Fluid Inclusion Thermobarometry as a
Tracer for Magmatic Processes
Thor H. Hansteen
IFM-GEOMAR, Leibniz-Institute for Marine Sciences
Dynamics of the Ocean Floor
D-24148 Kiel, Germany
thansteen@ifm-geomar.de
Andreas Klügel
Fachbereich Geowissenschaften
Universität Bremen
D-28334 Bremen, Germany
akluegel@uni-bremen.de
T.H. Hansteen & A. Klügel: Fluid Inclusions
1
Fluid inclusion:
Closed cavity in mineral containing one ore more
liquid, vapor and/or fluid phase(s).
May also contain daughter mineral(s) formed after
inclusion sealing.
Roedder´s rules (prerequisites for interpretations):
1) a single homogeneous fluid phase was trapped
2) the inclusion remained at a constant volume after trapping
3) nothing was added to or removed from the inclusion after
trapping
T.H. Hansteen & A. Klügel: Fluid Inclusions
2
20 µm
20 µm
T.H. Hansteen & A. Klügel: Fluid Inclusions
3
Boiling
(Vapor and liquid phases coexisting in separate cavities)
“Liquid” (now 2-phase)
“Vapor” (now 2-phase)
T.H. Hansteen & A. Klügel: Fluid Inclusions
4
Occurrence
and
Formation
„Necking down“
(Modified after Shepherd et al. 1985; Roedder 1984))
T.H. Hansteen & A. Klügel: Fluid Inclusions
5
Post-entrapment re-equilibration
Stretching: permanent, plastic deformation
(creep) of the enclosing crystal
Decrepitation (leakage): partial or total
Compositional re-equilibration (diffusion and/ or
reaction with host)
T.H. Hansteen & A. Klügel: Fluid Inclusions
6
Microthermometry
The measurement of phase transitions upon heating
(Problem: metastability)
Fluid inclusions are isochoric systems
(constant mass & volume => constant density & molar volume)
T.H. Hansteen & A. Klügel: Fluid Inclusions
7
Heating/ freezing stage on petrographic microscope
T.H. Hansteen & A. Klügel: Fluid Inclusions
8
The system CO2
Melting properties: Composition
Homogenization proporties: Density (molar volume)
(Modified after Van den Kerkhof 1988))
T.H. Hansteen & A. Klügel: Fluid Inclusions
9
Homogenization of CO2 inclusions: T increase from 30.0 to 30.5 °C within 30 sec
10 µm
T.H. Hansteen & A. Klügel: Fluid Inclusions
10
The system CO2
Isochores
Arbitrary inclusion (r=const.)
(Projection)
(Modified after Roedder 1984; Goldstein and Reynolds 1994)
T.H. Hansteen & A. Klügel: Fluid Inclusions
11
System H2O
Isochores
(Fisher 1976)
T.H. Hansteen & A. Klügel: Fluid Inclusions
12
The system H2O- NaCl
25 wt%
10 wt%
(Modified after Crawford 1981;
T.H. Hansteen & A. Klügel: Fluid Inclusions
13
Microthermometry cycle, system H2O- NaCl
Melting properties: Composition
Homogenization proporties: Density (molar volume)
(Hein 1989)
T.H. Hansteen & A. Klügel: Fluid Inclusions
14
Boiling in system H2O - NaCl
Vapor
Liquid
(Modified after Bodnar et al. 1985; Chou 1987)
T.H. Hansteen & A. Klügel: Fluid Inclusions
15
“Vapor” (now 2-phase)
“Liquid” (now 4-phase)
Sylvite
Halite
Boiling
(Vapor and liquid phases coexisting in separate cavities)
T.H. Hansteen & A. Klügel: Fluid Inclusions
16
Part 2: Tracking volcanic plumbing systems using CO2 inclusions
(after Hansteen et al. 1998; Klügel et al. 2005)
(after Zanon et al. 2003; Frezzotti and Peccerillo 2004;
Peccerillo et al. 2006)
Models of the Recent magma plumbing systems beneath
La Palma (Canary Islands) and Vulcano (Aeolian arc)
T.H. Hansteen & A. Klügel: Fluid Inclusions
17
Rationale: understanding the density distribution of FI
Density distribution of a gang of "Roedder's rule" inclusions
First level of
entrainment
Frequency
Second level of
entrainment
Homogeneous,
isochoric
& closed
Subordinate
entrainment
Real
Inclusion density
Interpretation: major entrainment levels
=> prolonged magma storage
=> magma ponding / reservoirs
x x
_
T.H. Hansteen & A. Klügel: Fluid Inclusions
18
Density distributions of REAL fluid inclusions
idealized
measured (example)
(data from
Neumann et al. 1995)
(data from
Zanon et al. 2003)
(data from
Hansteen et al. 1998)
T.H. Hansteen & A. Klügel: Fluid Inclusions
19
Work flow chart: how to obtain pressures from fluid inclusions
Microthermometry: determine
inclusion composition
Determine homogenization temp.
