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

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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.