Strontium isotope geochemistry of groundwater affected by human activities in Nandong underground river system, China

The Nandong Underground River System (NURS) is located in a typical karst area dominated by agriculture in SE Yunnan Province, China. Groundwater plays an important role in the social and economical development in the area. The effects of human activities (agriculture and sewage effluents) on the Sr isotope geochemistry were investigated in the NURS. Seventy-two representative groundwater samples, which were collected from different aquifers (calcite and dolomite), under varying land-use types, both in summer and winter, showed significant spatial differences and slight seasonal variations in Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios. Agricultural fertilizers and sewage effluents significantly modified the natural ⁸⁷Sr/⁸⁶Sr ratios signature of groundwater that was otherwise dominated by water-rock interaction. Three major sources of Sr could be distinguished by ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations in karst groundwater. Two sources of Sr are the Triassic calcite and dolomite aquifers, where waters have low Sr concentrations (0.1-0.2 mg/L) and low ⁸⁷Sr/⁸⁶Sr ratios (0.7075-0.7080 and 0.7080-0.7100, respectively); the third source is anthropogenic Sr from agricultural fertilizers and sewage effluents with waters affected having radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7080-0.8352 for agricultural fertilizers and 0.7080-0.7200 for sewage effluents, respectively), with higher Sr concentrations (0.24-0.51 mg/L). Due to the overlapping ⁸⁷Sr/⁸⁶Sr ratios, it is difficult to distinguish the sources of Sr in groundwater samples contaminated by agricultural fertilizers or sewage effluents based only on their ⁸⁷Sr/⁸⁶Sr ratios. However, ⁸⁷Sr/⁸⁶Sr ratios do provide key information for natural and anthropogenic sources in karst groundwater.     

湘中锡矿山锑矿床的Sr同位素地球化学

对湘中锡矿山锑矿床围岩灰岩、硅化灰岩、煌斑岩和脉石矿物进行了系统的Sr同位素研究。结果表明，矿区围岩发生了隐性蚀变，灰岩中Sr亏损，而^87Sr/^86Sr高于同时代的海相碳酸盐，这种隐性蚀变很可能是水／岩反应所致。矿体附近的硅化灰岩中Sr更加亏损，而^87Sr/^86Sr明显增加。成矿期方解石的^87Sr/^86Sr较高，成矿体系中变化的W／R比造成了方解石中^87Sr/^86Sr值的明显波动。成矿流体为一富放射成因^87Sr的溶液。成矿流体来自或流经基底地层，流体中的Sr由基底碎屑岩提供，矿质Sb也可能主要来自富Sb的元古宇基底。水／岩反应的理论模拟显示，锡矿山成矿流体中的Sr约为3．0μg/g，^87Sr/^86Sr为０．７１７；蚀变－成矿体系为一开放体系，矿石的沉淀机制主要为水／岩反应，成矿体系中W／R 比较高。     

The geochemistry of the Dunedin Volcano: Strontium isotope chemistry

The isotopic composition of strontium has been determined for samples from the alkaline lavas of the Dunedin Volcano covering the range basalt, basanite, intermediate compositions, phonolite and quartz normative trachyte. The basaltic, intermediate and phonolitic rocks appear to be comagmatic and have similar low initial Sr⁸⁷/Sr⁸⁶ ratios around 0.7030, comparable with those of other alkaline provinces. The quartz normative trachytes have initial ratios significantly higher than those of the other rocks (0.7040) although their age is comparable. Contamination by sea water or crustal material could explain the higher initial ratios of the trachytes but it does not account for important features of their chemistry. It is suggested that the trachytes formed by partial melting involving an alkali feldspar-rich portion of older igneous rocks. Rb-Sr ages obtained are comparable with published K-Ar dates. The Rb-Sr age for the trachytes is 14.± 7 m.y. and the other alkali-enriched rocks give ages ranging within the limits of 14.4 to 12.0 m.y.     

