A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Petrography and Petrology

Abstract

In the absence of a complete profile through fast‐spreading modern oceanic crust, we established a reference profile through the whole paleo crust of the Samail ophiolite (Sultanate of Oman), which is regarded as the best analogue for fast‐spreading oceanic crust on land. To establish a coherent data set, we sampled the Wadi Gideah in the Wadi‐Tayin massif from the mantle section up to the sheeted dikes and performed different analytical and structural investigations on the same suite of samples. This paper reports our studies of the lower crust, a 5 km thick pile of gabbros, focusing on petrographic features and on the results of mineral analyses. Depth profiles of mineral compositions combined with petrological modeling reveal insights into the mode of magmatic formation of fast‐spreading lower oceanic crust, implying a hybrid accretion mechanism. The lower two thirds of the crust, mainly consisting of layered gabbros, formed via the injection of melt sills and in situ crystallization. Here, upward moving fractionated melts mixed with more primitive melts through melt replenishments, resulting in a slight but distinct upward differentiation trend. The upper third of the gabbroic crust is significantly more differentiated, in accord with a model of downward differentiation of a primitive parental melt originated from the axial melt lens located at the top of the gabbroic crust. Our hybrid model for crustal accretion requires a system to cool the deep crust, which was established by hydrothermal fault zones, initially formed on‐axis at very high temperatures.

A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Whole Rock Geochemistry

“…This explains some rather primitive compositions found in the AML horizon. In the following, we show that our results for bulk rock geochemistry are in full accord with findings of , Koepke et al (2022), andMüller et al (2022).…”

A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Trace Element Systematics and REE Crystallization Temperatures—Implications for Hybrid Crustal Accretion

“…A striking feature of the Wadi Gideah layered gabbros is the tendency toward inverse zonation found in plagioclase Ca# and, to a more limited extent, in clinopyroxene Mg# (Figures 3b and 3i), a feature already observed by Müller (2015) and Koepke et al. (2022) for the whole sample set of the Wadi Gideah reference profile. In addition, although less systematic, this feature has been observed by Browning (1982) for a rhythmic series of layers in the Oman Wadi Shafan gabbroic complex and by Bosch et al.…”

“…Olivine data of the samples investigated here and discussion about ultramafic lithologies is beyond the scope of this paper but can be found in Koepke et al. (2022).…”

“…A successful model has to explain the lower crustal cumulate rock sequence. From MTZ to the AML: Modally layered gabbros with a layering parallel to the crust‐mantle transition, followed by foliated gabbros that show a typical lineation steepening toward shallower crustal levels, in turn, followed by varitextured gabbros, which are regarded as the frozen fillings of the AML (Koepke et al., 2022). In the gabbro glacier model (also referred to as ductile flow model), indicated by fabric pattern as described above, gabbros of the lower crust are regarded as cumulates from a top‐down crystal mush flow that forms a gabbro glacier at the ridge axis flanks originating at the AML.…”

“…By focusing on primary magmatic features as a function of crustal depth, this work provides a direct link between the on‐axis thermal rock record and numerical models of crustal accretion. This paper is part of a series of manuscripts that deal with different aspects of the Wadi Gideah transect sample suite obtained from the Oman ophiolite, which provides a coherent section through fast‐spread oceanic crust (Garbe‐Schönberg et al., 2022; Koepke et al., 2022; Mock, Ildefonse, et al., 2021).…”