Possible hydration anomaly in the upper mantle prior to Red Sea rifting: Evidence from petrologic modeling of the Wadi Natash alkali basalt sequence of eastern Egypt

Abstract

Basalt flows of ∼90-m.y. age occur in the Wadi Natash area of the Eastern Desert of Egypt and consist of an assemblage of alkali-olivine basalt (AOB), hawaiite-mugearite (HM), and benmoreite. The Wadi Natash occurrence is one of several upper Mesozoic to lower Cenozoic volcanic suites that surround the Red Sea and may be related to early mantle activity in the area prior to later rifting. The most mafic rock type at Wadi Natash is AOB, which could not have equilibrated with the mantle because of low Ni contents (50 ppm). The AOB9s presumably formed by extensive olivine fractionation from a primary picrite. The HM9s could have fractionated from the AOB9s by separation of a low-Ti amphibole. Alternatively, both suites may have formed by melting of an eclogitic mantle, with greater melting of jadeitic pyroxene contributing to the formation of the HM9s than to that of the AOB9s. The benmoreites apparently formed by equilibration of high-Ti (kaersutitic) amphibole with AOB. The Wadi Natash alkali basalt suite was probably generated from a mantle that contained high, and possibly varying, amounts of water. Mantle hydration is consistent with observations of the development of small extensional basins in the Red Sea area during the Cretaceous and may be an expected precursor to rifting of the type that now characterizes the Red Sea.