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1.
最近在青藏高原拉萨地体新发现了一条榴辉岩带,该榴辉岩带位于冈底斯岩浆岛弧带的北缘。对该带榴辉岩的岩石地球化学特征及SmNd、RbSr同位素组成研究表明： ① 该带榴辉岩的原岩主要是一套低钾的大洋拉斑系列玄武岩,其原岩类似于典型的NMORB,源于亏损地幔; ② 该带榴辉岩样品 （87Sr/86Sr）i＝0.70335~0.70457,变化范围较大,且与εNd（t）值及微量元素特征均没有好的对应关系,可能是由于该套岩石形成于大洋环境,形成后遭受了海水的蚀变作用;（3）该带榴辉岩全岩的SmNd等时线年龄为305.5±50Ma （2σ）,（143Nd/144Nd）i＝0.5126±0.00007 （2σ）,表明该带榴辉岩可能形成于早石炭－晚二叠世,在这一时期冈底斯北缘可能存在古特提斯洋盆及古特提斯洋盆深俯冲作用。 相似文献
2.
根据赣南会昌地区中基性火山岩高碱、富钾、低钛、贫铁,岩石中斜长石斑晶具钾长石环边,基质中存在大量钾长石微晶,以及富集轻稀土元素和大离子亲石元素等矿物岩石地球化学特征,确切厘定会昌地区的中基性火山岩为橄榄玄粗岩安粗岩组合,属典型的大陆板内橄榄玄粗岩系列火山岩。对会昌橄榄玄粗岩进行了RbSr同位素定年研究,确定其全岩RbSr等时线年龄为107.3±2.3Ma。 会昌橄榄玄粗岩系火山岩SrONdPb 同位素组成的特征为: 偏高的 ISr(0.7098~0.7115); 较低的δ18O值(5.3‰~7.0‰);中等的εNd(t)(-0.61~-3.60); 富放射性成因铅((206Pb/204Pb)i=17.32~18.29, (207Pb/204Pb)i=15.34~15.65, (208Pb/204Pb)i =37.51~38.60)。会昌橄榄玄粗岩的Δ7/4Pb值为-7.8~+16.8(平均值为+5.42), Δ8/4Pb值为27.3~97.3 (平均值为68.04), ΔSr值为96.2~114.1 (平均值为104.3),这表明存在典型的Dupal 同位素异常。根据SrO, SrNd, SrPb, NdPb, PbPb同位素相关特征, 判明会昌橄榄玄粗岩是由亏损地幔端元(DM)和岩石圈富集地幔端元(EM)在源区混合形成的。按SrNd双变量二元混合模型计算得出源区物质中亏损地幔端元和富集地幔端元所占份额各占50%左右。会昌早白垩世橄榄玄粗岩系火山岩带的形成反映了华南板块内部在燕山晚期发生的一起重要的伸展构造事件 相似文献
3.
海洋Sr同位素的变化主要是由陆地河流注入的Sr同位素的变化所引起。在全球河流中，流经喜马拉雅山地河流（恒河—布拉马普特拉河）表现出与世界上其它河流明显不同的特点，具有高87Sr/86Sr、高\[Sr\]的特征。恒河—布拉马普特拉河是世界上第四大河流，是当今世界海洋Sr的重要来源。国际上，目前人们对造成喜马拉雅河流Sr异常的原因（来源）的认识，仍存较大分歧。归纳起来，主要有3种认识：一是认为来源于硅酸盐岩；二是认为来源于碳酸盐岩的风化；三是认为来源于碳酸盐岩和硅酸盐岩的风化。近年来，作者对高喜马拉雅中央结晶岩系河流Sr同位素异常及其源岩进行的研究表明，高喜马拉雅河流Sr同位素受流域地质作用的强烈影响，呈现出高87Sr/86Sr、低\[Sr\]的特点。对岩石和单矿物的研究表明，中央结晶岩系变质岩（片岩、片麻岩）和花岗岩黑云母中的87Sr/86Sr与\[Rb\]成正比并具有高87Sr/86Sr、低\[Sr\]的特征。黑云母矿物具有的易风化性，为高喜马拉雅河流放射性87Sr提供了主要来源。恒河—布拉马普特拉河的Sr异常（高87Sr/86Sr和高\[Sr\]）则可能是反映了流经整个喜马拉雅造山带河流Sr混合平衡后的特征。 相似文献
4.
