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

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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.