Rb-Sr and K-Ar systems of biotite in surface environments regulated by weathering processes with implications for isotopic dating and hydrological cycles of Sr isotopes     

Gi Young Jeong  Chang-Sik Cheong 《Geochimica et cosmochimica acta》2006,70(18):4734-4749

Biotite is widely used for Rb-Sr and K-Ar isotopic dating and influences Sr isotope geochemistry of hydrological regimes. The isotopic system of biotite behaves diversely in response to surface weathering; i.e. the complete preservation of original Rb-Sr and K-Ar isotopic ages or dramatic reduction. In this study, we have explored the relation between the behavior of isotopic systems and complex weathering processes of biotites in the weathering profiles distributed on the Mesozoic granitoids in South Korea. In the lower parts of the profiles, biotite in the early stages of weathering was transformed into either oxidized biotite or hydrobiotite, with a mass release of ⁸⁷Sr and ⁴⁰Ar forced by the rapid oxidation of ferrous iron. During the transformation to oxidized biotite, ⁸⁷Sr and ⁴⁰Ar were preferentially released relative to Rb and K, respectively, via solid-state diffusion through the biotite lattice, resulting in a drastic reduction of original isotopic age. The reduction of Rb-Sr age was greater than that of K-Ar age because K was preferentially released over Rb whereas ⁸⁷Sr and ⁴⁰Ar were released proportionally to each other. However, during the transformation of biotite to hydrobiotite (i.e., to regularly interstratified biotite-vermiculite), ⁸⁷Sr, Rb, ⁴⁰Ar, and K were completely retained in the alternating biotite interlayer, and thus the original isotopic age can be preserved. In the upper parts of the profiles, where iron oxidation was almost completed, ⁸⁷Sr, Rb, ⁴⁰Ar, and K were gradually and proportionally released, with no further significant change in isotopic age during the gradual transformation of the early-formed oxidized biotite into hydrobiotite and vermiculite or during their final decomposition to kaolinite. The ratios and amounts of isotopes released from weathered biotites are dependent upon the degree of iron oxidation and the pathways of mineralogical transformation. Regional and local variations in isotopic systems affected by particular weathering processes should be considered when dating biotite or biotite-bearing rocks in weathering environments, modeling the transfer of Sr isotopes to hydrologic regimes, and tracking the provenance of sediments.  相似文献   

Effect of hydrogen limitation and temperature on the fractionation of sulfur isotopes by a deep-sea hydrothermal vent sulfate-reducing bacterium     

Joost Hoek  Anna-Louise Reysenbach  Donald E. Canfield 《Geochimica et cosmochimica acta》2006,70(23):5831-5841

The fractionation of sulfur isotopes by the thermophilic chemolithoautotrophic Thermodesulfatator indicus was explored during sulfate reduction under excess and reduced hydrogen supply, and the full temperature range of growth (40-80 °C). Fractionation of sulfur isotopes measured under reduced H₂ conditions in a fed-batch culture revealed high fractionations (24-37‰) compared to fractionations produced under excess H₂ supply (1-6‰). Higher fractionations correlated with lower sulfate reduction rates. Such high fractionations have never been reported for growth on H₂. For temperature-dependant fractionation experiments cell-specific rates of sulfate reduction increased with increasing temperatures to 70 °C after which sulfate-reduction rates rapidly decreased. Fractionations were relatively high at 40 °C and decreased with increasing temperature from 40-60 °C. Above 60 °C, fractionation trends switched and increased again with increasing temperatures. These temperature-dependant fractionation trends have not previously been reported for growth on H₂ and are not predicted by a generally accepted fractionation model for sulfate reduction, where fractionations are controlled as a function of temperature, by the balance of the exchange of sulfate across the cell membrane, and enzymatic reduction rates of sulfate. Our results are reproduced with a model where fractionation is controlled by differences in the temperature response of enzyme reaction rates and the exchange of sulfate in and out of the cell.  相似文献   

Increasing critical sensitivity of the Load/Unload Response Ratio before large earthquakes with identified stress accumulation pattern   总被引：2，自引：0，他引：2  

Huai-zhong Yu  Zheng-kang Shen  Yong-ge Wan  Qing-yong Zhu  Xiang-chu Yin   《Tectonophysics》2006,428(1-4):87-94

