Collaborative Research: Integrated He-CO2-N2 Isotope and Petrologic Study of Volatiles Cycling via the New Zealand Subduction System

了解水(H2O)、二氧化碳(CO2)和分子N(N2)等挥发物在地球会聚边缘的循环是理解流体在与地球板块构造相关的各种过程中所起作用的基础。为了充分评估与人类有关的(“人为”)过程对我们的表面环境(大气、海洋和生物圈)的影响，我们必须在所有时间尺度上了解促成环境变化的自然(非人类)过程。没有比大气中二氧化碳的积聚及其对全球平均表面温度的可能影响以及无数其他相关过程(海洋表面水温变化、海洋环流、极地冰川融化等)有关的需要更好的例子了。对这些过程的理解建立在对进入俯冲带的挥发物与弧前、火山前缘或弧后地区返回地表的挥发物的质量平衡以及运入地球更深处(地幔深度超过150公里)的比例的定量评估的基础上。直接返回地表的挥发物与被俯冲到地幔深处的挥发物的通量估计存在巨大的不确定性，这阻碍了对地球内部和外部储集层之间关键挥发物的长期和短期循环历史的评估。在这项研究中，研究小组将调查新西兰北岛俯冲系统中挥发物的循环，在该系统中，特定挥发物(二氧化碳、氮气以及惰性气体，如He和Ar)的输入和输出通量可以受到很好的限制。拟议的研究区是Hikurangi边缘、轴向山脉、陶波火山带和Hauraki裂谷带的组合，包括弧前-火山前缘-弧后剖面，提供了对Hikurangi海沟沉积岩石圈输入和海洋岩石圈输入之间这些关键挥发物的质量平衡的关键测试，并通过北岛不同构造制度的岩浆、地热和地下水活动输出。挥发分输入特征将使用来自ODP1124站点、DSDP站点317和即将到来的IODP分支的沉积物岩芯进行限制，而输出挥发特征和通量将通过在整个北岛战略地点对约100个站点进行广泛的陆上采样来提供。该方法将以利用Perple_X和其他软件对板坯挥发分释放进行热力学模拟和输入沉积物/基底的复合剖面为基础。总之，新西兰俯冲带的岩石学-地球化学-地热组合方法提供了诱人的前景，通过俯冲系统为关键挥发分的回收效率问题提供了良好的约束解决方案，并更清楚地了解了控制俯冲挥发分释放/保留性质的基本过程(例如，板片组成、P-T体制、板块耦合和上盘结构)。

Knowledge of the cycling of volatiles such as water (H2O), carbon dioxide (CO2), and molecular N (N2) at Earth's convergent margins is fundamental to understanding the role of fluids in a variety of processes related to plate tectonics on Earth. In order to fully assess the impacts of human-related ("anthropogenic") processes on our surface environment (atmosphere, oceans, and biosphere), we must understand the natural (non-anthropogenic) processes contributing to environmental change, at all time scales. There is no better example of this need than that relating to the buildup of CO2 in the atmosphere and its likely effect on global mean surface temperature and the myriad other related processes (change in ocean surface water temperature, ocean circulation, polar ice melting, etc.). Understanding these processes is predicated on a quantitative assessment of the mass balance of volatiles entering subduction zones versus the fraction returned to the surface in fore-arc, volcanic front, or back-arc localities, versus the proportion transported into the deeper Earth (to depths greater than 150 km, in the mantle). Huge uncertainties on flux estimates of volatiles returned directly to the surface versus those subducted to the deeper mantle remain, and prevent assessments of both long- and short-term cycling histories of key volatiles between Earth's interior and external reservoirs.In this study, the research team will investigate the cycling of volatiles in the well-characterized New Zealand North Island subduction system where input and output fluxes of specific volatiles (CO2, N2, and noble gases such as He and Ar) can be well constrained. The proposed study area, a combination of the Hikurangi Margin, Axial Ranges, Taupo Volcanic Zone and Hauraki Rift Zone, comprises a forearc-volcanic front-backarc profile providing a crucial test of the mass balance of these key volatiles between sedimentary and oceanic lithosphere inputs at the Hikurangi Trench to output via magmatic, geothermal and groundwater activity in various tectonic regimes of the North Island. The volatile input characteristics will be constrained using sediment cores from ODP Site 1124, DSDP Site 317, and an upcoming IODP leg, whereas output volatile characteristics and fluxes will be provided by extensive on-shore sampling of ~100 sites at strategically-placed localities throughout the North Island. The approach will be underpinned by thermodynamic modeling of slab volatile release utilizing Perple_X and other software and composite profiles of incoming sediment/basement. Taken together, the combination of combined petrologic-geochemical-geothermal approaches at the NZ subduction zone offers the tantalizing prospect of providing well-constrained solutions to the question of recycling efficiencies for key volatiles through subduction systems, and a clearer understanding of fundamental processes (e.g., slab composition, P-T regime, plate-coupling and upper plate structure) involved in controlling subducted volatile release/retention properties.