利用套柱法快速分离提纯Sr和Nd元素 总被引：1，自引：1，他引：0
样品放射性成因Sr-Nd同位素比值受控于源区初始同位素组成、放射性元素母体与子体相对丰度,以及衰变时间等因素。它们具有极强的示踪能力,因而在地质学领域有广泛的应用。传统的Sr-Nd同位素分析使用的是阳离子树脂,提纯Nd元素时往往涉及有机试剂以及调节pH值等操作,其分析效率较低。近年来特效树脂的出现使得分离这些元素变得简单,但是受硫酸根等因素影响,特效树脂使用次数有限。为了提高分析效率,缩短分析时间,本文开发了一种套柱法,该方法结合阳离子树脂和特效树脂,实现了Sr-Nd元素的快速分离,并且能延长特效树脂的使用寿命。实验采用阳离子树脂、Sr特效树脂和LN稀土特效树脂对玄武岩BCR-2标样进行了分析。Sr-Nd回收率均90%,BCR-2玄武岩~(87)Sr/~(86)Sr比值为0.705016±0.000016(n=36,1SD),~(143)Nd/~(144)Nd比值为0.512624±0.000012(n=39,1SD),与前人TIMS法获得的结果吻合(~(87)Sr/~(86)Sr:0.705000～0.705023;~(143)Nd/~(144)Nd:0.512630～0.512650)。最终分离提纯的溶液中~(85)Rb/~(86)Sr值小于0.01,~(147)Sm/~(144)Nd值小于0.001,表明该方法可以高效分离Rb-Sr和Sm-Nd,实现Sr、Nd同位素的准确分析。 相似文献
5.
济南辉长岩体的锶、钕同位素特征 总被引：6，自引：0，他引：6
济南辉长岩体的Rb Sr全岩 矿物参考等时线年龄为 144± 2 5Ma ,线性差的原因可能是该岩体来自于锶、钕同位素初始比值不均一的源区 ,结晶时处于同位素不平衡状态。钕同位素初始值εNd(t)值为 - 9.0 8～13.0 5 ,表明该岩体的源区为富集性质的地幔 ,并且表现出不均一性 ,εNd(t) 87Sr 86Sr(t)关系显示出源区满足DMM、EMⅠ和EMⅡ的三元混合关系。 相似文献
6.
三塘湖盆地二叠系陆相热水沉积方沸石岩特征及成因分析 总被引：1，自引：0，他引：1
在新疆三塘湖盆地野外露头及钻井中二叠统芦草沟组(P2l)厚层黑色泥岩中发现多层白色纹层状方沸石岩。根据主要造岩矿物特征,将其大致分为两类,第一类为方沸钾长白云岩,主要造岩矿物包括细粒方沸石、泥晶钾长石（透长石、正长石）、泥晶白云石、泥晶铁白云石等,碳酸盐矿物以高的Fe、Mn含量为特征;第二类为方沸硅质岩与硅质方沸石岩,以细粒方沸石及泥晶石英为主,含少量碳酸盐。扫描电镜观察方沸石以自形的四角三八面体少见,多为它形块状或无定形胶体状。样品主量元素以富SiO2、Al2O3、Na2O、贫TiO2为特征,微量元素Th、Nb、Ti强烈亏损,Sr/Nd比值为44.73~831.78,Nb*介于0.02~0.17之间。∑REE总量偏低7.33~20.619 μg/g,LREE/HREE为4.77~11.20,δEu 0.60~0.99略具负异常,δCe 0.96~1.04,基本无亏损,稀土配分曲线为较平缓的右倾。方沸石岩微量元素、稀土元素配分曲线与研究区早、中二叠世玄武岩、粗面岩及上地壳有较大差异。87Sr/86Sr介于0.705 118~0.706 438之间,低于同时期全球海水及中上二叠统海相碳酸盐的87Sr/86Sr比值,143PNd/144Nd介于0.512 454~0.512 713之间,εNd介于-0.81~3.84之间,表明造岩热液可能含有幔源组分。通过详细的岩石学、地球化学及Sr、Nd同位素研究,认为这是一类与陆相裂谷盆地湖底热泉喷流作用有关的热水沉积岩组合,热液组分除了有壳源物质及大气水等的混入外,可能还有深部物质的参与。 相似文献
7.
