Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

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).

Evolution of hydrothermal fluids in the Ashanti gold belt, Ghana; stable isotope geochemistry of carbonates, graphite, and quartz

Abstract: The oxygen and carbon isotope systematics of ankerite, siderite, calcite, quartz, and graphite from the Bogosu and Prestea mining districts of the Ashanti gold belt, Ghana are used to speculate on the processes leading to hydrothermal alteration and gold deposition. The geologic environment of this turbidite- and graywacke-hosted gold system was a sediment-dominated accretionary complex. The gold ores are localized within the Ashanti structural belt which we suggest became a conduit for deep-seated fluids during uplift and dilatancy.Ankerite and siderite in mineralized sedimentary rocks are broadly similar in oxygen and carbon isotope compositions to corresponding minerals in the metasedimentary country-rocks. Evidence of alteration that preceded gold mineralization is best preserved in spatially associated altered mafic dikes. The compositions of fluids responsible for the gold mineralization and associated alteration have been reconstructed using temperatures from carbonate-mineral geothermometry.Alteration of country rocks occurred under rock-dominant, greenschist facies conditions. The isotopic composition of the least altered mafic dikes was influenced by interaction with fluids generated from Birimian greenschist facies metasedimentary rocks. Later, extensive carbonate alteration of mafic dikes adjacent to the structural conduit occurred during slow upward migration of fluids. Lateral diffusion of hydrogen into the structural conduit permitted partial conversion of CO 2 to methane, resulting in 13 C enrichment of both the residual CO 2 and the deposited carbonate minerals.Rapid, even explosive, expulsion of deep-seated fluids and CO 2 -H 2 O phase separation occurred episodically throughout the evolution of the hydrothermal system. This caused precipitation of sulfides, arsenides, and gold at relatively high crustal levels and low ambient pressure and temperature. During mineralization, fluid ascent was too rapid for isotopic and chemical reequilibration between the deep-seated ore fluid and adjacent country rocks. The ore fluid had chemical and isotopic compositions consistent with deep-seated metamorphic fluids that were subsequently modified by preferential partitioning of 18 O and 13 C into a gas phase. The composition of the deep-seated metamorphic fluid determined using chemical equilibria (CO 2 -H 2 O rich; 7-25 mole % CO 2 ) is in approximate agreement with that required to produce the isotopic depletion of the residual fluid via phase separation.

Yamato 86029: Aqueously altered and thermally metamorphosed CI‐like chondrite with unusual textures

Abstract: We describe the petrologic and trace element characteristics of the Yamato 86029 (Y-86029) meteorite. Y-86029 is a breccia consisting of a variety of clasts, and abundant secondary minerals including coarse- and fine-grained phyllosilicates, Fe-Ni sulfides, carbonates, and magnetite. There are no chondrules, but a few anhydrous olivine-rich grains are present within a very fine-grained phyllosilicate-rich matrix. Analyses of 14 thermally mobile trace elements suggest that Y-86029 experienced moderate, open-system thermal metamorphism. Comparison with data for other heated carbonaceous chondrites suggests metamorphic temperatures of 500-­600 oC for Y-86029. This is apparent petrographically, in partial dehydration of phyllosilicates to incompletely re-crystallized olivine. This transformation appears to proceed through `intermediate' highly-disordered `poorly crystalline' phases consisting of newly formed olivine and residual desiccated phyllosilicate and their mixtures. Periclase is also present as a possible heating product of Mg-rich carbonate precursors. Y-86029 shows unusual textures rarely encountered in carbonaceous chondrites. The periclase occurs as unusually large Fe-rich clasts (300­-500 μm). Fine-grained carbonates with uniform texture are also present as small (10-­15 μm in diameter), rounded to sub-rounded `shells' of ankerite/siderite enclosing magnetite. These carbonates appear to have formed by low temperature aqueous alteration at specific thermal decomposition temperatures consistent with thermodynamic models of carbonate formation. The fine and uniform texture suggests crystallization from a fluid circulating in interconnected spaces throughout entire growth. One isolated aggregate in Y-86029 also consists of a mosaic of polycrystalline olivine aggregates and sulfide blebs typical of shock-induced melt re- crystallization. Except for these unusual textures, the isotopic, petrologic and chemical characteristics of Y- 86029 are quite similar to those of Y-82162, the only other heated CI-like chondrite known. They were probably derived from similar asteroids rather than one asteroid, and hence may not necessarily be paired.

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.