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1.
腾冲、长白山年轻火山岩铀系不平衡热电离质谱法年代学研究 总被引：9，自引：0，他引：9
在国内首次建立了铀系不平衡热电离质谱法年轻火山岩年龄测定的试验流程,并研究了云南腾冲、吉林长白山天池火山岩的年龄。划分出腾冲晚更新世以来的5个喷发期次：220 ka、80 ka、20 ka、12.8~13 ka、0.75~0.79 ka和长白山天池全新世以来的3次喷发：5.3 ka、1.0 ka、0.75 ka。这些年龄值和其他的方法所得的结果基本一致,表明本方法的可靠性。此外,还介绍了铀系不平衡法火山岩年代学的基本原理,讨论了样品的封闭性、火山岩中钍同位素初始组成均一性等问题。 相似文献
2.
活火山是指1万年来有过喷发历史的全新世火山。火山的高分辨年代学对火山灾害评估和火山分类具有重要意义。对于缺乏历史记载的全新世火山,直接对火山岩进行同位素定年很困难。本文利用具有高时间分辨率的镭-钍-铀非平衡确定中国东部年轻火山的年龄。根据镭-钍-铀同位素,海南岛的马鞍岭和雷虎岭是全新世火山(马鞍岭:4.3ka;雷虎岭:4.7ka);镜泊湖火山(4.9ka)也是全新世火山;龙岗火山存在晚更新世和全新世活动(7.0ka,15.0ka);大兴安岭阿尔山和诺敏河Ra/Th非平衡消失但~(230)Th/~(238)U非平衡显著,属于晚更新世喷发(阿尔山:63ka;诺敏河:71ka)。海南岛的马鞍岭火山、雷虎岭火山和东北地区的龙岗火山、镜泊湖火山,是4座活火山。至于东北地区的阿尔山和诺敏河火山是否是活火山,有待测试更多样品的Ra/Th同位素。五大连池老黑山和火烧山有历史喷发记录,这与它们都存在显著Ra/Th非平衡一致。五大连池老黑山和火烧山的岩浆滞留年龄分别小于4.2ka和3.2ka,岩浆上升速率 18～23m/y。 相似文献
3.
对采自全澳大利亚４４５个土壤样品中铀和镭的测量表明，土壤中铀的平均含量为２．０＊１０（－６），Ｕ／２２６Ｒａ的活度比为１．０９。虽然单个样品值的变化范围很宽，但总体上富铀土壤多于富镭土壤。Ｃｒｏｓｓ－Ｋａｍｂａｌｄａ地区（ＷＡ）南部，Ｅｙｒｅ半岛（ＳＡ）东南部和Ｃｏｂａｒ（ＮＳＷ）北部地区的样品，显示了区域（放射性）不平衡，偏２２６Ｒａ。以昆士兰东北部的两片石炭纪火山岩发育地带发现土壤富铀。活度比 相似文献
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5.
江西菖蒲盆地中生代火山系划分的新认识 总被引：4，自引：0，他引：4
本文分析了江西寻乌菖蒲盆地中生代晚期火山岩系划分的现状及存在的问题,提出了以不整合面为界,将火岩岩纱刑 分为上、下两个不同时期的火山岩系,即下火山岩系称为田 火山岩系称为版石群,依据岩性岩相特征,可进一步将余田 划分为水头迳组和菖蒲组,版石群划分为合水组和优胜组。余田群草蒲组玄武岩全岩Ｒｂ－Ｓｒ等时线年龄为１７６Ｍａ,流纹岩全岩Ｒｂ－Ｓｒ等时线年龄为１５０Ｍａ;版石群优胜组流纹岩全岩Ｒｂ－Ｓｒ等时 相似文献
6.
