Contrib Mineral Petrol (2011) 162:1249–1257
DOI 10.1007/s00410-011-0649-9
ORIGINAL PAPER
Microanalysis of the iron oxidation state in (Mg,Fe)O
and application to the study of microscale processes
Micaela Longo • Catherine A. McCammon
Steven D. Jacobsen
•
Received: 25 February 2010 / Accepted: 6 May 2011 / Published online: 20 May 2011
Ó Springer-Verlag 2011
Abstract We report application of the flank method using
the electron microprobe to a suite of twelve (Mg,Fe)O
samples with composition 2–47 wt% Fe and Fe3?/RFe = 1
to 11%, where Fe3?/RFe was determined independently
using Mössbauer spectroscopy on the same grains used for
the flank method measurements. A calibration curve of the
form Fe2? = A ? B 9 (RFe)2 ? C 9 (Lb/La) was fit to
the data and gave excellent agreement between Fe3?/RFe
calculated from the flank method and Fe3?/RFe determined
using Mössbauer spectroscopy. We found the method to be
sufficiently sensitive to determine meaningful variations in
Fe3?/RFe for geophysically relevant compositions of
(Mg,Fe)O (\25 wt% Fe), and calibration parameters
remained constant within experimental uncertainty over the
course of the entire study (20 months). Flank method
measurements on an inhomogeneous sample of synthetic
(Mg,Fe)O showed evidence of diffusion processes resulting
from rupture of the capsule during the high-pressure
experiment and the possibility to measure Lb/La variations
with a spatial resolution of a few microns. We detected the
presence of exsolved magnesioferrite in a suite of
Communicated by M. W. Schmidt.
M. Longo (&) C. A. McCammon
Bayerisches Geoinstitut, University of Bayreuth,
95440 Bayreuth, Germany
e-mail: [email protected]
S. D. Jacobsen
Department of Earth and Planetary Sciences,
Northwestern University, Evanston, IL 60208, USA
Present Address:
M. Longo
Department of Geosciences, University of Padua,
35122 Padua, Italy
(Mg,Fe)O single crystals using transmission electron
microscopy and Mössbauer spectroscopy. Flank method
measurements on the same suite of single crystals showed
enhanced Fe3?/RFe values, consistent with the presence of
magnesioferrite even though the grains were too small to
be resolved by conventional electron microprobe
measurements.
Keywords Iron oxidation state Flank method Electron microprobe Ferropericlase
Introduction
Ferropericlase, (Mg,Fe)O, is probably the most abundant
non-silicate oxide within the Earth. Current models suggest
that it makes up roughly 20% of the lower mantle, which
comprises more than half of the Earth by volume.
(Mg,Fe)O can incorporate a significant amount of Fe3?,
where its concentration and incorporation mechanism (e.g.
point defects) strongly influence the rheological and
transport properties. The strong contrast between these
properties of (Mg,Fe)O and those of (Mg,Fe)(Si,Al)O3
perovskite, believed to be the Earth’s most abundant phase,
suggests that the lower viscosity and higher atomic diffusivity and electrical conductivity of (Mg,Fe)O may dominate those properties of the lower mantle (e.g. Dobson et al.
1997; Yamazaki and Karato 2001; van Orman et al. 2003).
(Mg,Fe)O is stable over an extremely wide pressure and
temperature range, and it is the only lower mantle phase
that is also thermodynamically stable at ambient conditions. Additionally, it is the only lower mantle phase for
which natural samples have been recovered from the
Earth’s interior, namely as inclusions in diamonds of deep
mantle origin (e.g. McCammon 2001). The behaviour of
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Fe3? in (Mg,Fe)O and its relation to mantle properties can
therefore be studied either by experimental synthesis or
through examination of natural samples.
Studies of Fe3? in (Mg,Fe)O have been restricted up
until now with regard to sample size due to the limited
number of methods to analyse Fe3? on the microscale. For
example, Mössbauer spectroscopy measurements have
been performed on ferropericlase inclusions from lower
mantle diamonds using a high specific activity source
(‘point source’) (McCammon et al. 1997, 2004), but the
smallest inclusion that could be studied was 100 lm due to
physical limitations of the 57Co point source (McCammon
1994). Since the majority of inclusions in lower mantle
diamonds are less than 50 lm in diameter, the population
cannot be considered yet to have been adequately sampled.
The study of microscale processes in synthetic (Mg,Fe)O
has also been restricted with respect to Fe3? determination.
For example, diffusion of transition elements in MgO was
studied nearly 50 years ago (e.g. Wuensch and Vasilos
1962), but so far, it has not been possible to independently
measure diffusion coefficients of Fe2? and Fe3?.
One promising method to determine Fe3?/RFe on the
microscale is electron energy loss spectroscopy (EELS) on
a transmission electron microscope, which offers nanometre-scale spatial resolution (e.g. van Aken et al. 1998).
