Toward a Robust Elastic Geobarometry of Kyanite Inclusions in Eclogitic Diamonds

Abstract

Here we report the first results from elastic geobarometry applied to a kyanite inclusion entrapped within an eclogitic diamond (from Voorspoed mine, South Africa) using micro‐Raman and Fourier transform infrared spectroscopy, electron microprobe analysis, ab initio calculations, and finite element modeling. Application of elastic geobarometry to very elastically anisotropic kyanite inclusions is challenging, as current models do not allow for elastic anisotropy. In order to minimize the effects of anisotropy, we have explored the effects of deviatoric stress on Raman modes via ab initio density functional theory. The results allowed us to select the Raman mode (at ca. 638 cm−1) that is the least sensitive to deviatoric stress. The shift of this band in the inclusion while still trapped within the diamond relative to the inclusion in air (once liberated) was used under hydrostatic approximation to determine a residual pressure on the inclusion of 0.184 ± 0.045 GPa and an entrapment pressure of 5.2 ± 0.3 GPa (~160 km depth) for an FTIR N‐aggregation residence temperature of 1119 ± 50 °C. This is the first geothermobarometric determination for a diamond from the Voorspoed kimberlite. It overlaps with P–T estimates obtained by traditional chemical geobarometry for diamonds from other kimberlites from the Kaapvaal craton, suggesting that the hydrostatic approximation does not introduce significant errors in the geobarometric evaluation. Our protocol of Raman peak selection can be used for geobarometry of further kyanite‐bearing diamonds and may provide a guide for more robust geobarometry of other types of mineral inclusions in diamonds, both eclogitic and peridotitic.

Quartz‐in‐garnet and Ti‐in‐quartz thermobarometry: Methodology and first application to a quartzofeldspathic gneiss from eastern Papua New Guinea

“…For example, Raman measurements can be performed on a quartz inclusion in garnet, and the same inclusion can be subsequently exposed and polished for Ti-in-quartz measurements. Most previous applications of elastic thermobarometry have generally used a temperature estimate obtained from an external method or an assumed constraint to determine the P-T of mineral entrapment conditions on an isomeke (Anzolini et al, 2019;Ashley et al, 2014Ashley et al, , 2016Barkoff et al, 2017;Bayet, John, Agard, Gao, & Li, 2018;Castro & Spear, 2017;Enami et al, 2007;Endo et al, 2012;Kouketsu, Hattori, Guillot, & Rayner, 2016;Kouketsu, Enami, & Mizukami, 2010;Nestola et al, 2018;Soret et al, 2019;Spear, Thomas, & Hallett, 2014;Taguchi, Enami, & Kouketsu, 2016Wolfe & Spear, 2018;Zhong et al, 2019). For example, pseudosection modelling uses bulk rock geochemistry and mineral chemical compositions to reveal the ranges of stable P-T conditions where possible mineral assemblage can crystallize at chemical equilibrium.…”

“…Theoretical models for host‐inclusion mineral pairs with different elastic properties (e.g. quartz‐in‐garnet [QuiG], apatite‐in‐garnet, zircon‐in‐garnet, garnet‐in‐diamond, kyanite‐in‐diamond) were recently developed to estimate inclusion entrapment P–T conditions (Angel, Nimis, Mazzucchelli, Alvaro, & Nestola, ; Anzolini et al, ; Barkoff, Ashley, & Steele‐MacInnis, ; Campomenosi et al, ; Kohn, ; Nestola et al, ; Thomas & Spear, ). Low bulk modulus ‘soft’ minerals entrapped in high bulk modulus ‘stiff’ host minerals will develop remnant strains upon exhumation from depth (Angel et al, ).…”

“…Previous applications of elastic thermobarometry have combined elastic models with a Zr‐in‐rutile solubility model (Castro & Spear, ; Wolfe & Spear, ), mineral assemblage equilibrium thermobarometry (Ashley, Caddick, Steele‐MacInnis, Bodnar, & Dragovic, ; Barkoff et al, ), Fourier transform infrared spectroscopy (Nestola et al, ), Raman spectroscopy of carbonaceous material (Endo, Wallis, Tsuboi, Torres De León, & Solari, ), and isomekes of two coexisting inclusions (e.g. QuiG and zircon‐in‐garnet; Zhong, Andersen, Dabrowski, & Jamtviet, ) to estimate unique P–T conditions.…”

Poroelastic response to rapid decarbonatisation as a mechanism of the diamonds formation in the mantle wedge of Kamchatka

“…Dashed lines bracket PT conditions of the craton lithosphere, where typical diamonds are formed. For example, the hatched rectangle represents the conditions of the crystallization of eclogite diamonds from Yakutya [Smith et al, 2015] and S. Africa [Nestola et al, 2018]. The CO 2 release will increase the fluid pressure limited by the equilibrium parameters of the PT reaction (3) ( Figure 1) and can reach the diamond stability field.…”

“…Similar PT conditions are expected in the oceanic plate (with the age of 100-120 million years [Davies, 2013]) near Kamchatka. It is predicted that the temperature at a depth of 60 km is about 700 ∘ C [McKenzie et al, 2005].…”