Reactions, element transfer, and fracturing at ultramafic rock–sediment contacts: insights from reaction experiments at 500 °C and 1 GPa

Metasomatic reactions affect the mechanical properties of the interface between the mantle wedge and a subducting plate. However, the controls on fluid-mediated reactions that occur at the interface, involving mass transfer, volume change, and deformation, are poorly constrained. In this study, we experimentally investigated metasomatic reactions at the boundary between mantle rocks (serpentinite and harzburgite) and sedimentary rocks (quartzite or pelitic schist) at 500 °C and 1 GPa, using an assembly with a core of metasedimentary rock sandwiched between serpentinite and harzburgite. In all experimental runs, talc formed preferentially in the mantle rocks. In experiments using quartzite, talc formed along pre-existing fractures in serpentinite and as a layer within harzburgite, while no obvious alteration occurred in the quartzite. In experiments using pelitic schist, talc formed within tree-like fractures in serpentinite, whereas in harzburgite it formed as a layer or wedge-like filling. In the pelitic schist, albite porphyroblasts were preferentially replaced by Mg-rich saponite. These observations and mass balance calculations indicate that: (1) the solid volume-decreasing reactions in serpentinite accompanying reaction-induced fracturing and solid volume-increasing reaction in harzburgite caused the contrasting textures in the two rock types; and (2) Al-bearing minerals in the sedimentary rocks absorbed Mg, which facilitated the overall progress of the Mg-releasing, talc-forming reactions in the mantle rocks. These results suggest that compositional heterogeneity in subducting sediments can produce variations in the talc distribution and rheological properties at the slab–mantle interface, potentially affecting the location of episodic tremor and slip in a mantle wedge corner.