The origin of Nb and Ta anomalies in recent ejecta from Turrialba Volcano, Costa Rica

Volcanism is an important geologic process. It creates hazards for human societies and opportunities to understand the physics and chemistry of the Earth’s interior. Lavas from Turrialba Volcano, Costa Rica have unusually diverse chemical compositions for a subduction-related volcano. In particular, the relative concentrations of the elements Niobium (Nb) and Tantalum (Ta) compared to other trace elements vary in its lavas from those typical of subduction-related volcanoes (low-Nb) and intraplate volcanoes (high-Nb). Thus, Turrialba’s volcanic products may be used to investigate differences in sources and melting conditions responsible for generating magmas in these tectonic settings.

Turrialba began erupting explosively in 2010 after 144 years of quiescence. Volcanic glass shards were collected from ash layers associated with this new eruptive phase. Using micro-analytical techniques, major and trace element concentrations were measured on these glass shards to infer the structural, thermal and compositional controls of their genesis. A group of high-Nb glass shards were identified, as well as three low-Nb groups of glasses. The diverse compositions require varying proportions of subducted material in magma sources and varying melting conditions. Diapirs composed of mixtures of subducted and mantle material that rise off the subduction zone-mantle interface may represent appropriately diverse sources. Melting of these diapirs typically generates the diverse low-Nb magmas. However, unusually high temperatures beneath Turrialba may heat some diapirs to the point that they melt out Nb- and Ta-rich minerals, resulting in the generation of high-Nb magmas. Mixing between all these magmas results in the array of compositions observed at Turrialba.