Garnet growth as a proxy for progressive subduction zone dehydration
Ethan F. Baxter and Mark J. Caddick, Department of Earth & Environment, Boston University, 675 Commonwealth Ave., Boston, Massachusetts 02215, USA. Posted online ahead of print on 29 April 2013; http://dx.doi.org/10.1130/G34004.1.
The progressive dehydration of subducted rocks represents a fundamental part of the global geologic water cycle. This release of water is a crucial triggering process for arc volcanoes, earthquakes, and the growth and maturation of continents. While models exist to predict water release from the subducting oceanic slab, it is challenging to reconstruct and test these fluid fluxes in nature without a marker in the dehydrated rock residue that can be linked to the primary fluid production. Here, Ethan Baxter and Mark Caddick show that the growth of garnet may be used as a proxy for dehydration in subduction zones. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ~1.4-3.0 GPa pressures where geophysical models and observations predict dehydration. Thermodynamic analysis of garnet-forming dehydration reactions permits quantification of the average water:garnet production ratio during subduction for varying rock types. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for p
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