Collaborative Research: Apatite petrochronology and microtextural analyses: a new tool to directly date subduction processes at the base of the seismogenic zone

Subduction zones are the locus for the most destructive geological hazards on earth including megathrust earthquakes, volcanic eruptions, and tsunami. The portion of the subduction zone where large earthquakes nucleate and slow earthquakes occur, known as the base of the seismogenic zone, is also an area where intense chemical change and fluid flow occurs. Understanding the timing and processes associated with these rocks deformating and interacting with fluids in this part of the subduction zone is critical for further constraining these earthquakes and global element cycling. However, we currently lack robust tools to do so. This project aims to develop apatite petrochronology, the integration of chronologic, chemical, and textural data from single grains of the common mineral apatite, to directly date chemical and mechanical processes that occurred at the base of the subduction seismogenic zone. The proposed research will provide new tools for researchers studying subduction zones as well as deformation and fluid-rock interactions more broadly. The broader impacts of this work center around providing education and research opportunities that increase inclusivity and accessibility in geoscience through the development of virtual field video game modules that integrate field and lab observations and with petrologic, microstructural, and chronological results from this research. These video games will give students opportunities to gain field skills and link outcrop to microscale observations, while being inclusive and accessible to all students (i.e., no barriers associated with cost or able-bodiedness). This research supports two early career female scientists, a female postdoctoral researcher, and will support an undergraduate and graduate student at UNLV. The base of the subduction seismogenic zone, which occurs at depths of 30-50 kms and temperatures ~200-500°C, is where both large megathrust earthquakes nucleate and enigmatic fault zone behaviors such as episodic tremor and slip occur. This is also an area of intense chemical transformation including devolatilization, fluid flow, and metamorphism. Chemical, mechanical, and fluid processes occurring along the plate interface likely play an important role in influencing the deformation style of the base of the subduction seismogenic zone within the relatively cool greenschist and blueschist metamorphic facies (250°C - 500°C). We currently lack well constrained in situ chronometers in these relatively cold metamorphic rocks, making it challenging to place direct timing constraints on these chemical and mechanical processes in exhumed subduction complexes. Apatite, a common accessory mineral in many subduction zone lithologies, dynamically recrystallizes during deformation, dissolves and reprecipitates during fluid flow, and chemically tracks metamorphic and metasomatic reactions making it a potentially transformative tool for recovering linked microstructure-metamorphism-Temperature-time data. This project tests the hypothesis that apatite U-Pb petrochronology can directly date deformation, metamorphism, and(or) fluid flow in rocks exhumed from the base of the subduction seismogenic zone. Through coupled microstructural (petrographic, EBSD), geochemical (EPMA, LA-ICP-MS), and geochronological (LA-ICP-MS) techniques the researchers will directly date these processes in four exhumed subduction complexes (C. Alps, Catalina Schist, & Crete/Andros, Greece) representing different stages of the subduction evolution across a range of P-T-fluid conditions and lithologies. Their results will systematically constrain the physical and chemical behavior of apatite across different P-T and fluid conditions and facilitate method development of EPMA mapping of apatite, yielding transformative tools for recovering linked microstructure-T-t data. Ultimately, this will provide rheologic, geochronologic, and geochemical constraints on from exhumed subduction related rocks that can be integrated with remote observations (e.g., seismology, geodetic data) to better understand complexities of subduction earthquakes, creeping deformation, slow slip events, and chemical transformations during metamorphism, metasomatism, and fluid flow. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.