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Geochemical fingerprints: a critical appraisal
No full textThis review provides an overview of geochemical fingerprints currently used in the Earth sciences and discusses their strengths and weaknesses. Special emphasis is given to stable and radiogenic isotopes. Topics to be addressed include various fields of Earth sciences and range from basalts, different water types, biosignatures, bones to anthropogenic fingerprints
Theoretical and Experiment Principles
No full textIsotopes are atoms whose nuclei contain the same number of protons but a different number of neutrons
Hydrothermal alteration of biotite and plagioclase as inferred from intragranular oxygen isotope- and cation-distribution patterns
No full textThe alteration processes of biotite to chlorite and plagioclase to albite that have occurred during hydrothermal alteration of granites were investigated on a scale of less than or equal to250 mum using a combination of in situ oxygen isotope- and electron microprobe analyses. On an intragranular scale, oxygen isotope compositions parallel variations in the cationic composition. Steep chemical and isotopic gradients are obtained between secondary and primary phases, whereas both secondary and primary phases themselves appear chemically and isotopically homogeneous. Oxygen isotope- and cation-exchange between fluid and biotite or plagioclase is, therefore, best explained by dissolution-reprecipitation mechanisms, taking place along microfractures, micropores and grain boundaries. The breakdown of biotite to chlorite occurs once K+ is removed from biotite interlayers. This process is most likely accompanied by a redistribution of elements including oxygen isotopes in both tetrahedral and octahedral sheets of the biotite. Distribution patterns of oxygen isotopes within altered plagioclase indicate that the albitization of plagioclase does not simply reflect addition of SiO2 and Na+ to plagioclase. Instead, albite is precipitated from dissolved plagioclase accompanying a restructuring of aluminosilicate tetrahedra within the plagioclase
Isotopic history of seawater: the stable isotope character of the global ocean at present and in the geological past
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Mesozoic alteration of Permian volcanic rocks (Thuringer Wald, Germany): Ar, Sr and O isotope evidence
No full textBesides older granites and gneisses, the Thuringer Wald in central Europe consists predominantly of altered Permian volcanic and pyroclastic rocks The alteration history of these volcanic rocks was investigated by K-Ar, (39)Ar(K)-(40)Ar, Rb-sr, delta(18)O, deltaD and X-ray studies on feldspar and biotite separates. Alteration is accompanied by increases in the triclinicity and delta(18)O values (+14 parts per thousand to + 19 parts per thousand) of alkali feldspars, an increase in the delta(18)O values of low-potassium biotites (> + 10 parts per thousand) and in sericitized plagioclases (+ 12 parts per thousand to + 19 parts per thousand) and also the generation of apparent Mesozoic isotopic ages. In contrast, volcanic biotites with primary magmatic 6180 signatures still yield Early Permian ages, and biotites of the granitic basement give Carboniferous ages. The observed isotope changes of the volcanic feldspars indicate episodic fluid exchange with meteoric and formation brines (delta(18)O less than or equal to+ 5 parts per thousand) at low temperatures (200 to 100 degreesC) during tectonic fracturing of the basement. As a result, sanidines were replaced by porous microclines which were subject to isotope exchange and Ar loss. Feldspars and biotites of the granitic wall rocks seem to be less affected by alteration processes, which may be due to their lower permeabilities. Plateau-shaped (40)Ar/(39)Ar age spectra of volcanic alkali feldspar indicate complete resetting of the K-Ar system, whereas staircase spectra point to partial retention or repeated Ar loss. Because the feldspars represent mixtures of microcline and sanidine remnants, the apparent ages (260 to 160 Ma) represent maximum ages for the time of replacement. Rb-Sr feldspar model ages (180 Ma or 230 to 170 Ma, respectively) support the K-Ar data. The minimal (40)Ar/(39)Ar spectra ages are concordant with both K-Ar and (40)Ar/(39)Ar sericite model ages (160 to 150 Ma) of sericitized plagioclases. The model ages favour Jurassic episodic fluid reactions with alkali feldspars and plagioclases in the Thuringer Wald. (C) 2003 Elsevier Science B.V. All rights reserved
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