K. T. Koga

Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman ophiolite : Implications for the origin of the oceanic lower crust

Gabbroic sills intruding dunite in the crust-mantle transition zone (MTZ) of the Oman ophiolite have textures and compositions very similar to those in modally layered gabbros that form the lower part of the gabbro section in the ophiolite, and different from those in non-layered gabbros near the dike-gabbro transition. The presence of gabbroic sills in the MTZ indicates that modally layered gabbros can form far below the level of magmatic neutral buoyancy and far below the dike-gabbro transition. Minerals in the sills and lower, layered gabbros are in FeMg and trace element exchange equilibrium with liquids identical to those that formed the sheeted dikes and lavas in the ophiolite. In contrast, many of the upper, non-layered gabbros resemble crystallized liquid compositions, similar to the dikes and lavas. The lower, layered gabbros probably formed in sills similar to those in the MTZ. Mantle-derived magmas cooled in these sills, where they crystallized from a few percent to 50% of their mass. Residual liquids then rose to form upper gabbros, dikes and lavas. Sills may form beneath permeability barriers created by the crystallization of cooling liquid migrating by porous flow. Once permeability barriers are present, however, porous flow becomes a less important mode of magma ascent, compared to ponding in sills, gradual increase in magma pressure, and periodic ascent in hydrofractures. Thus, gabbroic sills in the MTZ may represent the transition in fast-spreading ridge environments from continuous porous flow in the mantle to periodic diking in the crust.

Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite

We studied trace element geochemistry and petrology of the crust-mantle transition zone (MTZ) in the Samail massif of the Oman ophiolite to constrain the location where different primitive magmas mix beneath an oceanic spreading ridge. The MTZ is the deepest location where crystallization took place and thus is an ideal place to determine the compositional diversity of melts leaving the mantle, with various sources and degrees of depletion. We have reached three main conclusions: (1) More than 90% of our samples record equilibration with compositionally indistinguishable parental melts, similar to mid-ocean ridge basalts (MORB) and the melts that formed the crust in Oman. This suggests that mixing of diverse, polybaric partial melts of mantle peridotite occurred at or below the depth of the MTZ. The presence of distinct heterogeneity in less than 10% of our samples is similar to the nature and frequency of heterogeneity observed in melt inclusions in olivines from MORB. (2) Among the samples recording trace element equilibrium with MORB-like liquids are wehrlitic rocks, previously suggested to be cumulates from an unusual parental melt on the basis of petrological observation. (3) Systematics of Eu distribution among plagioclase and clinopyroxene in “impregnated peridotites” demonstrate that these minerals did not crystallize from “trapped melt.” As a consequence, it is not possible to use the modal proportion or texture of plagioclase + clinopyroxene impregnations to estimate the instantaneous melt porosity or the shape of melt pores at any time during the formation of these rocks.