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Equilibres de phases et conditions de mise en place des magmas dans la cro û te

Equilibres de phases et conditions de mise en place des magmas dans la cro û te. Bruno Scaillet ISTO Orléans. Plan. Introduction, généralités Histoire du Vésuve Approche expérimentale Résultats Implications Contraintes de conductivités electriques. Why experiments?.

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Equilibres de phases et conditions de mise en place des magmas dans la cro û te

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  1. Equilibres de phases et conditions de mise en place des magmas dans la croûte Bruno Scaillet ISTO Orléans

  2. Plan • Introduction, généralités • Histoire du Vésuve • Approche expérimentale • Résultats • Implications • Contraintes de conductivités electriques

  3. Why experiments? To constrain P, T (at least) H2O, CO2 etc…. -Dry systems (MORBs at low P): MELTs, COMAGMAT -Wet systems: ?

  4. Even in dry… • Putirka et al, 2007: Tex at Hawaii= 268°C • Fallon et al, 2007 : Tex at Hawaii= 6°C • Tex = T hot spot - T MORB(P) • From Ol-Melt thermometry • (Chemical Geology, 241)

  5. The case of Vesuvius • Collaborations • Michel Pichavant - ISTO • Raffaello Cioni - Cagliari Univ • Paola Marianelli - Pisa Univ • Alessandro Sbrana - Pisa Univ • Priscille Lesne - ISTO, PhD • Anne Pommier - ISTO, PhD

  6. Vesuvius volcano • Vesuvius is an explosive volcano lying within a densely populated area: 700 000 peoples on the volcano slopes + 2 millions in Napoli • Alternation of effusive and explosive activity over the last 20 kyrs • In the past, several Plinian and sub Plinian eruptions have destroyed human settlements • last Plinian: Pompei 79 AD • last sub Plinian: 1631 (4000 died) • last eruption: 1944 • Since 1944: dormant volcano with fumerolles at T<100°C.

  7. Caldera rim: Produced by at least 4 Plinian eruptions: PdB - 17 kyrs Mercato - 8 kyrs Avellino -3.7 kyrs Pompei - 2 kyrs Vesuvius cone: Built since AD 79, Pompei

  8. Problems and objectives • Is there any magma chamber beneath the Vesuvius cone? • If yes, what is its size and depth? • If yes, when it is going to erupt again and how? Explosive or effusive? • Experimental approach: depth-temperature-H2O of magma storage and its evolution with time (past events)

  9. Current Vesuvius model • Reservoir in the shallow crust periodically refilled by input of K-rich basalts (tephrites) • Two possibilities: • During closed conduit conditions, a large chamber develops : phonolite reservoir feeding Plinian eruptions • During open conduit conditions, little fractionation occurs: strombolian-type activity with eruption of basalts (1906, 1944)

  10. Eruptive stratigraphy of VesuviusAndronico et al 1996

  11. Vesuvius reservoir on the basis of field and petrological evidence, Cioni et al., 1998, JGR Plinian (1-4 km3) Strombolian Effusive (0.01 km3) Sub-plinian (0.1-0.4 km3) felsic mafic Pollena 1631 1944 1906, Since 1631 Mercato Avellino Pompei

  12. Repose time and zoning pattern < 7 years 100-500 years 1000-5000 years Step-like continuous No zoning Plinian (1-4 km3) Strombolian Effusive (0.01 km3) Sub-plinian (0.1-0.4 km3) felsic mafic Pollena 1631 1944 1906, Since 1631 Mercato Avellino Pompei

  13. Magma compositions at Vesuvius <AD79 <AD79-8 kyrs< >8 kyrs

  14. Next expected eruption • Feeding rates of the last 8 kyrs is estimated to be 1-4 10-3 km3 /year • Since 1944 no eruption. • Volume of injected magma is 0.05-0.2 km3 or slightly less than a sub-plinian event (Pollena 472 AD, 1631) • Corresponds to a sphere radius of 360m • Seismic resolution ca 500 m.

  15. Current constraints on the pressure depth of magma storage Petrological evidence Past Fluid inclusions: 100-300 MPa Belkin et al., 1985 Melt inclusions: 100-600 MPa Marianelli et al., 1999 Lithics: 100-200 MPa Barberi and Leoni, 1981 Geophysical evidence Present day Gravimetry: no signal at depth < 6 km Magnetotelluric: no clear signal at depth < 6 km Seismic: yes, attenuation at 8 km (200 MPa Zollo et al. 1996 Auger et al. 2001

  16. Range of pressure from MI and FI: Decompression from common source or discrete levels of magma storage?

  17. Experimental Approach • Work on felsic (top of reservoir) or on most mafic (bottom) magmas of past eruptions • Establish hydrothermal phase equilibria of selected rocks • Compare experiments with observed phenocrysts in rocks • When match is good=> pre-eruption conditions of the top/bottom part of the reservoir • Test of crystal-liquid equilibrium in the reservoir

  18. Method • Perform crystallisation experiments under controlled P, T, fO2 and H2Omelt in internally heated pressure vessels • Error on P control : ± 1-2 MPa • Error on T control : ± 5°C • Quench, observation and analyses • EMPA, MEB, FTIR, KFT

