rich siderit s are most abundant in non-glauconitic sandstones and mudrocks. The whole-rock isotopic composition of the siderite is highly variable, with 13C values ranging from -20.3 to 7.9 PDB, and 18O values from 21.4 to 31.1 SMOW. This variation in isotopic composition correlates with the relative proportions of the two siderite types in a given sample. The Fe-rich siderites have low 13C and high 18O values, whereas the Mg-rich siderites have high 13C and low 18O values. Both the elemental and carbon isotopic compositions of the siderite result from modification of the original marine pore waters during successive stages of microbial decomposition of organic matter. The oxygen isotopic compositions of the siderite indicate that early pore waters were depleted in 18O, perhaps as a result of water/sediment interaction. Authigenic ankerite associated with the siderite-bearing samples is unzoned and has a mean composition of Ca1.11 Fe0.35Mg0.53(CO3)2. The ankerites have higher 13C values and lower 18O values than the Fe-rich siderite, but have 13C values less than and 18O values greater than (with one exception) that of the Mg-rich siderite. The isotopic data suggest that ankerite precipitation began prior to precipitation of Mg-rich siderite and ended subsequent to or during Mg-rich side ite precipitation.

112 citations


Journal ArticleDOI
TL;DR: In this article, a transect through the 2.0 Ga Gunflint Iron Formation of the Animikie Basin, North America, was carried out, and the isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water, or a result of evaporative processes.

76 citations


Journal ArticleDOI
01 Apr 1992-Fuel
TL;DR: The mineralogy of three coals of different rank (from high volatile to low volatile bituminous coal) from the Asturian Central basin (NW Spain) was characterized by X-ray diffraction.

36 citations


Journal ArticleDOI
TL;DR: The diagenesis of Permian siliciclastic sedimentary rocks of the Denison trough is interpreted in terms of evolving pore-water composition related to the burial history of the basin this paper.
Abstract: The diagenesis of Permian siliciclastic sedimentary rocks of the Denison trough is interpreted in terms of evolving pore-water composition related to the burial history of the basin. In the sandstones, early precipitation of pyrite, siderite, and grain-rimming illite-smectite was followed by development of quartz overgrowths that incorporated silica released by pressure solution, recrystallization of lithics, and smectite/illite reaction. Later, as the sequence reached maximum burial during the Late Triassic, strongly {18}O-depleted siderite and ankerite, which incorporated iron and magnesium liberated by leaching of detrital mafic minerals and continuing smectite/illite reaction, precipitated from "connate meteoric" pore water expelled upward from the underlying, overpre sured continental Reids Dome beds. This implies the existence of significant cross-formational pore-water flow prior to and during attainment of maximum burial. Regional Late Triassic to earliest Jurassic compression resulted in major uplift and exposure of the Permian sequence. Attendant strata-concordant invasion of meteoric water led to labile grain and ferroan carbonate dissolution, and to the formation of strongly {18}O-depleted kaolinite and a second generation of quartz overgrowths. Later postdepositional events, which occurred during a second cycle of burial and uplift, include the precipitation of {18}O-depleted calcite and dawsonite, and hydrocarbon entrapment.

32 citations


Journal ArticleDOI
TL;DR: A petrographic and isotopic study of the reservoir sandstones in three wells indicates that a similar suite of diagenetic quartz overgrowths, kaolinite, magnesian siderite, ankerite, and illite developed across the field.
Abstract: Isotopically distinct pore waters remained stratified in the Upper Jurassic Magnus Sandstone, reservoir for the Magnus oil field in the northern North Sea, throughout diagenesis. A petrographic and isotopic study of the reservoir sandstones in three wells indicates that a similar suite of diagenetic quartz overgrowths, kaolinite, magnesian siderite, ankerite, and illite developed across the field. However, the oxygen isotope ratios (delta{18}O) recorded in these diagenetic minerals from the crest of the field are consistently more negative (by about 3 o/oo) than those from the downdip flank. We propose that this consistent difference in delta{18}O values records the persistence of isotopically different and stratified pore waters in the Magnus sandstones. The sandstones w re deposited as marine turbidites, but were subaerially exposed during periods of structural uplift and tilting during the Early and middle Cretaceous. Meteoric water entered the structural crest, resulting in pore-water stratification. Deposition of thick Upper Cretaceous mudstones and marls sealed the Magnus structure during subsequent rapid burial. Diagenetic minerals in the crest of the oil field grew in a pore water with significantly more meteorically derived water than those downdip, assuming that each diagenetic phase developed at a similar temperature in each well. The pore waters remained stratified for at least 35 m.y., from middle Cretaceous (approximately 90 Ma) subaerial erosion of the sandstone until reservoir charging at 55 Ma. The sandstones and mudstones of the Kimmeridge Clay Formation behaved essentially as a closed system during diagenesis, in contrast to the open-system concept envisaged in most other diagenetic reconstructions.

