The petrology of seafloor rodingites: Insights from geochemical reaction path modeling

Thermodynamic reaction path models were applied to provide insights into the formation of rodingites at mafic/ultramafic boundaries. The models were set up to investigate the shifts in fluid–rock equilibria as fluids move from peridotite undergoing serpentinization into a gabbroic body. Phase relations were investigated in the direction of increasing extent of reaction of the serpentinization fluid with gabbro at 200 °C, 300 °C, and 400 °C. Phase assemblages typical of rodingite (grossular + diopside ± chlorite) are predicted to form at 200 °C and 300 °C, but only in areas where the fluid is essentially unaffected by reactions with gabbro, i.e., near the contact with ultramafic rock or adjacent to a fissure filled with serpentinization fluid. As the fluid becomes more affected by reactions with the gabbro, prehnite or epidote-ss replace garnet and tremolite replaces clinopyroxene. Once the fluid chemistry is completely reset by reactions with gabbro, the predicted assemblage is typical of greenschist facies: albitic plagioclase, actinolite, chlorite, and epidote-ss. These transitions predicted in the model are very similar to what is observed in natural rodingites from different settings. Our model results hence support the hypothesis that rodingites form during serpentinization and only in areas where fluids controlled by serpentinization reactions are present. Our calculations further indicate that the formation of mineral assemblages and spatial mineralogical variability within rodingites from the ocean floor is mainly driven by steep activity gradients in protons and aqueous silica in pore fluids. Mass transfer of Ca is likely by diffusion of the CaOH+ species, which is predicted to show a very steep concentration gradient across a mafic–ultramafic boundary. Rodingites act as Ca traps because of the great stability of diopside and garnet at low SiO2 activities.Our model calculations further predict the formation of diopsidite from gabbroic and clinopyroxenite precursor rocks at 200 °C and 300 °C, suggesting that very high temperatures on the order of 800 °C are not required in the formation of diopsidite veins.