Compositional heterogeneity of Archean mantle estimated from Sr and Nd isotopic systematics of basaltic rocks from North Pole, Australia, and the Isua supracrustal belt, Greenland
Compositional heterogeneity of Archean mantle estimated from Sr and Nd isotopic systematics of basaltic rocks from North Pole, Australia, and the Isua supracrustal belt, Greenland
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DOI：
10.1016/j.precamres.2020.105803
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发表时间：
2020-09-01
影响因子：
3.8
通讯作者：
Iwamori, Hikaru
中科院分区：
文献类型：
--
作者：
Nakamura, Hitomi;Sano, Ayane;Iwamori, Hikaru
现代洋中脊玄武岩（MORB）和洋岛玄武岩（OIB）中发现的成分变异反映了与地幔-地壳系统中物质循环相关的分化过程。为了研究这个回收系统建立的时间以及它如何转变为当今的系统，我们对西澳大利亚皮尔巴拉克拉通东部的北极（NP）和西格陵兰岛南部的伊苏亚表壳带（ISB）的太古代玄武岩进行了地球化学分析。这些岩石代表太古代增生杂岩，其年龄分别类似于 3.5 Ga 和 3.7-3.8 Ga。我们分析了 NP 和 ISB 玄武岩中的微量元素含量，包括稀土元素 (REE) 以及 Sr 和 Nd 同位素组成，这些玄武岩可能代表 MORB 和 OIB。它们的微量元素组成大致相似，但显示出不同的地球化学特征，特别是 REE，这可能反映了源地幔和熔融程度的差异。这种差异在它们的初始 Nd 同位素组成中也很明显，这些同位素组成是根据 REE 的平衡分配和明确的等时年龄估计的。相比之下，NP 和 ISB 玄武岩的 Sr 同位素组成变化很大，其等时线年龄与以前的研究不一致。此外，NP玄武岩中Rb和Sr的分配表明不平衡，表明Rb-Sr系统受到火成岩后蚀变和变质作用的干扰。基于这些观察，我们提出以下模型来解释太古代地幔地球化学成分的时间变化：（i）与3800 Ma相似：板块材料的回收和熔化非常容易发生，因此，MORB和OIB是由分化产生的地幔来源； (ii) 3460Ma至接近3800Ma：由于地幔倾覆等极端事件而发生地幔-地壳混合，减少了地幔的成分变化； (iii)在大约3460 Ma之后：物质循环系统中地幔异质性逐渐发展，重新建立了MORB和OIB之间的成分差异。该模型需要一个极端事件来驱动阶段（ii）期间的均一化，这可能为壳幔系统的演化提供新的见解。
Compositional variability found in modern mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) reflects differentiation processes associated with material recycling in the mantle-crust system. To investigate the timing at which this recycling system was established and how it transformed into the present-day system, we present geochemical analyses of the Archean basalts from North Pole (NP) in the East Pilbara Craton, Western Australia, and the Isua supracrustal belt (ISB), southern West Greenland. These rocks represent Archean accretionary complexes with ages of similar to 3.5 Ga and 3.7-3.8 Ga, respectively. We analyzed the trace element contents including rare earth elements (REEs), and Sr and Nd isotopic compositions of the basalts, which may represent MORBs and OIBs, from NP and ISB.Their trace-element compositions are broadly similar, but show distinct geochemical characteristics particularly with respect to REEs that probably reflect differences in both the source mantle and degree of melting. Such differences are also evident in their initial Nd isotopic compositions, which were estimated based on equilibrium partitioning of REEs and well-defined isochron ages. In contrast, the Sr isotopic compositions of the NP and ISB basalts are highly variable and their isochron ages are inconsistent with previous studies. Furthermore, the partitioning of Rb and Sr in the NP basalts indicates disequilibrium, suggesting that the Rb-Sr system has been disturbed by post-igneous alteration and metamorphism.Based on these observations, we propose the following model to explain the temporal variations in the geochemical composition of the Archean mantle: (i) similar to 3800 Ma: recycling of plate material and melting occurred quite readily and, therefore, MORBs and OIBs were produced from differentiated mantle sources; (ii) 3460 Ma to similar to 3800 Ma: mantle-crust mixing occurred as the result of an extreme event, such as mantle overturning, reducing the compositional variation of the mantle; and (iii) after similar to 3460 Ma: mantle heterogeneity gradually developed in the material-recycling system, re-establishing the compositional differences between MORBs and OIBs. This model requires an extreme event to drive the homogenization during stage (ii), which may provide new insights into the evolution of the crust-mantle system.