Flow porosities, permeabilities and preliminary data on fluid inclusions and fossil thermal gradients in the crustal sequence of the Sumail ophiolite (Oman)

Abstract

Systematic measurements of the orientation and spacing of hydrothermal veins, and sampling for petrological, mineralogical and fluid inclusion studies, have been carried out in the crustal sequence of the Cretaceous Sumail ophiolite, in order to estimate the geometry of the seawater circulation at a fossil spreading centre. Petrological and mineralogical data show that the whole crust, down to the petrological Mono, has been affected by a hydrothermal metamorphism, pervasive in the sheeted dyke complex and high-level isotropic gabbros and more localized around fractures in the underlying plutonic units. The sheeted dyke complex is overprinted by a greenschist facies metamorphism, whereas the underlying high-level isotropic gabbros are characterized by the association of calcic plagioclase and actinolite (transition facies), retromorphosed in the greenschist facies. Three main types of hydrothermal circulation were distinguished on the basis of field, mineralogical and fluid inclusion studies and analogies with oceanic hydrothermal systems: (1) at the top of the magma chamber, (2) along a cracking front at the transition between a hydrothermal and a magmatic system and (3) off-axis circulation in the whole crust. Fluids trapped in minerals during their growth or during later fracturing events consist of aqueous solutions with an average salinity close to that of seawater. Homogenization temperatures in the stockwork zone, underlying the massive ore deposits, are very high, up to 392°C, with no evidence of boiling. Temperatures measured in quartz-sulphide veins at the base of the volcanic unit exhibit a large range but are much lower. Furthermore, solutions from the sheeted dyke complex exhibit high trapping temperatures of around 400°C. These observations are interpreted as reflecting the mixing with cold seawater of solutions exiting the sheeted dyke complex. The highest trapping temperatures, recorded in the plutonic sequence, are around 500° C, and are assumed to represent accretion-stage circulation occurring directly after crystallization of part or all of the magma chamber. Localized hydrothermal circulation was active until cooling of the whole crust to a geothermal gradient of 30° C/km. Flow porosities and permeabilities are very high in the upper crust and decrease exponentially with depth, with a major jump at the interface between the volcanic unit and the sheeted dyke complex. Furthermore, at this level, the geometric pattern of the circulation changes drastically, evolving from a nearly chaotic system in the volcanic unit to a well-regulated system in the sheeted dyke complex and high-level isotropic gabbros, where seawater circulated along vertical planes, parallel to the fossil ridge axis. This geometry in the sheeted dyke complex and underlying gabbros suggests that seawater convected essentially along strike in vertical planes, rather than in a cylindrical, transverse system.