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Wendy A Bohrson - One of the best experts on this subject based on the ideXlab platform.
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diagnosing open system Magmatic processes using the Magma Chamber simulator mcs part ii trace elements and isotopes
Contributions to Mineralogy and Petrology, 2020Co-Authors: Jussi S Heinonen, Wendy A Bohrson, Frank J Spera, Guy A Brown, Melissa A Scruggs, Jenna V AdamsAbstract:The Magma Chamber Simulator (MCS) is a thermodynamic model that computes the phase, thermal, and compositional evolution of a multiphase–multicomponent system of a Fractionally Crystallizing resident body of Magma (i.e., melt ± solids ± fluid), linked wallrock that may either be assimilated as Anatectic melts or wholesale as Stoped blocks, and multiple Recharge reservoirs (RnASnFC system, where n is the number of user-selected recharge events). MCS calculations occur in two stages; the first utilizes mass and energy balance to produce thermodynamically constrained major element and phase equilibria information for an RnASnFC system; this tool is informally called MCS-PhaseEQ, and is described in a companion paper (Bohrson et al. 2020). The second stage of modeling, called MCS-Traces, calculates the RASFC evolution of up to 48 trace elements and seven radiogenic and one stable isotopic system (Sr, Nd, Hf, 3xPb, Os, and O) for the resident melt. In addition, trace element concentrations are calculated for bulk residual wallrock and each solid (± fluid) phase in the cumulate reservoir and residual wallrock. Input consists of (1) initial trace element concentrations and isotope ratios for the parental melt, wallrock, and recharge Magmas/stoped wallrock blocks and (2) solid-melt and solid–fluid partition coefficients (optional temperature-dependence) for stable phases in the resident Magma and residual wallrock. Output can be easily read and processed from tabulated worksheets. We provide trace element and isotopic results for the same example cases (FC, R2FC, AFC, S2FC, and R2AFC) presented in the companion paper. These simulations show that recharge processes can be difficult to recognize based on trace element data alone unless there is an independent reference frame of successive recharge events or if serial recharge Magmas are sufficiently distinct in composition relative to the parental Magma or Magmas on the fractionation trend. In contrast, assimilation of wallrock is likely to have a notable effect on incompatible trace element and isotopic compositions of the contaminated resident melt. The magnitude of these effects depends on several factors incorporated into both stages of MCS calculations (e.g., phase equilibria, trace element partitioning, style of assimilation, and geochemistry of the starting materials). Significantly, the effects of assimilation can be counterintuitive and very different from simple scenarios (e.g., bulk mixing of Magma and wallrock) that do not take account phase equilibria. Considerable caution should be practiced in ruling out potential assimilation scenarios in natural systems based upon simple geochemical “rules of thumb”. The lack of simplistic responses to open-system processes underscores the need for thermodynamical RASFC models that take into account mass and energy conservation. MCS-Traces provides an unprecedented and detailed framework for utilizing thermodynamic constraints and element partitioning to document trace element and isotopic evolution of igneous systems. Continued development of the Magma Chamber Simulator will focus on easier accessibility and additional capabilities that will allow the tool to better reproduce the documented natural complexities of open-system Magmatic processes.
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deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber simulator
Contributions to Mineralogy and Petrology, 2019Co-Authors: Jussi S Heinonen, Wendy A Bohrson, Frank J Spera, Arto V. LuttinenAbstract:The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase Magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial eNd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive Magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of Magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in Magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of Magma through shallower fractures next to colder wallrock.
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Deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber Simulator
Contributions to Mineralogy and Petrology, 2019Co-Authors: Jussi S Heinonen, Frank J Spera, Arto V. Luttinen, Wendy A BohrsonAbstract:The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase Magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial ε _Nd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive Magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of Magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in Magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of Magma through shallower fractures next to colder wallrock.
Jussi S Heinonen - One of the best experts on this subject based on the ideXlab platform.
