U-series dating of Lake Nyos maar basalts, Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam

Abstract From previously published 14 C and K–Ar data, the age of formation of Lake Nyos maar in Cameroon is still in dispute. Lake Nyos exploded in 1986, releasing CO 2 that killed 1750 people and over 3000 cattle. Here we report results of the first measurements of major elements, trace elements and U-series disequilibria in ten basanites/trachy-basalts and two olivine tholeiites from Lake Nyos. It is the first time tholeiites are described in Lake Nyos. But for the tholeiites which are in 238 U– 230 Th equilibrium, all the other samples possess 238 U– 230 Th disequilibrium with 15 to 28% enrichment of 230 Th over 238 U. The ( 226 Ra/ 230 Th) activity ratios of these samples indicate small (2 to 4%) but significant 226 Ra excesses. U–Th systematics and evidence from oxygen isotopes of the basalts and Lake Nyos granitic quartz separates show that the U-series disequilibria in these samples are source-based and not due to crustal contamination or post-eruptive alteration. Enrichment of 230 Th is strong prima facie evidence that Lake Nyos is younger than 350 ka. The 230 Th– 226 Ra age of Nyos samples calculated with the ( 226 Ra/ 230 Th) ratio for zero-age Mt. Cameroon samples is 3.7 ± 0.5 ka, although this is a lower limit as the actual age is estimated to be older than 5 ka, based on the measured mean 230 Th/ 238 U activity ratio. The general stability of the Lake Nyos pyroclastic dam is a cause for concern, but judging from its 230 Th– 226 Ra formation age, we do not think that in the absence of a big rock fall or landslide into the lake, a big earthquake or volcanic eruption close to the lake, collapse of the dam from erosion alone is as imminent and alarming as has been suggested.