Erebus is a well-studied open-vent volcano located on Ross Island, Antarctica (77◦ 32’ S, 167◦ 10’ E). The volcano is the focus of ongoing research aimed at combining petrologic data and experiments with surface gas observations in order to interpret degassing histories and the role of volatiles in magma differentiation, redox evolution, and eruptive style. This research focus has been driven in part by an abundance of studies on various aspects of the Erebus system, such as physical volcanology, gas chemistry, petrology, melt inclusion research, seismic, and more. Despite this large data set, however, interpretations of Erebus rocks, particularly mafic and intermediate lavas, which are thought to originate from deep within the magmatic plumbing system, have been hindered due to a lack of experimental data. Experimental petrology is a common tool used to understand volcanic plumb- ing systems and to tie observations made at the Earth’s surface to the deep pro- cesses responsible for driving volcanic activity. Experimental petrologists essen- tially recreate natural magma chambers in miniature by subjecting lavas to con- ditions of pressure, temperature, and volatile chemistry (P-T-X) relevant to a natural underground volcanic system. Because many important parameters can be constrained in the laboratory, the comparison of experimental products with naturally erupted ones allows for an understanding of the formation conditions of the rocks and gases we see at the surface. In this thesis, I have employed experimental and analytical petrological tech- niques to investigate the magmatic plumbing system of Erebus volcano. Broadly, the research is focused on volatiles (namely H2O, CO2, and S species) in the Ere- bus system: their abundances, solubilities, interactions, evolution, and ultimate contributions to degassing. Specifically, three key themes have been investigated, each employing their own experimental and analytical techniques. Firstly, the mixed volatile H2O-CO2 solubility in Erebus phonotephrite has been investigated under P-T-X conditions representative of the deep plumbing system of Erebus. Understanding the deep system is crucial because the constant supply of deeply derived CO2-rich gases combined with a sustained energy and mass input into the lava lake suggests a direct link between the phonolite lava lake and the volcano’s ultimate mantle source via a deep mafic plumbing system. Secondly, I have mapped the phase equilibria and evolution of primitive, inter- mediate, and evolved Erebus lavas. The chemistries of these experimental products span the full range of lavas on Ross Island and help to constrain magmatic evolu- tion from basanite to phonolite as well as to elucidate the geometry of the deep Ross Island plumbing system. Finally, lower-pressure experiments representing the shallow plumbing system at Erebus have been performed in order to understand the transport properties of sulfur in alkaline magma. Experiments were performed on natural Erebus basanite and phonolite, which represent the most primitive and evolved lavas from Erebus. A distinct cocktail of C-O-H-S fluid was equilibrated with each experiment, and a wide range of experimental oxygen fugacities was explored. Overall, experiments from this work are the first to place constraints on the en- tire magma plumbing system of Erebus volcano. In addition, experimental results foster a new understanding of non-ideal gas behavior at high pressure, the affinity of CO2 to deeply sourced rift magmas, and the effect of alkalis on fluid transport capabilities in melts.

Volcanoes are a key component of the Earth system, with volcanic activity reaching from deep in the Earth’s mantle and extending to interactions with volcanic gases and the atmosphere. Volatile trace metals degas from volcanic eruptions and at fumaroles, but their behaviour is poorly understood. I designed and built a benchtop fumarole, from which I degassed a silicate melt with trace metals, to simulate the volatilization and sublimation of trace metals from volcanic gases. I collected sublimates along a temperature gradient to examine the behaviour of the trace metals. The experimental sublimates were analysed for their chemical composition and phase identification. Lithium, Cu, As, Rb, Mo, Ag, Cd, Cs, W, Pt, Tl, Pb and Bi were found to be volatile and sublimed in elevated concentrations at various temperatures between 250-600°C. Compared to natural fumarole studies, similar volatile behaviour is seen for Cu, As, Ag and Tl. Variability between the experimental and natural fumarole sublimates is proposed to be from a lack of ligands in the experiments. Ligands can complex with trace metals, to transport and sublime mineralogical phases. Given the importance of ligands to metal complexation, I proceeded to examine the importance of chloride as a ligand in volatile transport and sublimation of trace metals. I degassed a silicate melt with trace metals and variable concentrations of Cl-, up to 2 wt% Cl-, in air. Sublimates produced from these experiments were analysed for mineralogical and chemical information. Raman spectroscopy and scanning electron microscopy helped to determine that silica polymorphs occur at all