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diagnosing open system Magmatic processes using the Magma Chamber simulator mcs part ii trace elements and isotopes
Contributions to Mineralogy and Petrology, 2020Co-Authors: Jussi S Heinonen, Wendy A Bohrson, Frank J Spera, Guy A Brown, Melissa A Scruggs, Jenna V AdamsAbstract:The Magma Chamber Simulator (MCS) is a thermodynamic model that computes the phase, thermal, and compositional evolution of a multiphase–multicomponent system of a Fractionally Crystallizing resident body of Magma (i.e., melt ± solids ± fluid), linked wallrock that may either be assimilated as Anatectic melts or wholesale as Stoped blocks, and multiple Recharge reservoirs (RnASnFC system, where n is the number of user-selected recharge events). MCS calculations occur in two stages; the first utilizes mass and energy balance to produce thermodynamically constrained major element and phase equilibria information for an RnASnFC system; this tool is informally called MCS-PhaseEQ, and is described in a companion paper (Bohrson et al. 2020). The second stage of modeling, called MCS-Traces, calculates the RASFC evolution of up to 48 trace elements and seven radiogenic and one stable isotopic system (Sr, Nd, Hf, 3xPb, Os, and O) for the resident melt. In addition, trace element concentrations are calculated for bulk residual wallrock and each solid (± fluid) phase in the cumulate reservoir and residual wallrock. Input consists of (1) initial trace element concentrations and isotope ratios for the parental melt, wallrock, and recharge Magmas/stoped wallrock blocks and (2) solid-melt and solid–fluid partition coefficients (optional temperature-dependence) for stable phases in the resident Magma and residual wallrock. Output can be easily read and processed from tabulated worksheets. We provide trace element and isotopic results for the same example cases (FC, R2FC, AFC, S2FC, and R2AFC) presented in the companion paper. These simulations show that recharge processes can be difficult to recognize based on trace element data alone unless there is an independent reference frame of successive recharge events or if serial recharge Magmas are sufficiently distinct in composition relative to the parental Magma or Magmas on the fractionation trend. In contrast, assimilation of wallrock is likely to have a notable effect on incompatible trace element and isotopic compositions of the contaminated resident melt. The magnitude of these effects depends on several factors incorporated into both stages of MCS calculations (e.g., phase equilibria, trace element partitioning, style of assimilation, and geochemistry of the starting materials). Significantly, the effects of assimilation can be counterintuitive and very different from simple scenarios (e.g., bulk mixing of Magma and wallrock) that do not take account phase equilibria. Considerable caution should be practiced in ruling out potential assimilation scenarios in natural systems based upon simple geochemical “rules of thumb”. The lack of simplistic responses to open-system processes underscores the need for thermodynamical RASFC models that take into account mass and energy conservation. MCS-Traces provides an unprecedented and detailed framework for utilizing thermodynamic constraints and element partitioning to document trace element and isotopic evolution of igneous systems. Continued development of the Magma Chamber Simulator will focus on easier accessibility and additional capabilities that will allow the tool to better reproduce the documented natural complexities of open-system Magmatic processes.
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deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber simulator
Contributions to Mineralogy and Petrology, 2019Co-Authors: Jussi S Heinonen, Wendy A Bohrson, Frank J Spera, Arto V. LuttinenAbstract:The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase Magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial eNd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive Magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of Magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in Magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of Magma through shallower fractures next to colder wallrock.
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Deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber Simulator
Contributions to Mineralogy and Petrology, 2019Co-Authors: Jussi S Heinonen, Frank J Spera, Arto V. Luttinen, Wendy A BohrsonAbstract:The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase Magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial ε _Nd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive Magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of Magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in Magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of Magma through shallower fractures next to colder wallrock.
Olivier Bachmann - One of the best experts on this subject based on the ideXlab platform.
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optimal depth of subvolcanic Magma Chamber growth controlled by volatiles and crust rheology
Nature Geoscience, 2019Co-Authors: Christian Huber, Meredith Townsend, Wim Degruyter, Olivier BachmannAbstract:Storage pressures of Magma Chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where Magmas accumulate and form such Chambers. However, the density of volatile-rich silicic Magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle–ductile transition around a Magma Chamber. Yet, typical storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2 ± 0.5 kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures ≲ ≲1.5 kbar, and for geologically realistic water contents, Chamber volumes and recharge rates, the presence of an exsolved Magmatic volatile phase hinders Chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures >rsim >rsim2.5 kbar, the viscosity of the crust in long-lived Magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible Magma reservoirs are able to develop only within a relatively narrow range of pressures around 2 ± 0.5 kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption.
