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Interpreting H2O and CO2 Contents in Melt Inclusions:
Constraints from Solubility Experiments and Modeling
Gordon Moore
Dept of Chemistry & Biochemistry
Arizona State University
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Outline
Focus: Review recent H2O-CO2 solubility experimental data, and to review
and assess solubility models for natural melts used in the interpretation of
melt inclusion measurements.
Review of experimental H2O-CO2 solubility data:
Brief review of a “good” solubility experiment, experimental apparatus, and analytical
techniques used.
Solubility data for pure and mixed H2O-CO2 fluids.
Review and assessment of H2O-CO2 solubility models for natural
silicate melts:
Models for pure and mixed H2O-CO2 fluids
Compositionally specific models (e.g. rhyolitic, basaltic)
General compositionally dependent models
Limitations of model use
Application of compositionally dependent H2O-CO2 solubility models
to melt inclusion data.
Scope of review
•To review solubility determinations
and modeling relevant to natural
melts and the interpretation of melt
inclusion data.
•Work done since “Volatiles in
Magmas” Rev. Mineral, v.30, 1994.
•Only natural melt compositions
(i.e. excludes haploid melts and
simple synthetic systems).
•For detailed information on H2OCO2 in silicate melts in general,
read the “bible”.
The “good” solubility experiment
1. Relatively large sample volume (to accommodate large
molar volume of fluid and enough sample to analyze).
2. Rapid quench from run temperature to form crystal-free,
glassy sample (difficult for hydrous melts).
3. Near-hydrostatic pressure conditions to minimize run
failure and error in run pressure estimate (solid media
apparatus).
4. Precise characterization of volatile content of run
product and composition of fluid (mixed fluid
experiments).
Experimental Apparatus:
•Rapid quench cold seal (to 200-300 MPa, max T~1100°C; Ihinger, 1991; Larsen and
Gardner, 2004).
•Rapid quench internally heated pressure vessel (to 500 MPa, ~1200°C; Holloway et al,
1992; DiCarlo et al, 2006)
•Large volume piston cylinder (>300 MPa; to 1600°C; Baker, 2004; Moore et al, 2008)
H2O-CO2 fluid
Basalt
Rhyolite
Ni-NiO
Moore et al, 2008
Analytical techniques for H2O and CO2 measurement
Two critical measurements: H2O-CO2 content of melt AND fluid composition
Determining glass H2O-CO2 contents (see Ihinger et al, 1994):
Bulk techniques (primary):
1. High T vacuum manometry (H2O and CO2)
2.
Karl-Fischer titration (H2O only)
3.
Elemental Analyzer (CO2 only)
Microbeam techniques for both H2O and CO2 (secondary):
1.
2.
Fourier-transform Infra-red spectroscopy (FTIR)
Secondary ion mass spectrometry (SIMS)
3. Raman spectroscopy
Determining fluid composition (H2O-CO2 fluids only) :
1. Mass balance/gravimetry
Simple, but error related to fluid mass and scale precision (20-100% error reported).
2. Low T vacuum manometry
Requires a vacuum line, precise to ± 10 micromoles of fluid (5-10% relative error).
Summary of solubility data for pure H2O and CO2 in
natural melts
Pure H2O (Table 1):
•Greater than 30 different melt compositions
~44-78 wt% SiO2; peralkaline to peraluminous
•Broad range in P and T
0.1 to 500 MPa; 800-1250°C; good coverage for most compositions
Pure CO2 (Table 1):
•Only 7 compositions
mostly mafic compositions (32-55 wt% SiO2); rhyolites studied earlier
•Dominated by high P (> 1000 MPa) and T (> 1200°C)
Due to low solubility of CO2 and increased solidus T; not extremely useful for
understanding melt inclusion measurements
General H2O solubility behavior
•Relatively large dissolved H2O contents (1-8
wt%; to 20-25 mol%) at magmatic P-T
conditions.
•Strong postive P dependence, with weaker
negative T dependence.
