Unravelling the Timescales and Processes of Hydrothermal Alteration at Tongariro Volcano, New Zealand

Tongariro is an active composite volcano located in the southern Taupo Volcanic Zone in New Zealand. Its eruptive products range from basaltic andesite to dacite. Tongariro volcano hosts distinct hydrothermal systems, typically formed around actively degassing eruptive vents. Te Maari is one of the recently active vents on Tongariro’s northern flanks that produced two small phreatic eruptions in August and November 2012, as well as a debris avalanche with a volume of 7.7×105 m3. The role of hydrothermal alteration in the sealing and eruption mechanisms has been already documented, yet the timescale of alteration and sealing that can lead to such events remains to be reconstructed.

This study is based on fresh and altered sample pairs aiming to reconstruct alteration processes and timescales for mineral precipitation using field geology, petrography, stable isotopes, and thermodynamic modelling. The samples vary in mineralogy, geochemistry, porosity and related physical properties. The fresh samples have abundant plagioclase, pyroxene, and magnetite as primary phases, while the altered samples are characterized by secondary phyllosilicate minerals, including kaolin- and smectite-group minerals, and precipitation of cristobalite and native sulfur.

The alteration minerals correspond to intermediate argillic alteration that is caused by acidic fluid flow at the near surface at 150-200 °C. Using samples that have stratigraphically and radiometrically well constrained aged from 45 kyr to a decade old, we show that hydrothermal alteration of the primary volcanic minerals and glass operates on timescales of decades to kyrs. Such rapid alteration may catalyse future flank collapses and phreatic eruptions by changing the mechanical and physical properties of the volcanic altered rocks, with implications for the development of more comprehensive hazard assessment strategies.