Transcript Volatiles & Magma - Michigan Technological University

Definition
Gases in magmas are called volatile
components, magmatic volatiles,
volatile species
“Fugitive elements”
Key Concept
Volatile species can be dissolved in melt
(accommodated in melt structure)
Or
They can be present as exsolved species
(bubbles)
Magmatic Phases - I
Basalt Melting Relations & Eruption Temps.
Magmatic Phases - II
Conceptual Models of Silicate Melts
From Carmichael et al. 1974
Masaya, Nicaragua; 1972
Quantifying Volcanic Emissions of Trace Elements to the
Atmosphere: Ideas Based on Past Studies
William I Rose
Geological Engineering & Sciences
Michigan Tech University
Fall AGU, San Francisco 8 December 2003, paper V12E-08 15:25
White Island, NZ
measured
Rose, Chuan, Giggenbach, Kyle & Symonds, 1986, Bull Volcanol 48: 181-188
Selected Direct samples of volcanic
gases from rift volcanoes.
Volcano
Erta Ale
Erta Ale
Surt sey
T ºC
1130
1032
1125
wt % H2O
77.2
69.4
81.1
CO2
11.3
17.2
9.29
SO2
8.34
9.46
4.12
H2
1.39
1.57
2.80
CO
0.44
0.75
0.69
H2S
0.68
1.02
0.89
HCl
0.42
nd
nd
HF
nd
nd
nd
S2
nd
0.59
0.25
COS
nd
0.02
nd
Symonds et al, 1994, Rev Mineralogy 30: 1-66.
Common Magmatic Volatile Species
• Volatiles are defined as those chemical species that at near atmospheric
P and high T appropriate for magmas, exist in a gas or vapor phase.
• Common chemical species include: H2O (steam), CO2, H2, HCl, HF, F,
Cl, SO2, H2S, CO, CH4, O2, NH3, S2, and noble gases He and Ar. H2O
and CO2 dominate!
• Most volatile species consist of only six low-atomic weight elements:
H, C, O, S, Cl, and F. Small but measurable amounts of these elements
can be dissolved in both the coexisting melt and crystalline phases.
• Oxygen is the major ion in all three phases in magmatic systems: solid,
liquid, and volatile.
State of Volatiles in Magmas
• Critical Point: for a volatile species is
the T, P at which there is no physical
distinction between liquid and gas
• Exsolved volatiles are above the
critical point. Called supercritical
fluids.
Phase Diagram of H2O
Specific Volume of Pure Water
Geothermal
Gradient
Pure H2O ->
218 bars; 371°C
Pure CO2 ->
73 bars; 31°C
Critical Point
At magmatic conditions no distinction between liquid and gas phases.
Refer to phase as volatile fluids if density < 2 g/cm3. From Burnham et al., 1969
Supercritical Fluids…….
Characteristics include:
• Density more like liquid
• Solubility like those of liquid
• Diffusivities like those of gas
• Viscosity like those of gas
Keep in mind that these are still P,T dependent
Supercritical Fluids in Magmas
• Density of supercritical fluid very
LOW
• Means the specific volume
(volume/mass) very LARGE
• 10-10,000 cm3/gm
• Why is this important?
Specific Volume of Water vs. Pressure
specific volume = 1 / r
0.1 g/cm3
rmagma = 2.2 g/cm3
What are the most important
volatile species?
• Most important are H2O and CO2
• Secondary importance are S in
the form of SO2 and H2S
• Additional importance are the
halides--Cl, F
Abundances of Dissolved Volatile Species in
Magmas
• One example of dissolved volatile (H2O) abundances shown
here.
• We will explore other abundances further in this lecture.
Example of Volatile Discharge at an
Active Volcano: Merapi, Indonesia
In tons
• 3000 CO2
• 400 SO2
• 250 HCl
• 50 HF
Why study volatile species?
• Play a fundamental role in forcing
magma to ascend, and erupt
• For example, typical percentage
by mass might be 0.1%;
equivalent to 90% bubbles in
magma!
Increase in Volume with Decreasing
Pressure
Exsolved volatile species
Depolymerization of Silicate Melts
Solubility of Volatiles in
Magmas
• Solubility measure of the concentration of
a volatile species that can be dissolved in a
melt (accommodated in melt structure)
• What this means is that for a particular
P, T, X, there is a maximum amount of
H2O that can be dissolved in a melt
Solubility is a function of:
• P, T, X--most important are
composition and pressure
Magmatic Volatile Reservoirs
PH2O < Pf
Px = Py = Pz
Isostatic = Lithostatic pressure
Solubility as a Function of
Pressure
Volume of volatile-rich melt << volatile-absent
melt + free volatile phase (bubbles)
• What happens at increasing P?