湘西渣滓溪钨锑矿床白钨矿的Sm-Nd和Sr同位素地球化学 总被引：4，自引：0，他引：4
渣滓溪钨锑矿床位于湘西雪峰山弧形构造带的中段,是我国典型的脉状充填型锑矿床。本文对渣滓溪矿区不同产状产出的白钨矿进行了Sm-Nd和Sr同位素研究。研究表明,渣滓溪矿区白钨矿Sm/Nd变化范围相对较宽（0.36~0.63）,143Nd/144Nd为0.51211~0.51288;在147Sm/144Nd-143Nd/144Nd图解中,该矿白钨矿样品没有明显的线性分布趋势,无法厘定出该矿的准确成矿时间。该矿白钨矿的Nd(t)明显可分为两组（-10.2~-14.7和-3.79~+0.01）,其成矿流体中的Nd主要有两个来源,一部分可能来自晚元古代地层或下伏陆壳基底的碎屑岩,另一部分很可能与冷家溪群的基性、超基性岩有关。与Nd同位素不同,渣滓溪成矿流体中Sr同位素组成均一化程度较高,该矿白钨矿87Sr/86Sr为0.7304~0.7329;该矿这种明显富放射成因87Sr的成矿热液,排除了成矿流体来自海水和赋矿围岩作为唯一矿源层的可能性,下伏陆壳的结晶基底很可能是这种高放射成因Sr的提供者。沃溪和渣滓溪矿区白钨矿Sr-Nd同位素组成的对比研究表明,两矿区的成矿物质来源有所不同,前者应来自一种更古老、更成熟、更富放射成因Sr的下伏陆壳基底。 相似文献
8.
松辽盆地北部地层水同位素特征及其地质意义 总被引：1，自引：1，他引：0
盆地流体是沉积盆地中非常活跃的因子,盆地流体的水文地球化学特征主要是受控于流体-岩石相互作用。本文以松辽盆地北部为研究对象,基于地层水的水文地球化学特征、锶-氢-氧同位素数据分析,初步探讨了水-岩反应与锶-氢-氧同位素的关系。这一研究对于丰富同位素示踪、水-岩相互作用等基础理论具有重要研究意义。分析表明,位于盆地西部地区地层水明显受到古大气降水的影响,中央凹陷地区δ18O正偏移可能是由水岩作用加强引起的。地层水为大气水与原生沉积水的混合,较轻的δD值反映出原始沉积水属陆相沉积水。地层水的87Sr/86Sr值略低而Sr2+含量又较高,是高87Sr/86Sr值流体端元与低87Sr/86Sr值流体端元的混合,即地表来源水与深部水的混合,也更多地受到高Sr2+含量低87Sr/86Sr值的火山-地热水等幔源深部水的补给。 相似文献
9.
《古地理学报》2021,23(5)
黄土物源研究对揭示第四纪以来东亚大气环流格局的演化和构造—气候之间的相互作用具有重要意义。目前已应用多种物源示踪方法对黄土物源开展了大量研究,但对黄土物源时空差异规律及其动力学机制尚未取得较为一致的认识。在综合分析黄土物源研究现状的基础上,重点从影响~(87)Sr/~(86)Sr值和~(143)Nd/~(144)Nd值组成及单颗粒碎屑锆石U-Pb年龄谱构成的因素角度,分析了第四纪黄土的物源及可能变化,获得以下认识:(1)第四纪黄土沉积物的~(87)Sr/~(86)Sr值变化幅度可达0.002 580～0.004 949,远远超出实验室分析测试误差(0.000 018)或由Sr同位素衰变带来的影响(2.6 Ma时段小于0.000 026,1个冰期旋回小于0.000 001);~(143)Nd/~(144)Nd值亦发生了较显著变化(0.000 095～0.000 240),其变化幅度远大于实验室的分析测试误差(0.000 010)及衰变导致的~(143)Nd/~(144)Nd值(0.000 013)变化。因此,在黄土中检测到的Sr-Nd同位素组成的变化应具有较明确的地质意义。(2)第四纪黄土沉积物中无论全岩还是主要粒径组分的~(87)Sr/~(86)Sr值和~(143)Nd/~(144)Nd值的变化都与气候代用指标变化不同步,难以从气候变化角度进行解释,可能更多地反映了源区的变化。(3)与Sr-Nd同位素示踪体系相比,单矿物或单颗粒的物源示踪体系对物源区变化的响应更为敏感,在追踪第四纪黄土沉积物源区时空差异方面具有较明显的优势。(4)基于碎屑锆石U-Pb年龄谱对黄土物源的有限研究揭示黄土高原的黄土在地质历史时期可能发生了原始物源区显著的时空分异,特别是不同剖面都反映出了1.2 Ma前后黄土主要源区的变化,表明通过锆石U-Pb年龄谱研究黄土物源时空差异规律具有揭示岩石圈、大气圈、水圈耦合作用过程及历史的巨大潜力。 相似文献
10.
