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Quantifying hydrothermal system timescales at Tongariro volcano, Aotearoa New Zealand

Rachelle Sanchez1, Gabor Kereszturi1, Antonio M. Álvarez-Valero2, Geoff Kilgour3, María Mercedes Suárez Barrios2, Georg F. Zellmer1,

  • Affiliations: ^ 1^Volcanic Risk Solutions, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand; 2Department of Geology, University of Salamanca, Spain; 3GNS Science, Wairakei Research Centre, Taupō, New Zealand 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 33

  • Programme No: 3.8.17

  • Theme 3 > Session 8

Abstract

Hydrothermal alteration is known to enhance conduit sealing, modulate phreatic eruptions, and contribute to the weakening of slope stability. However, the timescales of alteration are poorly understood and difficult to constrain. Here we use new field- and laboratory-based observations and analyses of fresh and altered lavas from Tongariro, New Zealand, to reconstruct the timescales, fluid composition, and impacts of alteration at this volcano. We focus on Tongariro as it hosts a moderate-sized hydrothermal system across a distributed vent complex. Importantly, it is the site of recent phreatic eruptions in 2012. Fresh Tongariro lavas (~500 years old) contain variable plagioclase and pyroxene phenocrysts in an aphanitic groundmass with trace titanomagnetite. Respective altered equivalents are characterized by secondary phyllosilicate minerals, including kaolin-group minerals, and other phases such as pyrite and alunite, reflecting (advanced) argillic alteration caused by acidic fluid flow and/or acidic steam percolation at shallow depths at ~150-200°C. Notable, albeit rare, hydrothermal carbonates are present (e.g., dolomite). The rates of alteration of the primary lavas (e.g., using stratigraphy and/or radiometric dating) indicate that hydrothermal alteration can occur within years to thousands of years. Our detailed analysis of secondary mineral assemblages indicates complex and temporally evolving hydrothermal fluid chemistry (including pH) and temperature. The analyzed secondary minerals may represent alteration processes associated with the 2012 eruptions, involving deep magmatic fluid discharge followed by neutral meteoric water. Understanding these processes provides insights into hydrothermal fluid circulation, alteration history, and potential volcanic hazards including flank collapse and conduit timescales contributing to phreatic eruptions.