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

半衰期为150万年的宇宙成因核素10Be在大气中产生，随后被吸收到海洋沉积物中。虽然地球形成时被困在地幔中的10Be应该已经衰变，但在俯冲区的火山岩中已证实有微量的10Be(每克0.1-2500万个原子)。10Be的存在被认为是弧火山作用中俯冲沉积物来源组分再循环的确凿证据。由于10Be的丰度很低，只有加速器质谱仪(AMS)才能测量其在岩石样品中的丰度，10Be的分析仍然很困难，特别是对低丰度的火山岩。本研究旨在建立火山岩样品中10Be的分析方法，并将其应用于有其他微量元素丰度和同位素示踪信息的伊豆弧区样品中。我们的最终目标是研究10Be丰度…的跨弧变化对大岛玄武岩、宫岛玄武岩和新岛玄武岩进行了跨弧变化分析。按照Shimaoka等人的方法，用稀酸浸取样品以去除10Be的表面污染。(2004)。浸出后，用HF、HNO3和HClO4消化约5克岩粉。从岩石的其他主要和次要成分中分离出Be。在东京大学核科学技术研究中心用AMS分析了10Be丰度。用东京大学ERI的ICPMS分析了9Be的丰度。测得大岛、宫岛和新岛样品的10Be丰度分别为0.79-0.80、0.61-1.26和0.78万个原子/g。我们的结果与之前报道的数值一致。大岛样品的10Be/9Be比值高于其他两个岛屿。流体活动元素与流体不活动元素的丰度比值，如Ba/Th和(238U/230Th)显示出明显的跨弧递减，而日本岛样品的10Be/9Be比值与宫崎岛样品相似。这可能是由于Be在矿物和流体之间的分配系数比Ba和U的分配系数大，从而降低了Be的流动性。较少

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