A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Insights From Crystallographic Preferred Orientations. Issue 6 (24th June 2021)

• Record Type: Journal Article
• Authors: Mock, Dominik; Ildefonse, Benoît; Müller, Tim; Koepke, Jürgen
• Is Part Of: Journal of geophysical research. Volume 126:Issue 6(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-24
• Subjects: Oman ophiolite -- crystallographic preferred orientations -- crystal size distributions -- lower oceanic crust; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB021864
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: We established a 5, 000 m thick profile through the paleo lower oceanic crust of the Samail ophiolite (Sultanate of Oman, Wadi Gideah), in order to investigate accretion processes beneath fast‐spreading mid‐ocean ridges. The Samail ophiolite is regarded as best on‐land analog for fast‐spread oceanic crust, and Wadi Gideah allows sampling of the entire lower crust along the Wadi bed. Here, we provide microstructural constraints to lower crustal accretion beneath fast‐spreading mid‐ocean ridges, which reveal changing microstructures and fabrics with depth. Grain size coarsening occurs from the foliated to the layered gabbro section. A ∼350 m thick zone of gabbroic rocks from the profile top, interpreted as frozen fillings of the axial melt lens and defined as varitextured gabbros, shows intergranular textures without crystallographic preferred orientations. Rocks from the varitextured/foliated gabbro transition (∼500 m thick) and from the upper foliated gabbros (∼600 m) are foliated and lineated. The lineation is absent or very weak in the underlying ∼800 m thick lower foliated gabbros. In layered gabbros, the fabric gradually strengthens and becomes more lineated down section with local scattering at small spatial scales. This implies distinct accretion mechanisms in the deep and shallow plutonic crust. For the layered and lower foliated gabbros, our data suggest in‐situ crystallizing individual magma reservoirs. Dominant lineation in the upper foliated gabbros and the varitextured/foliated gabbro transition suggests vertically transported crystal‐laden melts or mushes being consistent with subsiding crystal mushes from the axial melt lens, and/or crystallizing upward migrating melt expelled from the crystal mush beneath. Plain Language Summary: "Ophiolites" are fragments of ancient oceanic crust and shallow mantle thrusted on land mainly during mountain building processes. They provide the potential for a deep understanding how the oceanic crust forms. The largest, best‐preserved and best‐investigated ophiolite on our planet is the Samail ophiolite in the Sultanate of Oman, where we performed our study. We sampled a continuous lower crustal section, consisting of gabbro, from the upper mantle up to the transition with the upper basaltic crust in Wadi Gideah, near the town of Ibra in Northern Oman, with the aim to understand its formation, which is up to now controversially discussed. We used the Electron Backscattered Diffraction technique for investigating the microstructures of the sampled rocks, enabling insights into the mechanism of magmatic emplacement of the lower crust. These microstructures reveal that the upper third of the lower crust is dominated by vertical transport of melt and magmatic mush, while the lower two thirds were formed by intrusions and in‐situ crystallization. Key Points: A Reference section through fast‐spread lower oceanic crust was established with respect to microstructures Microstructural features of the lower oceanic crust of the Samail paleoridge vary with depth Data suggest a hybrid accretion model with different mechanisms for the upper and lower plutonic crust