磷灰石(U-Th)/He定年技术影响因素研究

磷灰石(U-Th)/He定年作为一种有效的低温热年代学定年技术,现已被广泛应用于地质研究的各个领域,而该技术测定的样品年龄会受到多方面因素的影响,其中磷灰石性状的影响尤为突出。通过归纳总结目前国内外磷灰石(U-Th)/He定年研究中,样品性状和成分、磷灰石颗粒尺寸及形貌、包裹体、147Sm含量、辐射损伤、U和Th不均匀分布等性状因素影响的研究进展,研究这些因素对测定年龄的影响及降低影响的方法。     

锆石(U-Th)/He定年技术研究

锆石(U-Th)/He定年是同位素热年代学体系中重要的定年手段,是记录地质体完整热历史重要的实验方法之一,在火山岩定年、造山带演化、地貌演化、沉积盆地热演化及限定矿床热液活动时代等应用方面发挥着重要的作用。中国地质科学院地质研究所同位素热年代学实验室成功建立了锆石(U-Th)/He定年的实验方法,实验主要包括三个独立的过程:样品前处理、He含量分析和U、Th含量分析。氦同位素质谱仪对锆石样品的氦同位素比值分析精度约为0.1%左右;ICP-MS对锆石的U、Th同位素比值分析精度通常情况下优于1%。对国际上普遍使用的FCT锆石和斯里兰卡锆石标样进行(U-Th)/He年龄测定。测试结果显示27粒FCT锆石(U-Th)/He年龄分布在25.81~30.72Ma之间,加权平均年龄为28.18±0.51Ma(1σ)(参考值为28.3±2.6Ma);20粒斯里兰卡锆石碎片(U-Th)/He年龄分布在445.5~489.5Ma之间,排除异常值后加权平均值为479.0±8.0Ma(1σ)(参考值为470±11Ma)。所测两个标准物质的年龄均与参考值一致,表明本实验室的实验流程准确可靠。本实验方法的建立填补了我国锆石(U-Th)/He定年实验方法的空白,为我国热年代学的发展提供了新的技术支撑。     

国际标样Durango磷灰石(U-Th)/He年龄测定

人们认识到可以用放射性成因4He对矿物进行定年已经有一百年的历史,但是利用富含U、Th的矿物进行(U-Th)/He定年是近三十年来快速发展的一种低温(热)年代学方法。由于U-Th-He同位素体系的封闭温度低(磷灰石4He的封闭温度为~75℃),该方法极大的拓展了中低温热年代学研究(如40Ar/39Ar,裂变径迹等)的温度范围下限,已经被广泛应用于浅表地质过程的研究中。Durango磷灰石是国际上广泛使用的磷灰石(U-Th)/He定年的标准样品,准确测定其年龄可以对实验方法及流程的可靠性进行验证。中国科学院地质与地球物理研究所(U-Th)/He定年实验室建立于2013年,经过一年多的实验与摸索,我们建立了完整、可行的化学分析流程和仪器测试流程,并采用Durango国际标样进行了流程验证。重复测定了4批共40个Durango磷灰石颗粒,40个年龄结果分布在28.95~34.11Ma之间,全部年龄的概率分布峰值为31.61±2.7Ma,与国际标定值在误差范围内一致;Th/U比范围为16.43~23.72,与国际报道值一致,表明我们所建立的实验流程准确可行,实验室已经可以稳定运行。     

矿物中4He同位素提取和含量测定研究

(U-Th)/He定年是一种有效的低温热年代学定年技术,现已被广泛应用于地质研究的各个领域,而矿物中4He同位素的有效提取和含量准确测定是该技术的关键。磷灰石和锆石是(U-Th)/He定年最常用的矿物,其4He提取条件及铀钍含量测定方法都较为成熟;而其他矿物(如磁铁矿、橄榄石、针铁矿、石榴子石等)的研究则相对较少。文章介绍了当前国内外(U-Th)/He研究中采用的4He同位素提取方法———真空炉加热法和激光加热法,激光加热法因具有低4He背景值和耗时短的优点而成为主要的提取方法。以磷灰石样品测试为例,介绍了成都地质矿产研究所建立的采用激光加热法和四极杆质谱提取4He同位素及其含量测量过程、含量计算和校正方法。指出未来(U-Th)/He测试技术除继续改进现有分析方法外,应加强对更多不同矿物的测试研究。     

