Variations of δSi and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture

In soils, silicon released by mineral weathering can be retrieved from soil solution through clay formation, Si adsorption onto secondary oxides and plant uptake, thereby impacting the Si-isotopic signature and Ge/Si ratio of dissolved Si (DSi) exported to rivers. Here we use these proxies to study the contribution of biogenic Si (BSi) in a soil-plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping. δ³⁰Si and Ge/Si ratios were determined in bulk soils (<2 mm), sand (50-2000 μm), silt (2-50 μm), amorphous Si (ASi, 2-50 μm) and clay (<2 μm) fractions: δ³⁰Si by MC-ICP-MS Nu Plasma in medium resolution, operating in dry plasma with Mg doping (δ³⁰Si vs. NBS28 ± 0.12‰ ± 2σ_SD), Ge/Si computed after determination of Ge and Si concentrations by HR-ICP-MS and ICP-AES, respectively. Components of the ASi fraction were quantified by microscopic counting (phytoliths, diatoms, ashes). Compared to fresh ash (δ³⁰Si = −0.38‰; Ge/Si = 2.21 μmol mol⁻¹), soil clay fractions (<2 μm) were enriched in light Si isotopes and Ge: with increasing weathering degree, δ³⁰Si decreased from −1.19 to −2.37‰ and Ge/Si increased from 4.10 to 5.25 μmol mol⁻¹. Sand and silt fractions displayed δ³⁰Si values close to fresh ash (−0.33‰) or higher due to saharian dust quartz deposition, whose contribution was evaluated by isotopic mass balance calculation. Si-isotopic signatures of bulk soils (<2 mm) were strongly governed by the relative proportions of primary and secondary minerals: the bulk soil Si-isotopic budget could be closed indicating that all the phases involved were identified. Microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation. δ³⁰Si and Ge/Si values of clay fractions in poorly developed volcanic soils, isotopically heavier and Ge-depleted in surface horizons, support the occurrence of a DSi source from banana phytolith dissolution, available for Si sequestration in clay-sized secondary minerals (clay minerals formation and Si adsorption onto Fe-oxide). In the soil-plant system, δ³⁰Si and Ge/Si are thus highly relevant to trace weathering and input of DSi from phytoliths in secondary minerals, although not quantifying the net input of BSi to DSi.     

Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201

Uranium-lead ratios (commonly represented as ²³⁸U/²⁰⁴Pb = μ) calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of μ values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range (²⁰⁶Pb/²⁰⁴Pb = 11.16-11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb-²⁰⁸Pb/²⁰⁴Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial Pb. This terrestrial Pb contamination generated a ²⁰⁶Pb-²⁰⁷Pb array in the QUE fractions that appears to represent an ancient age, which contrasts with a much younger crystallization age of 327 ± 10 Ma derived from Rb-Sr and Sm-Nd isochrons (Borg L. E., Nyquist L. E., Taylor L. A., Wiesmann H. and Shih C. -Y. (1997) Constraints on Martian differentiation processes from Rb-Sr and Sm-Nd isotopic analyses of the basaltic shergottite QUE 94201. Geochim. Cosmochim. Acta61, 4915-4931). Despite the contamination, and accepting 327 ± 10 Ma as the crystallization age, we use the U-Pb data to determine the initial ²⁰⁶Pb/²⁰⁴Pb of QUE 94201 to be 11.086 ± 0.008 and to calculate the μ value of its mantle source to be 1.82 ± 0.01. The μ value calculated for the QUE 94201 source is the lowest determined for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that μ values in martian source reservoirs vary by at least a factor of two. Additionally, the range of source μ values indicates that the μ value of bulk silicate Mars is approximately three. The amount of variation in the μ values of the mantle sources (μ ∼ 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate greater extents of U-Pb fractionation during formation of the mantle sources of martian basalts.     

