Fracture of andesite in the brittle and brittleductile transition regimes

Transcription

1 Fracture of andesite in the brittle and brittleductile transition regimes R. Smith 1,*, P.R. Sammonds 1, C.R.J. Kilburn 1, H. Tuffen 2,1 1. Department of Earth Sciences, UCL, Gower Street, London, WC1E 6BT 2. Department of Environmental Science, Lancaster University, Lancaster, LA1 4YQ * rosanna.smith@ucl.ac.uk

2 Outline Why is it important to understand the fracture mechanics of andesite in the brittle and brittle-ductile transition regimes? Evidence of widespread fracturing in lava domes, magma conduits and country rock in volcanic edifices How do we study this? Description of experimental equipment and methods Experimental results Strength and rheology Acoustic emissions Fracture surfaces Implications

3 Fracturing of lava domes Dome and spine complex at Soufriere Hills, Montserrat, Jan 1996 Lava spine from Unzen, Japan 1991 eruption covered in cataclasite (Nakada, 1999) Brittle-ductile shear textures (Cordonnier et al., in prep.) Extruding spine at Mount St Helens, USA, April 26

4 Fracturing of magma conduits Exposed rhyolitic conduit at Torfajokull, Iceland (from Tuffen et al., 23 and Tuffen and Dingwell, 25). Networks of shear-tensile fractures in obsidian, 5 m long Filled by obsidian ash with complex sedimentary structures Flux of fluidised gas-particle mixture through fractures Formed by shear fracture of lava Fracture and ductile textures (Tuffen et al., 23 and Tuffen and Dingwell, 25) Thorough sintering and welding faults healed Cohesive viscous deformation of healed cataclasite Flow banding generated Shows that faulting in eruptible magma

5 Seismic observations Jun-8 Aug-8 Oct-8 Dec-8 Feb-81 Apr-81 Jun-81 Aug-81 Oct-81 Dec-81 Feb-82 Apr-82 Jun-82 Aug-82 Oct-82 Dec-82 Feb-83 b) Feb-84 Apr-84 Jun-84 Aug-84 Oct-84 Dec-84 Feb-85 Apr-85 Jun-85 Aug-85 Oct-85 Dec-85 Feb-86 Apr-86 Jun-86 Aug-86 Oct-86 Dec-86 No. of earthquakes Number of VT earthquakes per day Number of VT earthquakes per day Eruption Mount St Helens, USA, lava dome growth Number of earthquakes per day during six years of intermittent lava dome extrusion from from June 198 until December 1986 Earthquakes located within 2 km depth and lateral distance of the lava dome (PNSN) a) 2-May 24-May 28-May 1-Jun 5-Jun Mount Pinatubo, Philippines, 1 st eruption after long repose interval Cumulative earthquake count before the 7 th June 1991 eruption, the first eruption after 5 years of repose Earthquakes located within 5 km depth and 3 km lateral distance of the vent (Hoblitt et al., 1996)

6 Laboratory Deformation Apparatus ACTUATOR APPLYING AXIAL LOAD Top Reaction Plate 2kN Actuator Internal Load Cell Cooling Rings Reaction Columns Pressure vessel Hydraulic Clamping Cylinder INSULATING FILLER AE WAVEGUIDE LOAD CELL TOP PYROPHILLITE ENCLOSING DISC ALUMINA COIL SUPPORT ACTUATOR NOSE 75. MM PRESSURE VESSEL 96. MM I.D MM O.D MM SLIGHTLY CONCAVE SURFACE TOP STEEL Fv52 PISTON WITH SLIGHTLY CONVEX TOP SURFACE, 25.MM DIA Internal Furnace Clamping Plates ALUMINA ROD 39. MM 25. MM DIA. PORE FLUID TUBE 1.5 MM DIA. WOUND HEATING ELEMENTS ROCK SPECIMEN 75. MM 25. MM DIA. Bottom Reinforcement Plate STEEL F.V.52 PISTON 78. MM 25. MM DIA. FIBROUS ALUMINA INSULATION BOTTOM PYROPHILLITE ENCLOSING BLOCK Temperatures up to 1 C Confining Pressure up to 5 MPa Axial pressure up to 45 MPa Strain rate from 1-6 to 1-3 Rocchi et al., 24, JVGR PRESSURE VESSEL I.D. 96. MM O.D MM BOTTOM PLUG 316 STAINLESS STEEL AE WAVEGUIDE HIGH PRESSURE FITTING