Th (LVL) / Th (LVV)
Get density from Th
• CO2: triple point at -56.6 °C
• additional Raman microspectrometry
• Microthermometry
• accuracy and precision better ±0.2 °C
• isobaric T-r section
• auxiliary equations (e.g. Span & Wagner 1996)
Calculate respective isochore
using an equation of state
Get/assume trapping temperature
and calculate pressure
Pressure => Depth
• different EOS available
• lack of experimental data in high P-T range
• use independent geothermometer
• proxy: eruption temperature of host magma
• surprisingly large source of error
T.H. Hansteen & A. Klügel: Fluid Inclusions
20
Obtaining density from measured homogenization temperature
liquid
vapor
Th (LVL): accurate to <0.2 °C
r accurate to 0.001-0.01 g/cm3 (0.1-2% relative)
(near Tcrit: 2-8% uncertainty @ T < 30.9 °C)
Th (LVV): accuracy ~1 °C
r accurate to 3-12% @ T < 30.2 °C)
T.H. Hansteen & A. Klügel: Fluid Inclusions
21
Calculation of isochores: example for an equation of state for CO2
Sterner SM, Pitzer KS (1994): An equation of state for carbon dioxide valid from zero to extreme pressures. Contrib Mineral Petrol 117: 362-374
Contains only 28 non-zero parameters:
T.H. Hansteen & A. Klügel: Fluid Inclusions
22
The MAIN problem of all equations of state
Realm of
igneous
petrologists
experimental
rPT data
(from Span & Wagner 1996)
T.H. Hansteen & A. Klügel: Fluid Inclusions
23
Calculation of isochores: comparison of different equations of state
r = 1.1 g/cm3 :
1030...1180 MPa,
~4-5 km uncertainty
r = 0.6 g/cm3 :
270...300 MPa,
~1 km uncertainty
DP is deviation from reference EOS: SP94 (Sterner & Pitzer 1994)
Eqs.: KJ81 (Kerrick & Jacobs 1981), BR81 (Bottinga & Richet 1981), H81 (Holloway 1981), SW96 (Span & Wagner 1996)
T.H. Hansteen & A. Klügel: Fluid Inclusions
24
Calculation of pressures: P-T relationships
50° C uncertainty:
r in g/cm3
30 MPa error (3%)
12 MPa error (4%)
2 MPa error (4%)
Isochores calculated using the EOS of Sterner & Pitzer (1994)
T.H. Hansteen & A. Klügel: Fluid Inclusions
25
Role of (possibly) missing H2O
Observation: H2O absent in most basalt-hosted phenocrysts + xenoliths
Explanation: H2O more prone to leakage than CO2: compositional reequilibration
Mechanisms: many! (diffusion along crystal defects, H and OH diffusion...)
Rates: fast! (hours to weeks)
For barometry: estimate former H2O content of CO2 inclusions and correct for it
notoriously difficult
straightforward
a = molar H2O/CO2 in trapped fluid, all H2O now lost:
rtrap = rmeas·(1 + a ·18/44)
Example: 10 mol% of H2O => correction factor = 1.045.
Equation of state for H2O-CO2 system (Kerrick and Jacobs 1981 @ 1150 °C):
Pressure correction is -7% for r = 0.3 g/cm3 , +23% for r = 0.8 g/cm3.
T.H. Hansteen & A. Klügel: Fluid Inclusions
26
Summary: effect of uncertainties and errors on P distribution
"Reference data" for 1150 °C
Effect of different temperature
Effect of different equation of state
Effect of correction for 10 mol% H2O
T.H. Hansteen & A. Klügel: Fluid Inclusions
27
Summary: effect of uncertainties and errors on P distribution
Determination of Th and r
Assumption of trapping temperature
Calculation of isochore / equation of state
Correction for former H2O content
Volumetric re-equilibration: stretching
Error magnitude
Stretching: probably the largest single source of error!
• systematic error, causes density decrease
• error magnitude difficult to assess
• e.g. CO2 inclusions in olivine @ 1100 °C:
1000 MPa decompression in 2 days = 8% density decrease (30% @ 1300 °C)
T.H. Hansteen & A. Klügel: Fluid Inclusions
28
Volumetric re-equilibration: a case study (if time permits...)
Data from the 1949 eruption on La Palma (Canary Islands), after Hansteen et al. (1998)
T.H. Hansteen & A. Klügel: Fluid Inclusions
29
Extra Overheads
T.H. Hansteen & A. Klügel: Fluid Inclusions
30
r in g/cm3
Host magma
Isochores calculated using the EOS of Sterner & Pitzer (1994)
T.H. Hansteen & A. Klügel: Fluid Inclusions
31
Microthermometry: Linkam THMS600
T.H. Hansteen & A. Klügel: Fluid Inclusions
32
Microthermometry: Linkam THMS600
T.H. Hansteen & A. Klügel: Fluid Inclusions
33
Microthermometry: Fluid Inc.-modified USGS stage
T.H. Hansteen & A. Klügel: Fluid Inclusions
34
T.H. Hansteen & A. Klügel: Fluid Inclusions
35
The system H2O-NaCl
T.H. Hansteen & A. Klügel: Fluid Inclusions
36
System H2O- NaCl
T.H. Hansteen & A. Klügel: Fluid Inclusions
37