Geology Reference
In-Depth Information
core about 6 6 mm. Secondary carbon ions of
masses 13 and 12 were simultaneously collected
using two Faraday cups. A combination of high
transmission and a high beam current was necessary
to obtain sufficient counts (1.5*10
6
cps on
13
C).
Charge neutralization was achieved using a
normal incidence electron gun. Each analysis
consisted of 20 blocks of 5 seconds measurement
time following a period of pre-sputtering. Analysis
of a group of 20 - 50 sample points alternated with
blocks of 10 analyses of standard UWC (University
of Wisconsin calcite, which has a composition of
22.14‰ (relative to VSMOW) determined by mul-
tiple bulk analyses by J. W. Valley). Analytical
points were spaced at 30 - 60 mm to avoid surface
charging effects and their X - Y coordinates were
recorded automatically. The 1-sigma precision on
10 successive spots on the UWC standard was nor-
mally ,0.6‰, but accuracy is more limited by vari-
able instrumental drift and is best judged by the
reproducibility of parallel scans on the sample as
presented later. For S and P analysis in May 2005,
a primary beam current of 4.5 nA were used with
K¨hler illumination to generate an analytical spot
around 20 - 25 mm in size. The instrument was
used in line scan mode (10 or 15 mm step) with a
pre-sputter reduced to 10 seconds. Ratios of the sec-
ondary ions of
31
P and
32
S (at a mass resolution of
2500 to distinguish from O
2
)to
13
C were counted
using an electron multiplier. Results have a pre-
cision of ,2% from counting statistics, but were
not standardized. Analysis of
31
P
2
and halogen
elements (
19
F
2
,
35
Cl
2
,
37
Cl
2
,
79
Br
2
,
81
Br
2
and
127
I
2
) was undertaken in November 2008 with a
1 nA primary beam focused to a diameter of
c. 10 mm. Analysis was in line scan mode, stepping
at 5 mm between analyses, which also had the effect
of cleaning the sample by pre-ablation. Analytical
precision is limited by counting statistics and is
,1% for Cl, 2 - 3% for P and 3 - 4% for Br and
I. Halogen concentrations were standardized by
direct comparison with a lead - silicate glass
K1053. The absolute ion yields for the halogens
were confirmed as being very high, varying from
5.0 (for F
2
) to 0.86 (for I
2
) times that of O
2
. The
effects of sample matrix on ion yields on the silicate
glass
were excited with monochromatic synchrotron radi-
ation of 2.9 keV in order to stimulate Ka radiation
from light elements (up to Cl). Ca was not analysed,
but the consistent beam conditions permit the
assumption that excitation conditions were consist-
ent; minor reductions in beam intensity of up to
10% were corrected for. Specific energy levels
within the resultant X-ray fluorescence spectrum
characteristic of particular elements were then
selected for study and scans and maps were gener-
ated. The X-ray fluorescence was predominantly
generated at depths of just a few microns in the
sample and maximum penetration of the exciting
radiation was 20 - 30 mm. The beam was focused to
either 5 mm (scans and maps) or, for a high-resolution
map using 1 mm pixels, to 0.6 0.3 mm, but resol-
ution is limited by the slightly larger excitation
volume.
At beamline ID22, the conditions are mostly
similar to those described in detail by Borsato
et al. (2007), but some differences are noted here.
The sample was prepared as a doubly-polished
wafer of thickness c. 150 mm, mounted over a
hole in the sample holder. Excitation energy used
was 23 keV, enabling in principle the detection of
the K-lines of all the elements up to atomic
number 47 and of the L-lines of other elements.
The spectrum was deconvolved by using PyMCA
software (Sol´ et al. 2007) and detection limits
were found to be sub-ppm level for Ca, Sr, Zn and
Pb. Detection of other elements was limited by
spectral interferences. Elemental mapping was
carried out with a 2 mm resolution over a length
of 1.1 mm and 5 mm resolution over a width of
0.1 mm. The fluorescence signal was integrated
for 2 seconds in each pixel. The information depth
is defined at the depth to which 63% of the charac-
teristic X-ray emission line intensity of an element is
being collected and depends on the X-ray energy. It
varies from around 17 mm for Ca to 110 mm for
Sr. Generation of X-ray fluorescence over a range
of depths for heavier elements results in the collec-
tion of data from within the crystals as well as from
the surface, and hence a more complex pattern is
observed than in a strictly two-dimensional image.
Unlike tomographic images, it is not possible to
distinguish which are shallower and
compared
with
carbonate
are
currently
which are
unknown.
Sample Obi84 was analyzed at the European
Synchrotron Research Facility using soft X-rays at
beamline ID21 in May 2005 and using hard
X-rays at beamline ID22 in December 2006. For
beamline ID21, experimental details are similar
to those given in detail in Frisia et al. (2005).
Samples were prepared as thin (,1 mm thick)
wafers up to around 22 19 mm, with polished
upper surfaces and were mounted with double-sided
tape to a circular 3 cm-diameter holder. The samples
deeper sites of X-ray generation.
Study site and dripwater characteristics
The local context of Obir is only briefly described
here, being summarized from Sp¨tl et al. (2005).
The cave is on the eastern flank of the Hochobir
massif (46.518N, 14.548E), 22 km SE of Klagenfurt,
Austria. One distinct group of cave passages
within the cave system is accessible only through
Search WWH ::
Custom Search