Testing For Regional Fluid Flow in Subducted Crust, Tinos Island, Cyclades (Greece)

近年来，有关俯冲地壳中流体迁移的性质和程度的深刻问题已经出现。区域流体流动和大流体通量的强有力证据可以在某些环境中找到（例如，加州），但在其他地方明显缺乏（例如，阿尔卑斯山、基克拉迪群岛）。后一种情况是特别发人深省的，因为粘土，蛇纹石，碳酸盐矿物的前体沉积物和热液蚀变的火成岩应释放丰富的挥发分在超高压/低温（HP/LT）变质。为了解决这一明显的矛盾，建议通过整合高精度数字现场测绘，岩石和矿物的化学分析以及耦合流体流动和反应的数值模型，在基克拉迪群岛（希腊）的逆冲俯冲和逆行折返以及构造叠瓦作用期间测试大规模区域流体流动的存在。具体的重点是蒂诺斯岛的HP/LT岩石，迄今为止，尽管在阿尔卑斯山造山带绿帘石-蓝片岩/榴辉岩相期间发生了逆冲变质作用，但没有产生区域流动的明确证据。此外，区域流动的潜力并不局限于进积阶段，因为基克拉泽斯的许多高压/低温岩石在折返过程中经历了流体渗透和绿片岩相退积。然而，流动的过程和长度尺度仍然不清楚。现场和实验室工作将集中在以下简单的标准上，适用于逆冲和逆行设置，以测试有限或无区域流动。(1)局部或有限的脱挥发分作用（脱挥发分情况）或挥发性吸收（逆行情况）;（2）与从贯穿流体中沉淀相反，从局部环境中导出静脉质量;（3）通过流动过程对非挥发性元素进行有限的质量转移;（4）估计的流体通量较小;（5）缺乏挥发性物质的区域管道。另一方面，区域流动要求违反这些标准中的一项或多项。数值模拟将建立什么类型的流态，无论是局部或区域范围内，是与观察到的净转移反应，反应进程和元素传输的量和空间分布，反应的体积变化，流体成分，和任何流动管道的分布兼容。主要目标将是直接确定挥发性生成过程和挥发性物质从下行板逃逸的途径。这些结果将限制俯冲带流体的主要和微量元素组成的模型，这对我们理解地幔交代作用和弧岩浆化学至关重要。此外，流体运动的一般方向，无论是近垂直或板平行将被估计。局限于区域构造单元的平行板状流与跨层的近垂直流形成强烈对比，可能剥夺地幔上盘的挥发分和非挥发性元素，从而限制地幔交代作用，并最终限制弧岩浆的生成。活跃俯冲带的构造并置使下行岩石单元经历逆冲变质作用，释放流体，使经历折返作用的高压/低温岩石退化。尚待解决的问题是，流体是否可以局部迁移，或者HP/LT封装是否局部吸收挥发物，从而防止流体大规模向外迁移到表面。

AgueEAR-0105927Profound questions regarding the nature and extent of fluid migration in subducted crust have arisen in recent years. Strong evidence for regional fluid flow and large fluid fluxes can be found in some settings (e.g., California), but is notably lacking in others (e.g., Alps, Cyclades). The latter case is particularly thought-provoking because clays, serpentine, and carbonate minerals in the precursor sediments and hydrothermally altered igneous rocks should have released abundant volatiles during prograde high-pressure/low-temperature (HP/LT) metamorphism. In an effort to resolve this apparent paradox, it is proposed to test for the presence of large-scale, regional fluid flow during both prograde subduction and retrograde exhumation and tectonic imbrication in the Cycladic Archipelago (Greece) by integrating high-accuracy digital field mapping, chemical analyses of rocks and minerals, and numerical models of coupled fluid flow and reaction. The specific focus is on the HP/LT rocks of Tinos island, which to date have yielded no clear evidence for regional flow despite prograde metamorphism during the Alpine orogeny to epidote-blueschist/eclogite facies. Futhermore, the potential for regional flow is not limited to the prograde stage, because many of the HP/LT rocks of the Cyclades underwent fluid infiltration and greenschist facies retrogression during exhumation. However, the processes and length scales of flow remain unclear.The field and laboratory work will focus on the following straightforward criteria, applicable to both prograde and retrograde settings, to test for limited or no regional flow. (1) Local or limited devolatilization (prograde case) or volatile uptake (retrograde case); (2) derivation of vein mass from local surroundings as opposed to precipitation from through-going fluids; (3) limited mass transfer of non-volatile elements by flow processes; (4) small estimated fluid fluxes and; (5) absence of regional conduits for volatiles. Regional flow, on the other hand, requires that one or more of these criteria be violated. The numerical modeling will establish what type of flow regimes, whether local or regional in scope, are compatible with observed net transfer reactions, amounts and spatial distribution of reaction progress and element transport, volume changes of reaction, fluid compositions, and distribution of any flow conduits. The major goal will be direct determination of processes of volatile generation and pathways of volatile escape from downgoing slabs. The results will constrain models of the major and trace element composition of subduction zone fluids, critical for our understanding of mantle metasomatism and arc magma chemistry. Furthermore, the general directions of fluid motion, whether subvertical or slab-parallel will be estimated. Slab-parallel flow confined to regional tectonic units, in strong contrast to subvertical flow across layers, may deprive the mantle hanging wall of volatiles and transported non-volatile elements, therefore limiting mantle metasomatism and, ultimately, arc magma generation. Tectonic juxtaposition in active subduction zones allows downgoing rock units undergoing prograde metamorphism to release fluids that retrograde HP/LT terranes undergoing exhumation. What remains to be resolved is if the fluids can migrate regionally, or if the HP/LT packages absorb volatiles locally, preventing large-scale outwards fluid migration to the surface.