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The composition of the lower oceanic crust in the Wadi Khafifah section of the southern Samail (Oman) ophiolite

Citation

VanTongeren, Jill et al. (2021), The composition of the lower oceanic crust in the Wadi Khafifah section of the southern Samail (Oman) ophiolite, Dryad, Dataset, https://doi.org/10.5061/dryad.c59zw3r6v

Abstract

The composition of the intrusive gabbroic lower oceanic crust remains poorly characterized in comparison to the extrusive portion of the oceanic crust, especially for intermediate-fast spreading mid-ocean ridges. This is a consequence of limited exposures of extant lower oceanic crust and of ophiolites similar to mid-ocean ridge crust. One of the best analogues for mid-ocean ridge crust is the southern Samail ophiolite that formed during a period of rapid seafloor spreading above a nascent subduction zone. Here, we focus on the geochemical stratigraphy (whole rock and mineral major and trace element compositions) of the 5200 m-thick, lower crustal, Wadi Khafifah section of the Wadin Tayin massif in the southern Oman ophiolite. Gabbros from the lowermost 3700 m of this section (the 'lower gabbros') show no systematic changes in composition with height above the Mantle Transition Zone. In contrast, gabbros from the uppermost 1500 m (the 'upper gabbros') display marked increases in incompatible trace element concentration with increasing height. Liquids in equilibrium with the lower gabbros have major and trace element compositions that overlap with those measured in the upper gabbros and sheeted dikes. Upper gabbros preserve mineral cores with primitive major element compositions that overlap with the range of lower gabbros, however, upper gabbro whole rock compositions are significantly more enriched in incompatible trace elements relative to the lower gabbros. Our data reveal that the upper gabbros are a composite of accumulated minerals derived from primitive melts and a large fraction of evolved melts derived from the fractionation of the lower gabbros. We propose a new “Full Sheeted Sills” model for the lower oceanic crust in which primitive magmas from the mantle are emplaced throughout the lower crust and crystallized in situ. After diking events, evolved magmas leave the lower gabbros and replenish the upper gabbros, thereby contributing to the higher incompatible trace element budget in the upper gabbros relative to the lower gabbros. Our reconstructed bulk compositions of the lower plutonic crust and the bulk oceanic crust from the Wadi Khafifah section yield a plausible primary mantle-derived magma composition in equilibrium with depleted MORB mantle.

Methods

All gabbro samples measured here are exceptionally fresh and largely free of low temperature alteration or metasomatism (Fig. 4b).

Sheeted dike samples were collected from a similar location to those studied by Pallister and Hopson (1981)(Table 1b). All 33 sheeted dike samples reported here are slightly to moderately altered at greenschist facies, but preserve chilled margin contacts and provide reliable compositions for comparision here for all but the more fluid-mobile elements.

3. 1 Compositional Analyses

We report data from 33 lower crustal samples spaced on average every 50-100 m through the lower crust stratigraphy, except for a stratigraphic sampling gap occurring between 2369 m and 2999 m. Whole rock major element compositions for gabbro and sheeted dikes lithologies were determined by XRF at the Washington State Geoanalytical Laboratory (Table 2). Trace element concentrations were determined by Inductively Coupled Plasma Mass Spectrometry (ICPMS) at the Université de Montpellier (the gabbros) and Cardiff University (the dikes).

Compositions of the major rock forming minerals were determined by electron microprobe analysis (EPMA) at the Massachusetts Institute of Technology JEOL EPMA facility and the American Museum of Natural History Cameca EPMA facility. Major and minor elements in pyroxenes (Table 3) and olivines (Table 4) were analyzed with 20nA, 15KeV beam, and 30 sec on peak and 15 sec on background. Major and minor elements in plagioclase (Table 5) were analyzed with a 10nA, 15 keV beam with 30 sec on peak and 15 sec on background. Modal abundances of minerals were determined by the least squares method using the major element compositions of the whole rock and its consituent minerals. This method does not distinguish between cumulus and intercumulus mineral abundances.

Trace element concentrations of plagioclase (Table 6) and clinopyroxene (Table 7) were measured by Laser Ablation ICPMS (LA-ICPMS) at Rutgers University. For plagioclase, we used a spot size of 65 microns and a fluence of 5.32 J/cm2. For pyroxene, we used a spot size of 40 microns and fluence of 2.95 J/cm2. Standards NIST 610, 612, and 614 were used to construct calibration curves, and basalt glasses BCR and BIR were regularly analyzed as unknowns to ensure accuracy. All data presented represent averages of mineral cores. No mineral rims were measured in this study.