388 research outputs found
Mineralogy, sulphur isotope geochemistry and the development of sulphide structures at the Broken Spur hydrothermal vent site, 29°10'N, Mid-Atlantic ridge
No full textA large collection of hydrothermal sulphides from the Broken Spur hydrothermal vent site, including representative samples of mound sulphide materials, has been characterized using optical mineralogy and sulphur isotope analysis. Young mound sulphides from Broken Spur have a pyrrhotite-dominated mineralogy unusual for bare ridge vent systems. However, pyrrhotite is metastable and is ultimately converted to iron disulphides. Mature sulphides are indurated, recrystallized and contain abundant quartz. Sulphide mound materials are developed by three major processes: (i) coalescing of chimney structures; (ii) accumulation of talus from mass wasting and (iii) precipitation and growth in response to hydrothermal flow. Progressive maturation of mound materials is by modification of primary textures, development of mineralogical zoning and replacement of metastable phases. Sulphur isotope analysis of 35 mineral separates returned δ34S values of – 0.5 to +3.2‰. These values are similar to those previously measured for Broken Spur and Snakepit, but are distinctly 32S enriched compared to the TAG active mound and some Pacific sites. Seawater entrainment and sulphate reduction within the subsurface feeder zone below Broken Spur mounds do not appear to be important processes at Broken Spur, in contrast to the TAG active mound
Zanchetta G., Leone G., Fallick A.E., Oxygen isotope composition of living land snail shells: data from Italy
No full textMicro-Fourier Transform Infrared (FT-IR) and ?D value investigation of hydrothermal vein quartz: interpretation of fluid inclusion ?D values in hydrothermal systems
No full textSome recent studies have suggested that the hydrogen isotopic composition (?D) of hydrothermal fluids, released in vacuo by thermal decrepitation of quartz, are not always accurately revealed. We report the results of a step-heating ?D value study of vein quartz, hosted by Lower Palaeozoic rocks in SW England, which was analyzed by micro-FT-IR for hydrogen speciation, before and after fluid extraction at temperatures between 750 and 1500 °C. The ?D values of individual aliquots of released water vary between ?3‰ and ?208‰, with the lowest values generally corresponding to the highest temperature fractions and samples of relatively low yield. The data show significant departures from geologically reasonable ?D. Micro-FT-IR analyses show that a variety of OH species are present within the vein quartz, with significant intra and inter sample variation. Typically a broad absorption due to molecular water, in the region 3400 cm?1 is observed, along with bands attributed to Li–OH and Al–OH. On heating, the broad absorption due to molecular water is reduced, accompanied by a measurable loss of Li–OH species. The latter becomes more pronounced in the higher temperature fractions (>750 °C). These data support earlier studies which indicated that contributions from the contrasting OH reservoirs in quartz can significantly influence the reported ?D values. These new data also suggest that the incorporation of OH released from Li–OH sites in the quartz may be the most important factor in the generation of the anomalous values for these samples
Retrogression by deep infiltration of meteoric fluids into thrust zones during late orogenic rapid unroofing
No full textHypersaline (Na-Ca-Cl) fluids are associated with late-stage quartz veining and retrogression of garnet, kyanite and other high P-T phases in the vicinity of thrusts and major lithological boundaries in the Scandian nappes of the Troms-Ofoten region, northern Norway. They record early Devonian fluid infiltration during rapid exhumation in the final stages of Caledonian orogenesis. The &dgr;18O and &dgr;D characteristics of these late fluids provide compelling evidence for deep circulation of meteoric fluids. The sub-greenschist to low greenschist facies retrogression (P=2±1 kbar; T =300-350 °C) suggests infiltration to depths of 7-9 km in a regime of supra-hydrostatic fluid pressure. Peak metamorphic quartz veins and associated fluids have &dgr;D and &dgr;18O characteristics consistent with a metamorphic origin (&dgr;D -47 to -75; &dgr;18O+8.6 to +17.4). However, late quartz veins and associated fluids show a broad spread of &dgr;D from -42 to -148, interpreted in terms of meteoric fluid infiltration. Such negative &dgr;D values are only recorded in present-day high-latitude or high-altitude settings, and since north Norway was in an equatorial setting (10° S) in the early Devonian, a high-altitude origin is deduced. By calculation, and by comparison with modern examples, the early Devonian mountains of the north Norwegian Caledonides are interpreted to have had a topography in excess of 5 km. The deep circulation of surface waters is interpreted in terms of topographically driven flow, linked with a hydrothermal system induced by elevated geothermal gradients due to rapid uplift. Whilst the case for deep penetration of surface-derived fluids has been promoted for Mesozoic and younger mountain belts, this study represents one of the first documented examples for a Palaeozoic orogenic belt. It suggests that many of the fundamental processes operating during the exhumation of mountain belts are similar irrespective of age. <br/
Hydrothermal clays as tracers of seafloor sulphide mound evolution (abstract of paper presented at 12th Annual V.M. Goldschmidt Conference, Davos, Switzerland, August 18-23, 2002)
No full text
The origin of clay minerals in active and relict hydrothermal deposits
No full textSamples of Fe-oxide-rich hydrothermal sediments were collected from active and inactive portions of the TransAtlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge. Clays separated from TAG metalliferous sediments in this study all consist of Al-poor nontronite. Oxygen isotope thermometry of the clays yields formation temperatures of 54-67[deg]C for samples from the inactive Alvin mound compared with 81-96[deg]C for samples from the active TAG site. The latter are the highest recorded temperatures for authigenic hydrothermal clays. Sr isotope analysis of the clays from the active mound suggests that they precipitated from seawater-dominated fluids, containing less than 15% hydrothermal end-member fluid. In contrast, nontronite from the inactive Alvin mound has 87Sr/86Sr values that closely resemble that of detrital North Atlantic clays, suggesting a dominantly continental source for the Sr. Rare earth element data are consistent with a significant detrital input to the inactive site but also demonstrate the extent of hydrothermal input to the low temperature fluid. Crystallographic fractionation of the trivalent REE is apparent in the heavy REE enrichments for all nontronite samples. The inferred formation-mechanism for nontronite-rich Fe-oxyhydroxide deposits at the surface of the active mound is by direct precipitation from low temperature fluids. At the inactive Alvin site, in contrast, the deposits form during alteration of pelagic sediments by diffuse fluids and replacement of biogenic carbonate with nontronite and Fe-oxyhydroxide. These two modes of formation are both important in seafloor hydrothermal settings where clay minerals are a significant component of the hydrothermal deposi
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