Nolwenn Le Gall1, Michel Pichavant1

1ISTO/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

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ΔPhn

ΔPhn = [ 16 π σ3 / 3 k T ln (J/J0) ]0.5

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Melt H2O contents: Equilibrium concentrations

No correlation with vesicularity

Melt CO2 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 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

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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 - 1400°C

→ Glass coring: ø 2.5 mm, l up to 10 mm

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Synthesis of the H2O-CO2-bearing basaltic glasses

- Use of AuPd capsules

- 3 different dissolved H2O/CO2:

XH2Oin. = 1 (#1) XH2Oin. = 0.55 (#2)

XH2Oin. = 0 (#3)

(XH2Oin. = H2O / (H2O + CO2)Charge)

- > 40h - 200 MPa - 1200°C 6

Glass

Distilled H2O

Ag2C2O4

Decompression experimentsT = 1200°C

Pin

Pin assumed = Psat

Pf investigated = 200, 150, 100, 50 and 25 MPa

vramp = 1.5 m/s

Rapid quench

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Pf

∆P

FTIR spectroscopy→ Concentrations and spatial distributions of H2O and CO2

X-ray microtomography→ Textures (vesicularity, bubble number, size and density)

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1mm 1mm

1 2 3 4 5

1mm

Homogeneous volatile contents and distributions 3 compositional groups consistent with their XH2Oin conditions of synthesis

5.07 wt%0 ppm

5.03 wt%0 ppm

5.06 wt%0 ppm 2.36 wt%

886 ppm2.37 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 #2 Group #3

1mmP = 200 MPaT = 1200°Ct = 50h

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Glasses decompressed to Pf > 50 MPa: Bubble-poor

Glasses decompressed to Pf < 50 MPa: Bubble-rich

Phn

No homogeneous bubble nucleation

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ΔPhn

Psat

H2Omelt ↘ with Pf

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H2Omelt ↘ with Pf

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Pre-decompression glasses S3 S4 S5 S8Post-decompression glasses

Analysis?

Synthesis?

CO2 progressively ↘ with Pf (at ~ constant H2Omelt)13

CO2 progressively ↘ with Pf (at ~ constant H2Omelt)14

H2O and CO2 contents ↘ with Pf

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Pre-decompression glass S3 Post-decompression glass Pf = 150 MPa

H2O and CO2 contents ↘ with Pf

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Pre-decompression glass S4 Post-decompression glass Pf = 100 Mpa Pf = 50 Mpa

Pre-decompression glass S5 Post-decompression glass Pf = 25 MPa

H2O and CO2 contents ↘ with Pf

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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

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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: ∆Phn = 150 - 175 MPa

Both equilibrium and disequilibrium decompression paths can be obtained

Final glass compositions can be CO2 supersaturated

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