Long in-situ sections in the Wadi Gideah, Oman ophiolite: The key for understanding the mechanism of accretion, magmatic evolution and cooling of lower fast-spread oceanic crust

位于苏丹国阿曼和阿拉伯联合酋长国的Samail蛇绿岩是陆地上最大、暴露最好、研究最多的一块大洋岩石圈。这是国际地球科学中心钻探项目“阿曼钻探项目”的目标，该项目将提高我们对大洋地壳和浅地幔从海底的起源到阿曼山区的现代背景的一系列过程的理解。在这一ICDP倡议的框架内，这里提出的项目侧重于通过热液冷却快速扩张的洋壳的沉积、演化和改变的机制，基于将在阿曼南部蛇绿岩的Wadi Gideah的三个不同地点钻探的样品。为了实现这一目标，我们将在微观和宏观尺度上调查连贯的部分。这是检验现有的地壳增长、MORB分异和深部热液冷却理论模型的唯一方法，方法是将成分-深度剖面与模型的预测进行比较。结合详细的岩相学调查，我们将使用广泛的分析方法对大宗岩石和矿物进行常量和微量元素分析，以及硫、锶和氧多种同位素的分析。在该项目中，我们讨论了四个具体目标：(1)地壳增长和分异机制。我们的初步结果表明，在深度上存在显著的原位结晶，这与常见的地壳吸积“辉长岩冰川”模型形成了鲜明对比。在GT1和GT2地点的钻探将穿透下地壳更深的关键地带，致力于揭示这一复杂问题的新线索。(2)关键岩脉/辉长岩过渡过程中的岩浆/变质/热液过程。变质岩中代表夹在轴向熔体透镜和片状岩脉之间的导电边界层的特征矿物分带将使我们能够提取轴向熔体透镜地球动力学的时间尺度。穿透轴向熔融透镜层的GT3站点致力于为这一问题提供关键剖面。(3)系统地横切辉长岩剖面的热液断裂带的作用。详细的岩石学和分析研究将表明，这些区域是否代表了海水衍生流体在极高温度下穿透地壳深层的通道，从而提供了一种有效的热量排出机制。钻探GT1和GT2的位置将准确地穿透这些带。(4)海水流体对下地壳岩石的强度、温度和程度。钻探GT1-3将提供适当的样品材料来解决这一复杂问题。多种硫、氧和锶同位素组成的变化将使我们能够量化背景蚀变过程的强度，以及可以追踪到的海水流体循环的深度。

The Samail ophiolite in the Sultanate Oman and the United Arab Emirates is the largest, best-exposed, and most-studied piece of oceanic lithosphere on land. It is the target for an ICDP drilling initiative, "the Oman Drilling Project", which will improve our understanding of the spectrum of processes that create and modify the oceanic crust and shallow mantle from its origin on the ocean floor to its modern setting in the mountains of Oman. Embedded within this ICDP initiative, the project presented here focuses on the mechanisms of accretion, evolution and alteration by hydrothermal cooling of fast-spreading oceanic crust, based on samples to be drilled at three different sites in the Wadi Gideah in the southern Oman ophiolite. To reach this goal we will investigate coherent sections on a micro- and macro-scale. This is the only way to test the available, theoretical models for crustal accretion, MORB differentiation, and deep hydrothermal cooling by comparing composition-vs.-depth profiles with the predictions of the models. In combination with detailed petrographic surveys, we will use a wide spectrum of analytical methods for bulk rock and mineral analysis of major and trace elements as well as multiple sulfur, strontium and oxygen isotopes. Within the project we address four specific objectives:(1) Crustal accretion and mechanisms of differentiation. Our preliminary results point to significant in-situ crystallization in the depth which is in contrast to the common "gabbro glacier" model for crustal accretion. The drilling at sites GT1 and GT2 will penetrate critical zones in the deeper parts of the lower crust, dedicated to shed new light on this complex issue.(2) Magmatic/metamorphic/hydrothermal processes within the critical dike/gabbro transition. Characteristic mineral zoning in metamorphic rocks representing the conducting boundary layer sandwiched between the axial melt lens and the sheeted dikes will enable us to extract time scales of the geodynamics of the axial melt lens. Site GT3, penetrating the axial melt lens horizon, is dedicated to provide key sections for this issue.(3) Role of hydrothermalized fault zones crosscutting the gabbro section in a systematic way. Detailed petrographic and analytical studies will show whether these zones represent pathways where channeled seawater-derived fluids penetrated at very high temperatures the deep crust, thereby providing an efficient mechanism of heat removal. The drilling GT1 and GT2 will be precisely sited to penetrate such zones.(4) Intensity, temperature, and extent of lower crustal rock alteration by seawater-derived fluids. The drillings GT1-3 will provide appropriate sample material to address this complex issue. Variation of the compositions of multiple sulfur, oxygen, and strontium isotopes will enable us to quantify the intensity of background alteration processes and how deep the circulation of a seawater-derived fluid can be traced.