Testing a new combined (U,Th)–He and U/Th dating approach on Plio-Pleistocene calcite speleothems

The geochronology of cave deposits in the Cradle of Humankind UNESCO World Heritage Site in South Africa provides a timeframe essential for the interpretation of its fossils. The uranium-lead (U–Pb) and uranium-thorium disequilibrium (U/Th) dating of speleothems, mostly flowstones that underlie and blanket the fossil-bearing sediments, have been effective in this sense, but U–Pb is limited by the requirement of ∼1 ppm U concentrations and low common Pb contents, and U/Th has a c. 500 ka limit of applicability. Here we report age results for calcite-aragonite speleothems obtained using a new combined uranium-thorium-helium ((U,Th)–He) and U/Th dating routine. We reproduced within analytical uncertainty, the published U–Pb or U/Th ages for (a) flowstone in three drill core samples in the range 2000–3000 ka, (b) a flowstone hand sample taken at surface with an age of 1800 ka, and (c) five underground flowstone samples in the range 100–800 ka. Calcite retentivity for He under cave conditions is thus demonstrated. In the few cases where helium loss was observed in speleothems, only some of the subsamples were affected, and to varying degrees, suggesting loss by lattice damage not related to diagenetic processes, rather than volume diffusion. In the 100 to 800 ka range, the combined U/Th disequilibrium and (U,Th)–He method also yielded reliable values for initial (²³⁰Th/²³⁸U) and (²³⁴U/²³⁸U) activity ratios. Importantly, most subsamples had high initial (²³⁰Th/²³⁸U) values, ranging from 1.0 to 19.7, although having low Th/U ratios. This is probably due to incorporation of Fe–Mn oxides-hydroxides dust, on which ²³⁰Th was previously adsorbed. Such samples are mostly not dateable by U/Th without the additional input from the He analysis. If not detected and corrected for, such high initial (²³⁰Th/²³⁸U) values can lead to inaccurate U/Th and U–Pb ages. Our study shows that the incorporation of He analysis in U/Th dating has broad potential application, with four methods for calculating the ages, in carbonates from different environments where U-Pb or U/Th dating would not work.     

ESR/U-series dating of Eemian human occupations of Northern France

Palaeolithic sites associated with the Eemian Interglacial (MIS 5e) are very rare in NW Europe, and especially in Northern France, where their preservation is restricted to very specific geological contexts, in association with carbonated tufa (Caours) or peat deposits (Waziers). In order to check the reliability of ESR/U-series method to date teeth recovered from archaeological levels in such specific geological environments, teeth were sampled on these two Middle Palaeolithic sites and systematic in situ dosimetry was performed using portable gamma spectrometer. The ESR/U-series ages obtained on the Caours site are very homogeneous allowing the calculation of a mean age equal to 125 ± 11 ka, in agreement with the geological age, mammal's association and other available geochronological data (U-series on tufa carbonate, TL on burnt flints, OSL on sediments), despite a relatively heterogeneous dosimetric environment (gamma dose rate ranging between ca 200–450 μGy/a). At Waziers, reducing (water logging) environment linked to the peat leads to very specific U-series data of the analysed teeth (U content lower than 0.1 ppm in all the dental tissues, evidence of leaching in some tissues), but the mean ESR/U-series age, 129 ± 4 ka, is also in agreement with the available geological and palaeoenvironmental data indicating that the dated teeth were associated with Late Glacial deposits prior to the climatic interglacial optimum. These two case studies then confirm the reliability of ESR/U-series method to date with good reliability and accuracy the archaeological levels linked to such relatively short climatic events (ca 10 ka).     

落石冲击混凝土棚洞力学特性研究

落石冲击棚洞结构作用过程复杂,缺乏统一的落石冲击力表达式。首先,将落石简化为刚性球体,基于Hertz接触理论,推导得到落石冲击力半正弦算法的理论表达式,考虑落石冲击下棚洞的非弹性特征,根据落石与材料碰撞过程中落石加速度曲线特征,采用函数拟合法推导得到落石法向冲击下其冲击力的理论计算方法;然后,基于ANSYS/LS-DYNA软件建立落石冲击棚洞数值计算模型,研究不同冲击速度下落石冲击棚洞动力特征;最后,与现存常见的多种方法进行对比,得出以下结论：Hertz半正弦法得到的落石冲击力远大于函数拟合法和数值法,而函数拟合法和数值法得到的落石冲击力时程曲线相接近,表明函数拟合法更能反映落石与棚洞接触碰撞动力关系;对比其他计算方法可以得到,Hertz算法适用于分析无能量损失下的弹性碰撞问题,而Logistic算法适用于材料大塑性变形的情况,弹塑性接触理论结果和动力有限元结果存在差异,而采用函数拟合推导的计算方法得到的落石最大冲击力和落石冲击作用时间与动力有限元法更接近,更能反映落石冲击棚洞动力响应特征,推导的落石冲击力计算方法可为工程实践中棚洞防护设计提供理论参考。     

