Showing papers on "Ankerite published in 1990"

A paleoweathering profile from Griqualand West, South Africa: evidence for a dramatic rise in atmospheric oxygen between 2.2 and 1.9 bybp

Abstract: A core drilled near Wolhaarkop in Griqualand West, South Africa, intersected highly oxidized Kuruman Iron Formation below red beds of the Gamagara Formation. The lateral equivalents of the Kuruman Iron Formation in this drill hole consist largely of siderite, ankerite, magnetite, greenalite, and quartz. The oxidation of the Kuruman Iron Formation in WOL 2 occurred almost certainly during weathering prior to the deposition of the Gamagara Formation. The date of this weathering episode is bracketed between about 2.2 and 1.9 bybp by the age of the Ongeluk lavas in the Transvaal sequence below the unconformity and by the age of the Hartley lavas in the Olifantshoek Group above the unconformity. The ratio of iron to SiO2 in the several facies of the weathered Kuruman Iron Formation in WOL 2 is nearly the same as that in their unweathered equivalents. Since SiO2 loss during weathering was almost certainly minor, the similarity of the Fe/SiO2 ratio in the weathered and unweathered BIF indicates that nearly all the "FeO" in the Kuruman Iron Formation was oxidized and retained as FeO3 during weathering. Such a high degree of iron retention is best explained by an O2 content of the atmosphere > or = 0.03 atm at the time of weathering. Such an O2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt > or = 2.2 bybp but is consistent with the highly oxidized nature of the 1.85 by Flin Flon paleosol. The new data suggest that PO2 rose dramatically from about 1 percent PAL (present atmospheric level) to > or = 15 percent PAL between 2.2 and 1.9 bybp.

The Influence of Calcite Dissolution on Reservoir Porosity in Miocene Sandstones, Picaroon Field, Offshore Texas Gulf Coast

Abstract: Middle Miocene sandstones occur in Picaroon field (Corsair trend; offshore Texas Gulf Coast) at depths of approximately 4.9 to 5.2 km (16-17,000 ft). These deltaic sandstones contain evidence of the following sequence of diagenetic events: 1) formation of chlorite coatings on detrital grains, 2) partial dissolution of detrital feldspar, 3) quartz cementation, 4) calcite cementation, 5) dissolution of calcite cement and grains, 6) ankerite cementation. The reservoir quality of the sandstones is largely a function of porosity enhancement by calcite dissolution. Calcite cement was emplaced at depths of approximately 1.8 to 2.6 kin. The calcite has 87/86Sr values of 0.7083-0.7086, eliminating coeval ( 15 myr.) seawater and marine carbonate (0.70873-0.70885) as the primary source of strontium. The 87/86Sr composition of calcite cement implies that mass transfer of calcite from older marine sources to younger sediments has occurred. Fluid inclusion measurements indicate that ankerite cement formed at minimum temperatures of 120-188°C (3.0-5.1 km). Oxygen isotope modeling predicts that at these depths shales would expel waters with 18OSMOW of +5 to +9 during smectite illite conversion. Ankerite (18OPDB = -7.8) would be in isotopic equilibrium with these waters at temperatures similar to those derived from fluid inclusions. Ankerite cements have relatively radiogenic 87/86Sr ratios (0.7097) which are consistent with their formation from shale-derived fluids. Calcite dissolution occurs between the precipitation of calcite and ankerite. It is therefore concluded that calcite cement dissolution occurred at burial depths of 2.6 to 3.0 km (107-120°C).

Linear coupling of carbon and strontium isotopes in Rotliegend Sandstone, North Sea: Evidence for cross-formational fluid flow

