Collaborative Research: He-CO2-N2 Isotopes and Dissolved Gases in Groundwaters of the Costa Rica Fore-arc Margin

通过俯冲带的流体流动是地球上的一个基本过程。随着大洋板块俯冲到地球内部，水和其他挥发物（如二氧化碳、氮气）从俯冲板块释放到上覆的地幔楔中。这些流体是地幔融化和富水岩浆产生的主要原因，而富水岩浆导致火山爆发。此外，挥发物要么通过与俯冲有关的火山（如氮气的情况）有效地再循环到大气中，要么被潜在地带入地幔深处（如二氧化碳的情况）。因此，俯冲带挥发性再循环的程度对地幔和大气的总体挥发性组成有重要影响。在这个提议中，研究小组将把目标放在哥斯达黎加俯冲带复合体内的地方，在那里富含气体的冷泉向大气释放挥发物。此外，还将连续监测几个泉的气体成分，以便调查与该地区发生的地震活动有关的时间变化。该项目的研究计划包括建立挥发性数据集，包括He、CO2、N2、CH4、O2和Ar的同位素系统和丰度特征，以及主要/微量元素、稳定同位素（D/H和18O/16O）和含水流体（地下水）的碳-14特征，旨在了解挥发性物源、通量和时间变异的细节。这些弹簧位于火山锋面的海沟方向，因此，在地幔岩石圈和/或俯冲板的地壳到达与火山锋面相应的深度之前，样品挥发性物质可能会从地幔岩石圈和/或地壳中释放出来。通过这种方式，这项工作旨在调查实际到达火山锋的挥发物的比例。此外，由于预计不同的挥发性物质在下降的板块上具有不同的释放/保留特性，我们可以通过平行于板块收敛方向的采样剖面来衡量挥发物如何相互分离。此外，通过利用对水文地质的良好构造控制，我们还将确定断层对哥斯达黎加弧前三个目标部分（尼科亚、中太平洋海岸和奥索半岛）挥发物损失的影响。最后，由于哥斯达黎加的弧前地区是发生在大量活动断层上的许多大地震的位置，因此建议通过(a)在弧前的三个部分进行广泛的采样活动，针对约100个采样点进行采样，研究断层位置，冷泉的发生，气体成分和地震活动之间的关系；(b)使用专门开发的仪器（Spartah设备）连续监测几个选定弹簧的气体成分。在局部地震的情况下，研究小组预计能够通过持续监测气体化学和记录弹簧气体成分的任何变化来捕捉到这样的事件。该项目的这一部分有可能更好地了解该地区的地震过程。为了进一步实现这些目标，并为当地科学家提供研究机会，这些项目将通过讲座、实践课程和实地培训模块，为哥斯达黎加大学的水文地质学硕士项目做出贡献，这些课程涵盖了水文地质学、水化学、火山学和构造地质学的各个方面。哥斯达黎加波阿斯火山的监测工作也将继续调查挥发物在火山即将发生危险情况时发出信号的潜在作用。

Fluid flow through subduction zones is a fundamental process on Earth. As oceanic plates get subducted into Earths interior, water and other volatiles (i.e. CO2, N2) are released from the subducting plate into the overlying mantle wedge. These fluids are the primary cause for the melting of the mantle and the production of water-rich magmas that cause explosive volcanic eruptions. In addition, volatiles are either efficiently recycled back to the atmosphere through subduction-related volcanoes (as is the case for N2) or are potentially carried into the deep mantle (as may be the case for CO2). The extent of volatile recycling in subduction zones, therefore, has significant bearing on the overall volatile composition of the mantle and the atmosphere. In this proposal, the research team will target localities within the Costa Rica subduction zone complex where gas-rich cold springs release volatiles to the atmosphere. In addition, several springs will be monitored continuously for gas composition in order to investigate temporal changes related to earthquake activity occurring in the region. The research plan of this project involves producing a volatile dataset - comprising the isotope systematics and abundance characteristics of He, CO2, N2, CH4, O2, and Ar together with major/trace element, stable isotope (D/H and 18O/16O) and carbon-14 signatures of aqueous fluids (groundwaters), with the aim of understanding details of volatile provenance, flux and temporal variability. These springs are located trench-ward of the volcanic front and therefore sample volatiles that are potentially released from the mantle lithosphere and/or crust of the subducting slab before it reaches a depth corresponding to the volcanic front. In this way, this work aims at investigating what proportions of the volatiles that are subducted actually reach the volcanic front. Additionally, as various volatile species are expected to have different release/retention characteristics on the down-going plate, we can gauge how volatiles fractionate from one another by sampling profiles parallel to the direction of plate convergence. Moreover, by taking advantage of well-mapped structural controls on the hydrogeology, we will also determine the influence of faulting on the loss of volatiles along three targeted sections of the Costa Rica fore-arc (Nicoya, Central Pacific Coast and Oso Peninsula). Finally, because the fore-arc region of Costa Rica is the location of numerous large earthquakes occurring on a large number of active faults, it is proposed to investigate the relationships between fault localities, the occurrence of cold springs, gas compositions, and earthquake activity through (a) a widespread sampling campaign targeting ~ 100 sampling locations at the three sections of the forearc, and (b) continuously monitoring the gas composition of several selected springs using specifically-developed instrumentation (Spartah device). In the case of a local earthquake, the team anticipates being able to capture such an event by continuous monitoring of gas chemistry and documenting any changes in the gas composition of the springs. This part of the project has the potential to better understand earthquake processes in this region. To further these aims and to provide research opportunities for local scientists, the PIs will contribute to the M.Sc. Hydrogeology program at the University of Costa Rica through lectures, practical sessions and field-based training modules covering various aspects of hydrogeology, hydrochemistry, volcanology and structural geology. Monitoring efforts at Poas volcano in Costa Rica will also continue to investigate the potential role of volatiles in signaling the onset of impending hazardous conditions at the volcano.