Rheology of hydrous minerals in the subduction multisystem

The relatively low strength of the hydrous minerals has been theorized to play a role in the initiation of subduction through the feedbacks between faulting, hydration reactions, and rheological weakening. To further explore the behaviour of hydrous magnesium silicate minerals under the high stress conditions relevant to propagating faults, we performed nanoindentation tests on three serpentine species—lizardite, antigorite, and chrysotile—from room temperature up to their respective dehydration temperatures. While all serpentine minerals exhibit markedly lower indentation hardness than olivine under the same conditions (H_ol = 13.1–14.9 GPa), we find that antigorite (H_atg = 5.7–6.7 GPa) is almost a factor of three harder than lizardite (H_liz = 2.2–2.6 GPa), which is itself an order of magnitude harder than chrysotile (H_ctl = 0.1 GPa). We also indented chlorite from room temperature up to 400 °C and found that it has a hardness between that of lizardite and antigorite (H_chl = 2.8–4.0 GPa). Chrysotile is even weaker than the mineral talc (H_tlc = 0.6 GPa), another hydrous magnesium silicate, which was tested in a previous study. The weakest hydrous magnesium silicates – talc and chrysotile – are approximately one order of magnitude weaker than antigorite and almost two orders of magnitude weaker than olivine. There is a systematic relationship between indentation hardness and the lattice spacing between c-planes in these sheet silicates. Geodynamic models of subduction initiation typically use an ad hoc finite yield stress to trigger localized deformation. This study confirms that hydrous magnesium silicates are a likely candidate for alteration products that can facilitate localized deformation both before and after subduction initiation. However, the degree of weakening is highly dependent on the specific reaction product.