Strontium isotope geochemistry of megacrysts and host basalts from southeastern Australia

Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The ⁸⁷Sr/⁸⁶Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the ⁸⁷Sr/⁸⁶Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.     

Strontium isotope geochemistry of Icelandic geothermal systems and implications for sea water chemistry

Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water.     

Strontium and hydrogen isotope geochemistry of fresh and metabasalt dredged from the Mid-Atlantic Ridge

Fresh basalt and metabasalt dredged from the Mid-Atlantic Ridge were studied for Na, K, Rb, Sr, and H₂O(+) contents, and strontium and hydrogen isotope ratios. Na, K, Rb, and Sr contents of these samples are within the range of those of oceanic tholeiite. H₂O(+) content, strontium, and hydrogen isotope ratios vary widely. The variation in water content of metabasalt is apparently related to the chlorite content. The metamorphic temperature was about 550 °C based on the estimated δD value of chlorite. There is positive linear relationship between water content and strontium isotope ratio. Based on this relationship, the variation of strontium isotope ratio of the metabasalt was interpreted as follows: complete exchange occurred between strontium in the chlorite portion of the metabasalt and strontium in sea water (⁸⁷Sr/⁸⁶Sr ratio=0.7090), while the original strontium (⁸⁷Sr/⁸⁶Sr∼0.7023) was retained in the non-altered portion of the basalts.     

Strontium isotope geochemistry of groundwater in the central part of the Dakota (Great Plains) aquifer,USA

The Dakota aquifer of the central and eastern Great Plains of the United States is an important source of water for municipal supplies, irrigation and industrial use. Although the regional flow system can be characterized generally as east to northeasterly from the Rocky Mountains towards the Missouri River, locally the flow systems are hydrologically complex. This study uses Sr isotopic data from groundwater and leached aquifer samples to document the complex subsystems within the Dakota aquifer in Nebraska and Kansas. The interaction of groundwater with the geologic material through which it flows has created spatial patterns in the isotopic measurements that are related to: long-term water–rock interaction, during which varying degrees of isotopic equilibrium between water and rock has been achieved; and the alteration of NaCl fluids by water-rock interaction. Specifically, Sr isotopic data distinguish brines from Kansas and western Nebraska from those in eastern Nebraska: the former are interpreted to reflect interaction with Permian rocks, whereas the latter record interaction with Pennsylvanian rocks. The Sr isotopic composition of groundwater from other parts of Nebraska and Kansas are a function of the dynamic interaction between groundwater and unlithified sediments (e.g., glacial till and loess), followed by interaction with oxidized and unoxidized sediments within the Dakota Formation. This study illustrates the power of combining Sr chemistry with more conventional geochemical data to obtain a more complete understanding of groundwater flow systems within regional aquifer systems where extensive monitoring networks do not exist.     

Major ion geochemistry of groundwaters from southern Nevada and eastern California, USA

The dissolved ionic constitutents of groundwaters are,in part,a recored of the minerals and rocks in aquifers through which the water has flowed.The chemical composition and association of these major ions in groundwaters have been used to trace groundwater flow paths and sources,In general,the chemical compostion of water in carbonate-rock aquifers in dominated by calcium,magnesium,and bicarbonate,whereas sodium,chloride,and sulfate can be dominant ions in the water that comes from volcanic aquifers or clay minerals.Since the 1990‘s,we have dealt with the geochemistry of groundwaters from more than 100 springs and wells in southern Nevada and eastrn california ,USA for major solutes and trace elements.This paper compiles the hydrochemical data of major ions of these groundwaters.Based on major ion geochemistry,groundwaters from southern Nevada and eastern California can be classified as carbonate aquifer water,volcanic aquifer water,and mixing water (either mixing of cabonate and volcanic aquifer waters or mixing with local recharges),Piper and stiff diagrams of major ions have graphically shown the general chemical characteristics,classification,and mixing relationships of groundwaters from southern Nevada and eastern California.     