Although sample preparation involves a degree of sample
destruction, the advent of focussed ion beam thinning may
offer improved possibilities to study rare inclusions from
the lower mantle as well as optimise selection of the
sample region to be studied, such as perpendicular to diffusion profiles. Another promising method for Fe3? studies
is X-ray Absorption Near Edge Structure (XANES) spectroscopy, which has successfully been applied at a spatial
resolution of roughly 5 lm (e.g. Berry et al. 2008). However, neither of these methods has yet been applied to
determine Fe3?/RFe in (Mg,Fe)O, and more crucially,
neither method can be considered to be a routine application accessible to a large proportion of the geoscience
community to the extent of, for example, the electron
microprobe.
In the past decades, many studies have been devoted to
Fe3?/RFe determination from the FeLb and FeLa emission
spectra using the electron microprobe. One of the most
successful is the so-called flank method, which has shown
an unambiguous correlation of measured intensity ratios of
Fe Lb/La X-ray emission with the oxidation state in garnet,
independent of composition (Höfer and Brey 2007 and
references therein). Höfer and Brey (2007) demonstrated
that the sensitivity of Lb/La to Fe3?/RFe is a function of
both the total iron content and the coordination number
of the iron atoms, where the large coordination number of
iron in garnet gives the highest sensitivity. Earlier work
applying the flank method to FexO demonstrated that Fe3?/
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Contrib Mineral Petrol (2011) 162:1249–1257
RFe could be measured, even though the Lb/La variation
was less than for garnet (Höfer et al. 2000). For (Mg,Fe)O,
particularly samples with iron compositions relevant for
the lower mantle (\25 wt% Fe), it is not obvious whether
the sensitivity of the flank method is sufficient to determine
meaningful variations in Fe3?/RFe, particularly since values in lower mantle compositions may range only between
0 and 10%. The goal of this study, therefore, is to investigate the potential of the flank method to measure Fe3?/
RFe in geophysically relevant compositions of (Mg,Fe)O
and to demonstrate its application to the study of microscale processes.
Experimental methods
Sample synthesis
We synthesised a suite of (Mg,Fe)O samples covering a
wide range of composition (2–47 wt% Fe) and Fe3?/RFe
(1–11%). To synthesise the samples, Mg and Fe metals
were mixed in stoichiometric proportions to give a wide
compositional range of Fe in (Mg,Fe)O. The mixture was
enriched in 57Fe to approximately 10% of the Fe total to
optimise Mössbauer measurements. Metals were first dissolved in HNO3, and then, the mixture was heated to 50°C
to allow HNO3 to slowly evaporate. Subsequently, NH4OH
was added to obtain oxide precipitates. Excess HNO3 and
NH4OH were then removed by drying the obtained gels
using a Bunsen burner (1,200–1,500°C). All synthetic
powders were then equilibrated in a gas-mixing furnace
under CO/CO2 at 1,300°C and controlled oxygen fugacity
in order to obtain a wide range of Fe3?/RFe. Oxygen fugacities were varied over the range from 10-7 to 10-11 and
were monitored during the experiments using an oxygen
fugacity sensor. X-ray powder diffraction was used to
verify the structure of the polycrystalline powders.
In order to obtain the high-quality surfaces needed for
electron microprobe measurements, the grain size of the
polycrystalline samples was coarsened through annealing
at high pressure and temperature using a multianvil apparatus. The (Mg,Fe)O polycrystalline powders were loaded
into Re capsules of 1.6 mm diameter and 2 mm long.
Samples were compressed up to 15 GPa and heated to
1,800–2,000°C for about 1 h. The resulting samples were
analysed using Mössbauer spectroscopy to determine Fe3?/
RFe and then were mounted on epoxy or glass for the
electron microprobe measurements.
For Mössbauer measurements, the multianvil run products were cut into sections 100 lm thick and centred
behind a hole of 150 lm diameter in 25 lm thick Ta
foil. Mössbauer spectra were recorded at room temperature
(293 K) in transmission mode on a constant acceleration
Contrib Mineral Petrol (2011) 162:1249–1257
spectrometer with a nominal 370 MBq 57Co high specific
activity source in a 12-lm Rh matrix. The velocity scale
was calibrated relative to 25 lm thick a-Fe foil using the
positions certified for (former) National Bureau of Standards standard reference material no. 1541; line widths of
0.36 mm/s for the outer lines of a-Fe were obtained at
room temperature. On average, spectra took one day each
to collect. The spectra were analysed using the commercially available fitting program NORMOS written by R.A.
Brand (distributed by Wissenschaftliche Elektronik GmbH,
Germany). We fitted the Mössbauer spectra of (Mg,Fe)O to
two Fe2? doublets and one Fe3? doublet, all with Voigt
lineshape according to previous models (e.g. McCammon
et al. 1998). Fe3?/RFe was calculated from the relative
areas of the Fe3? absorption corrected for differences in
recoil-free fraction of Fe2? and Fe3?. Waychunas et al.