Genesis of Pb–Zn Mineralization Beneath the Xiangshan Uranium Orefield,South China: Constraints from H–O–S–Pb Isotopes and Rb–Sr Dating

The volcanic‐hosted Xiangshan uranium orefield is the largest uranium deposit in South China. Recent exploration has discovered extensive Pb–Zn mineralization beneath the uranium orebodies. Detailed geological investigation reveals that the major metallic minerals include pyrite, sphalerite, galena, and chalcopyrite, whilst the major non‐metallic minerals include quartz, sericite, and calcite. New δ¹⁸O_fluid and δD_fluid data indicate that the ore‐forming fluids were mainly derived from magmatic, and the sulfide δ³⁴S values (2.2–6.9‰) suggest a dominantly magmatic sulfur source. The Pb isotope compositions are homogeneous (²⁰⁶Pb/²⁰⁴Pb = 18.120–18.233, ²⁰⁷Pb/²⁰⁴Pb = 15.575–15.698, and ²⁰⁸Pb/²⁰⁴Pb = 37.047–38.446). The ⁸⁷Sr/⁸⁶Sr ratios of sulfide minerals range from 0.7197 to 0.7204, which is much higher than volcanic rocks and fall into the range of metamorphic basement. Lead and strontium isotopic compositions indicate that the metallogenic materials probably were derived from metamorphic basement. Pyrite Rb–Sr dating of the ores yielded 131.3 ± 4.0 Ma, indicating that the Pb–Zn mineralization occurred in the Early Cretaceous.     

Variable initial Pb isotopic compositions of rocks associated with the UG2 chromitite, eastern Bushveld Complex

An investigation of the Pb isotopic compositions of plagioclase and sulfide in a stratigraphic interval including the UG2 chromitite of the eastern Bushveld Complex has been conducted to determine the Pb isotopic composition(s) of the magma(s) that crystallized to form this part of the intrusion, gain a better understanding of why coexisting plagioclase and sulfide commonly exhibit widely different Pb isotopic compositions, and explore the use of Pb isotopes in deducing post-accumulation history. Analyses were obtained in situ with a NuPlasma multicollector ICP-MS coupled with 193 nm Excimer or 213 nm lasers.Most plagioclase compositions fall on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb geochron of 2.06 Ga, which is the solidification age of the intrusion. The measured ratios have not been affected by radiogenic ingrowth, and plagioclase generally remained closed to Pb exchange after initial cooling. The array of plagioclase compositions on the geochron is significantly larger than that defined by analytical error. This indicates that in terms of Pb at least two different magma compositions were present. The composition of the least radiogenic magma was approximated by that of the contemporaneous BSE with μ (²³⁸U/²⁰⁴Pb) and ω (²³²Th/²⁰⁴Pb) values of ≈9.0 and 35, respectively, suggesting a mantle derivation with little or no involvement of the continental crust, while the second magma possessed a Pb isotopic composition similar to the upper crust with μ ≈ 9.6.Compared to plagioclase, sulfides generally possess slightly higher ²⁰⁶Pb/²⁰⁴Pb ratios for equivalent ²⁰⁷Pb/²⁰⁴Pb ratios such that their compositions fall between the 2.06 and 1.86 Ga geochrons. The latter age is much younger than the cooling age. The data are interpreted to mean that the Bushveld Complex remained buried in the crust at temperatures of several hundred °C for about 200 Ma after solidification, and that any sulfides accessible to fluid continued to re-equilibrated during this time with more radiogenic Pb. The sulfide Pb may have been transported into the Bushveld Complex by fluids from an external reservoir when the rocks were still partially molten and thus permeable. Alternatively, the sulfide Pb may have originated mainly from radiogenic decay of U and Th present in minerals other than the sulfides in the immediately surrounding Bushveld rocks, followed by local redistribution of Pb by whatever fluid was present. Indeed, some sulfides are characterized by ²⁰⁸Pb/²⁰⁴Pb ratios sufficiently high that an external source is unlikely. This observation and the fact that the sulfides display small-scale heterogeneity suggest that most, if not all, of the radiogenic sulfide Pb was locally derived. It also implies that during the post-solidification, re-equilibration period there was no large-scale fluid-flow through the microfracture network because otherwise the isotopic heterogeneities would not have been preserved. The minerals in 2 of the 19 samples studied contain young Pb, presumably introduced by meteoric waters that permeated the macrofracture network.     