The Load/Unload Response Ratio (LURR) method is proposed for short-to-intermediate-term earthquake prediction [Yin, X.C., Chen, X.Z., Song, Z.P., Yin, C., 1995. A New Approach to Earthquake Prediction — The Load/Unload Response Ratio (LURR) Theory, Pure Appl. Geophys., 145, 701–715]. This method is based on measuring the ratio between Benioff strains released during the time periods of loading and unloading, corresponding to the Coulomb Failure Stress change induced by Earth tides on optimally oriented faults. According to the method, the LURR time series usually climb to an anomalously high peak prior to occurrence of a large earthquake. Previous studies have indicated that the size of critical seismogenic region selected for LURR measurements has great influence on the evaluation of LURR. In this study, we replace the circular region usually adopted in LURR practice with an area within which the tectonic stress change would mostly affect the Coulomb stress on a potential seismogenic fault of a future event. The Coulomb stress change before a hypothetical earthquake is calculated based on a simple back-slip dislocation model of the event. This new algorithm, by combining the LURR method with our choice of identified area with increased Coulomb stress, is devised to improve the sensitivity of LURR to measure criticality of stress accumulation before a large earthquake. Retrospective tests of this algorithm on four large earthquakes occurred in California over the last two decades show remarkable enhancement of the LURR precursory anomalies. For some strong events of lesser magnitudes occurred in the same neighborhoods and during the same time periods, significant anomalies are found if circular areas are used, and are not found if increased Coulomb stress areas are used for LURR data selection. The unique feature of this algorithm may provide stronger constraints on forecasts of the size and location of future large events.  相似文献   

Polar and neutral isopranyl glycerol ether lipids as biomarkers of archaea in near-surface sediments from the Nankai Trough   总被引：1，自引：0，他引：1  

Masahiro Oba  Susumu Sakata  Urumu Tsunogai 《Organic Geochemistry》2006,37(12):1643

The molecular and carbon isotopic compositions of polar isopranyl glycerol ether lipids, which are direct indicators of viable archaea, and neutral isopranyl glycerol ether lipids, which are derived from polar lipids via hydrolysis, in near-surface sediments from a methane seep in the Nankai Trough (off central Japan) were investigated. Procedures for extracting, separating and derivatizing polar and neutral ether lipids for detection using gas chromatography were first examined with one sediment sample and a cultivated methanogen. For all sediment samples, archaeol and hydroxyarchaeol were detected in both the polar and neutral ether lipid fractions. Acyclic and cyclic biphytanes were also detected in both types of lipid fractions after treatment with HI/LiAlH₄ for ether cleavage and alkylation. The δ¹³C values of archaeol, sn-2-hydroxyarchaeol, and sn-3-hydroxyarchaeol in the sample from 0.82 m below the seafloor were lower than −100‰ relative to PDB, indicating that diverse living methanotrophic archaea are present in the seep sediments. Biphytanes released from polar ether lipids in the same sample were less depleted in δ¹³C (−71‰ to −36‰). The wide range of δ¹³C values suggests that the biphytanes were derived not only from methanotrophic but also from non-methanotrophic archaea, and that the relative contributions of the methanotrophic and non-methanotrophic archaea differed, depending on the biphytane compound. The vertical profiles and δ¹³C values of the neutral ether lipids were similar to those of the intact polar ether lipids, suggesting that neutral ether lipids derived from fossil archaea in the samples had mainly been lost by the time of sampling.  相似文献   

High precision analysis of all four stable isotopes of sulfur (32S, 33S, 34S and 36S) at nanomole levels using a laser fluorination isotope-ratio-monitoring gas chromatography–mass spectrometry     

