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Insights into Magma Storage Beneath a Frequently Erupting Arc Volcano (Villarrica, Chile) from Unsupervised Machine Learning Analysis of Mineral Compositions
A key method to investigate magma dynamics is the analysis of the crystal cargoes carried by erupted magmas. These cargoes may comprise crystals that crystallize in different parts of the magmatic system (throughout the crust) and/or different times. While an individual eruption likely provides a partial view of the sub-volcanic plumbing system, compiling data from multiple eruptions builds a picture of the whole magmatic system. In this study we use machine learning techniques to analyze a large (>2000) compilation of mineral compositions from a highly active arc volcano: Villarrica, Chile. Villarrica's post-glacial eruptive activity (14 ka–present) displays large variation in eruptive style (mafic ignimbrites to Hawaiian effusive eruptions) yet its eruptive products have a near constant basalt-basaltic andesite bulk-rock composition. What, therefore, is driving explosive eruptions at Villarrica and can differences in storage dynamics be related to eruptive style? We used hierarchical cluster analysis to detect previously undetected structure in olivine, plagioclase and clinopyroxene compositions, revealing the presence of compositionally distinct clusters. Using rhyolite-MELTS thermodynamic modeling we related these clusters to intensive magmatic variables: temperature, pressure, water content and oxygen fugacity. Our results provide evidence for the existence of multiple discrete (spatial and temporal) magma reservoirs beneath Villarrica where melts differentiate and mix with incoming more primitive magma. The compositional diversity of an erupted crystal cargo strongly correlates with eruptive intensity, and we postulate that mixing between primitive and differentiated magma drives explosive activity at Villarrica.
The concept of “ore magma” remains an obscure hypothesis in ore formation. The article considers the process of natural overgrowth of trivial primary basaltic magma into an ore-bearing granite melting and further into the ore-forming “ore magma” as the concentration of volatile and ore components. The dark side of the problem lies in the fact that during the ore formation the “ore magma” liquates into contrasting phases and leaves practically no traces of itself (with rare exceptions). But the concept of the ore magma has received a logical scientific justification from the standpoint of ore-magmatic systems.
<p>Silicic (i.e. dacitic-rhyolitic) magmatic systems have the potential to generate large, explosive caldera-forming eruptions which have global effects and consequences. How, and over what timescale, magma accumulates and is stored in the upper crust are key aspects in understanding such systems and their associated hazards. The absence of such eruptions in the historical record, however, has forced understanding of these systems to be developed through numerical models or the study of the deposits in the geological record. Numerical models primarily focus on the long-term generation but instantaneous eruption of single magma (i.e. melt-dominant) bodies. In contrast, the stratigraphic and geochemical nature of eruption deposits often show features more consistent with complex magmatic systems comprising multiple melt-dominant bodies that may have formed rapidly but erupted episodically. Further studies of past eruption deposits are valuable, therefore, in reconstructing silicic magmatic systems and highlighting the nature of melt-dominant body generation and storage. To this end, this thesis examines the 2.08 Ma, ∼2,500 km³ Huckleberry Ridge Tuff (HRT), Yellowstone Plateau volcanic field (YPVF), U.S.A, the deposit of the first and largest of three caldera-forming eruptions in the YPVF. The HRT comprises an initial fall deposit followed by three ignimbrite members (A, B and C) with a second fall deposit between members B and C. Despite emanating from an archetypal silicic volcanic field, minimal previous work has been undertaken on the geochemical nature of the HRT but it is thought to conform to traditional, unitary magma body ideas. A revised stratigraphic framework, detailing an episodic and prolonged initial fall deposit, identification of a weeks-months time gap between members A and B, and a similar but longer years-decades hiatus in activity between members B and C provides the context for this geochemical investigation. A large sample suite representative of the diverse range of physical characteristics of clasts and material found in the HRT was analysed. In situ micro-analysis of matrix glass (major and trace elements) and crystals (major elements) in the initial fall deposit are coupled with major and trace element, and isotopic compositions of single silicic clasts (i.e. pumice/fiamme) from all three ignimbrite members, supplemented by in situ analysis of their crystals and groundmass glass. These data are used to reconstruct the silicic magmatic system. Furthermore, major and trace element, andisotopic compositions of rare mafic (i.e. basaltic to andesitic) material