Determination of cooling rates of the lower oceanic crust on drilled rocks of the ICDP Oman Drilling Project.

沿全球大洋中脊网络形成的新洋壳约占地球-S岩浆预算的75%，每年涉及20千米~3的岩浆侵位。一个密切相关的过程是海水在新形成的地壳中循环，提取岩浆热并产生热液，形成大量硫化物矿床，为海底的化学合成生态系统提供养分。然而，有关岩浆结晶和冷却以形成洋壳的过程仍然知之甚少。现有的沿快速扩展的大洋中脊的地壳吸积的端元模型在下洋壳内不同深度的结晶比例不同。因此，这些模型预测了不同的热演化，以及不同的热液流体在洋壳中循环的深度。因此，这意味着冷却速度随深度的不同而变化。Samail蛇绿岩(阿曼，阿联酋)是世界上S最大、暴露最好、研究最深入的海底地壳块体。我们建议在萨迈勒蛇绿岩Wadi Gideah的ICDP阿曼钻探项目中，确定冷却速度作为辉长岩中深度的函数。将根据不同矿物之间元素随温度变化的扩散交换情况，从地球测速仪中提取定量冷却速率。两个独立的扩散计时器(斜长石中的镁和橄榄石中的钙)将用于相同的样品(只要有可能)，以检查内部一致性。根据来自Wadi Gideah的钻芯样本，将有可能确定Samail蛇绿岩整个下部洋壳的冷却速度剖面。这一新的冷却速率剖面将与最近在现代洋壳中测定的冷却速率剖面进行比较，这将提供现代洋壳冷却历史与阿曼蛇绿岩中暴露的古代大洋岩石圈之间的相似和潜在差异的信息。此外，新的冷却速度曲线将与之前确定的阿曼蛇绿岩不同区域的冷却速度曲线进行比较，这些曲线仅基于Ca-In-Ol数据。这种比较将被用来了解对比数据集，并处理较低地壳冷却速率的自然空间和时间变异性。与热学模型的比较将提供有关下部洋壳热排出的主要机制(传导冷却与离轴热液循环冷却)的信息。新的结果将与现有的洋壳形成模型结合起来，以约束：(A)在快速扩张的海脊形成辉长岩结壳的过程；(B)在这些海脊的热液循环的性质(范围、深度)及其对整个海洋水系统化学通量的贡献。

Formation of new oceanic crust along the global network of mid-ocean ridges accounts for ~75% of the Earth_s magmatic budget and involves the emplacement of 20 km3 of magma per year. An intimately linked process is the circulation of seawater through newly formed crust that extracts magmatic heat and produces hydrothermal fluids that form massive sulfide deposits and feed chemosynthetic ecosystems on the seafloor. However, the processes involved in crystallizing and cooling the magma to form the oceanic crust are still poorly understood. The existing end-member models of crustal accretion along fast-spreading mid-ocean ridges differ in the proportion of crystallization at different depths within the lower oceanic crust. Therefore, these models predict different thermal evolution, and different depths to which hydrothermal fluids circulate in the oceanic crust. As a consequence, this implies different variations of cooling rate as a function of depth. The Samail Ophiolite (Oman, U.A.E.) is the world_s largest, best-exposed, and most-studied subaerial block of oceanic crust. We propose to determine cooling rates as a function of depth within the gabbroic rocks to be drilled in the ICDP Oman Drilling Project in the Wadi Gideah of the Samail Ophiolite. Quantitative cooling rates will be retrieved from geospeedometers based on the temperature dependent diffusive exchange of elements between different minerals. Two independent diffusion chronometers (the Mg-in-plagioclase and the Ca-in-olivine geospeedometer) will be used on the same samples (whenever possible) to check for internal consistency. Based on drill core samples from the Wadi Gideah, it will be possible to determine a cooling rate profile over the complete section of the lower oceanic crust from the Samail Ophiolite. This new cooling rate profile will be compared to a cooling rate profile recently determined in modern oceanic crust, which will provide information about similarities and potential differences between the cooling history of modern oceanic crust and ancient oceanic lithosphere exposed in the Oman ophiolite. Furthermore, the new cooling rate profile will be compared to previously determined cooling rate profiles from different areas of the Oman ophiolite, that were based only on Ca-in-Ol data. This comparison will be used to understand contrasting data sets and to address the natural spatial and temporal variability in lower crustal cooling rates. A comparison with thermal models will provide information about the dominant mechanism of heat removal (conductive cooling vs cooling by off-axis hydrothermal circulation) in the lower oceanic crust. The new results will be combined with the existing models on formation of the oceanic crust to constrain: (a) the process by which gabbroic crust is formed at fast-spreading ridges; and (b) the nature (extent, depth) of hydrothermal circulation at these ridges, and its contribution to the overall chemical flux to the oceanic water system.