temperatures and that halite forms below 600°C. Additional phases, including hydrated phases transporting Mo, Cu and Pb also formed as sublimates. These hydrated phases are suggested to be hydrated post-experiment or are Cl--bearing analogues. The addition of Cl- to the experiments increases the concentration of Li, Rb, Cs, Ag, Cr, Cu, Mo and W in the sublimates compared to Cl-free experiments and Cl-bearing phases are likely hosts of volatile trace metals. Volcanic gases in nature do not have the oxygen fugacity of air and contain considerable S. To conduct sublimation experiments at various lower oxygen fugacities and with S as it is a redox sensitive ligand, I adapted my original benchtop fumarole design to a gas-mixing furnace, in which I degassed silicate melts containing S, Cl and trace metals. Substantial loss of S and Zn, Sn, As, Bi, Pb and Cd occurred from the starting material melt in the most reduced experiment at 4.6 log units below the FMQ buffer. This loss corresponded to increased concentrations of the same elements in the sublimates of the same experiment. These trace elements are likely hosted as sulfide minerals, as the fO2 conditions are in the sulfide stability field. This agrees with thermodynamic calculations that determine that sulfides should be stable in similar conditions to this experiment. Chlorides are sublimed in experiments from ~200-650°C and are likely subliming as a NaCl-KCl-FeCl3 solid solution. Halite is calculated to form at all temperatures in the experiments, based on modelling. These chlorides are probably hosting Cu, Cd, Bi, Li, Rb and Ag in the experiments. In nature, if these metals are in soluble salts, when leached they provide a source of metals to the environment where they are deposited. Overall, I demonstrated that trace metal behaviour in the sublimates from volcanic gases will be affected by available ligands and the oxygen fugacity of the melt and the gas. Chlorides are a likely phase to host trace metals and are ubiquitous in experiments, even with variable melt compositions, fO2 conditions and across a wide temperature range. / Graduate

Comprendre l’enrichissement des liquides sulfurés en éléments chalcophiles et sidérophiles pouvantmener à la formation de gisements de sulfures magmatiques.Le premier objectif de cette thèse a été de quantifier les variations des coefficients de partage (Dsul/sil)de Co, Ni, Cu, Pd, Ag, Pt et Au dans des conditions crustales typiques des intrusions de la région deNoril’sk-Talnakh (Russie), par le biais d’une étude expérimentale en autoclave à chauffage interne,effectuée à 1200 °C, 70 MPa, sous différentes fugacités d’oxygène.Les résultats montrent que les Dsul/sil de Pd, Ag, Pt et Au augmentent avec la teneur de ces élémentsdans le liquide sulfuré, indiquant qu’ils ne suivent pas la loi de Henry, au contraire de ceux de Co, Ni, etCu. Ces Dsul/sil permettent de reproduire les teneurs en métaux des sulfures naturels de la région deNoril’sk, à partir d’un magma parent plus enrichi en Pd et Pt que les laves de la région, et en invoquantl’interaction entre des masses de liquide silicaté et de liquide sulfuré pour pouvoir enrichir ce dernier(facteur R entre 300 et 1000 pour les sulfures massifs et entre 300 et 6000 pour les disséminés).Le second objectif de cette thèse a été de modéliser, par une approche thermodynamique, les variationsdes Dsul/sil en fonction des conditions magmatiques. Ce modèle décrit les variations des Dsul/sil avec latempérature, la pression, la fugacité d’oxygène et la chimie des liquides, et suggère que les magmasmafiques à l’équilibre avec les liquides sulfurés les plus enrichis sont ceux qui ont les plus faiblestempératures et pressions, et les fugacités d’oxygène les plus élevées. / A comprehensive knowledge of metal partitioning between sulfide liquid and silicate melt is essential tounderstand sulfide liquid enrichment in chalcophile and siderophile elements that can lead to theformation of magmatic sulfide ore deposits.The first aim of this thesis was to quantify the partition coefficients (Dsul/sil) for Co, Ni, Cu, Pd, Ag, Pt andAu at crustal conditions relevant to the Noril’sk-Talnakh region (Russia), through an experimental studyconducted in internal heated pressure vessels at 1200 °C, 70 MPa, and under variable oxygenfugacities.Our results show that Dsul/sil for Pd, Ag, Pt and Au increase with the content of the element in the sulfideliquid, showing that they do not follow Henry’s law, in contrast to those for Co, Ni, and Cu. These Dsul/silcan reproduce the metal contents of natural sulfides of the Noril’sk region, starting from a parent magmaPd- and Pt-richer than the lavas of the region, and invoking an interaction between the masses of silicateliquid and sulfide liquid in order to enrich the latter.The second objective of this thesis was to model Dsul/sil variations as a function of magmatic conditions,using a thermodynamic approach. This modeling allows investigating the variations of Dsul/sil withtemperature, pressure, oxygen fugacity and the compositions of both liquids and suggests that the maficmagmas with the lowest temperatures and pressures and the highest oxygen fugacities are those inequilibrium with the most enriched sulfide liquid.

