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

阿曼苏丹国和阿拉伯联合酋长国的Samail蛇绿岩是陆地上最大、暴露程度最高、研究最多的海洋岩石圈。这是ICDP钻井计划“阿曼钻井项目”的目标，该项目将提高我们对海洋地壳和浅层地幔从海底起源到阿曼山区现代环境的形成和改变过程的理解。作为ICDP计划的一部分，本项目基于在阿曼南部蛇绿岩Wadi Gideah的三个不同地点钻探的样品，重点研究了快速扩张的海洋地壳的热液冷却作用下的沉积、演化和蚀变机制。为了达到这个目标，我们将在微观和宏观尺度上研究连贯的章节。这是测试现有理论模型的唯一方法，即通过比较成分与成分来测试地壳吸积、MORB分化和深部热液冷却。-深度剖面与模型的预测。结合详细的岩石学调查，我们将使用广泛的分析方法对大块岩石和矿物进行主要和微量元素分析，以及多种硫、锶和氧同位素。在该项目中，我们解决了四个具体目标：(1)地壳增生和分化机制。我们的初步结果表明，在深部有明显的原位结晶，这与地壳增生的常见“辉长岩冰川”模型相反。GT1和GT2的钻探将穿透下地壳深处的关键区域，致力于为这个复杂的问题提供新的线索。(2)岩脉/辉长岩临界过渡区内的岩浆/变质/热液作用。变质岩中的特征矿物分带代表夹在轴向熔融透镜体和片状岩脉之间的导电边界层，这将使我们能够提取轴向熔融透镜体地球动力学的时间尺度。站点GT3穿透轴向熔融透镜视界，专门为这个问题提供关键部分。(3)热液化断裂带对辉长岩剖面的系统横切作用。详细的岩石学和分析研究将表明，这些区域是否代表了通道海水衍生流体在非常高的温度下穿透地壳深处的通道，从而提供了一种有效的散热机制。钻井GT1和GT2将精确定位以穿透这些区域。(4)海水流体对下地壳岩石蚀变的强度、温度和程度。GT1-3钻孔将提供合适的样品材料来解决这个复杂的问题。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，以及可以追踪海水衍生流体循环的深度。

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.