Geological and C–O–S–Pb–Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb–Zn deposit,Southwest China

Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ³⁴S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.561 to 18.768, ²⁰⁷Pb/²⁰⁴Pb = 15.701 to 15.920, and ²⁰⁸Pb/²⁰⁴Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ⁸⁷Sr/⁸⁶Sr ratios of sphalerite range from 0.7107 to 0.7136 and (⁸⁷Sr/⁸⁶Sr)_200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.     

Constraints of C–O–S–Pb isotope compositions and Rb–Sr isotopic age on the origin of the Tianqiao carbonate-hosted Pb–Zn deposit,SW China

The Tianqiao Pb–Zn deposit in the western Yangtze Block, southwest China, is part of the Sichuan–Yunnan–Guizhou (SYG) Pb–Zn metallogenic province. Ore bodies are hosted in Devonian and Carboniferous carbonate rocks, structurally controlled by a thrust fault and anticline, and carried about 0.38 million tons Pb and Zn metals grading > 15% Pb + Zn. Both massive and disseminated Pb–Zn ores occur either as veinlets or disseminations in dolomitic rocks. They are composed of ore minerals, pyrite, sphalerite and galena, and gangue minerals, calcite and dolomite. δ³⁴S values of sulfide minerals range from + 8.4 to + 14.4‰ and display a decreasing trend from pyrite, sphalerite to galena (δ³⁴S_pyrite > δ³⁴S_sphalerite > δ³⁴S_galena). We interpret that reduced sulfur derived from sedimentary sulfate (gypsum and barite) of the host Devonian to Carboniferous carbonate rocks by thermal–chemical sulfate reduction (TSR). δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcite range from –5.3 to –3.4‰ and + 14.9 to + 19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid was a mixture origin of mantle, marine carbonate rocks and sedimentary organic matter. Sulfide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.378 to 18.601, ²⁰⁷Pb/²⁰⁴Pb = 15.519 to 15.811 and ²⁰⁸Pb/²⁰⁴Pb = 38.666 to 39.571) that are plotted in the upper crust Pb evolution curve and overlap with that of Devonian to Carboniferous carbonate rocks and Proterozoic basement rocks in the SYG province. Pb isotope compositions suggest derivation of Pb metal from mixed sources. Sulfide minerals have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7125 to 0.7167, higher than Sinian to Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than basement rocks. Again, Sr isotope compositions are supportive of a mixture origin of Sr. They have an Rb–Sr isotopic age of 191.9 ± 6.9Ma, possibly reflecting the timing of Pb–Zn mineralization. C–O–S–Pb–Sr isotope compositions of the Tianqiao Pb–Zn deposit indicate a mixed origin of ore-forming fluids, which have Pb–Sr isotope homogenized before the mineralization. The Permian flood basalts acted as an impermeable layer for the Pb–Zn mineralization hosted in the Devonian–Carboniferous carbonate rocks.     

Initial Pb–Sr(–Nd) isotopic heterogeneity in a single allanite–epidote crystal: implications of reaction history for the dating of minerals with low parent-to-daughter ratios

We analyzed 17 fragments from a zoned allanite–epidote crystal (ca 2.2 mm × 4.0 mm), which had formed during different prograde and retrograde stages of ultra high pressure (UHP) and amphibolite facies metamorphism (240–230 Ma, Sulu Belt, E China), for the isotopic composition of Pb, Nd, and Sr and contents of Pb, U, and Th, Sr and Rb, and Nd and Sm. Since most fragments had ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb values less than 1, corrections for in situ Pb growth are small and uncertainties in the recalculation of the Pb isotopic compositions to 240 Ma are insignificant. The recalculated Pb falls on a linear trend in the ²⁰⁶Pb/²⁰⁴Pb vs ²⁰⁷Pb/²⁰⁴Pb diagram with the allanite defining the low–²⁰⁶Pb/²⁰⁴Pb end (17.07) of this trend and the epidote defining its high–²⁰⁶Pb/²⁰⁴Pb end (17.56). The recalculated data scatter in the ²⁰⁶Pb/²⁰⁴Pb vs ²⁰⁸Pb/²⁰⁴Pb diagram, which implies that the initial Pb isotopic variation reflects the involvement of at least three different Pb sources. The low ⁸⁷Rb/⁸⁶Sr values account for a change in ⁸⁷Sr/⁸⁶Sr by in situ ⁸⁷Sr growth of less than 0.0007, which implies that the isotopic heterogeneity of ⁸⁷Sr/⁸⁶Sr (0.70601–0.7200) is a primary feature. The Pb and Sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product. It is likely that such an initial isotopic heterogeneity also exists for Nd, but it could not be resolved in the present study. Initially heterogeneous Pb and Sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates, especially for minerals with relatively low parent-to-daughter ratios.     