《Chemical Geology》2006,225(1-2):30-39

The discovery of mass-independent isotope effects observed in Archean rocks, certain classes of meteorites, and atmospheric aerosols has had profound implications to our understanding of ancient and present atmospheric sulfur chemistry. We present a new technique that takes advantage of continuous He flow isotope-ratio-monitoring gas chromatography–mass spectrometry to achieve precise analysis of all four stable sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) at nanomole level samples. The technique involves fluorination of sulfide (silver sulfide or pyrite), and separation of product gas by gas chromatography and the removal of mass-131 interference by a liquid-nitrogen ethanol slush at − 110 °C. This technique works with an optimum sample size of 100 to 200 nmol with precision for Δ³³S and Δ³⁶S at 0.1 and 0.5‰ (2σ). Samples, as small as tens of nanomole, can be analyzed using this new method. One of the major sources of error in irm-GCMS is found to be tailing of the major ion beam (³²SF₅⁺) onto minor beams (³³SF₅⁺ and ³⁶SF₅⁺), which results in contraction of the measured δ³³S and δ³⁶S scales. This effect is corrected by measuring a series of reference sulfide samples with mass-dependent sulfur isotope compositions. This methodology increases the spatial resolution of the laser ablation in situ analysis and considerably reduces the analysis time as compared with conventional dual inlet methods.  相似文献   

Simulation of thermodynamic mixing properties of garnet solid solutions at high temperatures and pressures     

《Chemical Geology》2006,225(3-4):336-346

We present results of high temperature, high pressure atomistic simulations aimed at determining the thermodynamic mixing properties of key binary garnet solid solutions. Computations cover the pressure range 0–15 GPa and the temperature range 0–2000 K. Through a combination of Monte-Carlo and lattice-dynamics calculations, we derive thermodynamic mixing properties for garnets with compositions along the pyrope–almandine and pyrope–grossular joins, and compare these with existing experimental data. Across the pressure–temperature range considered, simulations show virtually ideal mixing behaviour in garnet on the pyrope–almandine join, while large excess volumes and enthalpies of mixing are predicted for garnet along the pyrope–grossular join. Excess heat capacities and entropies are also examined. These simulations shed additional light on the link between the behaviour at the atomic level and macroscopic thermodynamic properties: we illustrate the importance of certain atomistic Ca–Mg contacts in the pyrope–grossular solid solutions. For simulation techniques of this type to become sufficiently accurate for direct use in geological applications such as geothermobarometry, there is an urgent need for improved experimental determinations of several key quantities, such as the enthalpies of mixing along both joins.  相似文献   

Stable oxygen and carbon isotopes of carbonate concretions of the Miocene Yeonil Group in the Pohang Basin,Korea: Types of concretions and formation condition     

《Sedimentary Geology》2006,183(1-2):15-30

Carbonate concretions in the Miocene sedimentary rocks of the Yeonil Group in the Pohang Basin (Korea) were investigated in terms of stable oxygen and carbon isotope composition to delineate the origin and associated diagenetic environment for their formation. Carbonate concretions are widely distributed in all the sedimentary rocks in the Pohang Basin, showing that the calcitic concretions are preserved within the mass-flow deposits and the dolomitic ones mostly in the hemipelagic siliceous rocks (diatomites). Concretions can be classified into four different types, on the basis of the stable isotopic signatures, each of which represents its own geochemical range.Type I concretions are calcitic and are composed of micrite to microspar. They occur in the conglomerates and sandstones which were deposited by mass flows (debris flow to turbidity current). It shows relatively lower δ¹⁸O (− 14.0 to − 9.3‰) and δ¹³C (− 19.6 to − 8.4‰) values. These concretions grew in a sulfate reducing zone under the influence of residual ambient seawater which had been significantly modified by volcanogenic sediments. Type II concretions are also calcitic, composed mostly of micrite with minor microspar and found in the sandstones. These concretions are characterized by relatively high δ¹⁸O (+ 1.8 to + 2.4‰) and variable δ¹³C (− 17.3 to − 0.4‰) values. These isotopic signatures reflect that Type II concretions formed from just beneath the sediment/water interface down to the sulfate reducing zone through the early stage of methanogenesis. Type III concretions are also calcitic, and composed largely of micrite with a minor contribution of microspar. They are observed in hemipelagic mudrocks which were deposited under the influence of mass flows. They are characterized by intermediate to high δ¹⁸O (− 4.6 to + 1.6‰) and high δ¹³C (− 1.3 to + 8.8‰) values. These concretions grew in a methanogenic zone by residual ambient seawater and/or seawater slightly modified by reaction with volcanogenic sediments. Type IV concretions are dolomite with calcite inclusion, and occur in hemipelagic siliceous rocks. These concretions are mostly composed of micrite and characterized by variable δ¹⁸O (− 9.1 to + 0.7‰) and high δ¹³C (+ 3.1 to + 17.9‰) values, suggesting formation in the methanogenic zone, although the residual ambient seawater is slightly modified by volcanogenic sediments.The same type of the concretions is widely distributed throughout the basin and always shows its own distinctive stable isotopic signature. This means that the formation of the given type depends upon the lithology and composition of host sediments that are closely related to the depositional process of the fan-delta systems regardless of their localities. Further, the different types of concretions are also found at the different, but closely spaced stratigraphic levels in the same locality, displaying the distinctive diagenetic conditions for each type. Such preservation of the unique diagenetic signatures in individual type of concretion suggests that the concretions formed in a completely closed diagenetic system. Therefore, caution should be made to simplify and generalize the diagenetic condition for the formation of any concretions in a large sedimentary basin.  相似文献   

