高精度铅同位素比值MC-ICP-MS测试方法及岩石标样铅同位素组成

利用多接收电感耦合等离子体质谱仪（Neptune plus）建立了高精度铅同位素比值MC-ICP-MS测试方法。建立该方法的过程中,重点评估了加入的Tl标准溶液对铅同位素比值测试结果的影响,并最终确定Tl标准溶液的浓度为25 ng/mL,同时样品溶液Pb的浓度应该大于25 ng/mL（即Pb/Tl浓度比应大于1）。利用该方法对铅同位素标准物质SRM 981进行了长期监控（2020年7月—2021年6月）,测试结果为：206Pb/204Pb=16.9415±0.0010、207Pb/204Pb=15.4985±0.0009、208Pb/204Pb=36.7204±0.0023,与统计的文献报道值一致。长期监控的全流程空白均小于0.25 ng,能满足地质样品高精度铅同位素比值测试的需要。同时运用该方法,对4个元素含量标样（BCR-2、AGV-2、BHVO-2和BIR-1a）进行了铅同位素比值测试,测试结果与文献报道的测试结果和精度一致,表明建立的方法是准确、可靠的。     

Germanium isotope measurements of high-temperature geothermal fluids using double-spike hydride generation MC-ICP-MS

We present a double-spike isotope dilution MC-ICP-MS technique for the determination of germanium (Ge) isotope fractionation. Using this technique we determined Ge isotope compositions of geothermal spring fluids, a Columbia River Basalt sample, and an in-house diatom standard. Our technique uses a ⁷³Ge/⁷⁰Ge double spike in combination with hydride generation for Ge extraction from the sample matrix. Fractionation is determined on the ⁷⁴Ge/⁷²Ge mass ratio. The double spike allows us to effectively correct analytical isotope fractionation. Our external standard reproducibility is 0.4‰ (2 SD) over the course of several months. The minimum quantity of Ge needed for isotope analysis is approximately 2 ng. Consistent with previous work on geothermal fluids, Ge in the geothermal spring samples presented here is enriched over Si as compared to low temperature weathering signatures. This observation is typically interpreted as Ge exclusion during silicate mineral precipitation (e.g., quartz). Our isotope results indicate that the analyzed high temperature fluids fractionate Ge isotopes with a range in δ⁷⁴Ge between −0.4‰ and −1.4‰ relative to a Columbia River basalt. We cautiously interpret the observed fractionation as preferential removal of heavy Ge isotopes out of solution during cooling of the hydrothermal fluid and subsequent precipitation of quartz.     

Deciphering petrogenic processes using Pb isotope ratios from time-series samples at Bezymianny and Klyuchevskoy volcanoes,Central Kamchatka Depression

The Klyuchevskoy group of volcanoes in the Kamchatka arc erupts compositionally diverse magmas (high-Mg basalts to dacites) over small spatial scales. New high-precision Pb isotope data from modern juvenile (1956–present) erupted products and hosted enclaves and xenoliths from Bezymianny volcano reveal that Bezymianny and Klyuchevskoy volcanoes, separated by only 9 km, undergo varying degrees of crustal processing through independent crustal columns. Lead isotope compositions of Klyuchevskoy basalts–basaltic andesites are more radiogenic than Bezymianny andesites (²⁰⁸Pb/²⁰⁴Pb = 37.850–37.903, ²⁰⁷Pb/²⁰⁴Pb = 15.468–15.480, and ²⁰⁶Pb/²⁰⁴Pb = 18.249–18.278 at Bezymianny; ²⁰⁸Pb/²⁰⁴Pb = 37.907–37.949, ²⁰⁷Pb/²⁰⁴Pb = 15.478–15.487, and ²⁰⁶Pb/²⁰⁴Pb = 18.289–18.305 at Klyuchevskoy). A mid-crustal xenolith with a crystallization pressure of 5.2 ± 0.6 kbars inferred from two-pyroxene geobarometry and basaltic andesite enclaves from Bezymianny record less radiogenic Pb isotope compositions than their host magmas. Hence, assimilation of such lithologies in the middle or lower crust can explain the Pb isotope data in Bezymianny andesites, although a component of magma mixing with less radiogenic mafic recharge magmas and possible mantle heterogeneity cannot be excluded. Lead isotope compositions for the Klyuchevskoy Group are less radiogenic than other arc segments (Karymsky—Eastern Volcanic Zone; Shiveluch—Northern Central Kamchatka Depression), which indicate increased lower-crustal assimilation beneath the Klyuchevskoy Group. Decadal timescale Pb isotope variations at Klyuchevskoy demonstrate rapid changes in the magnitude of assimilation at a volcanic center. Lead isotope data coupled with trace element data reflect the influence of crustal processes on magma compositions even in thin mafic volcanic arcs.     