• Push reaction to side with smaller volume
• This means that solubility increases with
pressure
Solubility as a Function of
Pressure
Volume of volatile-rich melt << volatile-absent
melt + free volatile phase (bubbles)
• Easy way to remember this: soda
analogy
Water Solubility vs. Pressure
“Cold-Seal” bomb
Pressure medium
Platinum capsule:
contains melt +
dissolved volatiles
To pump to increase
pressure
From Moore et al., 1998
Interpreting Solubility Diagrams
Definitions
Undersaturated
Saturated
Oversaturated
Pressure Effects on Volatile-rich Systems
Speciation of Water in Silicate Melts
H2O + O2- = 2OHin melt
in melt
From Silver et al., 1990
CO2 Solubility in Silicate Melts
a
SiO2
Solubility with More than One
Volatile
Vesiculation Stages
Bubble Nucleation
Froth Saturation
New Nucleii and Growth
Fragmentation
From: Sparks (1978)
Summary of ascent issues
Cooling of magma
Saturation and supersaturation
Formation of bubble nuclei
Bubble growth
Formation of solid phase nuclei
Growth of crystals
Interpretation of textures
Cooling of magma
As magma rises from mantle levels to the surface, the wallrock
changes from ductile to brittle. Diapirs are thought to exist in
ductile regions, and above there are dikes and cylindrical
conduits.
With rise of magma, there is a cooling of the wallrock
temperature and an increase in the temp contrast between the
magma and the wallrock.
Geometry of feeder system influences rate of heat loss.
The strong relationship of viscosity with T is important in
causing magma viscosity to increase as the magma approaches
the surface.
Saturation and supersaturation
The availability of H2O and other volatiles in the source region
and the degree of partial melting gives rise to an unsaturated
magma with perhaps <1-7 % volatiles.
The effect of dissolving volatiles in silicate magma is
depolymerization--resulting in a dramatic viscosity decrease.
With rise, the solubility of volatiles decreases and eventually
the magma is saturated. This threshold is academic and barely
noticable, because volatiles do not begin to escape. Why?
If the magma continues to rise and pressure decreases, the
magma enters a state of supersaturation, which does lead
eventually to bubbles.
Formation of bubble nuclei
The most important activation energy barrier for bubble formation
is the formation of a nucleus---the accumulation of many molecules
of gas that can sustain enough pressure to avoid being resorbed by
the magma that surrounds them.
There is probably a critical minimum size of nuclei that can
survive, perhaps around one micron in diameter.
In a probabilistic sense the evolution of a nucleus of critical size,
one that has a >50% chance of growing rather than shrinking, is
quite unlikely so most nuclei get resorbed.
Only when a nuclei reach critical sizes can volatiles escape--this is
why supersaturation is needed.
An analogous concept is undercooling.
Growth of bubbles
If the magma is low in viscosity, bubbles will rise in the liquid after
growing to some critical size. Thus they may reach the top of a
magma chamber or even burst in an open vent, releasing gas
passively.
In a viscous magma bubbles never grow large enough to rise by
gravity, because resistence of the magma to flow is too great.
Diffusion of gas through a viscous liquid is slow--slowest in very
viscous ones. Each growing bubble has a gradient around it.
Overpressure develops in bubbles as bubbles grow, but as the
bubbles “feel” each other’s presence, their growth is inhibited.
Thus a particular vesicle size is reached when a foam is formed.
Why do Bubbles Grow
• Bubbles nucleate and grow when magma
reaches super-saturation.
• Equivalent to when vapor pressure equals or
more typically exceeds confining P. This allows
critical fluid to separate (equal to formation of
bubble)
Why do Bubbles Grow?
• Super-saturation can occur via:
Decompression = first boiling
Crystallization = second boiling
Bubble Growth
• Once bubbles successfully nucleate, they grow!
