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

俯冲地壳区域流体流动测试，基克拉泽斯群岛蒂诺斯岛（希腊）

• 批准号: 0105927
• 负责人: Jay Ague
• 金额: $ 21.88万
• 依托单位: Yale University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-08-01 至 2005-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0105927&HistoricalAwards=false
• 关键词: Testing; Regional; Fluid; Flow; Subducted

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.

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深刻问题。在某些地区（例如加利福尼亚州）可以找到区域流体流动和大流体通量的有力证据，但在其他地区（例如阿尔卑斯山、基克拉泽斯）则明显缺乏。后一种情况尤其发人深省，因为前体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进行的高压/低温（HP/LT）变质作用过程中释放出丰富的挥发物。为了解决这一明显的悖论，建议通过集成高精度数字现场测绘来测试基克拉迪群岛（希腊）顺行俯冲和逆行折返以及构造叠瓦期间是否存在大规模区域流体流动、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。特别关注的是蒂诺斯岛的高压/低温岩石，尽管在阿尔卑斯造山运动期间发生了渐进变质作用，形成了绿帘石-蓝片岩/榴辉岩相，但迄今为止，尚未提供区域流动的明确证据。此外，区域流动的潜力不仅限于前积阶段，因为基克拉泽斯群岛的许多高压/低温岩石在折返过程中经历了流体渗透和绿片岩相退缩。然而，流动的过程和长度尺度仍不清楚。现场和实验室工作将集中于以下简单的标准，适用于顺行和逆行设置，以测试有限或无区域流动。 (1) 局部或有限的脱挥发分（顺行情况）或挥发吸收（逆行情况）； (2) 静脉质量源自局部环境，而不是源自通过的流体的沉淀； (3)流动过程对非挥发性元素的有限传质； (4) 较小的估计流体通量； (5) 缺乏挥发物的区域通道。另一方面，区域流动则要求违反这些标准中的一个或多个。数值模拟将确定哪种类型的流态（无论是局部还是区域范围）与观察到的净转移反应、反应进程和元素传输的数量和空间分布、反应的体积变化、流体成分和任何流的分布兼容导管。主要目标是直接确定挥发物产生过程以及挥发物从下行板片逸出的途径。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，这对于我们理解地幔交代作用和弧岩浆化学至关重要。此外，还将估计流体运动的一般方向，无论是垂直的还是平板平行的。局限于区域构造单元的板片平行流，与跨层的垂直流形成鲜明对比，可能会剥夺地幔上盘的挥发物和输送的非挥发性元素，从而限制地幔交代作用，并最终限制弧岩浆的生成。活跃俯冲带中的构造并置使得经历顺变质作用的下行岩石单元释放出流体，使经历折返的高压/低温地体逆行。尚待解决的问题是流体是否可以局部迁移，或者 HP/LT 封装是否能局部吸收挥发物，从而防止流体大规模向外迁移到地表。