Geology Reference
In-Depth Information
Transmission electron microscopy
Two types of samples were used for TEM work: (1)
powders that were suspended for a few seconds in
distilled water then deposited on the membrane of
a lacey carbon-coated 200 mesh copper grid and
air dried; and (2) the FIB ultrathin foil. TEM obser-
vations were carried out at IMPMC on a JEOL
2100F microscope operating at 200 kV, equipped
with a field emission gun and a high resolution
UHR pole piece. A double-tilt sample holder was
used to orient samples during the collection of
TEM
where I is the transmitted intensity and I
0
is the inci-
dent beam measured in a region adjacent to the
foil that contains no sample. The resulting images
were aligned carefully. The carbonate OD of each
individual pixel was then fit using a routine written
with the statistical software R as a function of the
polarization angle using the following equation:
OD(a) ¼ C þ A
cos
2
(a f) (2)
where a is the polarization of the incident X-ray
beam (from 0 to 908), f is the angular orientation
of the c-axis of aragonite (from 0 to 1808), C is
the in-plane polarization-independent fraction of
the absorption, and A is the amplitude of the polar-
ization-dependent absorption. From these measure-
ments the crystallographic orientation of the c-axis
(f) of aragonite was plotted for each individual
pixel (here, at the 50-nm scale) on the whole FIB
foil providing the map reported in Figure 6.
STXM observations were performed at the
Advanced Light Source (ALS) (Lawrence Berkeley
National Laboratory) on MES branch line 11.0.2.2.
During our measurements, the ALS storage ring
operated at 1.9 GeV and 200 - 400 mA stored
current. A 1200 l/mm grating and 40 mm exit slits
in both dispersive and non-dispersive direction were
used for carbon imaging and spectroscopy, provid-
ing a theoretical energy resolution of 72 meV.
Energy calibration was accomplished using the
well-resolved 3p Rydberg peak at 294.96 eV of
gaseous CO
2
for the C K-edge.
images
and
electron
diffraction
patterns
from the samples.
Scanning transmission X-ray microscopy
(STXM)
STXM is a type of transmission microscopy that
uses a monochromated X-ray beam produced by
synchrotron radiation. The rationale for STXM
data acquisition and analysis and examples of appli-
cations can be found in Hitchcock (2001) and Bluhm
et al. (2006). For STXM imaging, the X-ray beam is
focused on an X-ray transparent sample using a zone
plate, and a 2-D image is collected by scanning the
sample at a fixed photon energy. The achieved spatial
resolution is dependent on the zone plate and was
c. 25 nm in the present study. The image contrast
results from differential absorption of X-rays, which
partly depends on the chemical composition of the
sample. In addition to imaging, it is possible to
perform at the same spatial resolution, near edge
X-ray absorption fine structure (NEXAFS) spec-
troscopy at the carbon K-edge (or other absorption
edges in the 80 - 2000 eV energy range) which gives
information on the speciation (i.e. type of functional
group and bonding) of carbon (or other elements). In
this present case, it was thus possible to detect an
absorption feature at 290.3 eV corresponding to
1s ! p* electronic transitions in the carbonate
group. It has been shown previously that the absorp-
tion intensity at 290.3 eV depends on the orientation
of the aragonite c-axis (i.e. the direction of the
carbonate p* orbitals) relative to the polarization
of the synchrotron X-ray beam (e.g. Metzler et al.
2008). This phenomenon is known as linear dichro-
ism. The absorption at 290.3 eV is maximum when
the c-axis of aragonite parallels the polarization
vector of the beam and minimum when they are
perpendicular (see Fig. 6). At 290.3 eV, we acquired
ten different images of the FIB foil, each at a differ-
ent polarization of the synchrotron X-ray beam
ranging from horizontal (noted as 08) to vertical
(noted as 908). These images were converted to a
linear absorbance (optical density, OD) scale
NanoSIMS
Spatially resolved determination of Mg/Ca and
Sr/Ca ratios along profiles within the carbonate
laminae of the Satonda stromatolites was carried
out using the Cameca NanoSIMS N50 at the
Mus
´
um National d'Histoire Naturelle in Paris
with the same settings as in Meibom et al. (2007,
2008). Using a primary beam of O
2
, secondary
ions of
24
Mg
þ
,
40
Ca
þ
, and
88
Sr
þ
were sputtered
from the sample surface and detected simul-
taneously in electron multipliers at a mass resolving
power of c. 4000. At this mass-resolution all poten-
tially problematic interferences are resolved. The
data were obtained from a pre-sputtered surface in
a series of line-scans with the primary ions
focused to a spot-size of c. 150 nm. Magnesium
and Sr concentrations were calibrated against car-
bonate standards of known composition.
Results
The modern subfossil stromatolites from Satonda
that have been analyzed in the present study
consist of aggregated mm-sized laminated circular
OD ¼ ln(I=I
0
)
(1)
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