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

AgueEAR-0105927近年来出现了有关俯冲地壳中流体迁移的性质和程度的问题。区域流体流动和大流体通量的有力证据可以在一些环境中找到(例如加利福尼亚)，但在其他环境中明显缺乏(例如阿尔卑斯山、旋风)。后一种情况特别发人深省，因为前驱沉积物和热液蚀变火成岩中的粘土、蛇纹石和碳酸盐矿物应该在推进高压/低温(HP/LT)变质过程中释放出大量的挥发分。为了解决这一明显的悖论，有人建议通过整合高精度的数字野外填图、岩石和矿物的化学分析以及流体流动和反应耦合的数值模型，来检验在希腊塞克拉迪奇群岛进行性俯冲和逆行折返以及构造叠置过程中是否存在大规模的区域性流体流动。特别关注的是蒂诺斯岛的HP/LT岩石，尽管在阿尔卑斯山造山到绿帘石-蓝片岩/榴辉岩相期间发生了进退变质作用，但至今仍没有明显的区域流动证据。此外，区域流动的潜力并不局限于进行性阶段，因为旋回的许多HP/LT岩石在折返过程中经历了流体渗透和绿片岩相退化。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，既适用于正向和逆行环境，以测试有限的或没有区域流动。(1)局部或有限的去挥发(顺行情况)或挥发吸收(逆行情况)；(2)来自局部环境的脉团，而不是来自通过的流体的沉淀；(3)流动过程对非挥发元素的传质有限；(4)估计的流体通量较小；(5)没有区域的挥发物管道。另一方面，区域流动要求违反这些标准中的一个或多个。数值模拟将确定哪种流型，无论是局部的还是区域的，与观察到的净转移反应、反应进度和元素迁移的数量和空间分布、反应的体积变化、流体组成和任何流动管道的分布相一致。主要目标将是直接确定挥发分的产生过程和挥发分从下行板中逃逸的途径。这一结果将制约俯冲带流体的主元素和微量元素组成的模式，这对我们理解地幔交代和弧岩浆化学至关重要。此外，还将估计流体运动的总体方向，无论是近垂直的还是平行的。局限于区域构造单元的板条平行流动与跨层次垂直流动形成强烈对比，可能会剥夺地幔上盘挥发分和输送的非挥发元素，从而限制地幔交代作用，并最终限制弧状岩浆的产生。活动俯冲带中的构造并置使经历递进变质作用的下行岩石单元释放出流体，使正在进行折返的HP/LT地体倒退。有待解决的问题是，流体是否可以区域性迁移，或者HP/LT组件是否能在当地吸收挥发物，防止流体大规模向外迁移到地表。

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.