Survival of the Brown Color in Diamond During Storage in the Subcontinental Lithospheric Mantle

Abstract

The common brown color in natural diamond is likely caused by vacancy clusters, produced by plastic deformation of the crystal structure. A brief treatment in a high-pressure -high-temperature (HPHT) vessel at 1800-2700°C can remove this color. As there has been speculation that a similar removal of color should occur continuously at depth in the subcontinental lithospheric mantle (SCLM), the effect of temperature on the time required to eliminate the brown color in diamond has been calculated from published data. A critical component of this calculation is the activation energy for the breakup of vacancy clusters. The time necessary to remove the brown color in the SCLM is significant on a geological time-scale. Crystals of brown diamond stored at or below about 1000°C should maintain their color for 10 8 years or more. In spite of some uncertainty in extrapolating the HPHT data to natural conditions in the SCLM, it appears that higher temperatures toward the base of the lithosphere could lead to reduction or elimination of the brown color within thousands of years. Plastic deformation needed to produce the brown color causes lattice distortion and strain-related mosaic spread, which is discernible by X-ray diffraction. The degree of mosaic spread was gauged in 18 untreated crystals of natural diamond by estimating -dimension peak-widths from micro-X-ray diffraction (mXRD). For these samples, there is a correlation between peak width and depth of brown color. None of the colorless crystals of diamond examined have large peak-widths, as should be expected for a crystal of diamond that has been deformed and turned brown, but later lost its brown color. The removal of the brown color thus does not seem to be a common natural occurrence. We thus conclude that survival of the brown color in the lithospheric mantle does not require the color to be formed late in the storage history, nor does it require metastable storage of diamond in the stability field of graphite.Keywords: brown diamond, high-pressure -high-temperature treatment, vacancy cluster, mosaic spread. SoMMaIrELa couleur brune habituelle du diamant naturel serait attribuable à des agrégats de lacunes qui seraient dues à une déformation plastique de la structure cristalline. Un bref traitement à haute pression et haute température à 1800-2700°C peut éliminer cette couleur. Afin d'évaluer les mérites de l'hypothèse voulant qu'une élimination de la couleur pourrait avoir lieu de façon continue dans le manteau lithosphérique subcontinental en profondeur, nous avons calculé l'effet de la température sur l'intervalle de temps requis pour éliminer la couleur brune à partir de données dans la littérature. Un facteur d'importance capitale dans ce calcul serait l'énergie d'activation de la dispersion de ces nanoagrégats de lacunes. Le temps requis pour éliminer la couleur brune dans le manteau lithosphérique subcontinental est appréciable sur une échelle de temps géologique. Les cristaux bruns tenus à environ 1000°C ou moins devraient maintenir leur couleur pour 10...

Non-Destructive In Situ Study of Plastic Deformations in Diamonds: X-ray Diffraction Topography and µFTIR Mapping of Two Super Deep Diamond Crystals from São Luiz (Juina, Brazil)

“…This color in diamond is commonly related to vacancy clusters, produced by plastic deformation of the crystal structure [30], as recently confirmed also by spectroscopic studies [31]. Both diamond crystals are partially flattened along the (111) plane.…”

Mécanismes de solidification des magmas basaltiques :

“…An objective of this study is to validate both the optical pétrographie observations of olivine deformation and the U.XRD technique recently developed for in-situ analysis purposes (Flemming, 2007;Izawa et al, 2009;Izawa et al, in review;Moser et al, 2009;Smith et al, 2010). A comparison between the pétrographie and XRD evidence of deformation for each crystal is presented in Table 1.…”

“…Dans la plupart des études antérieures, l'observation au microscope optique était le seul moyen de distinguer les cristaux déformés des cristaux non déformés. (Flemming, 2007), le zircon (Moser et al, 2009), l'orthopyroxène (Izawa et al, 2009, in review), le diamant (Smith et al, 2010), et l'olivine de météorites (McCausIand et al, 2010) ; aucune étude n'a examiné l'olivine terrestre. Par ailleurs, la caractérisation in situ basée sur la diffraction a été utilisée pour l'étude d'autres types de roches ou minéraux : des roches métamorphiques de ultra haute pression et les minéraux de coésite et quartz associés, par exemple (Ikuta et al, 2007).…”

“…Previous studies have examined quartz (Flemming, 2007), zircon (Moser et al, 2009), orthopyroxene (Izawa et al, 2009;Izawa et al, in review), and diamonds (Smith et al, 2010), but none have examined olivine. Consequently, our aim is to quantify deformation using such parameters as the full width at half maximum (FWHM) of peak intensity along the Debye rings.…”

Plastic deformation in natural diamonds: Rose channels associated to mechanical twinning

“…The bands are always parallel to the octahedral {111} faces. Later microtwins (lamellae) have been reported repeatedly using optical microscopy, but many X-ray diffraction studies (using Laue technique) did not reveal twins (see [25] and references therein). Microtwins have been suspected by Mineeva et al [26] who noticed unusual electron paramagnetic resonance (EPR) spectra in pink-purple diamond from the International"naya kimberlite (Siberia, Russia).…”