found in members A and B provide an insight into the thermal and chemical drivers of HRT silicic volcanism. This macro- and micro-analytical investigation using multiple techniques reveals remarkable complexity within the large-scale HRT magmatic complex. Four geochemically distinct magmatic systems are differentiated on single clast elemental and isotopic characteristics that are further reflected in crystal and glass compositions. Two of these systems (1 and 2) were active in the initial fall deposit and member A. Magmatic system 1 is volumetrically dominant in the HRT and is characterised by moderate-high Ba single clast (450-3540 ppm) and glass (100-3360 ppm) compositions, in contrast to the distinctly low-Ba (≤250 ppm single clast, <65 ppm glass Ba contents) magmatic system 2. Both these magmatic systems exhibit clustered glass compositions, indicating multiple, laterally-adjacent melt-dominant bodies were present, and shared moderate isotopic compositions (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.70950-0.71191) are explicable by a multi-stage partial melting-fractional crystallisation petrogenesis. The time break between members A and B is associated with mixing and mingling within magmatic system 1, related to a renewed influx of mafic material, and a cessation of activity of system 2, which is absent from member B. The time break between members B and C reflects significant changes within the magmatic complex. Magmatic system 2 is rejuvenated and melt-dominant bodies associated with two new magmatic systems (3 and 4) are formed, with at least system 3 comprising multiple bodies. These latter two magmatic systems strongly differ in their elemental characteristics (system 3: high SiO₂ [75-78 wt% SiO₂]; system 4: dacite-rhyolite [66-75 wt% SiO₂]). Despite this, they have similar and highly radiogenic (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.72462-0.72962) isotopic compositions indicating shared extensive incorporation of Archean crust. They also contrast in their relation to mafic compositions, with system 4 associated with olivine tholeiitic compositions erupted prior to and following the HRT in the YPVF. In contrast, system 3, like systems 1 and 2, is associated with high-Ba, high-Zr mafic compositions found co-erupted in HRT members A and B. These compositions are similar to lava flows erupted further west at the Craters of the Moon field, and are interpreted as representing partial melts from regions in the lithospheric mantle enriched by high-T, P fluids emanating from the subducted Farallon slab. Overall, the HRT magmatic complex was remarkably heterogeneous. Two contemporaneous mafic root zones drove four silicic magmatic systems, episodically active throughout the eruption. At least three of these systems comprised multiple laterally-adjacent melt-dominant bodies. Intra-eruption time breaks are associated with broad-scale reorganisation of the magmatic complex. This complexity highlights the utility of a detailed, systematic, multi-technique geochemical investigation, within a stratigraphic framework, of the deposits of large silicic caldera-forming eruptions, and breaks new ground in the understanding of such systems.</p>
<p>Silicic (i.e. dacitic-rhyolitic) magmatic systems have the potential to generate large, explosive caldera-forming eruptions which have global effects and consequences. How, and over what timescale, magma accumulates and is stored in the upper crust are key aspects in understanding such systems and their associated hazards. The absence of such eruptions in the historical record, however, has forced understanding of these systems to be developed through numerical models or the study of the deposits in the geological record. Numerical models primarily focus on the long-term generation but instantaneous eruption of single magma (i.e. melt-dominant) bodies. In contrast, the stratigraphic and geochemical nature of eruption deposits often show features more consistent with complex magmatic systems comprising multiple melt-dominant bodies that may have formed rapidly but erupted episodically. Further studies of past eruption deposits are valuable, therefore, in reconstructing silicic magmatic systems and highlighting the nature of melt-dominant body generation and storage. To this end, this thesis examines the 2.08 Ma, ∼2,500 km³ Huckleberry Ridge Tuff (HRT), Yellowstone Plateau volcanic field (YPVF), U.S.A, the deposit of the first and largest of three caldera-forming eruptions in the YPVF. The HRT comprises an initial fall deposit followed by three ignimbrite members (A, B and C) with a second fall deposit between members B and C. Despite emanating from an archetypal silicic volcanic field, minimal previous work has been undertaken on the geochemical nature of the HRT but it is thought to conform to traditional, unitary magma body ideas. A revised stratigraphic framework, detailing an episodic and prolonged initial fall deposit, identification of a weeks-months time gap between members A and B, and a similar but longer years-decades hiatus in activity between members B and C provides the context for this geochemical investigation. A large sample suite