Absolute healing of pyroclasts during rheomorphic welding of ignimbrites in the Snake River Plain, USA
The architectural description of ignimbrites often shows evidence for post-deposition development of a rheomorphic, ductile shear zone - a feature which may strongly affect the progression of pyroclastic flows; especially in large volcanic fields. Rheological experiments were performed on a welded rheomorphic unit from the Grey's Landing ignimbrite in the Snake River Plain to characterize its behaviour and assess the degree of welding. The investigated sample contains 5 vol.% open pores and is made of approximately 5 vol.% crystals bathing in a relatively degassed, peraluminous glass containing 79 wt.% SiO2. Pre-eruptive temperature determination from geothermometry on pyroxenes yielded values at around 900-1050 C. Dilatometric measurements suggest a calorimetric glass transition temperature during deposition of approximately 845 C and a H2O content of approximately 0.04 wt.%. Repeated series of heating and cooling using an advanced dilatometric technique shows an increase of the glass transition temperature to 880 C, which is in accordance with degassing of approximately 0.02 wt.% H2O. Complementary investigation using a uniaxial press revealed an absence of strain rate dependence of the viscosity (1010.78 Pa·s) at a temperature of 900°;C and at strain rates up to 2.5 x 10-4 s-1. Under similar conditions, a fully degassed lava with an equivalent composition would yield a comparable viscosity of 1010.89 Pa·s. Our findings may help constrain the flare up of the Grey's Landing ignimbrite. The presence of small amounts of water in the glass and the narrow temperature window between the residence in the reservoir and the transition to a glass (which would have mechanically locked this unit in place) in the flow indicates a high discharge rate and rapid post-fragmentation deposition, mass agglutination and welding. Moreover, the Newtonian character of this welded unit suggests that healing of the pyroclastic flow was absolute (that is, no thixotropic effects from the pores remain), and thus that the term 'lava-like' is adequate to rheologically describe rheomorphic pyroclastic flows.
Measuring Coupled Rock/Pore-Fluid Interaction in Volcanic Rock under Simulated Volcano- Tectonic Conditions
We report a suite of laboratory experiments in which basalt from mount Etna volcano is deformed and fractured using a triaxial deformation apparatus. Experiments were monitored using an array of piezoelectric transducers around the sample to permit full-waveform capture, microseismic (AE) location and analysis. A small conduit of 3.125 mm diameter was drilled axially to allow access of the pore water directly to and from the shear/damage zone. Location of AE events through time shows the formation of a classical ~30 degree fault plane and waveforms exhibit the typical high frequency characteristics of volcano-tectonic (VT) earthquakes when experiments are conducted in dry samples. With water saturated samples, both VT events and some Hybrid events are observed (exhibiting a high frequency initial onset with lower frequency coda). A subsequent rapid decompression of the water-filled pore volume and damage zone triggered swarms of Low Frequency events, analogous to Volcanic Long Period seismicity. Analysis of the frequency component show characteristic low-frequencies (~90 kHz) associated with pore fluid decompression; first motion analysis reveals these events to exhibit a low percent component of shear (double-couple) slip, consistent with recent field data. By repeating these experiments at elevated temperatures (up to 175ºC), we additionally observe more complex, coupled hydro-mechanical behaviour and long-lasting seismic events analogous to volcanic tremor. Quasi-continuous waveforms are observed during the water ‡ steam phase change at approximately 0.8 MPa, as fluid/gas is vented through the damage zone to atmosphere. Frequencies during this stage of the experiment are lower still, at approximately 25 kHz. Using a simple size-frequency scaling relation, our laboratory-measured frequencies and crack dimensions can be shown to scale well to observations of sources of the order of hundred of metres to kilometers, typical of those in volcanic edifices. Our data suggest that LF events in volcanic areas are likely to be generated through the interaction of hydrothermal fluids moving through a combination of pre-existing microcrack networks and larger faults, such as those we observe in forensic examination; and that volcanic tremor is likely to be enhanced by elevated temperature conditions leading to phase change in magmatic fluids.
Can Rock Deformation Experiments Help us to Forecast Volcanic Eruptions?
Volcanic eruption forecasting models show that the strength and mechanical properties of volcanic rocks are a primary control on the behaviour of volcanic systems, especially during the approach to eruptions. The progressive failure of these rocks, recorded as sequences of small volcano tectonic earthquakes, can lead to the formation of new magma pathways, allowing eruptions to begin at volcanoes that have not erupted in hundreds of years. Rates and patterns of these small earthquakes, which are typically located within a few km of the volcano summit, are monitored at many volcanoes and used to forecast eruptions. Models of crack growth and interaction have been used to constrain expected patterns in accelerating earthquake rates before the first eruption after a long repose interval. These deterministic models rely on laboratory mechanical data from volcanic rocks tested under the temperature and pressure conditions expected within and beneath a volcanic edifice. Here, we present data from high temperature uniaxial and triaxial deformation of andesite and dacite at temperatures of up to 1000°C, and under confining pressures of up to 50 MPa. These are typical rock types for volcanoes likely to erupt violently after hundreds of years of repose, where these forecasting models have previously been applied. The conditions tested cover the full range of temperatures and pressures expected within the upper 2-3 km of volcanic systems. The eruption forecasting model, with new constraints from this laboratory data, is applied to sequences of VT earthquakes before eruptions and to sequences of acoustic emissions before laboratory sample failure. Both types of data showed accelerating trends within the limits defined in the model, whilst sequences of earthquakes and acoustic emissions not resulting in eruption or sample failure exhibited accelerations outside the model limits. These results support the scaling of laboratory data to field scale and the use of laboratory data to constrain eruption forecasting models.
