Authigenic titanite in weathered basalts: Implications for paleoatmospheric reconstructions

The stability of titanite is sensitive to temperature and partial pressure of CO₂. The finding of authigenic titanite grains in weathering regolith formed on Paraná basalts, Brazil, under tropical climatic conditions, reveals the thermodynamically-driven conversion from calcite to titanite at elevated ambient temperatures. Being unusual nowadays, this phase transition provides important implications for the understanding of silicate weathering in earlier geological epochs.Two types of secondary titanites were identified in the weathering profile of the study area. The tiny grains of 10 ​μm are forming in the microscopic voids in the rock. Also, large fractures filled with Fe-rich clay minerals contain bigger specimens of up to 170 ​μm. The titanites of second type often coexist with chalcedony and barite. No carbonate minerals were found in the weathering profile. Weathering sphene can be discriminated from other titanite types by its strong positive Eu anomaly, increased Al₂O₃ content and low content of trace elements. Its specific chemical composition and reactive transport modeling link this secondary mineral with dissolution of plagioclase. The titanite precipitation is controlled by slow diffusion in poorly-aerated, highly-alkaline pore fluids.The subaerial weathering of basaltic rocks provides a significant reservoir for atmospheric CO₂. However, the deposition of carbonate minerals is thermodynamically avoided at the stability field of titanite. We demonstrate a complex feedback between CO₂ and soil carbonates. The rise in pCO₂ triggers the precipitation of calcite in the weathering regolith, but the greenhouse effect increasing the temperature can cease carbonate deposition. Secondary titanites were found in several paleosols and at least a part of them can be of weathering origin.