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

沿着全球洋中脊网形成的新洋壳占地球岩浆预算的75%，每年有20 km3的岩浆入位。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，形成大量硫化物沉积物，为海底的化学合成生态系统提供养分。然而，岩浆结晶和冷却形成海洋地壳的过程仍然知之甚少。现有的沿快速扩张洋中脊的地壳吸积端元模型在下洋壳不同深度的结晶比例存在差异。因此，这些模型预测了不同的热演化，以及热液流体在海洋地壳中循环的不同深度。因此，这意味着冷却速率作为深度函数的不同变化。Samail蛇绿岩（阿曼，阿联酋）是世界上最大、暴露程度最高、研究最多的海底地壳块体。我们建议在Samail蛇绿岩Wadi Gideah的ICDP阿曼钻井项目中，确定冷却速率作为钻取辉长岩深度的函数。定量冷却速率将根据不同矿物之间依赖于温度的元素扩散交换从地速计中获得。两个独立的扩散计时器（mg -in-斜长石和ca -in-橄榄石地质速度计）将在相同的样品上使用（只要可能）来检查内部一致性。基于Wadi Gideah的岩心样本，将有可能确定Samail蛇绿岩整个海底地壳的冷却速率剖面。这一新的冷却速率剖面将与最近在现代海洋地壳中确定的冷却速率剖面进行比较，这将提供有关现代海洋地壳冷却历史与阿曼蛇绿岩中暴露的古代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，新的冷却速率剖面将与先前仅基于Ca-in-Ol数据的阿曼蛇绿岩不同区域确定的冷却速率剖面进行比较。这种比较将用于理解对比数据集，并解决较低地壳冷却速率的自然时空变异性。与热模式的比较将提供关于下海洋地壳的主要散热机制（导电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式相结合，以约束：(a)辉长岩壳在快速扩张脊上的形成过程；(b)这些脊上热液环流的性质（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.