(1994) found that Mössbauer determinations of Fe3?/RFe
in (Mg,Fe)O were systematically higher than for thermogravimetric results, which is consistent with a higher
recoil-free fraction for Fe3? in the tetrahedral site compared to Fe2? in the octahedral site. We corrected the
relative areas as described by McCammon (2004) using
Mössbauer Debye temperatures of 390 and 550 K for Fe2?
and Fe3?, respectively.
Electron microprobe analysis
We determined the FeLa and FeLb positions on the Jeol
XA-8200 microprobe at Bayerisches Geoinstitut following
the procedure established by Höfer and Brey (2007). For
each electron microprobe, it is necessary to collect the
FeLa and FeLb emission spectra of standards in order to
establish the exact La and Lb positions needed for the
measurements. As references for Fe2? and Fe3?, we collected spectra on almandine and andradite standards,
respectively, at 15 kV and 100 nA (Table 1). Spectra were
collected using a TAP spectrometer with detector slits set
to the smallest aperture possible (which corresponds to
300 lm for a TAP spectrometer type) in order to reduce
as much as possible the background contribution during
flank method measurements. The FeLa and FeLb positions were identified from the difference spectrum as the
maximum and minimum points, respectively (Höfer and
Brey 2007). For comparison, FeO and Fe2O3 spectra were
also collected to determine the reproducibility of FeLb
and FeLa energies using Fe2?- and Fe3?-bearing phases
other than almandine and andradite. We found that La and
Lb positions determined using the two different Fe2?- and
Fe3?-bearing phase couples were consistent within the
estimated uncertainty (Table 1). We corrected the daily
shift of the FeLa and FeLb peak positions using the
software PEAK FIT (Höfer and Brey 2007) at 25 kV and
80 nA.
1251
Table 1 Flank method positions La and Lb for the Jeol JXA-8200
microprobe at Bayerisches Geoinstitut
Energy (eV)
mma
Almandine-andradite
FeO–Fe2O3
FeLa
FeLa
0.710
190.74
FeLb
0.700
188.09
0.711
190.82
FeLb
0.700
187.98
a
Distance between the position of the Fe La and Fe Lb lines on the
spectrometer crystal and the beam spot in the Rowland circle
geometry
The flank method parameter, Lb/La, is determined from
the intensity ratio of the count rates recorded at each of the
positions described above. In order to perform flank
method measurements at the same time as major element
analysis, we added two fictitious elements to the major
element list with energies set to the FeLa and FeLb positions determined above in order to measure the intensities
at those positions. Flank method measurements were
always performed on the TAP spectrometer.
We performed major element analysis and qualitative
analysis simultaneously at an acceleration voltage of 15 kV
and a current of 80 nA according to the procedure described by Höfer and Brey (2007). We measured the X-ray
emission counts for the two positions La and Lb for 180 s
each, for a total time of 360 s for each measured spot.
Whenever possible, we performed measurements along
line profiles with a minimum of 20–30 spots for each
sample during each microprobe session to obtain reliable
statistics. The Lb/La ratio was then used to determine the
calibration for Fe3?/RFe based on the Mössbauer
measurements.
To test the flank method on the Jeol XA-8200 electron
microprobe at Bayerisches Geoinstitut, we performed
simultaneous major element analysis plus flank method
measurements on a suite of natural garnets that had also
been measured using Mössbauer spectroscopy (Longo
2009). We confirmed the observations of Höfer and Brey
(2007) that Lb/La increased linearly as function of Fe2?
(wt%) and that flank method Fe3?/RFe values were in
excellent agreement with those from Mössbauer spectroscopy (Longo 2009). However, we did find that calibration
parameters were different (their Eq. 3), which is likely due
to slight differences in analyser crystal geometry between
the two electron microprobes.
All synthetic (Mg,Fe)O samples previously analysed by
Mössbauer spectroscopy were mounted on epoxy or glass
for the electron microprobe measurements. For major element analysis on synthetic (Mg,Fe)O, only Fe and Mg were
calibrated. Fe metal and MgO were used as standards for
Fe and Mg, respectively. Counting time for both was 20 s
(peak) and 10 s (background), and the point beam was
focused on the smallest diameter possible (1 lm). The
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correction used for self-absorption and matrix effect was
the phi-rho-z type, and the oxygen concentration was calculated indirectly from the FeO and MgO oxides.
Before starting the calibration measurements, the sample homogeneity was tested for all synthesised samples,
and outer regions of each sample that were potentially
affected by diffusion processes during synthesis using the
multianvil apparatus were excluded from the analysis. One
of the synthetic (Mg,Fe)O samples was chosen as a standard that was measured before and after the measurements
of unknown samples in order to monitor possible spectrometer drifts which could cause a shift in Lb/La ratios
and therefore affect the accuracy in the determination of
Fe3?/RFe.
