Description
The Oman ophiolite represents the largest segment of former oceanic lithosphere tectonically emplaced on land and has inspired and tested models of how fast-spreading oceanic crust forms and is modified. Although the ophiolite was part of an oceanic ridge spreading center 96 million years ago, it subsequently became the hangingwall of a subduction zone with convergent plate margin magmatism affecting both the existing oceanic mantle and crust. Studies of surface outcrops in the ophiolite have raised many questions and hypotheses on the details of magmatic, hydrothermal and tectonic processes during the evolution of the ophiolithe. On the basis of a set of objectives and hypotheses from previous work, the ICDP funded Oman Drilling Project (2016-2018) had the goal of sampling deeper, lessweathered domains of the southern part of the ophiolite. This was achieved by continuous coring as well as monitoring conditions in drill hole observatories.
This proposal is addressing objectives that center around the processes that formed and modified the lower crust and uppermost mantle in the Wadi Tayin massif of the ophiolite. Recently acquired data on lower crustal rocks and mantle rocks underneath show that both are in osmium isotopic disequilibrium, meaning the crust cannot derive by melting of the local mantle with transport pathways of crustal magmas still unknown. Crustal samples studied so far have osmium isotope compositions that require a sedimentary or “continental” contribution to the oceanic crust. Hydrothermal alteration of the crust cannot easily accomplish this effect for osmium isotopes. We propose to test different alternative hypotheses, such as the intrinsic origin of the radiogenic osmium from the Indian ocean mantle, by (1) increasing the number of samples of lower crustal lithologies for osmium isotope and chalcophile element abundances and (2) combining these data with Rb-Sr and Sm-Nd isotope studies of leached clinopyroxenes and Pb isotope work on leachates of sulfides and plagioclase from the same rocks. The main goal here is to link the isotope systematics to distinguish hydrothermal alteration effects from the true (magmatic) composition of the minerals. Another objective is to apply some of these methods to study gabbro veins and selected dunite samples from near the top of the mantle section to test the hypothesis that some of these rocks could represent transport pathways of magma with “continental” osmium isotope compositions. New chalcophile element abundance data generated in the study will provide new constraints on processes that control sulfide saturation and segregation in the deep oceanic crust. To address these questions, we will mostly use Oman Drilling Project core samples from the CM (crust-mantle transition) drill sites, and core material from the GT1-3 drill sites that sampled the main lower crustal gabbro mass and the differentiated gabbros produced in the former axial melt lens underneath the sheeted dikes.