Temperature-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids

The fractionation of lithium isotopes between synthetic spodumene as representative of Li-bearing clinopyroxene and Cl- and OH-bearing aqueous fluids was experimentally determined between 500 and 900°C at 2.0 GPa. In all the experiments, ⁷Li was preferentially partitioned into the fluid. The fractionation is temperature dependent and approximated by the equation Δ⁷Li_{(clinopyroxene–fluid)}=−4.61×(1,000/T [K]) + 2.48; R ²=0.86. Significant Li isotopic fractionation of about 1.0‰ exists even at high temperatures of 900°C. Using neutral and weakly basic fluids revealed that the amount of fractionation is not different. The Li isotopic fractionation between altered basalt and hot spring water (350°C) in natural samples is in good agreement with our experimentally determined fractionation curve. The data confirm earlier speculations drawn from the Li isotopic record of dehydrated metamorphic rocks that fluids expelled from a dehydrating slab carry heavier Li into the mantle wedge, and that a light Li component is introduced into the deeper mantle. Li and Li isotopes are redistributed among wedge minerals as fluids travel across the wedge into hotter regions of arc magma production. This modifies the Li isotopic characteristics of slab-derived fluids erasing their source memory, and explains the absence of cross-arc variations of Li isotopes in arc basalts.     

Post-glacial rebound and present-day three-dimensional deformations

The WEGENER^† activities related to the study of post-glacial rebound are presented together with a review of the present state-of-the-art in this study field. Post-glacial rebound research is an unique tool for studying the viscoelastic behaviour of the Earth's mantle on time scales of thousands of years. The viscosity structure of the Earth's mantle determined from an inversion of observations of glacially induced deformations is a basic requirement for modelling long-term phenomena such as the convection in the Earth's mantle, and for better understanding unsolved questions such as the nature of the mantle discontinuities or the vertical scale of convection.First, an introduction to the scientific background is given, and the three principal ingredients for post-glacial rebound studies, namely the ice model, the Earth model, and the observations are briefly considered. For the ice model, the uncertainties due to a trade-off between ice model and Earth rheology are outlined. The different approaches used to model the Earth and its deformations in post-glacial rebound studies are discussed emphasising the preliminary nature of the derived rheologies and depth dependencies. The observations, in particular the relative sea-level changes and three-dimensional surface deformations, are described with special emphasis on observational gaps. Based on the discussion of the ingredients, an outline of the future developments in post-glacial rebound research is attempted with particular emphasis on the Earth model and the theory of deformations.For several decades extreme efforts have been made to precisely monitor the land uplift in Scandinavia. However, for the height component the existing data still are associated with large uncertainties while reliable data on the horizontal component are practically nil. The ongoing long-term (longer than ten years) spacegeodetic measurements are likely to provide the three-dimensional deformations with the spatial resolution and accuracy required in order to substantially contribute to post-glacial rebound studies. Thus, present-day three-dimensional deformations of the Earth's surface beneath and around the former ice sheets as a constraint for the mantle rheology and viscosity structure will increasingly become important as they become known from space-geodetic measurements with high spatial resolution and an accuracy approaching the mm/a-level.     

Spatio-temporal Variability of Soil Water at Three Seasonal Floodplain Sites: A Case Study in Tarim Basin,Northwest China

The floodplain -egetation of the Tarim River in Northwest China is strongly influenced by irrigated agriculture. The abstrac- tion of river water disturbs; the natural dynamics of the floodplain ecosystem. The human impact on the hydrological system by bank dams and the irrigation of cotton plantings have caused adverse changes of the Tarim River and its floodplains, so the current stocks of the typical Tugai vegetation show significant signs of degradation. Field studies of soils and statistical analysis of soil moisture data have shown that the vitality of the Tugai vegetation is primarily determined by its position to the riverbank and the groundwater. There exist complex interactions between soil hydrological conditions and the vitality of the vegetation. But the availability of water is not only influenced by the groundwater level and seasonal flood events. The spatial distribution of stocks at different states of vitality seems also to be decisively influenced by physical soil properties. Our results show that the water supply of plant communities is strongly in- fluenced by the soil texture. Spatial differences of soil moisture and corresponding soil water tensions may be the decisive factors for the zonafion of vegetation. Physical soil properties control the water retention and rising of capillary water from deeper soil layers and the phreatic zone and may supply the root systems of the phreatophytic vegetation with water. Keywords： soil moisture;soil texture; soil water tensions; Tarim River; water retention     

Quantitative relationship between water-depth and sub-fossil ostracod assemblages in Lake Donggi Cona,Qinghai Province,China