Results and discussion
Calibration of the flank method on synthetic (Mg,Fe)O
We measured the variation of Lb/La as a function of RFe
for the twelve synthetic (Mg,Fe)O ferropericlase samples
that showed the highest homogeneity and followed a similar approach used for garnets by Höfer and Brey (2007)
using least-squares regression to fit the three variables RFe,
Lb/La and Fe2?. Through many fitting trials, we found that
the best and simplest fit for Fe2? in (Mg,Fe)O with RFe
between 2 and 47 wt% is given by:
Fe2þ ¼ A þ B ðRFeÞ2 þC ðLb=LaÞ
A comparison of Fe3?/RFe determined using the flank
method with Fe3?/RFe calculated using Mössbauer spectroscopy shows good agreement within experimental error
(Fig. 2). Error bars for the flank method were calculated by
ð1Þ
In all other formulations with more parameters, the
system was overdetermined, producing highly correlated
parameters with consequently large uncertainties. Our
calibration gave the following parameters: A =
-18.16(51), B = 0.0078(2) and C = 34.61(66) where
both Fe2? and RFe are in wt%. RFe was calculated from
electron microprobe data assuming no cation vacancies,
since inclusion of their effect would require a priori
knowledge of Fe3? concentrations. However, we estimated
the influence of non-stoichiometry on the calculations and
found it to be smaller than experimental error for
compositions with RFe \50 wt% and Fe3?/RFe \0.2.
The experimental data are plotted in Fig. 1 with the
calibration curves calculated for constant values of Fe3?/
RFe, which shows the variation of Lb/La with both RFe
and Fe3?/RFe. Similar to the results for garnet reported by
Höfer and Brey (2007), the sensitivity of the flank method
increases with increasing Fe concentration. From Eq. 1, the
detection limit for Fe3?/RFe can be estimated based on the
iron concentration and the precision of Lb/La determination. For example, a precision of 0.01 enables a minimum
difference in Fe3?/RFe of roughly 0.05 to be detected for
ferropericlase with a composition of Mg0.95Fe0.05O.
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P
Fig. 1 Lb/La variation as a function of
Fe (wt%) for twelve
synthetic (Mg,Fe)O samples plotted with isopleths of Fe3?/RFe
(values indicated on each line) determined using (1)
Fig. 2 Comparison of Fe3?/RFe determined using the flank method
according to (1) with Fe3?/RFe determined using Mössbauer
spectroscopy. Error bars are indicated for all points (solid circles)
except for those with RFe \3 wt% (open circles), where the error
bars were omitted due to their large size (± 0.015 for x values
and ± 0.20 for y values). The dotted line indicates 1:1
correspondence
Contrib Mineral Petrol (2011) 162:1249–1257
1253
Table 2 Flank method results for synthetic (Mg,Fe)O samples used for calibration
Run
RFe (wt%)
Lb/La
Fe3?/RFe Mössbauer
Fe3?/RFe Flank method
0.006 (205)
S4068
2.44 (15)
0.594 (20)
0.027 (15)
S4099
2.46 (23)
0.594 (16)
0.015 (15)
0.001 (195)
S4117
12.57 (20)
0.846 (11)
0.024 (15)
0.016 (37)
S4139
13.23 (20)
0.816 (11)
0.107 (25)
0.136 (35)
S4044
22.79 (6)
0.999 (15)
0.109 (20)
0.102 (22)
S4123
22.98 (20)
1.005 (15)
0.099 (25)
0.097 (23)
S3939
29.58 (75)
1.104 (18)
0.098 (30)
0.092 (32)
S3941
33.03 (47)
1.170 (15)
0.070 (30)
0.066 (22)
S3996
33.15 (32)
1.143 (11)
0.084 (30)
0.096 (18)
S4028
33.44 (7)
1.189 (17)
0.070 (25)
0.064 (16)
S4153
S4155
39.72 (26)
46.81 (26)
1.232 (27)
1.285 (32)
0.075 (20)
0.069 (20)
0.074 (19)
0.073 (17)
Fe3?/RFe determined by Mössbauer spectroscopy and by the flank method using (1)
propagating the uncertainties in the equation parameters, as
well as in RFe and Lb/La obtained from the mean value of
20–30 independent measurements at 15 kV and 80 nA.
Results obtained for flank method measurements plus Fe
bulk compositions, as well as a comparison between Fe3?/
RFe ratios determined by flank method and Mössbauer
spectroscopy data, are summarised in Table 2. We found
that subsequent measurements of the ferropericlase standards gave the same Lb/La values as previous measurements within experimental error and that the calibration
(Eq. 1) remained constant with time over the entire period
of the study (20 months).
