Heidy Mader, Michael Burton, Margherita Polacci. A combined physico-chemical model for passive degassing from Stromboli volcano.

Transcription

1 Heidy Mader, Michael Burton, Margherita Polacci A combined physico-chemical model for passive degassing from Stromboli volcano.

2 2 Degassing at Stromboli Degassing from basaltic volcanoes is the most common form of volcanic activity on Earth. Eruption Style: persistent, non-explosive, background degassing passive degassing interspersed with episodes of explosive activity active degassing Image: Passive degassing accounts for >99% of total gas emissions and is the primary source of Earth s atmosphere. Stromboli: SO 2 fluxes ~ 400tonnes/day, implying complete degassing of ~35,000m 3 /day of magma AIM: Understand mechanisms that would allow this volume of magma to ascend, degas passively (non-explosively) and then descend.

3 3 Passive Degassing conduit convection Image: Stromboli online Key Question: Transition between closed-system and open-system degassing. Ex: Stevenson and Blake, BV 1998

4 4 Conceptual Physico-Chemical Model of Passive Degassing fluid-dynamical model of conduit convection + constraints petrology: exsolved volatile content [Métrich et al, J. Petrol. 2001; Bertagnini et al, JGR 2003; Landi et al, Contrib. Mineral. Petrol. 2004] scoria textures: nature of shallow magma vesiculation [Lautze & Houghton, Geology 2005; Polacci et al, Geology 2006] gas geochemistry: closed- versus open-system degassing magma supply rate

5 5 Petrology: exsolved volatile content primitive, uncrystallised shallow, degassed (50% crystallised) viscosity Pas 14,000 Pas density 2500 kgm kgm -3 temperature 1145 C 1115 C [Métrich et al, J. Petrol. 2001; Bertagnini et al, JGR 2003; Landi et al, Contrib. Mineral. Petrol. 2004]

6 6 Gas Geochemistry: magma supply rate magma density 2500kgm 3 hence 1m 3 magma exsolves 5.68kg of S oxidizes to 11.35kg of SO 2 SO 2 flux of 4.6kg/s requires complete degassing of 0.41m 3 /s of primitive magma (~35,000m 3 /day) SO 2 fluxes on Stromboli observed to be around 400 tonnes/day ~4.6 kg/s

7 7 Gas Geochemistry: closed- versus open-system degassing Molar ratio for complete closed-system degassing: in closed-system degassing: FTIR measurements on Stromboli:

8 8 Scoria Textures: nature of shallow magma vesiculation products from mild explosive activity show no evidence of post-eruptive degassing processes gas volume fraction ~0.5 to ~0.65 bubble number densities m -3 >0.80 of the bubble volume fraction is interconnected clear evidence of interconnected bubble network open-system

9 9 Permeability: transition from closed- to open-system degassing permeability= f(gas volume fraction) percolation threshold is reached for gvf~0.30 maximum permeability reached for gvf~0.50 (for spherical bubbles regardless of size distribution or shape) [Rintoul & Torquato, J. Phys. A: Math. Gen. 1997; Sur et al, J. Stat. Phys. 1975] gvf~0.35 at 80MPa depth ~3000m VOLATILECALC calculates the equilibrium dissolved amount of H 2 O and CO 2 as a function of pressure during closedsystem degassing [Newman & Lowenstern, Comp. Geosci. 2002] gvf~0.50 at 45MPa depth ~1700m

10 10 Pre-Transitional Flow Dynamics magma flux down: 0.38m 3 /s Including gas volume fraction: where is the density contrast between the rising (vesicular) and sinking (dense, degassed) magma. Stevenson & Blake (BV 1998) show experimentally that for high viscosity contrasts buoyant material rises centrally. 14,000Pas 2700kgm -3 FLOW MODELLING RESULTS: Average magma rise speed between 200MPa (7.3km) and 45MPa (1.6km) is ~0.2m/s implying ascent duration of ~8hours. 20Pas 2500kgm -3 magma flux up: 0.41m 3 /s A unique conduit radius produces the required primitive magma flux of 0.41m 3 /s at 45MPa of R~1.47m.

11 11 Shallow, Open-System Degassing Region HCl and HF degassing at <15MPa [Metrich et al, EPSL 2004] halogen stripping volatile budget requires loss of 80% Cl: 1m 3 of magma produces ~13m 3 of HCl at 0.1MPa [Burton et al, Is there a physical control on halogen degassing at Mt Etna? Monday afternoon Session V14B on volatiles.] hypothesis: The apparent closed-system degassing signature seen in SO 2 /HCl ratios on Stromboli is the result of the superposition of a deep-sourced SO 2 - rich gas flux and a shallow-sourced HCl-rich gas flux. flow modelling estimate a conduit radius of ~1.63m sufficient to produce the required flux, slightly larger than ~1.47m for the conduit below the transition

12 12 Schematic Model

13 13 Thank you!