The origin of geochemical diversity of lunar mantle sources inferred from the combined U-Pb, Rb-Sr, and Sm-Nd isotope systematics of mare basalt 10017

The results of our combined U-Pb, Rb-Sr, and Sm-Nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources, as well as the U-Pb isotope systematics of the Moon. The Rb-Sr, Sm-Nd, and ²³⁸U-²⁰⁶Pb isotope systems yield concordant crystallization ages of 3.633 ± 0.057 Ga, 3.678 ± 0.069 Ga, and 3.616 ± 0.098 Ga, respectively. The ²³⁵U-²⁰⁷Pb isochron yields an older, though still concordant, age of 3.80 ± 0.12 Ga. Neither the ²⁰⁶Pb-²⁰⁷Pb system nor U-Pb concordia system yields an age for 10017 that is concordant with the age determined from the Sm-Nd, Rb-Sr, and ²³⁸U-²⁰⁶Pb systems. The initial ⁸⁷Sr/⁸⁶Sr of 10017 is 0.69941 ± 7 and the initial ε_Nd is +3.2 ± 0.4. Initial Pb isotopic compositions, determined from the U-Pb isochrons, are ²⁰⁶Pb/²⁰⁴Pb_i = 31 ± 11 and ²⁰⁷Pb/²⁰⁴Pb_i = 34 ± 15. Together, these initial Pb compositions constrain the μ value of the 10017 source to be 70 ± 30, assuming a single-stage Pb growth model. This is considerably lower than μ values typically estimated for mare basalt sources (∼100-600). Regardless, the μ values calculated for the sources of mare basalts, as well as other lunar samples, show a range that is larger than can be explained by fractionation of U from Pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources. The U-Pb isotope systematics may reflect late-stage formation of a sulfide phase, which strongly fractionates Pb from U but has minimal effect on Rb/Sr or Sm/Nd compositions, during crystallization of the lunar magma ocean.     

H-O-S-Cu-Pb Isotopic Constraints on the Origin of the Nage Cu-Pb Deposit, Southeast Guizhou Province, SW China

The Nage Cu-Pb deposit,a new found ore deposit in the southeast Guizhou province,southwest China,is located on the southwestern margin of the Jiangnan Orogenic Belt.Ore bodies are hosted in slate and phyllite of Neoproterozoic Jialu and Wuye Formations,and are structurally controlled by EW-trending fault.It contains Cu and Pb metals about 0.12 million tonnes with grades of 0.2 wt% to 3.4 wt% Cu and 1.1 wt% to 9.27 wt% Pb.Massive and disseminated Cu-Pb ores from the Nage deposit occur as either veinlets or disseminations in silicified rocks.The ore minerals include chalcopyrite,galena and pyrite,and gangue minerals are quartz,sericite and chlorite.The H-O isotopic compositions of quartz,S-Cu-Pb isotopic compositions of sulfide minerals,Pb isotopic compositions of whole rocks and ores have been analyzed to trace the sources of ore-forming fluids and metals for the Nage Cu-Pb deposit.The δ65CuNBS values of chalcopyrite range from-0.09% to +0.33‰,similar to basic igneous rocks and chalcopyrite from magmatic deposits.δ65CuNBS values of chalcopyrite from the early,middle and final mineralization stages show an increasing trend due to63Cu prior migrated in gas phase when fluids exsolution from magma.δ34SCDT values of sulfide minerals range from 2.7‰ to +2.8‰,similar to mantle-derived sulfur(0±3‰).The positive correlation between δ65CuNBS and δ34SCDT values of chalcopyrite indicates that a common source of copper metal and sulfur from magma.δDH2OSMOW and δ18OH2O-SMOW values of water in fluid inclusions of quartz range from 60.7‰ to 44.4‰ and +7.9‰ to +9.0‰(T=260°C),respectively and fall in the field for magmatic and metamorphic waters,implicating that mixed sources for H2O in hydrothermal fluids.Ores and sulfide minerals have a small range of Pb isotopic compositions(208Pb/204Pb=38.152 to 38.384,207Pb/204Pb=15.656 to 17.708 and 206Pb/204Pb=17.991 to 18.049) that are close to orogenic belt and upper crust Pb evolution curve,and similar to Neoproterozoic host rocks(208Pb/204Pb=38.201 to 38.6373,207Pb/204Pb=15.648 to 15.673 and 206Pb/204Pb=17.820 to 18.258),but higher than diabase(208Pb/204Pb=37.830 to 38.012,207Pb/204Pb=15.620 to 15.635 and206Pb/204Pb=17.808 to 17.902).These results imply that the Pb metal originated mainly from host rocks.The H-O-S-Cu-Pb isotopes tegather with geology,indicating that the ore genesis of the Nage Cu-Pb deposit is post-magmatic hydrothermal type.     

