Models of permeability contrasts in subduction zone melange: Implications for gradients in fluid fluxes, Syros and Tinos Islands, Greece
Models of permeability contrasts in subduction zone melange: Implications for gradients in fluid fluxes, Syros and Tinos Islands, Greece
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DOI：
10.1016/j.chemgeo.2006.08.012
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发表时间：
2007-04-30
期刊：
影响因子：
3.9
通讯作者：
Ague, Jay J.
中科院分区：
文献类型：
--
作者：
Ague, Jay J.
先前研究的地质和地球化学证据表明，在希腊基克拉泽斯岛的Syros和Tinos岛上，在超镁铁质或变质沉积岩混合基质中，许多洋壳俯冲块体内部的流体流动有限。这一发现具有挑衅性，因为在俯冲过程中，火成岩和沉积岩的变质脱挥发作用应该释放了大量的流体。为了解决这一问题，采用考虑渗透率各向异性和脱挥发的二维数值模型，研究了高压/低温（HP/LT）条件下俯冲带混合岩稳态流体流动的空间模式。模拟结果表明，随着混凝块体渗透率相对于混凝块体基质的降低，更多的流体从混凝块体周围流向基质。最大通量（在基质中）相对于最小通量（在块中）的比值随着基质和渗透性较差块之间的渗透率对比而增加。渗透率的数量级变化导致区块内部与周围基质之间流体通量的数量级空间变化；在非常短的长度尺度上（小到米尺度），可以出现10倍或更多的通量变化。最大的通量产生在靠近块体边缘的基质中，这些块体边缘与区域理面和流动方向近平行。在这些地区，块体边缘的流体-岩石反应最大，导致反应过程在块体周围的分布不对称（图中为Syros上一个混杂块体的现场图）。然而，同变质变形可以使块体旋转，并在其边缘剥离反应带，因此在自然界中很可能破坏反应带的不对称性。推断Syros和Tinos上混杂岩块体内部不存在强烈的流-岩反应，这可能反映了块体内部的低渗透率，这将流体转移到混杂岩基质中。因此，俯冲带流体通过混杂岩基质的通量可能非常大，而低渗透率在不同程度上屏蔽了块体内部的区域流动。(c) 2006 Elsevier B.V.版权所有
Geological and geochemical evidence from previous studies suggest limited fluid flow through the interiors of many blocks of subducted oceanic crust within meta-ultramafic or metasedimentary melange matrix on the islands of Syros and Tinos, Cyclades, Greece. This finding is provocative because metamorphic devolatilization of igneous and sedimentary rocks during subduction should have liberated substantial volumes of fluid. In an effort to address this problem, two-dimensional numerical models incorporating permeability anisotropy and devolatilization are used to investigate spatial patterns of steady-state fluid flow through subduction zone melange under high-pressure/low-temperature (HP/LT) conditions. The modeling shows that as the permeability of the melange blocks decreases relative to the melange matrix, more flow is diverted around the blocks into the matrix. The ratio of the maximum flux (in matrix) relative to the minimum flux (in blocks) increases with the permeability contrast between the matrix and the less permeable blocks. Order-of-magnitude variations in permeability produce order-of-magnitude spatial variations in fluid fluxes between block interiors and surrounding matrix; flux variations of a factor of 10 or more can be present over very short length scales (as little as meter scale). The largest fluxes are produced in the matrix adjacent to block margins lying sub-parallel to regional foliation and flow direction. Fluid-rock reaction on block margins would be greatest in these areas, leading to asymmetrical distributions of reaction progress around blocks (illustrated herein by field mapping of a melange block on Syros). However, syn-metamorphic deformation can rotate blocks and strip off reaction zones at their margins, so it is likely that reaction zone asymmetries are commonly disrupted in nature. The inferred absence of strong fluid-rock reaction within melange blocks on Syros and Tinos could reflect low permeabilities in block interiors which acted to divert flow into melange matrix. Consequently, fluxes of subduction zone fluids through the melange matrix could have easily been very large, while low permeabilities shielded block interiors from the regional flow to varying degrees. (c) 2006 Elsevier B.V. All rights reserved.