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Optimal depth of subvolcanic Magma Chamber growth controlled by volatiles and crust rheology
Nature Geoscience, 2019Co-Authors: Christian Huber, Meredith Townsend, Wim Degruyter, Olivier BachmannAbstract:Storage pressures of Magma Chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where Magmas accumulate and form such Chambers. However, the density of volatile-rich silicic Magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle–ductile transition around a Magma Chamber. Yet, typical storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2 ± 0.5 kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures $$\lesssim$$≲1.5 kbar, and for geologically realistic water contents, Chamber volumes and recharge rates, the presence of an exsolved Magmatic volatile phase hinders Chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures $$>rsim$$≳2.5 kbar, the viscosity of the crust in long-lived Magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible Magma reservoirs are able to develop only within a relatively narrow range of pressures around 2 ± 0.5 kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption.Volatile exsolution and crustal viscosity dictate that the optimum pressure for the growth of an eruptible Magma reservoir is 2 kbar in all tectonic settings and crustal compositions, according to thermomechanical modelling.
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the origin of a zoned ignimbrite insights into the campanian ignimbrite Magma Chamber campi flegrei italy
Earth and Planetary Science Letters, 2016Co-Authors: Francesca Forni, Olivier Bachmann, Silvio Mollo, Gianfilippo De Astis, S E Gelman, Ben S EllisAbstract:Abstract Caldera-forming eruptions, during which large volumes of Magma are explosively evacuated into the atmosphere from shallow crustal reservoirs, are one of the most hazardous natural events on Earth. The Campanian Ignimbrite (CI; Campi Flegrei, Italy) represents a classical example of such events, producing a voluminous pyroclastic sequence of trachytic to phonolitic Magma that covered several thousands of squared kilometers in the south-central Italy around 39 ka ago. The CI deposits are known for their remarkable geochemical gradients, attributed to eruption from a vertically zoned Magma Chamber. We investigate the relationships between such chemical zoning and the crystallinity variations observed within the CI pyroclastic sequence by combining bulk-rock data with detailed analyses of crystals and matrix glass from well-characterized stratigraphic units. Using geothermometers and hygrometers specifically calibrated for alkaline Magmas, we reconstruct the reservoir storage conditions, revealing the presence of gradients in temperature and Magma water content. In particular, we observe a decrease in crystallinity and temperature and an increase in Magma evolution and water content from the bottom to the top of the Magma Chamber. We interpret these features as the result of protracted fractional crystallization leading to the formation of a cumulate crystal mush at the base of the eruptible reservoir, from which highly evolved, crystal-poor, water-rich and relatively cold melts were separated. The extracted melts, forming a buoyant, easily eruptible cap at the top of the Magma Chamber, fed the initial phases of the eruption, until caldera collapse and eruption of the deeper more crystalline part of the system. This late-erupted, crystal-rich material represents remobilized portions of the cumulate crystal mush, partly melted following hotter recharge. Our interpretation is supported by: 1) the positive bulk-rock Eu anomalies and the high Ba and Sr contents observed in the crystal-rich units, implying feldspar accumulation; 2) the positive Eu anomalies in the matrix glass of the crystal-rich units, testifying to the presence of liquid derived from partial melting of low temperature mineral phases within the crystal mush (mostly feldspars); 3) the Ba and Sr-rich rims in the feldspars and positive Eu anomalies in clinopyroxene rims, suggesting late rim growth from a locally enriched melt following cumulate mush remelting and 4) the occurrence of An-rich plagioclase, relict from a more mafic recharge, which acted as a heat source. Our model reconciles many observations made over the years on zoned deposits of such high-magnitude explosive eruptions, and provides a framework to understand Magma Chamber processes leading up to cataclysmic events.
