Volatile transport of metals in the andesitic magmatic-hydrothermal system of White Island

<p>Volcanic gases observed at active volcanoes originate from the magma at depth. These volatiles exsolve as a result of decompression, crystallization and cooling of the silicate melt. The transport of metals in a magmatic volatile phase arises from complexation with the main volatile species, sulfur and halides. Composition of the magma, temperature, pressure and redox state have thus strong implications on metal mobility in these environments. Moreover, a variety of interactions and phase separations can affect these fluids after exsolution from the parental magma. This thesis aims at constraining the volatile transport of trace metals at White Island, a subduction-related magmatic-hydrothermal system, through a characterization and metal budget of the magmatic reservoir and the different atmospheric discharges. The metal content of the reservoir, as well as the effects of degassing and magma mixing on the magma are explored through the study of ejecta from the 1976-2000 eruptive cycle. CO₂, SO₂ and H₂O are degassing from a mafic melt at ~ 5 km depth, regularly feeding a shallower and evolved reservoir at ~ 800 m. Average contents of 164 ppm of Cu, 73 ppm of Zn, 12 ppm of Pb and 0.4 ppm of Au and Ag were detected in melt inclusions. A fraction of these metals partition into the exsolving aqueous fluid. Onset of magnetite crystallization may trigger exsolution of sulphide melt, found to contain around 30 wt% of Cu, and as much as 36 wt% Ni, 21 wt% Ag, 0.10 wt% Au in small inclusions, representing a considerable source of metals available for an aqueous fluid phase upon resorption. The volatile transport of metals is indicated by their enrichment in a variety of discharges at the surface. The hyperacidic waters of the crater lake absorb metals from the magmatic gases injected at subaqueous vents. Concentrations of ~ 12 ppm of As and Zn, 6 ppm of Cu and Pb were observed. Hydrolysis of the host rock by the reactive waters is responsible for the high cation contents of the fluids. Precipitation of secondary minerals such as silica, anhydrite, gypsum, sulfur and alunite occurs within and underneath the crater lake. The predicted speciation of metals greatly varies, dominated by CuI and FeII chloride complexes in the more reduced environment at the lake bottom, whereas CuII and FeIII are stable in the oxidized surficial waters. Arsenic is mainly present as As(OH)₃ at depth, with H₃AsO₄ dominating at the surface. Ag, Pb and Zn are complexed with chloride, and are not redox dependent. The presence of a body of molten sulfur at the bottom of the lake is indicated by sulfur spherules, both floating at the lake surface and in sediments. Pyrite crystals coat the surface of some globules, and chemical analyses reveal an enrichment in a variety of chalcophile metals (Tl, Sb, Bi, Au, As, Ag. Re, Cu). The volcanic gases emitted at fumaroles are enriched in metals compared to the magma. The effective transport of Se, Te, Sb, B, Au, As, and Bi is indicated by enrichment factors larger than 1000. In contrast, Cu is relatively depleted, suggesting deposition in the subsurface environment. Variations in composition are observed with time, mainly depending on temperature and major composition of the emissions. Values > 100 ppb of Sb, Bi, Ni, Zn, As and Se, > 10 ppb of Te, Pb, and Cu, and up to 8 ppb of Tl were recorded. Chloride is predicted to be the main ligand responsible for metal transport, even at higher temperature. The lack of thermodynamic data for complex solvated metal clusters may nevertheless bias our results. The low temperature of the studied fumaroles (maximum 192.5 °C) is in accordance with the small abundance of sulfides in the sublimates, whereas the high proportion of sulfates indicates oxidized conditions. The volcanic plume is enriched in metals such as Bi, Cd, Tl, Se, Te and Sb. The most common particles emitted are sulfates, halides, silicates, sulphuric acid and Zn ± Cu oxides. Metal emission rates are in the range of 1-10 kg/day for As, Se, Cu and Zn, 0.1-1 kg/day for Pb, Tl and Bi. Emissions of high-temperature magmatic gases are indicated by elevated SO₂/HCl ratio and the presence of Au in the particulate phase. Mass balance calculations in White Island magmatic-hydrothermal system indicate a segregation of around 4900 tons of copper per year, either accumulated from a dense brine at ~ 500 m depth, or deposited by low-density vapors on their way to the surface. Metal-rich sulfide blebs trapped in phenocrysts may also retain Cu at depth. These results thus reinforce the belief that White Island is an actively forming porphyry copper deposit.</p>

Volatile transport of metals in the andesitic magmatic-hydrothermal system of White Island

