Chemical Geology: Isotope Geoscience section最新文献

The use of stable isotopes of water as tracers in characterising the source of water in vegetation is tried in three field situations. Initially, methodological aspects of the uptake of water into plant tissue, water movement from roots to tops and recovery from stem-wood by distillation are considered. Using tracers of deuterium and oxygen-18, diffusion of water into lettuce leaf tissue was demonstrated to occur as the whole molecule, equilibrating in 24 h. It is considered from the similar tissue density that diffusion of water into roots would be similar. Distillation in toluene or kerosene to recover water from various materials found small decreases in deuterium composition of the recovered water: −2.3‰ when recovering water from pine wood, −1.4‰ from potting mix and −3.2‰ from wood of native plants, while water recovered from lettuce leaf was not subject to such effects. These decreases are correlated to the proportion of organic matter in the materials, and may relate to decomposition of a component of this. Uptake of water by potted Hordeum vulgare, Atriplex nummularia, A. rhagodiodes, A. vesicaria, Callitris preissii, Casuarina glauca, Eucalyptus oleosa, E. citriodora and Maireana sedifolia was found to be relatively free of further fractionation.

In field studies, comparison of the isotopic composition of water from stems of Hordeum vulgare, Maireana sedifolia and Pinus radiata and from adjacent soil profiles suggested that the depth of extraction of water by roots varied with water availability and rooting depth. Examination of the stable isotopes of soil and sap water in addition to soil water contents provides a more realistic interpretation of plant water source than obtained by studying soil water content alone.

Boron isotope and concentration data are presented for hydrothermal fluids from different tectonic settings that reflect derivation of boron from marine evaporites (Red Sea brines), non-marine evaporites (Salton Sea), elastic sediments (Escanaba Trough and Guaymas Basin), back-arc basalts (Mariana Trough) and mid-ocean ridge basalts (21° and 11–13°N East Pacific Rise; Juan de Fuca Ridge; and 23° and 26°N, Mid-Atlantic Ridge). Boron isotope systematics of these fluids are used to constrain the S″B-values of tourmalines found in ancient massive sulfide deposits and tourmalinites that formed under conditions analogous to those of modern hydrothermal systems. The data also provide insights into the physical conditions required for the formation of tourmaline in certain geologic environments.

Faraday cup efficiencies (FCE's) in the multicollector mass spectrometer were examined with a static multicollection technique. The relative Faraday cup efficiency (RFCE) was estimated by measurement of Nd-isotope ratios with five different cup configurations. The application of RFCE correction to the static multicollection technique will increase the reliability of analytical results by eliminating the errors caused by differences in FCE's, and will also enable us to distinguish which Faraday cups are damaged and should be renewed.

Brachiopods from the Demissa Bed (Middle Devonian, Hamilton Group) biologically regulated the incorporation of Mg, Sr and Na into their shell calcite. Significant taxonomic differences in the elemental contents of three species (Athyris spiriferoides, Mediospirifer audacula and Mucrospirifer mucronatus) may be related to differences in calcification processes. In contrast, no significant difference or “vital effects” were observed between the isotopic values of Mediospirifer audacula and Athyris spiriferoides from Erie (Δ¹⁸O=0.03‰, p=0.949; Δ¹³C=0.76‰, p=0.083) and Genessee Counties (Δ¹⁸O=0.39‰, p=0.471; Δ¹³C=0.06‰, p=0.854). This suggests that these brachiopods did not exert a biological control over their isotopic compositions, and that their shell calcites reflect ambient physicochemical conditions.

Isotopic compositions in unaltered shell calcites of brachiopods from Genessee County, which was close to the basin depocentre, are heavy for carbon (δ¹³C, $\text{χ}$=+5.01‰, PDB) and oxygen (δ¹⁸C, $\text{χ}$=−2.85‰, PDB) compared to the species sampled at the basin's edge (Erie County; (δ¹³C, $\text{χ}$=+2.79‰, PDB; δ¹⁸O, $\text{χ}$=−3.83‰, PDB). There is a significant separation in isotopic values between the deeper- and shallower-water brachiopods of the basin ((δ¹⁸O=0.98‰, p=0.0005; Δ¹³C=2.22‰, p=0.0005). The δ¹⁸O variation suggests a temperature/salinity change with water depth, whereas the change in δ¹³C composition probably records an enrichment /depletion of organic matter with water depth. This observation has significant implications for Paleozoic ocean isotopic-evolution studies, because many global changes in marine δ¹⁸O and δ¹³C are based on isotopic shifts of similar magnitude.

A high-precision analytical method for the measurement of ${}^{138}\text{Ce}{}^{142}\text{Ce}$ ratios is reported using static multicollection mass spectrometry. This technique reduced the data acquisition time for 2 hr. for 400 ratios and improved analytical reproducibility to ±0.002% (n-16) and precision to ±0.002–0.003%. The better precision and reproducibility were established collecting a large ion beam [¹⁴²Ce¹⁶O of (2–7) · 10⁻¹¹ A], short data acquisition time and in situ measurement of ${}^{18}\text{O}{}^{16}\text{O}$ ratios during the analysis.

To reduce the blank effect to the Ce isotope analysis, the chemical procedure for separation of Ce was refined using a small ion-exchange resin bed column (4 cm length × 3 mm diameter) with which the procedural total blank was lowered to 0.04 ng and the recovery yield of Ce from 20 mg BCR-1 was 90%. In order to confirm the reproducibility of this technique including the chemical procedure, six Ce isotope analyses individually separated from the USGS standard BCR-1, were carried out with an analytical reproducibility of ±0.002%.

With these analytical precision and reproducibility and normalization to BCR-1 in order to eliminate any inter-laboratory biases, it is now possible to apply the LaCe isotope system to the terrestrial and extraterrestrial samples combined with other isotope systems, such as SmNd and RbSr.