Title: Drillcore GT1 of the ICDP Oman Drilling Project: insights into magmatic processes beneath fast-spreading mid-ocean ridges
Dominik Mock (1,2), Benoit Ildefonse (2), Dieter Garbe-Schönberg (3), Samuel Müller (3), David Axford Neave (4), Jürgen Koepke (1), Oman Drilling Project Science Team (5)
Insitut für Mineralogie, Leibniz Universität Hannover, Germany (1); Géosciences Montpellier, Université de Montpellier, CNRS, Université des Antilles, Montpellier, France (2); Institut für Geowissenschaften, Christian-Albrechts-Universität zu Kiel (3); Department of Earth and Environmental Sciences, University of Manchester, UK (4); different institutions of Earth Sciences worldwide (5)
Event: GeoUtrecht 2020
The Samail ophiolite in Oman provides an ideal field laboratory for investigating the processes taking place beneath fast-spreading mid-ocean ridges (MORs). Drill site GT1, which was sampled by the Oman Drilling Project in the frame of the International Continental Scientific Drilling Program (ICDP), is located in Wadi Gideah (Wadi Tayin massif). Here, a reference profile through the entire Oman paleocrust was established  such that GT1 can be embedded into the surface profile. The core was investigated with diverse petrographic, petrological, and microstructural tools in order to better understand the accretion of the lower crust at fast-spreading MORs.
The majority of the obtained samples are gabbros or olivine gabbros. Mg#s (Mg#=Mg/(Mg+Fe)x100; molar) vary between 74 and 86 in clinopyroxene, and between 70 and 83 in some primary olivine relicts. Ca# (Ca#=Ca/(Ca+Na)x100; molar) in plagioclase varies between 68 and 87. Both Mg#s and Ca# show distinct minima with progressively increasing trends both up and down section at 1050 m above the Moho. To quantify the pole figure symmetry, i.e. fabric, of plagioclase, we use the BA index  which varies between 0 for a purely foliated and 1 for a purely lineated fabric. BA correlates with the petrological data indicating a more foliated plagioclase fabric that correlates with the minima in Mg#s and Ca#, indicating stronger compacted and/or less sheared material at this horizon. We interpret the correlation between petrological and microstructural data as indicator for the influx of more evolved melt leading to a concurrent weakening of plagioclase lineation.
 Koepke, J., Garbe-Schoenberg, D., Mueller, T., Mueller, M., Mock, D., Strauss, H., Schuth, S., Ildefonse, B., 2017. A Reference Section through the Lower Fast-spreading Oceanic Crust in the Wadi Gideah (Sumail ophiolite, Sultanate Oman): Drill Sites GT1A and GT2A within the ICDP Oman Drilling Project Abstract V43G-2737: presented at 2017 Fall Meeting, AGU, New Orleans.
 Satsukawa, T., Ildefonse, B., Mainprice, D., Morales, L. F. G., Michibayashi, K., & Barou, F. (2013). A database of plagioclase crystal preferred orientations (CPO) and microstructures-implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks. Solid Earth, 4(2), 511.
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