B–O–Th isotope systematics in Icelandic tephra

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• 作者：Estelle F. Rose-Koga ; Olgeir Sigmarsson
• 关键词：Boron ; Oxygen ; Thorium ; Isotope ; Tephra ; Iceland
• 刊名：Chemical Geology
• 出版年：2008
• 出版时间：15 October 2008
• 年：2008
• 卷：255
• 期：3-4
• 页码：454-462
• 全文大小：727 K

文摘

We report new boron concentrations and isotope compositions measured by ion microprobe on Holocene tephra samples from 6 Icelandic volcanoes. The B concentrations range over almost a factor of 10, from 1.50 ±0.10 to 13.10 ± 0.10 ppm in basalt and obsidian, respectively. The highest δ¹¹B ( + 16.9 ± 2.2 and + 6.1 ± 1.6‰) measured in rhyolites from the rift-related Askja and Krafla volcanoes. In contrast, the lowest δ¹¹B values (− 5.3 and − 5.2‰) observed in samples with the some of the highest δ¹⁸O (+ 4.92 and + 5.18‰, respectively) and (²³⁰Th/²³²Th) (1.067 and 1.030, respectively), correspond to a “normal-mantle” signature. The variations of boron isotope compositions in the Icelandic tephra and their negative correlations with both δ¹⁸O and (²³⁰Th/²³²Th) strongly support the mixing between mantle derived basalts and crustal rhyolites produced from anatexis of hydrothermally altered basaltic crust. The low δ¹⁸O in combination with high δ¹¹B values in rhyolite is best explained by the presence of seawater derived B in a hydrothermal fluid that is mostly of meteoric origin.