The role of plate-interface metasomatic rocks in nurturing subsurface microbial life

Fluid–rock interaction and microbial life are intimately connected. One process that is recognized to feed microbial communities is serpentinization, during which mantle minerals (olivine and pyroxenes) react with fluids to form serpentine and magnetite. This process also produces H₂ and, in the presence of C-rich units, CH₄. These serpentinization-derived, reduced C-H forms represent energy sources for microbial activities, as documented at multiple sites on present-day and ancient seafloor (i.e., mid-ocean ridges and ophiolites). Notably, however, serpentinization does not only occur at crustal levels that are shallow enough to overlap with T conditions permitting microbial life (the “biotic fringe”; ≤ 135 °C). Instead, several cases (e.g., the Monte Maggiore ultramafic massif, France) have been documented where oceanic lithosphere was majorly serpentinized only during subduction. This implies that major amounts of reduced energy sources may not be released until as deep as the plate interface, where microbial life is precluded.

We show that serpentinization occurring at P–T conditions that are prohibitive for microbial life can still play a fundamental role in nurturing microbial communities at shallow levels in the mantle wedge. At least from the Phanerozoic, slab-derived serpentinizing fluids have induced the formation of plate-interface metasomatic rocks (PIMRs) in the mantle wedge worldwide. Importantly, these PIMRs were recognized as fluid pathways by previous studies, and their exhumation path overlaps with the biotic fringe. Using micro-Raman spectroscopy, we identified CH₄, H₂ and N₂ in fluid inclusions in high-P/primary (e.g., jadeite) and lower-P/secondary (e.g., albite, analcime) minerals constituting Phanerozoic PIMRs all over the world, confirming their role in transferring deeply tapped, reduced energy sources from the plate interface to the biotic fringe. U-Pb geochronology of primary (i.e., zircon) and secondary (i.e., titanite) minerals in Phanerozoic-exclusive PIMRs confirms that such fluxes were protracted for tens–hundreds of millions of years, thus being able to sustain subsurface microbial communities in the mantle wedge. Our thermodynamic modelling confirms that, as subduction regimes became progressively cooler across geological time, reduced C-H forms like the detected CH₄ and H₂ became dominant over oxidized ones (e.g., CO₂). High-P serpentinization-derived fluids thus became optimal for sustaining microbial life in the Phanerozoic. However, serpentinization-derived energy sources may have never reached the biosphere in the mantle wedge without the emergence of fluid pathways like PIMRs, which emerged worldwide only in the Phanerozoic. Our study indicates that the emergence of these Phanerozoic-exclusive PIMRs, combined with cooler subduction regimes, may have played a pivotal role in promoting the proliferation and diversification of microbial life in this eon by boosting the supply of energy sources towards the biotic fringe.

While it is true that the release of slab-derived fluids induces potentially catastrophic geological processes that can disrupt life as we know it (e.g., high-magnitude earthquakes and highly explosive volcanic eruptions), if properly channelized and under the right geodynamic conditions, such fluids can also play a key role in sustaining hidden life on Earth.