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

1. Bubble nucleation in H2O-CO2-bearing basaltic melts: results of high temperature decompression experiments Nolwenn Le Gall1, Michel Pichavant1 1ISTO/CNRS, Orléans, France VUELCO conference Barcelona

2. Degassing of basaltic magmas (Task 4.1) ΔPhn ΔPhn = [ 16 πσ3 / 3 k T ln (J/J0) ]0.5 Volatiles initially dissolved into the melt During decompression volatile supersaturation increases until surface tensions are overcome → Homogeneous bubble nucleation

3. Diffusion-controlled decompression Vesiculation-controlled decompression Pichavant et al., submitted • Melt H2O contents: Equilibrium concentrations No correlationwithvesicularity • Melt CO2 contents: Equilibrium and disequilibrium concentrations Correlationwithvesicularity → Decompressionregimecontrastedwhengasbubbles are present/absent • Need of experimental information on the mechanisms of bubblenucleation in basalticmelts

4. Experimental simulation of basaltic magma ascent and vesiculation… 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 → To simulate the behaviour of H2O and CO2 during degassing → To obtain information on the conditions of homogeneous bubble nucleation (=f(P, ascent rate, dissolved H2O/CO2)) → To test equilibrium vs. disequilibrium degassing (interpretation of gas data) and explore the implications for the generation of explosive eruptions

5. Experimental methodology: startingmaterialpreparation • Starting material: natural basaltic pumice from Stromboli → Fusion: 3h - 1 atm - 1400°C → Glass coring: ø 2.5 mm, l up to 10 mm

6. Experimental methodology: equipment and runprocedure • Synthesis of the H2O-CO2-bearing basaltic glasses • Use of AuPd capsules • 3 differentdissolved H2O/CO2: XH2Oin. = 1 (#1) XH2Oin. = 0.55 (#2) XH2Oin. = 0 (#3) (XH2Oin. = H2O / (H2O + CO2)Charge) • > 40h - 200 MPa- 1200°C Glass Distilled H2O Ag2C2O4

7. Experimental methodology: equipment and runprocedure T = 1200°C Pin ∆P vramp = 1.5 m/s Pf Rapid quench Pinassumed = Psat Pf investigated = 200, 150, 100, 50 and 25 MPa Decompression experiments

8. Analytical techniques • FTIR spectroscopy → Concentrations and spatial distributions of H2O and CO2 • X-ray microtomography → Textures (vesicularity, bubble number, size and density) 1 2 3 4 5 1mm 1mm 1mm

9. Experimental results: H2O and CO2 concentrations of pre-decompression glasses Group #1 Group #2 Group #3 0.65 wt% 1033 ppm 2.37 wt% 805 ppm 5.07 wt% 0 ppm 5.03 wt% 0 ppm 5.06 wt% 0 ppm 2.36 wt% 886 ppm 0.65 wt% 995 ppm 0.65 wt% 1052 ppm 2.37 wt% 772 ppm P = 200 MPa T = 1200°C t = 50h 1mm • Homogeneous volatile contents and distributions • 3 compositional groups consistent withtheir XH2Oin conditions of synthesis

10. Experimental results: textures and vesicularities of post-decompression glasses • Glasses decompressed to Pf > 50 MPa: Bubble-poor • Glasses decompressed to Pf < 50 MPa: Bubble-rich Phn No homogeneous bubble nucleation ΔPhn Psat

11. Experimental results: volatile concentrations of group #1 post-decompression glasses H2Omelt↘ with Pf

12. Experimental results: volatile concentrations of group #1 post-decompression glasses Pre-decompression glasses S3 S4 S5 S8 Post-decompression glasses Analysis? Synthesis? H2Omelt↘ with Pf

13. Experimental results: volatile concentrations of group #3 post-decompressionglasses CO2 progressively ↘ with Pf (at ~ constant H2Omelt)

14. Experimental results: volatile concentrations of group #3 post-decompressionglasses CO2 progressively ↘ with Pf (at ~ constant H2Omelt)

15. Experimentalresults: volatile concentrations of group #2 post-decompression glasses • H2O and CO2 contents ↘ with Pf Pre-decompression glass S3 Post-decompression glass Pf = 150 MPa

16. Experimentalresults: volatile concentrations of group #2 post-decompression glasses • H2O and CO2 contents ↘ with Pf Pre-decompression glass S4 Post-decompression glass Pf = 100 Mpa Pf = 50 Mpa

17. Experimentalresults: volatile concentrations of group #2 post-decompression glasses • H2O and CO2 contents ↘ with Pf Pre-decompression glass S5 Post-decompression glass Pf = 25 MPa

18. Discussion 200 MPa Pre-decompression glasses #1 #2 #3 • Groups #1 and #3 decompression paths similar to theoretical decompression paths • Group #2 decompression paths differ from theoretical decompression paths • Final CO2 contents exceed the equilibrium concentrations 100 MPa Closed-system equilibrium decompression 50 MPa

19. Conclusions Homogeneous bubble nucleation occurs between 50 and 25 MPa → Large supersaturations are required: ∆Phn = 150 - 175 MPa Both equilibrium and disequilibrium decompression paths can be obtained Final glass compositions can be CO2 supersaturated

20. Thankyou for your attention