<p>Volcanic gases observed at active volcanoes originate from the magma at depth. These volatiles exsolve as a result of decompression, crystallization and cooling of the silicate melt. The transport of metals in a magmatic volatile phase arises from complexation with the main volatile species, sulfur and halides. Composition of the magma, temperature, pressure and redox state have thus strong implications on metal mobility in these environments. Moreover, a variety of interactions and phase separations can affect these fluids after exsolution from the parental magma. This thesis aims at constraining the volatile transport of trace metals at White Island, a subduction-related magmatic-hydrothermal system, through a characterization and metal budget of the magmatic reservoir and the different atmospheric discharges. The metal content of the reservoir, as well as the effects of degassing and magma mixing on the magma are explored through the study of ejecta from the 1976-2000 eruptive cycle. CO₂, SO₂ and H₂O are degassing from a mafic melt at ~ 5 km depth, regularly feeding a shallower and evolved reservoir at ~ 800 m. Average contents of 164 ppm of Cu, 73 ppm of Zn, 12 ppm of Pb and 0.4 ppm of Au and Ag were detected in melt inclusions. A fraction of these metals partition into the exsolving aqueous fluid. Onset of magnetite crystallization may trigger exsolution of sulphide melt, found to contain around 30 wt% of Cu, and as much as 36 wt% Ni, 21 wt% Ag, 0.10 wt% Au in small inclusions, representing a considerable source of metals available for an aqueous fluid phase upon resorption. The volatile transport of metals is indicated by their enrichment in a variety of discharges at the surface. The hyperacidic waters of the crater lake absorb metals from the magmatic gases injected at subaqueous vents. Concentrations of ~ 12 ppm of As and Zn, 6 ppm of Cu and Pb were observed. Hydrolysis of the host rock by the reactive waters is responsible for the high cation contents of the fluids. Precipitation of secondary minerals such as silica, anhydrite, gypsum, sulfur and alunite occurs within and underneath the crater lake. The predicted speciation of metals greatly varies, dominated by CuI and FeII chloride complexes in the more reduced environment at the lake bottom, whereas CuII and FeIII are stable in the oxidized surficial waters. Arsenic is mainly present as As(OH)₃ at depth, with H₃AsO₄ dominating at the surface. Ag, Pb and Zn are complexed with chloride, and are not redox dependent. The presence of a body of molten sulfur at the bottom of the lake is indicated by sulfur spherules, both floating at the lake surface and in sediments. Pyrite crystals coat the surface of some globules, and chemical analyses reveal an enrichment in a variety of chalcophile metals (Tl, Sb, Bi, Au, As, Ag. Re, Cu). The volcanic gases emitted at fumaroles are enriched in metals compared to the magma. The effective transport of Se, Te, Sb, B, Au, As, and Bi is indicated by enrichment factors larger than 1000. In contrast, Cu is relatively depleted, suggesting deposition in the subsurface environment. Variations in composition are observed with time, mainly depending on temperature and major composition of the emissions. Values > 100 ppb of Sb, Bi, Ni, Zn, As and Se, > 10 ppb of Te, Pb, and Cu, and up to 8 ppb of Tl were recorded. Chloride is predicted to be the main ligand responsible for metal transport, even at higher temperature. The lack of thermodynamic data for complex solvated metal clusters may nevertheless bias our results. The low temperature of the studied fumaroles (maximum 192.5 °C) is in accordance with the small abundance of sulfides in the sublimates, whereas the high proportion of sulfates indicates oxidized conditions. The volcanic plume is enriched in metals such as Bi, Cd, Tl, Se, Te and Sb. The most common particles emitted are sulfates, halides, silicates, sulphuric acid and Zn ± Cu oxides. Metal emission rates are in the range of 1-10 kg/day for As, Se, Cu and Zn, 0.1-1 kg/day for Pb, Tl and Bi. Emissions of high-temperature magmatic gases are indicated by elevated SO₂/HCl ratio and the presence of Au in the particulate phase. Mass balance calculations in White Island magmatic-hydrothermal system indicate a segregation of around 4900 tons of copper per year, either accumulated from a dense brine at ~ 500 m depth, or deposited by low-density vapors on their way to the surface. Metal-rich sulfide blebs trapped in phenocrysts may also retain Cu at depth. These results thus reinforce the belief that White Island is an actively forming porphyry copper deposit.</p>

A multi-purpose, multi-rotor drone system for long-range and high-altitude volcanic gas plume measurements

A multi-rotor drone has been adapted for studies of volcanic gas plumes. This adaptation includes improved capacity for high-altitude and long-range, real-time SO2 concentration monitoring, long-range manual control, remotely activated bag sampling and plume speed measurement capability. The drone is capable of acting as a stable platform for various instrument configurations, including multi-component gas analysis system (MultiGAS) instruments for in situ measurements of SO2, H2S, and CO2 concentrations in the gas plume and portable differential optical absorption spectrometer (MobileDOAS) instruments for spectroscopic measurement of total SO2 emission rate, remotely controlled gas sampling in bags and sampling with gas denuders for posterior analysis on the ground of isotopic composition and halogens. The platform we present was field-tested during three campaigns in Papua New Guinea: in 2016 at Tavurvur, Bagana and Ulawun volcanoes, in 2018 at Tavurvur and Langila volcanoes and in 2019 at Tavurvur and Manam volcanoes, as well as in Mt. Etna in Italy in 2017. This paper describes the drone platform and the multiple payloads, the various measurement strategies and an algorithm to correct for different response times of MultiGAS sensors. Specifically, we emphasize the need for an adaptive flight path, together with live data transmission of a plume tracer (such as SO2 concentration) to the ground station, to ensure optimal plume interception when operating beyond the visual line of sight. We present results from a comprehensive plume characterization obtained during a field deployment at Manam volcano in May 2019. The Papua New Guinea region, and particularly Manam volcano, has not been extensively studied for volcanic gases due to its remote location, inaccessible summit region and high level of volcanic activity. We demonstrate that the combination of a multi-rotor drone with modular payloads is a versatile solution to obtain the flux and composition of volcanic plumes, even for the case of a highly active volcano with a high-altitude plume such as Manam. Drone-based measurements offer a valuable solution to volcano research and monitoring applications and provide an alternative and complementary method to ground-based and direct sampling of volcanic gases.