山东蓬莱、临朐新生代碱性玄武岩的钕、锶同位素组成 总被引：16，自引：0，他引：16
本文报道了鲁东和鲁西新生代碱性玄武岩13个样品的Nd、Sr同位素组成,~(143)Nd/~(144)Nd=0.512967—0.512744,~(87)Sr/(86)Sr=0.70349—0.70450。它们在地质剖面上呈现规律性变化,可能与其地幔源区同位素组成的层状分带有关。鲁西地幔源区具有较鲁东更加亏损的组份。两地地幔源区在演化中都曾发生过地幔交代(或富集)作用,根据玄武岩Nd同位素模式年龄估计地幔交代作用发生的时间为0.45Ga。 相似文献
11.
Stratigraphic changes in the εNd of epeiric sea carbonates from central North America track the submergence history of the interior craton during the Late Ordovician. Fluctuations in sea level changed the Nd isotope balance of the epeiric sea by modifying the flux of Nd weathered from the highlands of the Taconic Orogen (εNd = −6 to −9) and from the low relief Precambrian basement (εNd = −22 to −15) of the Transcontinental Arch and Canadian Shield. Transgressions over the Arch and Shield, which diminished the weathering flux of Nd from the Precambrian basement, are recorded as positive shifts in the εNd profiles of carbonates. Negative εNd shifts reflect regression and reexposure of the Precambrian basement to erosion. Correlation of Upper Ordovician carbonates by use of the εNd profiles demonstrates the potential for Nd isotope stratigraphy. Comparison of stratigraphic variations in carbonate Sm/Nd ratios with sea level curves, conodont paleoecology, and the εNd profiles suggest that our observed variations in Sm/Nd ratios are related to changes in depth. Increasing Sm/Nd ratios correlate with increasing depth, whereas decreasing Sm/Nd ratios correlate with decreasing depth. This relationship between Sm/Nd ratios and depth suggests Sm/Nd profiles have potentially wide applications in understanding the paleoceanography of ancient epeiric seas. 相似文献
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A precise and accurate method for the determination of 143 Nd/144 Nd isotope ratio without Nd and Sm separation by multiple collector inductively coupled plasma mass spectrometry is demonstrated in this paper. 相似文献
14.