Geology and Isotope Geochemistry of the Dongzhongla Pb–Zn Deposit in Tibet: Implications for the Origin of the Ore‐Forming Fluids and Storage Condition of Certain Metals

As a newly discovered medium‐sized deposit (proven Pb + Zn resources of 0.23 Mt, 9.43% Pb and 8.73% Zn), the Dongzhongla skarn Pb–Zn deposit is located in the northern margin of the eastern Gangdese, central Lhasa block. Based on the geological conditions in this deposit of ore‐forming fluids, H, O, C, S, Pb, Sr, and noble gas isotopic compositions were analyzed. Results show that δ¹⁸O_SMOW of quartz and calcite ranged from −9.85 to 4.17‰, and δD_SMOW ranged from −124.7 to −99.6‰ (where SMOW is the standard mean ocean water), indicating magma fluids mixed with meteoric water in ore‐forming fluids. The δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from −1.4 to −1.1‰ and from 5.3 to 15.90‰, respectively, show compositions consistent with the carbonate limestone in the surrounding rocks, implying that the carbon was primarily sourced from the dissolution of carbonate strata in the Luobadui Formation. The ore δ³⁴S composition varied in a narrow range of 2.8 to 5.7‰, mostly between 4‰ and 5‰. The total sulfur isotopic value δ³⁴S was 4.7‰ with characteristics of magmatic sulfur. The ³He/⁴He values of pyrite and galena ranged from 0.101 to 5.7 Ra, lower than those of mantle‐derived fluids (6 ± 1 Ra), but higher than those of the crust (0.01–0.05 Ra), and therefore classified as a crust–mantle mixed source. The Pb isotopic composition for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of the ores were in the ranges of 18.628–18.746, 15.698–15.802, and 39.077–39.430, respectively, consistent with the Pb isotopic composition of magmatic rocks in the deposit, classified as upper‐crust lead. The ore lead was likely sourced partially from the crustal basement of the Lhasa Terrane. The initial (⁸⁷Sr/⁸⁶Sr)_i value from five sulfide samples ranged from 0.71732 to 0.72767, and associated ore‐forming fluids were mainly sourced from the partial melting of the upper‐crust materials. Pb isotopic compositions of ore sulfides from the Dongzhongla deposit are similar to that of the Yuiguila and Mengya'a deposit, indicating that they have similar sources of metal‐rich ore‐forming solution. According to basic skarn mineralogy, the economic metals, and the origin of the ore‐forming fluids, the Dongzhongla deposit was classified as a skarn‐type Pb–Zn deposit.     

Baseline determination of the atmospheric Pb,Sr and Nd isotopic compositions in the Rhine valley,Vosges mountains (France) and the Central Swiss Alps