赣南北部黄陂河流域离子型稀土矿地区水质与健康风险评价

水质安全和健康是保障中国赣南老区乡村振兴发展的重要因素。赣南离子型稀土矿长期开发利用,导致浸矿剂和矿体中重金属元素等危害人体健康的物质进入水循环系统,给周边乡村饮用水卫生安全带来了潜在风险。目前,针对当地复垦后稀土矿及周边地区水质和健康风险缺乏系统调查评价,本文以《生活饮用水卫生标准》(GB 5749—2006)为评价依据,选择赣南北部黄陂河流域典型离子型稀土矿及周边的水体开展调查研究,采用电感耦合等离子体发射光谱/质谱等技术测定锰、镉等元素含量,采用水质指数(WQI)、危害商(HQ)、致癌风险(CR)评价了锰和铅等9种指标及其健康风险。结果表明：地表水中的异常指标有氨氮(平均值750μg/L)、锰(平均值207μg/L),地下水中的异常指标有氨氮(平均值4533μg/L)、锰(平均值4009μg/L);世界卫生组织(WHO)公布的Ⅰ类致癌物砷在地表水及地下水均未见异常。WQI显示研究区内85.7%的地表水适宜饮用。地表水及地下水中氨氮的HQ平均值<1,对人类健康没有不良影响;地下水中锰的HQ平均值>1,可能会对人类健康产生不良影响。地表水及地下水中致癌元素砷的CR值分布区...     

甘肃厂坝-李家沟超大型铅锌矿床成矿金属来源——来自闪锌矿原位S-Pb和Zn同位素证据

甘肃厂坝-李家沟铅锌矿床位于西秦岭多金属成矿带内的西成矿集区,为矿集区内重要的超大型铅锌矿床。矿体赋存在中泥盆统安家岔组的白云石化大理岩及石英片岩中,其成因认识一直存在争议,主要分歧集中在是同生还是后生。文章对不同成矿阶段的闪锌矿,采用多接收电感耦合等离子体质谱(MC-ICP-MS)测定Zn同位素组成、采用激光剥蚀-多接收电感耦合等离子体质谱(LA-MC-ICP-MS)原位微区分析技术测定S、Pb同位素组成,示踪成矿物质来源,并分析矿物沉淀机制,为深入理解矿床成因提供新的精细证据。研究结果显示,Ⅰ、Ⅱ、Ⅲ三个成矿阶段闪锌矿的δ⁶⁶Zn分别为0.08‰~0.29‰,平均0.20‰;0.19‰~0.37‰,平均0.30‰;0.36‰~0.37‰,平均0.37‰。其中,Ⅰ阶段的闪锌矿δ³⁴S_Ⅰ值为20.9‰~26.1‰,平均24.4‰;Ⅱ阶段的闪锌矿δ³⁴S_Ⅱ值为12.2‰~21.9‰,平均19.1‰;Ⅲ阶段的闪锌矿δ³⁴S_Ⅲ值为18.2‰~24.7‰,平均21.5‰。3个阶段的矿石矿物Pb同位素组成变化不大,²⁰⁶Pb/²⁰⁴Pb为17.922~18.013,²⁰⁷Pb/²⁰⁴Pb为15.567~15.647,²⁰⁸Pb/²⁰⁴Pb为37.990~38.266。δ⁶⁶Zn同位素值显示,成矿金属早期来源于围岩海相碳酸盐岩,由于混合了岩浆热液或者是瑞利分馏作用,在成矿作用中后期δ⁶⁶Zn同位素逐渐上升。δ³⁴S同位素值显示,早期硫源主要为地层中的硫酸盐,中后期的δ³⁴S同位素值降低,可能是成矿流体中岩浆热液中的S^2-成分逐渐增多导致,闪锌矿为硫酸盐通过TSR反应沉淀成矿。Pb同位素指示成矿物质来源于上地壳,并混入了部分古老的变质基底的成分。笔者研究发现,厂坝-李家沟铅锌矿的成矿机制为不同来源的流体混合,随着pH值、成矿流体的温度发生变化而沉淀成矿。     

Does continental crust transform during eclogite facies metamorphism?

