Tracing subducted sediment inputs to the Ryukyu arc-Okinawa Trough system: Evidence from thallium isotopes
Tracing subducted sediment inputs to the Ryukyu arc-Okinawa Trough system: Evidence from thallium isotopes
复制标题
DOI：
10.1016/j.gca.2017.08.035
复制
发表时间：
2017-11
影响因子：
5
通讯作者：
Yunchao Shu;S. Nielsen;Z. Zeng;R. Shinjo;J. Blusztajn;Xiaoyuan Wang;Shuai Chen
中科院分区：
文献类型：
--
作者：
Yunchao Shu;S. Nielsen;Z. Zeng;R. Shinjo;J. Blusztajn;Xiaoyuan Wang;Shuai Chen
沉积物在世界范围内几乎每一条弧线上都处于活跃的俯冲状态。然而，量化它们对弧熔岩的贡献，从而建立沉积物如何参与板幔相互作用的预算，是具有挑战性的。本文利用琉球弧（包括南九州）及其弧后盆地冲绳海槽熔岩的铊（Tl）丰度和同位素组成，研究了弧到弧后沉积物的影响。我们还提供了琉球弧北部（DSDP地点296、442B、443和444）和中部（DSDP地点294和295）部分外的沉积物和蚀变洋壳（AOC）的广泛地球化学数据。沉积物的Tl同位素组成呈现出由北琉球弧外较轻向中琉球弧外较重的系统变化。这一特征反映了琉球弧北部外侧以陆源物质为主，中部以远洋沉积为主。琉球弧中、北段和冲绳海槽的Tl同位素变化幅度大于南段，这与琉球弧中、北段外侧沉积物比南段外侧沉积物具有更多的远洋物源是一致的。对比弧后熔岩，发现了相同的Tl、Sr、Nd和Pb同位素变化，说明冲绳海槽熔岩的Tl同位素变化也与沉积物通量有关。Tl与Pb、Sr和Nd同位素的双端元混合模式要求沉积物输入的重量分别为< 1%、0.1-1%和0.3-2%，以解释来自琉球弧北部、中部和冲绳海槽南部的熔岩的所有同位素组成。利用Tl、Sr、Nd和Pb同位素预测地幔和沉积物端元的整体混合预测了非常相似的沉积物通量，表明这些元素的分馏一定发生在地幔和沉积物混合之后。Sr/Nd分馏沉积物熔体与地幔楔混合的模型计算证实了这一结论，表明在混合线上没有弧状熔岩分布区。因此，来自琉球弧和冲绳海槽的熔岩的形成需要大量的沉积物混合，而不是沉积物融化。在亚弧地幔中微量元素分馏之前发生的大块沉积物混合的要求与在板块顶部形成的模子一致，模子是弧熔岩的主要来源物质。此外，在琉球弧和冲绳海槽熔岩中观测到的沉积物成分相似，表明向弧源区和弧后区输送的化学物质同样有效。这一特点最容易解释的是，从板坯作为底辟物运输的金属材料。
Sediments are actively subducted in virtually every arc worldwide. However, quantifying their contributions to arc lavas and thereby establishing budgets of how sediments participate in slab-mantle interaction is challenging. In this contribution we use thallium (Tl) abundances and isotopic compositions of lavas from the Ryukyu arc (including south Kyushu) and its back-arc basin, Okinawa Trough, to investigate the influence of sediments from arc to back-arc. We also present extensive geochemical data for sediments and altered oceanic crust (AOC) outboard of the northern (DSDP Sites 296, 442B, 443 and 444) and central (DSDP Sites 294 and 295) part of the Ryukyu arc. The Tl isotopic compositions of sediments change systematically from lighter outboard of northern Ryukyu arc to heavier outboard of central Ryukyu arc. The feature reflects the dominance of terrigenous material and pelagic sedimentation outboard of the northern and central Ryukyu arc, respectively. Central and northern sections of Ryukyu arc and Okinawa Trough display larger range of Tl isotopic variation than southern section, which is consistent with more pelagic provenance for sediments outboard of central and northern Ryukyu arcs than that of expected sediments outboard of southern Ryukyu arc. Identical Tl, Sr, Nd and Pb isotope variations are found when comparing arc and back arc lavas, which indicates that sediments fluxes also account for the Tl isotopic variations in the Okinawa Trough lavas. Two-end-member mixing models of Tl with Pb, Sr and Nd isotopes require sediment inputs of< 1%, 0.1–1% and 0.3–2% by weight to the depleted mantle source to account for all these isotopic compositions of lavas from northern, central and southern portion of the Ryukyu arc and Okinawa Trough. Bulk mixing between mantle and sediment end members predict very similar sediment fluxes when using Tl, Sr, Nd and Pb isotopes, which indicates that fractionation of these elements must have happened after mixing between mantle and sediments. This conclusion is corroborated by model calculations of mixing between sediment melts with fractionated Sr/Nd ratios and mantle wedge, which show that no arc lava plot on such mixing lines. Thus bulk sediment mixing, rather than sediment melt, is required for the generation of the lavas from the Ryukyu arc and Okinawa Trough. The requirement of bulk sediment mixing occurring before trace element fractionation in the sub-arc mantle is consistent with models where mélange layers form at the top of the slab and are the principle source material for arc lavas. In addition, the fact that sediment components observed in the Ryukyu arc and Okinawa Trough lavas are similar, suggests that transport of mélange material to the source regions of the arc and back arc is equally efficient. This feature is most readily explained if mélange material is transported from the slab as diapirs.