Figure 4
•Total H2O solubility has a significant
compositional dependence (e.g. Moore et al,
1998; Behrens & Jantos, 2001).
•Less data on mafic compositions due to higher
T and difficulty quenching H2O-rich mafic
melts to glass.
Dissolved H2O content in silicic melts (Behrens &
Jantos, 2001) as a function of alkali/alumina.
General pure CO2 solubility behavior
•Low dissolved concentration (100-1000’s
ppm) at fluid-saturated magmatic P-T
conditions.
•Strong P dependence, negative T
dependence.
•Strong compositional dependence (e.g.
Dixon, 1997), but much less data overall
relative to H2O solubility.
•Dominates fluid saturation behavior of
magmas.
•Two infra-red active species: carbonate
(mafic) and molecular CO2 (silicic).
•Mixed speciation in intermediate
composition melts such as dacite and
andesite (Behrens et al, 2004; King et al,
2002).
Dacite
Andesite
Solubility data for mixed H2O + CO2 fluids in natural
melts
•Most important for melt inclusion interpretation, yet only 8 new studies (see
Table 2).
•Good coverage for calc-alkaline rhyolite melts, but mafic and intermediate
studies are sparse, as are alkaline compositions.
Silicic: ~20-500 MPa, 800-1100°C (e.g. Tamic et al, 2001)
Mafic and intermediate: ~20-700 MPa, up to 1400°C (e.g. Dixon et al, 1995;
Botcharnikov et al, 2005, 2006, 2007).
•Difficult experimental solubility measurements:
Fluid composition measurement (low T manometry or weight-loss method).
Dissolved CO2 measurements can be problematic in mixed volatile bearing
glasses:
•multiple speciation in intermediate melts (Behrens et al, 2004; King & Holloway,
2002).
•potential matrix effects in calibrations of secondary techniques such as SIMS and
FTIR (Behrens et al, 2004; Moore and Roggensack, 2007).
General H2O + CO2 solubility behavior
Rhyolite
•Simple linear solubility dependence
as a function of fluid composition at
low P.
Less than 150 MPa for basalts (Dixon et
al, 1995; Botcharnikov et al, 2005), 200
MPa and lower for rhyolite (Tamic et al,
2001) and dacite (Behrens et al, 2004).
•More complex, non-linear
dependence for both H2O and CO2 at
higher P conditions.
500 MPa
200 MPa
Figure from Liu et al, 2005 (filled squares and circles
from Tamic et al, 2001; triangles, Blank et al, 1993;
open symbols, Fogel and Rutherford, 1990)
Dacite
•CO2 speciation changes with H2O
content (molecular CO2 decreases with
increasing H2O)
Figure from Behrens et al (2004)
Compositional dependence of H2O + CO2 solubility
•Dissolved CO2 is stabilized by
H2O in melt (non-Henrian),
particularly at high P.
XH2O(fluid)~0.45
P ~ 400 MPa
T = 1200°C
•Strong dependence of CO2 and
H2O content on melt composition
E.g. dissolved CO2 increases w/ increasing
CaO, Na2O, K2O, etc (Dixon, 1997;
Roggensack & Moore, 2008)
•Any H2O-CO2 solubility model
needs to take these complexities
into account.
Figure from Roggensack & Moore (2008)
Modeling the solubility of H2O-CO2 in natural melts
Types of models:
1. Regular solution (single
composition; Silver and Stolper,
1985)
2.
Empirical
3.
Compositionally dependent
(includes comp dependent
regular solution of Papale, 1997,
1999; Papale et al, 2006)
Limitations and caveats:
•
Extrapolation beyond range of
data (P, T, or compositionally)
•
Interpretation of fit parameters
(e.g. partial molar volume of H2O
and CO2)
Extrapolation leads to significant error when inverting melt inclusion volatile
contents to obtain saturation pressure!
Adventures in solubility model extrapolation
Pressure extrapolation
Compositional extrapolation
Figure comparing rhyolite-H2O solubility
models from Behrens & Jantos, (2001).
Figure showing the compositional variable (PI) from the
basalt-CO2 solubility model of Dixon (1997).
