中国科学院海洋研究所机构知识库

        冲绳海槽位于西太平洋中琉球岛弧的西侧，是一个活动的弧后扩张盆地。本研究分析了冲绳海槽北部熔岩和浮岩的主微量元素和Sr、Nd同位素组成特征，其岩石类型包括玄武质安山岩、安山岩、英安岩和粗面英安岩，并且这些中酸性岩石均富集大离子亲石元素，亏损高场强元素。根据地球化学特征可将这些中酸性岩石分为三种类型：类型I包括安山岩D5-1和粗面英安岩D10，其Sr和Nd同位素组成特征分别为0.70399和0.512788-0.512872，同时该类型火山岩富集轻稀土元素和不相容元素，因此推测其成因可能来源于玄武质岩浆的结晶分异作用；类型II包含安山岩D5-2、D5-3和D5-4，与类型I火山岩相比，具有略低的Sr同位素比值和略高的Nd同位素比值，该类型火山岩的稀土元素分配模式和微量元素比值均指示存在俯冲板片熔体的混入，由此推测这三个安山岩的成因为被俯冲板片熔体交代的地幔楔发生部分熔融产生熔体后，在上升过程中又混染了演化的玄武质岩浆；类型III为英安质浮岩D22，根据其微量元素、稀土元素和Sr、Nd同位素组成特征，推测该浮岩并非原地成因，而是由黑潮从南琉球火山前缘搬运至冲绳海槽北部。

        各个俯冲带地区的岛弧火山岩中普遍存在俯冲沉积组分的加入。然而定量研究沉积组分对岛弧火山岩的俯冲通量是学术界一大难题，确定俯冲板片与地幔楔相互作用时沉积组分的参与机制及进入地幔楔的物质通量具有重大意义。本研究利用琉球岛弧（包括九州岛南部）及其弧后盆地（冲绳海槽）火山岩中的铊（Tl）含量及稳定同位素组成特征，分析从岛弧到弧后盆地，沉积物对其影响的变化。本文还测试分析了分别位于琉球海沟北部和中部外侧、菲律宾海板块上，由深海钻探计划（DSDP）采集的沉积物及蚀变洋壳样品，给出了其微量元素、放射性成因（Sr、Nd和Pb）同位素及稳定同位素（Tl同位素）的组成特征。琉球岛弧外侧沉积物的Tl同位素组成特征存在系统变化：琉球岛弧北部外侧的沉积物Ɛ²⁰⁵Tl比值低，琉球岛弧中部外侧沉积物中的Ɛ²⁰⁵Tl比值高。沉积物中Tl同位素组成特征反映了沉积物Tl来源的差异：岛弧北部外侧沉积物以陆源碎屑沉积为主，岛弧中部外侧沉积物则为远洋沉积产物。琉球岛弧火山岩中Tl同位素组成特征沿岛弧方向发生变化，并且与沉积物中Tl同位素变化特征一致，这一发现暗示琉球岛弧火山岩中Tl同位素组成特征的控制因素为俯冲沉积物。琉球岛弧和冲绳海槽（弧后盆地）火山岩中Tl、Sr、Nd和Pb同位素的变化特征相同，表明俯冲沉积物同样控制了冲绳海槽火山岩中的Tl同位素变化特征。Tl同位素分别与Pb、Sr和Nd同位素的两端元混合模型表明：进入琉球岛弧北部、中部和南部下方地幔楔的俯冲沉积物通量分别为<1%、0.1-1%和0.3-2%。俯冲沉积物与亏损地幔楔的两端元混合可以解释琉球岛弧和冲绳海槽系统北部、中部和南部火山岩中的Tl（包括Sr、Nd和Pb）同位素变化特征。Tl同位素分别与Sr、Nd和Pb同位素的三个独立二元混合模型分别揭示了沉积物进入地幔楔的俯冲通量，并且结果相似，这一特征表明俯冲沉积物与地幔楔发生混合之前以及混合过程中，这四个元素之间并未产生强烈亏损或富集。沉积物发生部分熔融形成熔体的过程中会发生Sr和Nd元素的分馏，然而琉球岛弧和冲绳海槽火山岩无一例外均未落在沉积物熔体与地幔楔的二端元模拟曲线上，这一结果也证实了Sr和Nd元素是在沉积物与地幔楔混合之后才发生分馏。因此，琉球岛弧和冲绳海槽火山岩的岩浆源区存在沉积物（而非沉积物熔体）与地幔楔的混合作用。俯冲沉积物整体与岛弧下方地幔楔发生混合，而且混合前不存在微量元素的分馏，这一假设与混杂岩模型（mélange model）一致。除此之外，琉球岛弧北部、中部和南部火山岩中Tl（包括Sr、Nd和Pb）同位素变化特征分别与冲绳海槽北部、中部和南部类似，揭示岛弧和弧后盆地地区沉积物俯冲通量类似，表明混杂岩物质对岛弧和弧后盆地岩浆源区的贡献量并不存在差异，这一特征也暗示混杂岩物质从俯冲板片到地幔楔岩浆源区是通过底辟作用进行输送。

        分别位于冲绳海槽北部和琉球岛弧中部的两个火山岩具有比地幔楔略轻的Tl同位素组成特征，表明这两个火山岩并未受到沉积组分的影响，或沉积组分的加入较少，同时其还具有类似埃达克岩的地球化学特征，比如相对较高的Sr/Y和La/Yb比值，贫放射性成因Sr和Pb同位素组成特征。因此这两个火山岩可能受到俯冲板片中高温蚀变洋壳的影响，或者来源于低温蚀变洋壳熔体与上覆地幔楔的进一步相互作用。

      Okinawa Trough is an active back-arc basin behind the Ryukyu Arc in the western Pacific. We report the geochemical and Sr, Nd isotopic compositions of volcanic rocks, which are recovered as lavas and pumices, dredged from northern Okinawa Trough. The volcanic rocks are composed of basaltic andesite, andesite, dacite and trachydacite. Both intermediate and felsic rocks are characterized by enrichments in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Three groups of volcanic rocks have been identified. Andesite D5-1 and trachydacite D10 have identical Sr and Nd isotopic ratios, and more enriched REE patterns and incompatible element values, compared with those of basalts in middle Okinawa Trough (MOT). Andesite D5-1 and trachydacite D10 are derived from basaltic magma by crystal fractionation. Andesite D5-2, D5-3 and D5-4 have lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd than those of MOT basalts, transitional REE patterns and trace element values between Adakite rocks and MOT basalts. Features of these samples can be interpreted by mixing of the interaction magma of mantle wedge and subducted slab-melt, with evolved basaltic magma. Dacite D22, occurring as pumice, has identical trace elements, including REE, and Sr, Nd isotopic compositions with rhyolites from the southern Ryukyu volcanic front, which indicates that this pumice was not emplaced in situ and drifted from southern Ryukyu volcanic front by Kuroshio Current.

      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. Along-arc Tl isotopic variation of the lavas mirror those found for the sediments, which implies that sediment inputs are the main control on Tl budgets for lavas in the 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 subarc 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.

     Two lavas from northern Okinawa Trough and central Ryukyu arc reveal slightly lighter Tl isotopic compositions than the mantle wedge. These samples are also characterized by adakite-like geochemical characterizations such as high Sr/Y and La/Yb ratios and less radiogenic Sr and Pb isotope compositions than lavas from same locations. Thus, these lavas might represent melts derived from high-temperature AOC or subsequent interaction of low-temperature AOC-melts with the overlying mantle wedge.