A thermodynamic modelling approach to predict the outcome of carbonaceous fluid metasomatism on Earth and Mars

Abstract

The carbon flux between the atmosphere and the geosphere is linked by a broad range of geodynamic and magmatic processes which govern the deep carbon cycles on all telluric planets, asteroids, and moons. On Earth, carbon is introduced into the mantle by subduction. On Mars, deep carbon is mobilised by magmatism and mantle convection, with delamination the only likely mechanism to recycle crustal carbon. The mobilisation and transportation of phases across P-T-X gradients result in the destabilisation of carbon-bearing and hydrated minerals leading to the formation of melts and fluids. Both act as mass-transfer agents, mobilising and transferring carbon. The interaction of fluids and magmas with surrounding rocks results in metasomatism. The role of carbon-rich fluids in the formation of metasomatic minerals and methane reservoirs on Earth and Mars has been studied via thermodynamic modelling. Here I present the results of predictive simulations to express the evolution of metasomatic systems at different P-T-fO₂ conditions. This study finds that the traditional distinction of diamonds through paragenetic groups cannot be used as a genetic classification because fluid-rock metasomatism can produce the compositional range of garnet and clinopyroxene found as inclusions in diamonds. The amount of carbon in the system – and its speciation – controls the geochemistry of metasomatic silicates, highlighting how carbon is even more influential than previously thought. Furthermore, fluid metasomatism can convert depleted mantle rocks into fertile websterites without championing a mechanism involving partial melting. Fluids have all the rock-forming elements to precipitate anhydrous silicates, and the presence or absence of hydrated minerals is no reliable evidence to distinguish melt and fluid metasomatism. Finally, the reduced conditions of Mars favour the formation of methane, which can be stored in geological reservoirs and then transported to the surface, sustaining the CH₄-based greenhouse required for liquid water on the surface of Early Mars.