3.2 f. Oxygen-fugacity constraints in multianvil high pressure
experiments (C.K. Geßmann, C.A. McCammon and D.C. Rubie)
Despite the relatively large sample volume in multianvil experiments
it is difficult to apply usual buffering methods such as a double capsule
technique to control oxygen fugacity. It is therefore important to understand
the constraints on oxygen fugacity in experiments which are performed to
simulate mantle conditions.
Experiments with mantle phases have been performed to investigate the
reliability of common methods which are used to impose and/or deduce redox
Relative oxygen fugacities can be inferred from experiments performed at
fixed conditions of pressure, temperature, time and composition (P, T,
t, x) but using different capsule materials, assuming that the positions
of buffer curves relative to one another are preserved at high pressure.
The variation of starting material composition (e.g. oxygen content, reducing
components) can be used to impose different redox states while P, T, t
conditions of the experiments remain fixed.
The fO2 values (relative to a metal-metal oxide buffer)
can be estimated for experiments containing a metal M (e.g. Fe) and a corresponding
MOx-bearing phase (e.g. magnesio-wüstite, mw) according
to the relation: log fO2 (rel. to IW) = 2* log (aFeOmw
/ aFemet). This simple calculation assumes ideal activities
of both components.
The investigation of the samples allows conclusions to be made about (a)
the buffering capacity of the capsule material or the sample composition
and (b) the reliability of fO2 values calculated from
the phase relations of the sample:
The Fe3+ content of different mantle minerals (e.g.
majorite, perovskite) at experimental conditions were directly measured
with Mössbauer spectroscopy, and used to infer the relative redox
states provided by different capsule materials. Results suggested that
different capsule materials indeed impose different oxygen fugacities,
thus indicating that the capsule material may provide sufficient buffering
capacity for relatively short run times.
A systematic variation of Si and O contents in the starting material of
partitioning experiments between liquid metal and magnesiowüstite
at high pressures (9 and 18 GPa) and high temperature (2200°) was used
to impose different oxygen fugacities. The fO2 values
estimated using the above equation show a variation of 4 log bar units.
The results confirm that changes of the starting material at given P, T
conditions can indeed establish different oxygen fugacity levels.
A comparison of fO2 data calculated assuming that
Fe in Fe-Ni-liquids and FeO in magnesiowüstite display ideal mixing,
(i.e. the activity coefficients equal 1) with fO2
constraints employing activity coefficients given in the literature shows
that the two data sets agree within 0.5 log bar units. This result suggests
that such a calculation can be a valuable tool for obtaining fO2
estimates even if the respective activity relations at high pressures and
temperatures are not known.
Further information on the redox conditions in multianvil experiments is
obtained by a temperature-dependent set of magnesiowüstite - liquid
metal equilibria. Figure 3.2-5 shows the calculated oxygen fugacities as
a function of temperature.
Fig. 3.2-5: Calculated log fO2 values
for metal-magnesiowüstite assemblages as a function of temperature.
The oxygen fugacity is internally buffered below a 'critical' temperature
above which it rapidly increases.
Below a certain 'critical' temperature the calculated oxygen fugacity
is essentially constant while above it rapidly increases with increasing
temperature. The increase appears to be nearly linear with a value
0.4 log bar units per 100°C. The absolute value of the `critical´
temperature ( 2200°C for the investigated
composition at 9 GPa) may vary with experimental parameters. To further
constrain the nature of these relations, corresponding investigations at
different fO2 levels are under way (indicated by open
symbols in the figure). Preliminary results indicate that the fO2
values show also a weak dependence on pressure such that with increasing
pressure the redox conditions slightly increase. These investigations imply
that the buffering capacity of the sample composition can be restricted
to certain P, T, t conditions. As a consequence the results of high pressure
experiments, particularly those as a function of temperature, may be interpreted
erroneously if the influence of oxygen fugacity is not adequately considered.