Magnetic investigation of iron oxides of the Galápagos Archipelago and the relationship with island age and climate

The Galápagos Archipelago has been formed as the Nazca Plate moved over a volcanic hotspot. The islands' age increases from west to east, and they exhibit pronounced climatic zonation, ranging from arid lowlands to humid highlands. In this work, topsoil and parent material samples from four islands, Isabela, Floreana, Santa Cruz, and San Cristóbal, were analyzed to investigate iron mineralogy as a function of island age and climate. Samples were collected from six sites in the humid highlands on all four islands, as well as from two sites in very dry and dry zones on Floreana and San Cristóbal, respectively.

Room-temperature Mössbauer spectroscopy and magnetic measurements, including hysteresis loops (up to 3.0 T) and temperature-dependent magnetization (50 – 1000 K), were performed to identify and characterize the iron-bearing phases. Mössbauer spectra reveal the presence of Fe²⁺ and Fe³⁺ doublets attributed to iron silicates, as well as a sextet corresponding to hematite. In all samples, the relative contribution of the Fe²⁺ doublet decreases from parent material to topsoil, accompanied by an increase in the Fe³⁺ doublet contribution. Samples from the older islands (Santa Cruz and San Cristóbal), in addition to the presence of Fe³⁺, showed an important sexted associated with hematite in the topsoil and parent material samples. We cannot rule out the contribution of fine particle size and superparamagnetic goethite and/or ferrihydrite associated with the Fe³⁺ doublet.

Climate-dependent variations are also evident. Mössbauer spectroscopy data of topsoil samples from humid environments exhibit a higher hematite contribution (59%) compared to those from dry environments (49%). For parent materials, humid conditions yield a 54% hematite contribution, whereas samples from dry conditions show a 17% contribution from maghemite. The magnetic results are complemented by hysteresis loops, which indicate the presence of a high-magnetization phase, consistent with Ti-magnetite and/or Ti-maghemite. The absence of a Verwey transition near 120 K in low-temperature magnetization curves and a drop in magnetization near 580 °C in high-temperature magnetization curves further support the presence of Ti-magnetite. AC magnetic susceptibility curves exhibit a frequency dependency, which may indicate a broad distribution of particle sizes, due to the contribution of superparamagnetic iron phases.