In der vorliegenden Studie wurden kinetische Prozesse beim experimentell simulierten Aufstieg dazitischer Magmen, den Eruptionsprodukten vom Vulkan Taapaca nachempfunden, untersucht. Der Schwerpunkt lag auf der Entgasung der wassergesättigten Proben und der mit dem resultierenden Anstieg der Liquidustemperaturen einhergehenden Kristallisation von Plagioklas. Desweiteren wurden enthaltene Amphibole auf eine mögliche Reaktion als Folge der Dekompression untersucht. Die Entgasung während der Dekompression erfolgte bei einer Temperatur von 850°C in isothermen Dekompressionsraten von 6,3 bar/h bis 450 bar/h stets nahe an der Gleichgewichtskonzentration von Wasser, welche unabhängig durch Gleichgewichtsexperimente bestimmte wurde. So nehmen die Wassergehalte bei einer Dekompressionsrate von 50 bar/h von 4,93 Gew.-% bei 1550 bar zu niedrigeren Drücken kontinuierlich ab. Eine systematische Abhängigkeit von der Dekompressionsrate konnte für die Unterschiede in den jeweiligen Wassergehalten nicht gefunden werden. Die Anpassung der residualen Glaszusammensetzung während der Dekompression wird vor allem von der für die Reaktionen zur Verfügung stehenden Zeit kontrolliert, wie die in dieser Studie eingeführten Größen “Reequilibrationsindex” (REI) und “-geschwindigkeit” (RES) zeigen: Experimente mit Dekompressionsraten von 21,4 bar/h erreichen eine weitgehendere Reequilibration als die schneller dekomprimierten Proben, obwohl die letztgenannten RES aufweisen, die auf eine bis zu 10-fach schnellere Reaktionskinetik hinweisen. Dies kompensiert nicht die wesentlich kürzere, für die Reaktionen zur Verfügung stehenden Zeiten. Die Kristallisation von Plagioklas wurde (neben der Entgasung) als wichtigster Prozess während der Dekompression identifiziert. Weitere signifikante Reaktionen unter Beteiligung von Mineralen konnten nicht gefunden werden. Die Kristallrößenverteilungen (CSD) von Plagioklasen bei unterschiedlichen Dekompressionsraten unterscheiden sich signifikant bei 500 bar, wo langsamer dekomprimierte Proben eine steile CSD mit höheren Populationsdichten bei kleinen Kristallgrößenklassen erreichen. Bei 50 bar hingegen tritt dieser Unterschied nicht mehr auf. Wenngleich unterschiedliche Dekompressionsraten zur sehr ähnlichen CSDs bei geringen Drücken führen, so unterscheidet sich dennoch die Gewichtung mit welcher Keimbildung und Wachstum zur Kristallisation bei höheren Drücken im jeweiligen Experiment beitragen. Die Zusammensetzungen der Plagioklase aus Dekompressionsexperimenten sind im Schnitt An-reicher als die jeweiligen Kristalle aus Gleichgewichtsexperimenten. Während in Gleichgewichtsexperimenten bei niedrigen Drücken ein Anstieg der K2O-Komponente beobachtet werden kann, so tritt dies in den Dekompressionsexperimenten nicht auf. Die Kristallzusammensetzungen von Dekompressions- und Gleichgewichtsexperimenten unterscheiden sich demnach signifikant. In Amphibolen konnten keinerlei Reaktionen auf die Dekompression beobachtet werden. Die Zusammensetzungen der Amphibole bleibt konstant und es bilden sich keine Konzentrationsprofile von Kernen zu Rändern. Auch wurde keine Zerfallsreaktion  vorgefunden, obwohl während der Dekompressionspexperimente das Stabilitätsfeld von Amphibol früh verlassen wurde. Die Aufstiegsraten am natürlichen System des Vulkans Taapaca können anhand der vorliegenden Daten zur Entgasung, der Kristallisation von Plagioklas, sowie den Ergebnissen zu Amphibolen nicht eingegrenzt werden. Die Beobachtungen anhand von simulierten Dekompressionsraten von 6,3 bar/h bis 450 bar/h sind mit den natürlichen Proben vereinbar. Allerdings kann ein systematischer Fehler hier nicht ausgeschlossen werden, da sich das natürliche System vor der Eruption in einem Ungleichgewichtszustand befindet, der mittels der Experimente nicht rekonstruiert werden konnte. Darüber hinaus wurden Wasserlöslichkeiten in der dazitischen Schmelze bei 1000 bis 1250°C und 250 bis 2500 bar bestimmt. Bei 1200°C steigen die Wasserlöslichkeiten von 1,6 Gew.-% bei 250 bar auf 6,5 Gew.-% bei 2500 bar an. Mit steigenden Temperaturen nimmt die Wasserlöslichkeit bei konstantem Druck ab. Bei 1 kbar von 4,3 Gew.-% bei 1000°C auf 3,3 Gew.-% bei 1250 C, bei 2 kbar von 6 Gew.-% auf 5,5 Gew.-% im gleichen Temperaturintervall.

