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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Subduction controls of Hf and Nd isotopes in lavas of the Aleutian island arc
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Subduction controls of Hf and Nd isotopes in lavas of the Aleutian island arc

机译：阿留申群岛弧形熔岩中Hf和Nd同位素的俯冲控制

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摘要

The Hf and Nd isotopic compositions of 71 Quaternary lavas collected from locations along the full length of the Aleutian island arc are used to constrain the sources of Aleutian magmas and to provide insight into the geochemical behavior of Nd and Hf and related elements in the Aleutian subduction-magmatic system. Isotopic compositions of Aleutian lavas fall approximately at the center of, and form a trend parallel to, the terrestrial Hf-Nd isotopic array with ε_(Hf) of +12.0 to +15.5 and ε_(Nd) of +6.5 to +10.5. Basalts, andesites, and dacites within volcanic centers or in nearby volcanoes generally all have similar isotopic compositions, indicating that there is little measurable effect of crustal or other lithospheric assimilation within the volcanic plumbing systems of Aleutian volcanoes. Hafnium isotopic compositions have a clear pattern of along-arc increase that is continuous from the eastern-most locations near Cold Bay to Piip Seamount in the western-most part of the arc. This pattern is interpreted to reflect a westward decrease in the subducted sediment component present in Aleutian lavas, reflecting progressively lower rates of subduction westward as well as decreasing availability of trench sediment. Binary bulk mixing models (sediment+peridotite) demonstrate that 1-2% of the Hf in Aleutian lavas is derived from subducted sediment, indicating that Hf is mobilized out of the subducted sediment with an efficiency that is similar to that of Sr, Pb and Nd. Low published solubility for Hf and Nd in aqueous subduction fluids lead us to conclude that these elements are mobilized out of the subducted component and transferred to the mantle wedge as bulk sediment or as a silicate melt. Neodymium isotopes also generally increase from east to west, but the pattern is absent in the eastern third of the arc, where the sediment flux is high and increases from east to west, due to the presence of abundant terrigenous sediment in the trench east of the Amlia Fracture Zone, which is being subducting beneath the arc at Seguam Island. Mixing trends between mantle wedge and sediment end members become flatter in Hf-Nd isotope space at locations further west along the arc, indicating that the sediment end member in the west has either higher Nd/Hf or is more radiogenic in Hf compared to Nd. This pattern is interpreted to reflect an increase in pelagic clay relative to the terrigenous subducted sedimentary component westward along the arc. Results of this study imply that Hf does not behave as a conservative element in the Aleutian subduction system, as has been proposed for some other arcs.
机译：从沿阿留申岛弧线全长位置收集的71个第四纪熔岩的Hf和Nd同位素组成用于限制阿留申岩浆的来源，并提供对Nd和Hf以及在阿留申俯冲中相关元素的地球化学行为的认识。 -岩浆系统。阿留申熔岩的同位素组成大约落在地面Hf-Nd同位素阵列的中心，并与之形成平行的趋势，其ε_（Hf）为+12.0至+ 15.5，ε_（Nd）为+6.5至+10.5。火山中心或附近火山中的玄武岩，安山岩和dacits通常都具有相似的同位素组成，这表明在阿留申火山的火山管道系统中地壳或其他岩石圈同化作用几乎没有可测量的作用。 Cold同位素组成具有明显的沿弧增加的模式，这种弧从冷湾附近的最东端到弧形最西端的Piip Seamount连续。该模式被解释为反映了阿留申熔岩中俯冲沉积物成分向西减少，反映了向西俯冲率逐渐降低以及海沟沉积物的减少。二元散装混合模型（沉积物+橄榄岩）表明，阿留申熔岩中的Hf的1-2％来自俯冲的沉积物，表明Hf以与Sr，Pb和Nd。 Hf和Nd在含水俯冲流体中的溶解度较低，这使我们得出结论，这些元素从俯冲组分中动员出来，并作为整体沉积物或硅酸盐熔体转移到地幔楔中。钕同位素通常也从东向西增加，但是在弧的东三分之一没有这种模式，那里的沉积物通量很高，并且从东向西增加，这是由于在东面的海沟中存在大量的陆源沉积物。 Amlia断裂带正在Seguam岛的弧线下俯冲。在沿弧线更西边的位置的Hf-Nd同位素空间中，地幔楔和沉积物末端成员之间的混合趋势变得更平坦，这表明与Nd相比，西部的沉积物末端成员具有更高的Nd / Hf或更多的放射源。该模式被解释为反映了相对于弧线向西的陆源俯冲沉积成分，中上层粘土的增加。这项研究的结果表明，Hf在阿留申俯冲系统中不作为保守元素，正如其他一些电弧所提出的那样。

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