Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time
Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time
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DOI：
10.1016/s0016-7037(98)00274-9
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发表时间：
1999-02
影响因子：
5
通讯作者：
J. Vervoort;J. Blichert‐Toft
中科院分区：
文献类型：
--
作者：
J. Vervoort;J. Blichert‐Toft
Lu-Hf和Sm-Nd同位素在大多数岩浆过程中的协变行为早已被认识到，但这种行为在亏损地幔储集层中的细节还没有得到充分的研究。我们报道了新的全岩Hf和Nd同位素数据：1)太古宙中-中生代年轻地幔岩石；2)西格陵兰早太古宙片麻岩。年轻岩石的Hf和Nd同位素组成具有很好的相关性，最好的拟合方程为εHf=1.40εND+2.1，与陆地样品的εHf=1.36εNd+3.0的集体Hf-Nd关联相似。相比之下，早太古宙格陵兰片麻岩的εNd值(−4.4至+4.2；Bennett等人，1993年)的极端范围没有得到Hf同位素系统的反映。这些岩石的εHf值始终为正值，并且与其εNd值相比变化小得多(0到+3.4)。西格陵兰片麻岩的Hf同位素组成信息表明，太古宙早期地幔在3.8~3.6Ga是相对同位素均一的，不相容元素中等亏损。没有证据表明这些片麻岩中的任何一种来自富集型储集层。Hf同位素数据与Nd同位素记录形成鲜明对比，强烈暗示由Nd同位素指示的极端初始同位素不均一性图景不是西格陵兰片麻岩的真实特征，而是这些岩石的Sm-Nd同位素系统扰动产生的产物。尽管Hf和Nd同位素数据不能唯一地限制最早地壳的性质或地壳生长的时间，但最有可能的是在4.0Ga之前的地球中补充亏损地幔的富集型储集层是一种镁铁质海洋型地壳。为了解释太古宙早期岩石的εHf和εNd值主要为正，该地壳在返回地幔之前在地球表面停留了很短的时间，在那里它与亏损的地幔隔离了数亿年。接下来的3.5~2.7Ga时期可能标志着早期形成的镁铁质地壳逐渐混合回到枯竭的地幔储集层的过渡阶段。虽然不能以同位素理由排除今天4.0Ga的大陆地壳体积，但我们认为，基于缺乏直接的同位素和物理证据来证明其存在，这种情况不太可能发生。因此，地壳生长和演化的一个重要方面可能是富集型储集层的转变，从早期地球上的主要镁铁质到随着时间的推移逐渐变得更具涎质。
The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation εHf= 1.40 εNd+ 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of εHf= 1.36 εNd+ 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in εNdvalues (−4.4 to+4.2; Bennett et al., 1993) that is not mirrored by the Hf isotopic system. The εHfvalues for these rocks are consistently positive and have much less variation (0 to+3.4) than their εNdcounterparts. The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks. Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive εHfand εNdvalues for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.