Sample homogeneity and the potential to investigate
diffusion processes
Nearly all of the synthetic (Mg,Fe)O samples appeared to
be homogeneous as shown by the random variation of Lb/
La versus Fe (wt%) within the experimental error of the
measurements. However, one of the multianvil run products (S4149) showed a large variation in flank method
measurements taken over different parts of the sample. We
therefore omitted that particular sample from the calibration fitting procedure, but performed further analysis to
explore the potential of the flank method to quantify
inhomogeneity.
The sample prepared from multianvil run S4149 showed
a large incursion of metal into the ferropericlase as well as
uneven electron reflectivity of the ferropericlase itself
(Fig. 3). Energy dispersive X-ray (EDX) analysis showed
the metal to contain Re, which likely came from the sample
capsule, and also La and Cr, which likely came from the
LaCrO3 heater, strongly suggesting that the capsule broke
during the experiment. EDX analysis showed that ferropericlase also contains 2–4 wt% Cr and that the
Fig. 3 Electron backscattered image of synthetic ferropericlase
sample S4149 suggesting a fracture of the Re capsule (right), leading
to incursion of Re as well as La and Cr (from the LaCrO3 furnace)
into the sample
concentrations of Cr and Fe appear to be inversely correlated. We performed flank method measurement profiles on
two of the ferropericlase grains in the inner part of the
sample, as well as one on ferropericlase near the outer
edge. Results show variations of Lb/La versus RFe that are
displaced significantly below the calibration line for zero
Fe3?/RFe, indicating that a reasonable proportion of Fe3?
is present (Fig. 4a). The actual Fe3?/RFe values determined from the flank method calibration show a consistent
trend with total Fe concentration, as well as a measureable
difference between the inner and outer parts of the sample
(Fig. 4b). Possible explanations for the difference in Fe3?/
RFe values across the sample include a gradient in oxygen
fugacity, interactions between Fe2?/Fe3? and Cr2?/Cr3?
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(a)
(e.g. Eeckhout et al. 2007), or a matrix effect due to Cr that
changes the Fe3?/RFe calibration. While more detailed
interpretation of the processes that occurred during run
S4149 is not justified due to the uncontrolled nature of the
experiment, the results do illustrate the ability of the flank
method to resolve variations in Lb/La across a sample and
demonstrate how the method could be applied to study
dynamic processes such as diffusion.
Synthetic (Mg,Fe)O containing exsolved
magnesioferrite
Fig. 4 Flank method measurements for sample S4149: a Lb/La
variation as a function of RFe wt% for measurement profiles from a
region in the inner part of the sample (open circles), from a second
region in the inner part of the sample (solid circles), and from the
outer part of the sample (grey circles). The lines show the isopleths of
the calibration equation for the indicated values of Fe3?/RFe; b Fe3?/
RFe variation calculated using the flank method for the same regions
of sample S4149
We measured a set of synthetic (Mg,Fe)O single crystals
consisting of nine samples with an iron compositional
range between 8 and 67 wt% that had been synthesised by
interdiffusion of Fe and Mg between single-crystal MgO
and (Mg,Fe)O prereacted powders (Reichmann et al. 2000;
Jacobsen et al. 2002). Previous Mössbauer measurements
had revealed the presence of minor amounts of magnesioferrite (Reichmann et al. 2000; Jacobsen et al. 2002), so
to further investigate the microstructure of the crystals, we
performed TEM analysis. Results confirmed the presence
of sub-micron grains of magnesioferrite and showed that its
distribution in the crystal is not homogeneous (Fig. 5).
One possible explanation for the presence of magnesioferrite in the (Mg,Fe)O single crystals are the oxidising
conditions during the annealing experiments (near the NiNiO buffer). Jacobsen et al. (2002) reported two series of
synthesis experiments: the first involved annealing at
1,450°C and fO2 = 10-7, while the second involved
annealing at more reducing conditions, namely 1,300°C
and fO2 = 10-10 bar. Mössbauer spectroscopy detected
magnesioferrite in the samples from the first series of
Fig. 5 TEM dark field microphotographs showing a comparison
between (Mg,Fe)O samples with similar iron concentrations for a
single crystal from Jacobsen et al. (2002) (Fe15) (left) and a sample
from the present study (S3855 with 18.7 wt% Fe and 5% Fe3?/RFe)
(right). The left image clearly shows cubic-shaped magnesioferrite
impurities smaller than 1 lm in the (Mg,Fe)O matrix, whereas the
right image shows no evidence of impurities. Laminar structures
visible in both images are caused by stacking faults in the (Mg,Fe)O
structure. The insets illustrate electron diffraction images taken from
the respective samples. The left pattern shows a combination of
ferropericlase and magnesioferrite, while the right pattern shows only
ferropericlase
(b)
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annealing experiments, but not in the samples from the
second series. Although both series of oxygen fugacity
conditions are within the (Mg,Fe)O stability field, the first
series conditions are quite close to the phase boundary with
magnesioferrite, and cooling could cause the phase equilibrium to shift into the magnesioferrite stability field,
causing exsolution of a secondary magnesioferrite phase
from the primary (Mg,Fe)O crystals. Such exsolution is
already well known in the Fe–O system, where magnetite
can exsolve from single-phase FexO on cooling (e.g. Hazen
and Jeanloz 1984).