Volatile trace metals are variably enriched in volcanic gases. Metal concentrations in sub-aerially erupted magmas are also depleted in many of these metals. The causes of variable metal enrichment in volcanic gasses, however, remain enigmatic. The objective of this work is to place experimental constraints on kinetic and thermodynamic factors that influence the concentrations of trace metals in volcanic gases. To measure metal mobility in silicate melts, Pt crucibles packed with metal doped glasses of broadly basaltic composition were equilibrated with air and mixed gases at atmospheric pressure. The metals in the melt diffused to the gas/melt interface where they were released as a volatile species. The experiments produced concentration-distance profiles from which diffusivity was derived. Experiments were also conducted in a piston-cylinder apparatus at 1 GPa pressure. In these experiments, melts were equilibrated with Cl-bearing fluids at high temperature and pressure. At equilibrium, trace metals partitioned between the melt and fluid phase as a function of temperature and fluid composition. The diffusivity of Re in melts of natural basalt, andesite and a synthetic composition in the CaO-MgO-Al2O3-SiO2 (CMAS) system has been investigated at 0.1 MPa and 1250-1350C over a range of fO2 conditions from log fO2 = -10 to –0.68. Re diffusivity in natural basalt at 1300C in air is logDRe = -7.2  0.3 cm2/sec and increases to logDRe = -6.6 0.3 cm2/sec when trace amounts of Cl were added to the starting material. At fO2 conditions below the nickel-nickel oxide (NNO) buffer Re diffusivity decreases to logDRereducing = -7.60.2 cm2/sec and to logDReandesite = -8.4  0.2 cm2/sec in andesite melt. Cd, Re, Tl, Pb, Sb and Te diffusivity in CMAS and Na2O-MgO-Al2O3-SiO2 (NMAS) melts were also determined at 0.1 MPa and 1200-1350C. In the CMAS composition at 1300C, the fastest diffusing element was Cd having a logDCd = -6.5  0.2. The slowest element was Re with logDRe = -7.5  0.3. Diffusivities of Sb, Te, Pb and Tl have intermediate values where logDSb = -7.1  0.1, logDTe = -7.2  0.3, logDPb = -7.1  0.2, logDTl = -7.0  0.2 cm2/sec. In the NMAS composition, logDRe = -6.5  0.2, logDSb = -6.0  0.2, logDPb = -6.1  0.1, logDTl = -5.8  0.2 cm2/sec. Fluid/melt partition coefficients ( ) of Re, Mo, W, Tl and Pb between fluid (H2O + Cl) and a haplobasaltic melt in the CMAS system were measured between 1200 and 1400°C at 1 GPa and fluid chlorine molarities from 7.7 to 27 mol/L. At 1300°C and fluid molarity of 7.7 mol/L, = 9.8±1.8, = 11.8±1.6, = 3.7±1.6, = 4.5±1.4 and = 2.4 ±1.8. Both Mo and Re were shown to partition most strongly into the fluid at all temperatures and fluid chlorinities. Differences in diffusivity of volatile heavy metal ions to a lead to significant fractionation between these metals in magmas during degassing. Given the observed differences in Cd and Re diffusivities, an increase in the normalized Cd/Re ratio in the gas phase with increasing bubble growth rate is predicted. Monitoring of the Cd/Re ratios in aerosols from degassing volcanoes may provide a tool for predicting volcanic eruption. Modeling of Re using the values measured here support the contention that subaerial degassing is the cause of lower Re concentrations in arc-type and ocean island basalts compared to mid-ocean ridge basalts. The model results were also compared with emanation coefficients for trace metals from natural volcanoes. The magnitudes of the modeled Re/Tl and Re/Pb in fluids at 1300C and the lowest fluid chlorinities were less than that observed from their emanation coefficients. Re and Pb are more sensitive to fluid chlorinity than Tl. The ratios of Re/Tl and Re/Pb expected from emanation coefficients are closely matched if partitioning values for experiments having fluid chlorinities of ~16-20 MCl at 1300C are used.