Eclogites within exhumed continental collision zones indicate regional burial to depths of at least 60 km, and often more than 100 km in the coesite‐stable, ultra‐high pressure (UHP) eclogite facies. Garnet, omphacitic pyroxene, high‐Si mica, kyanite ± coesite should grow at the expense of low‐P minerals in most felsic compositions, if equilibrium obtained at these conditions. The quartzofeldspathic rocks that comprise the bulk of eclogite facies terranes, however, contain mainly amphibolite facies, plagioclase‐bearing assemblages. To what extent these lower‐P minerals persisted metastably during (U)HP metamorphism, or whether they grew afterwards, reflects closely upon crustal parameters such as density, strength and seismic character. The Nordfjord area in western Norway offers a detailed view into a large crustal section that was subducted into the eclogite facies. The degree of transformation in typical pelite, paragneiss, granitic and granodioritic gneiss was assessed by modelling the equilibrium assemblage, comparing it with existing parageneses in these rocks and using U/Th–Pb zircon geochronology from laser ablation ICPMS to establish the history of mineral growth. U–Pb dates define a period of zircon recrystallization and new growth accompanying burial and metamorphism lasting from 430 to 400 Ma. Eclogite facies mafic rock (~2 vol.% of crust) is the most transformed composition and records the ambient peak conditions. Rare garnet‐bearing pelitic rocks (<10 vol.% of crust) preserve a mostly prograde mineral evolution to near‐peak conditions; REE concentrations in zircon indicate that garnet was present after 425 Ma and feldspar broke down after 410 Ma. Felsic gneiss – by far the most abundant rock type – is dominated by quartz + biotite + feldspar, but minor zoisite/epidote, phengitic white mica, garnet and rutile point to a prograde HP overprint. Relict textures indicate that much of the microstructural framework of plagioclase, K‐feldspar, and perhaps biotite, persisted through at least 25 Ma of burial, and ultimately UHP metamorphism. The signature reaction of the eclogite facies in felsic rocks – jadeite/omphacite growth from plagioclase – cannot be deduced from the presence of pyroxene or its breakdown products. We conclude that prograde dehydration in orthogneiss leads to fluid absent conditions, impeding equilibration beyond ~high‐P amphibolite facies.     

内蒙古林西县北三段银铅多金属矿床碳、氧、硫、铅同位素特征及成矿物质来源

本文对内蒙古林西县北三段银铅多金属矿床矿体的碳、氧、硫、铅同位素进行了测定,并对其成矿物质来源及演化进行了探讨。矿区样品的δ34S‰变化范围为-0.24‰~2.37‰,平均值为1.29‰,初步认为矿石中的硫主要来源于深部岩浆,并有少量生物还原硫的加入。样品的δ13CPDB变化范围为-4.57‰~-7.33‰,平均值为-5.96‰,δ18O为-3.4‰~-4.8‰,平均值为-4.06‰;δ13CPDB-δ18OSMOW关系表明,矿体中的碳可能主要由基性-超基性岩浆提供,并有部分来源于地层,且受大气水影响明显。铅同位素表明北三段矿床的成矿物质可能由造山带物质和地幔物质两部分提供。     

BiOBr/铁改性蒙脱石对罗丹明B的光催化降解性能研究

在低温条件下通过铁改性蒙脱石与BiOBr复合成功合成了一种新型的光催化材料,称为BiOBr/Fe-Mt。运用X射线粉末衍射(XRD)、透射电镜(TEM)、傅立叶转换红外光谱(FTIR)、紫外可见漫反射光谱(UV-Vis DRS)对BiOBr/Fe-Mt材料进行表征。同时以λ400 nm的可见光为光源,开展该催化剂对罗丹明B(Rh B)的光催化降解实验以评估其光催化活性。结果表明,支撑材料投加量被固定在2.5 g的条件下,采用铁改性蒙脱石制备的BiOBr/Fe-Mt材料对Rh B的光催化降解活性高于采用钠基蒙脱石制备的BiOBr/Na-Mt。光催化活性的显著提高,主要归因于:1 BiOBr/Fe-Mt的禁带宽度降低至2.46 e V,或可能形成了BiOBr/铁氧化物异质结结构;2 BiOBr/Fe-Mt上仅发生较少的十六烷基三甲基溴化铵有机分子残留。这些结构因素均可以有效促进BiOBr/Fe-Mt上Rh B的光催化降解。     

