Nolwenn Le Gall 1, Michel Pichavant 1 1 ISTO/CNRS, Orléans, France VUELCO conference Barcelona

 Volatiles initially dissolved into the melt  During decompression volatile supersaturation increases until surface tensions are overcome → Homogeneous bubble nucleation 2 ΔP hn ΔP hn = [ 16 π σ 3 / 3 k T ln (J/J 0 ) ] 0.5

3  Melt H 2 O contents: Equilibrium concentrations No correlation with vesicularity  Melt CO 2 contents: Equilibrium and disequilibrium concentrations Correlation with vesicularity → Decompression regime contrasted when gas bubbles are present/absent  Need of experimental information on the mechanisms of bubble nucleation in basaltic melts Diffusion-controlled decompression Vesiculation- controlled decompression Pichavant et al., submitted

→ To simulate the behaviour of H 2 O and CO 2 during degassing → To obtain information on the conditions of homogeneous bubble nucleation (=f(P, ascent rate, dissolved H 2 O/CO 2 )) → To test equilibrium vs. disequilibrium degassing (interpretation of gas data) and explore the implications for the generation of explosive eruptions 4 Times series of volcanic gas amounts and radiating source temperature measured during quiescent and explosive degassing on Stromboli (9 april 2002) Burton et al., 2007

 Starting material: natural basaltic pumice from Stromboli → Fusion: 3h - 1 atm °C → Glass coring: ø 2.5 mm, l up to 10 mm 5

 Synthesis of the H 2 O-CO 2 - bearing basaltic glasses - Use of AuPd capsules - 3 different dissolved H 2 O/CO 2 : XH 2 O in. = 1 (#1) XH 2 O in. = 0.55 (#2) XH 2 O in. = 0 (#3) (XH 2 O in. = H 2 O / (H 2 O + CO 2 ) Charge ) - > 40h MPa °C 6 Glass Distilled H 2 O Ag 2 C 2 O 4

 Decompression experiments T = 1200°C P in P in assumed = P sat P f investigated = 200, 150, 100, 50 and 25 MPa v ramp = 1.5 m/s Rapid quench 7 PfPf ∆P∆P

 FTIR spectroscopy → Concentrations and spatial distributions of H 2 O and CO 2  X-ray microtomography → Textures (vesicularity, bubble number, size and density) 8 1mm

 Homogeneous volatile contents and distributions  3 compositional groups consistent with their XH2O in conditions of synthesis 5.07 wt% 0 ppm 5.03 wt% 0 ppm 5.06 wt% 0 ppm 2.36 wt% 886 ppm 2.37 wt% 772 ppm 2.37 wt% 805 ppm 0.65 wt% 1033 ppm 0.65 wt% 995 ppm 0.65 wt% 1052 ppm Group #1 Group #2Group #3 1mm P = 200 MPa T = 1200°C t = 50h 9

 Glasses decompressed to P f > 50 MPa: Bubble-poor  Glasses decompressed to P f < 50 MPa: Bubble-rich P hn No homogeneous bubble nucleation 10 ΔP hn P sat

 H 2 O melt ↘ with P f 11

 H 2 O melt ↘ with P f 12 Pre-decompression glasses S3 S4 S5 S8 Post-decompression glasses Analysis? Synthesis?

 CO 2 progressively ↘ with P f (at ~ constant H 2 O melt ) 13

 CO 2 progressively ↘ with P f (at ~ constant H 2 O melt ) 14

 H 2 O and CO 2 contents ↘ with P f 15 Pre-decompression glass S3 Post-decompression glass P f = 150 MPa

 H 2 O and CO 2 contents ↘ with P f 16 Pre-decompression glass S4 Post-decompression glass P f = 100 Mpa P f = 50 Mpa

Pre-decompression glass S5 Post-decompression glass P f = 25 MPa  H 2 O and CO 2 contents ↘ with P f 17

 Groups #1 and #3 decompression paths similar to theoretical decompression paths  Group #2 decompression paths differ from theoretical decompression paths  Final CO 2 contents exceed the equilibrium concentrations 18 Pre-decompression glasses #1 #2 #3 Closed-system equilibrium decompression 200 MPa 100 MPa 50 MPa

 Homogeneous bubble nucleation occurs between 50 and 25 MPa → Large supersaturations are required: ∆P hn = MPa  Both equilibrium and disequilibrium decompression paths can be obtained  Final glass compositions can be CO 2 supersaturated 19

20