Transcript 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 [email protected] Andreas.

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
[email protected]
Andreas Klügel
Fachbereich Geowissenschaften
Universität Bremen
D-28334 Bremen, Germany
[email protected]
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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
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20 µm
20 µm
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Boiling
(Vapor and liquid phases coexisting in separate cavities)
“Liquid” (now 2-phase)
“Vapor” (now 2-phase)
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Occurrence
and
Formation
„Necking down“
(Modified after Shepherd et al. 1985; Roedder 1984))
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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)
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Microthermometry
The measurement of phase transitions upon heating
(Problem: metastability)
Fluid inclusions are isochoric systems
(constant mass & volume => constant density & molar volume)
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Heating/ freezing stage on petrographic microscope
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The system CO2
Melting properties: Composition
Homogenization proporties: Density (molar volume)
(Modified after Van den Kerkhof 1988))
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Homogenization of CO2 inclusions: T increase from 30.0 to 30.5 °C within 30 sec
10 µm
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The system CO2
Isochores
Arbitrary inclusion (r=const.)
(Projection)
(Modified after Roedder 1984; Goldstein and Reynolds 1994)
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System H2O
Isochores
(Fisher 1976)
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The system H2O- NaCl
25 wt%
10 wt%
(Modified after Crawford 1981;
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Microthermometry cycle, system H2O- NaCl
Melting properties: Composition
Homogenization proporties: Density (molar volume)
(Hein 1989)
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Boiling in system H2O - NaCl
Vapor
Liquid
(Modified after Bodnar et al. 1985; Chou 1987)
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“Vapor” (now 2-phase)
“Liquid” (now 4-phase)
Sylvite
Halite
Boiling
(Vapor and liquid phases coexisting in separate cavities)
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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)
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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
_
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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)
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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
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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)
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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:
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The MAIN problem of all equations of state
Realm of
igneous
petrologists
experimental
rPT data
(from Span & Wagner 1996)
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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)
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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)
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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.
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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
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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)
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Volumetric re-equilibration: a case study (if time permits...)
Data from the 1949 eruption on La Palma (Canary Islands), after Hansteen et al. (1998)
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Extra Overheads
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r in g/cm3
Host magma
Isochores calculated using the EOS of Sterner & Pitzer (1994)
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Microthermometry: Linkam THMS600
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Microthermometry: Linkam THMS600
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Microthermometry: Fluid Inc.-modified USGS stage
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The system H2O-NaCl
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System H2O- NaCl
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