We measured the Fe3? concentration in the residual
ferropericlase as well as the abundance of magnesioferrite using Mössbauer spectroscopy (Fig. 6). The presence
of magnesioferrite can be detected even at low abundance due to its occurrence as two magnetic sextets
whose intense peaks do not overlap those of ferropericlase (De Grave et al. 1979), and its abundance could be
estimated from its relative area corrected for the compositions based on the electron microprobe data and
assuming that the Fe2?/Mg ratio in the residual ferropericlase and the exsolved magnesioferrite are approximately the same (Speidel 1967). We collected Mössbauer
spectra at two velocity ranges, one small (–5 to ?5 mm/
s) in order to resolve the Fe3? component in ferropericlase (Fig. 6), and one large (-9 to ?9 mm/s) in order
to observe all of the magnesioferrite peaks. The results
show that the crystals from series 1 contain significantly
more magnesioferrite than those from series 2, where the
abundance of magnesioferrite in the latter samples was
below the detection limit (Table 3). A sample that had
been annealed at even more oxidising conditions
(T = 1,300°C and fO2 = 10-7.5 bar) showed the highest
magnesioferrite concentration (sample Fe78ox, Table 3).
We calculated the bulk Fe3?/RFe ratio for each sample
based on the individual Fe3? contents of each phase,
their iron concentrations, and the phase abundance in the
assemblage.
Due to the small size of the exsolved magnesioferrite,
the phase will contribute to both the major element analysis
and the flank method measurements; thus, both RFe and the
Lb/La ratios will be affected. Figure 7 illustrates the flank
method results obtained for the nine (Mg,Fe)O samples,
five from the first series (open circles), three from the
second series (grey triangles) and one from a third experiment that was the most oxidizing of all the annealing
series (1,300°C and fO2 = 10-7.5; solid circles). Results
show that the samples from the most reducing conditions
(grey triangles) are closest to the Fe3?/RFe = 0 calibration
line as expected. Fe3?/RFe values for sample Fe24 fall
between 0 and 0.1, which is roughly consistent with the
results from Mössbauer spectroscopy, particularly considering the difference in spatial resolution (discussed in more
1255
(a)
(b)
(c)
Fig. 6 Room temperature Mössbauer spectra of (Mg,Fe)O: a pure
(Mg,Fe)O from the present study (S4074 with 18.8 wt%Fe and 8%
Fe3?/RFe); b sample Fe15 containing 0.3 mol% magnesioferrite
(seen as a small deviation in the residual in the regions marked by
arrows); c sample Fe37 containing 4 mol% magnesioferrite (fitted
with sextets corresponding to magnesioferrite and the most intense
lines in the displayed velocity range are marked by arrows). The Fe3?
in ferropericlase components are shaded grey, and the residuals are
shown above each spectrum
detail below). The other samples in series 2, Fe53 and
Fe78, fall outside the calibration range, and it is evident
that the curvature of the calibration curve affects the
accuracy of the Fe3?/RFe determination outside of the
range (Fig. 7).
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Table 3 Quantification of Fe3? content and magnesioferrite fraction for synthetic (Mg,Fe)O single crystals containing (Mg,Fe)Fe2O4 using
Mössbauer spectroscopy
Sample
Seriesa
Fe3?/RFe (Mg,Fe)O
Mole fraction (Mg,Fe)Fe2O4
Fe3?/RFe bulk
Fe6
1
0.016 (10)
0.001 (1)
0.04 (2)
Fe15
1
0.009 (10)
0.003 (2)
0.04 (2)
Fe24
2
0.006 (10)
0.000 (1)
0.01 (1)
Fe27
1
0.052 (30)
0.004 (2)
0.08 (4)
Fe37
1
0.060 (30)
0.038 (19)
0.20 (7)
Fe53
2
0.068 (30)
0.000 (1)
0.07 (3)
Fe75
Fe78
1
2
0.123 (50)
0.072 (30)
0.047 (24)
0.000 (1)
0.21 (7)
0.07 (3)
Fe78ox
3
0.090 (30)
a
Conditions for annealing—series 1: T = 1,450°C and fO2 = 10
fO2 = 10-7.5 bar
0.28 (14)
-7
bar; series 2: T = 1,300°C and fO2 = 10
Fig. 7 Lb/La variation as a function of RFe for nine previously
studied (Mg,Fe)O single crystals (open and solid circles and grey
triangles) (Reichmann et al. 2000; Jacobsen et al. 2002) and a single
crystal of magnetite (grey stars). The samples indicated by open
circles (Fe6, Fe15, Fe27, Fe37, Fe75) were annealed at 1,450°C and
fO2 = 10-7 bar, the sample indicated by the solid circles (Fe78ox)
was annealed at 1,300°C and fO2 = 10-7.5 bar, and the samples
indicated by grey triangles (Fe24, Fe53, Fe78) were annealed at
1,300°C and fO2 = 10-10 bar. Thirty to sixty measurements made on
each crystal are mostly clustered at discrete values of RFe, but show
significant data scatter with Lb/La. The lines show the isopleths of the
calibration equation for the indicated values of Fe3?/RFe; solid lines
indicate values within the calibration range, while dashed lines
indicate values outside the calibration limits
The flank method results from the crystals of series 1
show varying agreement with the estimated bulk Fe3?/
RFe values determined using Mössbauer spectroscopy.
