The neodymium isotopic compositions and rare earth patterns in highly depleted ultramafic rocks
The neodymium isotopic compositions and rare earth patterns in highly depleted ultramafic rocks
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DOI：
10.1016/s0016-7037(96)00280-3
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发表时间：
1996-11
影响因子：
5
通讯作者：
Mukul Sharma;G. Wasserburg
中科院分区：
文献类型：
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作者：
Mukul Sharma;G. Wasserburg
大量的陆地和地外储层是由主要含有橄榄石和正辉石的超晶岩取样的。然而，由于在处理大量样品时存在大量的分析问题，很少有人尝试确定这些样品的稀土元素含量和钕同位素组成。介绍了一种高收率、低污染的化学分离技术，适用于贫化超镁质岩石中稀土元素的提取。样品溶解后，我们利用岩石中存在的Fe进行稀土共沉淀，同时保持镁（超镁质岩石中的主要元素）在溶液中。该技术将有效样本量减少了一个数量级，从而允许使用现有的柱技术分离镧系元素。利用该方法，分析了与蛇绿岩杂岩地幔剖面相关的8个哈尔茨伯基岩和1个泥质岩的稀土元素和钕同位素组成。结果表明，轻稀土浓度从十亿分之一到小于10 ppb不等；代表ree的Yb浓度从1到70 ppb不等。岩石的/tfSm ndvalue范围为- 0.45至+3.4，ϵNd(0)值范围为- 12.5至+47.5。超镁质岩可根据其初始值ϵNd(t)分为两类。ϵNd(t) > +7的样品来源于枯竭的MORB地幔，表现出极度的LREE枯竭和HREE富集，没有u型模式的迹象。然而，ϵNd(t) < +7的样品产生的REE模式呈u型或向轻稀土端变平，被解释为受到了大陆地壳的污染。我们认为，在蛇绿岩杂岩的地幔剖面中发现的u型稀土元素模式是地壳污染的结果，而不是熔融过程中特殊分馏的结果。含有ϵNd(t) > +7的样品显示的稀土耗尽可以用平衡分数熔化来模拟。然而，观测到的（La SMx） nratio比熔融模型预测的要高几个数量级。我们认为，高（La - SM） nratio可能是玄武岩熔体再成矿的结果。本文开发的化学程序允许对来自上地幔的超贫岩石以及某些高度枯竭的陨石进行全面探索。
A large number of terrestrial and extraterrestrial reservoirs are sampled by ultradepleted rocks containing mainly olivine and orthopyroxene. However, very few attempts have been made to determine the REE contents and neodymium isotopic composition of such samples due to substantial analytical problems in handling large sample volumes. We describe a chemical separation technique with high chemical yield and low contamination suitable for extraction of REEs from depleted ultramafic rocks. After sample dissolution, we use the Fe present in the rock for coprecipitation of REEs while keeping Mg, the dominant major element in an ultramafic rock, in solution. This technique reduces the effective sample size by an order of magnitude, thus permitting the use of existing column techniques to separate the lanthanides. Utilizing this procedure, eight harzburgites and a dunite, associated with the mantle sections of ophiolite complexes, were analyzed for REE and neodymium isotopic composition. The results show the LREE concentrations varying from sub parts per billion to less than 10 ppb; the Yb concentrations representing the HREEs vary from 1 to 70 ppb. The rocks have /tfSm Ndvalues ranging from −0.45 to +3.4 and ϵNd(0) values varying from −12.5 to +47.5. The ultramafic rocks can be classified into two groups using their initial ϵNd(t) values. The samples with ϵNd(t) > +7 are derived from depleted MORB mantle and show extreme LREE depletion and HREE enrichment with no hint of a U-shaped REE pattern. However, samples with ϵNd(t) < +7 yield REE patterns that are either U-shaped or flatten towards the LREE end are interpreted as having been contaminated by continental crust. We conclude that the U-shaped REE patterns found in many harzburgites associated with mantle sections of ophiolite complexes are the result of crustal contamination and not the consequence of peculiar fractionation during melting. The HREE depletion shown by the samples with ϵNd(t) > +7 can be modeled with equilibrium fractional melting. However, the observed ( La SMx)Nratios are several orders of magnitude higher than predicted by the melting model. We suggest that the high ( La SM )Nratios in the ultradepleted samples may be a result of refertilization by basaltic melts. The chemical procedure developed in this paper permits a full exploration of ultradepleted rocks from the upper mantle as well as some classes of meteorites that are highly depleted.