Magma Ascent Rates
Malcolm J Rutherford
Dept. of Geological Sciences
Brown University
Providence RI
(Presentation for MSA short Course Dec. 13, 2008)
Outline
1. Why study magma ascent rates?
- Better understanding of subsurface magmas and processes.
- Prediction of eruption style changes, e.g., explosive vs. effusive.
2. How can we determine ascent rates?
-
Melt degassing produced by ascent-induced decompression.
Crystallization driven by decompression and degassing.
Crystal-melt reactions “
“.
Seismicity changes associated with magma ascent.
Theoretical modeling of magma ascent.
3. Caveats and problems involved?
- Determining where magma ascent began.
- Determining the exact nature of reactions involved.
- Involvement of magma convection and mixing.
- The size of the conduit.
Amph
Opx
4 km -
Phase equilibria of 2004-06 MSH dacite magma showing amphibole stability limit; inset
Shows internal zoning in the amphibole and Opx that requires two cycles of convection
from 300 to 120 MPa prior to final ascent (after Rutherford and Devine, 2008).
Using diffusion data for water, Humphries et al., (2008) calculate time
required to develop measured profile in tubular melt channels.
Assuming 4.6 wt% H2O initially, L= 5 km, C.R. = 25 m, v = 37- 49 m/s for
MSH May 18, 1980. Method similar to that of Anderson (1996) & Liu et
al., 2007.
Conduit Radius (effective) and Magma ascent velocity changes derived from
magma mass eruption rates at Mount St. Helens in 1980-82 eruptions
assuming single phase flow (after Geschwind and Rutherford, 1995).
- Ave. Ascent rate for 1980 explosive = 3 m/s for 8 km (C.R. = 33 m).
- Theoretical modeling V = 15-20 m/s (C.R.; 17-33m) Papale & Dobran(1994).
- Humphries et al., (2008) V = 37- 49 m/s.
* Conduit radius and depth both critical in determining rate of ascent.
Ascent rates (m/s)
I
0.045
I
0.022
I
0.015
I
0.01
Ascent rates based on crystallization induced by decompression in MSH 1980-86
dacite magma (after Geschwind and Rutherford, 1995). Assume 8 km starting
depth and conduit radius from mass eruption rate data (Swanson et al., 1987).
SH308
Amph
Phase equilibria of MSH dacite magma showing amphibole stability limit; inset shows
amphibole from a 2005 sample with a reaction rim (after Rutherford and Devine, 2008).
Amphibole reaction rim-width growth in dacitic and andesitic magmas as
a function of decompression time (constant rate decompression) from
experiments at 830-900 oC (after Rutherford and Hill, 1993; Rutherford
and Devine, 2003).
Images showing reaction rims on amphibole phenocryst in the Black Butte (CA) dacite;
also illustrated is the contrast in texture of the phenocrysts relative to the lineated
matrix.
After McCanta et al., (2007).
- Rim widths (34 m) in each of 4 lobes of BB lava dome erupted at 890oC, yield an ave.
magma ascent rate of 0.004-0.006 m/s for ascent from 200 MPa (~8 km) to the surface.
- ** Single stage ascent and no mixing indicated for these magmas.
840oC
New amphibole reaction rim growth rate study show how the rate varies with
P at a 840oC (Brown and Gardner, 2006).
Temperature affect of
decompression-induced
crystallization in MSH
1980-82 dacite
Modified after Blundy et al., (2006)
A
B
F wt %
F-rich amphibole rims develop in very slow ascending
2004-6 Mount St. Helens dacite magma preventing rim
growth (DeJesus and Rutherford, 2008)
- Zoning developed in Mt Unzen Timagnetite. (Nakamura, 1995). Profile
reflects time following magma mixing, and
yields a minimum ascent time assuming that
mixing accompanied onset of ascent.
- Ascent of Mt. Unzen mixed magma from 7
km was at a min. rate of 0.003-0.007 m/s (1130 da) based on 20 m profile.
1996-2002 Montserrat: TiO2 profiles for natural and experimental Timagnetite (after Devine et al., 2003). 830-860oC andesitic magma
reached surface from 5 km depth ~20 days after basaltic andesite influx.
Real Time Seismic Amplitude counts build-up at site near Mount St. Helens in 1986
along with focal depth shows magma rising from 1.4 km to surface over 12 hours;
this corresponds to an ave magma ascent rate of 0.32 m/s (Endo et al., 1996)
Formation at
7-15 km
(2)
(1)
Xenolith-melt reactions.
Two scales of reaction in olivinerich xenoliths carried up in basalt at
La Palma, Canary Islands. (1) Long
diffusion profiles in Olv = 8-110 yrs
storage at 7-15 km; (2) Melt bearing
fractures in Olv have zoning that
indicates 0-4 days origin and ascent
rates of > 0.06 m/s (Klugel et al.,
1998) assuming cracks began with
ascent.
