The role of subducted fluids on the genesis of deep earthquakes: evidence from deep diamonds and subduction zone thermal modeling

The role of subducted fluids on the generation of deep earthquakes (300 – 700 km) has been a topic of much research and debate for decades. While fluids are commonly believed to play a role in the genesis of intermediate depth earthquakes (70 – 300 km), it is often argued that fluids (i.e., water- or carbonate-bearing) cannot be transported to sufficient depth to play a role in the triggering or propagation of deep earthquakes. However, recent investigations show evidence of up to ~1.5 wt% water in a ringwoodite inclusion in a diamond from the mantle transition zone [1]. Additionally, heavy iron (δ⁵⁶Fe = 0.79–0.90‰) and unradiogenic osmium (187Os/188Os = 0.111) isotopic compositions of metallic inclusions in sublithospheric diamonds trace the pathway of serpentinized slabs from the trench to the top of the lower mantle [2]. Given this evidence for slab derived fluids at transition zone depths, we investigate the ability of fluids to reach these depths in subducted slabs by compiling a) new subduction zone thermal models, b) slab earthquake locations within these modeled subduction zones, and c) phase relations of hydrated or carbonated mantle peridotite and basaltic crust. Our results show a distinctive pattern that is consistent with the necessity of fluids in the generation of deep seismicity [3]. Specifically, those slabs capable of transporting water to the bottom of the transition zone (via dense hydrous magnesium silicates (DHMS)) produce earthquakes at transition zone depths. Conversely, virtually all slabs that do not transport water to these depths do not generate deep earthquakes. We also note that the depths of deep earthquakes coincide with the P/T conditions at which oceanic crust is predicted to intersect the carbonate-bearing basalt solidus to produce carbonatitic melts. We suggest that hydrous and/or carbonated fluids released from subducted slabs at these depths lead to fluid-triggered seismicity, fluid migration, diamond precipitation, and inclusion crystallization. Deep focus earthquake hypocenters would then track the general region of deep fluid release and migration in the mantle transition zone [3].

[1] Pearson, D. G., Brenker, F. E., Nestola, F., Mcneill, J., Nasdala, L., Hutchison, M. T., et al. (2014). Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, 507, 221–224. https://doi.org/10.1038/nature13080 [2] Smith EM, Ni P, Shirey SB, Richardson SH, Wang W, and Shahar, A (2021) Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor. Science Advances 7: eabe9773 https://doi.org/10.1126/sciadv.abe9773 [3] Shirey SB,  Wagner LS, Walter MJ, Pearson DG, and van Keken PE (2021) Slab Transport of Fluids to Deep Focus Earthquake Depths – Thermal Modeling Constraints and Evidence From Diamonds. AGU Advances: 2, e2020AV000304.    https://doi.org/10.1029/2020AV000304