5 Mössbauer Spectroscopy with High Spatial Resolution …
245
reflected light image of the thick section was imported and used to define the coordinate system; (2) the x–y locations of the Mössbauer analyses were identified and
entered into a data table with all fitting results and then imported; (3) scanning electron images at the micron scale were imported and keyed to their locations within
the reference frame; (4) chemical analysis data from the electron microprobe were
imported and aligned with the images; and (5) determinations of pressure, temperature and oxygen fugacity from adjoining garnet and clinopyroxene grains (based
on chemical analyses, Mössbauer Fe
3+ /Fe ratios and thermodynamic models) were
imported and keyed to the relevant clinopyroxene grains. The resulting QGIS map file
can be displayed within the QGIS environment with any combination of images and
data and facilitates interrogation of all information and exploration of spatial relationships. Clicking on individual layers brings up a list of every piece of data attached
to objects within that layer and allows individual analysis points to be identified.
After exploring the QGIS maps they can be exported in different formats for
publication. Figure 5.13b illustrates an image from the eclogite example that shows
the location and diameter of Mössbauer analyses combined with two determinations
of pressure, temperature and oxygen fugacity. These results combined with analyses
of other grains allowed the history of the rock to be elucidated—from subduction to
residence in the mantle to final exhumation (C. McCammon, unpublished).
5.6 Applications of High Spatial Resolution Measurements
The myriad of applications using the techniques described above are the showcase
of this chapter. Section 5.6 summarises studies organised according to the broad
research topic that was investigated, the property of the sample targeted in the study,
the specific observable that was measured, and the technique that was used. The
compilation targets all studies reported in the peer-reviewed literature involving small
beam size that address geoscience research questions or investigate materials of
geological importance.
5.6.1 In Situ High-Pressure Studies with a Radioactive
Source
The first Mössbauer spectra collected using a radioactive point source (RPS) were
in situ measurements under high pressure. Indeed, the very first high-pressure Mössbauer study [50] was conducted not long after the Mössbauer effect was first discovered. The source was prepared by diffusing 74 MBq (2 mCi) of
57 Co into an iron
disk of approximately 700 μm diameter, giving a specific activity of 20 GBq/cm
2
(0.5 Ci/cm
2 ), which was essentially the first
57 Co point source. Initial interest in high
pressure came from laboratories focussed on physics and chemistry. For example,
245
reflected light image of the thick section was imported and used to define the coordinate system; (2) the x–y locations of the Mössbauer analyses were identified and
entered into a data table with all fitting results and then imported; (3) scanning electron images at the micron scale were imported and keyed to their locations within
the reference frame; (4) chemical analysis data from the electron microprobe were
imported and aligned with the images; and (5) determinations of pressure, temperature and oxygen fugacity from adjoining garnet and clinopyroxene grains (based
on chemical analyses, Mössbauer Fe
3+ /Fe ratios and thermodynamic models) were
imported and keyed to the relevant clinopyroxene grains. The resulting QGIS map file
can be displayed within the QGIS environment with any combination of images and
data and facilitates interrogation of all information and exploration of spatial relationships. Clicking on individual layers brings up a list of every piece of data attached
to objects within that layer and allows individual analysis points to be identified.
After exploring the QGIS maps they can be exported in different formats for
publication. Figure 5.13b illustrates an image from the eclogite example that shows
the location and diameter of Mössbauer analyses combined with two determinations
of pressure, temperature and oxygen fugacity. These results combined with analyses
of other grains allowed the history of the rock to be elucidated—from subduction to
residence in the mantle to final exhumation (C. McCammon, unpublished).
5.6 Applications of High Spatial Resolution Measurements
The myriad of applications using the techniques described above are the showcase
of this chapter. Section 5.6 summarises studies organised according to the broad
research topic that was investigated, the property of the sample targeted in the study,
the specific observable that was measured, and the technique that was used. The
compilation targets all studies reported in the peer-reviewed literature involving small
beam size that address geoscience research questions or investigate materials of
geological importance.
5.6.1 In Situ High-Pressure Studies with a Radioactive
Source
The first Mössbauer spectra collected using a radioactive point source (RPS) were
in situ measurements under high pressure. Indeed, the very first high-pressure Mössbauer study [50] was conducted not long after the Mössbauer effect was first discovered. The source was prepared by diffusing 74 MBq (2 mCi) of
57 Co into an iron
disk of approximately 700 μm diameter, giving a specific activity of 20 GBq/cm
2
(0.5 Ci/cm
2 ), which was essentially the first
57 Co point source. Initial interest in high
pressure came from laboratories focussed on physics and chemistry. For example,