The carbon isotope composition (6l3C%0) of the tissues of benthic invertebrates usually ranges from -16 to -20.In contrast we report that several common bivalve molluscs belonging to the superfamily Lucinacea and several small species of Pogonophora show much greater depletions, ranging from -23 to -31 in the bivalves and from -35 to -46 in the pogonophores.These bivalves and pogonophores live in reducing sediments where the concentration of dissolved sulphide is low, usually < 1 PM.The gills of the bivalves and the trophosome tissue of the pogonophores contain intracellular or sub-cuticular bacteria.The bacteria are autotrophs, as shown by ribulosebisphosphate carboxylase activity of extracts of the bacteria-containing tissues.Comparisons of the 613C values of the bacteria-containing regions and the rest of the tissues indicate substantial transfer of organic carbon, half or more of the nutritional needs of the hosts.The organic carbon is produced by fixation of CO, by the autotrophic bacteria, which oxidize reduced inorganic compounds, notably sulphide, to obtain energy for the process.Similar depletions of 13C were previouslt reported for other autotrophic symbiont-containing bivalves that live in habitats where dissolved sulphide concentrations are 2 or 3 orders of magnitude higher than in the sediments analysed here.This form of nutrition, involving symbiosis with autotrophic bacteria, is evidently not dependent on high levels of dissolved sulphide and appears to be widespread in calcareous reducing sediments of the shelf and continental slope.
The Strathy Complex of the Scottish Caledonides is a bimodal association of amphibolites and siliceous grey gneisses that structurally underlies adjacent metasediments of the Moine Supergroup. Both rock units record a common polyphase Caledonian tectonometamorphic history. New elemental and radiogenic isotope data indicate that both end-members of the Strathy suite were derived from a depleted mantle source, that they are cogenetic and that they may have been related by crystal fractionation. δ 18 O values and their correlations with major and trace elements suggest that the protoliths were hydrothermally altered at temperatures below 200 °C. Tectonomagmatic discrimination based on relatively immobile elements and isotope systems, plus comparison with geochemically similar bimodal supracrustal associations elsewhere, strongly support the conclusion that the igneous protoliths of the Strathy Complex formed in an oceanic destructive margin setting. If T DM model ages of c. 1000 Ma approximate protolith crystallization, the Strathy Complex may have formed as juvenile crust in the peri-Rodinian ocean broadly contemporaneous with the Grenville orogenic cycle.
The Rogart pluton is a typical example of high Ba–Sr granitic magmatism, emplaced in northern Scotland towards the end of the Caledonian Orogeny. It consists of three granitoid facies (tonalite, granodiorite, granite) that are locally associated with large enclaves of coeval mafic appinite. The overall range of compositions is therefore extreme, with MgO from 11.6 to <0.1 wt%, SiO 2 from 47.50 to >73.0 wt%, with relatively high Na 2 O+K 2 O especially for the mafic rocks (up to 8.4 wt%), associated with K 2 O/Na 2 O≈1.5. Trace element abundances vary extensively and coherently, and the typical high Ba–Sr elemental signature of the pluton is also carried by the appinites. This is consistent with a genetic relationship throughout the suite. Sr, Nd and O isotope ratios are sufficiently similar to support this contention, but vary systematically with magma evolution. The appinites were derived from an enriched mantle source ( 143 Nd/ 144 Nd 400 ≈0.51194, 87 Sr/ 86 Sr 400 ≈0.7061) with high δ 18 O (≈+8‰), probably related to active contemporaneous subduction. Quantitative elemental and isotopic modelling suggests that the granitoid magmas evolved from the appinites by crystal fractionation accompanied by minor crustal contamination. Early fractionation from appinite to tonalite was driven by crystallization of pyroxene plus biotite with minor plagioclase, replaced by a feldspar-dominated assemblage to produce granodiorite and granite. The total amount of crust assimilated was less than 25%, highlighting the juvenile nature of the high Ba–Sr granite class.
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 H 2 O‐rich and CH 4 ‐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 18 O‐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 18 O‐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.
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