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conditions for the growth of a long lived shallow crustal Magma Chamber below mount pelee volcano martinique lesser antilles arc
Journal of Geophysical Research, 2008Co-Authors: Catherine Annen, Olivier Bachmann, Michel Pichavant, Alain BurgisserAbstract:[1] The compositional homogeneity of silicic andesites emitted during the 13,500-year-long last eruptive cycle of Mount Pelee suggests that the physical state of the Magma Chamber was largely unmodified during this period. Experimental phase equilibria on Mount Pelee recent products indicate that pre-eruptive Magma temperatures and pressures were in the range of 875–900°C and 2 ± 0.5 kbar, respectively. The estimated average eruption rate was 7.5 × 10−4 km3/a with an average volume of about 0.3 km3 per eruption. An analytical model for a spherical Magma Chamber indicates that a Magma flux of 4–5 × 10−4 km3/a can maintain a Magma Chamber of 0.3 km3 above 875°C below Mount Pelee. However, observations of plutons suggest that Magma Chambers may grow by addition of sheet-like intrusions. With numerical simulation we show that a minimum sheet accretion rate of a few centimeters per year is required to grow a persistently active Magma Chamber independently of the intruded volumetric flux. This minimum injection rate is higher if the heat transfer is enhanced by convection processes. The limited ability of an arc crust to extend and accommodate dikes suggests that Magma injections are sills or, if they are dikes, that most of the volume injected in the Magma Chamber is eventually erupted. In a conductively cooling igneous body formed by sills injected at a rate of a few centimeters per year, most of the injected Magma completely solidifies and only a small part of the body (about 10–20%) is above 875°C and able to feed eruptions.
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deciphering Magma Chamber dynamics from styles of compositional zoning in large silicic ash flow sheets
Reviews in Mineralogy & Geochemistry, 2008Co-Authors: Olivier Bachmann, George W BergantzAbstract:The understanding of the dynamic processes in Magmatic systems has grown and changed markedly in the last decade. Old models for Magmatic systems as vats of near-liquidus material have been revised by observations from seismology (Sinton and Detrick 1992), crystal chemistry and zoning (Davidson et al. 2007) and the geochronology and geochemistry of both plutonic and volcanic systems (Hildreth 2004; Charlier et al. 2007; Miller et al. 2007; Peressini et al. 2007; Walker et al. 2007). New views emphasise Magmatic systems as temporally dominated by crystal mushes (Magma bodies with a high fraction of solid particles; see definition in Miller and Wark 2008), that wax and wane in temperature and crystallinity, and are subject to significant open system processes (Charlier et al. 2007; Hildreth and Wilson 2007; Walker et al. 2007; Bachmann and Bergantz 2008). These processes, such as Magma reintrusion, mixing, gas sparging, and subsequent thermal rejuvenation, may be significantly more important in producing the characteristics of a Magmatic system than the previous closed-system, near-liquidus behaviour would predict. There are a number of recent reviews that summarize these observations (for example see Eichelberger et al. 2006; Bachmann et al. 2007b; Lipman 2007). Our aim here is to illustrate how this new perspective to Magma dynamics is motivated by observations of heterogeneities (or lack thereof) in erupted rocks (and to a lesser amount in plutons). Owing to rapid withdrawal and quenching of Magma during explosive volcanic eruptions (hours to a few days), large-volume (>1 km3) pyroclastic deposits (also referred to as ignimbrites or ash-flow tuffs ) provide an instant image of the state of the Magma Chamber evacuated during eruption. A first-order observation that characterizes these pyroclastic deposits of intermediate to silicic composition is that many do not tap into chemically …
Agust Gudmundsson - One of the best experts on this subject based on the ideXlab platform.
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forecasting Magma Chamber rupture at santorini volcano greece
Scientific Reports, 2015Co-Authors: John Browning, Kyriaki Drymoni, Agust GudmundssonAbstract:How much Magma needs to be added to a shallow Magma Chamber to cause rupture, dyke injection, and a potential eruption? Models that yield reliable answers to this question are needed in order to facilitate eruption forecasting. Development of a long-lived shallow Magma Chamber requires periodic influx of Magmas from a parental body at depth. This redistribution process does not necessarily cause an eruption but produces a net volume change that can be measured geodetically by inversion techniques. Using continuum-mechanics and fracture-mechanics principles, we calculate the amount of Magma contained at shallow depth beneath Santorini volcano, Greece. We demonstrate through structural analysis of dykes exposed within the Santorini caldera, previously published data on the volume of recent eruptions, and geodetic measurements of the 2011-2012 unrest period, that the measured 0.02% increase in volume of Santorini's shallow Magma Chamber was associated with Magmatic excess pressure increase of around 1.1 MPa. This excess pressure was high enough to bring the Chamber roof close to rupture and dyke injection. For volcanoes with known typical extrusion and intrusion (dyke) volumes, the new methodology presented here makes it possible to forecast the conditions for Magma-Chamber failure and dyke injection at any geodetically well-monitored volcano.