representative of the diverse range of physical characteristics of clasts and material found in the HRT was analysed. In situ micro-analysis of matrix glass (major and trace elements) and crystals (major elements) in the initial fall deposit are coupled with major and trace element, and isotopic compositions of single silicic clasts (i.e. pumice/fiamme) from all three ignimbrite members, supplemented by in situ analysis of their crystals and groundmass glass. These data are used to reconstruct the silicic magmatic system. Furthermore, major and trace element, andisotopic compositions of rare mafic (i.e. basaltic to andesitic) material found in members A and B provide an insight into the thermal and chemical drivers of HRT silicic volcanism. This macro- and micro-analytical investigation using multiple techniques reveals remarkable complexity within the large-scale HRT magmatic complex. Four geochemically distinct magmatic systems are differentiated on single clast elemental and isotopic characteristics that are further reflected in crystal and glass compositions. Two of these systems (1 and 2) were active in the initial fall deposit and member A. Magmatic system 1 is volumetrically dominant in the HRT and is characterised by moderate-high Ba single clast (450-3540 ppm) and glass (100-3360 ppm) compositions, in contrast to the distinctly low-Ba (≤250 ppm single clast, <65 ppm glass Ba contents) magmatic system 2. Both these magmatic systems exhibit clustered glass compositions, indicating multiple, laterally-adjacent melt-dominant bodies were present, and shared moderate isotopic compositions (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.70950-0.71191) are explicable by a multi-stage partial melting-fractional crystallisation petrogenesis. The time break between members A and B is associated with mixing and mingling within magmatic system 1, related to a renewed influx of mafic material, and a cessation of activity of system 2, which is absent from member B. The time break between members B and C reflects significant changes within the magmatic complex. Magmatic system 2 is rejuvenated and melt-dominant bodies associated with two new magmatic systems (3 and 4) are formed, with at least system 3 comprising multiple bodies. These latter two magmatic systems strongly differ in their elemental characteristics (system 3: high SiO₂ [75-78 wt% SiO₂]; system 4: dacite-rhyolite [66-75 wt% SiO₂]). Despite this, they have similar and highly radiogenic (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.72462-0.72962) isotopic compositions indicating shared extensive incorporation of Archean crust. They also contrast in their relation to mafic compositions, with system 4 associated with olivine tholeiitic compositions erupted prior to and following the HRT in the YPVF. In contrast, system 3, like systems 1 and 2, is associated with high-Ba, high-Zr mafic compositions found co-erupted in HRT members A and B. These compositions are similar to lava flows erupted further west at the Craters of the Moon field, and are interpreted as representing partial melts from regions in the lithospheric mantle enriched by high-T, P fluids emanating from the subducted Farallon slab. Overall, the HRT magmatic complex was remarkably heterogeneous. Two contemporaneous mafic root zones drove four silicic magmatic systems, episodically active throughout the eruption. At least three of these systems comprised multiple laterally-adjacent melt-dominant bodies. Intra-eruption time breaks are associated with broad-scale reorganisation of the magmatic complex. This complexity highlights the utility of a detailed, systematic, multi-technique geochemical investigation, within a stratigraphic framework, of the deposits of large silicic caldera-forming eruptions, and breaks new ground in the understanding of such systems.</p>
<p>Hong Kong represents a microcosm of the magmatic and tectonic processes that are related to formation of the Southeast China Magmatic Belt (SCMB, ~1,300 km long by 400 km wide). The SCMB is dominated by extensive Mesozoic (Yanshanian Orogeny) igneous rocks, which form part of an extensive, long-lived circum-Pacific igneous province. In Hong Kong, large silicic ignimbrites, produced from several calderas identified through geological mapping, together with their sub-volcanic plutons record a ~26-Myr period of magmatic activities from ~164 to 138 Ma. This work studies these volcanic-plutonic assemblages with the associated Lantau and High Island caldera complexes, with an emphasis on the ~143-138 Ma period from the latter complex. This study uses multiple techniques, including field studies, zircon geochronology and trace element analyses, and zircon and apatite low-temperature thermochronology, to gain new insights into the Mesozoic tectono-magmatic history in this region. Field studies demonstrate that the High Island caldera complex (with its main collapse at 140.9±0.4 Ma in association with the High Island Tuff) is structurally more complex than previously suggested and represents a long-lived, large (320 km²) feature. The volcanic strata exposed in the eastern part of the caldera are inferred to have been tilted during