Deep crustal seawater penetration and circulation at ocean ridges: Evidence from the Oman ophiolite

Abstract

Detailed field mapping of the plutonic sequence and sheeted dyke complex in several areas of the Sumail Ophiolite Nappe, mainly in the Haylayn Block (Wadi Haymiliyah area) and comparisons with other well-studied areas has shown that early hydrothermal circulation affected the whole crust, down to the petrological Moho. Five major vein systems have been distinguished in the field and their mineralogy is as follows: amphibole, zoisite-epidote, epidote, quartz and prehnite vein systems. The amphibole veins occur in the lower plutonic sequence and are interpreted as the fossil trace of a cracking front at the interface between a hydrothermal and a magmatic system. The plutonic sequence and the zoisite-epidote and epidote vein systems were formed in an off-axis environment after the formation of the magma chamber. In the sheeted dyke complex, the epidote and quartz-sulphide veins are restricted to the dyke margins and represent early hydrothermal circulation at the top of a magma chamber. These two systems formed contemporaneously, the quartz-sulphide veins marking zones of mixing with colder seawater in the stockwork zone. Prehnite veins, along with minor calcite veinlets, are the fossil trace of the latest hydrothermal circulations and occur down to the petrological Moho in a diffused system. Shear zones are rare in the whole ophiolite and are essentially developed in the northern Fizh Block. Diffusing from these veins, seawater penetrates the crust and induces retrograde metamorphic transformations. Mineralogical and petrological data show that the entire crust, down to the petrological Moho, has been affected by hydrothermal metamorphism, pervasive in the sheeted dyke complex, more localized around veins in the underlying plutonic unit. The mineralogical parageneses in the sheeted dyke complex are typical of the greenschist facies (albite, chlorite, actinolite and sphene) whereas the high-level isotropic gabbros are characterized by the association of calcic plagioclase and actinolite (actinolitic facies) retrogressive in the greenschist facies. The almost unaltered underlying units, although all samples show traces of metamorphic alteration, exhibit mineral parageneses typical of the amphibolite to greenschist facies. The main crustal transformations during this metamorphism are hydration, a Na gain, and leaching of Ca. Superimposed Ca-rich and Na Mg-poor epidotised zones in the sheeted dyke complex are produced by Ca-rich upflowing (discharge) fluids. High Cl contents were found in the amphibole veins suggesting that they formed during the circulation of seawater, or of its modified equivalent. The hydration of the Wadi Haymiliyah area plutonic sequence and sheeted dyke complex (Haylayn Block) was calculated as requiring a minimum seawater consumption of a column of 260 m. Fluids trapped in minerals during their growth (quartz, epidote, amphiboles, calcite and sphalerite) or during a fracturing event (quartz and plagioclase) consist of aqueous solutions with an average salinity close to that of seawater (3.2% NaCl equivalent). Thermal gradients deduced from fluid-inclusion microthermometric studies decrease exponentially with depth and vary between 300°C/km in the volcanic unit and 30°C/km in the plutonic sequence, with temperatures up to 530°C. Circulation was active until cooling of the crust to a general thermal gradient of 30°C/km.