Long in-situ sections in the Wadi Gideah, Oman ophiolite: The key for understanding the mechanism of accretion, magmatic evolution and cooling of lower fast-spread oceanic crust
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Oceanic crust covers more than two-thirds of Earth’s surface but, due to limited accessibility, no single coherent compositional profile that would allow for understanding the processes how oceanic crust is formed has been obtained up to now. The Oman ophiolite (Samail massif; Sultanate Oman) is the largest sub-aerial exposure of oceanic lithosphere on Earth and provides the opportunity to study the accretion and alteration of oceanic lithosphere formed under fastspreading conditions. This ophiolite was target for the drilling campaign Oman Drilling Project in the frame of ICDP, where three 400 m long cores GT1, GT2, and GT3 of the “Gabbro Transect” have been drilled in order to penetrate the paleocrust of the ophiolite. We also sampled a transect through 6.5 km of this fossil lower oceanic crust from outcrops along Wadi Gideah. The core of our methodological concept is to apply a wide variety of analytical techniques to the same samples integrating results from petrographical, petrological, bulk geochemical, in situ micro-chemical, and micro-structural work (lattice preferred orientation using electron backscattering diffraction). Overarching goal of our research is testing hypotheses for crustal accretion and mechanisms of differentiation where in situ crystallization in lower gabbros and/or fractionation in the axial melt lens play a major role but also the effects of (sea)water along hydrothermally active fault zones. Special emphasis was laid on establishing a thermal profile from crystallization temperatures of the major mineral phases clinopyroxene and plagioclase. The work was organized in four themes: (1) internal differentiation and the thermal structure of the Oman oceanic crust; (2) bulk differentiation trends and average crustal composition; (3) the role and nature of hydrothermal fault zones in the context of cooling the lower crust; (4) detailed mapping and structural analysis to refine the final positions of drill sites GT1-GT3. For samples from the OmanDP were not available in time for this project all results presented here are based on outcrop samples. Work on OmanDP drill core samples, however, is continued in subsequent projects under the same group of PIs. Both REE-in-plagioclase-clinopyroxene and anorthite thermometry suggest a homogeneous temperature structure for the lower, and a variable temperature record for the upper gabbros. Crystallization temperatures of the layered gabbros cover a narrow range of 1216 ± 14°C. Considerable temperature variability of 1077–1231°C is observed further up-section, featuring a thermal minimum within the foliated gabbros at ~3,525 m above the mantle-crust boundary. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels. Layered gabbro parental melts correspond to mid-ocean-ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady-state boundary conditions. Further up-section, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open-system fractional crystallization, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). Whole rock data show systematic compositional trends correlating with stratigraphic depth. Distribution trends of incompatible trace elements suggest that lower, layered gabbros are made of cumulus crystals that crystallize in situ, separating from the ascending parental magma. We also find gabbros with variable compositions at intermediate depth intervals that could originate from melts crystallizing and differentiating in sills. In contrast, compositions of upper non-layered gabbros above a discontinuity at ~3,525 m above the mantle-crust boundary are controlled by fractional crystallization and mixing processes in transient melt lenses. New average compositions of (paleo) oceanic crust are reported for major and 38 trace elements. Comparison with new data from Wadi Khafifah in 10 km distance to Wadi Gideah suggests robustness of accretion processes in both space and time.
Publications
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A Crystallization-Temperature Profile Through Paleo-Oceanic Crust (Wadi Gideah Transect, Oman Ophiolite): Application of the REE-in-Plagioclase-Clinopyroxene Partitioning Thermometer. In AGU Fall Meeting Abstracts (Vol. 2017, pp. V43G-2956)
Müller, S.; Hasenclever, J.; Garbe-Schönberg, D.; Koepke, J. & Hoernle, K.
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Trace Element Systematics In Gabbros From The Wadi Gideah Transect (Wadi Tayin Massif, Oman Ophiolite)? Constraints On Accretion Processes In Fast-spread Oceanic Crust. IODP/ICDP Kolloquium 2017 61-62
Garbe-Schönberg, D.; Koepke, J.; Müller, S.; Müller, T. & Strauss, H.
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Hydrothermal fault zones in the lower oceanic crust: An example from Wadi Gideah, Samail ophiolite, Oman. Lithos, 323(c(2018, 12)), 103-124.
Zihlmann, Barbara; Müller, Samuel; Coggon, Rosalind M.; Koepke, Jürgen; Garbe-Schönberg, Dieter & Teagle, Damon A.H.
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Mass transfer at hydrothermal fault zones in the lower oceanic crust: An example from Wadi Gideah, Samail ophiolite, Oman. Annual DMG Meeting GeoBonn 2018
Müller, S.; Zihlmann, B.; Garbe-Schönberg, D.; Teagle, D. A.H. & Koepke, J.
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Oman Drilling Project Phase 1 Science Party (2018): Understanding Lower Crust Accretion At Fastspreading Mid-ocean Ridges: New Insights From Drill Cores Obtained By The ICDP Oman Drilling Project: First Results. IODP/ICDP Kolloquium 2018 63-64
Mock, D.; Ildefonse, B.; Garbe-Schönberg, D.; Müller, S.; Neave, D.A. & Koepke, J.
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The Role Of Hydrothermal Fault Zones In The Lower Oceanic Crust: Evidence From Wadi Gideah (OmanDP Drill Site, Samail Ophiolite, Oman). IODP/ICDP Kolloquium 2019 55
Müller, S.; Zihlmann, B.; Coggon, R.M.; Garbe-Schönberg, D.; Teagle, D.A.H.; Wenke, H. & Koepke, J.
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Accretion of fast-spread lower oceanic crust: drill core GT1 from the ICDP Oman Drilling Project. American Geophysical Union (AGU).
Mock Dominik, Neave David Axford; Müller Samuel, Garbe-Schönberg Dieter; Ildefonse Benoit, Koepke Jürgen & Science Team Oman Drilling Project
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A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Petrography and Petrology. Journal of Geophysical Research: Solid Earth, 127(1).
Koepke, J.; Garbe‐Schönberg, D.; Müller, T.; Mock, D.; Müller, S. & Nasir, S.
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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. Journal of Geophysical Research: Solid Earth, 127(3).
Müller, Samuel; Garbe‐Schönberg, Dieter; Koepke, Jürgen & Hoernle, Kaj
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A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Whole Rock Geochemistry. Journal of Geophysical Research: Solid Earth, 127(6).
Garbe‐Schönberg, Dieter; Koepke, Jürgen; Müller, Samuel; Mock, Dominik & Müller, Tim
