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| Titel |
Survival of Brown Colour in Diamond During Storage in the Subcontinental Lithospheric Mantle |
| VerfasserIn |
Evan Smith, Herwart Helmstaedt, Roberta Flemming |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038836
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| Zusammenfassung |
| Brown is the most common colour of natural diamond. This colour is generally
associated with plastic deformation of the crystal structure, which is imparted during
residence in the mantle. Dislocation movement generates vacancies, which aggregate
into clusters of perhaps 30–60 vacancies. The resulting electronic configuration
leads to the broad, featureless absorption pattern associated with common brown
colour.
It is well-established that the common brown colour can be removed by
high-pressure–high-temperature (HPHT) treatment. The process involves pressures and
temperatures in the range of 5–9 GPa and 1800–2700 Ë C, respectively. The treatment may
take several minutes or hours.
It has been suggested that the same colour removal process operates continuously in the
subcontinental lithospheric mantle, causing any brown diamonds in the diamond window to
quickly lose their colour. The present study examined the validity of this suggestion.
Temperature is the important difference between HPHT conditions and diamond window
conditions. Higher temperatures result in faster colour removal. HPHT treatment occurs at
1800–2700 Ë C, whereas inclusion thermometry places most lithospheric diamonds in the
range of 900–1400 Ë C.
Destruction of the brown colour centre involves breaking up vacancy clusters.
How quickly this can be done depends on the concentration of clusters as well as
the rate constant. The rate constant is changes with temperature, according to the
Arrhenius equation. The key to this relationship is the activation energy required for the
breakup of a vacancy cluster. This activation energy can be estimated as the energy of
an isolated monovacancy, minus the energy per vacancy of the cluster, plus the
vacancy migration energy. A value of 7.7±0.3 eV is obtained using data from recent
literature.
For any given brown diamond, the rate constant determines the time needed to
remove a certain amount of colour. The Arrhenius equation can be rearranged to
show how this reaction time varies with temperature. Thus, the activation energy
can be used in conjunction with experimental HPHT data to extrapolate reaction
times from HPHT temperatures to lithospheric mantle temperatures. For a certain
reduction in brown colour produced by HPHT (T1, t1), the equation below shows
the time required (t2) to produce the same reduction at a different temperature
(T2):
ln(t2) = ln(t1) + (Ea/k)(1/T2-1/T1)
where Ea is activation energy, k is the Boltzmann constant, and temperatures are in
kelvins. The error in activation energy causes large, increasing error as the temperature
difference between T1 and T2increases.
Nevertheless, the time required to destroy brown colour in the lithospheric mantle is
significant at the scale of geological time. Brown diamonds should easily maintain brown
colour during cooler mantle storage at or below 1000 Ë C. Warmer temperatures toward the
base of the lithosphere may be able to reduce or eliminate brown colour within a reasonable
geological time frame. However, the survival of brown colour in the lithospheric mantle does
not require the colour to be formed late in the storage history nor does it require metastable
storage in the graphite stability field.
Additionally, preliminary measurements of diamond crystal strain suggest that brown
colour removal in the lithospheric mantle is, at least, not a common occurrence. Mosaic
spreading was gauged in 18 untreated natural diamonds using micro-X-ray diffraction
(μXRD) Ï-dimension peak widths. None of the colourless diamonds examined have a large
residual mosaic spread, as should be expected for a diamond that has been deformed
and turned brown, but later lost its brown colour. Despite the limited sample size,
the results support some capacity for the survival of brown colour in the mantle. |
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