通过详细的野外地质调查和室内分析研究认为寿昌－－梅城盆地是一个大型的中生代Ⅲ级火山构造，其岩相和结构相当复杂，盆地中岩石主要为钙碱性火山岩系的英安－－流纹质火山岩类，火山岩可分七个类型，是晚侏罗世构造岩浆作用的产物。经调查表明盆地内有６个Ⅳ级火山机构和５７个Ⅴ级火山机构，盆地的火山活动可划分为二个喷发旋回及五个喷发阶段，该盆地的发展经历了如下五个阶段：①褶皱基底断陷、火山岩浆间歇性喷发；②火山岩浆 相似文献
7.
铀系组分法是测定火山岩不同组分中的２３０Ｔｈ／２３２Ｔｈ值和２３８Ｕ／２３２Ｔｈ值、计算年龄的方法。岩石中的各组分是用磁选或浮选分离的。经过论证，组分法与等时线方法是等价的，但不需要从全岩中精选纯的单矿物，不会发生铀和钍同位素的分馏。根据组分法模式测定了云南腾冲盆地北来凤山、老龟坡山和马鞍山火山岩样品的年龄，分别为０．１１ＭａＢ．Ｐ．、５１０００。Ｂ．Ｐ．和２５０００ａＢ．Ｐ，与地层顺序相吻合。组分法对于年青火山岩定年有着极大的潜力。 相似文献
8.
曾本兰 《华东地质学院学报》2001,24(4):295-297
福建 地处我国东南沿海，火山岩分布面积广，火山岩型铀矿化类型多，包括火山沉积型、远温地下热水淋滤型、火山期后热液型、富铀次火山岩体热液交代型等。铀矿化表现为多形态、多产状、多部位和多期、 多阶段，矿物 组合简单等特征。赋铀火山岩普遍发育有水解作用，岩石水解后，铀含量增加5－8倍，而分散铀量则减少50－80％。表明火山岩系中铀的活化与岩石的水解作用关切密切。另外，区内火山岩型铀矿化均赋存于火山杂岩系中的反差效应场中，显示出火山杂岩系中的反差效应场与铀的矿化也有密切关系。从总体上看，水解活化、渗析对流和半封闭强反差是福建火山岩型铀矿化的重要成矿机制。 相似文献
9.
近10年来国际上用热电离质谱代替α谱仪测定不平衡铀系样品的年龄，在造礁珊瑚、洞穴堆积物、年轻火山岩和湖泊沉积物等领域已取得了显著的成果。该技术有明显的优点，如样品用量少，测试时间短，测量精度高，测年范围宽，是中更新世至全新世以来有效的测年方法之一，可为全球变化研究提供高精度和高灵敏度的年龄谱，展示了热电离质谱技术如同加速器质谱技术一样，有广阔的应用前景。我们曾用MAT－262质谱仪对国内石笋标样(GBW04412，GBW04413)和国际珊瑚标样(RKM－4)进行过测试，取得了与标准值一致的结果。 相似文献
10.
近10年来国际上用热电离质谱代替α谱仪测定不平衡铀系样品的年龄，在造礁珊瑚、洞穴堆积物、年轻火山岩和湖泊沉积物等领域已取得了显著的成果。该技术有明显的优点，如样品用量少，测试时间短，测量精度高，测年范围宽，是中更新世至全新世以来有效的测年方法之一，可为全球变化研究提供高精度和高灵敏度的年龄谱，展示了热电离质谱技术如同加速器质谱技术一样，有广阔的应用前景。我们曾用MAT－262质谱仪对国内石笋标样（GBW04412，GBW04413）和国际珊瑚标样（RKM－4）进行过测试，取得了与标准值一致的结果。 相似文献
11.