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numerical modelling of dykes deflected into sills to form a Magma Chamber
Journal of Volcanology and Geothermal Research, 2014Co-Authors: Zoe Barnett, Agust GudmundssonAbstract:Abstract Most shallow Magma Chambers are thought to evolve from sills. For this to happen, several conditions must be met. (1) There must be a discontinuity, normally a contact, that deflects a dyke (or an inclined sheet) into a sill. (2) The initial sill must have a considerable thickness, normally (depending on dyke injection rates) not less than some tens of metres. (3) The resulting sill must receive Magma (through dykes) frequently enough so as to stay liquid and expand into a Chamber. (4) The resulting Magma Chamber must remain at least partially molten and receive multiple Magma injections over a given period of time to build up a volcano on the surface above. In this paper we present numerical models based upon field data and geophysical data as to how sills are emplaced and may subsequently evolve into shallow Magma Chambers. We suggest that most sills form when dykes meet contacts, particularly weak ones, which are unfavourable to dyke propagation. A contact may halt (arrest) a dyke altogether or, alternatively, deflect the dyke into the contact. The three main mechanisms for dyke deflection into a contact are (1) the Cook–Gordon debonding or delamination, (2) rotation of the principal stresses, generating a stress barrier, and (3) an elastic mismatch across a contact between adjacent layers. Elastic mismatch means that the layers have contrasting Young's moduli and varying material toughness. Once a sill is initiated, the developing Magma Chamber may take various forms. Many shallow Magma Chambers, however, tend to maintain a straight sill-like or somewhat flat (oblate) ellipsoidal geometry during their lifetimes. For a sill to evolve into a Magma Chamber there must be elastic-plastic deformation of the overburden and, to some extent, of the underburden. So long as the sill stays liquid, subsequent dyke injections become arrested on meeting the sill. Some Magma Chambers develop from sill complexes. For the sill complex to remain partially molten it must receive a constant replenishment of Magma, implying a high dyke-injection rate. Alternatively, an initial comparatively thick sill may absorb much of the Magma of the dykes that meet it and evolve into a single shallow Magma Chamber.
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deflection of dykes into sills at discontinuities and Magma Chamber formation
Tectonophysics, 2011Co-Authors: Agust GudmundssonAbstract:Abstract Many rift zones contain composite volcanoes (central volcanoes), most of which are supplied with Magma from shallow crustal Magma Chambers. Volcanotectonic studies of eroded rift zones, such as in Iceland, as well as geophysical studies indicate that many shallow Magma Chambers evolve from sills. Here I provide field description of sills and extinct Magma Chambers in the Quaternary and Tertiary palaeorift zones of Iceland, focusing on the felsic pluton of Slaufrudalur, Southeast Iceland, as a fine example of a Magma Chamber developed through the injection of sills. In this paper I review, analyse, and develop further two related mechanisms for the deflection of dykes into sills, and thus the potential initiation of a Magma Chamber, namely: (a) Cook–Gordon debounding (delamination), and (b) stress barriers, and propose a third mechanism, (c) favourable material-toughness ratios due to elastic mismatch (difference in Young's moduli or stiffnesses of layers in contact). In the Cook–Gordon mechanism, a weak contact opens up as a result of dyke-induced tensile stress. This mechanism is likely to operate primarily at shallow depths, as is supported by field observations and numerical models. A stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture, here a dyke. Field observations and numerical models show that on meeting a stress barrier, a dyke either changes into a sill or becomes arrested. The material-toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer (hosting the dyke), there is little tendency for the dyke to become deflected into the contact. However, when the upper layer is stiffer, such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer, the dyke tends to become deflected into the contact to form a sill. Field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together, particularly where the rock consists of alternating soft and stiff layers. Since the Quaternary lava pile in Iceland has many more soft (hyaloclastite) layers than the Tertiary lava pile, these mechanisms were probably very efficient in generating sills and, by implication, shallow Magma Chambers, during the Quaternary. This may be one reason why, in comparison with the Tertiary lava pile, the Quaternary lava pile contains so many sills and extinct (plutons) and active shallow Magma Chambers.
