The dynamics of magma chamber refilling at

Transcription

1 Vertical deformation Seismogram Gravity change The dynamics of magma chamber refilling at the Campi Flegrei caldera A. Longo 1, C. Montagna 1, M. Vassalli 2, P. Papale 1, D. Giudice 1, G. Saccorotti 1 INGV Pisa

2 New reconstruction of ground deformation at Serapeo (Pozzuoli) during last two centuries Present unrest started in 1950 About 60 years of unrest From Del Gaudio et al., INGV-DPC Project /V3_2

3 Seismic tomography Melt zone at 7-8 km below the surface (from Zollo et al., GRL 2008)

4 0 0 pressure (MPa) Pressure (MPa) Top of carbonatic basement Minopoli 2 Agnano Monte Spina Campanian Ignimbrite Seismic discontinuity Campi Flegrei Depth (km) depth (km) From Civetta, Arienzo, Mangiacapra, Moretti, et al., INGV-DPC Project /V3_ CO 2 gas (wt fraction) composition of the gas phase (wt% CO 2 ) pressure (MPa) top of carbonatic basement Top of carbonatic basement Seismic discontinuity Vesuvius depth (km) composition of the gas phase (wt% CO 2 )

5 Seismic reflection From RU Faccenna, INGV-DPC Project /V3_2 Na 09

6 Plinian phase D1 of the 4100 BP Agnano Monte Spina eruption CO 2 ~ wt% in the gas phase km depth From Rutherford et al., INGV-DPC Project /V3_2

7 Agnano Monte Spina eruption shallow phonolite deep trachyte A few tens of hours before eruption From Rutherford et al., INGV-DPC Project /V3_2

8 Chemical and isotopic evidence of mixing-mingling preceeding many CF eruptions AMS Eruption Pb/ 204 Pb AMS Eruption A1 wr E1 wr E gray glass Sr/ 86 Sr AVERNO Eruption 0, IC Eruption SM sp SMs SMc Mond.15U3 0, Sr/ Sr 0, Mond.152a2 From Civetta et al., INGV-DPC Project /V3_2 0, w.r Mg-cpx. big Fe-cpx.small Fe-cpx.big Magn. Feld. big 0, glass 2 3 Mg-4 5 Fe-cpx.int 6 7 Biot. Feld.frantz Apat 12 13

9 Global view of the CAMPI FLEGREI system INGV-DPC Project /V3_2

10 CAMPI FLEGREI 0 CO 2 -depleted phonolite - 4 (Project V3_2) CO 2 -rich shoshonite - 8 km a.s.l. - 12

11 p t o p = 70 MPa (~3 km) Shallow oblate CO 2 poor phonolite H 2 O 2.5 wt% CO wt% (Rutherford, 2004, Civetta, pers. comm.) Shallow prolate CO 2 rich shoshonite H 2 O 2 wt% CO 2 1 wt% (D'Antonio et al., 1999) T = 1533 K No crystals Mingling magmas Exsolution law: Papale et al. (2006) Viscosity: Giordano et al. (2008)

12 Initial physical properties p t o p = 70 MPa (3 km) 6 gas vol%, 2240 kg/m 3 viscosity 620 Pa s 5 gas vol%, 2270 kg/m 3 ρ = 40 kg/m 3 ρ = 20 kg/m 3 10 gas vol%, 2230 kg/m 3 4 gas vol%, 2290 kg/m 3 9 gas vol%, 2270 kg/m 3 3 gas vol%, 2440 kg/m 3 viscosity 420 Pa s p b o t t o m = 210 MPa (9 km)

13 GALES Developed at INGV Pisa Finite Elements Method Galerkin Weighted Residuals Stabilization: Least Squares (streamwise direction) Discontinuity Capturing (solution gradient direction) Double discretization in space and time Primitive variables (y, p, u, T) C++ programming language Parallel computation (Linux cluster) ~longo/gales/gales.htmlgales.html Accurate, robust, and adaptable Solves from compressible to incompressible flows Suitable for the simulation of the space-time evolution of magmatic systems in a wide domain (from the deep regions of magma chamber to the volcanic crater)

14 MAGMA multiphase, multicomponent separated flow phase- and composition-dependent properties Newtonian to non-newtonian rheology phase changes (volatile exsolution, crystallization) multicomponent gas-liquid reactions (H 2 O, CO 2, S, ) kinetics of phase change wide range of flow regimes (incompressible to compressible, low-to-high-re, ) Red: accounted for Black: future implementations

15 Oblate chamber Evolution of composition

16 Physical properties at t = 7h 45' Upper chamber: Composition: 100 wt% -> ~70 wt% phonolite Density: kg/m 3 -> ~ kg/m 3 Gas content: 6-5 vol% -> ~8-6 vol% gas ~1 km 130 m/h v max ~1 m/s Mass changes (relative): upper chamber bottom chamber v max ~ 0.1 m/s

17 Prolate chamber Evolution of composition Slower convection/mixing lower density contrast larger friction in the chamber

18 Upper chamber ~ 8 hours bars 0 bars overpressure bars -0.2 bars -0.4 bars -0.3 bars overpressure Lower chamber +1 bar

19 ~ 8 hours overpressure prolate oblate Chamber top overpressure prolate oblate Chamber middle overpressure prolate oblate Chamber bottom

20 Mass of shoshonitic magma entering the shallow chamber oblate prolate

21 Conclusions The buoyant magma invading the shallow chamber soon looses its identity as a separate component (in optimum agreement with the observations) The geometry of the shallow chamber has large effects on the dynamics of magma refilling and mixing: Oblate: faster / more efficient Prolate: slower / less efficient The pressure evolves in a complex way, overall decreasing in the shallow chamber, and increasing in the deep chamber At least in the oblate chamber case, the pressure decrease in the shallow system is partially recovered over the range of ~10 hours, letting however a residual negative overpressure of the order of a few tenths of a bar