Variant across-forearc compositions of slab-fluids recorded by serpentinites: Implications on the mobilization of FMEs from an active subduction zone (Mariana forearc)
Variant across-forearc compositions of slab-fluids recorded by serpentinites: Implications on the mobilization of FMEs from an active subduction zone (Mariana forearc)
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DOI：
10.1016/j.lithos.2020.105525
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发表时间：
2020-07
期刊：
影响因子：
3.5
通讯作者：
E. Albers;Wolf-Achim Kahl;Lena Beyer;W. Bach
中科院分区：
文献类型：
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作者：
E. Albers;Wolf-Achim Kahl;Lena Beyer;W. Bach
马里亚纳前弧的蛇纹岩泥火山作用为研究活跃俯冲带的浅部提供了一个窗口。通过研究由蛇纹岩泥火山带入海底的板源流体和蛇纹岩化地幔楔物质的微量元素组成，可以评估流体-岩石相互作用和相关的地幔楔质量传递。我们研究了国际海洋发现计划第366次探险中从Yinazao、Fantangisña和Asùt Tesoru泥火山中回收的不同蛇纹石化超镁铁质碎屑，以研究流体可移动元素（FMEs）从板块到楔块的转移。这些泥火山取样于深度约13-18公里的板楔界面，估计温度为80-250°C。我们的样品代表了蛇纹石化的前弧，并表现出多阶段的蛇纹石化历史，这从微组构、矿物学和不同代蛇纹石的原位主要和痕量元素分析中可以明显看出。弧前地幔的初始水化作用发生在富硅流体的还原条件下。早期蛇纹石的特征是Li、Sr、Rb、Cs和Ba的浓度普遍较高。随后的流-岩相互作用主要由富Si和贫fme流体驱动，后期由泥火山导管中的贫Si和贫fme流体驱动，后者导致富铁水镁石的大量形成。爱荷华岩和赤铁矿表明，碎屑在海底就位后的蚀变过程中，还原条件较少。B的浓度普遍较高，但我们的数据无法区分来自板岩和海水的B。来自浅源Yinazao的蛇纹石的Rb/Cs比值高，≤37，Li浓度最高，但Rb、Sr、Ba和Cs含量最低。蛇纹岩浆液来源于沉积孔隙水的排出和俯冲沉积物中蛋白石的分解。中间来源Fantangisña的蛇纹石Rb/Cs比值<10，Li、Sr和Ba浓度与Yinazao相似，但Rb和Cs含量较高。这些模式可能反映了俯冲沉积物中粘土的脱水和fme释放。深层来源Asùt Tesoru的流体也来自粘土破碎，但Rb、Sr、Cs和Ba浓度的增加进一步表明了蚀变海洋地壳开始脱水。包括来自南查莫罗蛇尾岩泥火山（18 km板块深度；Kahl等，2015,Lithos）的数据，我们提供了弧前浅深度板块脱水反应的详细记录，以及FMEs的相关动员及其向地幔楔体的运输。我们的研究表明，在与地幔楔橄榄岩相互作用过程中，板源流体发生了广泛的蚀变。因此，来自蛇纹岩泥火山的孔隙水提供了对深层过程的不完全了解；在蛇纹石等早期水化产物中，可以最好地记录板-楔界面的流体特征及其跨弧变化。
Serpentinite mud volcanism in the Mariana forearc provides a window into the shallow portions of an active subduction zone. Fluid–rock interactions and related mass transfers into the mantle wedge can be assessed by studying the trace element compositions of slab-derived fluids and serpentinized mantle wedge materials brought to the seafloor by the serpentinite mud volcanoes. We investigated variably serpentinized ultramafic clasts from the Yinazao, Fantangisña, and Asùt Tesoru mud volcanoes recovered on International Ocean Discovery Program Expedition 366 to examine the transfer of fluid-mobile elements (FMEs) from the slab to the wedge. These mud volcanoes sample the slab–wedge interface at depths of ~13–18 km and estimated temperatures of 80–250 °C. Our samples represent the serpentinized forearc and exhibit a multi-phase serpentinization history, as apparent from microfabrics, mineralogy, and in situ major and trace elemental analyses of distinct generations of serpentine. Initial hydration of the forearc mantle occurred under reducing conditions by Si-rich fluids. Early serpentine is characterized by generally high concentrations of Li, Sr, Rb, Cs, and Ba. Subsequent fluid–rock interactions were driven by Si-rich and FME-poor fluids and at later stages by Si- and FME-poor fluids in the mud volcano conduits, the latter of which resulted in the abundant formation of Fe-rich brucite. Iowaite and hematite indicate that less reducing conditions prevailed during the alteration of clasts after their emplacement at the seafloor. Concentrations of B are generally high but our dataset does not allow distinguishing slab- from seawater-derived B.Serpentine from the shallow-sourced Yinazao exhibits high Rb/Cs ratios of ≤37, highest concentrations of Li, but lowest Rb, Sr, Ba, and Cs contents. The serpentinizing fluids were derived from expulsion of sedimentary pore waters and by the breakdown of opal in the subducted sediments. Serpentine at the intermediate-sourced Fantangisña has Rb/Cs ratios of <10, similar Li, Sr, and Ba concentrations as Yinazao, but higher Rb and Cs contents. These patterns likely reflect dewatering and FME-release from clays in the subducted sediments. Fluids at the deeply sourced Asùt Tesoru as well originate from clay-breakdown, but increased concentrations of Rb, Sr, Cs, and Ba are further indicative of beginning dehydration of altered oceanic crust.Including data from the South Chamorro serpentinite mud volcano (18 km slab depth; Kahl et al., 2015, Lithos), we provide a detailed record of slab dehydration reactions at shallow forearc depths and the related mobilization of FMEs as well as their transport into the mantle wedge. Our study demonstrates that slab-derived fluids undergo extensive alteration during the interaction with mantle wedge peridotite. Pore waters from the serpentinite mud volcanoes hence provide incomplete insight into the processes at depth; fluid signatures at the slab–wedge interface as well as their across-forearc changes are best recorded in early hydration products such as serpentine.