A 230Th-234U-238U dating study on pedogenic silica-carbonate clast rinds and matrix laminae from alluvium in Crater Flat, Nevada was conducted using small-sample thermal-ionization mass spectrometry (TIMS) analyses on a large suite of samples. Though the 232Th content of these soils is not particularly low (mostly 0.1-9 ppm), the high U content of the silica component (mostly 4-26 ppm) makes them particularly suitable for 230Th/U dating on single, 10 to 200 mg totally-digested samples using TIMS. We observed that (1) both micro- (within-rind) and macro-stratigraphic (mappable deposit) order of the 230Th/U ages were preserved in all cases; (2) back-calculated initial 234U/238U fall in a restricted range (typically 1.67±0.19), so that 234U/238U ages with errors of about 100 kyr (2σ) could be reliably determined for the oldest, 400 to 1000 ka rinds; and (3) though 13 of the samples were >350 ka, only three showed evidence for an open-system history, even though the sensitivity of such old samples to isotopic disruption is very high. An attempt to use leach-residue techniques to separate pedogenic from detrital U and Th failed, yielding corrupt 230Th/U ages. We conclude that 230Th/U ages determined from totally dissolved, multiple sub-mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples, chemical enhancement of 238U/232Th, or isochrons. 相似文献
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13.
U-series disequilibria measured in waters and rocks from a chalk aquifer in France have been used as an analog for long-term radionuclide migration. Drill core samples from a range of depths in the vadose zone and in the saturated zone, as well as groundwater samples were analyzed for 238U, 234U, 232Th and 230Th to determine transport mechanisms at the water/rock interface and to quantify parameters controlling the migration of radionuclides. Isotope measurements in rocks were done by TIMS, whereas (234U/238U) and (230Th/232Th) activity ratios in water samples were measured by multi-collector-ICP-MS. Both depletion and enrichment in 234U relative to 238U were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water-table oscillation, respectively. The correlation between (230Th/232Th) activity ratios and 87Sr/86Sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples. Water samples are all characterized by (234U/238U) > 1 resulting from α-recoil effect of 234Th. Groundwaters are characterized by a more radiogenic signature in 87Sr/86Sr than the rocks. Moreover, (230Th/232Th) activity ratios in the waters are lower than in the rocks, and increase with distance from the water divide, which suggests that Th transport is controlled by colloids formed during water infiltration in the soil. A 1-D transport model has been developed in order to constrain the U-series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time-dependent composition for the solid phase. This model permits a prediction of the time scale of equilibration of the system, and an estimation of parameters such as weathering rate, distribution coefficients and α-recoil fractions. Retardation factors of 10-35 and from 1 × 104 to 2 × 105 were predicted for U and Th, respectively, and can be used to predict the migration of radionuclides released as contaminants in the environment. At the scale of our watershed (∼32 km2), a characteristic migration time from recharge to riverine discharge of 200-600 yr for U and 0.2-3.7 Myr for Th was obtained. 相似文献
14.
TIMS测定铀矿石样品中^234U/^238U、^230Th/^232Th、^228Ra/^226Ra的方法研究 总被引：1，自引：0，他引：1
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。 相似文献
15.
In situ U-series dating by laser-ablation multi-collector ICPMS: new prospects for Quaternary geochronology 总被引：2，自引：0，他引：2
Stephen M. Eggins Rainer Grün Malcolm T. McCulloch Alistair W.G. Pike John Chappell Les Kinsley Graham Mortimer Michael Shelley Colin V. Murray-Wallace Christoph Sptl Lois Taylor 《Quaternary Science Reviews》2005,24(23-24):2523-2538
The capabilities and potential applications of in situ dating of Quaternary materials using laser ablation-MC-ICPMS are explored. 234U/238U and 230Th/234U can be measured with precision sufficient for dating at a spatial resolution of 100 μm or better in samples that contain as a little as 1 ppm uranium. Moreover, U and Th concentrations and U-series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth (e.g. in speleothems). These capabilities additionally permit the dating of bones, teeth and possibly molluscs, which are subject to post-mortem open-system behaviour of U-series isotopes, and can be employed to elucidate processes of U-series migration during weathering and diagenesis. A drawback of laser ablation-MC-ICPMS is that it cannot in general provide U-series age estimates with the high precision and accuracy of conventional TIMS or solution MC-ICPMS methods. However, sample preparation is straightforward, the amount of sample consumed negligible, and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods. Given further instrumental developments and the establishment of suitable matrix-matched standards for carbonates and other materials, we foresee that laser ablation-MC-ICPMS will play an increasingly important role in Quaternary dating research. 相似文献
16.