A multi-purpose, multi-rotor drone system for long range and high-altitude volcanic gas plume measurements

A multi-copter drone has been adapted for studies of volcanic gas plumes. This adaptation includes improved capacity for high altitude and long range, real-time SO2 concentration monitoring, long range manual control, remotely-activated bag sampling, and plume speed measurement capability. The drone is capable of acting as a stable platform for various instrument configurations including: MultiGAS instruments for in-situ measurements of SO2, H2S, CO2 and H2O concentrations in the gas plume, MobileDOAS instruments for spectroscopic measurement of total SO2 emission rate, remotely-controlled gas sampling in bags and sampling with gas denuders for posterior analysis on the ground of isotopic composition and halogens. The platform we present has been field-tested during three campaigns in Papua New Guinea: in 2016 at Tavurvur, Bagana and Ulawun volcanoes, in 2018 at Tavurvur and Langila volcanoes and in 2019 at Tavurvur and Manam volcanoes; as well as in Mt. Etna in Italy in 2017. This paper describes the drone platform and the multiple payloads, the various measurement strategies, an algorithm to correct for different time-responses of MultiGAS sensors. Specifically, we emphasise the need for an adaptive flight path, together with live data transmission of a plume tracer (such as SO2 concentration) to the ground station, to ensure optimal plume interception when operating beyond visual line of sight. We present results from a comprehensive plume characterization obtained during a field deployment at Manam volcano in May 2019. The Papua New Guinea region, and particularly Manam volcano, has not been extensively studied for volcanic gases due to its remote location, inaccessible summit region and high level of volcanic activity. We demonstrate that the combination of a multi-rotor with modular payloads is a versatile solution to obtain the flux and composition of volcanic plumes, even for the case of a highly active volcano with a high-altitude plume such as Manam. Drone-based measurements offer a valuable solution to volcano research and monitoring applications, and provide an alternative and complementary method to ground-based and direct sampling of volcanic gases.

The BrIdge voLcanic LIdar—BILLI: A Review of Data Collection and Processing Techniques in the Italian Most Hazardous Volcanic Areas

Volcanologists have demonstrated that carbon dioxide (CO2) fluxes are precursors of volcanic eruptions. Controlling volcanic gases and, in particular, the CO2 flux, is technically challenging, but we can retrieve useful information from magmatic/geological process studies for the mitigation of volcanic hazards including air traffic security. Existing techniques used to probe volcanic gas fluxes have severe limitations such as the requirement of near-vent in situ measurements, which is unsafe for operators and deleterious for equipment. In order to overcome these limitations, a novel range-resolved DIAL-Lidar (Differential Absorption Light Detection and Ranging) has been developed as part of the ERC (European Research Council) Project “BRIDGE”, for sensitive, remote, and safe real-time CO2 observations. Here, we report on data collection, processing techniques, and the most significant findings of the experimental campaigns carried out at the most hazardous volcanic areas in Italy: Pozzuoli Solfatara (Phlegraen Fields), Stromboli, and Mt. Etna. The BrIdge voLcanic LIdar—BILLI has successfully obtained accurate measurements of in-plume CO2 concentration and flux. In addition, wind velocity has also been retrieved. It has been shown that the measurements of CO2 concentration performed by BILLI are comparable to those carried out by volcanologists with other standard techniques, heralding a new era in the observation of long-term volcanic gases.

The control of magma crystallinity on the fluctuations in gas composition at open vent basaltic volcanoes

Basaltic open vent volcanoes are major global sources of volcanic gases. Many of these volcanoes outgas via intermittent Strombolian-type explosions separated by periods of passive degassing. The gas emitted during the explosions has high molar CO2/SO2 and SO2/HCl ratios, while during the passive degassing these ratios are lower. We present new laboratory experiments in a model volcanic conduit, which suggest that these differences in gas geochemistry are a consequence of gas migration through crystal-rich magma. We show that gas may flow along channels through the particle-laden liquid and, at a critical depth, the gas may displace an overlying crystal-rich plug en masse, producing a growing slug of gas. Owing to the friction on the walls of the conduit, this plug becomes progressively sheared and weakened until gas enriched in the least soluble volatiles breaks through, causing an explosion at the surface. When the gas slug bursts, liquid is drawn up in its wake, which exsolves the more soluble volatile components, which then vent passively at the surface until the next explosive slug-bursting event.