Terrestrial constraints on H2 generation during Martian serpentinization

Serpentinization, the water-driven alteration of olivine-rich rocks, plays an integral role in solar system evolution. While much attention has been directed towards the role of serpentinization in the evolution of our own planet, it has also been proposed as a mechanism for warming and stabilizing liquid water on early Mars, controlling the fate of the Martian hydrosphere, and originating life early in the planet’s history. Because olivine is widespread on the Martian surface and highly reactive in the presence of water, many researchers have hypothesized that serpentinization would have been common during periods of Martian history when liquid water was present. Observations of serpentine, the most abundant by-product of serpentinization, in intimate association with olivine on the Martian surface lends fundamental support to this hypothesis.

H₂ and organic carbon production during typical serpentinization on Earth is fundamentally limited by the modest quantities of Fe in terrestrial mantle olivine, which is typically composed of just 10% of the Fe-endmember, fayalite. To explore how this limitation would differ during Martian serpentinization, we compiled analyses of olivines in Martian meteorites and those analyzed by Curiosity in Gale Crater, Mars. The results show that even the most magnesian Martian olivines contain around twice the Fe content of terrestrial mantle olivine, and most contain much more. Thus, to gain a better understanding of H₂ and organic carbon production during Martian serpentinization, we must study serpentinization of atypical, Fe-rich olivines on Earth. To this end, we have performed and compiled analyses of serpentinites of the Duluth Complex (USA), which solidified from tholeiitic magmas broadly similar to those that produced the Martian crust and contains ferroan olivines representative of those on Mars. The data show increases in Fe(III)/Fe(tot) with increasing extents of serpentinization (as measured by H₂O content) that mimic the trends observed during serpentinization of terrestrial mantle rocks.  However, because of the much higher primary fayalite content, the Duluth Complex serpentinites produced around 5 times the H₂ at any given extent of serpentinization than those in an equivalent compilation of terrestrial serpentinites. This observation implies that even weakly serpentinized (20%) rocks on Mars would have produced as much H₂ as fully serpentinized terrestrial mantle peridotite, and a formation as large and stratigraphically continuous as the Olivine Bearing Unit in Jezero Crater could have produced a very substantial amount of H₂, even if it were only partially serpentinized. Thus, although orbiter observations suggest serpentine may be uncommon on the Martian surface, this does not necessarily indicate that serpentinization, and the reduced gases that it produced, did not play a significant role in the planet’s biogeochemical evolution.