Chemical Geology: Isotope Geoscience section最新文献

The possibility of dating peat by the uranium-series disequilibrium method is discussed. In principle, this method can be used to date peat to ∼ 350 ka. The application of the U/Th disequilibrium method (UTD) on peat provides us with the probability of constructing a new chronology for the Late Pleistocene paleoclimatic record in NW Europe. The reliability of the obtained ages will be discussed as well as open-system behaviour and the contamination with detrital Th. By studying in detail interglacial peat profiles from the Tenagi Philippon site, Greece (a long terrestrial record), of an expected age of 125 ka and the Fenit site in Ireland of unknown age, we were able to explain the results in terms of the suspected open-system behaviour of top and bottom parts of these layers and how to avoid it by careful sampling. Peals contaminated with detrital Th were also analysed. Two peat layers, which were interpreted on basis of pollen analyses, stratigraphic position and TL dates to be early Last Glacial in age, were sampled. The first one is the Alit Odhar organic layer near Inverness, Scotland, and gave an age of 106 ka. The second is the key site to the British Last Glacial stratigraphy, the Chelford organic layer at Chelford, Cheshire, yielded an age of 86 ka which is in good agreement with the recently obtained TL dates.

Tourmaline is a ubiquitous mineral in the Mid-Proterozoic, peraluminous, syn- to post-tectonic granites and aplites and the related hydrothermal rocks of the Karagwe-Ankolean belt in northwest Tanzania. Electron microprobe analysis indicates that tourmalines from all of the intrusive and hydrothermal lithologies: (1) belong to the schorl-dravite solid-solution series; and (2) plot within the field occupied by tourmaline from Li-poor granitoids on the FeAlMg classification diagram. Oxygen isotope compositions range from +12.2 to +11.6‰ (SMOW) for magmatic tourmalines and from +10.8 to +9.8‰ for those of hydrothermal origin. Hydrogen isotope compositions vary from −79 to −65‰ (SMOW ) for magmatic tourmalines and from −99 to −84‰ for hydrothermal tourmalines. Water contents measured by manometry are constant at 3.0–3.2 wt.%. Within the broad grouping there arc systematic variations in both chemical [particularly $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$] and isotopic composition that relate to evolving magmatic and hydrothermal conditions. Igneous differentiation [increasing $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$ in magmatic tourmaline] has produced trends with higher δ¹⁸O in quartz, lower δ¹⁸O in tourmaline, and larger Δ_QTZ.-TOUR.-values, that reflect a combination of a reduction of crystallization temperature and an increase of $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$ in the residual melt. Subsequent cooling and interaction of an exsolved, B-rich magmatic fluid with the pelitic country rocks, resulted in the deposition of hydrothermal tourmaline with increasing $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$ ratios, and progressively lower δ¹⁸O and δD -values.

Newly developed techniques for boron chemical separation and isotopic analysis in natural silicate rocks and waters are described. Sample dissolution and the subsequent ion-exchange chromatography were conducted using hydrofluoric and hydrochloric acids in the presence of mannitol which suppresses boron volatilization and isotopic fractionation by the formation of a boron-mannitol complex. Thermal ionization mass spectrometry using the Cs₂BO₂⁺-graphite method was employed for the determination of boron isotopic composition. No boron isotopic fractionation was observed in the course of chemical separation and mass spectrometry. In the whole analytical procedure, procedural blank and recovery yield of boron were 3–4 ng and 99±1%, respectively. The analytical precision and reproducibility of measured ¹¹B/¹⁰B ratios were ±0.1−0.1% and ±0.2‰ for the measurements of basalt and seawater, respectively. The present method enables us to determine the isotopic composition of < 1 μg B in silicate samples and in natural fluids with the above-mentioned analytical errors. This method also provides a remarkable improvement in the measurement of boron concentration by isotope dilution mass spectrometry because of the achievement of complete mixing between sample and spike during sample decomposition.

The source of sulfate salts in the Great Plains region of North America is not well understood. Sulfur and oxygen isotope data of sulfate salt efflorescences and other sulfur-bearing species from selected locations in Saskatchewan were used to understand the sources, mechanism of formation and association of these salts with different facies of the sedimentary rocks in the area. The δ³⁴S_SO4² and δ¹⁸O_SO4²-values of the solid salt samples varied widely and ranged from −42.9 to 10.5‰ and −8.3 to +15.1‰, respectively. The δ³⁴S-values of the pyrite, coal, jarosite and gypsum samples also ranged from −39.0 + 4.0‰. The δ¹⁸O-values of the jarosite and gypsum samples varied between −10.3 and +14.4‰, indicating their close relationships with the salt crusts. Oxidation of pyrite and hydrolysis of natrojarosite appear to play a major role in the salt formation. The very low negative δ³⁴S-values associated with Cretaceous marine shales suggest a bacterial SO₄²⁻ reduction during the formation of pyrite at the bottom of the sea (pelagic facies), which once occupied the area. The salts with positive δ¹⁸O-values are associated with Tertiary continental environment and are consistent with the high sulfur isotopic composition of coal samples in this sediment. This suggests that the δ³⁴S-values depend on the type of lithofacies of the sediment and can be utilized to establish the formation conditions of sulfur species. The positive δ¹⁸O-values of majority of the salt crust samples indicate an incorporation of greater amount of atmospheric oxygen into the SO₄²⁻, hence a drier (semi-arid) environment during their formation, than for those salts with negative values, which are likely formed under a more humid environment or regions with high moisture regimes in the landscape. The oxygen isotopic composition of the salt crust from Chaplin Lake showed that incorporation of oxygen from the atmosphere into the sulfate was far more significant than salts from the rest of the study sites.