TIMS测定铀矿石样品中^234U/^238U、^230Th/^232Th、^228Ra/^226Ra的方法研究

研究了TIMS测定铀矿石样品中234U/238U、230Th/232Th、228Ra/226Ra的方法。建立了铀矿石密闭混酸一次溶样的方法和采用阴离子、阳离子和Sr特效树脂逐级离子交换分离纯化U、Th和Ra的流程,满足了TIMS测量要求。测定结果表明：100～1000 ng的天然铀中234U/238U,其测量精密度从静态多接收的2.34%提高到动态多接收的0.47%;对230Th与232Th丰度接近、质量为1μg左右的钍,采用三带点样技术和法拉第多接收技术测定230Th/232Th,其内精度平均值为0.0048%,外精度为0.028%;采用单带加钽发射剂,ETP跳峰测定50～100 fg镭-228稀释剂中的228Ra/226Ra,其内精度小于0.10%,外精度小于0.20%。比较TIMS和HR-ICP-MS、α能谱法测定234U/238U、230Th/232Th、228Ra/226Ra结果,三者结果相吻合。TIMS测量法样品用量少、快速、准确、精密度高,是U、Th、Ra同位素比值测定方法的又一补充。     

Fish Canyon Tuff榍石He扩散及(U- Th)/He年代学研究

榍石富含U和Th,是(U-Th)/He定年的理想矿物之一。本文以Fish Canyon Tuff榍石为例,开展了榍石He扩散行为和榍石(U-Th)/He定年实验方法研究。榍石分步加热扩散实验结果表明He扩散系数ln(D/a²)与温度倒数呈负相关,与期望的热活化扩散过程一致。测试Fish Canyon Tuff榍石(U-Th)/He年龄分布在28.3～24.6 Ma之间,平均值为26.7±1.2 Ma (1σ),Th/U分布在4.6～5.5之间,平均值为5.2±0.2,在误差范围内与国际上已出版数据一致,表明建立的榍石(U-Th)/He定年实验方法可靠。本次测试15粒榍石碎片外表层(～20μm)存在不同程度的磨蚀(即不完整晶体),且榍石表层磨蚀厚度随着等效半径的增加而增大。榍石碎片(U-Th)/He年龄介于完整晶体(U-Th)/He年龄和真实(U-Th)/He年龄之间,且随着榍石等效半径及表层磨蚀厚度(<20μm)的增大,(U-Th)/He年龄更接近真实年龄,这表明榍石(U-Th)/He年龄不确定度与等效半径大小和表层磨蚀厚度有关。     

LA-MC-ICPMS分析古老熔体包裹体Pb同位素组成中的误差评价

目前应用LA-MC-ICPMS分析熔体包裹体Pb同位素,由于没有同时测试U和Th的信号,导致熔体包裹体Pb同位素的研究仅局限于中生代以来的样品。本文应用LA-MC-ICPMS分析了玻璃样品以及存在显著U-Th衰变影响的古老熔体包裹体的Pb同位素组成,评价了U/Pb和Th/Pb分析误差对初始Pb同位素比值校正的影响。实验中以国际玻璃标样NKT-1G为外部标样,采用"标样-样品-标样法"进行仪器漂移和质量歧视校正,结果表明,国际玻璃标样BHVO-2G、TB-1G的208Pb/206Pb和207Pb/206Pb分析精度优于0.30%(2RSD),与推荐值的偏差小于0.30%,然而232Th/206Pb和238U/206Pb分析结果显示了较大分散性(外精度约5.0%)。根据误差传递计算,样品的年龄对初始铅的误差有很大影响。对于古生代以来的样品(年龄小于540 Ma),即使测试的232Th/206Pb和238U/206Pb与真值偏差达到10%,经过U-Th衰变校正后的Pb同位素比值与真值的偏差依然小于0.80%。因此本方法可以将熔体包裹体等地质样品的Pb同位素研究由新生代样品(年龄小于65 Ma)扩展到古生代样品。     

不纯碳酸盐U-Th等时线定年及同位素分馏对年龄的影响

应用U-系L/R等时线模式测定了用天然碎屑沉积物与已知年龄的纯CaCO3按不同比例混合而成的不纯碳酸盐样品的年龄。实验分两个序列进行:其一研究碎屑沉积物化学前处理过程中U、Th同位素的萃取行为及其分馏效应。另一序列用不同浓度的溶剂对均匀混合的不纯碳酸盐样品进行溶解,确定稀酸淋滤碳酸盐的效率。各组分的238U、234U、230Th和232Th含量分析数据表明,稀酸溶解碎屑沉积物样品将U和Th同位素等比例的提取出来,不会发生U和Th同位素分馏现象。对于不纯碳酸盐样品,所有数据点落在一条直线上,良好的线性关系反映真实年龄含意。3个不纯碳酸盐样品的等时线年龄与纯碳酸盐年龄一致表明实验技术是可信的。同时,也证明对于碎屑含量较高的不纯碳酸盐U-Th定年,L/R等时线技术是适应的。     

4He同位素含量测试技术研究

4He同位素含量的准确测定是(U-Th)/He同位素定年方法的关键,因为4He同位素是气体,所以其含量测定难度是比较大的,国内其他实验室还没有建立4He同位素含量的测定方法.本文详细介绍了国外(U-Th)/He同位素定年实验室4He同位素含量的测试技术和本实验室基于现有仪器条件建立的4He同位索含量的测试方法.笔者等利...     

