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Lowenstein, Tim, Jenkins, David, Demicco, Robert, Graney, Joseph, Renaut, Robin
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2022-05-16
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Abstract
Petrographic, XRD, EDS, SEM and SIMS microanalysis demonstrate that
Pleistocene Magadi cherts formed primarily from the precipitation of an amorphous
siliceous gel and occasionally from hydrous sodium silicates, such as magadiite or
kenyaite. Brine simulations and oxygen isotope analysis support the formation of these
cherts from highly evaporated brines at modern lake temperatures near surface.
Fossils within chert, including, microorganisms, diatoms, gastropods, ostracods,
and plant and insect fragments provide valuable paleoenvironmental information,
including the presence of alkaliphilic fungal communities found in chert. Identifiable
crystal habits in evaporite pseudomorphs of trona and gaylussite suggest saline conditions
were present throughout much of the basin history.
Labyrinth patterns discovered in Magadi cherts indicate the precipitation of silica
gels in the saline-alkaline Lake Magadi system near the surface in conjunction with chert
formation. Hele-Shaw cell experiments conducted to replicate Magadi labyrinth patterns,
however some of these patterns in nature occurred along non-horizontal planes. The
orientations of the Magadi patterns imply invasion of air from above if confined to open
cracks or infiltration from all sides if the gels dried and cracked in the vadose zone.
Cherts developed from siliceous gel precursors have elevated d18O values, from +41‰ to
+47‰ indicating the formation in the presence of brines. Such brines, with total
dissolved solids of ~ 220,000 to 300,000 mg/L, exist in the modern Magadi basin during dry periods. Cherts with magadiite and other hydrous sodium silicates precursors form
brines with lower salinities than cherts formed from gels. Cementing chalcedony and
megaquartz crystals in chert have lower d18O values that require formation from waters at
elevated temperatures, which suggests a hydrothermal origin. Finally, siliceous mudstone
cherts show relatively wide ranges of d18O, from +37‰ to +46‰, which suggests
formation from subsurface brines.
Based on this new information, we have further classified the origins of chert in
Lake Magadi, Kenya and extended the presence of a high salinity paleoenvironment to
~500 ka before present.