Deep root chemistry
Andri Stefánsson
University of Iceland
Natural fluid chemistry
Experiments
Fluid chemistry
Molecular modelling
Geochemical modelling
• Natural systems
– Volcanic geothermal systems
– Volcanic systems
– Fluid inclusions ...
• Experiments
– Ex-situ (phase equilibria, solubility and quenching)
– In-situ (spectroscopy, conductivity ...)
• Molecular modelling
– MD, density functional ...
• Geochemical modelling
– EoS
– Reaction
– Reactive transport ...
Our deep fluid
and then what happens when we add
CO2, HCl, HF, SO2, NaCl ...
pure water
T+p comfort zone
We usually run into major problems
with multi-component “mixed”
solvent systems and high T+p
And we are even still unsure
about the chemical forms (compounds)
of the components (elements) at high T+p
Volcanic gas composition
• Major gases include H2O, HCl, HF,
S, CO2 …
• Many minor gases both metals
and metalloids
• Some are enriched in the
volcanic fluid relative to
quenched magma (rock)
CO2
0
Vulcano (this study)
Solfatara (this study)
100
10
Two-phase wells
Kudryavy
Colima
Augustine
Momotombo
Kilauea
Vulcano
90
20
80
30
70
40
60
50
50
60
40
100
70
30
80
10
20
mol %
90
HCl+HF
1
0
10
10
20
30
40
50
60
70
0.1
0.01
World volcanoes
Two-phase wells
Vulcano (this study)
Solfatara (this study)
Kaasalainen et al. (in prep.)
0.001
H2O
CO2
SO2
H2S
HCL
HF
80
90
STOT
Trace elements in volcanic gas
1000
100
condensate (ppm)
10
1
0.1
0.01
0.001
0.0001
B
Ca
Al
As
Zn
Pb
Mg
Fe
Pb
Zn
Sn
Mn
Cr
Al
Ba
Ni
Ba
Cu
Ei = (X/B)vapor/(X/B)rock
0
-1
-2
-3
-4
-5
-6
As
Sn
Cd
Cr
Ni
Cu
Mg
Ca
P
Fe
Ti
V
Mn
Metal transport, precipitation and gas
condensation
10
1000
1
Ni (ppm) in vapor
B (ppm) in vapor
100
Vulcano (this study)
Solfatara (this study)
Two-phase wells
Vulcano
Kudryavy
Augustine
Colima
Merapi
Momotombo
Cerro Negro
Masaya
Telica
San Cristobal
Poas
10
1
0.1
0.01
0.1
0.001
0.01
0.0001
0.001
200
400
600
Temperature °C
800
1000
200
400
600
800
Discharge temperature (°C)
1000
Magmatic gas-sea water-basalt interaction
Basalt
Seawater
H 2O+acid
(CO2,SO4,H2 S,HCl,HF)
Secondary minera ls+
dissolved elements
Secondary minerals
Basalt
Magmatic gas
basalt + H2O + acid => secondary minerals + solutes
Magmatic gas-sea water-basalt interaction
300°C
magmatic gas
per 1 kg seawater
0.1%
0.5%
1%
5%
90
H2S (ppm)
pH
6
5
a
300°C
magmatic gas
per 1 kg of seawater
0.1%
0.5%
1%
5%
60
30
4
0
C
200
300°C
0
300
b
0.1%
0.5%
1%
5%
6000
100
200
300
grams basalt reacted per 1 kg seawater
-6
logaFe2+
CO2 (ppm)
9000
100
magmatic gas
grams basalt reacted per 1 kg
perseawater
1 kg seawater
-7
3000
300°C
-8
magmatic gas
per 1 kg seawater
0.1%
0.5%
1%
5%
0
0
100
200
300
grams basalt reacted per 1 kg seawater
Padilla e al. (in prep.)
0
100
200
300
grams basalt reacted per 1 kg seawater
• Most elements
seems to be rock
buffered at ξ = 50 g
basalt and 0.1-5%
magmatic acid
supply
• CO2 is the only
major reactive
element that seems
to be largely
influenced by gas
supply
Aquifer volatile (CO2, H2S & H2) concentrations
• Do we see evidences for magmatic input in surface
geothermal fluids?
• Need to reconstruct the aquifer fluid first from data on
surface fluid (v+lq) composition
• Need to look for volatiles that may not be overwritten by later
stage reactions and transport processes
Boiling and aquifer fluid composition
Sample (lq+v)
Closed system boiling
Open system boiling: hv+hlq>hfluid
Closed system boiling
mi ,total  misteam X  miwater (1  X )
Open system boiling
phase segregation
conductive heat transfer
Aquifer
Tmeasured => hfluid
liquid
liquid+steam
Aquifer volatile (CO2, H2S & H2) concentrations
– The Hellisheidi geothermal system
Scott et al. (in prep.)
Non-reactive elements (Cl) concentrations –
The Námafjall geothermal system
Steam addition
in aquifer
Magmatic gas addition
2500
Enthalpy (kJ/kg)
• Two phase aquifer
(lq+v)
• Indication of
magma degassing
using non-reactive
elements like Cl
3000
2000
1500
Aquifer fluid at 275°C
Liquid only
1000
500
0
0
50
100
150
200
Cl (ppm)
Stefánsson (2011)
Cl and B in geothermal fluids in Iceland
• B and Cl are both
conservative elements
• Originate from
Rock dissolution
– Rock
– source water
– magmatic gas
• There is a Cl-B shift
(addition) in fluids
associated with volcanic
geothermal systems
Cl/B shift
Giroud (2008)
Non-reactive elemental ratios (Cl/B) concentrations
– The Nesjavellir, Krafla and Námafjall geothermal systems
• Mixing lines between
Cl and B are observed
between two endmember components meteoric water and
magmatic gas
• The fluids produced
have then reacted to
various degree with
the rock
• The slope of the mixing
suggest different Cl/B
ratio in the degassed
magma
Giroud (2008)
Conclusions
• The chemistry of geothermal and volcanic fluids may
be studied by various methods ( Theoretical molecular
calculations, experiments, natural fluid chemistry,
geochemical modelling)
• “non”-reactive compounds and mass-transfer limited
compounds may be used to trace volcanic-gas input to
geothermal systems and gas-water-rock interaction
• Understanding the geochemistry of these “tracer”
compounds may help us looking into the interaction of
magma body and the convective cell of geothermal
systems