Melt Segregation in Deep Crustal Hot Zones: a Mechanism for Chemical Differentiation, Crustal Assimilation and the Formation of Evolved Magmas
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Mantle-derived basaltic sills emplaced in the lower crust provide a mechanism for the generation of evolved magmas in deep crustal hot zones (DCHZ). This study uses numerical modelling to characterize the time required for evolved magma formation, the depth and temperature at which magma formation occurs, and the composition of the magma. The lower crust is assumed to comprise amphibolite. In an extension of previous DCHZ models, the new model couples heat transfer during the repetitive emplacement of sills with mass transfer via buoyancy-driven melt segregation along grain boundaries. The results shed light on the dynamics of DCHZ development and evolution. The DCHZ comprises a mush of crystals plus interstitial melt, except when a new influx of basaltic magma yields a short-lived (20–200 years) reservoir of melt plus suspended crystals (magma). Melt segregation and accumulation within the mush yields two contrasting modes of evolved magma formation, which operate over timescales of c. 10 kyr–1 Myr, depending upon emplacement rate and style. In one, favoured by emplacement via over-accretion, or emplacement at high rates, evolved magma forms in the crust overlying the intruded basalt sills, and is composed of crustal partial melt, and residual melt that has migrated upwards out of the crystallizing basalt. In the other, favoured by emplacement via under- or intra-accretion, or by emplacement at lower rates, evolved magma forms in the intruded basalt, and the resulting magma is composed primarily of residual melt. In all cases, the upward migration of buoyant melt yields cooler and more evolved magmas, which are broadly granitic in composition. Chemical differentiation is therefore driven by melt migration, because the melt migrates through, and chemically equilibrates with, partially molten rock at progressively lower temperatures. Crustal assimilation occurs during partial melting, and mixing of crustal and residual melt occurs when residual melt migrates into the partially molten crust, yielding chemical signatures indicative of a mixed crustal and mantle origin. However, residual melt is volumetrically more significant than crustal melt, except at the highest emplacement rates. Contamination of crustal melt by residual melt from basalt crystallization appears to be an inevitable consequence of melt segregation in DCHZ, and can explain the mixed crust–mantle origin of many granites.Keywords:
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The Loch Scridain Sill-complex on the Isle of Mull affords an opportunity to examine how magma moves through and builds sill-complexes (Holness & Humphreys 2003). For example, field and petrological evidence indicates the Tràigh Bhàn na Sgùrra Sill is segmented and comprised several thick channels, separated by thin sill portions, that facilitated longer-lived magma flow (Holness & Humphreys 2003; Stephens et al. 2017). We integrate structural field analysis and rock magnetic techniques in an attempt to identify and characterise magma flow pathways in the LSSC, with a view to expanding the study to understand how the entire sill-complex was constructed.
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Sills and dykes are commonplace items frequently seen in the field and rarely given a second glance. The mechanism of intrusion of sills and dykes is a relatively simple concept, readily taught to students, and rarely given a second thought. This book, however, is full of second thoughts, insights and models about the intrusion of magma and the formation of many different types of high-level sub-volcanic or plutonic body. It is a volume that makes you think about old …
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The best place to seek evidence of the style of past magma flow through a conduit is in the country rock. Heat flow has been studied in country rock adjacent to two Tertiary dolerite sills intruding the Caledonian schists and quartzites, on the Isle of Mull, Scotland. Radiogenic 40 Ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the Ultra‐Violet Laser Ablation Micro‐Probe, to discriminate between a history of prolonged magma flow, a history of conductive cooling following laminar flow, and instantaneous emplacement of the intrusions. The 40 Ar/ 39 Ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3–5 months resulted in extensive argon loss from the micas, country rock melting, and mineral breakdown adjacent to a 6‐m sill. These features were absent from the wall rocks of a smaller 2.7‐m‐thick sill, which exhibited even less argon loss than might have been predicted for an instantaneous intrusion. If the heat loss from the 6‐m sill observed in one locality had been repeated along its length, it would have formed an important magma conduit to the Mull volcano, but dolerite is not a common flow composition on Mull. If on the other hand, the heat loss from the sill varies along strike, it constitutes strong evidence for channelling and heterogeneous flow within the sill.
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The Great Whin and Midland Valley dolerite sill complexes appear to have been emplaced laterally from the walls of feeder dykes. The overall thickness of each sill increases with depth, as would be expected if the magma finally reached a state of hydrostatic equilibrium. Variations in the thickness of the sills with estimated intrusion depth imply that the head of magma was about 100 m below the contemporary ground surface in the areas of the present-day outcrops at the end of the intrusive episodes. Before hydrostatic equilibrium was established, the magma pressure would probably have been somewhat greater, so it is likely that intrusion of the sills was accompanied by the extrusion of flood basalts. Step-and-stair transgressions of the bedding are commonly found within the sills, mostly stepping downwards in the direction of bedding dip. The reason for this directionality is that the weight of sediments floating on an intruding sill has a downdip component that applies a tensile stress to intersecting fractures below the sill when magma is moving downdip, and to intersecting fractures above the sill when magma is moving updip.
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Abstract Caldera unrest is often caused by kilometer‐sized kilometer‐deep sills. Still unanswered questions include the following: How do sills spread? Why can magma propagate for kilometers without solidifying? Why do ground deformation data rarely, if ever, detect sill propagation? We show that kilometer‐sized kilometer‐deep magmatic sills spread like hydraulic fractures in an infinite medium. How magma propagates depends on overburden pressure, magma viscosity, injection rate, and difference between magma and rock temperatures. A small lag, filled with vapors from the fluid and/or the rock, exists between the propagating magma and fracture fronts. If the sill spreads along an interface, the lag slightly affects isothermal sill spreading but takes a key role in the case of nonisothermal propagation: A sill would stop after few tens of meters without it, unless magma intrudes rocks that are as hot as the solidification temperature or has unrealistic overpressures, because spreading velocity decreases soon to the critical value at which the tip becomes blocked with solidified magma. The lag defers magma solidification as heat exchange between the magma and the rock is effective only behind the thermal‐insulating lag, where magma has some finite thickness and sill opening grows with distance from the tip faster than thickness of solidified magma. Thus, the critical velocity decreases, allowing greater maximum sill sizes. We also show that the ground deformation pattern does not change appreciably over time if the final sill radius is smaller than 2 to 3 km, explaining why deformation is usually attributed to the inflation of a stationary source.
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