Strontium isotopic geochemistry of Tianqiao Pb-Zn deposit,Southwest China

Tianqiao carbonate-hosted Pb-Zn deposit, controlled by NW-trending F37 thrust fault and NW-trending Tianqiao anticline, is located in the eastern part of Sichuan-Yunnan-Guizhou （SYG） Pb-Zn metallogenic province, southwestern Yangtze Block, southwest China. Ore bodies in this deposit are hosted in the Devonian-Carboniferous carbonate rocks, and ore minerals include sphalerite, galena and pyrite, while the gangue minerals are dominated by calcite and dolomite. Using high-precision solid thermal ionization mass spectrometry （TIMS）, this paper reports the strontium isotopic compositions （0.7119 to 0.7167） of sulfide samples from the Tianqiao deposit in order to trace the origin of hydrothermal fluids. Compared with the country rocks, the calculated 87Sr/86Sr200 Ma values of sulfide range from 0.7118 to 0.7130, higher than those of the age-corrected Devonian to Permian sedimentary rocks （0.7073 to 0.7101） and the Middle Permian Emeishan flood basalts （0.7078 to 0.7039）, but lower than those of the age-corrected Proterozoic basement rocks （such as the Kunyang and Huili Groups, 87Sr/86Sr200 Ma=0.7243 to 0.7288）. This implies a mixed strontium source between the older basement rocks and the younger cover sequences. Together with geologic and previous isotopic evidences, we considered that the fluids＇ mixing is a possible mechanism for sulfide precipitation in the Tianqiao deposit.     

Strontium isotope geochemistry of a low potassium olivine tholeiite and two basalt-pyroxene andesite magma series from the Medicine Lake Highland,California

Three contemporaneous yet mineralogically and chemically distinct lava series which range in composition from olivine basalt through basaltic andesite through two-pyroxene andesite, have been analyzed for their strontium isotopic composition. The Mammoth-Modoc lavas produce a mantle isochron which suggests derivation from a source approximately 243 m.y. old. The Medicine Lake shield-forming extrusives produce a minimum mantle isochron; the age of this source is likely greater than 760 m.y. old. Encompassing the entire time span of eruption for the two basalt-andesite series are a number of low potassium high alumina olivine tholeiite lavas which, when analyzed as a coherent group, produce a zero age. Lavas related to these three magma series have been extruded during the last 10⁵ years and cover a limited geographic area. The isotope data suggest a model with substantial heterogeneity in the subcontinental lithosphere and asthenosphere, at least beneath the Medicine Lake Highland portion of the southern Cascade petrographic province.Contribution No. 126     

Strontium and neodymium isotope study of Bohemian basalts

Summary The Cenozoic alkaline basalts of northern and western Bohemia are part of the Central European Volcanic Province (CEVP) which extends from the Rhineland (Eifel, Germany) to Moravia (Czechoslovakia) and the Lover Silesia (Poland). Seven samlpes from locations within Czechoslovakia have been analyzed isotopically for the Rb-Sr and the Sm-Nd systems. Present-day, normalised 87Sr/86Sr ratios range from 0.7031 to 0.7036, and the corresponding 143Nd/144Nd ratios range from 0.51279 to 0.51286. An eigth sample from the Silurian basalts of the St. Jan type (K-Ar age: 420 Ma) occuring in the Barrandian basin in Central Bohemia is also analysed. Its Present-day, normalised, 87Sr/86Sr ratio is 0.7031, and the corresponding 143Nd/144Nd ratio 0.51288.The Nd ratios of the Cenozoic basalts are similar, but more restricted than those from Germany, but are lower than those from Lower Silesia; a trend which is converse for the Sr ratios.Comparison of the results with the systematics ofZindler andHart (1986), suggests that the mande reservoir source of the Bohemian Cenozoic CEVP basalts is similar to the HIMU (High- ocean island basalts), with transition to PUM (primitive upper mantle) or BSE (bulk silicate earth). The reservoir for the Silurian Barrandian basin basalts suggest some affinity to MORB (mid-ocean ridge basalts) or HIMU.

