The Mode of Formation of Chromium-Poor Megacryst Suites from Kimberlites
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Abstract:
The nature of magma bodies associated with the formation of Cr-poor megacryst suites included in kimberlites is considered with particular regard to the possibility of a virtually isobaric body containing crystals at widely varying temperatures. A model is proposed in which a complex of magma sheets and other apophyses exists in a temperature gradient, thereby allowing the simultaneous existence of different states of magma differentiation and crystallization.Keywords:
Isobaric process
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Solutions of water and a salt or sugar make excellent experimental analog magmas for teaching concepts of igneous petrology because of the comparatively low temperatures involved, the simplicity of the apparatus needed, and the responsiveness of familiar chemical systems. Boiling of these aqueous solutions on a hot plate can be used to increase the concentration of a dissolved salt or sugar to levels that may be predicted by steam-saturation curves. Sufficiently concentrated solutions will crystallize, partially or completely, upon cooling to room temperature. Binary temperature–composition phase diagrams for H 2 O and KCl, NaCl, MgCl 2 , CaCl 2 , or C 12 H 22 O 11 have been drawn to provide guidance for experiments, and equations are given for the saturation curves. Possible instructional activities with these simple systems include: (1) determination of saturation (liquidus) curves on binary phase diagrams, (2) measurement of the relative proportions of liquid and solid in a system that has partially crystallized, and comparison with predictions of the lever rule, (3) observation of some consequences of peritectic reactions on crystallization, (4) observation of the kinetic effects of temperature and concentration on crystallization, (5) simulation of a magma chamber with crystals settling because of their density and rising owing to convection, and (6) observation of simultaneous boiling and crystallization that buffer temperature, which can lead to a solid with vapor cavities. Movies of interesting aspects of these experiments are available online as supplementary documents.
Igneous petrology
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Basaltic magmas at temperatures of 1100 degrees to 1200 degrees C commonly ascend in dikes through the Earth9s upper crust, where temperatures are considerably lower. As heat exchange between an invading magma and wall rocks commences, temperatures near dike margins are well below solidus temperatures, and a layer of cooled, solidified magma is thus present between the wall rocks and the uncooled magma flowing in the center of the dike. The rate of growth (or decay) of this layer is controlled by dike thickness, magma flow rate, distance from the source region, temperature difference between magma and host rocks, and temperature dependence of magma viscosity. In agreement with available data obtained during observation of volcanic fissure eruptions and from study of ancient dikes, we calculate that magmas flowing distances more than a few kilometers from an upper crustal source region and at initial velocities of 1 m/s in dikes that are 2 m thick solidify within a few hours. Temperatures at the contact between magma and wall rocks may remain well below that of the initial, uncooled magma for the duration of heat transfer. The short duration of heat transfer and small temperature increase in the wall rocks explains the lack of contact metamorphic effects near the contact of many dikes. Magmas leaving the source region at different times become juxtaposed. As the solidified layer grows inward from the dike walls, this juxtaposition is preserved and may account for the compositional and textural zonation of some dikes.
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Magma chamber
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Silicic
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Abstract Between 2015 and 2021, Nyiragongo's lava lake level experienced a linear increase punctuated by fast intermittent drops. These drops occurred synchronously to seismic swarm at approximately 15 km below the surface and extending laterally NE from the volcano. To interpret these lava lake level patterns in terms of reservoirs pressure evolution within Nyiragongo, we consider the following simplified plumbing system: a central reservoir is fed by a constant flux of magma, distributing the fluid up into the lava lake and laterally into a distal storage zone. Magma transport is driven by a pressure gradient between the magma storage bodies, accommodating influx and outflow of magma elastically, and the lava lake. Lateral transport at depth occurs through a hydraulic connection for which the flow resistance is coupled to the magma flux. When the right conditions are met, lateral magma transport occurs intermittently and triggers intermittent lava lake level drops matching the observations.
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Abstract Large volume effusive eruptions with relatively minor observed precursory signals are at odds with widely used models to interpret volcano deformation. Here we propose a new modelling framework that resolves this discrepancy by accounting for magma buoyancy, viscoelastic crustal properties, and sustained magma channels. At low magma accumulation rates, the stability of deep magma bodies is governed by the magma-host rock density contrast and the magma body thickness. During eruptions, inelastic processes including magma mush erosion and thermal effects, can form a sustained channel that supports magma flow, driven by the pressure difference between the magma body and surface vents. At failure onset, it may be difficult to forecast the final eruption volume; pressure in a magma body may drop well below the lithostatic load, create under-pressure and initiate a caldera collapse, despite only modest precursors.
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Electrical conduit
Volcanic Gases
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Dike
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This study combines petrology, thermal modelling and magma rheology to characterise timescales and modalities of magma emplacement in the Earth’s crust. Thermal modelling was performed to determine the influence of magma injection rates in the crust on the temperature evolution of a magmatic body. The injected tonalitic magma was considered to contain dioritic enclaves, common in plutons. The contrast in chemical composition between host and enclaves leads to different crystallinities of these magmas during cooling and produce a rheological contrast that permits reciprocal deformation only in restricted temperature ranges. Characterising the thermal and rheological evolution of host magma and enclaves, we traced the evolution of strain recorded by these inclusions during the construction of an intrusion, showing that the strain recorded by enclaves distributed in different portions of a pluton can be used to constrain thermal evolution in time, magmatic fluxes and timescale of assemblage of magmatic bodies in the crust.
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