Distribution of Vapor and Condensate in a Hydrothermal System: Insights From Self‐Potential Inversion at Mount Tongariro, New Zealand

Abstract

Inversion of self‐potential data for source current density, js, in complex volcanic settings, yields hydrological information without the need for a prior groundwater flow model; js contains information about pH, pore saturation, and permeability, from which we infer the distribution of liquid and vapor phases. To understand the hydrothermal flow dynamics and hydraulic connectivity between surface thermal features at Mount Tongariro volcano, New Zealand, we undertook a reconnaissance scale self‐potential survey and developed an inversion routine for js, constrained by an existing 3‐D conductivity model from magnetotelluric measurements. The 3‐D distribution of js at Mount Tongariro reveals a discontinuous zero js zone interpreted as vapor or residually saturated pore space, surrounded by low to moderate js interpreted as circulating condensate liquid. Bounding faults act as conduits for down flowing groundwater or condensate, as well as barriers for the hydrothermal system. Localized small‐scale circulation associated with individual surface thermal features, rather than a single circulating system, accounts for the lack of widespread anomalous geochemical observations prior to the 2012 Te Maari eruption.

Hydrothermal Alteration on Composite Volcanoes: Mineralogy, Hyperspectral Imaging, and Aeromagnetic Study of Mt Ruapehu, New Zealand

“…Aeromagnetic and hyperspectral data show that Mt Ruapehu currently has an active but spatially confined vent‐hosted hydrothermal system. This is in sharp contrast with the neighboring Tongariro volcanic complex, which has abundant hydrothermal features, including fumaroles and hot pools (Miller et al, 2018; Moore & Brock, 1981). While there is a limited surface hydrothermal alteration on Mt Ruapehu (i.e., intermediate and advanced argillic styles), covering 2.6 km 2 or only 5% of the total area (Figure 8), the location of this alteration is scattered throughout the mapped areas.…”

Three‐Dimensional Mapping of Mt. Ruapehu Volcano, New Zealand, From Aeromagnetic Data Inversion and Hyperspectral Imaging

“…Tongariro is also dissected by many more faults on its east and west flanks compared to Ruapehu where active faults are nearly entirely absent (however, they may be buried by younger lava flows). These faults offset the basement by several hundred meters and may act as impermeable barriers (Miller et al, 2018) retaining circulating fluids within the hydrothermal system, promoting alteration. An eruptive hiatus between 80 and 50 ka (Conway et al, 2016), together with removal of hydrothermal alteration through deglaciation or flank collapse also contribute to the patchwork pattern at Ruapehu.…”

Electrokinetic Contributions to Self‐Potential Signals From Magmatic Stressing

“…Anomalous electrical signals have been observed during periods of volcanic unrest at several volcanoes (Hashimoto & Tanaka, 1995; Zlotnicki et al., 2001, 2005) and even appear prior to other geophysical signals (e.g., seismicity; Zlotnicki, 2015, and references therein). SP tomography and 3D time‐lapse inversion algorithms can accurately identify water flow patterns and SP source locations if the subsurface resistivity structure is known (e.g., Crespy et al., 2008; Jardani et al., 2008, 2007) with application to active volcanic systems (e.g., Vulcano [Revil et al., 2008] and Tongariro [Miller et al., 2018]).…”