For samples Fe6, Fe15 and Fe27, all of which showed
the presence of magnesioferrite in Mössbauer spectra,
Fe3?/RFe is systematically overestimated by the flank
method (Fig. 7) compared to the Mössbauer estimates,
which fall in the range 0.04–0.08 (Table 3). In contrast,
for sample Fe37, Mössbauer data indicate that the bulk
123
0.44 (6)
-10
bar; series 3: T = 1,300°C and
Fe3?/RFe ratio should be *0.2, while the flank method
shows values less than 0.1 (Fig. 7). The difference in
behaviour may lie in the different spatial resolution of
the two methods, combined with the effect on Lb/La
ratios of different crystal structures (rocksalt versus
spinel).
Mössbauer spectroscopy as applied in this study is a
transmission method that probes the entire sample and
gives information about the average environment in each of
the phases. In contrast, the electron microprobe (and hence
the flank method) probes only the upper few microns of the
sample with a spatial resolution also on the order of a few
microns. Our TEM study showed that the distribution of
magnesioferrite is not homogeneous (Fig. 5), which means
that its distribution between the surface and the bulk may
also vary. If magnesioferrite were concentrated near the
surface, flank method measurements would overestimate
the amount of Fe3?, as we observed in our study (samples
Fe6, Fe15 and Fe27).
For samples with greater amounts of iron, the size of
the magnesioferrite grains would be larger, for example,
Reichmann et al. (2000) reported that magnesioferrite in
sample Fe75 could be observed with the optical microscope. In this case, the structure of magnetite would have
a stronger influence on the Lb/La ratios. We collected
flank method measurements on a single crystal of magnetite, which gave Lb/La values (grey stars, Fig. 7) that
represent the limit of maximum exsolution. Indeed, the
Lb/La values for the most oxidised samples (Fe75 and
Fe78ox) tend towards this limit. However, our calibration
model predicts that Lb/La values should be much lower
for such oxidised compositions (dashed curve labelled
0.67 in Fig. 7), which is likely due to both applying the
model outside its calibration limits and the effect of
different structures in modifying the Lb/La ratio (Höfer
and Brey 2007).
Contrib Mineral Petrol (2011) 162:1249–1257
Conclusions and further work
We successfully demonstrated the use of the flank method
to determine Fe3?/RFe ratios in geophysically relevant
compositions of (Mg,Fe)O with an uncertainty comparable
to that of Mössbauer spectroscopy. The flank method offers
a number of advantages over Mössbauer spectroscopy,
however: measuring times of only a few hours (compared
to 1–2 days for Mössbauer spectroscopy) and spatial resolution of 1–10 lm (compared to no less than 100 lm for
Mössbauer spectroscopy). Thus, the flank method is a
suitable technique to determine Fe3?/RFe in ferropericlase
inclusions in diamond, which are commonly less than
50 lm in size.
Results from measurements on inhomogeneous samples
demonstrate the possibility to quantify the variation of
Fe3?/RFe with a spatial resolution of a few microns in
samples subject to dynamic processes such as diffusion or
exsolution. For example, the diffusion coefficients of Fe2?
and Fe3? could be measured independently, providing
important insight into the kinetics of oxidation–reduction
reactions. Measurements of (Mg,Fe)O single crystals containing exsolved (Mg,Fe)Fe2O4 show enhanced Fe3?/RFe
in (Mg,Fe)O up to compositions of 40 wt% Fe, consistent
with the presence of ferrite grains even when these are too
small to be resolved by conventional electron microprobe
measurements.
Acknowledgments We are grateful to Stephen Mackwell for supplying some of the samples examined in this study, to Hugh O’Neill
for valuable advice concerning (Mg,Fe)O synthesis, to Heidi Höfer
for her support on the flank method calibration, to Detlef Krauße for
his technical support at the electron microprobe, to Nobuyoshi Miyajima for TEM investigations and to Florian Heidelbach for SEM
analyses. The manuscript was significantly improved by the comments of Andrew Berry and an anonymous reviewer. ML received
financial support from the European Commission under the Marie
Curie Action for Early Stage Training of Researchers within the 6th
Framework Programme (contract number MEST-CT-2005-019700).
