Résumé
As largest and best-exposed paleo-fast-spread oceanic crust on land, the Samail ophiolite in the Sultanate of Oman represents an ideal natural laboratory for investigating processes at fast-spreading mid-ocean ridges. We studied two layered gabbro sequences from different stratigraphic depths: one from the middle of the plutonic crust showing dm-scale modal layering with olivine abundance gradually decreasing from layer base to top (Wadi Somerah, Sumail block) and one located near the crust-mantle boundary showing mm-scale layers being enriched in olivine (Wadi Wariyah, Wadi Tayin block). Petrological analyses for major and minor element compositions of the phases olivine, clinopyroxene and plagioclase were performed at the Institut für Mineralogie (Leibniz Universität Hannover, Germany) using a Cameca SX100 microprobe. Core and rim analyses on olivine, clinopyroxene and plagioclase were done using a beam current of 15 nA with 15 kV accelerating voltage and 10 seconds measurement time on the peak and 5 seconds on each background. On clinopyroxene, the beam was defocused to 20 µm, olivine and plagioclase analyses were performed with a beam defocused to 2 µm. Calcium in olivine and Mg in plagioclase profiles for the estimation of closure temperatures and cooling rates were performed with a focused beam with an accelerating voltage of 15 kV, a beam current of 100 nA, and 240 seconds peak time and 120 seconds on each background. Electron backscattered diffraction (EBSD) was performed at Géosciences Montpellier (Université de Montpellier, France) using a Jeol JSM 5600 SEM equipped with an Oxford/Nordlys EBSD detector. The accelerating voltage was 15 kV, the scanning step size varied from 16.5 to 30 µm depending on the grain size of the sample.Major element measurements reveal mineral compositions being more primitive in Wadi Wariyah than in Wadi Somerah. Compositions vary along the profiles representing no systematic trend, neither along the entire profiles nor within single layers. Weak zonation is observed in the Mg# and the TiO2 content of clinopyroxene in Wadi Somerah. The modeled cooling rates vary from 1.2 to 21°C per thousand years in Wadi Somerah and 3 to 130°C per thousand years in Wadi Wariyah. The closure temperatures of olivine cores vary from 910 to 965°C in Wadi Somerah and from 1074 to 1106°C in Wadi Wariyah. The closure temperatures of plagioclase rims vary from 850 to 930°C. The fabric strength is generally low in Wadi Somerah and slightly increased in Wadi Wariyah. BA and BC indices give a stronger lineation in Wadi Wariyah and the grain orientation spread is also higher for the samples from Wadi Wariyah.