The Pb, Sr and Nd isotopic compositions of biomonitors (lichen, moss, bark) and soil litter from different regions in the Rhine valley, as well as of <0.45 μm particles separated out of ice of the Rhône and Oberaar glaciers and lichens from the Swiss Central Alps, have been determined in order to deduce the natural baseline of the atmospheric isotopic compositions of these regions, which are suggested to be close to the isotopic compositions of the corresponding basement rocks or soils at the same sites. ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr isotope ratios are positively correlated. Most polluted samples from traffic-rich urban environments have the least radiogenic Pb and Sr isotopic compositions with ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr ratios of 1.11 and 0.7094, respectively. These ratios are very different from those of the atmospheric baseline for the Vosges mountains and the Rhine valley (²⁰⁶Pb/²⁰⁷Pb: 1.158–1.167; ⁸⁷Sr/⁸⁶Sr: 0.719–0.725; ε_Nd: −7.5 to −10.1). However, this study indicates that the baseline of the atmospheric natural Pb and Sr isotopic compositions is affected by anthropogenic (traffic, industrial and urban) emissions even in remote areas. Lichen samples from below the Rhône and Oberaar glaciers reflect the baseline composition close to the Grimsel pass in the Central Swiss Alps (⁸⁷Sr/⁸⁶Sr: 0.714 − 0.716; ε_Nd: −3.6 to −8.1). The ¹⁴³Nd/¹⁴⁴Nd isotope ratios are highly variable (8ε units) and it is suggested that the variation of the ¹⁴³Nd/¹⁴⁴Nd is controlled by wet deposition and aerosols originating from the regional natural and industrial urban environments and from more distant regions like the Sahara in North Africa. The least anthropogenetically affected samples collected in remote areas have isotopic compositions closest to those of the corresponding granitoid basement rocks.     

K-Ar ages and Pb,Sr isotopic characteristics of Cenozoic volcanic rocks in Shandong Province,China

28 samples of Cenozoic volcanic rocks collected from Shandong Province have been dated by K-Ar method. They are mainly Neogene with an age range of 4–19 m. y. The basalts from Linqu and Yishui in west Shandong Province are Miocene and those from Penglai and Qixia in east Shandong Province are Miocene and Pliocene in age. The basalts from Wudi in north Shandong Province are Middle-Early Pleistocene in age. In each area the duration of volcanic eruption was estimated at about 2–3 m. y. Pb and Sr isotopic compositions and U, Th, Pb, Rb, Sr, and major elements in most of the samples were determined. The isotopic compositions are:²⁰⁶Pb/²⁰⁴Pb—16.92-18.48,²⁰⁷Pb/²⁰⁴Pb—15.30-15.59,²⁰⁸Pb/²⁰⁴Pb—37.83-38.54, and (⁸⁷Sr/⁸⁶Sr)_i—0.70327-0.70632. There are some positive or negative linear correlations between²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb, Pb isotopes and Pb content, Pb isotopes and Sr isotopes, and Sr isotopes and other elements. The basaltic rocks from east and west Shandong Province have somewhat differences in isotopic composition and element content. The basalts probably are products of multi-stage evolution of the mantle. They have preserved the primary features of the source, although they were influenced, to some extent, by the contamination of crustal materials.     

Geothermal waters from the Taupo Volcanic Zone,New Zealand: Li,B and Sr isotopes characterization

Chemical and isotopic data for 23 geothermal water samples collected in New Zealand within the Taupo Volcanic Zone (TVZ) are reported. Major and trace elements including Li, B and Sr and their isotopic compositions (δ⁷Li, δ¹¹B, ⁸⁷Sr/⁸⁶Sr) were determined in high temperature geothermal waters collected from deep boreholes in different geothermal fields (Ohaaki, Wairakei, Mokai, Kawerau and Rotokawa geothermal systems). Lithium concentrations are high (from 4.5 to 19.9 mg/L) and Li isotopic compositions (δ⁷Li) are homogeneous, ranging between −0.5‰ and +1.4‰. In particular, it is noteworthy that, except for the samples from the Kawerau geothermal field having slightly higher δ⁷Li values (+1.4%), the other geothermal waters have a near constant δ⁷Li signature around a mean value of 0‰ ± 0.6 (2σ, n = 21). Boron concentrations are also high and relatively homogeneous for the geothermal samples, falling between 17.5 and 82.1 mg/L. Boron isotopic compositions (δ¹¹B) are all negative, and display a range between −6.7‰ and −1.9‰. These B isotope compositions are in agreement with those of the Ngawha geothermal field in New Zealand. Lithium and B isotope signatures are in a good agreement with a fluid signature mainly derived from water/rock interaction involving magmatic rocks with no evidence of seawater input. On the other hand, Sr concentrations are lower and more heterogeneous and fall between 2 and 165 μg/L. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.70549 to 0.70961. These Sr isotope compositions overlap those of the Rotorua geothermal field in New Zealand, confirming that some geothermal waters (with more radiogenic Sr) have interacted with bedrocks from the metasedimentary basement. Each of these isotope systems on their own reveals important information about particular aspects of either water source or water/rock interaction processes, but, considered together, provide a more integrated understanding of the geothermal systems from the TVZ in New Zealand.     

