Detecting and quantifying magmatic tracers in cold springs on Mount Taranaki Volcano, New Zealand

Abstract:

Volatile emissions from subsurface magmas can be subtle at dormant volcanoes. In particular, carbon dioxide (CO₂) emissions from deep magmatic sources can easily go undetected. CO₂ can react with cold groundwaters in subsurface aquifers, providing a mechanism for long-term CO₂ transfer to the surface. However, this CO₂ is rapidly lost to the atmosphere as water flows away from the point of discharge. The magnitude of magmatic CO₂ released from cold springs on dormant volcanoes is globally unconstrained, and while rarely used as a monitoring tool, understanding how such emissions vary in time could provide important insight into the reawakening of dormant volcanic systems. Furthermore, the location and output of these springs may also help to understand alteration and the development of planes of weakness forming in long-lived unstable stratovolcanoes.

Here we present the initial findings comprehensive geochemical survey of mineral springs associated with Mount Taranaki volcano in New Zealand. The springs emerge on flanks of the volcano typically between 1.5 to 5 km from the summit. In February 2020 one mineral spring was sampled for the first time in 40 years. The chemistry is of the water is largely unchanged since the last sampling campaign, indicating a stable reservoir likely feeds the mineral springs. The water was 5^oC with a pH of ~ 5.5 with carbonic acid as the dominant anion. The waters are enriched in Fe and Mg due to the dissolution of volcanic rocks. The δ¹³C of the dissolved inorganic carbon (DIC) is -5.2, suggesting a magmatic source for the CO₂, and CO₂ concentrations in the air several meters from the spring source were several hundred ppm above background, with minor amounts of H₂S detected. Concentrations of Cl, SO₄ and SiO₂ were low, indicating no hydrothermal component in the spring water. Helium isotope analysis and tritium dating of the waters are underway. This study is the first step in a comprehensive plan to estimate the amount of volcanic volatiles released by individual springs, and develop a baseline of CO₂ emission from Mount Taranaki to underpin future assessment of any volcanic unrest. Eventually this study aims to build a better conceptual model of the ongoing rock alteration and the deep magmatic system at Mount Taranaki.