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(Peyssonnelia and Lithoporella) and nubecullinid
foraminifers. The laminated overgrowths show
alternation of a poorly crystallized Mg - Si-rich
mineral phase (thin and dark in transmitted light
optical microscopy) described by Arp et al. (2003)
and aragonite, forming thick and bright laminae in
transmitted light optical microscopy. We focused
on the aragonite laminae for the textural analyses.
A detailed description of the texture of aragonite
laminae might help to better understand the poten-
tial role of microorganisms in their formation.
Aragonite laminae can be as thick as a millimetre
and most of them show fine striations in transmitted
light microscopy parallel to the accretion planes of
the laminae (Fig. 1). This striated texture could
also be observed by SEM on samples etched
gently with formic acid. These striations were also
described by Kempe & Ka´mierczak (1993), but
their origin remains mysterious. Interestingly,
these striations show a striking similarity with the
growth bands observed in the fibrous aragonite skel-
eton of scleractinian corals (Fig. 2, e.g. Meibom
et al. 2004). As these bands correlate with signifi-
cant Mg and Sr variations in corals, we measured
the same elemental profiles within the aragonite
laminations by NanoSIMS (Fig. 1). In contrast to
what has been observed for corals (e.g. Meibom
et al. 2007, 2008), no significant variations of the
Sr/Ca ratio were observed in the aragonite
laminae. The slight increase of Mg/Ca at 36
microns was very small compared to what is
usually observed in corals and moreover, this ratio
was constant for the rest of the profile and did not
show systematic variations within the striations
observed by SEM.
TEM observations on powdered samples of the
crystal units forming the Satonda stromatolites
showed only a poorly crystallized Mg - Si-rich
phase and aragonite, consistent with bulk XRD
measurements. Although powdering destroys the
textural relationship between crystals, these obser-
vations were nonetheless useful because they
showed that the aragonite can have two different
morphologies (Fig. 3): (1) some aragonite appears
as single-crystal fibres that are a few micrometres
in length and a few hundred nanometres in width;
and (2) the remainder of the aragonite appears as
small globules a few tens of nanometres in size
and forms polycrystalline clusters. This latter
texture is usually called micritic and has been
observed by many authors in the past (e.g. Riding
2000; Dupraz et al. 2004).
To obtain more information on the spatial distri-
bution of these types of morphologies in the arago-
nite laminae of Satonda stromatolites, non-etched
petrographic thin sections were examined by SEM.
As previously reported by Kempe & Ka´mierczak
(1993) and Arp et al. (2003), the aragonite lami-
nations appear at first sight to be mostly fibrous,
with aragonite fibres perpendicular to the laminae
surface (Fig. 4). However, at higher magnification,
these fibres seem to consist of clusters of nanocrys-
tals that are similar in size to the nanocrystals found
in the powdered samples. SEM observations of such
tiny domains may however be misleading as they
probe the very near surface of the sample which
might be very sensitive to the sample preparation
protocol, to slight etching during the polishing
stage and/or to artifactual nanograins that may
have formed during the gold-palladium coating pro-
cedure (Gibbs & Powell 1996; Steele et al. 1998).
The use of other techniques that require minimal
sample preparation such as cryo-SEM and/or
environmental SEM would be a useful approach to
better address this issue (e.g. Dupraz et al. 2004).
Another approach consists of preparing the
samples for techniques that are not surface-sensitive
such as TEM or STXM.
Fig. 2. Comparative SEM images of Porites sp. skeleton (a) and Satonda stromatolites (b). Both samples were gently
etched with formic acid (1%). (a) Centres of rapid accretion can be observed as pits in the centre of the image.
Concentric growth bands as well as aragonite fibres are visible on this etched sample. (b) Two pits can be observed at the
center of the structure. They have classically been interpreted as remnants of green algae around which the carbonates
precipitate. Concentric growth bands can be observed. Aragonite fibres can be observed at higher magnification
(see Fig. 4).
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