Alan D. Brandon Thomas J. Lapen Brian L. Beard Clive Neal 《Geochimica et cosmochimica acta》2009,73(20):6421-17609
The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Mars-sized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142Nd/144Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142Nd-143Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369-1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505-516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth.To further examine this issue, the same six lunar mare basalt samples measured in Rankenburg et al. [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369-1372] were re-measured for high-precision Nd isotopes using a multidynamic routine with reproducible internal and external precisions to better than ±3 ppm (2σ) for 142Nd/144Nd ratios. The measurements were repeated in a distinct second analytical campaign to further test their reproducibility. Evaluation of accuracy and neutron fluence corrections indicates that the multidynamic Nd isotope measurements in this study and the 3 in Boyet and Carlson [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505-516] are reproducible, while static measurements in the previous two studies show analytical artifacts and cannot be used at the resolution of 10 ppm to determine a bulk Moon with either chondritic or superchondritic Sm/Nd ratios. The multidynamic data are best explained by a bulk Moon with a superchondritic Sm/Nd ratio that is similar to the present-day average for depleted MORB. Hafnium isotope data were collected on the same aliquots measured for their 142Nd/144Nd isotope ratios in order to assess if the correlation line for 142Nd-143Nd systematics reflect mixing processes or times at which lunar mantle sources formed. Based on the combined 142Nd-143Nd-176Hf obtained we conclude that the 142Nd-143Nd correlation line measured in this study is best interpreted as an isochron with an age of 229+24−20Ma after the onset of nebular condensation. The uncertainties in the data permit the sources of these samples to have formed over a 44 Ma time interval. These new results for lunar mare basalts are thus consistent with a later Sm-Nd isotope closure time of their source regions than some recent studies have postulated, and a superchondritic bulk Sm/Nd ratio of the Moon and Earth. The superchondritic Sm/Nd signature was inherited from the materials that accreted to make up the Earth-Moon system. Although collisional erosion of crust from planetesimals is favored here to remove subchondritic Sm/Nd portions and drive the bulk of these bodies to superchondritic in composition, removal of explosive basalt material via gravitational escape from such bodies, or chondrule sorting in the inner solar system, may also explain the compositional features that deviate from average chondrites that make up the Earth-Moon system. This inferred superchondritic nature for the Earth similar to the modern convecting mantle means that there is no reason to invoke a missing, subchondritic reservoir to mass balance the Earth back to chondritic for Sm/Nd ratios. However, to account for the subchondritic Sm/Nd ratios of continental crust, a second superchondritic Sm/Nd mantle reservoir is required. 相似文献
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16.
利用MC-ICPMS精确测定143Nd/144Nd和Sm/Nd比值 总被引：43，自引：14，他引：43
多收集器等离子体质谱(MCICPMS)用于分析SmNd同位素时,质量分馏系数(β)与同位素的质量数呈线性关系.可以采用两种方法进行质量分馏校正:双分馏系数内部校正法(DFIC)和单分馏系数外部校正法(SFEC).采用DFIC法,对国际标样ShinEtsuJNdi1和实验室标样NdGIG进行了为期五个月的143Nd/144Nd比值测量统计,结果分别为0.512120±0.000012(2σSD)、0.511532±0.000013(2σSD).采用SFEC法,对NdGIG标样的测量统计结果为0.511525±0.000015(2σSD).两种方法的测量结果在分析误差范围内与其推荐值或TIMS测量值完全一致.对加Ce和Sm的NdGIG混合溶液分别进行了Ce和Sm对143Nd/144Nd比值分析的干扰校正研究和Sm/Nd比值测量,结果显示,143Nd/144Nd比值分别与Ce/Nd、Sm/Nd测量值呈线性关系,Sm/Nd测量值与其质量比值亦呈很好的线性关系.这表明利用MCICPMS可以快速精确地测定存在Ce、Sm干扰的样品的143Nd/144Nd比值,同时可获得精确的Sm/Nd比值,而无需加入稀释剂.这就使直接测定地质样品的SmNd等时线年龄成为可能. 相似文献
17.
David Thomas Murphy Alan D. Brandon Ray Burgess 《Geochimica et cosmochimica acta》2010,74(2):738-4475
Here we search for evidence of the existence of a sub-chondritic 142Nd/144Nd reservoir that balances the Nd isotope chemistry of the Earth relative to chondrites. If present, it may reside in the source region of deeply sourced mantle plume material. We suggest that lavas from Hawai’i with coupled elevations in 186Os/188Os and 187Os/188Os, from Iceland that represent mixing of upper mantle and lower mantle components, and from Gough with sub-chondritic 143Nd/144Nd and high 207Pb/206Pb, are favorable samples that could reflect mantle sources that have interacted with an Early-Enriched Reservoir (EER) with sub-chondritic 142Nd/144Nd.High-precision Nd isotope analyses of basalts from Hawai’i, Iceland and Gough demonstrate no discernable 142Nd/144Nd deviation from terrestrial standards. These data are consistent with previous high-precision Nd isotope analysis of recent mantle-derived samples and demonstrate that no mantle-derived material to date provides evidence for the existence of an EER in the mantle.We then evaluate mass balance in the Earth with respect to both 142Nd/144Nd and 143Nd/144Nd. The Nd isotope systematics of EERs are modeled for different sizes and timing of formation relative to ε143Nd estimates of the reservoirs in the μ142Nd = 0 Earth, where μ142Nd is ((measured 142Nd/144Nd/terrestrial standard 142Nd/144Nd)−1 * 10−6) and the μ142Nd = 0 Earth is the proportion of the silicate Earth with 142Nd/144Nd indistinguishable from the terrestrial standard. The models indicate that it is not possible to balance the Earth with respect to both 142Nd/144Nd and 143Nd/144Nd unless the μ142Nd = 0 Earth has a ε143Nd within error of the present-day Depleted Mid-ocean ridge basalt Mantle source (DMM). The 4567 Myr age 142Nd-143Nd isochron for the Earth intersects μ142Nd = 0 at ε143Nd of +8 ± 2 providing a minimum ε143Nd for the μ142Nd = 0 Earth. The high ε143Nd of the μ142Nd = 0 Earth is confirmed by the Nd isotope systematics of Archean mantle-derived rocks that consistently have positive ε143Nd.If the EER formed early after solar system formation (0-70 Ma) continental crust and DMM can be complementary reservoirs with respect to Nd isotopes, with no requirement for significant additional reservoirs. If the EER formed after 70 Ma then the μ142Nd = 0 Earth must have a bulk ε143Nd more radiogenic than DMM and additional high ε143Nd material is required to balance the Nd isotope systematics of the Earth. 相似文献
18.
