Silicic volcanic rocks, a main regional source of geogenic arsenic in waters: Insights from the Altiplano-Puna plateau, Central Andes

Abstract

Volcanic rocks are a common, worldwide source of geogenic arsenic (As) that can affect water quality detrimentally. Nonetheless, variations of As concentration within different types of volcanic rocks and questions related to the original source of As in magma are not yet fully understood. We compiled published As data from the abundant Cenozoic volcanic rocks of the Altiplano-Puna plateau, considered the main source of As for waters in the region. These data indicates that volcanic rocks in the Altiplano-Puna have a mean As concentration of 9.1 mg/kg, which is almost 2 times higher than the upper continental crust. Arsenic increases within the more silicic compositions such as dacites and rhyolites as well as from mafic monogenetic volcanic edifices to stratovolcanoes of intermediate composition, silicic calderas and ignimbrite fields. The rate and composition of crustal assimilation by the magmas (enhanced by the crustal thickness up to >70 km) strongly influence the As concentration in volcanic rocks. The assimilation of argillaceous shale-type sediments represents the most important source of As in the magmas. The eruption dynamics has an enormous effect on As concentration. The higher the explosivity of an eruption, the higher the amount of ash that will be formed. Arsenic can be sorb onto ash particles so that fall deposits produced during explosive volcanic eruptions are prone to high As concentrations. Large volume ignimbrites formed after caldera collapses in the Altiplano-Puna are often the result of sustained explosive eruptions related to low, dense pyroclastic fountaining eruptions where highly concentrated pyroclastic density currents (PDC, flowing pyroclastic mixtures of particles and gas) are able to maintain high temperatures for long periods. Then As released from volatiles may remain for longer time in the emplacing PDC and can be sorbed onto ash particles, resulting in deposits with a higher As concentration. Post eruption hydrothermal alteration and sulfide mineralization produce As enrichment up to 100 times in the volcanic rocks (957 mg/kg) of the region, but can be very heterogeneous. Volcanism and its related products are the main source of As in the region. Once As is released from the volcanic sources, the semiarid climate, the occurrence of Na-HCO3 water types, and alkaline pH further enhance As concentrations in surface water and groundwater.