长石中Pb同位素的LA-MC-ICPMS测定

单矿物的微区原位Pb同位素分析是微区原位分析技术一个重要的研究方向,能够得到矿物晶体间甚至晶体本身不同区域同位素组成的变化信息.本文采用LA-MC-ICPMS对三件标准样品和两件长石样品进行了微区原位测试,标准样品结果在误差范围内与推荐值一致,两件长石样品测试结果在误差范围内与溶液法测试结果一致,测试精度达到1‰以内.     

Hafnium isotope results from mid-ocean ridges and Kerguelen

¹⁷⁶Hf/¹⁷⁷Hf ratios are presented for oceanic volcanics representing both extremes of the range of mantle HfNdSr isotopic variation. Hf from critical mid-ocean ridgebasalts shows that ¹⁷⁶Hf/¹⁷⁷Hf does indeed have a greater variability than ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr in the depleted mantle. This extra variation is essentially of a random nature, and can perhaps be understood in terms of known Rb/SrSm/NdLu/Hf fractionation relationships. At the other extreme of mantle isotopic composition, ¹⁷⁶Hf/¹⁷⁷Hf ratios for igneous rocks from the Indian Ocean island of Kerguelen show a closely similar variation to published ¹⁴³Nd/¹⁴⁴Nd ratios for the same samples. Comparison of HfNdSr c relationships for Tristan da Cunha, Kerguelen and Samoa reveals divergences in the mantle array for ocean island magma sources, and perhaps suggests that these irregularities are largely the result of an extra component of ⁸⁷Sr/⁸⁶Sr variation.     

氩同位素用于库车坳陷天然气主力气源岩判识

稀有气体是天然气中指示其地球化学特征的重要组分,放射成因氩( Arr)的年代积累效应广泛应用于同位素定年和天然气的气源对比.本文系统地讨论了 40Ar的形成机理及其运移进入气藏的过程,利用 40Ar的母体元素 K在煤和煤系泥岩中的丰度差异,分析了以煤和煤系泥岩为母源的天然气 40Ar/36Ar同位素组成上的不同,并利用这种差异对煤系中煤岩或煤系泥岩为源岩的天然气藏的主力源岩进行了判识,继而建立了利用氩同位素组成判识煤系烃源岩主力源岩的方法.利用此方法对塔里木盆地库车坳陷三叠系-侏罗系煤与煤系泥岩互层的烃源岩进行了探讨, 数学计算结果表明该区主力源岩为煤系泥岩, 煤对天然气藏的贡献较小.     

Determination of the extent of anthropogenic Pb migration through fractured sandstone using Pb isotope tracing

