Shallow forearc mantle dynamics and geochemistry: New insights from IODP Expedition 366
Shallow forearc mantle dynamics and geochemistry: New insights from IODP Expedition 366
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DOI：
10.1016/j.lithos.2018.10.038
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发表时间：
2019-02-01
期刊：
影响因子：
3.5
通讯作者：
Williams, H.
中科院分区：
文献类型：
--
作者：
Debret, B.;Albers, E.;Williams, H.
马里亚纳前弧是地球上一个独特的环境，蛇纹岩泥火山从15公里或更深的前弧地幔中挖掘出碎屑。这些橄榄岩碎屑是通过与板片流体相互作用而被蛇纹石化的，并提供了弧前地幔动力学和地球化学随深度变化的记录。在国际海洋发现计划（IODP）第366次考察期间，我们发现了三个不同泥火山的蛇纹岩泥中所含的蛇纹化超镁铁质碎屑，这些泥火山距离马里亚纳海沟的距离越来越远，距离板/幔界面的深度也越来越深：（到海沟的距离：55公里/到板块/地幔界面的深度：13公里）、凡坦吉斯纳（62公里/ 14公里）和亚洲特索鲁（72公里/ 18公里）。四种不同类型的超镁铁质碎屑被回收：蓝色蛇纹岩，lizardite-serpentinite，叶蛇纹石/lizardite-和叶蛇纹石-蛇纹岩。Lizardite-serpentinites主要由橙子蛇纹岩组成，形成网状和碧云母结构。拉曼和微探针分析表明，这些纹理包含富铁水镁石（XMg类似于0.84）和利蛇纹石/温石棉的混合物。叶蛇纹石/利蛇纹石-和叶蛇纹石-蛇纹岩记录了网状和粗面玄武岩结构向叶蛇纹石、磁铁矿和纯贫Fe水镁石（XMg类似于0.92）的渐进再结晶。碎屑和孔隙流体的氧同位素组成表明，从利蛇纹石到叶蛇纹石的转变是由于温度从200摄氏度上升到约400摄氏度以上的板/幔界面的弧前区域内。利蛇纹石、叶蛇纹石/利蛇纹石和叶蛇纹石蛇纹岩显示U型球粒陨石标准化稀土元素（REE）配分模式，其特征是流体移动的元素（Cs、Li、Sr、As、Sb、B、Li）相对于深海橄榄岩和/或原始地幔具有较高的浓度。利蛇纹石重结晶为叶蛇纹石的过程伴随着块状碎屑中Cs、Li和Sr的减少以及As和Sb浓度的增加，而B浓度相对恒定。一些碎屑被蓝色蛇纹石覆盖，通常与硫化物有关。这些蓝色蛇纹岩中的大多数是在Yinazao和Fantangisfia和Asia Tesoru的最高单位发现的，这表明前弧较浅部分的蚀变，可能是在碎屑的剥露过程中。这一事件的蚀变导致了平坦的稀土元素光谱和蛇纹岩中的锌浓度的增加。此外，没有系统的变化，超镁铁碎屑化学或矿物学观察到的板深度增加。这些样品记录了马里亚纳俯冲带浅部以前未描述的俯冲变质事件，与俯冲过程中蛇纹石化弧前地幔的连续埋藏一致。类似的过程，由与流体的相互作用所引起的释放从下行板，可能发生在世界各地的俯冲带。在更深处，水镁石和叶蛇纹石的破裂将导致流体和流体移动的元素如As、Sb和B的大量迁移。找到了弧岩浆的来源(C)2018 Elsevier B. V.版权所有。
The Mariana forearc is a unique setting on Earth where serpentinite mud volcanoes exhume clasts originating from depths of 15 km and more from the forearc mantle. These peridotite clasts are variably serpentinized by interaction with slab derived fluid, and provide a record of forearc mantle dynamics and changes in geochemistry with depth. During International Oceanic Discovery Program (IODP) Expedition 366, we recovered serpentinized ultramafic clasts contained within serpentinite muds of three different mud volcanoes located at increasing distance from the Mariana trench and at increasing depth to the slab/mantle interface: Yinazao (distance to the trench: 55 km / depth to the slab/mantle interface: 13 km), Fantangisna (62 km / 14 km) and Asia Tesoru (72 km / 18 km). Four different types of ultramafic clasts were recovered: blue serpentinites, lizardite-serpentinites, antigorite/lizardite- and antigorite-serpentinites. Lizardite-serpentinites are primarily composed of orange serpentine, forming mesh and bastite textures. Raman and microprobe analyses revealed that these textures contain a mixture of Fe-rich brucite (XMg similar to 0.84) and lizardite/chrysotile. Antigorite/lizardite- and antigorite-serpentinites record the progressive recrystallization of mesh and bastite textures to antigorite, magnetite and pure Fe-poor brucite (XMg similar to 0.92). Oxygen isotope compositions of clasts and pore fluids showed that the transition from lizardite to antigorite is due to the increase in temperature from 200 degrees C to about 400 degrees C within the forearc area above the slab/mantle interface. Lizardite-, antigorite/lizardite- and antigorite-serpentinites displayed U-shaped chondrite normalized Rare Earth Element (REE) patterns and are characterized by high fluid mobile element concentrations (Cs, Li, Sr, As, Sb, B, Li) relative to abyssal peridotites and/or primitive mantle. The recrystallization of lizardite to antigorite is accompanied by a decrease in Cs, Li and Sr, and an increase in As and Sb concentrations in the bulk clasts, whereas B concentrations are relatively constant. Some clasts are overprinted by blue serpentine, often in association with sulfides. Most of these blue serpentinites were recovered at Yinazao and the uppermost units of Fantangisfia and Asia Tesoru suggesting alteration in the shallower portions of the forearc, possibly during exhumation of the clasts. This episode of alteration resulted in a flattening of REE spectra and an increase of Zn concentrations in serpentinites. Otherwise, no systematic changes of ultramafic clasts chemistry or mineralogy were observed with increasing depth to the slab. The samples document previously undescribed prograde metamorphic events in the shallow portions of the Mariana subduction zone, consistent with a continuous burial of the serpentinized forearc mantle during subduction. Similar processes, induced by the interaction with fluids released from the downgoing slab, likely occur in subduction zones worldwide. At greater depth, breakdown of brucite and antigorite will result in the massive transfer of fluids and fluid - mobile elements, such as As, Sb and B. to the source of arc magmas. (C) 2018 Elsevier B.V. All rights reserved.