Nd isotopic evolution of the upper mantle during the Precambrian: models, data and the uncertainty of both
Nd isotopic evolution of the upper mantle during the Precambrian: models, data and the uncertainty of both
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前寒武纪上地幔的 Nd 同位素演化：模型、数据及其不确定性
DOI：
10.1016/s0301-9268(98)00051-5
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发表时间：
1998
影响因子：
3.8
通讯作者：
J. Kramers
中科院分区：
文献类型：
--
作者：
T. Nägler;J. Kramers
本文提出了一种解决Sm-ND系统陆地演化的双重方法：严格回顾了前寒武纪Nd初始同位素比值的编制，并独立地对上地幔的Nd同位素发展进行了正演模拟。所使用的输运平衡模型与以前发表的模型相同，该模型允许协调Th-U-Pb系统的计算参数和观测参数。为了表达成分和年龄的不均一性，陆壳被划分为四个域：上和下以及年轻和老。地幔由上、下两个储集层组成。成功用于Th-U-Pb和Sm-ND的模型版本为上地幔提供了非常窄范围的ϵNd发展曲线。所有这些情景都表明，在2-1.6Ga之前，大陆地壳几乎没有再循环进入上地幔。资料和模型表明，上地幔的ϵND[T]值从大约3GA开始稳定增加到现在的+10。在太古代早期，出现了一个准恒定的ϵND[T]Nd值。该模型预测，在这个早期，上地幔与块体硅酸盐地球(BSE)没有明显不同。然而，这些数据通常比公认的球粒陨石均匀储集层(CHUR)演化高出一个ϵ单位。月球岩石也显示了这种偏移量，特别是似乎表明它在4.5Ga以前就存在了。这意味着初始的BSE和整体硅酸盐月亮明显与常规的初始CHUR略有不同。对于这种明显的偏差，可能的解释是丘尔模型的一些不确定性、分析伪影和实验室间的偏见。公布的与可接受的Chur值的偏差和不同分馏校正过程所造成的影响都是相同的，这使得地面和月球数据相对于Chur的明显偏差是可能的。在CHUR模型上方一个ϵNd单位应用一个明显的BSE演化，使上地幔的地面模拟和数据保持良好的一致性。由于该模型描述的是内部完全均质的油藏，由于内部混合不完善，预计模型开发曲线周围的实际数据会分散。给出了由该模型生成的亏损上地幔ϵ演化曲线的三次多项式，作为计算ND模型年龄的新参考，公式为：ϵND[T]Sample=0.164T3−0.566T2−2.79T+10.4。
This paper presents a dual approach to resolve the terrestrial evolution of the Sm–Nd system: a critically reviewed compilation of Precambrian Nd initial isotope ratios and, independently, a forward modelling effort of the Nd isotopic development of the upper mantle. The transport balance model used is identical to a previously published one which allows to reconcile calculated and observed parameters of the Th–U–Pb system. In order to express compositional and age heterogeneity, the continental crust is divided into four domains: upper and lower as well as younger and older. The mantle is represented by an upper and a lower reservoir. Model versions successful for Th–U–Pb and Sm–Nd produce a very narrow range of ϵNddevelopment curves for the upper mantle. All these scenarios show very little recycling of continental crust into the upper mantle before 2–1.6Ga ago. The data as well as the model show a steady increase of the ϵNd[T] value of the upper mantle from ca 3Ga on to a present day value of +10. In the Early Archean, a quasi constant ϵNd[T] Nd value is seen. The model predicts that the upper mantle was not significantly different from bulk silicate Earth (BSE) in this early period. The data, however are generally one ϵNdunit above the accepted Chondritic Uniform Reservoir (CHUR) evolution. Lunar rocks also show this offset and, in particular, appear to indicate its existence since >4.5Ga ago. This implies that the initial BSE and bulk silicate Moon are apparently slightly different from the conventional initial CHUR. Possible explanations for this apparent offset are some uncertainty of the CHUR model, analytical artefacts and interlaboratory bias. Published deviations from the accepted CHUR values and the effects caused by different fractionation correction procedures are all in the same direction, and make an apparent offset of ca +1 ϵNdunit of terrestrial and lunar data relative to CHUR plausible. Applying an apparent BSE evolution with an initial at one ϵNdunit above the CHUR model brings terrestrial modelling of the upper mantle and data into good alignment. As the model describes internally fully homogeneous reservoirs, a scatter of real data around the model development curves is expected as a result of imperfect internal mixing. A third order polynomial fit to ϵNdevolution curves generated by the model for the depleted upper mantle is given as a new reference for Nd model age calculations, via the relation: ϵNd[T]sample=0.164T3−0.566T2−2.79T+10.4.