Geochemical characteristics of stream sediments,sediment fractions,soils, and basement rocks from the Mahaweli River and its catchment,Sri Lanka

Geochemical variations in stream sediments (n = 54) from the Mahaweli River of Sri Lanka have been evaluated from the viewpoints of lithological control, sorting, heavy mineral concentration, influence of climatic zonation (wet, intermediate, and dry zones), weathering, and downstream transport. Compositions of soils (n = 22) and basement rocks (n = 38) of the catchment and those of <180 μm and 180–2000 μm fractions of the stream sediments were also examined. The sediments, fractions, soils and basement rocks were analyzed by X-ray fluorescence to determine their As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc, Fe₂O₃, TiO₂, MnO, CaO, P₂O₅ and total sulfur contents. Abundances of high field strength and ferromagnesian elements in the sediments indicate concentration of durable heavy minerals including zircon, tourmaline, rutile, monazite, garnet, pyriboles, and titanite, especially in <180 μm fractions. The sediments show strong correlation between Ti and Fe, further suggesting presence of heavy mineral phases containing both elements, such as ilmenite and magnetite. The basement rocks range from mafic through to felsic compositions, as do the soils. The river sediments lack ultrabasic components, and overall have intermediate to felsic compositions. Elemental spikes in the confluences of tributary rivers and high values in the <180 μm fractions indicate sporadic inputs of mafic detritus and/or heavy minerals to the main channel. Al₂O₃/(K₂O + Na₂O) and K₂O/Na₂O ratios of the sediments and LOI values of the soils correlate well with the climatic zones, suggesting intense weathering in the wet zone, lesser weathering in the intermediate zone, and least weathering in the dry zone. Low Sr and CaO contents and Cr/V ratios in stream sediments in the wet zone also suggest climatic influence. Fe-normalized enrichment factors (EFs) for As, Pb, Zn, Cu, Ni and Cr in stream sediments in the main channel are nearly all <1.5, indicating there is no significant environmental contamination. The chemistry of the sediments, rocks and the soils in the Mahaweli River are thus mainly controlled by source lithotype, weathering, sorting, and heavy mineral accumulation.     

Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada,California

This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.70597–0.70653; ²⁰⁶Pb/ ²⁰⁴Pb=18.862–19.018; ²⁰⁷Pb/²⁰⁴Pb=15.640–15.686; ²⁰⁸Pb/ ²⁰⁴Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (⁸⁷Sr/⁸⁶=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high ⁸⁷Sr/⁸⁶Sr (71141–0.71240) and low ²⁰⁶Pb/²⁰⁴Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H₂O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.     

Nd,Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (_Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, ⁸⁷Sr/⁸⁶Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (²⁰⁶Pb/²⁰⁴Pb=18.35–18.84, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.97 and ²⁰⁸Pb/²⁰⁴Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.     

Sr,Nd and Pb Isotopic Systematics of the Cenozoic Basalts of the Korean Peninsula and Their Implications for the Permo-Triassic Continental Collision Boundary

