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

沿着全球洋中脊网络形成的新洋壳约占地球岩浆收支的75%，每年有20立方公里的岩浆侵位。一个密切相关的过程是海水在新形成的地壳中循环，提取岩浆热量，产生热液，形成块状硫化物矿床，并为海底的化学合成生态系统提供营养。然而，对岩浆结晶和冷却形成洋壳的过程仍然知之甚少。现有的关于沿着快速扩张的洋中脊的地壳增生的端元模型在下洋壳内不同深度的结晶比例上有所不同。因此，这些模型预测了不同的热演化，以及不同的深度，热液流体在洋壳中循环。因此，这意味着冷却速率作为深度的函数的不同变化。Samail蛇绿岩（阿曼，阿联酋）是世界上最大、暴露最好、研究最多的陆上洋壳块体。我们建议，以确定冷却率作为一个功能的深度内的辉长岩钻在ICDP阿曼钻探项目在瓦迪Gideah的Samail蛇绿岩。定量冷却速率将从地质速度计检索的基础上，温度依赖于不同矿物之间的元素扩散交换。两个独立的扩散计时器（斜长石中镁和橄榄石中钙地质速度计）将用于相同的样品（只要可能），以检查内部一致性。根据Wadi Gideah的岩芯样本，将有可能确定Samail蛇绿岩下洋壳整个剖面的冷却速率剖面。这一新的冷却速率剖面将与最近在现代洋壳中确定的冷却速率剖面进行比较，这将提供有关现代洋壳和阿曼蛇绿岩中暴露的古海洋岩石圈冷却历史之间的相似性和潜在差异的信息。此外，新的冷却速率配置文件将进行比较，以前确定的冷却速率配置文件从不同地区的阿曼蛇绿岩，这是只基于钙在Ol数据。这一比较将用于了解对比数据集，并解决地壳冷却率较低的自然空间和时间变化。与热模型的比较将提供关于下洋壳排热的主要机制（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相结合，以限制：（a）辉长岩壳在快速扩张的海脊形成的过程;（B）这些海脊热液循环的性质（范围、深度）及其对海洋水系统总化学通量的贡献。

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.