syneruptive, asymmetric collapse of the caldera floor, whereas those in other parts have been affected by block faulting but not overall tilting. Two ignimbrites (e.g. Long Harbour: 141.4±1.0 Ma) exposed within the caldera outline are now interpreted to have accumulated in local volcano-tectonic basins, confined by faults that were later exploited by dyke intrusions. Field observations offer important constraints on the ages of volcanic and plutonic units, which have been tested by zircon U-Pb dating in this study. The field evidence also negates a previous interpretation that there was an overall tilting of the High Island caldera complex. U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) techniques have been carried out on zircons separated from 21 samples, chosen from both volcanic and plutonic samples within the Lantau and High Island Caldera complexes. The SIMS age datasets reveal two groups: (1) seven samples with unimodal age spectra; and (2) fourteen samples yielding multiple age components. Five samples in group 1 yield mean ages indistinguishable from their previously published ID-TIMS ages, demonstrating that the SIMS techniques have generated results fully in agreement with the ID-TIMS methods, although with overall less precision. Of the two other samples, one is slightly younger than the published ID-TIMS age, and the other has no previous age determination. Thirteen samples in group 2 are interpreted to have crystallisation/eruption ages that are younger (although often within 2.s.d. uncertainties) than their corresponding ID-TIMS values. The remaining sample from this group has no previous age determination. The overall age patterns from both groups suggest that, instead of separate phases of activity at ~143 and 141-140 Ma as previously inferred, magmatic and volcanic activities were continuous (within age analytical uncertainties) over a ~5 Myr period. Direct linkages between several plutonic and volcanic units in this period of activity (e.g. High Island Tuff and the Kowloon Granite) are no longer supported by the age data, and magmatic activity represented by exposed plutons continued until 137.8±0.8 Ma, as with the Mount Butler Granite. Under CL imagery, a wide variety of zircon textures is evident, indicative of complex processes that operated in the magmatic systems. Zircon trace element data coupled with textural characteristics enable identification of some common petrogenetic processes. Overall, the intra-grain (cores-rims, sector-zoned zircons) and intra-sample variations in trace element abundance and elemental ratios are more significant than the differences between individual samples. Zircon chemistries in samples from both the volcanic and plutonic records indicate that there are two groups of volcanic-plutonic products through the history of the High Island Caldera magmatic system. Two evolutionary models are proposed here to explain these two groups. In the first model, the magmatic system comprises a single domain that fluctuated in temperature through varying inputs of hotter melts (and was randomly tapped). In the second model the intrusive and extrusive products represent interplay of two magmatic domains in the crust, with contrasting characteristics. Zircon and apatite fission track analyses have been carried out on several of the rocks dated by U-Pb methods (either SIMS or TIMS), together with a selection of other Mesozoic igneous rocks and post-magmatic Cretaceous and Eocene sediments to cover the geographic area of Hong Kong. The fission-track dataset and associated thermal modelling show that the igneous rocks and Cretaceous sediments (but not the Eocene sediments) together experienced post-emplacement or post-depositional heating to >250 ºC, subsequently cooling through 120-60 ºC after ~80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. The enhanced geothermal gradient is interpreted to represent continuing Yanshanian magmatic activity at depth, without any documented surface eruption products, until ~100-80 Ma. The data also indicate a long-term, slow cooling (~1 ºC/Myr) since the early Cenozoic, linked to ~2-3 km of erosion-driven exhumation. The thermo-tectonic history of Hong Kong reflects the mid-Cretaceous transition of southeast China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic. The inferred cessation of magmatism at depth below Hong Kong at ~100-80 Ma is broadly coincident with the cessation of plutonic activity in many other circum-Pacific magmatic provinces related to reorganisation of Pacific Plate motion.</p>