The extent of vertical migration of anthropogenic Pb beneath a medieval smelting site in Derbyshire, U.K. has been estimated using the determination of total Pb concentrations and ²⁰⁶Pb/²⁰⁷Pb isotope ratio from samples taken down 6 m of drill core. Preliminary studies of total Pb concentrations established that the surface slag derived from the smelting contained up to 16% Pb and that the normal background levels in uncontaminated sandstone were 10±2 ppm. Sample analyses beneath the site revealed elevated Pb concentrations in fracture infill clays (270 ppm Pb) and sandstone (76–83 ppm Pb). Both are well above the background Pb concentration.Lead isotope analysis of the slag wastes, the underlying contaminated sandstone and fracture infill has shown that all 3 contain very similar isotope ratios for ²⁰⁶Pb/²⁰⁷Pb (1.1802–1.1820). However, matched control sandstone samples show that the background ²⁰⁶Pb/²⁰⁷Pb isotope ratio (1.1670 ± 0.003) is distinctly different. This would indicate that both the sandstone and fracture infill underlying the historical smelting site contain a substantial proportion of Pb that has been derived from the overlying contamination.The application of total Pb concentrations along the core and isotope analysis suggest that anthropogenically derived Pb from the smelting site (that was operated between 665 and 445 a BP) has migrated to a depth of 4.50 m. Assuming a uniform migration rate and a mean time of migration of 555 a, then the mean migration rate is estimated to be 8 ± 2 mm/a.The proportion of natural versus anthropogenic Pb in the samples has been estimated from small variations in the ²⁰⁶Pb/²⁰⁷Pb isotope ratio. If the slag is considered to contain 100% anthropogenic Pb and the uncontaminated sandstone considered to contain 100% natural Pb, the linear interpolation can be applied between the 2 end members of the isotope ratio. The use of this approach to the ²⁰⁶Pb/²⁰⁷Pb ratio measurements has shown that 88% of the Pb in the contaminated sandstone (i.e. 69 ppm from a mean total Pb concentration of 78.5 ppm) has been derived from the anthropogenic Pb at the surface. For the fracture infill sample taken at a depth of 4.50 m, and with a total Pb concentration of 270 ppm, the % of Pb that has been derived from the slag wastes is approximately 98% (equivalent to 265 ppm Pb). The remaining Pb in both these samples (9.4 and 5 ppm, respectively) is deduced to have originated from the natural background concentration of Pb in the sandstone.The closeness of these estimates to the measured background concentration, suggests that a simple two-source model of Pb contamination is valid for this site.     

Mantle structure and dynamics under East Russia and adjacent regions

We present seismic images of the mantle beneath East Russia and adjacent regions and discuss geodynamic implications. Our mantle tomography shows that the subducting Pacific slab becomes stagnant in the mantle transition zone under Western Alaska, Bering Sea, Sea of Okhotsk, Japan Sea, and Northeast Asia. Many intraplate volcanoes exist in these areas, which are located above the low-velocity zones in the upper mantle above the stagnant slab, suggesting that the intraplate volcanoes are related to the dynamic processes in the big mantle wedge above the stagnant slab and the deep slab dehydration. Teleseismic tomography revealed a low-velocity zone extending down to 660 km depth beneath the Baikal rift zone, which may represent a mantle plume. The bottom depths of the Wadati–Benioff deep seismic zone and the Pacific slab itself become shallower toward the north under Kamchatka Peninsula, and the slab disappears under the northernmost Kamchatka. The slab loss is considered to be caused by the friction between the slab and the surrounding asthenosphere as the Pacific plate rotated clockwise at about 30 Ma ago, and then the slab loss was enlarged by the slab-edge pinch-off by the hot asthenospheric flow and the presence of Meiji seamounts.     

Calcium isotope ratios in animal and human bone

Calcium isotopes in tissues are thought to be influenced by an individual’s diet, reflecting parameters such as trophic level and dairy consumption, but this has not been carefully assessed. We report the calcium isotope ratios (δ^44/42Ca) of modern and archaeological animal and human bone (n = 216). Modern sheep raised at the same location show 0.14 ± 0.08‰ higher δ^44/42Ca in females than in males, which we attribute to lactation by the ewes. In the archaeological bone samples the calcium isotope ratios of the herbivorous fauna vary by location. At a single site, the archaeological fauna do not show a trophic level effect. Humans have lower δ^44/42Ca than the mean site fauna by 0.22 ± 0.22‰, and the humans have a greater δ^44/42Ca range than the animals. No effect of sex or age on the calcium isotope ratios was found, and intra-individual skeletal δ^44/42Ca variability is negligible. We rule out dairy consumption as the main cause of the lower human δ^44/42Ca, based on results from sites pre-dating animal domestication and dairy availability, and suggest instead that individual physiology and calcium intake may be important in determining bone calcium isotope ratios.     