Abstract: Isotopic analyses of carbon and strontium in diagenetic dolomite and ankerite present in the eolian Lower Permian Rotliegend Sandstone, southern North Sea, show an excellent linear correlation between {delta}{sup 13}C and {sup 87}Sr/{sup 86}Sr (r = 0.93), suggesting that the sources of these elements were interlinked. The isotopic data indicate that Late Permian Zechstein seas flooded the basin and displaced the interstitial meteoric Rotliegent pore fluids, producing early dolomite with predominantly marine bicarbonate ({delta}{sup 13}C = {minus}15{per thousand}) and strontium ({sup 87}Sr/{sup 86}Sr = 0.7076) isotopic signatures. The isotopic values of this dolomite form a trend to merge with the diagenetically later ankerite, which has lower {sup 13}C/{sup 12}C ({delta}{sup 13}C = {minus}4{per thousand}) and more radiogenic strontium ({sup 87}Sr/{sup 86}Sr = 0.7112). The obvious source of low-{delta}{sup 13}C carbon was thermal decarboxylation of organic matter in the underlying Upper Carboniferous mudstones. The authors also suggest that the increasing quantities of radiogenic strontium ({sup 87}Sr/{sup 86}Sr = 0.720) were released when organic acids dissolved silicates (feldspar ) in the same underlying mudstones. The upward movement of fluid or diffusion of ions across formation boundaries, therefore, progressively became the dominant control on carbonate precipitation in the Rotliegend Sandstone.

Carbonates of the magnesite–siderite series from four carbonatite complexes

Abstract: Carbonates of the magnesite-siderite series have been found and analysed in carbonatites from the Lueshe, Newania, Kangankunde, and Chipman Lake complexes. This series has been represented until now only by a few X-ray identifications of magnesite and three published analyses of siderite and breunnerite (magnesian siderite). Most of the siderite identified in carbonatites in the past has proved to be ankerite, but the new data define the complete solid-solution series from magnesite to siderite. They occur together with dolomite and ankerite and in one rock with calcite. The magnesites, ferroan magnesites and some magnesian siderites may be metasomatic/hydrothermal in origin but magnesian siderite from Chipman Lake appears to have crystallized in the two-phase calcite + siderite field in the subsolidus CaCO3-MgCO3-FeCO3 Textural evidence in Newania carbonatites indicates that ferroan magnesite, which co-exists with ankerite, is a primary liquidus phase and it is proposed that the Newania carbonatite evolved directly from a Ca-poor, Mg-rich carbonatitic liquid generated by partial melting of phlogopite-carbonate peridotite in the mantle at pressures >32 kbar.

The carbonatites and fenites of Chipman Lake, Ontario

Abstract: Simple and composite dikes of dolomitic and ankeritic carbonatite, up to one meter thick, cut and fenitize Archean country rocks around Chipman Lake, Ontario. The carbonate compositions define a complete series from dolomite to ankerite, and coexist with calcite. Cathodoluminescence studies indicate an early dolomitic phase brecciated by later ankeritic carbonate. DropJike inclusions of burbankite within the dolomite are taken as evidence for an early Na-rich fluid or immiscible liquid. Compositional data are given for burbankite and other phases in the carbonatite, including synchysite, parisite, bastnasite, norsethite, barytocalcite (alstonite?), strontianite, apatite, strontian apatite, monazite and F-rich phlogopite. The fenites comprise Na- and K-rich feldspars, magnesio-arfvedsonite and aegirine, with phlogopite particularly concentrated along carbonatite contacts. The evolution of the carbonatite liquid from magnesian to more iron-rich compositions paralleled a decrease in the Na:K ratio, which was responsible for early fenitization of a predominantly sodic character changing later to potassium fenitization.

The effect of carbonate cementation of underthrust sediments on deformation styles during underplating

Abstract: Fluids expelled during deformation at convergent margins lead to a variety of diagenetic reactions, including carbonate cementation, which alter the rheology of the deforming sediment. The volume of carbonate cement in shallow sediments at modern convergent margins is small, but sandstones in an ancient accretionary complex in Alaska contain significant amounts of carbonate cement that formed before underplating occurred. Other than vein calcite, carbonates are early calcite replacing framework grains or filling pore spaces, followed by ankerite that occurs mainly as pore-filling cement. Amounts range from 2–16 weight percent in typical samples, averaging about 9 weight percent. Mean porosity filled by carbonate in Kodiak sandstones corresponds to porosities expected at depths of 4–5 km in normal compacting sands underthrust at modem convergent margins. The temperatures of formation are difficult to determine precisely, but were probably between 100°C and 200°C. The bulk of cementation occurred before the formation of thrust faults, folds, and slaty cleavage related to underplating. Lithification by cementation changed the rheology of sandstones by (1) filling pore space and increasing peak strength, and (2) increasing cohesion so that paniculate flow was no longer a viable deformation mechanism. Lithification before significant deformation contributed to underplating of the Kodiak Formation as relatively coherent duplexes.