Garnet-melt reaction in kimberlite (Canil and Fedortchouk, 1999). Garnet
dissolution features (~25m) interpreted using experimental data yield exposure
times; minimum ascent rates depending on depth where garnet is exposed and T.
Water loss profiles in olivine in garnet peridotite xenolith carried in alk basalt
(Demouchy et al., 2006; Peslier and Luhr, 2006). Assuming 40 and 60-70 km depth
for inclusion enclosure in basalt respectively, initial water = 300 ppm, and 1200oC,
D et al., calculate 6.3 m/s ascent; P & L ascent rate = 0.2 - 0.5m/s.
Kimberlite Magma Ascent Rates from theoretical flow modeling
1.
2.
Calculation is for buoyancy-driven dike flow (Stage 2), does not consider effect
of gas expansion that would be particularly important in Stage 3 (1- 0 km).
Calculation agrees well with estimates from garnet dissolution and with
xenolith transport requirements (Spera, 1984).
TABLE 2. Magma ascent rate estimates from different observations *
Explosive
Ascent rate (m/s)
>> 0.2
Ext rusive
Ascent rate (m/s)
0.01 - 0.02
> 0.18
0.04 - 0.15
1-2
0.01 - 0.1
Mount St. Helens
Calculation fr om mass
eruption rate
Seismicity
0.6
0.007 - 0.01
Soufriere Hills, Mont.
Amphibole rims
> 0.2
0.001 - 0.012
Soufriere Hills, Mont.
magnetite
> 0.2
0.003 – 0.015
Mount Unzen
Magnetite zonation
not present
0.002
Black Butte CA
not present
0.004-0.006
Hualali, HI alk basalt
Amphibole rims &
plagioclase gr owth
Xenolith transport
not present
> 0.1
La Palma, Canary Is.
Olivine zonation
not present
> 0.06
Volcano
Mount St. Helens
Mount St. Helens
Mount St. Helens
Xenocrysts in Alk basalt
Observation*
groundmass
crystallization
Hornblend e rims
Hydroge n zoning in olv
Kimberlite
Theoretical modeling
Kimberlite
Garnet dissolution
0.2 to 0.5 m/s
> 4 -16
1.1 to 30 m/s (final 2 km)
dissolved CO 2 (ppm
1600
Path 1
1400
300 MPa
A
1200
400 MPa
Path 2
B
1000
800
200 MPa
600
Path 3
400
200
100 MPa
50 MPa
0
0
1
2
3
4
5
6
dissolved H 2O
Dissolved volatiles in Min2 shoshonite and F.R latite olv- and cpx-hosted
melt inclusions (FTIR) after Mangiacapra et al., 2008, GRL.
7
Lobe 1, SHM-22,
< 50um Slope: 0.2260, intercept: -12.45.
50-600 um, slope: 0.0153, intercept: -22.53
Plagioclase Phenocrysts
and microlites in Black Butte
CA dacite. CSD for the two
phases of crystallization gives
growth rate
TABLE 2. Magma ascent rate estimates from different observations*
Explosive
Ascent rate (m/s)
>> 0.2
Ext rusive
Ascent rate (m/s)
0.01 - 0.02
> 0.18
0.04 - 0.15
1-2
0.01 - 0.1
Mount St. Helens
Calculation fr om mass
eruption rate
Seismicity
0.6
0.007 - 0.01
Soufriere Hills, Mont.
Amphibole rims
> 0.2
0.001 - 0.012
Soufriere Hills, Mont.
magnetite
> 0.2
0.003 – 0.015
Mount Un zen
Magnetite zonation
not present
0.002
Black Butte CA
not present
0.004-0.006
Hualali, HI alk basalt
Amphibole rims &
plagioclase gr owth
Xenolith transport
not present
> 0.1
La Palma, Canary Is.
Olivine zonation
not present
> 0.06
Volcano
Mount St. Helens
Mount St. Helens
Mount St. Hele ns
Xenocrysts in Alk basalt
Observation*
groundmass
crystallization
Hornblend e rims
Hydroge n zoning in olv
Kimberlite
Theoretical modeling
Kimberlite
Garnet dissolution
0.2 to 0.5 m/s
> 4 -16
1.1 to 30 m/s (final 2 km)
Objectives
1. Why study magma ascent rates?
- Better general understanding of subsurface volcanic processes.
- Prediction of eruption style, e.g., explosive vs. effusive.
2.
How can we determine ascent rates?
-
Magma degassing rates from melt phase.
Crystallization driven by decompression and degassing.
Crystal-melt reactions “
“.
Seismicity associated with magma ascent.
Zoning developed in crystals by decompression.
Theoretical modeling of magma ascent.
3. Caveats and problems involved?
- determining where magma ascent began, the nature of reaction
observed, the parameters controlling the reaction, involvement of
magma convection and mixing, the size of the conduit.
Magma storage zone at Mount Pinatubo in
1991 based on seismicity and petrological
phase equilibria of the phenocryst - melt
assemblage (after Pallister et. al., 1996
and Hammer, 2003).