A new approach to geobarometry by combining fluid inclusion and clumped isotope thermometry in hydrothermal carbonates

This study presents a new approach to geobarometry by combining fluid inclusion and clumped isotope (\u39447) thermometry on carbonate minerals. The offset between homogenisation temperatures of primary fluid inclusions with known composition and \u39447 temperatures of the host mineral allows a direct estimation of the fluid pressure at the time of carbonate crystallisation. This new approach is illustrated via hydrothermal dolomite samples from the Variscan foreland fold-and-thrust belt in northern Spain. Clumped isotope analyses yield crystallisation temperatures (107\u2013168\ub0C) which are higher than homogenisation temperatures in corresponding samples (95\u2013145\ub0C). The calculated pressure values suggest that dolomitizing fluids were overpressured during formation of zebra dolomite textures, whereas lower pressures are obtained for dolomite cement from breccia textures. This new approach to geobarometry opens up the possibility of estimating the pressure of carbonate crystallisation and has potential applications in diagenesis, basin analysis, ore geology and tectonics

Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

We present a study of pressure and temperature evolution in the passive continental margin under the Oman Ophiolite using numerical basin models calibrated with thermal maturity data, fluid-inclusion thermometry, and low-temperature thermochronometry and building on the results of recent work on the tectonic evolution. Because the Oman mountains experienced only weak post-obduction overprint, they offer a unique natural laboratory for this study. Thermal maturity data from the Adam Foothills constrain burial in the basin in front of the advancing nappes to at least 4&thinsp;km. Peak temperature evolution in the carbonate platform under the ophiolite depends on the burial depth and only weakly on the temperature of the overriding nappes, which have cooled during transport from the oceanic subduction zone to emplacement. Fluid-inclusion thermometry yields pressure-corrected homogenization temperatures of 225 to 266&thinsp;∘C for veins formed during progressive burial, 296–364&thinsp;∘C for veins related to peak burial, and 184 to 213&thinsp;∘C for veins associated with late-stage strike-slip faulting. In contrast, the overlying Hawasina nappes have not been heated above 130–170&thinsp;∘C, as witnessed by only partial resetting of the zircon (U-Th)/He thermochronometer. In combination with independently determined temperatures from solid bitumen reflectance, we infer that the fluid inclusions of peak-burial-related veins formed at minimum pressures of 225–285&thinsp;MPa. This implies that the rocks of the future Jebel Akhdar Dome were buried under 8–10&thinsp;km of ophiolite on top of 2&thinsp;km of sedimentary nappes, in agreement with thermal maturity data from solid bitumen reflectance and Raman spectroscopy. Rapid burial of the passive margin under the ophiolite results in sub-lithostatic pore pressures, as indicated by veins formed in dilatant fractures in the carbonates. We infer that overpressure is induced by rapid burial under the ophiolite. Tilting of the carbonate platform in combination with overpressure in the passive margin caused fluid migration towards the south in front of the advancing nappes. Exhumation of the Jebel Akhdar, as indicated by our zircon (U-Th)/He data and in agreement with existing work on the tectonic evolution, started as early as the Late Cretaceous to early Cenozoic, linked with extension above a major listric shear zone with top-to-NNE shear sense. In a second exhumation phase the carbonate platform and obducted nappes of the Jebel Akhdar Dome cooled together below ca. 170&thinsp;∘C between 50 and 40&thinsp;Ma before the final stage of anticline formation.</p

Phylogenetic and environmental context of a Tournaisian tetrapod fauna

The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palaeontological record, especially for tetrapods. This interval encapsulates the time of increasing terrestriality among tetrapods, but only two Tournaisian localities previously produced tetrapod fossils. Here we describe five new Tournaisian tetrapods ($\textit{Perittodus apsconditus}$, $\textit{Koilops herma}$, $\textit{Ossirarus kierani}$, $\textit{Diploradus austiumensis}$ and $\textit{Aytonerpeton microps}$) from two localities in their environmental context. A phylogenetic analysis retrieved three taxa as stem tetrapods, interspersed among Devonian and Carboniferous forms, and two as stem amphibians, suggesting a deep split among crown tetrapods. We also illustrate new tetrapod specimens from these and additional localities in the Scottish Borders region. The new taxa and specimens suggest that tetrapod diversification was well established by the Tournaisian. Sedimentary evidence indicates that the tetrapod fossils are usually associated with sandy siltstones overlying wetland palaeosols. Tetrapods were probably living on vegetated surfaces that were subsequently flooded. We show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary, and did not inhibit the evolution of terrestriality. This wealth of tetrapods from Tournaisian localities highlights the potential for discoveries elsewhere.NERC consortium grants NE/J022713/1 (Cambridge), NE/J020729/1 (Leicester), NE/J021067/1 (BGS), NE/J020621/1 (NMS) and NE/J021091/1 (Southampton

