Preferential Formation of Chlorite Over Talc During Si-Metasomatism of Ultramafic Rocks in Subduction Zones

Talc formation via silica-metasomatism of ultramafic rocks is believed to play key roles in subduction zone processes. Yet, the conditions of talc formation remain poorly constrained. We used thermodynamic reaction-path models to assess the formation of talc at the slab-mantle interface and show that it is restricted to a limited set of pressure-temperature conditions, protolith, and fluid compositions. In contrast, our models predict that chlorite formation is ubiquitous at conditions relevant to the slab-mantle interface of subduction zones. The scarcity of talc and abundance of chlorite is evident in the rock record of exhumed subduction zone terranes. Talc formation during Si-metasomatism may thus play a more limited role in volatile cycling, strain localization, and in controlling the decoupling-coupling transition of the plate interface. Conversely, the observed and predicted ubiquity of chlorite corroborates its prominent role in slab-mantle interface processes that previous studies attributed to talc.

Automated summary

The research used thermodynamic reaction-path models to evaluate the conditions of talc formation, finding that talc formation at the slab-mantle interface is restricted to specific pressure-temperature conditions and fluid compositions. In contrast, chlorite formation is predicted to be ubiquitous under relevant conditions at subduction zone interfaces, with chlorite abundance and talc scarcity evident in the rock record.