Potassium metasomatism of Precambrian paleosols

• K content in Precambrian paleosols greatly increased during post-burial alterations.
• The composition of fluids promoted the resistance of primary basaltic minerals.
• Pores were filled and alterations ceased at depth corresponding to 350 °C.
• Na, Mg, Ca were mobile elements; Al, Si, P, Ti, Fe contents did not change.
• The Fe(II)/Fe(III) ratio was transformed during subsidence of weathering profiles.

Independent of substratum composition, ancient regoliths formed by subaerial weathering are characterized by accumulation of K and other alkaline elements. Potassium enrichment due to the formation of clay-minerals is a trend typically observed in regoliths developed by modern hypergene processes. However, the K content in Precambrian paleosols is much higher than in modern weathering profiles. K-metasomatism affecting buried hypergen targets is a possible mechanism for K accumulation. A novel reaction-transport dissolution–precipitation model designed as part of this work proved able to reproduce many essential features of post-burial alterations of a subaerial weathering profile formed on basaltic rocks, including potassium enrichment, dehydration, compaction and pore filling that impacted the buried regolith. The phenomena of preservation of the primary basaltic structure, the apparent selective alteration of soil levels and immobility of Al, Ti and P could be explained by the model.