RELATIONSHIP BETWEEN MANGANESE AND METAMORPHIC-DRIVEN WATER CYCLING IN SUBDUCTION ZONES

The element manganese has an interesting history on Earth: its various oxidation states influence mineral stability in the surface environment and in the deep crust where redox reactions may release or sequester oxygen, a crucial component for the survival of life. In metamorphic systems, Mn is related to water retention through the mineral assemblages that exist at varying pressure and temperature conditions: rocks at higher metamorphic temperatures naturally retain less water as the proportion of hydrous silicates decline. Mn most notably stabilizes the mineral garnet, an anhydrous silicate product of many dehydration reactions (Dragovic et al. 2012; 2015). Thus, garnet growth influences water cycling in subduction zones where metamorphic dewatering contributes greatly to the global water cycle by hydrating mantle arcs and wedges (Hacker 2008).

We therefore set out to test the hypothesis that when more Mn is present in a rock composition it allows for less water to be retained in its intracrystalline structure during metamorphism. The program Theriak-Domino (de Capitani & Petrakakis 2010) was utilized to explore mineral properties and create phase diagrams for subduction zone conditions. Preliminary thermodynamic modeling revealed the results of the average retained water with varying Mn composition in a metamorphosed Mariana Trench clay sediment composition (Plank & Langmuir 1998). We find that metamorphosed Mn-rich sediments release approximately 30% more water at peak P-T conditions in comparison to otherwise identical low-Mn sediments. Furthermore, Mn-driven dehydration resulted in greater water production at cooler conditions in subducting slabs. These results require future analysis on how the stability of specific hydrous minerals cycle throughout metamorphism. Metamorphosed Mn have been an important link between hydration and oxygenation throughout geological history.