28 citations


Journal ArticleDOI
TL;DR: In this paper, a method of estimating an aqueous-solution composition from that of solid-solutions believed to be in equilibrium with it is derived, and the low-temperature ternary Ca-Fe-Mg phase relations of the rhombohedral carbonates are investigated.

27 citations


Journal ArticleDOI
TL;DR: Authigenic ankerite in the gas-bearing mid-Permian Aldebaran Sandstone (Denison Trough, Queensland, Australia) has an anomalously light oxygen-isotopic composition ( δ 18 O SMOW range: +7.6 to +14.4 ) and exhibits a trend of 18O-enrichment from the base to the top of the unit as mentioned in this paper.

18 citations


Journal ArticleDOI
TL;DR: In this paper, the lower Triassic sandstones of northeastern Belgium were deposited in channels and by sheet floods, and the introduction of clays, formation of iron oxides and alteration of feldspars in these sediments occurred at near-surface conditions.

13 citations


Journal ArticleDOI
TL;DR: In the district of Vall de Ribes in the Pyrenees as discussed by the authors, gold-bearing arsenopyrite veins are known to have formed at a temperature around 350 degrees to 400 degrees C and 2 kbars pressure.
Abstract: In the district of Vall de Ribes, Cambro-Ordovician schists contain several disseminated gold-bearing arsenopyrite occurrences. Close to these occurrences quartz veins parallel to the Hercynian foliation also contain arsenopyrite gold mineralization. Most of the ore is, however, within brittle structures (fractures and thrusts) crosscutting the Hercynian foliation. They occur as veins enclosed in schists and dolomites, and as replacement bodies in dolomites, and are representative of a large number of gold-bearing sulfide deposits that are known to occur in the Hercynian basement of the Pyrenees.The postfoliation veins mainly consist of quartz, siderite, ankerite, arsenopyrite, pyrite, and several minor sulfides. Gold is found in close association with the sulfides. According to fluid inclusion and arsenopyrite geothermometry, the veins formed at a temperature around 350 degrees to 400 degrees C and 2 kbars pressure. Mineralizing fluids were CO 2 -free NaCl + CaCl 2 + (KCl) brines with salinities ranging from 10 to 26 wt percent NaCl equiv. The pH of the fluid was near neutral, the sulfur fugacity was constrained by the stability of pyrite and arsenopyrite, and the oxygen fugacity was close to or higher than pyrite-pyrrhotite-magnetite equilibrium. Under these conditions, and as suggested by available experimental data, gold was most likely transported as AuCl (super -) 2 . The high dependence of gold solubility on temperature suggests a cooling of the solution as the most efficient precipitating mechanism, which is in accordance with the close association of native gold to sulfides and quartz found in the veins.The enclosing and vein carbonates are strongly depleted in 13 C and 18 O (delta 13 C = -10ppm; delta 18 = + 13ppm in vein ankerite) with respect to the regional dolomite value (delta 13 C = -0.4ppm; delta 18 O = +24ppm) but show a positive correlation between delta 13 C and delta 18 O, interpreted as the result of an exchange process between regional dolomite and an external fluid. The delta 18 O values of quartz range from 9.8 in synfoliation quartz to 15.8 per mil in postfoliation vein quartz from the Tarragona mine. Calculated delta 18 O values of water in equilibrium with carbonates and quartz range from 6.7 to 10.8 per mil. The delta D values of chlorites associated with quartz vary between -45 to -61 per mil. The delta D values of these fluids gave values between -5 and -30 per mil. These delta 18 O and delta D data are compatible either with a fluid of metamorphic origin or seawater infiltrating through the metamorphic pile. The heavy sulfur isotope composition of disseminated arsenopyrite (delta 34 S = 11ppm) and vein arsenopyrite (delta 34 S from 13-17ppm) indicates a sulfur of crustal origin. The carbon isotope composition of vein carbonates (delta 13 C 2 O-CO 2 -NaCl fluid generated during metamorphism (at 500 degrees -550 degrees C and 5-6 kbars), contemporaneously to the main foliation, is favored. This fluid could contain metals in solution as evidenced by the deposition of native gold and sulfides in quartz veins parallel to the foliation. The subsequent rise of the thermal gradient from 25 degrees to 45 degrees -70 degrees C/km caused the thermal expansion of the fluids, their migration into faults and thrusts, and the evolution of their chemistry to a CO 2 -free brine by the unmixing of a CO 2 -rich phase. The transport of gold and other metals as chloride complexes was ensured throughout the process. Although a surficial fluid, i.e., seawater, infiltrating through the metasediments could acquire a similar isotopic composition, several geologic uncertainties make this hypothesis less reliable.