Variations in chemical composition and particle size of dune sediments along the west coast of Jutland,Denmark     

《Sedimentary Geology》2006,183(3-4):217-242

Geochemical and textural variations in frontal dune sediments along the western coast of Jutland, Denmark, have been investigated in order to identify possible sediment ‘provinces’ and transport pathways. An understanding of sediment sources and sinks is important for both for an understanding of the nature of sedimentary environments and for applied coastal engineering and management purposes. Four coastal sectors were identified on the basis of geochemical composition. One sector is compositionally different from the other three units, having higher concentrations of the trace elements Ni, Cr, V, Sc, Zn, Pb, Ba, Zr and many rare earth elements. Dune sediments on this section of coast also have higher Al₂O₃ to K₂O ratios and lower Al₂O₃ to Fe₂O₃ ratios, reflecting a lower content of feldspar and higher content of heavy minerals. It is inferred that different, or additional, sediment source(s) have supplied sediment to this section of coast. Beach nourishment has contributed to the observed compositional differences, but previous data obtained from the Lodbjerg area indicate that aeolian sands on this section of coast naturally have a relatively high Si, Fe and Ti content, reflecting high quartz / feldspar ratio and relatively high content of heavy minerals, compared with those on other sections of the coast. Frontal dunes along the most northern section show high abundances of K₂O, Rb and Ba, reflecting a relatively high K-feldspar content derived from local sources. Alongshore trends were also identified in the mean particle size and sorting of the frontal dunes, although there is no direct correspondence with the observed geochemical differences. Three coastal units can be identified on the basis of particle size. Frontal dune sediments in the middle section are relatively coarser and less well-sorted than those to the north and south, probably reflecting both the addition of beach nourishment material and greater exposure to strong westerly winds, which are a capable of transporting a wide range of particle sizes. The three units defined on this basis do not correlate directly with the units defined on the basis of geochemical composition.  相似文献   

Element fractionation by sequential extraction in a soil with high carbonate content     

