Cosmogenic helium in a terrestrial igneous rock
Cosmogenic helium in a terrestrial igneous rock
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陆地火成岩中的宇宙成因氦
作者：
M. Kurz
地球样品中高的3He/4He比值通常被解释为表明存在原始或原始气体1，2。尽管陨石中普遍存在宇宙成因的惰性气体，但在陆地样品中从未观察到宇宙成因的氦同位素特征3，而且在大多数情况下，被认为在地球上是微不足道的。我在这里介绍了对夏威夷哈雷阿卡拉火山库拉组样品的新的氦同位素测量，这一测量最好地解释了3He中相当大一部分的原位宇宙成因。粉碎和逐步加热实验的结果，以及对样品在表面的暴露年龄和宇宙射线通量的考虑，有力地支持了这一假设。尽管在之前的4，5中已经提出了地壳成因氦，但这是它在陆地样本中的第一次明确的识别。
High 3He/4He ratios in terrestrial samples are generally interpreted as indicating the presence of primitive or primordial gas1,2. Despite the ubiquitous presence of cosmogenic noble gases in meteorites, cosmogenic helium isotopic signatures have never been observed in terrestrial samples3 and have, for the most part, been considered to be insignificant on the Earth. I present here new helium isotopic measurements on samples from the Kula formation of Haleakala volcano (Hawaii) that are best explained by an in situ cosmogenic origin for a significant fraction of the 3He. Results from crushing and stepwise heating experiments, and consideration of the exposure age of the sample at the surface and the cosmic-ray fluxes strongly support this hypothesis. Although crustal cosmogenic helium has been proposed previously4,5, this represents its first unambiguous identification in a terrestrial sample.