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how local stresses control Magma Chamber ruptures dyke injections and eruptions in composite volcanoes
Earth-Science Reviews, 2006Co-Authors: Agust GudmundssonAbstract:Abstract To assess the probability of a volcanic eruption during an unrest period, we must understand Magma-Chamber rupture and dyke propagation to the surface, as well as dyke arrest at depth in the volcano. Dyke propagation and arrest depend strongly on the local stresses in the individual mechanical layers which constitute the volcano. The local stresses are primarily determined by the loading conditions (tectonic stress, Magmatic pressure, or displacement) and the mechanical properties of the layers. In the absence of stress monitoring of volcanoes, the local stresses must be inferred from models, either analytical or numerical. This paper reviews many analytical and numerical models of local stresses around Magma Chambers, as well as analytical models and numerical examples of dyke-injection and eruption frequencies. Most analytical models of Magma Chambers ignore the mechanical properties of the individual layers and their contacts, assume the volcano to behave as a homogeneous, isotropic, elastic half space or a semi-infinite plate, and are of two main types: nuclei of strain and cavities. The best-known nucleus of strain is the point-source Mogi model, used to explain surface deformation as a result of either increase or decrease in Magma pressure in a Chamber whose depth is also inferred from the surface data. The model explains stresses and displacements far away from the Chamber, but neither the stress concentration around the Chamber, which determines if and where Chamber rupture and dyke injection take place, nor the shape, size, and likely tectonic evolution of the Chamber. In the cavity or (two-dimensional) hole model the Magma Chamber has a finite size. Thus, the local stresses at, and away from, the boundary of a Chamber can be calculated. For various loading conditions, an analytical cavity model gives a crude indication of the local stresses in a volcano and its surface deformation. However, variation in mechanical properties, and contacts, between layers are ignored. The analytical cavity model thus cannot be used for detailed analyses of the local stresses in a composite volcano. The numerical models presented here show that the local stresses in a volcano depend strongly on the Magma-Chamber geometry and the mechanical properties of its layers which are often contrasting, particularly at shallow depths. For example, lava flows, welded pyrolastic units, and intrusions may be very stiff (with a high Young's modulus), whereas young and non-welded pyroclastic and sedimentary units may be very soft (with a low Young's modulus). Consequently, the local stresses may change abruptly from one layer to the next; for example, one layer may favour dyke propagation while an adjacent layer favours dyke arrest. No dyke-fed eruption can occur if there is any layer along the potential path of the dyke to the surface where the stress field is unfavourable to dyke propagation. If such a layer occurs, the dyke normally becomes arrested and an eruption is prevented. The present results indicate that during unrest periods composite volcanoes commonly develop local stresses that arrest dykes and prevent eruptions, in agreement with field observations. These results underline the need for in situ stress monitoring of volcanoes to assess the probability of dyke-fed eruptions.
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The Las Cañadas caldera (Tenerife, Canary Islands): an overlapping collapse caldera generated by Magma-Chamber migration
Journal of Volcanology and Geothermal Research, 2000Co-Authors: Joan Martí, Agust GudmundssonAbstract:Abstract The Las Canadas caldera is one of the most important geological structures of Tenerife. Stratigraphic, structural, volcanological, petrological, geochronological, and geophysical data suggest that the Las Canadas caldera resulted from multiple vertical collapse episodes that occurred during the construction of the Las Canadas edifice Upper Group. Three long-term (≥200 ka) cycles of phonolitic explosive activity, each culminating with a caldera collapse, have been identified in the Upper Group. During the construction of the Upper Group, the focus of felsic volcanism migrated from west to east. Using the results of field observations, experimental analogue models and numerical studies, we propose that the formation of the overlapping Las Canadas collapse caldera is related to the migration of the associated Magma Chamber. Our model implies that each collapse of this overlapping caldera partly, or completely, destroyed the feeding Magma Chamber. This destruction led to changes in the local stress field that favoured the formation of a new Chamber at one side of the previous one, resulting in Magma-Chamber migration. The proposed model accounts for the formation of the Las Canadas caldera. In particular, it explains the geometrical relationships, stratigraphy and chronology of the caldera wall deposits. Comparison with other overlapping collapse calderas suggests that our model may apply to other overlapping calderas.