<p>Hong Kong represents a microcosm of the magmatic and tectonic processes that are related to formation of the Southeast China Magmatic Belt (SCMB, ~1,300 km long by 400 km wide). The SCMB is dominated by extensive Mesozoic (Yanshanian Orogeny) igneous rocks, which form part of an extensive, long-lived circum-Pacific igneous province. In Hong Kong, large silicic ignimbrites, produced from several calderas identified through geological mapping, together with their sub-volcanic plutons record a ~26-Myr period of magmatic activities from ~164 to 138 Ma. This work studies these volcanic-plutonic assemblages with the associated Lantau and High Island caldera complexes, with an emphasis on the ~143-138 Ma period from the latter complex. This study uses multiple techniques, including field studies, zircon geochronology and trace element analyses, and zircon and apatite low-temperature thermochronology, to gain new insights into the Mesozoic tectono-magmatic history in this region. Field studies demonstrate that the High Island caldera complex (with its main collapse at 140.9±0.4 Ma in association with the High Island Tuff) is structurally more complex than previously suggested and represents a long-lived, large (320 km²) feature. The volcanic strata exposed in the eastern part of the caldera are inferred to have been tilted during syneruptive, asymmetric collapse of the caldera floor, whereas those in other parts have been affected by block faulting but not overall tilting. Two ignimbrites (e.g. Long Harbour: 141.4±1.0 Ma) exposed within the caldera outline are now interpreted to have accumulated in local volcano-tectonic basins, confined by faults that were later exploited by dyke intrusions. Field observations offer important constraints on the ages of volcanic and plutonic units, which have been tested by zircon U-Pb dating in this study. The field evidence also negates a previous interpretation that there was an overall tilting of the High Island caldera complex. U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) techniques have been carried out on zircons separated from 21 samples, chosen from both volcanic and plutonic samples within the Lantau and High Island Caldera complexes. The SIMS age datasets reveal two groups: (1) seven samples with unimodal age spectra; and (2) fourteen samples yielding multiple age components. Five samples in group 1 yield mean ages indistinguishable from their previously published ID-TIMS ages, demonstrating that the SIMS techniques have generated results fully in agreement with the ID-TIMS methods, although with overall less precision. Of the two other samples, one is slightly younger than the published ID-TIMS age, and the other has no previous age determination. Thirteen samples in group 2 are interpreted to have crystallisation/eruption ages that are younger (although often within 2.s.d. uncertainties) than their corresponding ID-TIMS values. The remaining sample from this group has no previous age determination. The overall age patterns from both groups suggest that, instead of separate phases of activity at ~143 and 141-140 Ma as previously inferred, magmatic and volcanic activities were continuous (within age analytical uncertainties) over a ~5 Myr period. Direct linkages between several plutonic and volcanic units in this period of activity (e.g. High Island Tuff and the Kowloon Granite) are no longer supported by the age data, and magmatic activity represented by exposed plutons continued until 137.8±0.8 Ma, as with the Mount Butler Granite. Under CL imagery, a wide variety of zircon textures is evident, indicative of complex processes that operated in the magmatic systems. Zircon trace element data coupled with textural characteristics enable identification of some common petrogenetic processes. Overall, the intra-grain (cores-rims, sector-zoned zircons) and intra-sample variations in trace element abundance and elemental ratios are more significant than the differences between individual samples. Zircon chemistries in samples from both the volcanic and plutonic records indicate that there are two groups of volcanic-plutonic products through the history of the High Island Caldera magmatic system. Two evolutionary models are proposed here to explain these two groups. In the first model, the magmatic system comprises a single domain that fluctuated in temperature through varying inputs of hotter melts (and was randomly tapped). In the second model the intrusive and extrusive products represent interplay of two magmatic domains in the crust, with contrasting characteristics. Zircon and apatite fission track analyses have been carried out on several of the rocks dated by U-Pb methods (either SIMS or TIMS), together with a selection of other Mesozoic igneous rocks and post-magmatic Cretaceous and Eocene sediments to cover the geographic area of Hong Kong. The fission-track dataset and associated thermal modelling show that the igneous rocks and Cretaceous sediments (but not the Eocene sediments) together experienced post-emplacement or post-depositional heating to >250 ºC, subsequently cooling through 120-60 ºC after ~80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. The enhanced geothermal gradient is interpreted to represent continuing Yanshanian magmatic activity at depth, without any documented surface eruption products, until ~100-80 Ma. The data also indicate a long-term, slow cooling (~1 ºC/Myr) since the early Cenozoic, linked to ~2-3 km of erosion-driven exhumation. The thermo-tectonic history of Hong Kong reflects the mid-Cretaceous transition of southeast China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic. The inferred cessation of magmatism at depth below Hong Kong at ~100-80 Ma is broadly coincident with the cessation of plutonic activity in many other circum-Pacific magmatic provinces related to reorganisation of Pacific Plate motion.</p>