• Rate of growth function of a number of
variables:
concentration of volatiles
rate of diffusion (diffusivity)
density and viscosity of magma
surface tension of bubble
• Size of bubble in part result of competition
between nucleation and growth
Idealized Bubble Growth
• Defined as growth of a bubble at
constant pressure, in a stable (nonmoving) melt
• Two stages:
 Growth by diffusion (viscosity limited)
 Growth by expansion (diffusion limited)
Bubble Growth: Stage 1
• Early in bubble growth history, diffusion efficient.
• Bubbles grow by addition of volatile(s) by diffusion.
• Bubbles can commonly maintain equilibrium between
volatile(s) in bubble and volatile(s) in melt.
• Growth is viscosity-limited. That is, although volatiles
are diffusing into bubble, bubble still has to expend
energy to grow against surrounding melt.
• Higher viscosity makes bubble growth more difficult.
Bubble Growth: Stage 1
• Stage 1 referred to as
exponential growth
stage.
• See Stage A on radius
vs. time diagram
Bubble Growth: Stage 2
• Later in bubble growth, diffusion can not keep
pace with growth of bubble.
• Bubble may get too big for diffusion to maintain
equilibrium with melt.
• Thus, diffusive flux of H2O or CO2 into bubble
can not maintain equilibrium (saturation)
pressure.
• Consequence is rate of growth slows.
• This stage limited by diffusion.
Bubble Growth: Stage 2
• Stage 2 referred to as
parabolic growth stage.
• See Stage B on radius
vs. time diagram
Bubble Growth: In “Real Systems”
• During “real” bubble growth, other factors
contribute to bubble growth.
• What might they be?
• Proximity to neighboring bubbles
• Decompression
Bubble Growth: “Real Systems”
• Note that bubble growth
history falls below
parabolic line.
• This is because neighbor
bubbles can limit bubble
growth by competing for
volatile “resources.”
• See segment C.
Bubble Growth during Decompression
• Rate of decompression also very important:
• At depth in conduit, bubble grows initially by diffusion.
• As magma accelerates upward in conduit, bubbles grow by
expansion.
• That is critical fluid/gas phase is expanding against melt.
• Expansion limited by viscous resistance of melt and
neighboring bubbles.
• Thus “excess” pressure develops.
Bubble Growth during Decompression
• “Excess” pressure may lead to fragmentation.
• In general, fragmentation is favored by rapid rates
of decompression AND
• High viscosity (because difficult for bubbles to
maintain equilibrium, and thus more common for
bubbles to become over-pressured).
• More on exact mechanisms of fragmentation when
we talk about Plinian eruptions.
Bubble Growth: Basaltic vs. Silicic
Systems
• In general, silicic magmas form smaller bubbles
(0.001-0.1 cm) compared to basaltic (0.1-5 cm).
• WHY?
• Diffusivity is slower in silicic magmas
• Viscosity is higher, so more resistance to bubble
growth.
Formation of solid phase nuclei
Crystallization is also driven by cooling and volatile loss from the
magma.
Undercooling, or cooling at a rate faster that crystallization can
keep pace with, creates impetus for overcoming activation energy
barriers.
In the same way as vesicle formation happens, critical crystal
nuclei must attain a size which ensures their survival from
reabsorption by the magma.
At some critical undercooling there is a peak nucleation rate for
each mineral in each magma. If the magma is appropriately cooled
it will nucleate some phase readily (and others perhaps not).
Growth of Crystals
Phenocrysts form mostly long before eruption. They can record tidal
effects, mixing effects and magma movements of other sorts.
Rapid growth features include skeletal or bow-tie crystals, elongate
spinifex crystals and spherulites.
Textures in the groundmass of many lavas record events that occur
near the surface before eruption. Examination of these requires high
magnification--back scattered xray images using SEM or
microprobe.
Interpretation of textures
Petrographic interpretation of volcanic rocks can be used to
interpret subsurface events.
What happened before eruption?
What was the temp, pressure and pH2O of phenocryst formation?
When did saturation and volatile loss events occur?
Are mixing events recorded?
What happened in the weeks and days before eruption?
Measuring Magmatic Volatiles
What is the challenge in accurately
measuring/estimating amount of volatiles in
magmas?
• When gas samples taken at surface, they
can become contaminated with atmosphere
• If magma saturated and bubbles formed,
lost some of its volatile supply prior to
eruption
Measuring Magmatic Volatiles
Method 1: Phase Equilibria
• As pressure increases, so
does solubility of H2O
• Thus, T at which
minerals form decreases
• Amphibole is exception
because water-bearing
Method 2: Glasses and Melt
Inclusions
• Can measure abundances in
submarine glasses because little
to no degassing invoked; magma
cools on contact with seawater
• Melt inclusions, which are blobs
of melt (glass) surrounded by
crystal.