Interplay between sulphur sources in polysulphide mineralisation in the Lower Palaeozoic of the Anglo-Brabant Fold belt

Polysulphide mineralisation in the Lower Palaeozoic of the Anglo-Brabant Fold belt is the subject of a detailed sulphur isotopic investigation to constrain possible sulphur sources. A distinction can be made between sedimentary pyrite, with low delta(34)S values and identified as pyrite with a bacteriogenic origin, and pyrite with high delta(34)S, formed during hydrothermal circulation along strain zones. Both kinds of pyrites are present in the important orogenic mesozonal polysulphide mineralisation in the Marcq area. The hydrothermal sulphur is presumed to be H2S released through thermal decomposition of kerogen present in Lower Palaeozoic strata. Pyrite with intermediate delta(34)S values occurs in less important strain zones. These values are explained by mixing of hydrothermal sulphur and sulphur with a sedimentary origin

The influence of supergene reworking on variations in the isotopic composition of Cu in the Dikulushi Cu-Ag deposit (Democratic Republic of Congo)

info:eu-repo/semantics/publishe

Tectonic processes and the flow of mineralising fluids.

International audienc

Conditions of meteoric calcite formation along a Variscan fault and their possible relation to climatic evolution during the Jurassic-Cretaceous

Two calcite cements, filling karst cavities and replacing Lower Carboniferous limestones at the Variscan Front Thrust, were precipitated after mid-Jurassic Cimmerian uplift and subsequent erosion but before late Cretaceous strike-slip movement. The first calcite (stage A) is nonferroan and crystals are coated by hematite and/or goethite. These minerals also occur as inclusions along growth zones. The calcite lattice contains < 0.07 mol.% Fe, but Mn concentrations can be as high as 0.72 mol.% in bright yellow luminescent zones. Primary, originally one-phase, all-liquid, aqueous inclusions have a final melting temperature between -0.2 degrees and +0.2 degrees C, indicating a meteoric origin of the ambient water. The delta(13)C and delta(18)O values of the calcites are between -7.3 parts per thousand, and -6.3 parts per thousand, -7.8 parts per thousand, and -5.5 parts per thousand on the Vienna PeeDee Belemnite (VPDB) scale, respectively. The second calcite (stage B) consists of ferroan (0.13-0.84 mol.% Fe) blocky crystals with Mn concentrations between 0.34 and 0.87 mol.%. Primary, single-phase aqueous fluid inclusions indicate precipitation from a meteoric fluid below 50 degrees C. The delta(13)C values of stage B calcites vary between -7.3 parts per thousand and -2.1 parts per thousand VPDB and the delta(18)O values between -7.9 parts per thousand and -7.2 parts per thousand VPDB. A precipitation temperature below 50 degrees C for the stage A calcites and the presence of iron oxide/hydroxide inclusions in the crystals indicate near-surface precipitation conditions. Within this setting, the geochemistry of the nonferroan stage A calcites reflects precipitation under oxic to suboxic conditions. The ferroan stage B calcites precipitated in a reducing environment. The evolution from the stage A to stage B calcites and the associated geochemical changes are interpreted to be related to the change from semiarid to humid conditions in western Europe during late Jurassic-Cretaceous times. A change in humidity can explain the evolution of groundwater from oxic/suboxic to reducing conditions during calcite precipitation. The typically higher delta(13)C values of the stage B compared to the stage A calcites can be explained by a smaller contribution of carbon derived from soil-zone processes than from carbonate dissolution in the groundwater under humid conditions. The small shift to lower delta(18)O between stage A and B calcites may be caused by a higher precipitation temperature or a decrease in the delta(18)O value of the meteoric water. This decrease could have been caused by a change in the source of the air masses or by an increase in the amount of rainfall during the early mid-Cretaceous. Although the latter interpretation is preferred, it cannot be proven.status: publishe