Some effects of viscosity on the dynamics of replenished magma chambers
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Some aspects of the dynamical behavior of magma chambers, replenished from below with hotter but denser magma, have been modeled in a series of laboratory experiments. In previously reported work the fluids used were aqueous solutions of comparable viscosity, and thus the results should be applicable to basaltic magma chambers, in which the magmas do not vary greatly in viscosity. In that case, the lower layer cools by convective heat transfer to the fluid above, and crystallization causes the density of the residual liquid in the lower layer to decrease. When the density becomes equal to that in the upper layer, sudden overturning and intimate mixing take place. The present paper reports experimental results that allow us to extend the application to systems in which there is a large viscosity ratio between the resident and the injected fluid, for example, to calcalkaline magmas, where magma viscosity can vary by as much as 5 orders of magnitude. The largest viscosity ratio in our experiments (about 3000) was achieved using cold glycerine for the upper layer, above a hot denser KNO 3 solution. The most striking new feature with the very viscous upper layer is that now less dense fluid is released immediately and continuously from the interface and rises as plumes through the upper layer. Further crystallization occurs in the plumes, and the crystals fall out, but there is little mixing, and a layer of depleted KNO 3 solution is eventually deposited at the top. The transfer process between the layers is dominated by interfacial effects, with the high‐viscosity upper layer acting as a nearly rigid lid that allows buoyant fluid to accumulate just below the interface and then rise in localized plumes across the interface into the viscous layer. This physical picture is supported by a series of experiments in which the viscosity ratio is varied systematically; the mixing behavior changes gradually between that described above for a large viscosity ratio and the sudden over‐turning characteristic of layers with comparable viscosity. The importance of the viscosity ratio, rather than just an increase in viscosity, is confirmed by experiments in which both viscosities are increased by the same factor; the overturning process is then slower, but symmetrical. Other variations suggested by previous experiments are also described: the release of gas by a chemical reaction, to model the release of volatiles following an overturning event in a magma chamber; the effect of a cold, immiscible layer above the cooling crystallizing fluid; the influence of two viscous layers with a density step between them; and the constraining effects of a density (with corresponding viscosity) gradient in the upper region. The experiments indicate that whatever the stratification, whether it be in layers or continuous, the form of the initial motion in the upper fluid is determined by the viscosity ratio between the two fluids immediately adjacent to the interface. Geological applications are not examined in detail in this paper, but the experiments suggest that both sudden overturning (characteristic of magmas of nearly equal viscosity) and continuous release (when the upper layer is much more viscous) are viable mechanisms for magma mixing in the appropriate circumstances.Keywords:
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Abstract In the continental crust, the probability of dike propagation out of magma chambers is governed by thermal, rheological, and pressure conditions of magma chamber‐wall rock systems. Incremental injection of melt into an average‐size, laccolith‐shaped, midcrustal magma chamber produces a volume of mobile magma at the bottom of the chamber that has the potential to escape as dikes through the upper, immobile portion of the chamber and the roof. Here we numerically model the conditions needed for dike propagation out of a magma chamber during continuous and episodic injections of melt into the chamber. The roles of magma buoyancy and overpressure from melt injections in generating dikes are explored within 1.78 × 10 4 to 1.78 × 10 8 Pa·s range of magma viscosities ( μ mag ), 10 to 40 GPa range of elastic moduli ( E ) of the immobile top portion of the magma chamber, and 10 and 20 kyr durations of chamber growth. During episodic, high‐flux melt injections (tens of km 3 /yr), magma overpressure can reach >100 MPa and initiate dike propagation even when μ mag and E are near the high ends of the examined ranges. The probability of generating dikes diminishes when the injection flux is lower. Continuous low‐flux injections favor magma accumulation because injection overpressure never exceeds 20 MPa. During either continuous or episodic growth of magma chamber, there is never a sufficient amount of mobile magma in the chamber for dikes to be induced by magma buoyancy alone.
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Abstract Several recent studies have argued that large, long-lived and molten magma chambers 1–10 may not occur in the shallow Earth’s crust 11–23 . Here we present, however, field-based observations from the Bushveld Complex 24 that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering was found to continuously drape across a ~4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within such a thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated to be >5-km-high in thickness and >380,000 km 3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history 25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces 26 . This suggests that super-large, entirely molten and long-lived magma chambers, at least occasionally, occur in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’ 1–10 remains a viable research concept for some of Earth’s magmatic provinces.
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Assessing volcano hazard and forecasting eruptions require knowledge of the geometry and properties of a volcano's magma chamber. However, there are few high‐resolution seismic imagery studies of magma chambers. Paulatto et al. used seismic tomography along with numerical models of magma chamber growth to get a better picture of the magma chamber beneath the active Soufrière Hills volcano on the island of Montserrat. Their approach reveals details of the magma system that have not been shown in previous studies. The authors' analysis shows that the magma chamber contains about 13 cubic kilometers of magma, with more than 30% melt faction between about 5.5‐ and 7.5‐kilometer depth. The researchers suggest that the magma chamber could have formed through sill intrusion over several thousand years. ( Geochemistry, Geophysics, Geosystems (G 3 ), doi:10.1029/2011GC003892, 2012)
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