Volatile trace metals are variably enriched in volcanic gases. Metal concentrations in sub-aerially erupted magmas are also depleted in many of these metals. The causes of variable metal enrichment in volcanic gasses, however, remain enigmatic. The objective of this work is to place experimental constraints on kinetic and thermodynamic factors that influence the concentrations of trace metals in volcanic gases. To measure metal mobility in silicate melts, Pt crucibles packed with metal doped glasses of broadly basaltic composition were equilibrated with air and mixed gases at atmospheric pressure. The metals in the melt diffused to the gas/melt interface where they were released as a volatile species. The experiments produced concentration-distance profiles from which diffusivity was derived. Experiments were also conducted in a piston-cylinder apparatus at 1 GPa pressure. In these experiments, melts were equilibrated with Cl-bearing fluids at high temperature and pressure. At equilibrium, trace metals partitioned between the melt and fluid phase as a function of temperature and fluid composition. The diffusivity of Re in melts of natural basalt, andesite and a synthetic composition in the CaO-MgO-Al2O3-SiO2 (CMAS) system has been investigated at 0.1 MPa and 1250-1350C over a range of fO2 conditions from log fO2 = -10 to –0.68. Re diffusivity in natural basalt at 1300C in air is logDRe = -7.2  0.3 cm2/sec and increases to logDRe = -6.6 0.3 cm2/sec when trace amounts of Cl were added to the starting material. At fO2 conditions below the nickel-nickel oxide (NNO) buffer Re diffusivity decreases to logDRereducing = -7.60.2 cm2/sec and to logDReandesite = -8.4  0.2 cm2/sec in andesite melt. Cd, Re, Tl, Pb, Sb and Te diffusivity in CMAS and Na2O-MgO-Al2O3-SiO2 (NMAS) melts were also determined at 0.1 MPa and 1200-1350C. In the CMAS composition at 1300C, the fastest diffusing element was Cd having a logDCd = -6.5  0.2. The slowest element was Re with logDRe = -7.5  0.3. Diffusivities of Sb, Te, Pb and Tl have intermediate values where logDSb = -7.1  0.1, logDTe = -7.2  0.3, logDPb = -7.1  0.2, logDTl = -7.0  0.2 cm2/sec. In the NMAS composition, logDRe = -6.5  0.2, logDSb = -6.0  0.2, logDPb = -6.1  0.1, logDTl = -5.8  0.2 cm2/sec. Fluid/melt partition coefficients ( ) of Re, Mo, W, Tl and Pb between fluid (H2O + Cl) and a haplobasaltic melt in the CMAS system were measured between 1200 and 1400°C at 1 GPa and fluid chlorine molarities from 7.7 to 27 mol/L. At 1300°C and fluid molarity of 7.7 mol/L, = 9.8±1.8, = 11.8±1.6, = 3.7±1.6, = 4.5±1.4 and = 2.4 ±1.8. Both Mo and Re were shown to partition most strongly into the fluid at all temperatures and fluid chlorinities. Differences in diffusivity of volatile heavy metal ions to a lead to significant fractionation between these metals in magmas during degassing. Given the observed differences in Cd and Re diffusivities, an increase in the normalized Cd/Re ratio in the gas phase with increasing bubble growth rate is predicted. Monitoring of the Cd/Re ratios in aerosols from degassing volcanoes may provide a tool for predicting volcanic eruption. Modeling of Re using the values measured here support the contention that subaerial degassing is the cause of lower Re concentrations in arc-type and ocean island basalts compared to mid-ocean ridge basalts. The model results were also compared with emanation coefficients for trace metals from natural volcanoes. The magnitudes of the modeled Re/Tl and Re/Pb in fluids at 1300C and the lowest fluid chlorinities were less than that observed from their emanation coefficients. Re and Pb are more sensitive to fluid chlorinity than Tl. The ratios of Re/Tl and Re/Pb expected from emanation coefficients are closely matched if partitioning values for experiments having fluid chlorinities of ~16-20 MCl at 1300C are used.

