Geology Reference
In-Depth Information
carbonates can be very small (in the few tens of
nanometre size range).
In the present study, we focus on the morphology
and arrangement of carbonate crystals in recent
subfossil (possibly several hundreds of years old)
lacustrine stromatolites. The combination of micro-
scopy and microspectroscopy tools such as scanning
and transmission electron microscopy (SEM and
TEM), NanoSIMS and synchrotron-based scanning
transmission X-ray microscopy (STXM) provides
unique information on the compositional variations
and mineral textures or microfabrics of stromato-
lites at the submicrometre scale. Several textural
features show a remarkable similarity to those
observed in modern corals. These similarities as
well as differences are discussed in light of the
mineralization processes by which modern stroma-
tolites and corals form. Although these results are
of interest to the scientific community working on
microbialites, the approach and the basic mineral
building units that are described should be of
general interest to communities working on carbon-
ate precipitates such as tufas and speleothems.
layers. Some fragments of the Satonda stromatolites
were powdered for XRD and TEM analyses to
characterize the nature and morphology of the
crystals comprising them. In order to spatially
resolve textural and chemical information within
the structure of Satonda stromatolites, petrographic
thin sections were prepared and gold-coated for
SEM and NanoSIMS analyses. Some spots were
selected for preparation of ultrathin electron-
transparent foils (less than 100 nm in thickness)
by Focused Ion Beam milling. These foils were
further analysed at the few nanometer spatial scale
by scanning transmission X-ray microscopy (STXM)
and TEM.
Scanning electron microscopy
SEM analyses were performed on the Zeiss Supra
55 SEM microscope in the Laboratory Magie at
University Pierre et Marie Curie (Paris, France).
The microscope was operated at 10 kV with a
working distance of 3 mm. Two different detectors
were used: an in-Lens detector for secondary elec-
tron imaging (nano-topography of the sample) and
an Angle selective Backscattered (AsB) detector
for low-angle backscattered electrons which pro-
vide a contrast more sensitive to crystal orientation.
Following NanoSIMS analyses, the gold coating
was removed from the petrographic sections and the
samples were etched using a slightly acidic solution
consisting of formic acid diluted to 0.1% in
de-ionized milliQ water with 2% glutaraldehyde.
The samples were re-coated with gold-palladium
and observed by SEM.
Material and methods
Stromatolite samples
Modern subfossil stromatolite samples were col-
lected from the crater lake of Satonda, a small vol-
canic island in Indonesia previously described in
detail by Kempe & Ka´mierczak (1993, 2007),
Arp et al. (2003) and Kazmierczak & Kempe
(2004). We studied sample S-47, collected with
from the stromatolitic reefs at station 10 above the
water table (see Kempe & Kazmierczak, 1993).
These stromatolites are actually subfossil and,
although we do not know precisely their age, may
be as old as several hundred years. Some features
may have been modified by aging (e.g. some
organic functions), but the transformation of
mineral textures seems unlikely considering that:
(1) the samples have remained at the surface;
(2) they are still composed of aragonite and not
calcite as would be expected from prolonged inter-
action with meteoric water; and (3) the same
mineral textures are observed in other genuinely
modern microbialites from alkaline lakes (e.g.
from Lake Van in Turkey). As a comparison, the
earliest steps of diagenesis of aragonite structures,
which can be observed after a few years, have
been studied for example by Perrin & Smith
(2007). Satonda Lake is a slightly alkaline (pH
8.55) quasi-seawater system that harbors calcareous
stromatolites along the shore. The laminations
of Satonda stromatolites consist of alternating
50 - 500 mm thick aragonite laminae and 1 to a
few tens of micrometres thin Mg - Si-containing
Focused ion beam milling
Focused ion beam (FIB) milling was performed
with a FEI Model 200 TEM FIB system at the Uni-
versity Aix-Marseille III. The FIB lift-out method
was used to prepare a cross-section across one
Mg - Si-rich area surrounded by two aragonite
laminae. This method is described in detail in
Heaney et al. (2001) and Benzerara et al. (2005).
A thin strip of platinum was deposited on the area
of interest in order to protect it during the milling
process. A 30 kV Gaþ beam operating at c.20nA
excavated the sample from both sides of the Pt
strip to a depth of 5 mm. Before removal of the
thin foil, it was further thinned to c. 100 nm with a
glancing angle beam at much lower beam currents
of c. 100 pA. Finally, a line pattern was drawn
with the ion beam along the side and bottom edges
of the thin foil allowing its removal from the
sample. The slide was transferred at room pressure
with a micromanipulator onto the membrane of a
formvar-coated 200 mesh copper grid for TEM
and STXM analyses.
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