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Résumé Les basaltes alcalins du Cénozoique du nord et de l'ouest de la Bohème appartiennent à la Province Volcanique de l'Europe Centrale (PVEC) qui s'étend de la région du Rhin (Allemagne) jusqu'en Moravie et Basse-Silésie (Pologne). Nous avons analysé les isotopes de Rb-Sr et de Sm-Nd dans sept échantillons de Tchécoslovaquie. Les valeurs mesurées, et normalisées, du rapport 87Sr/86Sr sont comprises entre 0.7031 et 0.7036 tandis que celles du rapport 143Nd/144Nd varient entre 0.51279 et 0.51286. Un huitième échantillon provient des basaltes Siluriens du type St. Jan (avec un age K-Ar de 420 Ma) dans le bassin Barrandien de la Bohème centrale. La valeur normalisée de son rapport actuel 87Sr/86Sr est de 0.7031, celle de 143Nd/144Nd est de 0.51288.Les rapports isotopiques du Nd de ces basaltes Cénozoiques sont analogues á ceux des basaltes de l'Allemagne, bien qu'ayant un domaine de variation plus restreint, mais sont plus faibles que ceux des basaltes de Basse-Silésie. Les rapports isotopiques du Sr évoluent de façon opposée.L'interprétation de ces données suivant la systématique de ces systemes isotopiques proposée par Zindler et Hart (1986) suggere que le réservoir mantellique source des basaltes de la PVEC en Boheme, est proche du pôle HIMU transitionel vers PUM ou BSE. Le réservoir mantellique source du basalt Silurien du bassin Barrandien montre des affinités avec les pôles MORB ou HIMU.

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With 5 Figures     

Geochemistry and health aspects of F-rich mountainous streams and groundwaters from sierras Pampeanas de Cordoba,Argentina

Symptoms of dental fluorosis have been observed in rural communities located in the Sierras Pampeanas de Córdoba, a mountainous area in Central Argentina. The clinical assessment was performed in the Charbonier Department, where the fluoride (F⁻) intake was determined to be 3.90 ± 0.20 mg day⁻¹ (n = 16). In this community, mild and severe fluorosis reach an incidence of 86.7% (total teeth surface = 636 teeth) among the children population. To determine the origin and distribution of fluorine in natural waters from the Charbonier Department and nearby regions, sampling was performed in the area covering the San Marcos River basin. The obtained results show that F⁻ concentrations vary between ~1 to ~2.5 mg l⁻¹, with an outlier value of 8 mg l⁻¹. The spatial distribution of F⁻ shows that the lowest concentrations are found at the basin’s catchments. Maximum values are located in two sectors of the basin: the Charbonier depression in the eastern part and at the San Marcos village, downstream the main collector, in the western part of the basin. In these two regions, the F⁻ contents in ground- and surface waters are >2.0 mg l⁻¹ and nearly constant. Dissolved F⁻ in natural waters from the study area has its origin in the weathering of F-bearing minerals present in the region’s dominant lithology. The extent of mineral weathering is mostly determined by the residence time of water within the aquatic reservoir. Longer residence times and a major solid–water interaction lead to enhanced release of F⁻. This explains the higher F⁻ concentrations found in basin areas with lower run off. The removal of F⁻ from water appears to occur by neither fluorite precipitation, nor by adsorption. Hence, variations in F⁻ concentrations seem to be more related to regional hydrological conditions.     

Strontium isotope geology of the South Mountain batholith,Nova Scotia

The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}$ ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.70942 ± 35}$); porphyry (361.2 ± 1.4 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.71021 ± 119}$); using λ⁸⁷Rb = 1.42 × 10^?11yr^?1.A suite of Meguma country rock samples showed a variation of ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7113?0.7177}$ at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{372}\text{= 0.7081}$ of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known.     

