Low-Temperature Hydrothermal Systems Response to Rainfall Forcing: An Example From Temperature Time Series of Fumaroles at La Soufrière de Guadeloupe Volcano

Volcanoes with highly-developed and shallow hydrothermal systems may be subject to sudden increases of their surface steam emission at vents in response to either deep forcing (e.g. increase of heat flux coming from the magma chamber) or external forcing (e.g. sudden decrease of atmospheric pressure or variation of meteoric water input). Because the vent plumbing has a limited heat and mass transfer capacity, the rise of steam pressure accompanying the increase of flux may destabilize the system in order to augment its net transfer capacity. This reorganization may, for instance, take the form of an enlargement of existing conduits and vents or to the creation of new ones. In such a case, local and extremely dangerous blast phenomena are likely to occur with devastating consequences several hundreds of meters around. Even volcanoes with a moderate activity and considered safe by the local population are exposed to such abrupt and dangerous events. The detection of early warning signals through temperature monitoring in the vents is of a primary importance and a main difficulty is to correctly interpret temperature jumps in order to reduce false alarms. We analyze time series of the temperature measured in three fumaroles located at the top of La Soufrière volcano in Guadeloupe, which are characterized by their relatively low temperature around 99°C, slightly above the boiling temperature of water at this altitude. Thanks to the long duration of the records from January to August 2017 and to their short 1-s sampling interval, a multiscale analysis can be performed over several orders of magnitude. We show that, despite their complex and sometimes erratic appearance, the temperature variations observed in the vents contain components highly correlated with rain input variations. Some remarkable patterns recurrently appear at different periods and we show that the main temperature variations of more than 10°C are related to the rainfall intensity. Our results illustrate the importance of external forcing on the otherwise complex and possibly chaotic dynamics of the shallow hydrothermal system of La Soufrière. They also reveal that a careful analysis of rainfall forcing must be done to be able to draw any conclusion concerning changes caused by the underlying hydrothermal system.

Ore Mineralogy, Geochemistry, and Dating (Re-Os, Ar-Ar) of the El Volcán Porphyry/Epithermal System, Maricunga Gold Belt, Northern Chile

The El Volcán gold project (8.9 Moz Au @ 0.71 g/t Au) is located in the Maricunga gold belt in northern Chile, on the flank of the large Cenozoic Copiapó Volcanic Complex. Precious metal mineralization is hosted in two zones (Dorado and Ojo de Agua) of (pervasively) altered Miocene porphyry intrusions and lava flows of andesitic to rhyolitic composition, and in breccias. The ore zones reflect an evolving magmatic-hydrothermal system with mineral assemblages of magnetite-ilmenite-pyrite-molybdenite (early), bornite-chalcopyrite-pyrite-rutile (stage I), chalcocite-chalcopyrite-enargite-fahlore-pyrite (stage II), and chalcopyrite-covellite-pyrite (stage III). Alteration is dominantly of Maricunga-style (illite-smectite-chlorite ± kaolinite), partly obscured by quartz-kaolinite-alunite ± illite ± smectite alteration. Powdery quartz-alunite-kaolinite alteration with native sulfur and cinnabar forms shallow steam-heated zones. Early K-feldspar ± biotite alteration is preserved only in small porphyry cores and in deep drill holes. Most gold is submicrometer size and is in banded quartz veinlets, which are characteristic of the Maricunga gold belt. However, some gold is disseminated in zones of pervasive quartz-kaolinite-alunite alteration, with and without banded quartz veinlets. Minor visible gold is related to disseminated chalcocite-chalcopyrite-enargite-fahlore-pyrite. The lithogeochemical database identifies a pronounced Au-Te-Re signature (>100× bulk crust) of the hydrothermal system. Molybdenum-rich bulk rock (100–400 ppm Mo) has an Re-Os age of 10.94 ± 0.17 Ma (2σ). 40Ar-39Ar ages on deep K-feldspar alteration and on alunite altered rock have the same age within error and yield a combined age of 11.20 ± 0.25 Ma (2σ). The formation of the El Volcán gold deposit took place during the establishment of the Chilean flat-slab setting in a time of increasing crustal thickness when hydrous magmas were formed in a mature arc setting. The vigorous nature of the hydrothermal system is expressed by abundant one-phase vapor fluid inclusions recording magmatic vapor streaming through a large rock column with a vertical extent of ≥1,500 m.

Molecular Weight Distribution and Dissolution Behavior of Lignin in Alkaline Solutions

Lignin, as the sole renewable aromatic resource in nature, has great potential for replacing fossil resources. However, the complexity of its structure limits its high value utilization, and the molecular weight distribution and dissolution behavior of lignin in alkaline solutions is still unclear. In this study, a conventional lignin separation during the pulping process in an alkaline hydrothermal system was performed by controlling the amount of NaOH, reaction temperature and holding time. Various analysis methods, including GPC, 2D–HSQC NMR and FTIR were used to study the characteristics of lignin fragments dissolved from wood. We were aiming to understand the rule of lignin dissolution and the recondensation mechanism during the process. The results showed dissolution of lignin due to ether bond fracturing by OH− attacking the Cα or Cβ positions of the side chain with penetration of NaOH, and the lignin fragments in solution recondensed into complex lignin with more stable C–C bonds. The experimental results also prove that the average molecular weight increased from 4337 g/mol to 11,036 g/mol and that holding time from 60 min to 120 min at 150 °C with 14 wt% of NaOH.

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>

Shrinking of Ischia Island (Italy) from Long-Term Geodetic Data: Implications for the Deflation Mechanisms of Resurgent Calderas and Their Relationships with Seismicity

The identification of the mechanisms responsible for the deformation of calderas is of primary importance for our understanding of the dynamics of magmatic systems and the evaluation of volcanic hazards. We analyze twenty years (1997–2018) of geodetic measurements on Ischia Island (Italy), which include the Mt. Epomeo resurgent block, and is affected by hydrothermal manifestations and shallow seismicity. The data from the GPS Network and the leveling route show a constant subsidence with values up to −15 ± 2.0 mm/yr and a centripetal displacement rate with the largest deformations on the southern flank of Mt. Epomeo. The joint inversion of GPS and levelling data is consistent with a 4 km deep source deflating by degassing and magma cooling below the southern flank of Mt. Epomeo. The depth of the source is supported by independent geophysical data. The Ischia deformation field is not related to the instability of the resurgent block or extensive gravity or tectonic processes. The seismicity reflects the dynamics of the shallow hydrothermal system being neither temporally nor spatially related to the deflation.