• Interpretation is that these blobs
of melt do not lose volatiles
because “armored” by solid
crystal.
Volatiles and Eruptions
Explosive Eruptions
Types of volcanic eruptions
Explosive
Gas-particle dispersion flows out of
the vent
Extrusive
Lava flows or domes
Explosive
volcanic
eruptions
Strombolian
Vulcanian
Plinian
Eruption size distribution
Yellowstone
2.2 My BP
Yellowstone
630,000 y BP
3
2500 km
3
1000 km
Yellowstone
1.3 my BP
3
Pinatubo 1991
Mount St Helens 1980
280 km
Tambora 1815
Mount Mazama 7,600 y BP
Krakatau 1883
The Volcanic Explosivity Index
VEI
Tephra volume
(m3)
0
1
2
3
4
5
6
7
8
< 104
104-106
106-107
107-108
108-109
109- 1010
1010-1011
1011-1012
> 1012
Eruption
column height
(km)
< 0.1
0.1 –1
1–5
3 – 15
10 – 25
> 25
> 25
> 25
> 25
Stratospheric
injection
General
description
None
None
None
Possible
Definite
Significant
"
"
"
Non-explosive
Small
Moderate
Moderate-large
Large
Very large
"
"
"
VEI damages
VEI 0: quiet, effusive eruptions of lava; typically a
threat to local property only
VEI 1-3: progressively more violent explosive
eruptions capable of local damage
VEI 4-5: moderate explosive eruptions capable of
regional damage and disruption
VEI 6-7: large to gigantic explosive eruptions
capable of global impact through climate
modification
VEI 8: super-eruptions capable of severe global
climate modification
3
Tephra volume (m )
6
10
10
8
10
1 0
1 2
10
100,000
Eruptions per
thousands of years
10,000
Nevado del Ruiz 1985
Number of eruptions
1,000
Galunggung 1982
Mount St. Helens
1980
100
Krakatau
1883
10
Tambora 1815
1
Several small
explosive eruptions
every year.
Moderate explosive
events occur every
decade or so and have
regional impact
Large explosive
eruptions have return
periods of a century or
more
~2 VEI 8 events every
100 millennia
0.1
Yellowstone
2 my BP
0.01
2
3
5
6
7
Volcanic Explosivity Index (VEI)
8
Extrusive
eruptions
• Lava flows
• Lava domes
Photo: Copyright Marco Fulle - Stromboli On-Line http://stromboli.net
Structure of the
flow during
explosive
eruption
gas outflux
LAVA
DOME
Qout
Qin
Structure of the
flow during
extrusive eruption
Physical properties of magma
• Magma: melt + crystals + gas.
• Melt: Temperature 800-1300 оС, pressure 103 -10-1
MPa, Viscosity 102 -1012 Pa•s.
• Crystals: size 10-7-10-1 m, number density up to 1017
m-3, fraction up to 95 %.
• Gas: H2O - 60-95, CO2- 0-35%, mass fraction 0.1-7
%.
• Ascent velocity: V =10-4- 500 м/c.
Special features: high viscosity, strongly
dependent on chemical composition and
temperature, gas solubility and diffusion,
complicated crystal growth kinetics.
!
Magma rheology
F/A = µ (V/2h)
m - shear viscosity
•Viscosity depends on:
•chemical composition - more SiO2 - more viscous
•Temperature - higher temperature - less viscous
•Water content - higher water content - less viscous
•Crystal content - higher crystal content - more viscous
Shear stress(force)
NonNewtonian rheology
Bingham
Yield
strength
Power-law
fluid
Newtonian
Strain rate (velocity)
Questions--As magma ascends:
A. Will bubbles rise or not?
B. Can gas escape through a foam?
C. If both A and B are no, and gas escape continues, overpressure
will build. Will it explode?
D. As magma cools, degasses and crystallizes will it continue to
flow?
E. If flow stagnates, what happens below?
F. Flow of magma is unsteady because it does stagnate, and
pressurizes. Then what?
Vertical explosions of ash and bombs occur every 20-100
minutes from a vertical conduit of viscous dacite magma. This
has been true for decades and it happens during both slow and
rapid conduit flow.
3770 m
2500 m