The 2012 Eruption of Te Mari, New Zealand: Characteristics of a shallow magmatic degassing event

The 6 August 2012 phreatic eruption from Upper Te Mari crater, Mt Tongariro marked the re-awakening of this volcano which last erupted in 1896. This eruption was accompanied by precursory volcano-tectonic earthquakes, and by marked changes in chemical signatures of gases emitted from the Lower Te Mari fumaroles. Fumarolic gas samples collected on the western rim of Lower Te Mari crater on 22 May 2012 showed no significant changes from earlier (baseline) values. Earthquake activity commenced on 13 July, and intensified over the period of 18-21 July with approximately 120 located hybrid and volcano-tectonic events. Lower Te Mari fumarolic gases collected on 21 July revealed significant changes in gas signatures from the fumaroles by this time, with key ratios of CO₂/CH₄ and N₂/Ar increasing from 5,400 to 91,200 and 91 to 1,245 respectively, and increasing further to 121,000 and 1,300 respectively by 27 July. During this time there were no significant changes in fumarolic temperatures or pressures, suggesting that permeabilities in the hydrothermal system were low, and were throttling gas transfer from below. In May, C/S_t mole ratios for Te Mari fumaroles were close to 5. Immediately prior to the 6 August eruption, however, C/S_t had fallen to ca. 3, and post eruption fell to < 1, the lowest ratios yet observed for a NZ volcano. S_t/HCl ratios also peaked prior to the eruption, and then subsequently declined as shallow degassing and heating of the conduit system ensued, and HCl was volatilised from the system. A second, short-lived pulse of CO₂-rich gas was recognised in late-September, but this did not lead to further eruptive activity. A second, smaller phreatic eruption did occur on 21 November, but without precursory seismicity, and without any observed changes in pre-eruption gas signatures or gas emission rates. We currently regard this as a minor, vent-clearing eruption. Gas emissions have steadily declined since 21 November, and the volcano remains largely quiet seismically. The current model for this activity involves intrusion and open-system degassing of at least two shallowly emplaced dikes beneath the upper edifice of the volcano in mid to late July. The initial CO₂ pulse recognised in Lower Te Mari fumarolic emissions relates to early decoupling of this gas from the magma during dike emplacement, and is thought to have pressurised a partially sealed hydrothermal system, ultimately leading to the gas-driven eruption of 6 Aug. We suggest that the late September degassing event relates to a second, and largely aseismic intrusion into the fault/conduit system beneath the volcano. By the end of March 2013, an estimated 65 kT of SO₂ had been emitted from the system which, assuming complete degassing of a melt with an initial S content of 1000 ppm, equates to a degassed magma volume of some 13E6 m³. However, remobilisation of earlier precipitated S from the hydrothermal system cannot be ruled out, and would reduce the estimated magma volume involved in this activity.