Lead isotope variability in olivine-hosted melt inclusions from Iceland

The lead isotope and trace element compositions of a suite of olivine-hosted melt inclusions in primitive lava flows from the Reykjanes Peninsula in Iceland show extreme variability. Much of this variability is present in the composition of inclusions from one hand specimen of Háleyjabunga, a depleted picrite lava shield that erupted 13 ka. ²⁰⁸Pb/²⁰⁶Pb compositions in this sample span 50-90% of the total range found in Atlantic MORB, indicating that high-amplitude compositional heterogeneity is present in the mantle source of melts that aggregated to form a single eruption. The trace element and isotopic trends in the melt inclusions are coincident with those in whole rock samples from young lava flows of the Reykjanes Peninsula, and extend the total range of variation towards more depleted compositions. The incompatible trace element and lead isotope compositions of the inclusions are strongly coupled and lie close to binary mixing trends between the extreme melt inclusion compositions. These relationships indicate that the trace element variation in the melt inclusions reflects heterogeneity in the composition of the mantle source entering the melting region under the Reykjanes Peninsula. Large positive Sr concentration anomalies are present in three of the inclusions, but do not correlate with indicators of mantle melting or source variations and are likely to arise by reaction with plagioclase during crustal storage. Fractional melting of heterogeneous mantle is predicted to generate melts with a wide range of compositions, filling a large volume in trace element-isotope space. However, the compositional variations observed in the melt inclusions lie close to binary mixing curves. These observations may be accounted for by a two-stage model of melt mixing. The first stage occurs in porous channels that transport melt in the mantle and takes place before inclusion entrapment. This mixing stage generates a bimodal distribution of melt compositions that is supplied from the channels to sub-Moho and lower crustal magma lenses. The second stage of mixing occurs in these chambers, producing the binary mixing trends recorded in the inclusion compositions. The distribution of isotopic compositions observed in the melt inclusions and whole rock samples from the Reykjanes Peninsula is therefore controlled by melt mixing. These results have important implications for the interpretation of basalt composition in terms of distinct compositional entities within the upwelling solid mantle under mid-ocean ridges and ocean islands.     

Determination of isotope ratios in the natural gas components CH4 and N2 separated by gas chromatography

To determine stable-isotope ratios of natural gas components a system is constructed to separate CH₄ and N₂ from milliliter quantities of natural gas by gas chromatography. Having passed through the separation column and a catharometer detector the gas flow is distributed consecutively into two lines, one in which N₂ is trapped in a helium cryostat at 4.2 K and one in which CH₄ is immediately combusted by CuO to CO₂ and H₂O which are trapped separately.The δ ²H-, δ ¹³C- and δ ¹⁵N-values are determined by conventional isotope mass spectrometry.     

Oxygen isotope ratios in magnetite core-and-mantle structures

In a polymetamorphic, felsic, biotite-bearing gneiss, biotite has reacted to form magnetite and microcline. The resulting structure is a magnetite core surrounded by a mantle of feldspar and quartz normally not exceeding 20mm in diameter. Measurements of oxygen isotope ratios disclose disequilibrium between mantle microcline and mantle quartz and also between mantle and matrix minerals of the same species. A clustering of temperature estimates from the oxygen isotope distribution between magnetite and quartz and between magnetite and microcline in the interval 550 to 600°C suggests an approach to oxygen isotope equilibrium. No signs of a re-equilibriation of the reacting biotite can be found.     

Mantle Degassing and Diamond Genesis：A Carbon Isotope Perspective

The effect of Co2 and CH4 degassing from the mantle on the carbon isotopic composition of diamond has been quantitatively modeled in terms of the principles of Rayleigh distillation.Assuming the δ^13 C value of -5‰ for the mantle,the outgassing of CO2 can result in the large negative δ^13 C values of diamond,whereas the outgassing of CH4 can drive the δ^13C values of diamond in the positive direction.The theoretical expectations can be used to explain the full range of δ^13 C values from-34.4‰5 to 5‰ observed for natural diamonds.It is possible that diamond formation was triggered by the degassing of Co2 and/or CH4 from the mantle and the associated fractional crystallization of carbonate-bearing melt.     

