Lithium and boron isotope systematics in lavas from the Azores islands reveal crustal assimilation
Lithium and boron isotope systematics in lavas from the Azores islands reveal crustal assimilation
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DOI：
10.1016/j.chemgeo.2014.02.024
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发表时间：
2014-05-12
期刊：
影响因子：
3.9
通讯作者：
Haase, Karsten M.
中科院分区：
文献类型：
--
作者：
Genske, Felix S.;Turner, Simon P.;Haase, Karsten M.
Li和B稳定同位素体系是研究洋岛玄武岩成因的有用工具。我们已经将这两种系统应用于亚速尔群岛上一套全面的特征良好的样品，以进一步评估地幔源的性质。这些海洋岛屿代表了低浮力地幔柱的表面表现。整个群岛的锂和B浓度及其同位素组成的变化建立在最近的放射性同位素研究中大西洋中脊（弗洛雷斯和科尔沃）以西的岛屿。这两个系统的结果产生了一些典型的洋岛玄武岩的值;然而，在原始熔岩中观察到的δ Li-7和δ B-11的变化在弗洛雷斯岛和科尔沃岛的西部最为极端（δ Li-7=+3.5%至8.2%，δ B-11=-3.5%至11.8%）。同位素组成的大范围扩散很可能是由于岩浆在上升过程中通过热液蚀变的洋壳受到污染。同化分数结晶模型最好地解释了在每个岛屿上观察到的变化，并允许强大的估计地幔源。这些观察结果的含义是，如果稳定同位素记录了热液蚀变物质同化造成的污染，并且Li和B作为地幔源示踪剂的使用可能会被浅层次过程所掩盖，则对放射性同位素和微量元素的解释可能必须谨慎对待。此外，Sr和Nd同位素中观察到的大范围似乎不太可能反映来自再循环的低T物质（如沉积物或蚀变MORB）的可变贡献，因为放射成因和稳定同位素数据之间没有相关性。(C)2014年爱思唯尔B。V.保留所有权利。
The stable isotope systems of Li and B are useful tools for the understanding of ocean island basalt genesis. We have applied both systems to a comprehensive set of well- characterised samples across the Azores islands in order to further evaluate the nature of the mantle source. These ocean islands represent the surface expression of a low- buoyancy mantle plume. The variability of Li and B concentrations and their isotopic compositions across the archipelago builds on recent radiogenic isotope studies from the islands to the west of the Mid-Atlantic Ridge (Flores and Corvo). The results for both systems yield some values that are typical for ocean island basalts; however, the variability of both delta Li-7 and delta B-11 observed in primitive lavas is most extreme on the western islands of Flores and Corvo (delta Li-7=+ 3.5 to+ 8.2% and delta B-11=- 3.5 to+ 11.8%). The large spread in isotopic composition is most likely due to contamination of the magmas during ascent through hydrothermally altered oceanic crust. Models of assimilation fractional crystallisation best explain the variability observed on each island and allow for robust estimates of the mantle source. The implication of these observations is that the interpretation of the radiogenic isotopes and trace elements may have to be treated with care if the stable isotopes record contamination by assimilation of hydrothermally altered material and that the use of Li and B as mantle source tracers may be obscured by shallow level processes. Further, it seems unlikely that the large range observed in Sr and Nd isotopes can reflect variable contributions from recycled low- T materials such as sediment or altered MORB, since there is no correlation between the radiogenic and stable isotope data. (C) 2014 Elsevier B. V. All rights reserved.