Mineral reactions in the sedimentary deposits of the Lake Magadi region, Kenya

Abstract

The authigenic minerals, principally zeolites, in the Pleistocene to Holocene consolidated and unconsolidated sediments of the Magadi basin in the Eastern Rift Valley of Kenya have been investigated. Samples were available from outcrops as well as drill cores. The following reactions can be documented: (1) trachytic glass + H 2 O → erionite, (2) trachytic glass + Na-rich brine → Na-Al-Si gel, (3) erionite 4- Na + → analcime + K + + SiO 2 + H 2 O, (4) Na-Al-Si gel → analcime + H 2 O, (5) calcite + F-rich brine → fluorite + CO 3 −− , (6) calcite + Na-rich brine → gaylussite, and (7) magadiite → quartz + Na + + H 2 O. Erionite is the most common zeolite present, but minor amounts of chabazite, clinoptilolite, mordenite, and phillipsite were also recognized. Erionite can form directly from trachytic glass by the addition of H 2 O only. It is characteristic of the Magadi basin because of the low content of alkaline earths in the volcanic glasses and in the solutions interacting with them. Analcime is common in outcrops of the High Magadi and Oloronga Beds. It forms from erionite by a reaction probably initiated by a lowering of the silica activity, which results from the transformation of magadiite to chert. Analcime in the drill-core samples grew at the expense of a Na-Al-Si gel. This gel forms at the lake shore and is washed into the lake during flooding conditions. Fluorite is common in the core samples and can be explained by reaction of the fluoride-rich brines with calcium in the sediments, principally detrital calcite. Authigenic albite and potassium feldspar were not recognized, probably for reasons of reaction kinetics. The presence of authigenic minerals can be accounted for by considering the chemical compositions of the starting materials, mainly volcanic glasses, and the brines they come in contact with. Lake Magadi represents a unique opportunity for studies of diagenesis because authigenic minerals are forming there at the present time and because the evolution of its waters is well known.