Volatiles in the Central American Volcanic Arc: Constraints on Subduction Processes From I-129, C, N and Noble Gas Determinations

洋壳与板块交界处沉积物的俯冲作用导致了各种地质过程，包括新陆壳的堆积、火山弧系的发展和矿床的形成。尽管就俯冲过程的大致轮廓达成了一致，但诸如沉积物循环的程度、火山系统中流体的来源以及这些地点的陆壳、洋壳和地幔之间的相互作用等重要问题仍然知之甚少。为了解决这些问题，我们计划使用带有时间信号的同位素体系碘-129来研究火山流体，它被分配到流体相中，并与海洋沉积物有很强的相关性。这些测定将与在相同环境下的气体系统学研究相联系，特别是氦、氮、二氧化碳和甲烷的浓度和同位素特征。这些系统的组合为确定火山流体源岩的类型和年龄提供了强有力的工具。本研究选择的背景是中美洲南部火山弧，这是一个经过充分研究和容易获得的活跃俯冲带的例子。我们计划将研究重点放在三个火山中心，哥斯达黎加、尼加拉瓜和萨尔瓦多各一个，样本主要来自地热井和相关的温泉和喷气孔。这项调查的目的是确定在这些环境中的沉积物循环量，存在的沉积物的年龄，以及陆壳、洋壳和地幔对火山流体的影响。研究结果将为大陆地壳的形成、洋壳和沉积物的再循环以及活弧火山作用的机制提供重要的信息。

9902919FehnThe subduction of oceanic crust and sediments at convergent plate boundaries gives rise to a variety of geologic processes including the accumulation of new continental crust, the development of volcanic arc systems and the formation of ore deposits. Although agreement exists on the general outline of the subduction process, important questions such as the extent of sediment recycling, the origin of fluids in volcanic systems and the interaction between continental crust, oceanic crust and mantle at these locations are only poorly understood. In order to address these questions, we plan to study volcanic fluids using an isotopic systems, iodine-129, which carries a time signal, partitions into the fluid phase and has a strong association with marine sediments. These determinations will be linked to an investigation of the gas systematics in the same settings, in particular of He, N2, CO2 and CH4 concentrations and isotopic characteristics. The combination of these systems provides a powerful tool to determine type and age of source formations for volcanic fluids. The setting chosen for this study is the southern Central American Volcanic Arc, a well studied and accessible example of an active subduction zone. We plan to focus the study on three volcanic centers, one each in Costa Rica, Nicaragua and El Salvador, with samples taken predominantly from geothermal wells and associated hot springs and fumaroles. The goal of this investigation is to determine the amount of sediment recycling in these settings, the age of the sediments present, and the influence of continental crust, oceanic crust and mantle on volcanic fluids. The results will give important information on principal processes associated with the formation of continental crust, the recycling of oceanic crust and sediments and the mechanics of active arc volcanism.