GARNET NUCLEATION OVERSTEPPING: A POTENTIAL MECHANISM FOR FLUID PRODUCTION WITHIN THE SEISMIC-ASEISMIC TRANSITION IN SUBDUCTION ZONES

Metamorphic dehydration of sediment and ocean crust during subduction likely drives brittle deformation and seismicity. Thus, accurately constraining the pressure-temperature (P-T) conditions of devolatilization is important to understand depths and distributions of seismicity. Garnet nucleation and growth occur in response to metamorphic dehydration reactions, and can be used as a proxy for subduction zone devolatilization. Nearly all metamorphic models assume chemical equilibrium and a minimal kinetic barrier to nucleation. These models predict that the majority of prograde devolatilization occurs at shallow depths above the seismic-aseismic transition (SAT). Recent geophysical research, however, has identified seismicity and devolatilization at an intermediate depth within and below the SAT that is not easily explained by traditional thermodynamic modeling. In addition, significant garnet nucleation overstepping (delayed reaction) has been inferred in diverse tectonic settings, including oceanic subduction.

In this study, we explore whether garnet nucleation overstepping can explain earthquakes within and below the SAT by quantifying the potential effects of overstepped garnet nucleation and growth on the P-T conditions of subduction zone devolatilization. We employ the maximum driving force method of Spear and Pattison (2017) to model the overstepped nucleation of garnet along representative P-T paths from Penniston-Dorland et al. (2015). Reaction affinities required for nucleation were derived from the literature, and models will be run assuming moderate, intermediate, and large values of reaction affinity of 300, 700, and 1000 J/mol*O, respectively. Preliminary results for pelitic bulk compositions indicate a minimum P-T overstep of ~5 kbar and >100 °C along the average P-T path, and ~2 kbar and 100-150 °C along the hot path. Garnet nucleation occurs at the base or within the SAT along an average P-T path for all affinity values. Along the hot P-T path, however, nucleation occurs within the SAT at affinities of ~1 kJ/mol*O or higher. These results suggest that overstepped garnet nucleation may be a trigger for devolatilization within the SAT, particularly along colder P-T paths where less affinity is necessary for nucleation at the required depths.