Novel determination method of geochemical response time for volcanic activity monitoring

火山活动将岩浆深处的元素释放到浅层地下流体中，这些流体以温泉的形式渗出。氦是一种高度富含于岩浆中的元素，因此在温泉水中很容易检测到它作为岩浆同位素信号。在一些火山，岩浆氦信号似乎对喷发周期作出反应，但其变化与观测到的火山活动之间的联系往往是不确定的，因此很难将其用作喷发预测工具。在这些研究中，来自温泉的火山信号的时间变化与采集样本的火山活动时间进行了比较。然而，火山信号可能会在地下传播很长一段时间，然后从泉水中渗出，因此我们探测到的火山信号可以反映数十年前的岩浆活动。为了评估可能延迟的时间尺度，拟议的项目将利用Kr和Ar的放射性同位素确定携带岩浆氦信号的浅层地下流体的旅行时间。日本的Ontake火山和Unzen火山因其相对较新的火山活动和丰富的氦监测记录而被选为候选地点。Kr和Ar的放射性同位素将由阿贡国家实验室的原子俘获痕量分析(ATTA)和伯尔尼大学的低水平衰变计数进行分析。这些年代学信息将被用来重新解释当地大地构造活动与氦同位素记录之间的关系，以及评估恢复“化石”岩浆特征的可能性。这是这些新工具首次应用于火山含水层系统。这项工作将为评估使用包括氦在内的地球化学示踪剂预测火山喷发的可行性提供基础知识。两名学生将接受国际实地考察和实验室工作方面的培训。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Volcanic activities release elements from magma at depth into shallow underground fluids which seep as hot springs. Helium is an element highly enriched in magma, thus it can be easily detected as magmatic isotope signal in the hot spring water. The magmatic helium signal appears to respond to eruption cycles at some volcanos, but the association between its variation and observed volcanic activities is often indecisive, making it difficult to use as an eruption forecasting tool. In those studies, the temporal variations of the volcanic signals from hot springs were compared with the volcanic activities of time that the samples were collected. However, the volcanic signal may travel for substantial time period underground before seeping from the springs so that the volcanic signature we detect could reflect the magmatic activity of tens of years ago. In order to evaluate the time scale of the possible delay, the proposed project will determine the travel time of the shallow subsurface fluids that carry the magmatic helium signals using radioisotopes of Kr and Ar. Ontake and Unzen volcanos, Japan, were selected as candidate sites because of their relatively recent volcanic activities and abundant helium monitoring records. Radioisotopes of Kr and Ar will be analyzed by Atom Trap Trace Analysis (ATTA) at Argonne National Laboratory and by low level decay counting at University of Bern. The chronological information will be used for re-interpreting the relationship between local geotectonic activities and He isotope records as well as for assessing the possibility of recovering 'fossil' magmatic signatures. This is the first application of these novel tools to a volcanic aquifer system. This work will provide fundamental knowledge to evaluate the feasibility of forecasting volcanic eruptions using geochemical tracers including helium. Two students will be trained in the international fieldwork and laboratory work.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.