 |
 |
 |
 |
Sensitivity
Strong PIXE X-ray production and the large
solid-angle, close detection geometry of the new NMP enable detection
limits as low as 0.2 ppm to be achieved in sub-regions of
images. Minimum detection limits in sulfides range between 1-2
ppm and light matrices, such as diamond, give detection limits
below 50 ppb. |
|
|
The images on the left show major element Fe and trace Br distributions
in a quenched melt inclusion
in clinopyroxene. Note the clear distribution of Br that is
discernible at an average concentration of 2.6 ppm over the
area of the inclusion. The detection limit (99% confidence)
is 0.3 ppm. |
Resolution
High sensitivity analysis demands beam-currents of between 0.5
and 1 nA for fluid inclusions and 5-15
nA for sulfide and silicate imaging. An important design feature
of the new NMP is a slow rate of growth of beam-spot with increasing
current. So from a spot-size of 1.3 µm at 0.5 nA the spot
grows only to 1.8-2 µm at 10 nA, ideal for imaging applications.
In order to benefit from high resolution, a microprobe must
also provide ample sensitivity to obtain sufficient statistics
to render fine spatial detail. The large detector solid-angle
and high beam-currents of the new
NMP provide the sensitivity to image major and minor elements
at full beam resolution and to discern subtle trace element
images. The image on the right shows the fine-detail observed
in some minor elements in sulfides. |
|
 |
 |
 |
 |
 |
Spatial Accuracy
PIXE and electron probe images can suffer from artefacts due to
element overlaps, detector artefacts and pile-up. Images produced
by the new NMP use a powerful technique (Dynamic
Analysis) developed at the CSIRO to un-mix complex spectral
signatures. |
|
|
The resulting images are quantitative, stored in ppm-charge
units, and strongly reject artefacts due to element overlaps and
detector response effects. These operations are now available
using the new GeoPIXE II
software package.
This is particularly important for imaging trace elements such
as Au in sulfide assemblages. The
image of Au to the left shows the spatial variation of Au from
100 ppm (blue) to 5000 ppm (red) in pyrite from the Emperor Mine,
Fiji, and is free of artefacts that can potentially arise from
Zn, As, W and background. Image counting time: 45 minutes.
|
|
Quantitative
Analysis
The Dynamic Analysis transform
for imaging is built on a standardless PIXE analysis method developed
at the CSIRO for the quantitative analysis of PIXE spectra. The
resulting images are stored in ppm-charge units, and can be interrogated
directly to determine the concentrations of all detected elements
in portions of the images, using the new GeoPIXE
II software package.
The example on the right illustrates this function for images
of a sulfide growth feature from a sea-floor "black- smoker",
Manus Basin, north of Papua New Guinea. The figure shows concentrations
extracted directly from the images for a sub-set of detected
elements (25 in all) for a small sub-region near the core of
this growth feature (values in ppm or wt%). |
|
 |
 |
 |
 |
 |
Depth of Penetration
MeV energy protons penetrate several tens of microns into minerals,
and provide sensitivity to buried structures. Two important examples
of the application of this characteristic are in situ fluid
inclusion analysis, and the detection of rare precious metal
inclusions and phases in sulfides.
|
|
|
The images show an example that illustrates rare Au detection.
The small gold-electrum phases in this sulfide assemblage are
contained within Cu-rich veins locked within pyrite enclosed
within magnetite. The observation of this trapped gold texture,
which used Au detected at any depth throughout the thin-section,
may help explain low recoveries in the associated ore deposit.
|
|
Non-destructive
Analysis
MeV protons deposit very little energy over the first ~30 microns
of their path in minerals, and cause negligible damage. This enables
the in situ analysis of fluid inclusions,
preserved within minerals such as quartz. The protons excite X-rays
and gamma-rays from elements within the inclusion enabling the
imaging of fluid inclusons
and the quantitative analysis of their contents.
The figure on the right shows Fe and Cu images (images for 14
elements obtained) from two coexisting brine and vapour inclusions
in quartz from a porphyry-Cu-Au deposit. The brine Fe image
shows Fe in solution and a hematite daughter around a large
vapour bubble and daughters of halite and sylvite (inferred
from Cl and K images). The vapour inclusion Cu image shows the
high concentration of Cu in solution around a large central
vapour bubble and the presence of a chalcopyrite daughter.
Note that solid phases outside the inclusion can be recognized
and avoided using this NMP PIXE imaging approach. That is, they
are not included in the integration performed for quantitative
analysis. |
|
 |
 |
Graphics for these pages have been prepared using IDL and Amiga rendering and
image processing software.
+61-3-8344 8375 
+61-8-6436 8586