Amorphous phases - A new indicator for deep weathering

Amorphous phases are intermediate weathering products that result from primary mineral dissolution and transform into crystalline secondary phases like clays during rock weathering. Thus, amorphous phases are important indicators of the onset of beginning soil formation at depth. Yet, their abundance in deeper sections of the weathering zone, the saprolite, is rarely investigated. We analysed five granitoid drill cores from a climate gradient in the Chilean Coastal Cordillera by means of sequential extractions to study amorphous phases and to determine the concentrations of the elements they contain. Here, amorphous phases are operationally-defined based on the sequential extraction method. Scanning electron microscopy (SEM) of selected samples was used to investigate the morphology and chemistry of such phases.

Unexpectedly, we find non-negligible concentrations of extractable elements even in visually classified fresh bedrock. A comparison to primary mineral element composition and dissolution rates suggests that likely micro-scale pre-weathering of primary minerals has primed release of these elements. Higher in the profile, when sufficient fluid flow enables solubilisation of elements from pre-weathered minerals, loss of these elements into the dissolved phase (quantified by elemental loss balances) is the main process driving the evolution of the weathering profile. At the surface, their concentration strongly correlates with rainfall, but this correlation is diminished at depth. Amorphous phases were mainly observed along grain boundaries of biotite, in etch-pits on plagioclase, and in the dissolution structure of amphiboles and they are enriched in O, Si, Al, and Fe.

We use the concentrations to assign different zones in the weathering profile. These zones show distinguishable weathering processes that are reflected in the concentration of extractable Al and Fe: in soil and subsoil both dissolution and formation of reactive secondary weathering products are most important. In saprolite, dissolution and mobilisation into the dissolved phase are the main mechanisms. In bedrock, pre-weathering of primary minerals likely induced by post-magmatic processes, hydrothermalism, groundwater flow or gaseous O₂ diffusion releases  elements into amorphous phases such that water flow higher in the profile mobilises these elements during denudation. The onset of soil formation in the means of primary mineral dissolution is hence located much deeper than expected.