Removal of Brown Colour From Diamonds During Storage in the Lithospheric Mantle

Brown colour in natural diamonds is produced by plastic deformation during residence in the mantle. Dislocation movement generates vacancies, which aggregate into clusters of about 30-60 vacancies. The resulting electronic configuration at each cluster leads to the broad, featureless absorption pattern associated with common brown colour. Less commonly, other brownish colours can be attributed to hydrogen or isolated nitrogen atoms, H4 centres, or possibly oxygen. The common brown colour can be removed by high-pressure-high-temperature (HPHT) treatment. This process involves pressures and temperatures of 5-9 GPa and 1800-2700 ° C, respectively. Treatment may take several minutes or hours. It has been suggested that brown diamonds stored in the lithospheric mantle should lose their colour by analogy to HPHT treatment. If so, brown colour must be the product of late deformation (close to kimberlite eruption) or the brown diamonds must be stored above the diamond stability field (in a cooler part of the lithosphere). Is it reasonable to expect brown colour to be destroyed in the lithospheric mantle? An objective analysis of this question must consider temperature. Higher temperatures result in faster colour removal. HPHT treatment occurs at 1800-2700 ° C, whereas inclusion thermometry places most lithospheric diamonds in the range of 900-1400 ° C. Destruction of the brown colour centre involves breaking up vacancy clusters. The activation energy required to do this can be estimated as the energy of an isolated monovacancy, minus the energy per vacancy of the cluster, plus the vacancy migration energy. Data from recent literature produces a value of 7.7 eV. The Arrhenius equation can be modified to show how reaction time varies with temperature. 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 kelvin. A relatively potent treatment of 1 hour at 2500 ° C is comparable to about 109 years at 1150 ° C. Most diamonds would lose any brown colour in this scenario. Modest colour reduction is detectable after 5 hours at 1800 ° C. This is comparable to about 109 years at 950 ° C, or about 105 years at 1150 ° C. The time required to destroy brown colour in the lithospheric mantle is significant at the scale of geological time. Brown diamonds should easily maintain their colour during cooler mantle storage at or below 1000 ° C. Warmer temperatures toward the base of the lithosphere may be able to reduce or eliminate brown colour within a reasonable geological time frame. 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.