5 Mössbauer Spectroscopy with High Spatial Resolution …
251
high pressure using other methods allows a more robust interpretation of the bulk
data [229–231].
Mössbauer measurements of Fe
3+ /Fe in lower mantle minerals have delivered
many profound revelations regarding mantle chemistry [35, 232–241]. These include
the discovery that ferric iron is predominant in the lower mantle instead of ferrous
iron, that disproportionation of ferrous iron produces metallic iron throughout the
lower mantle, and that deep blobs of material near the core-mantle boundary can
be explained by the density contrast caused by different iron oxidation states. Mössbauer spectroscopy also provides information on crystal chemistry that can be used to
constrain mantle properties and speciation of volatiles [242–248] as well as isotopic
fractionation [249–251]. Finally, measurements of phase stability are important
ingredients for determining the composition of different regions in Earth’s interior
[252–254].
There are a number of other micro-techniques used to determine Fe
3+ /Fe ratios
in geological samples (for example XANES [255] and the flank method [256]), but
Mössbauer spectroscopy has an important advantage over these methods in that it
requires no calibration to obtain accurate results. It is therefore ideally suited for
calibrating synthetic XANES and flank method standards, and a small beam size
enables the same grain to be used for all methods (Sect. 5.4.2) [33, 34, 257, 258].
5.6.4 Inclusions in Diamond
Diamonds are natural samples from Earth’s interior. When they crystallise in Earth’s
mantle, they can incorporate mineral phases from their surroundings and protect them
from alteration during rapid exhumation of diamonds during kimberlite eruptions.
Mössbauer measurements of mineral inclusions therefore provide important information about properties and processes inside Earth. Since Mössbauer spectroscopy
is non-destructive and measurements can be made directly through host diamonds, it
is ideally suited for measuring these rare and precious samples (Sect. 5.4.2). Initially
measurements were carried out using RPS Mössbauer spectroscopy, which limited
the diameter of inclusions that could be studied to greater than around 200 μm (since
there is no possibility to enrich samples in
57 Fe), but the use of SMS provides the
capability to measure smaller inclusions.
Most studies have focussed on determination of Fe
3+ /Fe ratios of minerals
enclosed by diamond (Table 5.6). One obvious application is to decipher conditions
during diamond formation [23, 41, 259, 260], but studies have also been carried
out to determine oxygen fugacity [39, 261, 262] and hydration conditions [263] in
regions of the mantle where diamond formation takes place. Iron oxidation state is
also influenced by other volatile species such as carbon, which provides an ancient
tracer of the deep carbon cycle [264].
251
high pressure using other methods allows a more robust interpretation of the bulk
data [229–231].
Mössbauer measurements of Fe
3+ /Fe in lower mantle minerals have delivered
many profound revelations regarding mantle chemistry [35, 232–241]. These include
the discovery that ferric iron is predominant in the lower mantle instead of ferrous
iron, that disproportionation of ferrous iron produces metallic iron throughout the
lower mantle, and that deep blobs of material near the core-mantle boundary can
be explained by the density contrast caused by different iron oxidation states. Mössbauer spectroscopy also provides information on crystal chemistry that can be used to
constrain mantle properties and speciation of volatiles [242–248] as well as isotopic
fractionation [249–251]. Finally, measurements of phase stability are important
ingredients for determining the composition of different regions in Earth’s interior
[252–254].
There are a number of other micro-techniques used to determine Fe
3+ /Fe ratios
in geological samples (for example XANES [255] and the flank method [256]), but
Mössbauer spectroscopy has an important advantage over these methods in that it
requires no calibration to obtain accurate results. It is therefore ideally suited for
calibrating synthetic XANES and flank method standards, and a small beam size
enables the same grain to be used for all methods (Sect. 5.4.2) [33, 34, 257, 258].
5.6.4 Inclusions in Diamond
Diamonds are natural samples from Earth’s interior. When they crystallise in Earth’s
mantle, they can incorporate mineral phases from their surroundings and protect them
from alteration during rapid exhumation of diamonds during kimberlite eruptions.
Mössbauer measurements of mineral inclusions therefore provide important information about properties and processes inside Earth. Since Mössbauer spectroscopy
is non-destructive and measurements can be made directly through host diamonds, it
is ideally suited for measuring these rare and precious samples (Sect. 5.4.2). Initially
measurements were carried out using RPS Mössbauer spectroscopy, which limited
the diameter of inclusions that could be studied to greater than around 200 μm (since
there is no possibility to enrich samples in
57 Fe), but the use of SMS provides the
capability to measure smaller inclusions.
Most studies have focussed on determination of Fe
3+ /Fe ratios of minerals
enclosed by diamond (Table 5.6). One obvious application is to decipher conditions
during diamond formation [23, 41, 259, 260], but studies have also been carried
out to determine oxygen fugacity [39, 261, 262] and hydration conditions [263] in
regions of the mantle where diamond formation takes place. Iron oxidation state is
also influenced by other volatile species such as carbon, which provides an ancient
tracer of the deep carbon cycle [264].