  19. Target eruptions felsic mafic Mercato Avellino PompeiPollenaPollena Phono Phono PhonoPhono Teph-Phono SiO2 59.3 56.4 56.0 51.3 50.2 Al2O3 21.0 22.8 21.9 21.6 18.5 FeO 2.2 1.8 2.3 4.5 6.0 MgO 0.1 0.1 0.2 0.7 3.1 CaO 2.0 2.1 2.8 5.9 9.5 Na20 7.9 8.8 6.2 5.9 3.6 K20 6.9 7.7 10.2 9.4 8.1 TiO2 0.1 0.1 0.2 0.4 0.8 F 0.2 0.3 0.1 0.1 - total 100 100 100 100 100 Na2O/K20 1.2 1.1 0.6 0.6 0.4

  20. Mercato 775°C, 200MPa 6 wt H2O gt cpx feld glass

  21. Mercato-50 775°C, 250MPa 5 wt H2O Amph feld cpx glass

  22. Mercato-51 775°C, 250MPa 3 wt H2O Neph Feld glass Amph

  23. Mercato-isobaric-200 MPa minerals in rock: <10% -feld -amph -cpx -garnet -biotite

  24. Mercato-isothermal-800°C minerals in rock: <10% -feld -amph -cpx -garnet -biotite No solution

  25. Mercato-isothermal-775°C minerals in rock: <10% -feld -amph -cpx -garnet

  26. Pompei-isobaric-200 MPa minerals in rock: ca 15 % -feld -amph -cpx -garnet -biotite -leucite

  27. Pollena-isobaric-200 MPa minerals in rock: ca 10 % -leucite -amph -cpx -garnet -biotite -feld -nepheline No solution

  28. Pollena-isothermal-800°C minerals in rock: ca 10 % -leucite -amph -cpx -garnet -biotite -feld -nepheline

  29. summary

  30. Error on pressure via phase equilibria: depends on composition Example of Pompei: Amphibole in crystal-poor phonolite: ± 20 MPa

  31. Error on pressure via phase equilibria: depends on composition Example of Pollena: Coexistence of Neph-Cpx under near liquidus conditions: ± 10 MPa

  32. Experimental Approach - 2 • Work on felsic (top of reservoir) or on most mafic (bottom) magmas of past eruptions • Establish hydrothermal phase equilibria of selected rocks • Compare experiments with observed phenocrysts in rocks • When match is good=> pre-eruption conditions of the top/bottom part of the reservoir • Test of crystal-liquid equilibrium in the reservoir

  33. Phase equilibria of mafic end member: the Pollena eruption Depth of the bottom part of a sub-plinian reservoir Volume (DRE): 0.4 km3 Reservoir radius: ca 450 m Top-bottom difference: ca 1 km or 20-30 MPa

  34. cpx bt cpx Tephriphonolite, Pollena 1050°C, 200 MPa 5 wt H2O 950°C, 200 MPa 5 wt H2O

  35. Tephriphonolite, Pollena 1000°C, 200MPa 2 wt H2O cpx glass lc

  36. Tephriphonolite Po-21, 900°C, 150 MPa 2 wt H2O cpx leucite glass

  37. Phase equililibria of mafic Pollena

  38. Melt composition: effect of cpx 1100°C 1050°C 1000°C 950°C T zonation in Pollena reservoir : 800°C-1050°C

  39. Melt composition: effect of leucite 950°C 1000°C 1050°C 1100°C Pollena reservoir>50 MPa

  40. Melt compositions: from Pompei to Pollena Source for bulk rocks: Cioni et al., 1995, 1999; Marianelli et al., 1995; 1999; Rosi and Santacroce, 1987; Rosi et al., 1993 Shift in pressure goes along with shift in composition

  41. Pressure evolution with time ? PdB, Landi et al, 1999 Avellino Pompei Mercato 1631 Pollena ? 1944 ?

  42. Key questions • Origin of pressure shift? • Magma properties (density) • External stress field (regional tectonic) • Edifice load (plinian decapitation) • Others… • To what extent the study of old volcanic deposits can be used to forecast the style of next eruptive event if evolution is irreversible and not cyclic.

  43. Further constraints from conductivity measurementsPhD - Anne PommierANR-Electrovolc F Gaillard • Measurements on representative magmas of Vesuvius under various P, T, H2O, fO2 • Phonolite (Pompei) • Tephrite (1944) • Comparison with magnetotelluric surveys

  44. Increasing P, decreases s Increasing T, increases s Results • Influence of temperature • Influence of pressure

  45. Influence of composition Increasing the degree of polymerisation, increases s

  46. Influence of water 1300°C 1050°C The more hydrous the melt, the higher the conductivity

  47. Toward a prediction of the dynamics of the next eruption ? T and H2O content from phase relation works (Scaillet et al., submitted work)

  48. Conclusions • Phase equilibria provide robust constraints on P (+T, H2O, fO2..): ±20 MPa • ephemeral reservoirs in upper crust

  49. From Vesuvius toward MORB • Melt fraction? • Reservoirs? • - erupted volume during single events? • - existence of summit caldera? • residence time of MORB in reservoir • Monogenetic volcanoes? • What is a submarine volcanoe?

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