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Silica mobility and fluid movement during metamorphism of the Connemara schists, Ireland 总被引：5，自引：1，他引：5
Quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of Connemara, and attest to considerable migration of silica. Contrary to common assumptions, there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower P–T down the regional geothermal gradient. Under amphibolite facies conditions, a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated, while cooling over the temperature interval from 650 to 500° C. The absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through-flowing, externally derived fluids. The oxygen isotopic composition of matrix quartz shows considerable differences between different rock types (quartzite, pelite and marble), with a range of δ18OSMOW from c.+ 11.5% (quartzite) to + 18.5% (marble). In each rock type, vein quartz compositions closely match those of the matrix quartz. These results demonstrate the importance of local segregation processes in the formation of veins, and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations, if at all. Both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies, with only a relatively minor component of through flow of externally derived fluid. Veins are clearly not the major pathways of metamorphic dewatering. It is proposed that abundant veins in the predominantly pelitic Ballynakill Formation formed during peak metamorphic D3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble. Hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins, while irreversible loss through the rest of the succession was a much less important process. In the central mountains of Connemara, rather pure, unreactive quartzites are cut by widely spaced, laterally extensive quartz veins that are axial planar to D3 folds. These veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile, but here too, oxygen isotope data indicate considerable segregation of locally derived quartz, reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow. 相似文献
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Lower Cretaceous mudstones exposed at Speeton in North Yorkshire, UK, contain lobsters and burrows preserved in diagenetic phosphate concretions. Isotopic compositions of sulphur in both diagenetic sulphide and structural sulphate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry. The occurrence of phosphatized and pyritized lobsters and similarly preserved burrows, allows a detailed comparison of these microenvironments with the host sediments. Host sediments are extensively bioturbated and characterized by very light sulphide isotopic compositions (mean sulphide δ34S = –48·3 ± 3‰ (1σ, n = 19)) and sulphate isotopic compositions that are lighter than Lower Cretaceous seawater sulphate (mean sulphate δ34S = +8·7 ± 3·2‰ (1σ, n = 19)). These isotopic values can be explained by the action of bioturbating macrofauna; the introduction of oxygen in ventilating seawater results in the oxidation of early formed isotopically light pyrite, resulting in porewater sulphate that is enriched in 32S. Subsequent pyrite formation via bacteriogenic reduction of isotopically light porewater sulphate leads to the formation of isotopically light pyrite, whilst residual porewater sulphate apparently remains relatively enriched in the 32S isotope. Sulphur isotopic values for the burrows are very different; sulphide isotopic compositions average –34·4 ± 0·4‰ (1σ, n = 3) and sulphate isotopic compositions average –14·4 ± 6·6‰ (1σ, n = 4). These isotopic compositions are the result of rapid development of closed system conditions in burrows, resulting in the build-up of acidity necessary for phosphate precipitation and coprecipitation of isotopically light sulphate (formed by oxidation of isotopically light sulphide surrounding the burrows). Lobster shell isotopic compositions fall between these two groups. On the basis of their isotopic compositions, some lobsters appear to have died close to the sediment–water interface, whilst others appear to have been buried (in burrows) prior to death. 相似文献
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ROBERT J. G. MORTIMER AMANDA M. J. GALSWORTHY SIMON H. BOTTRELL LUCY E. WILMOT ROBERT J. NEWTON 《Sedimentology》2011,58(6):1514-1529
Fe (III) reduction is a key component of the global iron cycle, and an important control on carbon mineralization. However, little is known about the relative roles and rates of microbial (biotic) iron reduction, which utilizes organic matter, versus abiotic iron reduction, which occurs without carbon mineralization. This paper reports on the capacity for salt marsh sediments, which typically are rich in iron, to support abiotic reduction of mineral Fe (III) driven by oxidation of sulphide. Sediment was reacted with amorphous FeS under strictly anaerobic conditions at a range of temperatures in biotic and abiotic microcosm experiments. Fe (III) reduction driven by sulphide oxidation occurs abiotically at all temperatures, leading to Fe (II) and elemental sulphur production in all abiotic experiments. In biotic experiments elemental sulphur is also the oxidized sulphur product but higher bicarbonate production leads to FeCO3 precipitation. Abiotic reduction of Fe (III) occurs at rates that are significant compared with microbial Fe (III) reduction in salt marsh sediments. The solid phases produced by coupled abiotic and biotic reactions, namely elemental sulphur and FeCO3, are comparable to those seen in nature at Warham, Norfolk, UK. Furthermore, the rates of these processes measured in the microcosm experiments are sufficient to generate siderite concretions on the rapid time scales observed in the field. This work highlights the importance of abiotic Fe (III) reduction alongside heterotrophic reduction, which has implications for iron cycling and carbon mineralization in modern and ancient sediments. 相似文献
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SIMON H. BOTTRELL ROBERT J.G. MORTIMER IAN M. DAVIES† S. MARTYN HARVEY‡ MICHAEL D. KROM 《Sedimentology》2009,56(4):1159-1173
In this study, the biogeochemical transformations of sulphur in organic‐rich marine sediments in a Scottish fjord are investigated by a combination of pore water and sediment geochemistry with sulphide diffusive gradient thin‐film probes and sulphate isotopic data (δ34S and δ18O). Particular attention is paid to sulphur cycling in the upper sediment profile where sulphate reduction occurs but free sulphide is below the detection limits of conventional pore water geochemical analysis but quantifiable by sulphide diffusive gradient thin film. In the uppermost part of the sediment core, δ18O sulphate decreased from near‐sea water values to +7‰, indicating that anoxic sulphide oxidation dominated during this interval. Sulphate δ34S remained unchanged as there was no net sulphate reduction (i.e. reduction was balanced by re‐oxidation). Below 4 cm depth, there was a slight increase in sulphate δ34S from 20‰ to 23‰ associated with minor accumulation of iron sulphide. The δ18O of the sulphate also increased, to around +10‰ at 10 cm depth, as a result of the isotopic exchange of sulphate–oxygen with pore water and/or sulphur disproportionation reactions mediated during sulphur cycling. These processes continued to increase the δ18O of the sulphate to 14‰ at 20 cm depth with no further change in the δ34S of the sulphate. Below 20 cm depth, free sulphide is detectable in pore waters and both the δ34S of the sulphate and sulphide increase with depth with an offset controlled by kinetic fractionation during bacterial sulphate reduction. The δ34S of the sedimentary organic fraction shifted towards lower, more bacteriogenic, values with depth in the profile, without any increase in the size of this sulphur pool. Thus, the organic sulphur fraction was open to interaction with bacteriogenic sulphide without the occurrence of net addition. Therefore, caution should be exercised when using sulphur isotopic compositions to infer simple net addition of bacteriogenic sulphide to the organic sulphur fraction. 相似文献
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S. H. BOTTRELL P. B. GREENWOOD B. W. D. YARDLEY T. J. SHEPHERD B. SPIRO 《Journal of Metamorphic Geology》1990,8(1):131-143
A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low-grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H2O-rich and CH4-rich fluids coexisted. Late-stage inclusions are of calcium-rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz-oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end-Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of 18O-enriched calcites and micas. In the case of vein minerals it is possible that these crystallized directly from late-stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re-equilibrated with later 18O-enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re-equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late-stage fluids observed as inclusions. 相似文献
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