In recent decades there has been an increased interest in the carbon content of Earth’s geochemical reservoirs due to the impact of atmospheric carbon on the habitability of our planet. Earth’s interior likely hosts a greater mass of carbon than that of the oceans, atmosphere and crust combined, which has buffered the carbon content of the atmosphere over geological time. Yet only a few direct measurements of carbon from the upper mantle, and none from the lower mantle, have been made. Undegassed basalts erupted at mid-ocean ridges have previously been used to estimate the carbon content of the upper mantle. However, due to the low solubility of carbon within silicate melt, these undegassed basalt suites are rare. The majority of basalts have lost their mantle carbon information en route to eruption through the crust. Various crustal processes act to modify the geochemistry of melts before eruption, therefore it is important to be able to characterise the effect of these processes to better interpret the volatile signals preserved in erupted products. Pressure, and therefore depth, is a key parameter controlling volatile solubility and can be estimated using a variety of igneous barometers. This thesis presents results from crys- tallisation experiments conducted on basaltic glass from the Miðfell eruption, Iceland. The experiments provide new data that has been used to test a variety of barometers and crystalli- sation models used by igneous petrologists, and could aid future barometer recalibration. A key part of this work was the development of an experimental method for stabilising 5 kbar conditions in a piston cylinder apparatus. The experiments have shown that clinopyroxene- liquid barometry is more reliable than multi-reaction barometry. However, knowledge of equilibrium clinopyroxene compositions is crucial for accurately determining pressure using the clinopyroxene-liquid barometer. More experiments conducted at mid-crustal pressures are required for a full recalibration of these barometers. The results of testing igneous barometers and crystallisation models have been applied to two suites of olivine-hosted melt inclusions from the Kistufell and Miðfell eruptions to help determine the melt evolution history of these basalts. These eruptions were targeted due to previously measured noble gas isotopic ratios that suggest a primordial mantle component present in their melting regions, and therefore evoking the possibility that they could hold information about deep mantle carbon. Barometry suggests that Miðfell phases equilibrated, and therefore crystallised, at mid-crustal pressures (5–7 kbar), which could allow for the entrapment of undegassed melt inclusions within olivine. The two melt inclusion suites were found to differ in trace element variability, with the observation that the more trace element enriched eruption, Kistufell, had lower relative trace element variability than the more depleted eruption, Miðfell. Several processes, both in the crust and the mantle, are likely responsible for the level of trace element enrichment and variability, including extent of mantle melting, source heterogeneity, and melt transport. The depleted nature of the Miðfell melt inclusions has allowed them to preserve some of the highest CO$_2$/Ba and CO$_2$/Nb ratios ever recorded in basaltic glass, with ratios over five times greater than undegassed mid-ocean ridge basalt values. This carbon enrichment is not due to any crustal melt modification process, but rather pertaining to lower mantle carbon-rich lithologies that have been tapped by the Icelandic mantle plume. The carbon reservoir beneath Miðfell is estimated to contain 744 $\pm$ 188 ppm carbon, 15 times greater than the depleted upper mantle. This value matches estimates of bulk mantle carbon from planetary mass balance calculations and provides evidence for carbon-rich domains within the Earth.

