Magnetic mineralogy of a weathered tropical basalt, Hainan Island, South China

Numerous studies have shown that mineral magnetic properties are sensitive indicators of weathering and soil formation (pedogenesis). However, the formation pathways and subsequent alteration mechanisms of specific weathering products are poorly constrained. We report a detailed magnetic investigation through a basalt profile that has been weathered under tropical conditions in South China. The rock magnetic analyses show that hot and humid conditions led to the neoformation of fine-grained magnetite and promoted mineral transformations in the form of maghemitization and hematization. Thus, although the basaltic parent material is rich in titanomagnetite, the final products of the weathering process are maghemite and hematite. Magnetic grain-size varies through the profile, with a combination of pseudo-single domain and single-domain particles at the base, with fining to single-domain in the middle and a mixture of superparamagnetic and single-domain particles in the uppermost profile. Importantly, the magnetic mineral assemblage through the profile is influenced by the mobilization and migration of iron through the sequence. This process is illustrated by concentration-dependent magnetic parameters, which reach maxima at the assembly depth of major and trace elements and minima at the oxic front where Fe content is its lowest.