Metamorphic fluids, i.e. fluids that are produced by metamorphic reactions, carry life-essential elements such as carbon and hydrogen from the deep Earth to shallower reservoirs. These elements may combine to form energy sources such as molecular hydrogen and methane, which play a fundamental role in metabolic processes in the subsurface biosphere.

However, how such geologic energy sources seep from the deep is still unclear.

Using the fossil rock-record accessible on the surface in the Italian Western Alps, our study shows that the fluids migrate along tectonic discontinuities in subduction zones, and that they may trigger earthquakes. 

In detail, we show evidence of brecciation (i.e. brittle failure leading to fragmentation) of rocks rich in mineral omphacite, a dense and stiff mineral forming at very high pressure where tectonic plates collide. This strong rock type is in contact with other rock types producing molecular hydrogen and methane, such as rocks called serpentinites, at depths in the range of 30-80 km in the Alpine subduction zone.

We suggest that the migration and accumulation of methane and hydrogen rich fluids below a permeability seal led to supralithostatic fluid pressure conditions that triggered brittle, and potentially seismic, failure of the host rock. Thermodynamic models indicate that methane- and hydrogen-rich fluids can produce fluid overpressure more easily than just aqueous fluids.

These processes play an important role in promoting the migration of deep energy sources in the subduction zone from deep source areas towards shallower reservoirs, including the subsurface biosphere where microbial life can take advantage of them through metabolic processes.