Volcanic arcs have been the locus of continental growth since at least the Proterozoic eon. In this dissertation, I seek to shine more light on arc processes by inferring the lower crustal mineralogy of an ancient arc by geochemical and structural modelling of its exposed levels. Arcs characteristically have high concentrations of incompatible elements, thus I also experimentally assess the ability of alkaline melts and fluids associated with sediment melting to carry lithophile and chalcophile elements in the sub-arc. I measured the chemical composition of 18 plutonic samples from the Bonanza island arc, emplaced between 203 and 164 Ma on the Wrangellia terrane on Vancouver Island, British Columbia. Models using trace elements with Nd and Sr isotopes indicate < 10% assimilation of the Wrangellia basement by the Bonanza arc magmas. The Bonanza arc rare earth element geochemistry is best explained as two lineages, each with two fractionation stages implicating < 15% garnet crystallization. My inference of garnet-bearing cumulates in the unexposed lower crust of the Bonanza arc, an unsuspected similarity with the coeval Talkeetna arc (Alaska), is consistent with estimates from geologic mapping and geobarometry indicating that the arc grew to > 23 km total thickness. The age distribution of the Bonanza arc plutons shows a single peak at 171 Ma whereas the volcanic rock age distribution shows two peaks at 171 and 198 Ma, likely due to sampling and/or preservation bias. Numerous mechanisms may produce the E-W separation of young and old volcanism and this does not constrain Jurassic subduction polarity beneath Wrangellia. Although a small component of arc magmatism, alkaline arc rocks are associated with economic concentrations of chalcophile elements. The effect of varying alkalinity on S Concentration at Sulfide Saturation (SCSS) has not been previously tested. Thus, I conducted experiments on hydrous basaltic andesite melts with systematically varied alkalinity at 1270°C and 1 GPa using piston-cylinder apparatus. At oxygen fugacity two log units below the fayalite magnetite quartz buffer, I find SCSS is correlated with total alkali concentration, perhaps a result of the increased non-bridging oxygen associated with increased alkalinity. A limit to the effect of alkalis on SCSS in hydrous melts is observed at ~7.5 wt.% total alkalis. Using my results and published data, I retrained earlier SCSS models and developed a new empirical model using the optical basicity compositional parameter, predicting SCSS with slightly better accuracy than previous models. Sediment melts contribute to the trace element signature of arcs and the chalcophile elements, compatible in redox-sensitive sulfide, are of particular interest. I conducted experiments at 3 GPa, 950 – 1050°C on sediment melts, determined fluid concentrations by mass balance and report the first fluid-melt partition coefficients (Dfluid/melt) for sediment melting. Compared to oxidized, anhydrite-bearing melts, I observe high Dfluid/melt for chalcophile elements and low values for Ce in reduced, pyrrhotite-bearing melts. Vanadium and Sc are unaffected by redox. The contrasting fluid-melt behaviour of Ce and Mo that I report indicates that melt, not fluid, is responsible for elevated Mo in the well-studied Lesser Antilles arc. / Graduate

abstract: Volcanic eruptions can be serious geologic hazards, and have the potential to effect human life, infrastructure, and climate. Therefore, an understanding of the evolution and conditions of the magmas stored beneath volcanoes prior to their eruption is crucial for the ability to monitor such systems and develop effective hazard mitigation plans. This dissertation combines classic petrologic tools such as mineral chemistry and thermometry with novel techniques such as diffusion chronometry and statistical modeling in order to better understand the processes and timing associated with volcanic eruptions. By examining zoned crystals from the fallout ash of Yellowstone’s most recent supereruption, my work shows that the rejuvenation of magma has the ability to trigger a catastrophic supereruption at Yellowstone caldera in the years (decades at most) prior to eruption. This provides one of the first studies to thoroughly identify a specific eruption trigger of a past eruption using the crystal record. Additionally, through experimental investigation, I created a novel diffusion chronometer with application to determine magmatic timescales in silicic volcanic systems (i.e., rhyolite/dacite). My results show that Mg-in-sanidine diffusion operates simultaneously by both a fast and slow diffusion path suggesting that experimentally-derived diffusion chronometers may be more complex than previously thought. When applying Mg-in-sanidine chronometry to zoned sanidine from the same supereruption at Yellowstone, the timing between rejuvenation and eruption is further resolved to as short as five months, providing a greater understanding of the timing of supereruption triggers. Additionally, I developed a new statistical model to examine the controls on a single volcano’s distribution of eruptions through time, therefore the controls on the timing between successive eruptions, or repose time. When examining six Cascade volcanoes with variable distribution patterns through time, my model shows these distributions are not result of sampling bias, rather may represent geologic processes. There is a robust negative correlation between average repose time and average magma composition (i.e., SiO2), suggesting this may be a controlling factor of long-term repose time at Cascade volcanoes. Together, my work provides a better vision for forecasting models to mitigate potential destruction. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2020

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