![]() Iceland is situated where a mantle plume underlies the Mid-Atlantic Ridge. We then use a simple numerical model to explore how variations in magma chamber depth, country rock density and fracture strength, can facilitate the eruption of dense magmas to the surface. In this study, we explore the relationship between physical properties and erupted volumes for magmas from Iceland's Northern Volcanic Zone (NVZ). The eruption of these dense magmas cannot be simply explained by neutral buoyancy alone. However, dense olivine-phyric magmas with whole-rock MgO contents >15 wt.% and Mg#>0.8 have been erupted on Iceland's neovolcanic zones. This fluid-dynamical approach predicts that basalts with MgO contents >10 wt.% are very unlikely to erupt ( Huppert and Sparks, 1980). For mid-ocean ridge tholeiites erupting through typical oceanic crust, the window of eruptibility is found at melt MgO contents between 7 and 10 wt.%, or magnesium numbers (Mg#, molar Mg/(Mg+Fe 2+) around 0.55–0.70. ![]() The range of compositions that can pass through this window depends firstly on the crustal density, and secondly on the position of the density minimum, which will itself vary depending on the initial composition and liquid line of descent of the parental melt. Stolper and Walker (1980) defined a “window of eruptibility” for potential eruptive magmas, whereby the crust acts as an effective filter to high-density magmas. Further crystallization of the residual liquid causes the density to increase once more, reducing the buoyancy and hence the eruptibility of the melt. Typical suites of mid-ocean ridge basalts (MORB) reach this density minimum at ~7–10 wt.% MgO ( Sparks and Huppert, 1984) the buoyant residual liquids may then ascend to their level of neutral buoyancy, resulting in an eruption if the liquid remains buoyant all the way to the surface. As melt crystallizes within the chamber, the density of the residual liquid evolves until a density minimum is reached. (1980) considered a fluid-dynamical approach to explore the conditions under which magma stored in stratified chambers may be erupted at the surface. Huppert and Sparks (1980) and Sparks et al. ![]() Neutral buoyancy concepts can also be applied within magma chambers. The ponding of magmas at their level of neutral buoyancy in the crust ( Ryan, 1993) is considered to be an important factor in the prolonged existence of shallow magma chambers ( Ryan, 1987). Primary melts generated at depth are expected to rise through the crust until ρ m–ρ c≈ 0, that is, the point at which the magma density ρ m and crustal density ρ c are equal. It has long been understood that the range of lava compositions sampled at mid-ocean ridges is strongly linked to the buoyancy of magma in the oceanic crust ( Huppert and Sparks, 1980 Sparks et al., 1980) and the viscosity of the magma ( Walker, 1971). This conclusion is in agreement with petrological constraints on the depths of crystallization under Iceland. We use a simple numerical model to demonstrate that the eruption of magmas with higher densities and viscosities is facilitated by the generation of overpressure in magma chambers in the lower crust and uppermost mantle. ![]() However, a number of small-volume eruptions with densities greater than typical Icelandic upper crust are also found in Iceland's neovolcanic zones. These magmas are buoyant with respect to the Icelandic upper crust. Over 85% of the total volume of erupted material lies close to a density and viscosity minimum that corresponds to the composition of basalts at the arrival of plagioclase on the liquidus. We find a strong correlation between magmatic density and observed erupted volumes on the NVZ. By studying subaerial flows of known age and volume, we are able to directly relate erupted volumes to magmatic physical properties, a task that has been near-impossible when dealing with submarine samples dredged from mid-ocean ridges. In this study, we investigate the extent to which magmatic physical properties control the eruptibility of magmas from Iceland's Northern Volcanic Zone (NVZ). ![]() Magmatic density and viscosity exert fundamental controls on the eruptibility of magmas. 2Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom.1School of Earth and Environmental Sciences, University of Manchester, Manchester, United Kingdom.Margaret Hartley 1,2 * and John Maclennan 2 ![]()
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