原位U-Th/He同位素定年技术研究进展及其低温矿床学应用前景

应用传统单颗粒方法对目标矿物进行定年具有较高要求(如U、Th等母体同位素均匀分布),需要耗时的酸溶过程,同时还需进行α粒子射出效应校正。原位U-Th/He同位素定年技术是近年发展起来的一种定年技术,其主要原理是采用激光加热目标矿物,并通过与激光系统连接的稀有气体质谱(Alphachron)和电感耦合等离子体质谱(ICP-MS)分别完成 ~4He和U、Th等母体同位素分析,将 ~4He和U、Th分析结果代入年龄公式计算即可获得目标矿物的U-Th/He年龄。本文阐述了原位U-Th/He同位素定年技术的主要原理、实验测试流程、适用矿物等,重点对原位U-Th/He同位素定年的技术难点和低温矿床学应用前景进行了分析。相对于传统单颗粒方法,原位测试方法解决了两个关键问题:1无需进行α粒子射出效应的校正,提高了定年结果的可靠性和准确度;2能完成母体同位素分布不均匀样品的测试,扩展了U-Th/He同位素定年的应用范围。尽管原位U-Th/He同位素定年技术在侧向加热效应、剥蚀坑体积测定以及标准矿物等方面尚存在一些亟待解决的问题,但已在硅酸盐、磷酸盐、钛铁氧化物等矿物的年代学研究方面展示了良好的应用前景。随着原位U-Th/He同位素定年技术的发展和进步,尤其是硫化物的U-Th/He同位素定年的发展,将为解决低温矿床的年代学问题提供一种新的思路。     

The behavior of 232Th and the 238U decay chain nuclides during magma formation and volcanism

Concentrations of the members of the ²³⁸U decay chain and ²³²Th were determined in volcanic rocks from convergent plate margins, intraplate volcanoes and oceanic spreading centers. Contemporary and historical volcanic rocks from Mt. St. Helens, Arenal, El Chichon, Hawaii and Iceland and submarine basaltic glass from the Galapagos spreading center all show no fractionation of U and Th in the mantle source or during magma formation at least for the past 300,000 years. Mauna Kea (Hawaii) rocks of alkaline composition greater than 4000 years old and an old submarine basalt show disequilibrium for several of the nuclides in the ²³⁸U decay chain. We interpret these as resulting from post-emplacement processes.     

Use of 234U/238U disequilibrium in measuring chemical weathering rate of rocks

The rate of chemical weathering of rocks has been determined by using uranium as a natural isotopic tracer. The concentration of uranium and ²³⁴U/²³⁸U ratio in natural waters, rocks, and soils of the Preto river basin (Bahia State, Brazil) was measured by alpha-ray spectroscopy.The activity ratio U234/U238 measured in the various samples indicates the uranium fraction which is dissolved from rocks during the weathering process. The results obtained show that 1 m of rock needs 25,000 yr to be weathered in this region under present climatic conditions.     

高分辨电感耦合等离子体质谱法测定铀矿石样品中234U/238U、230Th/232Th和228Ra/226Ra同位素比值 

采用氢氟酸-硝酸-盐酸混合酸密闭消解含铀矿石样品,用阴离子交换树脂、阳离子交换树脂和锶特效树脂逐级分离富集铀、钍和镭。使用高分辨电感耦合等离子体质谱(HR-ICPMS)测定分离纯化液中234U/238U2、30Th/232Th和228Ra/226Ra同位素。比值的测量精密度取决于比值的大小和对应核素浓度的大小。对质量浓度为10 ng/mL天然铀测量液,234U/238U的测量精密度优于1.2%;对230Th质量浓度为0.6ng/mL且230Th和232Th质量浓度接近的测量液,230Th/232Th的测量精密度为1.2%;对228Ra质量浓度为0.48 pg/mL且228Ra和226Ra质量浓度接近的测量液,228Ra/226Ra的测量精密度为4.0%。     