SDJ is supported in part by the US NSF (EAR-0748707) and by the
David and Lucile Packard Foundation.
References
Berry AJ, Danyushevsky LV, HStC O’Neill, Newville M, Sutton SR
(2008) The oxidation state of iron in komatiitic melt inclusions
indicates hot Archean mantle. Nature 455:961–963
De Grave E, Govaert A, Chambaere D, Robbrecht G (1979)
Mössbauer effect study of MgFe2O4. Physica 96B:103–110
Dobson DP, Richmond NC, Brodholt JP (1997) A high-temperature
electrical conduction mechanism in the lower mantle phase (Mg,
Fe)1-xO. Science 275:1779–1781
1257
Eeckhout SG, Bolfan-Casanova N, McCammon C, Klemme S,
Amiguet E (2007) XANES study of the oxidation state of Cr
in lower mantle phases: periclase and magnesium silicate
perovskite. Am Mineral 92:966–972
Hazen RM, Jeanloz R (1984) Wüstite (Fe1-xO): a review of its defect
structure and physical properties. Rev Geophys Space Phys
22:37–46
Höfer HE, Brey GP (2007) The iron oxidation state of garnet by electron
microprobe: its determination with the flank method combined
with major-elements analysis. Am Mineral 92:873–885
Höfer HE, Weinbruch S, McCammon CA, Brey GP (2000) Comparison of two electron probe microanalysis techniques to determine
ferric iron in synthetic wüstite samples. Eur J Mineral 12:63–71
Jacobsen SD, Reichmann HJ, Spetzler HA, Mackwell SJ, Smyth JR,
Angel RJ, McCammon CA (2002) Structure and elasticity of
single-crystal (Mg,Fe)O and a new method of generating shear
waves for gigahertz ultrasonic interferometry. J Geophys Res
107(B2):2037. doi:10.1029/2001JB000490
Longo M (2009) Iron oxidation state of (Mg,Fe)O: calibration of the
flank method (EMPA) on synthetic samples and applications on
natural samples from lower mantle diamonds. Dissertation.
University of Bayreuth, Germany
McCammon CA (1994) A Mössbauer milliprobe: practical considerations. Hyperfine Interact 92:1235–1239
McCammon C (2001) Deep diamond mysteries. Science 293:813–814
McCammon CA (2004) Mössbauer spectroscopy: Applications. In:
Beran A, Libowitsky E (eds) Spectroscopic Methods in Mineralogy, vol 6. Eötvös University Press, Budapest, pp 369–398
McCammon CA, Hutchison M, Harris J (1997) Ferric iron content of
mineral inclusions in diamonds from São Luiz: a view into the
lower mantle. Science 278:434–436
McCammon CA, Peyronneau JP, Poirier J-P (1998) Low ferric iron
content of (Mg, Fe)O at high pressures and high temperatures.
Geophys Res Lett 25:1589–1592
McCammon CA, Stachel T, Harris JW (2004) Iron oxidation state in
lower mantle mineral assemblages II. Inclusions in diamonds
from Kankan, Guinea. Earth Planet Sci Lett 222:423–434
Reichmann HJ, Jacobsen SD, Mackwell SJ, McCammon CA (2000)
Sound wave velocities and elastic constants for magnesiowüstite
using gigahertz interferometry. Geophys Res Lett 27:799–802
Speidel DH (1967) Phase equilibria in the system MgO-FeO-Fe2O3:
the 1300°C isothermal section and extrapolations to other
temperatures. J Amer Ceram Soc 50:243–248
van Aken PA, Liebscher B, Styrsa VJ (1998) Quantitative determination of iron oxidation state in minerals using FeL23-edge
electron energy-loss near-edge structure spectroscopy. Phys
Chem Mineral 25:323–327
van Orman JA, Fei Y, Hauri EH, Wang J (2003) Diffusion in MgO at
high pressures: constraints on deformation mechanisms and
chemical transport at the core-mantle boundary. Geophys Res
Lett 30: doi:10.1029/2002GL016343, 2003
Waychunas GA, Dollase WA, Ross C II (1994) Short-range order
measurements in MgO-FeO and MgO-LiFeO, solid solutions by
DLS simulation-assisted EXAFS analysis. Am Miner 79:274–288
Wuensch BJ, Vasilos T (1962) Diffusion of transition metal ions in
single-crystal MgO. J Chem Phys 36:2917–2922
Yamazaki D, Karato S (2001) Some mineral physics constraints on
the rheology and geothermal structure of Earth’s lower mantle.
Am Miner 86:385–391
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