[Garbe-Schönberg] Establishing a reference profile for fast-spreading oceanic crust: Petrology and geochemistry of the "Wadi Gideah" cross section in the Oman ophiolite
German Title: Ein Referenzprofil für schnell spreizende ozeanische Kruste: Petrologie und Geochemie des Wadi Gideah-Profils im Oman Ophiolith-Komplex
Current Status: completed
Main Applicant:Dr. Dieter Garbe-Schönberg
Prof. Dr. Jürgen Koepke
Prof. Dr. Harald Strauß
Fast-spreading oceanic crust covering a large part of our planet is regarded as layered and homogenous. In spite of tremendous efforts by ship-based research, a complete modern chemical/petrological profile based on natural samples is still missing. In order to establish a reference profile through fast-spreading oceanic crust, the present study aims to perform a geochemical/petrological investigation of a crustal section in the Cretaceous ophiolite of Oman, which is regarded as the best example of fast-spreading oceanic lithosphere on land. For this, a complete section (gabbros, dikes, basalts) representative of approximately 6.5 km of fast-spreading oceanic crust was sampled at the Wadi Gideah in the Wadi Tayin massif. Within this project we focus on (1) petrography and phase analytical studies, (2) bulk-analytical characterization, (3) quantifying sulfur cycling and (4) characterizing the evolution of hydrothermal alteration with depth by using Sr and O isotopes. This profile will be unique in terms of completeness of the crustal components and quality and coherence of analytical data to be obtained. Hence, it is well-suited for advancing our understanding of crustal accretion processes and the evolution of primary and secondary geochemical cycles at fast-spreading oceanic crust. Finally, the Oman reference profile provide scientific support for the drilling of the Integrated Ocean Drilling Program (IODP) at Site 1256 (equatorial Pacific) where the deepening of the present hole down into the gabbro crust is scheduled for 2011.
n absence of a complete profile through fast-spreading oceanic crust in modern oceans, we established a reference profile through the whole paleocrust of the Samail Ophiolite in the Sultanate Oman, which is regarded as best analogue for fast-spreading oceanic crust on land. We sampled the Wadi Gideah in the Wadi-Tayin massif from the mantle section up to the pillow basalts. The Wadi Gideah is well suited for this purpose, since from all Oman ophiolite massifs, the Wadi-Tayin massif is best suited due to its affinity to primary magmatic mid-ocean ridge processes, and the widespread absence of a second magmatic phase initiated by flux-induced mantle melting in a subduction zone environment (absence of crustal wehrlite, gabbronorite, and large felsic intrusions in the Wadi Gideah). For establishing a coherent data set for advancing our understanding of processes on crustal accretion and cooling at fast-spreading mid-ocean spreading systems, we performed different analytical and structural investigations on the same suite of samples (pool sample concept). Major and trace element investigations on minerals and rocks from the Wadi Gideah plutonic crust reveal an up section trend of chemical evolution, with different characteristics for the layered gabbro and for the foliated/varitextured gabbros, implying a possible change in differentiation process especially in the layered to foliated gabbro transition zone. Petrological modelling in a hydrous MORB-type system with water contents between 0.6 and 1.0 wt% shows that the overall Mg# trend with depth can be well produced by fractional crystallization in combination with replenishments with fresh parental melt in the layered gabbro and pure fractional crystallization for the foliated gabbros in the upper part of the gabbroic crust. This trend is also associated with the presence of significant mineral zonation and by the crystallization of late magmatic phases in the foliated gabbros. Thus, major and trace element data on minerals and rocks reveal significant in-situ crystallization in the deep crust, thus strongly supporting a hybrid accretion model, characterized by sheeted sill intrusion in the lower part of the plutonic crust and gabbro glacier features in the upper part. This hybrid model is also supported by results on crystallographic preferred orientations (CPO) of the minerals within the gabbros, which clearly calls for distinct formation mechanisms in the upper and lower gabbro sections. The genesis of the upper foliated gabbro can be at least partly explained by the gabbro glacier model, while the continuous emplacement of sheeted sills at various depths is a more plausible model for the lower layered gabbro section. A requirement for our hybrid model based on in-situ crystallization in the depth, is significant hydrothermal cooling in the lower crust for the consumption of the latent heat of crystallization. This was probably facilitated by channelled hydrothermal flow zones, preserved today in multiple, up to 100 m wide, often faulted zones of extensively altered gabbro (metagabbro) cutting both layered and foliated gabbros. These metagabbros show significantly higher Sr87/Sr86 ratios compared to the uninfluenced background gabbro, the presence of late stage mineral phases as amphibole, orthopyroxene, and apatite, and in addition evidence for hydrous partial melting, as consequence of hydrothermal activity at very high temperatures.