Fabric Development in a Late-Hercynian Magmatic Strike-Slip Shear Zone in Southern Corsica: Indications of Melt-Supported Large-Scale Deformation Localization
The calc-alcaline granitoids of the Hercynian Corsica Batholith show a large-scale magmatic flow pattern, outlined by the alignment of large (mm-cm) euhedral feldspar crystals. The trend of the steep magmatic foliation is generally N-S in the northern part of the island, swings to approximately E-W orientation in the central part of the Batholith and back again to approximately N-S orientation in the southern part. This pattern is intensified by large-scale magmatic layering, mainly kilometer long lenses and layers of mafic and intermediate intrusions into the granitoids. On the macro- to micro-scale, magma mingling and mixing are present, reflecting the complex intrusion history and the compositional variability of the Corsica Batholith on different scales. Around the Golf of Valinco, a steep, sinistral magmatic shear zone is represented by E-W trending magmatic layering in mingled dioritic, tonalitic, and granitic magmas - previously misleadingly interpreted as migmatites - and by a magmatic flow foliation formed by the alignment of platy feldspar crystals, as well as amphibole and biotite. Characteristic magmatic structures include multiple thin layering, boudinage, monoclinic folding, melt-injected micro shear zones, and fragmenting and back- veining of dioritic enclaves. The intensity of grain alignment roughly correlates with the thickness of layers. It is low in thick and short boudins and high in cm-thin and cm-m long layers. The monoclinic folds refold the magmatic layering. Flat faces of amphibole and biotite grains are aligned in the axial planes of the folds. The feldspar crystals are locally recrystallized to a few large polygonal grains (up to 1 mm across), and quartz commonly shows chessboard subgrain patterns. No further indications of solid-state deformation are present. Field observations, as well as pattern quantification on variably oriented rock surfaces, indicate variations of crystal alignment and fabric anisotropy in cm- to more than 100m-wide bands parallel to the E-W oriented layering, and various stages of melt-present fragmentation. These variations are interpreted as variations of flow intensity and possibly strain-rate variation. The observations on the macro- as well as the micro-scale point to repeated injection of mafic to felsic magma and crystallization in the presence of a regional stress field. The resulting km-scale sinistral, sub-horizontal synmagmatic shear zone reflects large-scale movements during late-Hercynian crustal reorganization and represents an excellent example of localization of deformation into magma-enriched parts of the continental crust.
Syntectonic Flow and Crystallization Processes Investigated by Qualitative and Quantitative Fabric Analysis: the Piquiri Syenite Massif (Southern Brazil)
Macro- and microstructures of syntectonic granitoids provide numerous information about (i) crystallization, deformation and rheology history of these rocks, (ii) development of fabric anisotropy and heterogeneity during flow processes, and (iii) relationships between local and regional kinematics. The Piquiri Syenite Massif (611+-3 Ma) is part of the post-collisional magmatism related to the Neoproterozoic Brasiliano-Pan-African Orogenic Cycle in southern Brazil. The approximately 150 km2 large pluton is composed of mainly coarse- grained perthite syenites and quartz-syenites and was intruded by high-grade gneisses, syntectonic monzonites and granites, medium-grade metapelites, and acid metavolcanic rocks. In the center of the pluton, co-genetic granites occur with gradational contacts. Border features include local development of chilled margins in the pluton, as well as hornfelses and brecciation of metamorphic wall rocks which are cemented by syenitic or granitic material. The Piquiri Syenite Massif shows several mesoscopic magmatic structures, such as cm-dm thick, locally boudinaged and folded mafic layering, mafic microgranular enclaves, and schlieren structures. Mafic layers are enriched in Cpx and Hb and partly formed by flow segregation. Platy, mm-cm sized K-feldspars as well as pyroxenes and amphiboles form a steep magmatic foliation, locally sub-parallel to the pluton's border. Based on a series of outcrop photographs from three perpendicular cuts, scans of oriented rock slabs, and thin sections, both structures have been analyzed and quantified by modified and automated conventional fractal-geometry methods. The anisotropy obtained on the foliation plane reveals the sub-horizontal orientation of a weak magmatic lineation, not visible in the field or in thin section. The magmatic foliation is oriented parallel to the outer rims of the pluton and locally anastomoses around sub-vertical axes. Together with the lineation, these features indicate sub-horizontal magmatic flow and, at least locally, shearing during emplacement of the pluton. On the micro-scale, nearly no solid-state deformation structures are visible in any of the different minerals. In quartz, a weak, coarse chessboard subgrain pattern is locally developed. Even where the syenite comprises up to 90% well-aligned K-feldspars, neither magmatic fracturing nor high-T deformation structures are visible. This, together with overgrowth structures, indicates that during magmatic flow feldspars were only present to a minor extent or with much smaller sizes. In general, the Piquiri Syenite Massif is a syntectonic pluton with strong flow structures that develop during an early period of emplacement of a crystal-melt mush and, subsequently, static microstructures indicating stress decrease during cooling and final crystallization. The emplacement is related to weak sub-horizontal shearing along NNE-trending transcurrent brittle-ductile shear zones of the Southern Brazilian Shear Belt.