A calibration data set of 51 surface sediment samples from Lake Donggi Cona on the northeastern Tibetan Plateau was investigated to study the relationship between sub-fossil ostracod assemblages and water depth. Samples were collected over a depth range from 0.6 to 80 m. A total of 16 ostracod species was identified from the lake with about half of the species restricted to the Tibetan Plateau and its adjacent mountain ranges and poorly known in terms of ecological preferences, and the other half displaying a mainly Holarctic distribution. Living macrophytes and macroalgae were recorded in Lake Donggi Cona down to a depth of about 30 m, and bivalve (Pisidium cf. zugmayeri) and gastropod (Gyraulus, Radix) shells were found down to depths of 43 and 48 m, respectively. The ostracod-water-depth relationship was assessed by multivariate statistical analysis and ostracod-based transfer functions for water depth were constructed. Weighted averaging partial least squares (WA-PLS) regression provided the best model with a coefficient of determination r ² of 0.91 between measured and ostracod-inferred water depth, a root mean square error of prediction of 8% and a maximum bias of 10.6% of the gradient length, as assessed by leave-one-out cross-validation. Our results show the potential of ostracods as palaeo-depth indicators in appropriate settings. However, transfer-function applications using fossil ostracod assemblages for palaeo-depth estimations require a thorough understanding of the palaeolimnological conditions of lakes and therefore detailed multi-proxy analysis to avoid misinterpretation of ostracod-based inferences.     

Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation

The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess. In one series of experiments, [B]_fluid/[B]_tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline. ¹¹B always preferentially fractionated into the fluid. For experiments where [B]_fluid/[B]_tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ¹¹B_{(tour–fluid)} = −4.20 · [1,000/T (K)] + 3.52; R ² = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰ at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where [B]_fluid/[B]_tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations.     

Lithium speciation in aqueous fluids at high P and T studied by ab initio molecular dynamics and consequences for Li-isotope fractionation between minerals and fluids

Ab initio molecular dynamics simulations are performed to study the speciation changes in lithium bearing aqueous fluids at high temperature (T = 1000 K) and high pressures, P, between about 0.3 and 6.0 GPa. The simulations show a linear increase in Li coordination with fluid density, from 3.2 to about five in the considered pressure range. Towards low densities, associated LiF complexes are becoming increasingly stable, which is quantified by evaluating the dynamic behavior of the respective species. In the high-density region, HF complexes are observed. The differences in speciation may be related to structural changes of the solvent under compression. At a fluid density of 1.2 g/cm³, kinks in the pressure dependences of the oxygen–oxygen nearest neighbor distance and the oxygen–oxygen coordination are observed, which indicates a change in compaction mechanism. Assuming that the Li coordination difference between crystal and fluid is a major determinant for the isotopic fractionation between minerals and fluids, we expect only a small pressure dependence of the Li isotopic fractionation between Li bearing fluids and minerals. Our simulation results are consistent with experimental data that show reverse fractionation of ⁷Li between fluid and mineral, when Li is in tetrahedral instead of octahedral coordination in the crystal.     

The Global Stratotype Section and Point （GSSP） for the base of the Holocene Series/Epoch （Quaternary System/Period） in the NGRIP ice core

The Greenland ice core from NorthGRIP （NGRIP） contains a proxy climate record across the Pleistocene-Holocene boundary of unprecedented clarity and resolution. Analysis of an array of physical and chemical parameters within the ice enables the base of the Holocene, as reflected in the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase, to be located with a high degree of precision.     

Episodic mineralization of hydrothermal illite in the Soultz-sous-Forêts granite (Upper Rhine Graben, France)

Episodic and localized illite mineralization is documented in the hydrothermally altered Soultz-sous-Forêts granite (Upper Rhine Graben, France). Separated grain-size fractions of altered granite and argillite vein samples contain mixtures of 2M₁ and 1M trans-vacant illite varieties. The platy pseudohexagonal 2M₁ illite phases dominate the vein fillings, whereas the 1M illite occurs largely as a fibrous pore-filling variety, which is particularly abundant in the granite matrix. Multiple phases of fluid injections into the granite body have resulted in different illite assemblages, each sample containing a mixture of polytype generations formed during different crystal growth events. On the basis of mineralogical and K–Ar isotopic constraints, the ages of these vein-mineralizing events are determined by plotting the K–Ar values of the various grain-size fractions against polytype abundance and the fitted volume-weighted crystallite thickness distributions. The results suggest a Permian age for the formation of the studied argillite veins, characterized by successive injections of hydrothermal fluids. Secondary episodes of illite crystallization occurred during Jurassic and Cretaceous (or even younger times) in both the veins and the granite matrix. There are indications that the polytype structure and composition of illite were strongly influenced by variations in fluid chemistry and the degree of fluid–rock interaction as the granite was progressively sealed during post-Variscan, episodic hydrothermal activity.