《Applied Geochemistry》2006,21(1):16-28

The influence of carbonate and other buffering substances in soils on the results of a 3-step sequential extraction procedure (BCR) used for metal fractionation was investigated. Deviating from the original extraction scheme, where the extracts are analysed only for a limited number of metals, almost all elements in the soils were quantified by X-ray fluorescence spectroscopy, in the initial samples as well as in the residues of all extraction steps. Additionally, the mineral contents were determined by X-ray diffractometry. Using this methodology, it was possible to correlate changes in soil composition caused by the extraction procedure with the release of elements. Furthermore, the pH values of all extracts were monitored, and certain extraction steps were repeated until no significant pH-rise occurred. A soil with high dolomite content (27%) and a carbonate free soil were extracted. Applying the original BCR-sequence to the calcareous soil, carbonate was found in the residues of the first two steps and extract pH-values rose by around two units in the first and second step, caused mainly by carbonate dissolution. This led to wrong assignment of the carbonate elements Ca, Mg, Sr, Ba, and also to decreased desorption and increased re-adsorption of ions in those steps. After repetition of the acetic acid step until extract pH remained low, the carbonate was completely destroyed and the distributions of the elements Ca, Mg, Sr, Ba as well as those of Co, Ni, Cu, Zn and Pb were found to be quite different to those determined in the original extraction. Furthermore, it could be shown that the effectiveness of the reduction process in step two was reduced by increasing pH: Fe oxides were not significantly attacked by the repeated acetic acid treatments, but a 10-fold amount of Fe was mobilized by hydroxylamine hydrochloride after complete carbonate destruction. On the other hand, only small amounts of Fe were released anyway. Even repeated reduction steps did not destroy the amorphous Fe oxides completely, showing that 0.1 mol L⁻¹ hydroxylamine hydrochloride was not strong enough to attack these oxides effectively.The extraction sequences were carried out not only on the soil samples, but also on their coarse and fine fractions (> or <2 μm). The fine fraction of the calcareous soil contained only 10% dolomite, but was enriched in organic matter and clay minerals, which also resulted in increased extract pH-values during the sequential extraction. Hence, the effects on ion release in the fine fraction were similar to those of the whole soil. Since the destruction of the organic matter was incomplete after regular oxidation, the H₂O₂-treatment of the fine fraction had to be repeated. The addition of the extractable amounts of the two fractions showed good agreement to the results obtained for extraction of the whole soils. Likewise the pH-values of the carbonate-free soil extracts did not increase significantly, therefore it was concluded that repetitions of extraction steps for this soil were not necessary.Extract-pHs should always be controlled so that extraction conditions are comparable; to be able to use the BCR extraction scheme or similar ones for carbonate- and organic-rich samples this is mandatory. Single extraction steps should be repeated if pH rises too much; additionally the oxidizing step should be performed more than twice for samples rich in organic substances, depending upon the violence of the reaction with H₂O₂. If these precautions are neglected the validity of the extraction data is likely to be questionable.  相似文献   

Application of Sr isotopes to geochemical mapping and provenance analysis: The case of Aichi Prefecture,central Japan     

《Applied Geochemistry》2006,21(3):419-436

Geochemical maps expressing areal distributions of chemical elements in the earth’s land surface have been published in several countries in relation to various global environment issues. The authors have applied a radiogenic isotope ratio, ⁸⁷Sr/⁸⁶Sr, to geochemical mapping in order to understand the geological origin, transportation and dispersion system of chemical elements in the earth’s land surface. The Sr isotope ratio is a useful tracer for distinguishing the geological origin of surficial deposits, especially in areas where surface exposure of bedrocks is low, because it is not significantly altered by the processes of weathering and transportation. Most bedrocks in the Japanese islands are covered by plants, soils and urban areas. In this study, 142 of 1219 stream sediments (<180 μm) collected from the northeastern part of Aichi Prefecture, in the central part of Japan (75 km × 30 km), were analyzed. Their Sr isotope ratios range from 0.7086 to 0.7315 with an average of 0.7129, except for one sample. This average is higher than the mean of the upper crust of the Japan Arc (the Japanese Island Crustal Composite, JICC), 0.7077. This difference can be attributed to the below-average presence of young volcanic rocks, generally having lower ⁸⁷Sr/⁸⁶Sr values, and the above-average presence of granitic rocks, in the study area compared with the surface exposure of the Japan Arc. The first factor controlling the distribution of Sr isotope ratios is the bedrock distributed around the sampling points. Regional variation in the ⁸⁷Sr/⁸⁶Sr value shows that it is higher in the western and southeastern parts, where sedimentary rocks and metamorphic rocks are distributed, and that it is lower mainly in the central part, where granitic rocks are distributed. The ⁸⁷Rb/⁸⁶Sr–⁸⁷Sr/⁸⁶Sr plot for stream sediments more clearly reveals the differences and similarities of bedrocks. In some locations, the distribution of Sr isotope ratios does not correspond to that of bedrocks on the geological map. One reason is the existence of unmapped bedrock, for example, small intrusive masses of granite. The other is fluvial transportation and dispersion. The distribution of the isotope ratios suggests that some stream sediments include surficial deposits from a few km upstream. Application of the Sr isotope ratio to geochemical mapping is useful for revealing both the distribution of unexposed bedrocks and the transportation of surficial deposits. Information on unexposed bedrocks will be expected to contribute to the improvement of geological mapping.  相似文献