5 Mössbauer Spectroscopy with High Spatial Resolution …
249
melt densities and their relative buoyancy compared to bulk mantle material; hence
analogue studies of glass can place constraints on the dynamics of molten mantle
[182–187]. Also, stability studies of phases that undergo transformations involving
exchange of oxygen provide information on deep Earth volatile cycles [188, 189].
Most studies reported in Table 5.4 use detailed fits of NFS and/or SMS spectra
to monitor the electronic environment around the nucleus. The challenge of nonunique fitting models for NFS spectra (Sect. 5.3.1) is addressed through consistency
checks with energy domain spectra collected on the same sample (usually with RPS)
and using fitting software (Sect. 5.5) that incorporates realistic models for atomic
environments (including hyperfine parameter distributions) as well as exploration of
parameter space (for example using a Monte Carlo approach) to find other possible
solutions [48]. In some cases, however, a simpler approach can be used for NFS
measurements, particularly when monitoring processes in situ. For example, chemical reaction rates can be determined with high temporal resolution (30 s) by integrating counts within time windows [21] and melting points can be determined by
monitoring signal intensity [190, 191].
5.6.3 Ex Situ High-Pressure and/or High-Temperature
Studies
High-pressure experiments allow controlled studies of minerals at conditions
relevant to Earth’s interior. Mineral phases can be quenched to preserve chemistry,
oxidation state, isotopic composition and often defect structure; hence post mortem
analysis can provide important information about mineral history. Such studies have
focussed primarily on samples quenched from large volume high-pressure experiments, mainly due to the high degree of control that can be maintained during experiments (for example temperature, oxygen fugacity, hydrogen fugacity, and so on) as
well as straightforward procedures for sample recovery. Not surprisingly, therefore,
most studies have used RPS Mössbauer spectroscopy since beam sizes can be large
(>100 μm diameter) and lab-based Mössbauer measurements are more convenient
(Table 5.5).
Research questions involving hydration dominate the list of studies. The incorporation of hydrogen into iron-bearing phases (both crystalline and glass) can alter
charge balance; hence determination of Fe
3+ /Fe values provides information on
charge balance mechanisms that help for inferring hydrogen solubility, calculating
deep water reservoirs and quantifying the deep Earth water cycle [195–210]. Sample
characterisation combined with physical property measurements is also a common
application. For example electrical conductivity studies of mantle minerals can be
combined with Mössbauer measurements of Fe
3+ /Fe to determine charge carrier
249
melt densities and their relative buoyancy compared to bulk mantle material; hence
analogue studies of glass can place constraints on the dynamics of molten mantle
[182–187]. Also, stability studies of phases that undergo transformations involving
exchange of oxygen provide information on deep Earth volatile cycles [188, 189].
Most studies reported in Table 5.4 use detailed fits of NFS and/or SMS spectra
to monitor the electronic environment around the nucleus. The challenge of nonunique fitting models for NFS spectra (Sect. 5.3.1) is addressed through consistency
checks with energy domain spectra collected on the same sample (usually with RPS)
and using fitting software (Sect. 5.5) that incorporates realistic models for atomic
environments (including hyperfine parameter distributions) as well as exploration of
parameter space (for example using a Monte Carlo approach) to find other possible
solutions [48]. In some cases, however, a simpler approach can be used for NFS
measurements, particularly when monitoring processes in situ. For example, chemical reaction rates can be determined with high temporal resolution (30 s) by integrating counts within time windows [21] and melting points can be determined by
monitoring signal intensity [190, 191].
5.6.3 Ex Situ High-Pressure and/or High-Temperature
Studies
High-pressure experiments allow controlled studies of minerals at conditions
relevant to Earth’s interior. Mineral phases can be quenched to preserve chemistry,
oxidation state, isotopic composition and often defect structure; hence post mortem
analysis can provide important information about mineral history. Such studies have
focussed primarily on samples quenched from large volume high-pressure experiments, mainly due to the high degree of control that can be maintained during experiments (for example temperature, oxygen fugacity, hydrogen fugacity, and so on) as
well as straightforward procedures for sample recovery. Not surprisingly, therefore,
most studies have used RPS Mössbauer spectroscopy since beam sizes can be large
(>100 μm diameter) and lab-based Mössbauer measurements are more convenient
(Table 5.5).
Research questions involving hydration dominate the list of studies. The incorporation of hydrogen into iron-bearing phases (both crystalline and glass) can alter
charge balance; hence determination of Fe
3+ /Fe values provides information on
charge balance mechanisms that help for inferring hydrogen solubility, calculating
deep water reservoirs and quantifying the deep Earth water cycle [195–210]. Sample
characterisation combined with physical property measurements is also a common
application. For example electrical conductivity studies of mantle minerals can be
combined with Mössbauer measurements of Fe
3+ /Fe to determine charge carrier