Frank J Spera - One of the best experts on this subject based on the ideXlab platform.
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diagnosing open system Magmatic processes using the Magma Chamber simulator mcs part ii trace elements and isotopes
Contributions to Mineralogy and Petrology, 2020Co-Authors: Jussi S Heinonen, Wendy A Bohrson, Frank J Spera, Guy A Brown, Melissa A Scruggs, Jenna V AdamsAbstract:The Magma Chamber Simulator (MCS) is a thermodynamic model that computes the phase, thermal, and compositional evolution of a multiphase–multicomponent system of a Fractionally Crystallizing resident body of Magma (i.e., melt ± solids ± fluid), linked wallrock that may either be assimilated as Anatectic melts or wholesale as Stoped blocks, and multiple Recharge reservoirs (RnASnFC system, where n is the number of user-selected recharge events). MCS calculations occur in two stages; the first utilizes mass and energy balance to produce thermodynamically constrained major element and phase equilibria information for an RnASnFC system; this tool is informally called MCS-PhaseEQ, and is described in a companion paper (Bohrson et al. 2020). The second stage of modeling, called MCS-Traces, calculates the RASFC evolution of up to 48 trace elements and seven radiogenic and one stable isotopic system (Sr, Nd, Hf, 3xPb, Os, and O) for the resident melt. In addition, trace element concentrations are calculated for bulk residual wallrock and each solid (± fluid) phase in the cumulate reservoir and residual wallrock. Input consists of (1) initial trace element concentrations and isotope ratios for the parental melt, wallrock, and recharge Magmas/stoped wallrock blocks and (2) solid-melt and solid–fluid partition coefficients (optional temperature-dependence) for stable phases in the resident Magma and residual wallrock. Output can be easily read and processed from tabulated worksheets. We provide trace element and isotopic results for the same example cases (FC, R2FC, AFC, S2FC, and R2AFC) presented in the companion paper. These simulations show that recharge processes can be difficult to recognize based on trace element data alone unless there is an independent reference frame of successive recharge events or if serial recharge Magmas are sufficiently distinct in composition relative to the parental Magma or Magmas on the fractionation trend. In contrast, assimilation of wallrock is likely to have a notable effect on incompatible trace element and isotopic compositions of the contaminated resident melt. The magnitude of these effects depends on several factors incorporated into both stages of MCS calculations (e.g., phase equilibria, trace element partitioning, style of assimilation, and geochemistry of the starting materials). Significantly, the effects of assimilation can be counterintuitive and very different from simple scenarios (e.g., bulk mixing of Magma and wallrock) that do not take account phase equilibria. Considerable caution should be practiced in ruling out potential assimilation scenarios in natural systems based upon simple geochemical “rules of thumb”. The lack of simplistic responses to open-system processes underscores the need for thermodynamical RASFC models that take into account mass and energy conservation. MCS-Traces provides an unprecedented and detailed framework for utilizing thermodynamic constraints and element partitioning to document trace element and isotopic evolution of igneous systems. Continued development of the Magma Chamber Simulator will focus on easier accessibility and additional capabilities that will allow the tool to better reproduce the documented natural complexities of open-system Magmatic processes.
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deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber simulator
Contributions to Mineralogy and Petrology, 2019Co-Authors: Jussi S Heinonen, Wendy A Bohrson, Frank J Spera, Arto V. LuttinenAbstract:The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase Magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial eNd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive Magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of Magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in Magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of Magma through shallower fractures next to colder wallrock.
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Deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber Simulator
Contributions to Mineralogy and Petrology, 2019Co-Authors: Jussi S Heinonen, Frank J Spera, Arto V. Luttinen, Wendy A BohrsonAbstract:The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase Magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial ε _Nd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive Magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of Magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in Magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of Magma through shallower fractures next to colder wallrock.
