Advancing diamond exploration - novel techniques for
the interpretation of diamond indicator minerals
Gregory M Yaxley1Hugh O'Neill1 and
Andrew Berry2
1 Research School of Earth Sciences, Australian
National University, Canberra, ACT 0200, Australia
2Department of Earth Science and Engineering, Imperial
College, London, UK
Top left panel: Representative
raw XANES spectra for synthetic garnets with Fe3+/∑Fe
= 1.0, 0.46 and 0.01 (red, green, blue respectively - compositions
known from stoichiometry of skiaigite - almandine - andradite series,
kindly supplied by Prof. Alan Woodland, University of Frankfurt)
showing Fe K-edge and near edge structure in inset. Data was collected
on the Australian National Beamline Facility, KEK, Tsukuba, Japan;
Bottom left panel: Expanded view of inset in A, showing pre-edge
spectral features with backgrounds (dashed lines); Bottom right
panel: Pre-edge peaks after background subtraction and best fit
from fitting a number of Gaussian components; Top right panel:
Pre-edge peak centroids as a function of Fe3+/∑Fe, showing linear
correlation, the basis of the calibration.
We have developed new mineralogical tools applicable to the search for
diamonds in Australia and overseas. This project was an ARC Linkage Project
with industry partners BHP-Billiton, de Beers and Rio Tinto Exploration,
through AMIRA International.
Techniques of high-pressure experimental petrology were used to develop
mineral thermometers based on partitioning of Zn and Mn between upper
mantle minerals. A new synchrotron-based technique for determining the
redox state of the upper mantle is also being developed. These tools
are being applied to fragments of garnet peridotite transported from
the deep lithosphere to the surface by deeply-derived, occasionally diamondiferous
kimberlite magmas. The resulting temperature and redox information will
provide fundamental constraints on lithospheric diamond stability, and
will be important in diamond exploration programs at a strategic level
for targeting cratonic lithospheric domains more likely to contain high
grades of diamonds, and at a more local level for targeting particular
kimberlites for grade assessment by expensive bulk sampling techniques.
High-pressure experiments, focussing on Mn partitioning between garnet
and olivine under upper mantle pressure-temperature conditions, have
been completed. Algorithms for a Mn partitioning thermometers based on
statistical fitting of experimental Mn-Mg and Fe-Mn exchange data between
garnet and olivine have been determined. Experiments measuring Zn partitioning
between Cr-spinel and olivine were also conducted and an olivine-spinel
Zn-based thermometer was developed. These thermometers were applied to
a comprehensive range of natural garnet ± spinel peridotite xenoliths
samples from kimberlites erupted through the Kaapvaal and Slave Cratons.
They performed extremely well in most cases, when compared with conventional
thermometers (eg two-pyroxene thermometry etc).
The calibration of a synchrotron-based, X-ray Absorption Near Edge Structure
Spectroscopy (XANES) method for determining Fe3+ contents of mantle garnets
has proved unexpectedly challenging, despite a promising start (Figure
1), due to complexities relating to the compositional variations of natural
garnets. We therefore adopted the alternative approach of obtaining the
Fe3+ data by conventional Mössbauer Spectroscopy in collaboration with
Prof Alan Woodland (Uni of Frankfurt) and by the newly developed electron
microprobe based Flank Method in collaboration with Dr Heidi Höfer (Uni
of Frankfurt). This will still allow determination of the oxygen-fugacity
depth profiles through the lithospheric section represented by the garnet
peridotite xenoliths supplied by the sponsor companies. These calculations
are currently underway.