<p>This thesis research focuses on clast and crystal-specific studies to investigate the pre- and syn-eruptive magmatic processes of two supereruptions in the TVZ: the 1.21 Ma Ongatiti (>500 km3) and the 1.0 Ma Kidnappers (~1200 km3), together with the smaller (~200 km3) 1.0 Ma Rocky Hill eruption from the Mangakino Volcanic Centre (MVC). Crystallisation histories determined through SIMS U-Pb dating of zircons reveal that the paired Kidnappers and Rocky Hill eruptions were products of a common magmatic system, which built over ~200 kyr, in the time break after the Ongatiti eruption. U-Pb age spectra from the Ongatiti show a protracted crystallisation history (over ~250 kyr), in which the majority of zircon crystallised ~100 kyr prior to eruption in a crystal mush. Zircons then ascended with melt during accumulation of the final erupted magma body in the shallow crust. Zircons remained stable in the melt dominant body but underwent little further crystallisation. Zircons from all three systems record common geochemical processes governed by the fractionating assemblage (predominantly plagioclase and amphibole). In particular, the MREE/HREE ratios and Sr concentrations of zircons from the Ongatiti record imply two contrasting source regions governed by different proportions of crystallising amphibole. The in-situ major and trace element chemistry of glass shards and crystals from the Kidnappers fall deposit reveal that magma within the Kidnappers was stored in three discrete bodies, which were systematically tapped during the early stages of eruption. Temperature and pressure (T-P) estimates from amphibole and Fe-Ti oxide equilibria from each magma type are similar and therefore the three magma bodies were adjacent, not vertically stacked, in the crust. Amphibole model T-P estimates range from 770 to 840 °C and 90 to 170 MPa corresponding to pre-eruptive storage depths of ~4.0-6.5 km. The systematic evacuation of the three independent magma bodies implies that there was tectonic triggering and linkage of eruptions. The termination of fall deposition and onset of the overlying ignimbrite emplacement marks the point of widespread caldera collapse and the catastrophic evacuation of a wider variety of melt during the Kidnappers eruption. Pumice compositions from the Kidnappers ignimbrite fall into three groups, two of which (KI-1 and KI-2) can be matched to bodies tapped during the fall phase of the eruption, with the addition of a further discrete batch of lower SiO2 (KI-3) magma. Core-rim textural and chemical variations in major crystal phases (plagioclase, amphibole and orthopyroxene) suggest each compositional group was sourced from a common mush but underwent a unique magmatic history during the development of melt-dominant bodies in the final stages prior to eruption. The field relationships and distinctive appearance of the Rocky Hill ignimbrite (~200 km3 DRE) and the underlying Kidnappers ignimbrite suggests that the two deposits are from distinct eruption events. However, major and trace element chemistry of matrix glass, coupled with the textural and chemical signatures of crystals suggests the magma erupted during the Rocky Hill was generated from the same source or mush zone as the Kidnappers. The two largest melt-dominant bodies (KI-1 and KI-2) within the Kidnappers were renewed, underwent mixing and incorporation of marginal material to form two magma types (RH-1 and RH-2) in the time break prior to the Rocky Hill eruption. Fe-Mg interdiffusion timescales in orthopyroxenes from the Kidnappers and Rocky Hill deposits suggest the establishment of the final melt-dominant bodies, through extraction of melt and crystals from a common mush, occurred within 1000 years, and peaked within centuries of each eruption. In addition, one discrete batch of Kidnappers melt has evidence for interaction with a lesser evolved melt within 50 yrs prior to eruption. This rejuvenation event was not the eruption trigger but may have primed the magma for eruption. The difference in timescales from common zones from both the Kidnappers and Rocky Hill orthopyroxene, recording the same processes reveal the time break between the two eruptions was ~20-40 years. This work highlights the rapidity of rejuvenation and renewal of the melt-dominant bodies within the Kidnappers/Rocky Hill magmatic system. The textural and in-situ compositional signatures of crystals from the Ongatiti ignimbrite imply the final erupted magma body was assembled from a thermally and chemically zoned mush, which extended to the base of the quartzofeldspathic crust (~15km). The mush was close to water saturation and was dominated by amphibole crystallisation. Melt and crystals (including the majority of zircons) were extracted from the mush and ascended to 4-6 km depths during the development of a crystal-rich (20-30%), but melt-dominant body. Significant crystallisation of plagioclase (and lesser proportions of orthopyroxene and amphibole) occurred in an event involving the gradual heating and/or increase of water in the rhyolite, from a broadly andesitic underplated