Global structure of mantle isotopic heterogeneity and its implications for mantle differentiation and convection
Global structure of mantle isotopic heterogeneity and its implications for mantle differentiation and convection
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DOI：
10.1016/j.epsl.2010.09.014
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发表时间：
2010-11
影响因子：
5.3
通讯作者：
H. Iwamori;F. Albarède;Hitomi Nakamura
中科院分区：
文献类型：
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作者：
H. Iwamori;F. Albarède;Hitomi Nakamura
为了进一步认识全球地球化学结构和地幔动力学，本文采用独立分量分析方法（ICA）对一组海洋玄武岩全球同位素数据进行了分析。该数据集由2773个洋中脊玄武岩（MORB）和1515个洋岛玄武岩（OIB）组成，具有Pb、Nd和sr的5种同位素比值。该数据集在空间上覆盖了主要海洋，使我们能够将结果与全球地球物理观测结果进行比较。已经发现了三个独立成分（ic），其中两个与之前在大西洋和印度洋的玄武岩中发现的成分基本相同。两个集成电路（IC1和IC2）跨越一个组成平面，占样本方差的95.7%，而第三个集成电路（IC3）占3.7%。根据碳化硅的地球化学性质和微量元素和同位素组成正演模型，讨论了碳化硅的成因。IC1区分OIB和MORB，可能与熔解相关的元素分馏和随后的放射成因生长有关，平均循环时间为0.8 ~ 2.4Ga。IC2追踪了MORB和OIB的区域物源，可能与含水流体-岩石相互作用和随后的放射成因长进有关，平均循环时间为0.3 ~ 0.9Ga。在有限的地理和构造背景下，上陆地壳物质的IC3指纹及其高值出现。地幔中继承的熔融成分（IC1）和含水流体成分（IC2）的变化很可能分别反映了洋中脊和俯冲带的作用。富含IC1熔体组分的致密物质在对流地幔底部的长期积累，导致其循环时间长于富含水流体组分（IC2）的致密物质。IC2与最下地幔的地震速度结构和地幔过渡带周围的电导率有广泛的相关性。我们认为IC2反映了地幔内的氢分布，并且几个富含氢的全球域可能作为垂直扇区一直延伸到核幔边界。
In order to further our understanding of the global geochemical structure and mantle dynamics, a global isotopic data set of oceanic basalts was analyzed by Independent Component Analysis (ICA), a relatively new method of multivariate analysis. The data set consists of 2773 mid-ocean ridge basalts (MORB) and 1515 ocean island basalts (OIB) with five isotopic ratios of Pb, Nd and Sr. The data set spatially covers the major oceans and enables us to compare the results with global geophysical observations. Three independent components (ICs) have been found, two of which are essentially identical to those previously found for basalts from the Atlantic and Indian Oceans. The two ICs (IC1 and IC2) span a compositional plane that accounts for 95.7% of the sample variance, while the third IC (IC3) accounts for 3.7%. Based on the geochemical nature of ICs and a forward model concerning trace elemental and isotopic compositions, the origin of the ICs is discussed. IC1 discriminates OIB from MORB, and may be related to elemental fractionation associated with melting and the subsequent radiogenic in growth with an average recycling time of 0.8 to 2.4Ga. IC2 tracks the regional provenance of both MORB and OIB and may be related to aqueous fluid–rock interaction and the subsequent radiogenic ingrowth with an average recycling time of 0.3 to 0.9Ga. IC3 fingerprints upper continental crustal material and its high value appears in limited geographical and tectonic settings. Variations in the melt component (IC1) and in the aqueous fluid component (IC2) inherited in the mantle most likely reflect mid-ocean ridge and subduction zone processes, respectively. Long-term accumulation of dense materials rich in the IC1 melt component at the base of the convective mantle accounts for its longer recycling time with respect to that for less dense materials rich in the aqueous fluid component (IC2). IC2 broadly correlates with the seismic velocity structures of the lowermost mantle and electric conductivity around the mantle transition zones. We propose that IC2 reflects hydrogen distribution within the mantle and that several global domains enriched in hydrogen could exist as vertical sectors extending all the way down to the core–mantle boundary.