Abstract
Magmatic density and viscosity exert fundamental controls on the eruptibility of magmas. In this study, we investigate the extent to which magmatic physical properties control the eruptibility of magmas from Iceland's Northern Volcanic Zone (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. We find a strong correlation between magmatic density and observed erupted volumes on the NVZ. 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. These magmas are buoyant with respect to the Icelandic upper crust. However, a number of small-volume eruptions with densities greater than typical Icelandic upper crust are also found in Iceland's neovolcanic zones. 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. This conclusion is in agreement with petrological constraints on the depths of crystallisation under Iceland.
Highlights
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)
We explore the relationship between physical properties and erupted volumes for magmas from Iceland’s Northern Volcanic Zone (NVZ)
This inference is in agreement with petrological constraints on the depth of magma chambers, which indicate that crystallization of picrites and highMg# basalts under Theistareykir occurs at depths of 15–30 km (Maclennan et al, 2001, 2003; Winpenny and Maclennan, 2011)
Summary
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). 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 ascend to their level of neutral buoyancy, resulting in an eruption if the liquid remains buoyant all the way to the surface. The presence of modally layered gabbroic sills intruding dunite in the Moho transition zone of the Semail ophiolite suggests that magmas can be stored at, and undergo crystallization at, depths far greater than their level of magmatic neutral buoyancy (Kelemen et al, 1997) before the residual liquids ascend and erupt. The lava shields include large-volume eruptions of more evolved basalts (e.g., Stóravíti, ∼30 km; Skjaldbreiður, ∼11 km3) and smaller volumes of dense picrite containing up to 20% olivine macrocrysts (e.g., Háleyjabunga, 0.013 km3)
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