magma. Homogeneous crystal rim and matrix glass compositions imply the final erupted volume of magma was effectively mixed through convection. Eu/Eu* values of whole-rock and matrix glass suggest little crystal-melt separation occurred in the melt-dominant magma body prior to eruption. This work has implications for understanding the generation, storage and eruption of large-scale silicic magma systems. The Ongatiti ignimbrite does not represent either an erupted mush, or a stratified magma chamber, suggesting an alternative model for the development of eruptible magma within large-scale silicic systems. The Kidnappers/Rocky Hill sequence records a complex interplay of multiple melt-dominant bodies, which were established and renewed on rapid timescales. The rapid timescales for the development of melt-dominant bodies and the systematic tapping of magmas in the Kidnappers/Rocky Hill system imply that tectonics may have had a strong external control on the eruptions at Mangakino.</p>
<p>This thesis research focuses on clast and crystal-specific studies to investigate the pre- and syn-eruptive magmatic processes of two supereruptions in the TVZ: the 1.21 Ma Ongatiti (>500 km3) and the 1.0 Ma Kidnappers (~1200 km3), together with the smaller (~200 km3) 1.0 Ma Rocky Hill eruption from the Mangakino Volcanic Centre (MVC). Crystallisation histories determined through SIMS U-Pb dating of zircons reveal that the paired Kidnappers and Rocky Hill eruptions were products of a common magmatic system, which built over ~200 kyr, in the time break after the Ongatiti eruption. U-Pb age spectra from the Ongatiti show a protracted crystallisation history (over ~250 kyr), in which the majority of zircon crystallised ~100 kyr prior to eruption in a crystal mush. Zircons then ascended with melt during accumulation of the final erupted magma body in the shallow crust. Zircons remained stable in the melt dominant body but underwent little further crystallisation. Zircons from all three systems record common geochemical processes governed by the fractionating assemblage (predominantly plagioclase and amphibole). In particular, the MREE/HREE ratios and Sr concentrations of zircons from the Ongatiti record imply two contrasting source regions governed by different proportions of crystallising amphibole. The in-situ major and trace element chemistry of glass shards and crystals from the Kidnappers fall deposit reveal that magma within the Kidnappers was stored in three discrete bodies, which were systematically tapped during the early stages of eruption. Temperature and pressure (T-P) estimates from amphibole and Fe-Ti oxide equilibria from each magma type are similar and therefore the three magma bodies were adjacent, not vertically stacked, in the crust. Amphibole model T-P estimates range from 770 to 840 °C and 90 to 170 MPa corresponding to pre-eruptive storage depths of ~4.0-6.5 km. The systematic evacuation of the three independent magma bodies implies that there was tectonic triggering and linkage of eruptions. The termination of fall deposition and onset of the overlying ignimbrite emplacement marks the point of widespread caldera collapse and the catastrophic evacuation of a wider variety of melt during the Kidnappers eruption. Pumice compositions from the Kidnappers ignimbrite fall into three groups, two of which (KI-1 and KI-2) can be matched to bodies tapped during the fall phase of the eruption, with the addition of a further discrete batch of lower SiO2 (KI-3) magma. Core-rim textural and chemical variations in major crystal phases (plagioclase, amphibole and orthopyroxene) suggest each compositional group was sourced from a common mush but underwent a unique magmatic history during the development of melt-dominant bodies in the final stages prior to eruption. The field relationships and distinctive appearance of the Rocky Hill ignimbrite (~200 km3 DRE) and the underlying Kidnappers ignimbrite suggests that the two deposits are from distinct eruption events. However, major and trace element chemistry of matrix glass, coupled with the textural and chemical signatures of crystals suggests the magma erupted during the Rocky Hill was generated from the same source or mush zone as the Kidnappers. The two largest melt-dominant bodies (KI-1 and KI-2) within the Kidnappers were renewed, underwent mixing and incorporation of marginal material to form two magma types (RH-1 and RH-2) in the time break prior to the Rocky Hill eruption. Fe-Mg interdiffusion timescales in orthopyroxenes from the Kidnappers and Rocky Hill deposits suggest the establishment of the final melt-dominant bodies, through extraction of melt and crystals from a common mush, occurred within 1000 years, and peaked within centuries of each eruption. In addition, one discrete batch of Kidnappers melt has evidence for interaction with a lesser evolved melt within 50 yrs prior to eruption. This rejuvenation event was not the eruption trigger but may have primed the magma for eruption. The difference in timescales from common zones from both the Kidnappers and Rocky Hill orthopyroxene, recording the same processes reveal the time break between the two eruptions was ~20-40 