CONDITIONS OF FORMATION OF MAGADI-TYPE CHERTS FROM SIMS OXYGEN ISOTOPE MICROANALYSIS

Paleoenvironmental conditions in the Magadi basin, Kenya, were constrained from the oxygen isotope composition of Pleistocene cherts in cores drilled for the Hominin Sites and Paleolakes Drilling Project (HSPDP). The novel use of secondary ion mass spectrometry (SIMS) in cherts allowed for precision, 10 μm, multi-spot analyses within individual chert samples. SIMS is the preferred method in order to isolate cryptocrystalline quartz from pore filling chalcedony and megaquartz. Most cherts analyzed exhibited high δ¹⁸O values, ranging from +41 to +46‰, indicating hypersaline environments for much of the basin history, from 750 ka to today. These cherts contain uncompacted, randomly oriented crystal pseudomorphs, plant fragments, and other detritus indicative of syndepositional formation from a siliceous gel. Fewer samples showed petrographic evidence for traditional “Magadi-type” chert formed after hydrous sodium silicates, such as magadiite. Isotopic analyses of these samples yielded slightly lower δ¹⁸O, from +38‰ to +40‰. At the base of the section (Oloronga Beds), cherts with abundant gastropods and ostracods had relatively low δ¹⁸O values, ranging from +26‰ to +35‰, indicating lower salinity waters from ~ 1Ma to ~750 ka. Microanalysis of megaquartz within voids and fractures in Oloronga chert showed the lowest δ¹⁸O, +23‰ to +30‰, indicating formation from relatively low salinity, warm, likely hydrothermal waters. Petrographic observations combined with SIMS oxygen isotope analyses of cherts from the Magadi basin constrained paleoenvironmental conditions, such as water temperature and salinity, at the time of chert formation.