Sr, Nd and Pb isotopic compositions of the Cenozoic basalts were analyzed from Baengnyeongdo Island, Jeongok, Ganseong, and Jejudo Island of Korea. They reveal relatively enriched Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.70330∼0.70555, ¹⁴³Nd/¹⁴⁴Nd = 0.51298∼0.51256) compared with MORB.²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of all the analyzed Korean basalts lie above the Northern Hemisphere Reference Line (NHRL) defined by Hart (1984). Pb isotopic compositions of basalts from Jejudo Islands (²⁰⁶Pb/²⁰⁴Pb = 18.61∼19.12, ²⁰⁷Pb/²⁰⁴Pb = 15.54∼15.69, ²⁰⁸Pb/²⁰⁴Pb = 38.98∼39.72) are significantly more radiogenic than the rest (²⁰⁶Pb/²⁰⁴Pb = 17.72∼18.03, ²⁰⁷Pb/²⁰⁴Pb = 15.44∼15.58, ²⁰⁸Pb/²⁰⁴Pb = 37.77∼38.64). The Cenozoic Korean basalts thus can be divided into two groups based on their Sr, Nd and Pb isotopic compositions. The north group reveals mixing between DMM and EM1 while the south group displays DMM-EM2 mixing. Such a distribution is the same as Chinese Cenozoic basalts and it can be interpreted that the subcontinental lithospheric mantle under Korea represents simple lateral continuation of the South and North China Blocks. We suggest that Korean continental collision zone cross the Korean Peninsula through the region between the north and south basalt groups of Korea.     

Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (δ¹⁸O_olivine = 5.17 ± 0.08‰; ⁸⁷Sr/⁸⁶Sr = 0.7029 to 0.7031; ε_Nd = +5.7 to +7.1; ¹⁸⁷Os/¹⁸⁸Os = 0.1481 to 0.1750; ²⁰⁶Pb/²⁰⁴Pb = 19.1 to 19.7; Pd/Ir = 6 ± 3) versus those from La Palma (δ¹⁸O_olivine = 4.87 ± 0.18‰; ⁸⁷Sr/⁸⁶Sr = 0.7031 to 0.7032; ε_Nd = +5.0 to +6.4; ¹⁸⁷Os/¹⁸⁸Os = 0.1421 to 0.1460; ²⁰⁶Pb/²⁰⁴Pb = 19.5 to 20.2; Pd/Ir = 11 ± 4) are revealed from the dataset.Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (?110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai’ian tholeiites.The HIMU affinities of the lavas, in conjunction with the low δ¹⁸O_olivine and high ²⁰⁶Pb/²⁰⁴Pb for La Palma, and elevated ¹⁸⁷Os/¹⁸⁸Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic ²⁰⁶Pb/²⁰⁴Pb, ¹⁸⁷Os/¹⁸⁸Os, elevated Re/Os and Pt/Os, and low-δ¹⁸O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50 km).     

Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania

 Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb isotopic compositions. The ²⁰⁶Pb/²⁰⁴Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria vary from insignificant in the case of nepheline, to Δ ⁸⁷Sr/⁸⁶Sr of 0.0002 and Δɛ_Nd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data to identify source reservoirs. However, mass balance calculations reveal that the ²⁰⁶Pb/²⁰⁴Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination. Received: 15 February 1995 / Accepted: 4 May 1996     

黔西南紫木凼金矿床成矿物质来源:S⁃C⁃O⁃Pb⁃Sr同位素制约

紫木凼金矿床是黔西南卡林型金矿区一个重要的大型金矿床,其成矿物质来源尚不明确.对紫木凼金矿床不同类型矿石和赋矿围岩进行了S、C、O、Pb和Sr同位素组成对比研究.矿石中硫化物的δ34S值为-13.49‰~17.91‰（主要为-0.99‰~3.58‰）,赋矿围岩的δ34S值为-26.23‰~-19.63‰,矿床成矿期硫主要来源于岩浆,部分来源于赋矿地层中成矿前黄铁矿.热液期方解石的δ13C和δ18O分别为-9.10‰~0.59‰和15.65‰~23.82‰,与赋矿围岩、区域地层的碳、氧同位素组成差别较大,成矿流体的碳、氧部分来源于碳酸盐岩溶解,部分可能来源于岩浆.矿石中硫化物的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值分别为18.064~18.973、15.585~15.670和38.219~39.054,赋矿围岩的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值分别为18.136~18.650、15.574~15.656和38.423~38.812,矿石铅的来源较复杂,赋矿地层和岩浆可能都为其提供了部分铅.矿石中石英和方解石（87Sr/86Sr）_i比值为0.707 26~0.708 11,赋矿围岩的（87Sr/86Sr）_i比值为0.707 28~0.707 31,成矿流体中的锶主要来源于赋矿地层.紫木凼金矿床成矿物质具壳幔混合来源特征,成矿物质主要来自矿床深部隐伏岩浆岩,部分来自二叠系-三叠系赋矿地层.