Ž . Earth and Planetary Science Letters 146 1997 475–488 Geochemistry of gabbro sills in the crust–mantle transition zone of the Oman ophiolite: implications for the origin of the oceanic lower crust Peter B. Kelemen ) , Ken Koga, Nobu Shimizu Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 19 April 1996; revised 4 November 1996; accepted 12 November 1996 Abstract Ž . 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. Keywords: mid-ocean ridges; igneous rocks; petrology; lower crust; sills; gabbros; cumulates; Mohorovicic discontinuity 1. Introduction Observations of the Oman ophiolite place impor- tant constraints on models of oceanic crustal genesis. Because the ophiolite includes a continuous layer of sheeted dikes overlain by pillow basalts, it is clear that most of the igneous crust formed at an oceanic ) Corresponding author. Fax: q1 508 457 2183. Tel.: q1 508 289 2956. E-mail: peterk@cliff.whoi.edu spreading center. The main lava series in the ophio- lite, the Geotimes and Lasail volcanics, are tholeiitic Ž . basalts and andesites with rare earth element REE and trace element contents similar to mid-ocean ridge Ž . w x basalt MORB 1–3 . In detail, concentrations of REE and other incompatible trace elements are lower in Oman lavas, at a given Cr concentration, than in MORB. Also, in some of the northern massifs, an- desitic lavas comprise part of the extrusive section. These characteristics suggest that the ophiolite may have formed in a super-subduction zone setting. 0012-821Xr97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. Ž . PII S0012-821X 96 00235-X