Timescales and processes of hydrothermal alteration at Te Maari Tongariro, New Zealand: Insights utilizing petrographic and mass balance techniques

Hydrothermal alteration is known to enhance conduit sealing and modulate phreatic eruptions. 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 represent alteration processes associated with the 2012 eruptions, which may have involved 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 phreatic eruptions. • Approximate timescales of alteration at Te Maari, Tongariro. • Identified alteration style and conditions. • Mass balance calculations applied to hydrothermal alteration reactions.