Strontium isotope evolution of Late Permian and Triassic seawater

The ⁸⁷Sr/⁸⁶Sr values based on brachiopods and conodonts define a nearly continuous record for the Late Permian and Triassic intervals. Minor gaps in measurements exist only for the uppermost Brahmanian, lower part of the Upper Olenekian, and Middle Norian, and only sparse data are available for the Late Permian. These 219 measurements include 67 brachiopods and 114 conodont samples from the Tethyan realm as well as 37 brachiopods and one conodont sample from the mid-European Middle Triassic Muschelkalk Sea. The Late Permian/Lower Triassic interval is characterized by a steep 1.3 × 10⁻³ rise, from 0.7070 at the base of the Dzhulfian to 0.7082 in the late Olenekian, a rate of change comparable to that in the Cenozoic. In the mid-Triassic (Anisian and Ladinian), the isotope values fall to 0.7075, followed again by a rise to 0.7081 in the Middle/Late Norian. The ⁸⁷Sr/⁸⁶Sr values decline again in the Late Norian (Sevatian) and Rhaetian to 0.7076.The sharp rise in the ⁸⁷Sr/⁸⁶Sr values during the Late Permian/Early Triassic was coincident with widespread clastic sedimentation. Because of the paucity of tectonic uplifts, the enhanced erosion may have been due to intermittent humid phases, during mainly an arid interval, coupled with the absence of a dense protective land plant cover following the mass extinction during the latest Permian. The apex of the ⁸⁷Sr/⁸⁶Sr curve at the Olenekian/Anisian boundary coincides with cessation of the large-scale clastic sedimentation and also marks the final recovery of land vegetation, as indicated by the renewed onset of coal formation in the Middle Triassic. The rising ⁸⁷Sr/⁸⁶Sr values from the Middle Carnian to the Late Norian coincide with the uplift and erosion of the Cimmeride-Indosinian orogens marking the closure of the Palaeotethys. The subsequent Rhaetian decline that continues into Jurassic (Pliensbachian/Toarcian boundary), on the other hand, coincides with the opening of the Vardar Ocean and its eastern continuation in the Izmir-Ankara Ophiolitic Belt.Samples from the Upper Muschelkalk are more radiogenic than the global trend. This may reflect separation of the basin from the open ocean. Due to strong meteoric influx from a large land mass in the north, the Germanic Basin became increasing brackish up section in the north and east, but because of the high evaporation rates, the salt content was not much reduced in the southern and central basin where a rich, but increasingly endemic, marine fauna survived.     

Germanium isotope geochemistry: analysis,fractionation mechanisms and geochemical tracing

<正>Introduction Since Ge isotope is a new nontraditional isotope,the accumulated Ge isotope literatures are quite limited.The available researches mainly focused on two aspects:(1)the measurement of Ge isotopic compositions of geological and extraterrestrial materials,such as igneous rocks,marine sediments,seafloor hydrothermal fluids,hydrothermal Fe-oxyhydroxides,terrestrial high-temperature geothermal fluids,sphalerite,and iron meteorites;and(2)theoretical prediction of germanium isotope fractionation.     

Strontium isotope composition and petrogenesis of the Kirkpatrick basalt,Queen Alexandra Range,Antarctica

The initial ⁸⁷Sr/⁸⁶Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO₂ (56.50%) and K₂O (1.29%) and are depleted in Al₂O₃ (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts.The initial ⁸⁷Sr/⁸⁶Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial ⁸⁷Sr/⁸⁶Sr ratios of successive flows and their concentrations of SiO₂, FeO (total iron), Na₂O, K₂O, P₂O₅, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO₂, Al₂O₃, MgO, CaO and MnO increase upward. The initial ⁸⁷Sr/⁸⁶Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial ⁸⁷Sr/⁸⁶Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain.Mixtures of two components having different ⁸⁷Sr/⁸⁶Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial ⁸⁷Sr/⁸⁶Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their ⁸⁷Sr/⁸⁶Sr ratios. The chemical composition of the parent basalt (⁸⁷Sr/⁸⁶Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (⁸⁷Sr/⁸⁶Sr=0.720) is enriched in strontium (173 ppm), SiO₂, FeO (total iron) and the alkalies but is depleted in Al₂O₃, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.Laboratory for Isotope Geology and Geochemistry, Contribution No. 33.     