•Note good fit of Moore model to data up
to 200 MPa, and instability when
extrapolated above 300 MPa.
•Note that calc-alkaline basalts have significantly
different CaO/Al2O3 (strong effect on CO2 solubility).
•Some give zero or negative PI values.
•Basis for Newman & Lowenstern (2002) VolatileCalc
H2O-CO2 model that is widely used for melt inclusions.
Melt compositional variation in melt inclusions and H2O + CO2
solubility models
How significant is compositional variation in melt
inclusion suites? (See Fig 10 for ref’s)
Only 2 compositionally dependent mixed H2O-CO2
solubility models available:
1.
VolatileCalc (Newman & Lowenstern, 2002)
Rhyolite: regular solution model for calc-alkaline rhyolite
(Silver et al, 1990; Blank et al, 1993). Note: No melt
compositional dependence for H2O or CO2 solubility.
Basalts: regular solution model w/ compositional dependence
for CO2 calibrated by alkali-rich basalts (Dixon et al,
1995; Dixon, 1997). No compositional dependence for
H2O in model.
2.
Papale et al (2006)
Uses most C-O-H solubility measurements to calibrate a
compositionally dependent regular solution model across a
broad range of P, T, and melt composition.
Recently made available for general use by Dr. Mark Ghiorso.
(http://ctserver.ofm-research.org/Papale/Papale.php)
Comparison of VolatileCalc and Papale to silicic solubility data
Calculated fluid compositions and saturation
pressures for rhyolite (77 wt% SiO2) and
dacite (66 wt% SiO2) versus experimental
values
•Good agreement for both VolatileCalc and
Papale with the rhyolite data.
•Note failure of VC to estimate the dacite
fluid compositions and pressures (no
compositional dependence), while Papale
matches data quite well.
Rhyolite data from Tamic et al (2001)
Dacite data from Behrens et al (2004)
Comparison of VolatileCalc and Papale models to basaltic solubility data
1.
VolatileCalc
Calcic and calc alkaline basalt data (45-53 wt% SiO2) from
Moore et al (2006) and Moore et al (2008).
Some of data beyond the stated 500 MPa
limit of VC, but majority is at or below.
Systematic overestimation of saturation
pressure and underestimation of mole
fraction of H2O in fluid.
Significant error (up to 50%) in pressure
estimate due to extrapolation of the
compositional parameter used for CO2
solubility. The model is unable to
account for the higher CO2 solubility in
calc-alkaline compositions.
Comparison of VC and Papale to basaltic experimental data
2. Papale et al (2006)
Basalt data same as for VC, andesite (57 wt% SiO2) from
Botcharnikov et al (2007)
For calculated saturation pressures and fluid
compositions, values scatter around 1:1
line. Up to 30% error in pressure
estimates.
Large amount of scatter in fluid composition
estimates (systematic error for calcic
basalt). Possibly due to error in fluid
measurements used to calibrate model.
Best model currently available that can
account for broad melt compositional
variation over magmatic P-T range.
Papale et al (2006)
Application of VC and Papale et al (2006) to basaltic melt
inclusions
Isobars and degassing paths calculated using VolatileCalc for Cerro Negro inclusions.
•Significant error in pressure using VC
for calc-alkaline basalts.
•Isobars and degassing paths do not
account for melt composition variation
(49-52 wt% SiO2; 9.5-13 wt% CaO).
Cerro Negro MI data from Roggensack (2001)
Application of Papale et al (2006) to basaltic
melt inclusions
Calculated minimum saturation pressures versus
calculated fluid composition, measured CO2 and H2O
content of Cerro Negro melt inclusions using Papale et al
(2006).
•More precise pressure estimates for calc-alkaline melts
(usually lower estimated P).
•Accounts for solubility dependence on compositional
variation in melt inclusions.
•Recast data allows identification of pressure regions
critical to fluid/melt evolution of the magma (150-250
MPa).
•Theoretical degassing behavior (e.g. open vs closed) in a
compositionally variable system is not easily visualized.