9 citations


Journal ArticleDOI
TL;DR: In this article, an analysis of the abundances and distributions of silt-sized heavy minerals from the U.S. mid-Atlantic outer continental shelf, slope and rise shows that heavy minerals constitute a substantially greater weight percent of the silt fraction than that of the sand fraction regardless of environment and sediment texture.

01 Jan 1992
TL;DR: In this paper, shipboard and shore-based analyses of -230 interstitial-water samples gathered at six sites during ODP Legs 127 and 128 indicate that diagenetic reactions within sediment columns throughout the Japan Sea record the influence of bacterial degradation of organic matter, transformation of amorphous biogenic opaline silica to opal-CT, and formation and alteration of carbonate phases, as well as alteration of crystalline basement and volcanic ash.
Abstract: Shipboard and shore-based analyses of -230 interstitial-water samples gathered at six sites during ODP Legs 127 and 128 indicate that diagenetic reactions within sediment columns throughout the Japan Sea record the influence of bacterial degradation of organic matter, transformation of amorphous biogenic opaline silica to opal-CT, and formation and alteration of carbonate phases, as well as alteration of crystalline basement and volcanic ash. Organic matter oxidation not only affects the distribution of oxidants (SO4, Mn, and CO2) and dissolved metabolites (alkalinity, NH4, and PO4), but also controls the distribution of many authigenic solid phases. Site 794, located deep in the Yamato Basin, is the only site where sulfate is not completely consumed, whereas Sites 798 and 799 record the total consumption of sulfate at shallow depths and concomitant extreme generation of alkalinity, NH4, and PO4. Authigenic carbonates, phosphates, and barite also record the importance of bacterial activity. The diagenetic opal-A/opal-CT transition is a notable chemical, diagenetic, and stratigraphic horizon that occurs more shallowly in the basinal sites because of their greater geothermal gradient. Due to its effect on the porosity and permeability of the involved lithologies, the siliceous phase transformation effectively restricts diffusion between reactions occurring in the uppermost sediment column from those involved in deep basement alteration. Processes that influence dissolved Ca, Mg, Sr, and alkalinity tend to be preferentially dominant at either basinal (Site 794, 795, and 797) or ridge (all other) sites, due to the distinctly different depths and sedimentation rates of the two depositional regimes. At basinal sites, where sediment accumulation is relatively slow and occurs below the CCD, basement alteration is the dominant influence on dissolved Ca and Mg profiles. At ridge sites, where accumulation is relatively fast and occurs above the CCD, diagenetic reactions such as calcite dissolution, inorganic carbonate precipitation, dolomitization, and siderite/ankerite formation control Ca and Mg behavior. Basement alteration reactions are responsible for large overall decreases in Mg, K, Rb, Li, B, δ l8 0, δD, 87Sr/86Sr, and Na with depth, as well as for increases in Ca and Sr. Alteration of ash in the sediment column also influences these downhole profiles. Site 795, in the Japan Basin, preserves the greatest diagenetic signal of basement alteration. Gradients resulting from basement alteration document the importance of diffusive chemical exchange between the endogenic and exogenic reservoirs.

Journal ArticleDOI
TL;DR: The most common paragenetic equivalents to the fence-textured dolomite are dolomsite-filled rhombic moulds and two dolomedite intergrowth fabrics as mentioned in this paper.