浙江武义一东阳地区萤石矿床的锶同位素地球化学研究

本文通过对浙中火山岩区中最著名的武义—东阳萤石成矿区9个矿床和地层岩石的38个样品的锶同位素地球化学研究,证实了赋矿岩层下伏基底的前寒武纪陈蔡群变质岩是该区萤石矿床的主要矿源层;~(87)Sr/~(86)Sr和F/sr比值表明,萤石成矿过程中约57—76％的氟和40—50％的锶(钙)来自陈蔡群变质岩,来源于上地幔的氟平均不超过24％。~(87)Sr/~(86)Sr比值和其它稳定同位素等资料,也揭示这些萤石矿床的形成主要与古地热水环流汲取作用有关,从而对其成因属于火山热液矿床的传统认识提出了异议。     

The occurrence and geochemistry of arsenic in groundwaters of the Newark basin of Pennsylvania

Elevated As concentrations in groundwater in the eastern United States have been recognized predominantly in the accretionary geologic terranes of northern New England. A retrospective examination of more than 18,000 existing groundwater samples from the Pennsylvania Department of Environmental Protection (PA DEP) Drinking Water and Sampling Information System database indicates that elevated groundwater As concentrations occur throughout the northern half of the Piedmont Province of Pennsylvania. Chemical analyses of 53 samples collected in 2005 from drinking water wells in this area all had detectable As, and 23% of these samples contained elevated (>133 nmol/L or >10 μg/L) concentrations of As. Elevated concentrations of As in the groundwater samples were most common in the Mesozoic sedimentary strata composed of sandstone and red mudstone with interbedded gray shale, and gray to black siltstone and shale. Arsenic was typically not elevated in groundwater of diabase intrusions of the Newark Basin or in crystalline and calcareous aquifers to the north of the Newark Basin. Geochemical parameters such as pH and oxidation–reduction potential can indicate mobility mechanisms of As in some regions. In this area, measured groundwater conditions were predominantly oxidizing (Eh > +50 mV), and more than 85% of samples contained arsenate as the dominant As species. Variations in pH were strongly correlated to the As concentration, with highest As concentrations observed at pH values greater than 6.4. The original source of As is most likely the black and gray shales that contain some arsenian pyrite with groundwater concentrations likely to be controlled by adsorption/desorption reactions with Fe oxides in the red mudstone aquifer materials.     

The geochemistry of boron and its isotopes in groundwaters from marine and non-marine sandstone aquifers

The geological evolution of B in two UK sandstone aquifers is followed from precipitation chemistry through to groundwaters in both the unconfined and confined zones. Measurements have been made of major element geochemistry, B concentrations and B isotopic ratios. The isotopic measurements were carried out using ICP/MS following a simple preconcentration step. Isotopic measurements of rainfall show a bimodal distribution and it is suggested that enriched signatures are characteristic of Atlantic air over Britain and depleted signatures representative of continental air. In the marine Lower Greensand aquifer dissolution of glauconite results in the mobilisation of B and a correlation with SO₄ suggests that this dissolution is related to the oxidation of pyrite which appears to be the SO₄-forming reaction in the aquifer. In the non-marine Hastings beds isotopic ratios and a correlation with HCO₃ suggest that B is associated with the dissolution of ferroan carbonates. In both aquifers the geochemical evolution of B is complex and more information is needed on the behaviour of B isotopes during evapotranspiration and groundwater recharge.