| 英文摘要: | Fluids play a prominent role in tectonic processes associated with plate tectonic subduction zones including mountain building, volcanic activity, fluid-fault interaction, and earthquakes. Most of what is known about fluid sources and flow pathways is from geochemical characterization of hot springs and fumaroles near active volcanoes located along subduction zone arcs (e.g., the Pacific Ring of Fire). In contrast, the source and composition of fluids along subduction zones segments that lack active volcanism is not well known. This project investigates the source and chemistry of fluids in this type of setting using an innovative set of geochemical techniques applied to hot springs in the Peruvian Andes. This part of the Andes is targeted because it is the best example on Earth today of 'flat-slab' subduction, that manifests as a prominent gap in volcanic activity along an arc. Research on fluids in this geological setting is important for the fundamental understanding of their role in this stage of the subduction cycle, and this work is transportable to other ancient and modern subduction zones worldwide. Springs can contain high concentrations of metals, and this research will identify spring systems that pose potential water quality issues for the Peruvian public. Beyond these scientific and societal impacts, the project is contributing to training of graduate and undergraduate students and workforce development in a STEM discipline. It is facilitating scientific collaboration between U.S. and Peruvian scientists, and is providing training opportunities for Peruvian students in field and analytical methods. The project will also support a workshop on water quality and geothermal issues in the western U.S.
The primary research goal is to map the distribution of mantle- and slab-derived volatiles in the crust above a flat-slab subduction setting using the isotope geochemistry of thermal springs in Peru. This proposal builds on the prior results that identified up to 25 percent of mantle-derived helium in Cordillera Blanca hot springs with a geochemical sampling transect from the Cordillera Blanca (~9° South), southward including the Cordillera Huayhuash, a proposed 'slab-tear', the Nazca ridge, and the transition to steep subduction beneath the Altiplano plateau (~15° South). These data test the primary hypothesis that mantle helium in fluids above the Peruvian flat slab segment are geochemical manifestations of slab foundering north of the Nazca Ridge and ongoing slab dehydration and fluid transfer to the overriding lithosphere. A secondary goal targets the role of fault/shear zones in transferring fluids to the surface by comparing spring geochemistry and the paleofluid geochemistry preserved in fault rocks in the Cordillera Blanca versus the Cordillera Huayhuash. Multiple analytical tools will be used to address these objectives, including thermal spring water and gas chemistry, helium isotope ratios (3Helium/4Helium), 129Iodine, stable isotope analysis of fluids (carbon, nitrogen, chlorine, oxygen, and hydrogen) and fault rocks (hydrogen isotope ratios of micas), and fluid inclusion analysis of fault rocks. This project addresses the data gap on the nature of fluids in a flat-slab subduction setting, potentially identifying segments of the Peruvian trench that have subducted sediments, and providing a surface geochemical test for a proposed slab tear north of the Nazca ridge identified by geophysical investigations. Outcomes of this project may also serve as a modern analog for the role of fluids in ancient flat-slab subduction systems such as the Farallon slab under western North America. |