Fossilized high pressure from the Earth's deep interior: The coesite-in-diamond barometer

Abstract

Mineral inclusions in diamonds provide an important source of information about the composition of the continental lithosphere at depths exceeding 120 -150 km, i.e., within the diamond stability field. Fossilized high pressures in coesite inclusions from a Venezuela diamond have been identified and measured by using laser Raman and synchrotron x-ray microanalytical techniques. MicroRaman measurements on an intact inclusion of remnant vibrational band shifts give a high confining pressure of 3.62 (؎0.18) GPa. Synchrotron single-crystal diffraction measurements of the volume compression are in accord with the Raman results and also revealed direct structural information on the state of the inclusion. In contrast to olivine and garnet inclusions, the thermoelasticity of coesite favors accurate identification of pressure preservation. Owing to the unique combination of physical properties of coesite and diamond, this ''coesite-in-diamond'' geobarometer is virtually independent of temperature, allowing an estimation of the initial pressure of Venezuela diamond formation of 5.5 (؎0.5) GPa.S pecimens of Earth materials from great depths often contain mineralogical or textural clues, such as metastable highpressure polymorphs or characteristic mineral assemblages implicating their high-pressure genealogy (1-3). The actual pressure, however, is seldom preserved and observed. For such observations, the sample must be retained in a strong container with appropriate relative thermoelastic properties, and nondestructive analytical techniques must be developed to probe the fossilized pressure condition in situ in the container. Diamond is the strongest possible container. Infrared absorption spectroscopy has been used as the probe for fluid inclusions in diamond (4), and high residual pressures have been reported on the basis of infrared spectra of quartz and CO 2 inclusions in the material (5, 6). In these studies, bulk spectra of diamond and numerous microscopic inclusions were measured; pressures and phases of different inclusions were not determined individually. Herein, we present direct measurements of pressures up to 3.6 GPa in individual coesite inclusions in diamond, a thermoelastic couple most favorable for pressure preservation. We use micro-Ramanand micro-single-crystal x-ray diffraction techniques to probe the pressure of each inclusion separately and precisely. Application to coesite inclusions in diamond from Venezuela provides a determination of the pressure-temperature (P-T) conditions of its formation.Diamond and its inclusions contain rich information about the petrogenesis and geochemistry of the Earth's deep interior (7,8). Although coesite-and diamond-bearing rocks (9-12) undoubtedly have a high-pressure origin, individually, coesite and diamond cover a wide P-T domain of stability (13, 14) that precludes their use as a precise and accurate geobarometer, because they give only a lower bound on the pressure of formation. In many cases, much higher pressures are suggested (15) based on mineral equilibria d...

Raman mapping of coesite inclusions in garnet from the Kokchetav Massif (Northern Kazakhstan)

“…[5] for results). The maximum of overpressure value remains nearly constant 2.0-2.4 GPa (526-527 cm −1 ), although an overpressure of ∼3.2 GPa was identified for coesite inclusions in diamond [7]. This may perhaps indicate that garnet and zircon are not such good containers as claimed by Refs.…”

“…Raman spectra of all "monomineralic" coesite inclusions contain main quartz band (464 cm −1 at ambient pressure) located at 464-477 cm −1 , indicating that in addition to coesite optically undetectable quartz is present within the inclusions. The maximum Raman shift of the main coesite band (521 cm −1 at ambient pressure) does not exceed 524 cm −1 for inclusions in kyanite, 526 cm −1 for inclusions in garnet and zircon, while in kimberlitic diamonds is the main coesite band appears at about 532 cm −1 [7]. Both coesite and quartz remain under high pressure as reflected on the shift of their Raman bands.…”

“…Because of the sluggish reaction kinetics of the coesite to quartz transformation, fast cooling and high exhumation rates will promote the preservation of monomineralic coesite inclusions [3,4]. The recent discovery of single coesite inclusions, without any trace of radial cracks, preserving significant overpressure [5][6][7], highlights the importance of Raman spectroscopy as a very powerful tool in UHP research.…”

“…The most intense coesite band at 521 cm −1 (at ambient pressure) has a strong, characteristic pressure shift of 2.9 ± 0.1 cm −1 /GPa [8], while the most intense band of quartz at 464 cm −1 (at ambient pressure) is used for pressure measurements in the present study (characteristic pressure shift of 9 ± 0.5 cm −1 /GPa [9]). Applying these geobarometers for "monomineralic" coesite inclusions (following the definition of Parkinson [6]) in refractory minerals such as diamond, garnet, zircons and kyanite from different ultrahigh-pressure metamorphic (UHPM) complexes [5,6,10] and some kimberlitic diamonds [7] reveals several particular features, which can be summarized as follow. Raman spectra of all "monomineralic" coesite inclusions contain main quartz band (464 cm −1 at ambient pressure) located at 464-477 cm −1 , indicating that in addition to coesite optically undetectable quartz is present within the inclusions.…”

Is quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets

“…Coesite-to-quartz transformation is accomplished with significant volume changes; very high residual pressures can be preserved in ''monomineralic'' (following the definition of Parkinson, 2000) coesite inclusions, as well as in partly transformed coesite (Parkinson & Katayama, 1999;Sobolev et al, 2000;Ye et al, 2001;Korsakov et al, 2007). The Raman shift of quartz was recently tested on natural samples in order to estimate the effect of peak metamorphic pressure on residual pressure within the quartz inclusions (Enami et al, 2007).…”