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Geochemical constraints on the time scale of magma activity in island-arc area using multi-isotope systematics

利用多同位素系统学对岛弧区岩浆活动时间尺度的地球化学约束

基本信息

批准号：
15540465
负责人：
NAKAI Shun'ichi
金额：
$ 2.24万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2003
资助国家：
日本
起止时间：
2003 至 2004
项目状态：
已结题

项目摘要

Cosmogenic nuclide 10Be with a half-life of 1.5 million years is produced in the atmosphere and subsequently absorbed onto marine sediment. Although 10Be trapped in the mantle at the formation of the Earth should have been decayed away, trace amount of 10Be has been confirmed in the volcanic rocks derived from subduction areas (0.1-25 million atoms per gram). The presence of 10Be is considered as a rigid evidence indicating the recycle of subducted sediment derived component in arc volcanism. Since the abundance of 10Be is very low, only Accelerator Mass Spectrometry (AMS) can measure its abundance in rock samples.The 10Be analyses are still very difficult especially for volcanic rocks with low abundance. This study aims at establishing analytical method of 10Be in volcanic rock samples and applying it to the samples from Izu arc where information on other trace element abundances and isotopic tracers are available. Our final goal is to investigate across-arc variation of 10Be abundanc … More e and its relation with other trace element and isotopic systems.Samples of basalt from Oshima, Miyakejima and Niijima were used for analyses to investigate across-arc variation. The samples were leached with dilute acid to remove surface contamination of 10Be following the method described by Shimaoka et al. (2004). After the leaching, about five grams of rock powder was digested with HF, HNO3 and HClO4. Be was separated from other major and minor constituents of rocks. 10Be abundances were analyzed with an AMS at the Research Center for Nuclear Science and Technology in the University of Tokyo. Abundances of 9Be were analyzed with a ICPMS at ERI in the University of Tokyo. 10Be abundances of the samples from Oshima, Miyakejima and Niijima have been determined as 0.79-0.80, 0.61-1.26 and 0.78 million atoms/g, respectively. Our results are consistent with previously reported values. 10Be/9Be ratios are higher in the samples from Oshima than other two islands. While other abundance ratios between fluid mobile to fluid immobile elements, such as Ba/Th and (238U/230Th) shows clear across-arc decrease, 10Be/9Be ratio of a sample from Niijima was similar to those from Miyakejima. This may result from the relatively larger partition coefficient of Be between mineral and fluid than those of Ba and U, which decreases mobility of Be. Less

半衰期为 150 万年的宇宙成因核素 10Be 在大气中产生，随后被海洋沉积物吸收。虽然在地球形成时被困在地幔中的 10Be 应该已经被衰变掉了，但在俯冲区域衍生的火山岩中已经证实了微量的 10Be（每克 0.1-2500 万原子）。 10Be的存在被认为是弧火山活动中俯冲沉积物衍生成分再循环的有力证据。由于 10Be 的丰度很低，只有加速器质谱（AMS）才能测量岩石样品中 10Be 的丰度。特别是对于丰度较低的火山岩，10Be 的分析仍然非常困难。本研究旨在建立火山岩样品中 10Be 的分析方法，并将其应用于伊豆岛弧样品中，该样品可以获得其他微量元素丰度和同位素示踪剂的信息。我们的最终目标是研究 10Be 丰度 e 的跨弧变化及其与其他微量元素和同位素系统的关系。使用来自大岛、三宅岛和新岛的玄武岩样品进行分析以研究跨弧变化。按照 Shimaoka 等人描述的方法，用稀酸浸出样品以去除 10Be 的表面污染。（2004）。浸出后，用HF、HNO3和HClO4消化约5克岩粉。铍与岩石的其他主要和次要成分分离。东京大学核科学技术研究中心使用 AMS 分析了 10Be 丰度。使用东京大学 ERI 的 ICPMS 分析了 9Be 的丰度。大岛、三宅岛和新岛样品的 10Be 丰度分别为 0.79-0.80、0.61-1.26 和 78 万原子/克。我们的结果与之前报告的值一致。大岛样品中的 10Be/9Be 比值高于其他两个岛屿的样品。虽然流体可移动元素与流体不可移动元素之间的其他丰度比（例如 Ba/Th 和 (238U/230Th)）显示出明显的跨弧下降，但新岛样品的 10Be/9Be 比值与三宅岛样品相似。这可能是由于Be在矿物和流体之间的分配系数比Ba和U相对较大，从而降低了Be的流动性。较少的