U-series dating of Lake Nyos maar basalts, Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam
U-series dating of Lake Nyos maar basalts, Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam
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DOI：
10.1016/j.jvolgeores.2008.04.009
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发表时间：
2008-09-20
影响因子：
2.9
通讯作者：
Hell, Joseph
中科院分区：
文献类型：
--
作者：
Aka, Festus T.;Yokoyama, Tetsuya;Hell, Joseph
根据先前发表的C-14和K-Ar数据，喀麦隆尼奥斯马尔湖的形成时代仍有争议。尼奥斯湖在1986年爆炸，释放二氧化碳，造成1750人和3000多头牛死亡。在这里，我们报告的主要元素，微量元素和U系列不平衡的第一次测量结果在10个玄武岩/粗面玄武岩和两个橄榄石拉斑玄武岩从尼奥斯湖。这是尼奥斯湖首次描述拉斑玄武岩。但对于处于U-238-Th-230平衡的拉斑玄武岩，其他样品均具有U-238-Th-230不平衡，Th-230相对于U-238富集15 - 28%。这些样品的（Ra-226/Th-230）活性比表明小的（2至4%）但显著的Ra-226过量。玄武岩和尼奥斯湖花岗质石英分离物的U-Th系统学和氧同位素证据表明，这些样品中的U系列不平衡是基于源的，而不是由于地壳污染或喷发后的蚀变。Th-230的富集是尼奥斯湖年龄小于350 ka的有力证据。用零年龄Mt.的（Ra-226/Th-230）比值计算Nyos样品的Th-230-Ra-226年龄。喀麦隆样本的年龄为3.7 +/- 0.5 ka，但这是一个下限，因为根据测量的Th-230/U-238平均活度比，实际年龄估计超过5 ka。尼奥斯湖火山碎屑坝的总体稳定性令人担忧，但从其Th-230-Ra-226形成年龄判断，我们认为，在没有大岩坠落或滑坡进入湖中、没有大地震或靠近湖泊的火山爆发的情况下，大坝仅因侵蚀而倒塌并不像人们所认为的那样迫在眉睫和令人担忧。(C)2008 Elsevier B. V.保留所有权利。
From previously published C-14 and K-Ar data, the age of formation of Lake Nyos maar in Cameroon is still in dispute. Lake Nyos exploded in 1986, releasing CO2 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 U-238-Th-230 equilibrium, all the other samples possess U-238-Th-230 disequilibrium with 15 to 28% enrichment of Th-230 over U-238. The (Ra-226/Th-230) activity ratios of these samples indicate small (2 to 4%) but significant Ra-226 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 Th-230 is strong prima facie evidence that Lake Nyos is younger than 350 ka. The Th-230-Ra-226 age of Nyos samples calculated with the (Ra-226/Th-230) 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 Th-230/U-238 activity ratio. The general stability of the Lake Nyos pyroclastic dam is a cause for concern, but judging from its Th-230-Ra-226 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. (C) 2008 Elsevier B.V. All rights reserved.