Dynamic control on serpentine crystallization in veins:: Constraints on hydration processes in oceanic peridotites
Dynamic control on serpentine crystallization in veins:: Constraints on hydration processes in oceanic peridotites
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DOI：
10.1029/2006gc001373
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发表时间：
2007-02-28
影响因子：
3.5
通讯作者：
Escartin, J.
中科院分区：
文献类型：
--
作者：
Andreani, M.;Mevel, C.;Escartin, J.
在大洋岩石圈中，变形和水化过程密切相关，但它们之间的反馈作用仍然知之甚少，特别是在超镁铁质岩石中，蛇纹岩作用导致岩石密度降低，这意味着体积增加和/或质量转移。蛇纹岩化伴随着丰富的脉络，以不同世代的充满静脉的蛇纹石为标志，具有高度多样化的形态和结构，对应于不同的形成机制和条件。我们使用这些矿脉来限制在缓慢扩张的海脊上大洋橄榄岩水化过程中变形和质量转移过程的作用。我们从Kane变换断层(北纬23度)附近大西洋中脊的海底蛇纹岩中选取了一组具有代表性的脉体，并用光学显微镜、电子显微镜和电子探针相结合的方法对其显微结构和化学特征进行了详细的表征。伴随蛇纹岩化的有四个主要脉状幕(V1~V4)。第一幕，被识别为V1脉，被解释为构造控制的早期海水主导的流体在橄榄岩中的渗透，在3-4公里到8公里深，在T<；300-350℃加强了热破裂和网状结构的起始。随后的两个脉阶段(V2和V3)形成在一个封闭的扩散系统中，并容纳了达到原岩近50%蛇纹岩化所需的体积膨胀。这些矿脉所开采的裂缝是由沿滑脱断层在约4~2千米的深度内逐渐剥离造成的。蛇纹岩化的程度和速度似乎受到系统创造空间和驱动正在进行的蛇纹岩化所需的传质能力的控制，而这种能力反过来又与折返速率和当地构造有关。在此期间，水合作用消耗的水可能会阻止对流热水池的建立。浅层岩石圈(<；2公里)的开放热液系统的开始，在那里脆性破裂和平流转移占主导地位，并使蛇纹岩作用得以完成，标志着最后一次脉状生成(V4)。这些结果显示了一段完整的蚀变历史，不同类型的蛇纹岩结晶记录了不同的构造事件、化学条件和岩石圈的热液蚀变模式。
Deformation and hydration processes are intimately linked in the oceanic lithosphere, but the feedbacks between them are still poorly understood, especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and/or mass transfer. Serpentinization is accompanied by abundant veining marked by different generations of vein-filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation. We use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow-spreading ridges. We have selected a representative set of veins from ocean floor serpentinites of the Mid-Atlantic Ridge near Kane transform fault (23 degrees N) and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy (SEM, TEM) with electron microprobe analyses. Four main veining episodes (V1 to V4) accompany the serpentinization. The first episode, identified as vein generation V1, is interpreted as the tectonically controlled penetration of early seawater-dominated fluid within peridotites, enhancing thermal cracking and mesh texture initiation at 3-4 km up to 8 km depth and at T < 300-350 degrees C. The two following vein stages (V2 and V3) formed in a closed, diffusive system and accommodate volume expansion required to reach almost 50% serpentinization of the protolith. The cracks exploited by these veins were caused by the progressive unroofing at depths of similar to 4 to similar to 2 km along a detachment fault. Degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization, and this capacity is in turn linked to the exhumation rate and local tectonics. During this period, water consumed by hydration may prevent the establishment of convective hydrothermal cells. The onset of an open hydrothermal system in the shallow lithosphere (< 2 km), where brittle fracturing and advective transfer dominate and enable the completion of serpentinization, is marked by the last vein generation (V4). These results show a complete history of alteration, with the crystallization of different types of serpentine recording different tectonic events, chemical conditions, and modes of hydrothermal alteration of the lithosphere.