7 Ancestral Mount Shasta Andesite Reasonably isotropic and homogenous andesite/ dacite with no distinct flaws required. Ancestral Mount Shasta andesite from Northern California, USA, freshly exposed on road cutting. Isotropic, porphyritic texture, 2% phenocrysts (<2mm), 7% porosity, <1% glass. 61.6% SiO 2, % Al 2 O 3, 5.59% CaO, 3.62% Na 2 O, 2.33% MgO, 1.58% K 2 O,.19% P 2 O 5, <.1% SO 3

8 Experimental Conditions 1-5 s -1 strain rate this is within the typical rates of lava dome, conduit, and edifice deformation of 1-7 s -1 to 1-4 s -1 (Tuffen et al., 23, Rust et al., 23) Temperatures of 25ºC, 3ºC, 6ºC, and 9ºC Uniaxial and triaxial with confining pressures of 1 MPa, 3 MPa, and 5 MPa Dry with atmospheric pore pressure Nitrogen confining medium Recording AE

9 Deformation of Ancestral Mount Shasta Andesite peak differential stress (MPa) Young's Modulus (GPa) uniaxial MPa confining pressure MPa confining pressure 5 Mpa confining pressure 12 3MPa, load rate controlled uniaxial 1 MPa confining pressure 3 MPa confining pressure 5 MPa confining pressure 3MPa, load rate controlled Temperature (deg C) Peak differential stress (MPa) 4 Temperature (degc) Room temp to 6C 2 9C P wave velocity (km/s)

10 Uniaxial Compression, brittle field Stress (bar) and AE hit rate ºC Axial Stress AE hits per 5 sec Cumulative Energy AE Energy Release Stress (bar) & AE hit rate Time (sec) 3ºC Axial Stress AE hits per 5 sec Cumulative Energy AE Energy Release Stress (bar) & AE hit rate Time (sec) 6ºC Axial stress AE hits per 5 sec cumulative energy Time (sec) AE Energy Release

11 Triaxial Compression, brittle field ºC, 3 MPa 2 Differential Stress (MPa) & AE hits/ sec 5 1 Time (seconds) 3 Cumulative AE Energy Units b values Differential Stress (MPa) & AE hits/ sec Cumulative AE Energy Units Time (sec) b values Differential stress (MPa) & AE hits/ sec Time (seconds) 5 Cumulative AE Energy Unit b values Differential Stress (bar) & AE hits/ sec Cumulative AE Energy Units a) 3 ºC, 1 MPa b) 6 ºC, 1 MPa c) 6 ºC, 1 MPa time (sec) b values d) 25 ºC, 3 MPa ºC, 1 MPa e) 25 ºC, 5 MPa Differential stress (MPa) & AE hits/ sec Cumulative AE Energy Units Time (seconds) Differential stress AE hit rate b values Cumulative AE Energy b values

12 Uniaxial and triaxial compression in the brittle-ductile transition 1 9 Differential Stress (MPa) ºC Uniaxial compression Axial Strain (%) 1 9 axial stress cumulative energy 1 9 9ºC Triaxial compression 1 MPa confining pressure stress (MPa) AE hits per 2 sec b values AE hit count & b value x time (sec)

13 Preliminary SEM results Brittle fracture surfaces Brittle - ductile fracture surfaces

14 Implications Flaws too small to detect in acoustic velocity measurements may dominate the strength of crystalline igneous rocks making it difficult to discern changes in rheology due to temperature or strain rate from sample variability AE precursors to sample failure were not consistent for similar or the same experimental conditions The more consistent precursors seen before lava dome eruptions may relate to the interaction of more scales of cracking and/ or the magma feeding system geometry At the high strain rates observed in volcanic systems, earthquakes may occur in hotter material than that considered seismogenic in traditional earthquake science. Andesite country rock within a volcanic edifice would have to be within 1-2m 1 of the magma conduit to leave the brittle regime, especially if strain rates are high Brittle shear fracture is a plausible source for hybrid earthquakes in volcanic conduits and lava dome systems

15 Future Work We are currently redesigning the high temperature triaxial deformation apparatus in order to achieve: Improved temperature control In order to look more closely at changes in behaviour with small changes in temperature and strain rate within the brittle-ductile transition Full waveform recording of acoustic emissions Improve insulation so cooling system that masks AE is no longer necessary Embed AE transducers in pistons instead of at the end of long waveguides These waveforms can then be compared to volcanic earthquake characteristics Measurement of permeability and acoustic velocities under different hydrostatic conditions and during deformation Pore fluid inlets at either end of sample attached to permeameter