01 Jan 1992
TL;DR: In this article, geochemical and isotopic studies indicate that the Tianapapan Unit had undergone at least 9 stages of cementation which were responsible for complete occlusion ofinterstitial pores.
Abstract: The 'Tigapapan Unit' is one ofseveral Miocene hydrocarbon­ bearing reservoirs in the northwestern Sabah Basin. This unit consists of bioclast-rich, clastic-carbonate mixture, and has been interpreted as progradation storm shoal deposits. Cathodoluminescence, geochemical and isotopic studies indicate that the sequence had undergone at least 9 stages of cementation which were responsible for complete occlusion ofinterstitial pores. Each stage represents one distinctive cement texture which was precipitated at specific temperature and burial condition. The diagenetic evolution was first introduced by (Cal) a syndepositional, accicular and bladed iron-free marine calcite, followed by (Dol) early methane-derived dolomite resulting from degradation of biogenic methane during uplifting and induced faulting, (Ca2) bladed calcite, (Ca31 Ca4) blocky, vein-filling Fe-calcites, (D02) clay-associated dolomite, (D031 D04) Fe-dolomite and ankerite, and (Ca5) late iron-rich calcite. Each cement stage is composed of distinctive Sr4+,Mn2+, Mga., Fe2+, Ca2+, Si4., and Na+ concentrations, reflecting changes in palaeopore fluid systems with time. The cement stages are correlatable over the whole unit. The cementation occured from very early to late phases of diagenesis at near surface to 2.0 km depth. With progressive burial and temperature increased, oxygen isotope values become strongly negative (for example, stage Ca4: 81aC = +2.70 %0 PDB, 8180 = -5.70 %0 PDB). Oxygen isotope data indicate that the cements were precipitated at temperatures in the range of 20-75°C. Carbon isotope data suggest that the source of carbon varies from marine dissolved carbonates to fermentation of organic matter, biogenic methane, and possibly hydrothermal fluids. Strontium isotope dating of Dol (with 81SC = -34.7 %0 PDB, 8180 = +2.60 %0 PDB) and D02 dolomites indicates that these two dolomitization events took place at 10.5 Ma and 8.9 Ma, respectively. The Dol dolomite was precipitated from the methane-derived brine exactly at the boundary between the Serravallian anl.. .cortonian stages after a major uplifting, induced faulting, and beginning of sealevel fall. The D02 dolomite was precipitated at an intermediate to deep burial diagenesis duringsmectite-illi te transformation.

01 Jan 1992
TL;DR: Petrographic comparison of sandstone compositions and textures inside and outside the carbonate-cemented zones revealed that the precipitation of the ankerite was a late-diagenetic event.
Abstract: Zones of preferential ankerite cementation are common within non-marine and marine Lower Tuscaloosa sandstone reservoirs of southwestern Mississippi. These carbonate-cemented zones occur as nodules (average radius 6 cm), streaks (average thickness 1 cm), and patches which reduce and often completely impede porosity. Petrographic comparison of sandstone compositions and textures inside and outside the cemented zones revealed that the precipitation of the ankerite was a late-diagenetic event. With the exception of carbonate cement infilling porosity, the overall mineralogy and textural relationships both inside and outside of the carbonate-cemented zones are alike. Stable carbon and oxygen isotope analyses of whole-rock samples also indicate that the cemented zones are products of late diagenesis. Light 18O values of -9 to -12 indicate that the carbonates were precipitated after considerable burial. Negative 13C values of -11 to -14 indicate that carbonate precipitation was from pore waters enriched in 12C because of thermal decomposition of interbedded organic-rich zones.

01 Jan 1992
TL;DR: In this paper, the mineralogy of Stromatolite is studied by means of X-ray fluorescence analysis, IR(infra-red) absorption analysis and Xray diffraction analysis.
Abstract: Stromatolite is the fossil of the marine life of Infra-Cambrian era ( ca.650-800Ma), having layered structure similar to the annual ring of woods. The mineralogy of these ring structures are studied by means of X-ray fluorescence analysis, IR(infra-red) absorption analysis and Xray diffraction analysis. Dark-colored rings and light brown-colored intermediates are found to be consist of CaC03(Calcite) and Ca(FeMg)(C03 )2(Ankerite), respectively. Magnesium of chrolophyl origin is suspected.