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

了解地球汇聚边缘的挥发物，如水（H2O）、二氧化碳（CO2）和分子氮（N2）的循环，是理解流体在地球板块构造相关的各种过程中所起作用的基础。为了全面评估与人类有关的（“人为”）过程对我们的地表环境（大气、海洋和生物圈）的影响，我们必须了解在所有时间尺度上导致环境变化的自然（非人为）过程。没有比大气中二氧化碳的积累及其对全球平均表面温度和无数其他相关过程（海洋表面水温的变化、海洋环流、极地冰融化等）的可能影响更好的例子了。对这些过程的理解是基于对进入俯冲带的挥发物的质量平衡的定量评估，以及在弧前、火山锋或弧后区域返回到地表的挥发物的比例，以及被输送到地球深处（地幔深处超过150公里）的比例。直接返回地表的挥发物与俯冲到更深地幔的挥发物的通量估算存在巨大的不确定性，这阻碍了对地球内部和外部储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，研究小组将研究具有良好特征的新西兰北岛俯冲系统中挥发性物质的循环，该系统可以很好地约束特定挥发性物质（CO2， N2和稀有气体如He和Ar）的输入和输出通量。建议的研究区域是Hikurangi边缘、轴向山脉、Taupo火山带和Hauraki裂谷带的组合，包括弧前-火山弧前-弧后剖面，为Hikurangi海沟的沉积和海洋岩石圈输入到北岛不同构造制度下的岩浆、地热和地下水活动之间的这些关键挥发物的质量平衡提供了重要的测试。挥发性输入特征将使用来自ODP站点1124、DSDP站点317和即将到来的IODP站点的沉积物岩心进行限制，而输出挥发性特征和通量将通过在整个北岛战略位置的约100个站点进行广泛的岸上采样来提供。该方法将通过利用Perple_X和其他软件以及传入沉积物/基底的复合剖面对板状挥发物释放进行热力学建模来支持。总的来说，在新西兰俯冲带，岩石学-地球化学-地热方法的结合提供了诱人的前景，为通过俯冲系统回收关键挥发性物质的效率问题提供了良好的解决方案，并更清楚地了解了控制俯冲挥发性物质释放/保留特性的基本过程（例如，板块成分、P-T状态、板块耦合和板块上部结构）。

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.