19.
Zhuyin Chu Jinghui Guo Yueheng Yang Liang Qi Chaofeng Li 《Geostandards and Geoanalytical Research》2014,38(1):61-72
In this study, a high‐precision method for the determination of Sm and Nd concentrations and Nd isotopic composition in highly depleted ultramafic rocks without a preconcentration step is presented. The samples were first digested using the conventional HF + HNO3 + HClO4 method, followed by the complete digestion of chromite in the samples using HClO4 at 190–200 °C and then complete dissolution of fluoride formed during the HF decomposition step using H3BO3. These steps ensured the complete digestion of the ultramafic rocks. The rare earth elements (REEs) were separated from the sample matrix using conventional cation‐exchange chromatography; subsequently, Sm and Nd were separated using the LN columns. Neodymium isotopes were determined as NdO+, whereas Sm isotopes were measured as Sm+, both with very high sensitivity using single W filaments with TaF5 as an ion emitter. Several highly depleted ultramafic rock reference materials including USGS DTS‐1, DTS‐2, DTS‐2b, PCC‐1 and GSJ JP‐1, which contain extremely low amounts of Sm and Nd (down to sub ng g?1 level), were analysed, and high‐precision Sm and Nd concentration and Nd isotope data were obtained. This is the first report of the Sm‐Nd isotopic compositions of these ultramafic rock reference materials except for PCC‐1. 相似文献
20.
We present new ultra-high precision 142Nd/144Nd measurements of early Archaean rocks using the new generation thermal ionization mass spectrometer Triton. Repeated measurements of the Ames Nd standard demonstrate that the 142Nd/144Nd ratio can be determined with external precision of 2 ppm (2σ), allowing confident resolution of anomalies as small as 5 ppm. A major analytical improvement lies in the elimination of the double normalization procedure required to correct our former measurements from a secondary mass fractionation effect. Our new results indicate that metasediments, metabasalts, and orthogneisses from the 3.6 to 3.8 Ga West Greenland craton display positive 142Nd anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial ε143Nd value of 1.9 ± 0.6 ε-units, coupled 147Sm-143Nd and 146Sm-142Nd chronometry constrains mantle differentiation to 50-200 Ma after formation of the solar system. This chronological constraint is consistent with differentiation of the Earth’s mantle during the late stage of crystallization of a magma ocean. We have developed a two-box model describing 142Nd and 143Nd isotopic evolution of depleted mantle during the subsequent evolution of the crust-mantle system. Our results indicate that early terrestrial protocrust had a lifetime of ca. 0.7-1 Ga in order to produce the observed Nd isotope signature of Archaean rocks. In the context of this two box mantle-crust system, we model the evolution of isotopic and chemical heterogeneity of depleted mantle as a function of the mantle stirring time. Using the dispersion of 142Nd/144Nd and 143Nd/144Nd ratios observed in early Archaean rocks, we constrain the stirring time of early Earth’s mantle to 100-250 Ma, a factor of 5 shorter than the stirring time inferred from modern oceanic basalts. 相似文献