Hydrogen and oxygen isotope ratios in nodular and bedded cherts

Hydrogen and oxygen isotopic compositions of cherts (δD for hydroxyl hydrogen in the chert, δ¹⁸O for the total oxygen) have been determined for a suite of samples from the central and western United States. When plotted on a δD-δ¹⁸O diagram, Phanerozoic cherts define domains parallel to the meteoric water line which are different for different periods of geologic time. The elongation parallel to the meteoric water line suggests that meteoric waters were involved in the formation of many cherts.The existence of different chert δ-values for different geologic times indicates that once the granular microcrystalline quartz of cherts crystallizes its isotopic composition is preserved with time. An explanation for the change with time of the isotopic composition of cherts involving large changes with time in the isotopic composition of ocean water is unlikely since δ¹⁸O of the ocean would have had to decrease by about 3‰between Carboniferous and Triassic time and then increase about 5%.` from Triassic to Cretaceous time. Such isotopic changes cannot be accounted for by extensive glaciation, sedimentation of hydrous minerals, or input of water from the mantle into the oceans.The variation with time of the chert δ-values can be satisfactorily explained in terms of past climatic temperature fluctuations if the chert-water isotope fractionation with temperature is approximated by 1000 lnα = 3.09 × 10⁶T^?2 – 3.29. Crystallization temperatures so inferred suggest that the average climatic temperatures for the central and western U.S. decreased from about 34 to 20°C through the Paleozoic, increased to 35–40°C in the Triassic, and then decreased through the Mesozoic to Tertiary values of about 17°C. A few data for the Precambrian suggest the possibility that Earth surface temperatures may have reached about 52°C at 1.3 b.y. and about 70°C at 3 b.y.     

Determining the source of felsitic lithic material in southeastern New England using neodymium isotope ratios

We introduce the use of Nd isotopes as a high‐precision technique that can be used in conjunction with visual, petrographic, and trace element analyses to source indistinct felsitic debitage. This approach allows sourcing of debitage from southeastern New England, including samples that previously could not be constrained to one source. Combined with trace element data, isotopic data also provide the possibility of sourcing debitage to a particular quarry site within a volcanic complex, and even to a particular ash flow within a quarry site. Determining the origin of debitage so precisely is important for understanding acquisition, trade, and exchange networks in southeastern New England, where distances separating quarry sites within an individual volcanic complex (Lynn–Mattapan) are greater than distances between different volcanic complexes (Lynn–Mattapan, Blue Hill, and Wamsutta). The sourcing of debitage to Mattapan quarries suggests that Middle Archaic populations in the northwest part of the Boston Basin obtained nonlocal lithic material primarily from sources south of the basin. © 2000 John Wiley & Sons, Inc.     

Tracing the decomposition of dissolved organic carbon in artificial groundwater recharge using carbon isotope ratios

Reducing the concentration of dissolved organic C (DOC) in water is one of the main challenges in the process of artificial groundwater recharge. At the Tuusula waterworks in southern Finland, surface water is artificially recharged into an esker by pond infiltration and an equal amount of groundwater is daily pumped from the aquifer. This groundwater study was conducted to consider the role of redox processes in the decomposition of DOC. The isotopic composition of dissolved inorganic C (δ¹³C_DIC) in the recharged water was used as a tracer for redox reactions. The isotopic composition of O and H in water was determined in order to calculate mixing ratios between the local groundwater and the infiltrated surface water. Three distinct processes in the reduction of the DOC content were traced using isotopic methods and concentration analyses of DIC and DOC: (1) the decomposition of DOC, (2) adsorption of DOC on mineral matter, and (3) the dilution of artificially recharged water by mixing with local groundwater. The largest decrease (44%) in the DOC content occurred during the early stage of subsurface flow, within 350 m of the infiltration ponds. The reduction of DOC was accompanied by an equal increase in DIC and a significant drop in δ¹³C_DIC. This change is attributed to the oxidative decomposition of DOC. A further 23% decrease in DOC is attributed to adsorption and a final drop of 14% to dilution with local groundwater.     