years. This work highlights the rapidity of rejuvenation and renewal of the melt-dominant bodies within the Kidnappers/Rocky Hill magmatic system. The textural and in-situ compositional signatures of crystals from the Ongatiti ignimbrite imply the final erupted magma body was assembled from a thermally and chemically zoned mush, which extended to the base of the quartzofeldspathic crust (~15km). The mush was close to water saturation and was dominated by amphibole crystallisation. Melt and crystals (including the majority of zircons) were extracted from the mush and ascended to 4-6 km depths during the development of a crystal-rich (20-30%), but melt-dominant body. Significant crystallisation of plagioclase (and lesser proportions of orthopyroxene and amphibole) occurred in an event involving the gradual heating and/or increase of water in the rhyolite, from a broadly andesitic underplated magma. Homogeneous crystal rim and matrix glass compositions imply the final erupted volume of magma was effectively mixed through convection. Eu/Eu* values of whole-rock and matrix glass suggest little crystal-melt separation occurred in the melt-dominant magma body prior to eruption. This work has implications for understanding the generation, storage and eruption of large-scale silicic magma systems. The Ongatiti ignimbrite does not represent either an erupted mush, or a stratified magma chamber, suggesting an alternative model for the development of eruptible magma within large-scale silicic systems. The Kidnappers/Rocky Hill sequence records a complex interplay of multiple melt-dominant bodies, which were established and renewed on rapid timescales. The rapid timescales for the development of melt-dominant bodies and the systematic tapping of magmas in the Kidnappers/Rocky Hill system imply that tectonics may have had a strong external control on the eruptions at Mangakino.</p>
This document summarizes the outcomes of the Modeling Collaboratory for Subduction Zone Science (MCS) Volcanic Systems Workshop and presents a vision for advancing collaborative modeling of volcano-magmatic systems. The U.S. Geological Survey (USGS) has identified 161 potentially active volcanoes in the United States and its territories, of which 57 are considered to be high or very high threats (Ewert et al., 2018). All western states, including Alaska and Hawaii, have potentially active volcanoes. Eruptions range from the quiet effusion of sluggish lava flows over hours to decades to immense explosive ejections of tephra which produce massive calderas.Understanding these volcanoes and assessing their threat to society requires the development of quantitative models, rooted in physics and chemistry, which can be used to interpret diverse observations including real-time monitoring data. Existing models have tremendously advanced our understanding of volcanic systems and have improved our ability to assess hazards and forecast future activity, contributing directly to reductions in the number of lives lost to volcanic eruptions and helping mitigate their costs to society. Magmatic system models also provide a quantitative framework for understanding processes that occur at depth beneath volcanoes, linking volcanic systems with a broad range of deeper processes associated with the production, transport, and storage of magma and associated fluids above subducting slabs.Despite this exciting progress much remains to be accomplished and workshop participants identified several important opportunities. First and foremost is the recognition that enhanced support for the development and dissemination of volcano-magmatic system models and associated methodologies will enable advances in ways not currently possible. A key outcome of the workshops is a recognition of the transformative potential of diverse groups of scientists working together on common problems. Support for collaborative working groups will enable communication across disciplines and between modelers and non-modelers, leveraging expertise from scientists studying different aspects of volcano-magmatic systems, and between geoscientists and outside experts from fields such as mathematics, statistics, and material sciences. Better support will also enable modelers to more fully verify, validate, benchmark, and document their codes, and also provide new training opportunities. Enhanced model sharing and interoperability will reduce the need for different groups to independently duplicate (re-invent) code and increase confidence in published results. This report lays out a proposal for a collaborative modeling environment that is centered in large part around community working groups manifested as workshops, summer schools, and sustained long-term research collaborations involving diverse groups of scientists working on common problems. Programmatic support is envisioned in the form of enhanced student and postdoc funding for model development, incentives and support for cross-disciplinary collaborative research projects, and related support for these activities. This support will fundamentally improve our ability to integrate and interpret observations using volcanic and magmatic system models.
