When a mid-ocean ridge encroaches a continent: Seafloor-type hydrothermal activity in Lake Asal (Afar Rift)

Abstract

At the place where the submarine Aden Ridge encroaches on the African continent and interacts with the East African Rift system, two small basins form: Ghoubbet-al-Kharab and Lake Asal. Whereas Ghoubbet-al-Kharab is connected to the open ocean, Lake Asal is a typical example of oceanic “embryo”, which is defined as a system that is detached from the ocean, but has features of a marine basin with an oceanic type crust and a seawater-based water body. In order to shed light on the source of water, type of hydrothermal activity and hydrothermal deposits, and controls on the water chemistry in an oceanic “embryo”, we undertook a mineralogical-geochemical study of the lake water, hydrothermal fluids and hydrothermal carbonate deposits of Lake Asal. The geochemical analyses of lake water and hydrothermal fluids show that Lake Asal (located in an arid zone with strong evaporation and with no riverine input) is fed by seafloor-type hydrothermal fluids according to the following scenario: percolation of seawater along faults and cracks of extension in the rift, reaction of seawater with the hot basaltic rocks and hydrothermal fluid generation, discharge of the hydrothermal fluid in the Asal depression and accumulation of the Lake Asal water body. The fluid venting at the Lake Asal bottom is a mixture of 97% end-member hydrothermal fluid and 3% lake water. The calculated end-member hydrothermal fluid of this oceanic “embryo” is poorer in metals than the seafloor hydrothermal fluids of an open and evolved ocean. In addition to the seawater/rock interaction, the chemistry of Lake Asal is also controlled by evaporation leading to hyper salinity. In a hyper saline water body a number of hydrothermally supplied metals are stabilized as chloride complexes and accumulate. This results in a metal rich and mildly acidic “embryonic” ocean. Unlike an open and evolved modern ocean, the “embryonic” ocean located in an arid zone has heavy C and O isotope composition and light Zn and Fe isotope composition. Calcium isotope compositions of both types of ocean are similarly heavy. There are two genetically different sources of elements to the Lake Asal that are vertically separated: hydrothermal (lower, or bottom) and aeolian (upper, or surficial). Another important control on the lake water chemistry is the formation of carbonate spires at the lake bottom. Ca‑carbonate precipitation immobilizes substantial amount of hydrothermally supplied Ca and drives up the (Mg/Ca)mol of the lake water. Increasing (Mg/Ca)mol of the evolving lake water leads to changes in the mineralogy of spires: from low-Mg calcite to aragonite. Thus, the spire formation exerts a self-control on its mineralogy. Carbonate spire deposition affects also the Ca, Zn and Fe isotope composition of the lake water through adsorption or/and co-precipitation induced isotope fractionation.