湖南省龙山县莲花洞两根大型石笋LLl(文石-方解石型)和LL5(文石型)ICP-MS230Th结果表明,82个年龄数据并不完全符合石笋生长层序律.根据U/Th同位素比值、沉积和矿物学特征,分析了同位素体系开放度对建立石笋正确年代学模式的影响.莲花洞LLl石笋全新世以来234U/238U对230Th/238U的比值具有谐和性特征并且230Th年龄层序正常,说明文石矿物基本接近U/Th同位素封闭系统,实测年龄基本可靠.10～40 ka期间234U/238U与230Th/238U离散度较大和矿物具有溶蚀、风化现象,表明体系发生U加入/流失作用.LL5石笋60～80 ka期间封闭性较好,实测年龄可信.上述结果表明,同一洞穴中文石石笋U/Th同位素体系开放度与时间的关系并不是线性关系,沉积时水文和物理化学性质以及随后的保存状况是决定洞穴文石石笋同位素封闭性的关键因素. 相似文献
17.
《Applied Geochemistry》2002,17(6):781-792
Samples of tuff from boreholes drilled into fault zones in the Exploratory Studies Facility (ESF) and relatively unfractured rock of the Cross Drift tunnels, at Yucca Mountain, Nevada, have been analysed by U-series methods. This work is part of a project to verify the finding of fast flow-paths through the tuff to ESF level, indicated by the presence of ‘bomb’ 36Cl in pore fluids. Secular radioactive equilibrium in the U decay series, (i.e. when the radioactivity ratios 234U/238U, 230Th /234U and 226Ra/230Th all equal 1.00) might be expected if the tuff samples have not experienced radionuclide loss due to rock-water interaction occurring within the last million years. However, most fractured and unfractured samples were found to have a small deficiency of 234U (weighted mean 234U/238U=0.95±0.01) and a small excess of 230Th (weighted mean 230Th/234U 1.10±0.02). The 226Ra/230Th ratios are close to secular equilibrium (weighted mean=0.94±0.07). These data indicate that 234U has been removed from the rock samples in the last ∼350 ka, probably by pore fluids. Within the precision of the measurement, it would appear that 226Ra has not been mobilized and removed from the tuff, although there may be some localised 226Ra redistribution as suggested by a few ratio values that are significantly different from 1.0. Because both fractured and unfractured tuffs show approximately the same deficiency of 234U, this indicates that pore fluids are moving equally through fractured and unfractured rock. More importantly, fractured rock appears not to be a dominant pathway for groundwater flow (otherwise the ratio would be more strongly affected and the Th and Ra isotopic ratios would likely also show disequilibrium). Application of a simple mass-balance model suggests that surface infiltration rate is over an order of magnitude greater than the rate indicated by other infiltration models and that residence time of pore fluids at ESF level is about 400 a. Processes of U sorption, precipitation and re-solution are believed to be occurring and would account for these anomalous results but have not been included in the model. Despite the difficulties, the U-series data suggest that fractured rock, specifically the Sundance and Drill Hole Wash faults, are not preferred flow paths for groundwater flowing through the Topopah Spring tuff and, by implication, rapid-flow, within 50 a, from the surface to the level of the ESF is improbable. 相似文献
18.
U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th and U concentrations and (238U)/(232Th), (230Th)/(232Th), (230Th)/(238U) and (234U)/(238U) for 43 samples, most of which are minimally differentiated, along with (226Ra)/(230Th) and (231Pa)/(235U) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm and 0.5 and 6 ppm, respectively. Initial (230Th)/(238U) ranges from 1.1 to 1.6. (226Ra)/(230Th)o ranges between 0.9 and 1.8 while (231Pa)/(235U)o ranges between 1.0 and 2.0.Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 ± 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume.Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (± other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole ± phlogopite ± sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia. 相似文献
19.
Activity ratios of 234U/238U, 230Th/234U, and 230Th/232Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of 230Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time. 相似文献