平面型高纯锗低能γ谱仪测定铀矿样中^235U,^238U,^226Ra及^232Th含量

本文对选用低能γ射线测定铀矿中的~(235)U、~(238)U、~(226)Ra、~(232)Th含量进行了讨论,提出了一种对样品进行自吸收校正的简单准确方法,对铀矿样中的~(235)U、~(238)U、或~(235)U/~(238)U比值测定获得了较高的分析精度和准确度。     

State of disequilibrium between 238U, 234U, 226Ra and 222Rn in groundwater from bedrock

Approximately one thousand drilled wells were investigated for their natural radioactivity. The determinations of ²³⁸U, ²³⁴U, ²²⁶Ra and ²²²Rn from 310 samples showed a high state of radioactive disequilibrium between the members of the uranium series present in water. The ${}^{238}\text{U}{}^{226}\text{Ra}$ activity ratio usually fell in the range 1–20 and the ${}^{238}\text{U}{}^{222}\text{Rn}$ activity ratio in the range 1–20 × 10^?4, the highest activity ratios being from samples with an elevated uranium content. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio varied between 0.76 and 4.67, the most frequent values showing a 60% excess of ²³⁴U in the samples. Most of the ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios near unity were found in samples with a high uranium content. Several drilled wells with anomalously high uranium contents were found in southern Finland. The average ²²⁶Ra and ²²²Rn contents of these wells were not exceptionally high, which suggests high mobility of uranium in groundwater from the areas involved.     

Tracing the metasomatic and magmatic evolution of continental mantle roots with Sr, Nd, Hf and and Pb isotopes: A case study of Middle Atlas (Morocco) peridotite xenoliths

We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr-Nd-Hf isotope data. However, we focus in particular on Pb isotope data and ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated ²³⁸U/²⁰⁴Pb, ²³²Th/²³⁸U and ²³²Th/²⁰⁴Pb in continental mantle roots and (c) the ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM).Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU.U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between ²⁰⁶Pb/²⁰⁴Pb = 19.93-20.25, ²⁰⁷Pb/²⁰⁴Pb = 15.63-15.66 and ²⁰⁸Pb/²⁰⁴Pb = 39.72-40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle.The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower ²³⁸U/²⁰⁴Pb, ²³²Th/²⁰⁴Pb and ²³²Th/²³⁸U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high ²³²Th/²³⁸U also suggest that the metasomatism was due to carbonatitic liquids.When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have ²³²Th/²⁰⁴Pb ratios exceeding those known from terrestrial basalts.     

Helium emanometry under Australian conditions — Preliminary results

Helium has been proposed as a pathfinder in exploration for uranium deposits, hydrocarbons and geothermal energy sources, as an indicator of faults and fissures and as a possible tool for earthquake prediction. The rationale is that during the decay of ²³⁸U to ²⁰⁶ Pb, eight ⁴He atoms are produced via the intermediary of alpha particle emission. Similarly, six ⁴He atoms are produced by the decay of ²³²Th to ²⁰⁸Pb. Some of the common isotopes of the rare earths Ce, Nd and Sm are also alpha emitters, but they are unlikely to give rise to detectable helium anomalies.     

Investigation of natural background radiation of sediments in Rameswaram Island,Tamil Nadu,India

The aim of the study is to assess the natural background radiation in and around Rameswaram Island. In this context, samples were collected to measure the gamma radiations of ²³⁸U, ²³²Th, and ⁴⁰K using NaI(Tl) detector-based gamma ray spectrometer. The average activity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K are noted to be well below the world average values. The calculated absorbed dose rate, radium equivalent activity, and hazard index values were below the prescribed limit. The grain size of the sediment was analyzed following Trefethen’s nomenclature, and its correlation with the natural background radioactivity was studied. The sample that contained clay show elevated activity of ⁴⁰K. The percentage of heavy minerals shows concomitant variation in natural radioactivity, especially in the activity of ²³⁸U and ²³²Th.     

Uranium-series chronology for sediments of Lake Hovsgol, Mongolia, and the 1-Ma records of uranium and thorium isotopes from the HDP-04 drill core

Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvironmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in ²³⁸U–²³⁴U–²³⁰Th–²³²Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42, 14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8, 122 ± 11 and 128 ± 22 ka, respectively. The ²³⁸U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high ²³⁸U peak at 23.8 m in depth, while the ²³²Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk ²³⁸U into authigenic and terrigenous ²³⁸U fractions was attempted, based on the measured ²³²Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic ²³⁸U was higher in interglacials and lower in glacials. This depth profile of authigenic ²³⁸U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic ²³⁸U contents were associated with the environmental change around Lake Hovsgol.