205-5 Garnet as an archive of fluid flow processes during metamorphism: Evidence from in situ measurements of H and Li

Session: MSA 2025 Awards Lectures and Presidential Address

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Abstract:

Fluids released during metamorphism affect fundamental Earth processes, including seismicity, the generation of arc magmas, the formation of continental crust, the geochemical evolution of the mantle, and the concentration of elements critical for societal needs. However, there is still much that we do not know about processes of fluid transport in different tectonic settings, including a clear picture of how fluids emanate from and travel through metamorphic rocks, how episodic fluid flow is in different settings, and how long fluid flow events last. Light elements, including H and Li, can provide useful information about metamorphic fluid-rock interactions. Garnet is a useful recorder of metamorphic processes, as zoning within garnet can preserve details of the temporal history experienced by metamorphic rocks. In situ measurements of H and Li (both concentrations and isotopes) in garnet from several metamorphic localities reveals variations within crystals over a scale of tens to hundreds of microns. Some of these variations are associated with evidence for fluid release and fluid-rock reaction. Both H and Li have a valence state of +1, so consideration of incorporation and coupled substitution partners is important for the understanding of their behavior and incorporation in garnet. Some of the variations in H and Li found within garnet show evidence for coupled substitution with other trace elements. In many cases, the coupling is with more slowly diffusing elements, effectively sequestering the H and Li in the garnet and preserving a record that may be useful in discerning fluid flow sources, episodicity of fluid flow events, or closed system processes such as mineral breakdown. On the other hand, some of the variations in H and Li within garnet are consistent with intracrystalline diffusion, suggesting that these fast-diffusing elements may have potential to act as a diffusion chronometer recording the duration of relatively short-lived events such as cooling or fluid flow pulsing. Ongoing work is focusing on investigating garnet from a wide range of tectonic settings. Additionally, experiments determining the diffusivity of Li within garnet are being performed in order to constrain timescales of intracrystalline diffusion quantitatively.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7423

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