Fe substitution for Al in glauconite with increasing diagenesis in the first Wilcox sandstone (lower Eocene), Livingston Parish, Louisiana

Abstract: Glauconites in early ankerite concretions, ferroan calcite-cemented sandstones, and unce- mented sandstones in the first Wilcox sandstone of the Lockhart Crossing field, Livingston Parish, Lou- isiana, show a progressive substitution of Fe for octahedral A1 with increasing diagenesis. An octahedral Fe content of 0.50 atoms was calculated from glauconite located in early ankeritic concretions. Octahedral Fe averaged 0.60 and 0.90 atoms in later ferroan calcite-cemented sandstone and uncemented sandstone, respectively. Corresponding octahedral AI averages were 1.16, 1.03, and 0.67, respectively. A systematic increase in average interlayer K from 0.49 to 0.54 to 0.61 was also observed, with apparent increases in diagenesis. All element determinations were made with an electron microprobe and recast on an anion equivalent basis to structural formulae based on the O jo(OH)2 unit. The clay preserved in the early ankerite concretions was found to be an illite/smectite containing about 20% expandable layers, and the mineral in the glauconite pellets from uncemented areas of the sandstone, an ordered glauconite. "Minus cement" porosities of the sandstone indicate that glauconitization may have taken place at burial depths greater than 0.6 to 1.8 km, but the mechanism for the incorporation of Fe 3§ in the glauconite at that depth is not apparent.

Geochemistry of carbonate cements in the Sag River and Shublik Formations (Triassic/Jurassic), North Slope, Alaska: implications for the geochemical evolution of formation waters

Abstract: Carbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have 87Sr/86Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe-rich (up to 5·0 mol%) and are characterized by increasing 87Sr/86Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age). These geochemical variations appear principally to result from changes in pore-water chemistry during diagenesis. The increase in 87Sr/86 Sr and Sr content of the cements is most likely due to interaction between pore waters and 87 Sr-rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock 87Sr/86 Sr signatures for the mudrocks are > 0·716). Pore-water Fe2+ concentration was probably controlled by diagenetic alterations involving Fe-bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak. The low δ18 O value of some calcite cements (-11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid-Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with mid-Neocomian rifting.

Estimation of mineralogy and lithology from geochemical log measurements

Abstract: Mineralogy, chemistry, and physical properties of core samples from a research well in Grand County, Utah, have been determined to assess the applications of geochemical logging tool measurements to reservoir characterization. The cored interval (over 827 ft or 252 m of continuous core) samples a wide range of lithologies, including subarkose, sublitharenite, litharenite, feldspathic litharenite, quartz wacke, lithic wacke, siltstone, mudstone, and coal. Total clay mineral contents range from 2 to almost 60 wt.% and consist primarily of illite and kaolinite with subordinate ordered, mixed-layer illite/smectite. Dolomite cement is pervasive; locally, large amounts (up to 25 wt.%) of siderite or ankerite are present. Total feldspar minerals comprise 5-18 wt.% with sodium-ri h plagioclase slightly more abundant than potassium feldspar. Continuous geochemical well logs of 12 elements (K, U, Th, Al, Si, Ca, Fe, S, Gd, Ti, Cl, and H) were obtained using logging tools that measure neutron capture, neutron activation, and natural gamma-ray activities. Applications of these geochemical log measurements to assess mineralogy and lithology for the research well are complicated by (1) the near compositional colinearity of the assemblage illite + kaolinite + K-feldspar, (2) the presence of dolomite and iron-bearing carbonates, (3) the predominance of Na-plagioclase over K-feldspar, and (4) the abundance of sedimentary and metamorphic rock fragments in the litharenite group of lithologies. Because of these mineral and textural features, successful application of the log measurements to reservoir characterization requires calibration to core analyses. Mineral logs calculated using least-squares inversion programs accurately describe changes in mineralogy and mineral proportions once the mineral assemblage is identified. Lithology is best estimated based on these mineral logs. Alternatively, up to seven general lithology types can be discriminated using the lithology ratio (Al[2]O[3] + Fe[2]O[3])/(SiO[2] + Al[2]O[3] + Fe[2]O[3]), when chemical data are calibrated with petrographic data.