Oxygen isotope geothermometry and origin of smectites in the Atlantis II Deep, Red Sea

Abstract

The sediments underlying the hot brine pool of the Atlantis II Deep, a localised area of geothermal activity in the Red Sea, comprise a diversity of facies characterised by combinations of one or more of five species assemblages, sulphide, sulphate, silicate, oxide and carbonate, each including several mineral phases. The silicate mineral assemblage is dominated by geothermal authigenic smectites. Previous studies of these smectites have reported iron-rich varieties only, nontronite in particular, and only one environment of formation. In three cores from the Southwest Basin of the Atlantis II Deep, of the present study, three smectites comprising two species have been distinguished [10,21] evidently from three different environments of formation. Two of these smectites are nontronites, one from sulphide/silicate/amorphous facies, the other from silicate/carbonate/oxide facies. The third is a montmorillonite/beidellite from sulphate/sulphide/silicate/oxide facies. The oxygen isotopic compositions of samples of the three smectites have been determined from which formation temperatures have been calculated. Six samples of the “anoxic” nontronite have formation temperatures in the range 90–140°C. A single sample of the “oxic” nontronite has a formation temperature of about 80°C. Four samples of the montmorillonite/beidellite have formation temperatures in the range 160–200°C. The formation temperature range of the two nontronites is intermediate between the temperature of the brine at or prior to discharge (up to 250°C [12]) and the temperature of the brine pool in the Deep (about 50–60°C [13,14]). The nontronite formation temperature range reflects genesis by combination of isotopically light silica supplied by the incoming brine and isotopically heavier iron oxyhydroxide settling from the upper layers of the brine pool. Evidently, the “anoxic” nontronite forms at greater depth (hotter) in the brine pool than the “oxic” nontronite resulting in a relatively greater contribution from silica but diminished contribution from iron oxyhydroxide in the former compared to the latter. The wide range of the formation temperatures for the “anoxic” nontronite is related to the different actual locations of the samples in the sulphide/silicate/amorphous facies. The formation temperature range of the montmorillonite/beidellite is approaching the estimated temperature of the brine at or prior to discharge. The montmorillonite/beidellite formation temperature range reflects genesis by combination of isotopically light silica and aluminium, both supplied by the incoming brine, at the site of an active discharge vent. The wide range of the formation temperatures for the montmorillonite/beidellite may in part reflect a possible thermal event at the brine source, likely to have occurred during deposition of the sulphate/sulphide/silicate/oxide facies and which, it appears, has contributed to the formation of this facies [10,20].