Efficient outgassing of shallow magma bodies reduces the risk of explosive eruption. Silica-rich magmas are too viscous for exsolved gas bubbles to escape the system through buoyant forces alone, and so volatile overpressure is often released through deformation-related processes. Here we present a case study on magma-emplacement-related deformation in a shallow (500 m depth) rhyolite intrusion (the Sandfell laccolith, Eastern Iceland) to investigate the establishment and evolution of degassing and outgassing networks in silicic sub-volcanic intrusions. We observe viscous and brittle deformation features: from vesiculated flow bands that organized into 'pore channels' in the ductile regime, to uniform bands of tensile fractures (‘fracture bands’) that grade into breccia and gouge in the brittle regime. Through field mapping, structural analysis, and anisotropy of magnetic susceptibility (AMS), we show that the deformation spectrum, observed all over the laccolith, represents stages of degassing (viscous processes) and outgassing (brittle processes) that resulted in the formation of interconnected permeable networks through the growth and linkage of fracture bands. Areas with concentrations of higher degrees of brittle deformation are proximal to abruptly changing AMS fabrics and point to laccolith-scale strain partitioning in the magma linked to different stages of laccolith growth. The establishment of intrusion-scale permeable networks through the cumulation of discrete magma fractures would have profoundly assisted the outgassing of the entire laccolith. Therefore, fracture banding captures viscous and brittle processes working in tandem as an efficient outgassing mechanism, and should be considered in sub-volcanic intrusions elsewhere.

Volcanic and magmatic outgassing mechanisms can determine eruptive behavior of shallow silicic magma bodies. Most outgassing mechanisms proposed take place along conduit margins, where highest strain rates drive ascending magma to brittle failure. However, these mechanisms do not account for outgassing large volumes of magma away from the conduit walls. Here, we present a continuum of porosity preserved in the microcrystalline rhyolitic Sandfell laccolith, Eastern Iceland. Three stages in the continuum are described: porous flow bands, pore channels, and fracture bands. These deformation features are present throughout the entire exposed volume of the Sandfell laccolith in meter-long band geometries, ranging from mm- to dm-scale thickness, and interlayered with coherent, undeformed rhyolite. Using microstructural analytical methods, we show that emplacement-related deformation induced strain partitioning around a crystal content of 45% that resulted in the segregation of melt-rich and melt-poorer flow bands. Subsequent deformation induced by continued magma emplacement caused strain partitioning in the melt-rich flow bands. Depending on strain rate, different types of deformation features developed, through dilation or porosity redistribution (porous flow bands), cavitation (pore channels), or tensile fracture (fracture bands). Porous flow bands have permeability values ~4 orders of magnitude higher than undeformed rhyolite. Pore channels and fracture bands have much larger length scales, and so permeability increase dramatically in those systems. Hence, the abundance and interconnectivity of deformation features preserved in the Sandfell laccolith provided an efficient outgassing mechanism for the bulk of the intrusion. Outgassing due to viscous-brittle magma deformation during magma emplacement should therefore be considered for crystal-rich magmas, e.g., during effusive lava dome extrusion.

Demand for metals is increasing with the advancement of technology. Silicic magmas contain metals of interest in trace quantities which need to be released from the magma and concentrated by many orders of magnitude to form economically viable mineral deposits. Mechanisms that can act on such large volumes of magma and allow metal-rich fluids to move through the magma are keenly debated. Here we present evidence from the Sandfell laccolith, an exposed rhyolitic magmatic intrusion in Eastern Iceland. Sandfell hosts a rarely-preserved magmatic fracture network that contains extensive mineralization of hydrous Fe-silicates, hydrous Mn-oxides, and La-oxides. The bands of tensile fractures are pervasive throughout the exposed intrusion and have, remarkably, survived 11 Ma of weathering processes post-emplacement. Their excellent preservation and exposure have allowed the first thorough documentation of what role magmatic fractures play in a metal-partitioning process. We propose that small-scale yet extensive fracturing of silicic magma during emplacement provides essential pathways for vapor-rich volatiles to decouple fluid-mobile elements from the melt, and sufficiently localize them for later transport into high-grade veins. Understanding the processes behind metal partitioning and fluid transport in the magmatic environment will increase the efficiency of mineral exploration, contribute to geothermal applications, and shed light on degassing mechanisms occurring at depth under active and hazardous volcanoes today.

The rheology of magma largely dictates its ability to release pressurized volatiles through outgassing. Degree of outgassing governs explosive potential of silicic magma in the shallow crust. Rheological studies have been conducted to constrain the outgassing mechanisms in magmas with one, two, or three phases (pure melt, crystal or bubble suspensions, and crystal and bubble suspensions). While synthetic materials reduce uncertainties of results, applying the same methods to natural material allows for comparison to deformation textures preserved in nature. Here we deform natural rhyolite from the Sandfell laccolith, Eastern Iceland in torsion, in an attempt to recreate tensile fracture arrays that have previously been created on synthetic HPG8 melt by the same method. We used a Paterson-type torsion apparatus to deform partially melted (3-phase) rhyolite at 900 and 850 degrees C at controlled twist-rates (constant strain rate). We performed 9 successful experiments at strain rates between 2.92 x 10^-5 and 1.87 x 10^-3 s^-1 and reaching a final strain of ~1. Mechanical results show steady-state viscous deformation took place in all experiments, with apparent viscosity decreasing and maximum stress generally increasing with increasing strain rate. Preliminary sample imaging has shown strain partitioning between bubbly melt and ‘stiff’ clusters of microlites, and approximate crystal and bubble fractions of 0.26 and 0.03, respectively. We compared our results to rheological models of crystal suspensions, and crystal + bubble suspensions. Relative viscosity is higher than crystal suspension models at 0.26 crystal fraction, which may be explained by the influence of bubbles. The microlite clusters ‘rolling’ through melt channels may have also contributed to the apparent viscosity being higher than expected at 0.26 crystal fraction. We propose future work to simulate natural rhyolite fracturing by lowering deformation temperature to increase melt viscosity, increasing crystal fraction, and running experiments to higher final strain values.