High-precision Pb isotope analysis by multicollector-ICP-mass-spectrometry using 205Tl/203Tl normalization: Optimization and calibration of the method for the studies of Pb isotope variations

The development of the MC-ICP-MS method, which was launched about one decade ago and was largely stimulated by the need to solve geological problems, has opened a new avenue in isotope mass spectrometry. One of the advantages of this method is the possibility of applying a newly developed approach to the correction of analytical results for the effect of mass discrimination by normalizing the measured isotope ratios of an element to a reference (standard) isotope ratio of another element. This makes it possible to overcome the main disadvantage of conventional thermal ionization mass spectrometry (TIMS), in which the effect of mass discrimination cannot be fully taken into account during isotope analysis, and thus to implement a highly accurate method for the analysis of Pb-isotope composition. In application to the capability of the NEPTUNE MC-ICP mass spectrometer, we optimized and calibrated a method for high-accuracy Pb isotope analysis in solutions spiked with Tl, with all currently measured Pb-isotope ratios normalized to the standard ²⁰⁵Tl/²⁰³Tl ratio (TLN-MC-ICP-MS). The factors affecting the random and systematic analytical errors were examined, and the optimal operating regime and analytical conditions were determined. Much attention was paid to the correlation of the measurement results and the mass discrimination effect determined from the ²⁰⁵Tl/²⁰³Tl ratio. The value of the ²⁰⁵Tl/²⁰³Tl normalizing ratio was analytically determined through isotope analyses of the NIST SRM 981, and SRM 982 standard samples of Pb-isotope composition. The data obtained for two mixtures Tl + Pb (SRM 982) and Tl + Pb (SRM 981) in ten replicate analyses were 2.38898 ± 12 and 2.38883 ± 20, respectively. These results are in good mutual agreement, and their general mean ²⁰⁵Tl/²⁰³Tl = 2.3889 ± 1 coincides (within the error) with the recently published values of 2.3887 ± 7 [Collerson et al., 2002] and 2.3889 ± 1 [Thirlwall, 2002]. The precision of the method (±2SD), which was assayed by the long-term reproducibility of the results of replicate analyses of SRM 981 and seven galena samples (90 analyses) was 0.016–0.018% for the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios and 0.005 and 0.009% for the ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios, respectively. The precision of the isotope analysis of common Pb was significantly improved (by factors of 6–10 for various isotope ratios) compared with the precision of TIMS techniques acceptable in isotope studies during three decades. The described method was applied to examine the Pb-isotope composition of approximately 250 samples of galena, scheelite, and pyrite from a number of well known (including large) gold, sulfied, and base-metal deposits. The precision of the method (0.01–0.02%) makes it possible to study small inter-and intra-phase differences in Pb-isotope ratios in hydrothermal and magmatic rocks, to assay the scale of regional and variations in the isotope composition of ore Pb, and to correlate the Pb-isotope composition of rocks and ores and reveal its evolutionary trends.     

伟晶岩中不同副矿物U-Pb同位素定年和示踪的问题与应用

定年和示踪一直是伟晶岩成岩成矿过程和稀有金属富集机制研究的关键问题。副矿物不仅是伟晶岩中稀有稀土元素的重要载体,还蕴含丰富的微量元素并常常具有较高的U- Th含量,是研究伟晶岩年代学、成岩成矿过程和物质源区的“理想探针”。伟晶岩中常用的适合于U- Pb同位素定年的副矿物有锆石、铌钽铁矿、独居石、锡石、榍石、褐帘石、磷钇矿和磷灰石等。由于封闭温度、矿物学特性和不同性质流体中元素行为的差异,伟晶岩中不同副矿物的U- Pb系统常表现出复杂的年龄谱系,可能记录了伟晶岩中潜在的后期地质过程,如：自交代、后期变质与流体改造等。因此,基于前期光学显微镜、扫描电镜、冷阴极发光、激光拉曼光谱分析等矿物微观结构研究,对不同期次或世代的副矿物进行原位微区U- Pb定年及主微量元素和同位素地球化学分析,对于全面认知多期地质事件和伟晶岩成岩成矿过程演化历史,进而更准确地构建其构造- 岩浆- 热液- 成矿作用时空框架具有重要的科学意义。