Collaborative Research: Developing crystal clocks in metamorphic rocks: Using lithium in subduction zone garnets to decipher fluid release timescales

Subduction zones are places where one of Earth's tectonic plates dives beneath another. They are the location of many natural hazards, including volcanic eruptions such as Mt St Helens (1980) and Mt Pinatubo (1991), and earthquakes and tsunamis, like Tohoku, Japan (2011). In subduction zones, the release of water and other fluids plays an important role in generating deadly hazards, but fluid flow processes and duration are not well understood.

Metamorphic rocks that formed deep in ancient subduction zones and have been brought to the surface can contain minerals that record the release and movement of such fluids. These minerals provide a way to directly sample materials formed in subductions zones. The mineral garnet, found in many of these metamorphic rocks, grows over a period of time in concentric growth zones.

Growth zones are like tree rings, recording information about the geologic history of the mineral. Ratios of different masses (or isotopes) of an element called lithium change across growth zones within a garnet and variations in these ratios can act as a “stopwatch” for fluid flow. This work will measure lithium isotopes in garnets collected from six different sites and use laboratory experiments to determine fluid flow timescales.

The results of this work will link fluid flow processes recorded in the rocks with processes occurring in active subduction systems, including different types of seismic events. The proposed work includes development of an online mini-lesson for introductory geoscience classes. Through this lesson, students will explore crystal clocks in different geologic settings to better understand the timescales over which geologic processes operate.

This proposal aims to determine fluid flow timescales using a two-pronged approach. The first approach is to investigate natural samples from six different subduction-related localities that contain evidence for fluid flow events. Measurements of lithium isotopes across garnets from these samples will address the following key questions: 1) Do subduction-related fluids revisit former pathways and do existing timescales reflect a single pulse or multiple pulses of fluid? 2) If fluid flow occurs in multiple pulses, what is the degree of cyclicity of those fluid flow events? 3) What are the durations of individual fluid pulses?

The second approach is to use experiments designed to determine the diffusivity of Li in garnets. The experiments will address the following key questions: 4) What is the diffusivity of Li in garnet? 5) What is the role of oxygen fugacity? Two types of experiments will be used: sealed silica tube experiments to address the effects of temperature and garnet composition on diffusion and gas mixing experiments to address the role of oxygen fugacity on diffusion.

The combination of the results from the diffusion experiments applied to the proposed measurements within natural garnet will allow us to quantify the duration of fluid pulses described in question 3.

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