The Paleoceanography during the Time with High δ13C of Phanerozoic marine carbonates

Carbon isotopic composition of marine carbonates is a record for various important geological events in the process of earth development and evolution. The carbonates of Carboniferous, Permian and Triassic, as the transition from Paleozoic to Mesozoic-Cenozoic have very high ¹³C value. Taking this as the main point, and combined with the oxygen, strontium isotopic composition in carbonates, distribution of carbonate basin area through geologic time, the correlation of carbon isotopic composition of marine carbonates to sea level change, organic carbon burial flux, exchange of CO₂ content in atmosphere and ocean, and long cycle evolution of the earth ecosystems were approached. The results are shown as follows: ①The interval of ¹³C >3‰ during Phanerozoic was concentrated in Carboniferous, Permian and the beginning of Triassic, but the beginning of Triassic was characterized by higher frequency and larger fluctuations in ¹³C value during a short time, whereas the Carboniferous-Permian presented a continuously stable high ¹³C value, indicating a larger amount of organic carbon accumulation in this time interval. Relatively high ¹⁸O values during this time was also observed, showing a long time of glaciations and cold climate, which suggest a connection among rapid organic carbon burial, cold climate, as well as pCO₂ and pO₂ states of atmosphere. ②The over consumption of atmosphere CO₂ by green plants during the time with high ¹³C of seawater forced CO₂ being transferred from ocean to atmosphere for the balance, but the decrease in the seawater amount and water column pressure caused by the global cooling could weaken dissolution capacity of CO₂ in seawater and carbon storage of marine carbonates, and also reduce the carbonate sedimentary rate and decrease the carbonate basin area globally from Devonian to Carboniferous and Permian. During the middle-late Permian carbonate was widely replaced by siliceous sediments even though in shallow carbonate platform, which resulted in the decrease of marine invertebrates, suggesting the Permian chert event should be global. ③The Phanerozoic ⁸⁷Sr/⁸⁶Sr trend of seawater showed a sharp fall in Permian and drop to a minimum at the end of the Permian, indicting input of strontium from the submarine hydrothermal systems (mantle flux). Such process should accompany with a supplement of CO₂ from deep earth to atmosphere and ocean system, but the process associated with widespread volcanism and rises of earth’s surface temperature pricked up the mass extinction during the time of end Permian. ④Cold climate and increase of continental icecap volume, the amalgamation of northern Africa and Laurentia continentals were the main reasons responsible for the sea level drop, but the water consumption result from the significantly increased accumulation of organic carbon should also be one of the reasons for the sea level drop on the order of tens of meters. ⑤The mass extinction at the end Permian was an inevitable event in the process of earth system adjustment. It was difficult for marine invertebrates to survive because of the continuously rapid burial of organic carbon, and of the decrease of sea water amount and its dissolution ability to CO₂. At last, at the end of Paleozoic, the supplement of CO₂ to atmosphere and ocean by widely magma activities resulted in a high temperature of earth surface and intensified mass extinction.     

苏北盆地建湖隆起碳酸盐岩储层中的硫酸盐型热矿水成因

碳酸盐岩储层中的硫酸盐型热矿水是宝贵的旅游疗养资源。我国苏北盆地北部分布有丰富的热矿水,其中,盱眙县的一口热水井的单井流量达到3 616t·d-1,矿化度(TDS)达到4g·L-1,温度为53.5℃,为优质硫酸盐型热矿水。采用水化学和同位素方法,结合钻孔地质资料,研究了其形成机理。该地区热矿水的SO2-4离子含量较高,部分热矿水的阴离子中SO2-4离子占主导,成为硫酸盐型水。热水的氢氧稳定同位素(δ2 H、δ18 O)值证明了其来源于大气降水。建湖隆起存在石膏层,主要分布在白垩系浦口组,该组属盐湖相沉积,发育大范围的含膏质泥岩,其中石膏、硬石膏含量较多。根据水样的Sr/Ca摩尔比,该区热水中的硫酸盐来源于石膏的溶解。基于碳酸盐岩热水的pH值和SI值、14 C含量等数据的分析,排除了硫酸盐型热矿水的形成与金属矿的风化或氧化的关系,认为是石膏或硬石膏溶解形成。