DOI: 10.5061/dryad.c59zw3r6v
title: The composition of the lower oceanic crust in the Wadi Khafifah section of the southern Samail (Oman) ophiolite

description: 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.
description: 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. <i>3. 1 Compositional
 Analyses</i> 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/cm<sup>2</sup>. For
 pyroxene, we used a spot size of 40 microns and fluence of 2.95
 J/cm<sup>2</sup>. 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.