Protogenetic garnet inclusions and the age of diamonds

Abstract

OrientXplot calculationsThe information we report below is extracted from the "Users manual" of OrientXplot (Angel et al., 2015) and explains how it is possible to plot the orientation data from single-crystal X-ray diffraction or from other sources (i.e. Electron Backscatter Diffraction, hereafter EBSD).One of the main problems when plotting the relative crystallographic orientations of inclusions in different hosts is the possible ambiguity in indexing the diffraction patterns (or data from other sources, e.g. EBSD) coming from the symmetry of both the inclusion and the host. For example, in an orthorhombic system, the symmetry makes the [100] direction equivalent to [-100], and [010] equivalent to [0-10], and others. There is no physical approach to distinguish between these two directions. Therefore, in this case, an 'a-axis' pointing vertically upwards could equally be described as '-a' or [-100]. The possible equivalent orientations that maintain a right-hand description of the unit-cell axes of a crystal with a certain symmetry point group are described by the symmetry elements of the point sub-group that does not include any inversion symmetry and mirror planes. Accordingly, both garnet and diamond will be considered as having a point group 432 instead of m3m. In detail, to eliminate the symmetry ambiguity, we rotate the orientation matrix (determined by single-crystal X-ray diffraction) of each garnet inclusion by the symmetry elements of point group 432, oriented to coincide with the unit-cell axes of the diamond host. This generates 24 possible crystallographic orientations of garnet with respect to its diamond host (symmetrically equivalent and physically indistinguishable by any method). For each of them, there are 24 orientations, which are symmetrically-equivalent orientations of the garnet because of its symmetry.

Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

“…In the other diamonds with multiple inclusions, sulfides showed no specific orientations. The iso-orientation of multiple inclusions showing no epitaxial relationship with the diamond can only be explained if the inclusions are remnant portions of single preexisting grains, as interpreted for other inclusions in diamonds (e.g., Nestola et al, 2019). Therefore, we only state with certainty that sulfide inclusions in two diamonds (V2-19, V2-20) are protogenetic, but a similar origin is likely for many if not all other inclusions investigated.…”

“…An important aspect to consider in dating diamonds is whether the mineral inclusions formed at the same time as the diamond (syngenetic), as assumed by early studies (Harris, 1968), or whether the diamonds enclosed pre-existing mineral grains (protogenetic) (Thomassot et al, 2009;Nestola et al, 2014Nestola et al, , 2017. For example, garnet inclusions in diamonds from the main diamond-producing cratonic areas have been recently determined to be protogenetic, based on crystallographic relations (Nestola et al, 2019). However, despite being protogenetic, the majority of radiometric ages based on garnets have been shown to effectively correspond to the time of diamond formation, provided certain conditions are met (Nestola et al, 2019).…”

“…For example, garnet inclusions in diamonds from the main diamond-producing cratonic areas have been recently determined to be protogenetic, based on crystallographic relations (Nestola et al, 2019). However, despite being protogenetic, the majority of radiometric ages based on garnets have been shown to effectively correspond to the time of diamond formation, provided certain conditions are met (Nestola et al, 2019). Thus, inclusions and diamond hosts can be synchronous even if not syngenetic (Nestola et al, 2017).…”

Diamond's depth distribution systematics

“…Therefore, either many of the dated garnets were protogenetic and were not isotopically reset during diamond formation (cf. Nestola et al, 2019a) or both old and young diamonds occur at Kimberley. The occurrence of multiple generations of diamonds at Kimberley is likely, because the gem-or neargem quality diamonds dated by Richardson et al (1984) and the cloudy or cuboid-coated diamonds studied by Weiss et al (2018) belong to totally different genetic populations.…”

Mineral Inclusions in Lithospheric Diamonds

“…Occurrence of both syngenetic and protogenetic relationships was subsequently documented through a combination of cathodoluminescence imaging and crystallographic orientation (EBSD) data (Davies et al 2018). Based on small inclusion sizes (see above) and fast diffusion speeds in hot mantle environments, pre-existing host-rock minerals are nevertheless expected to generally equilibrate during fluid-mediated diamond growth events and thus the term "synchronous" inclusion was introduced (Nestola et al 2017(Nestola et al , 2019aPamato et al 2021). That a lack of syngeneity does not necessarily preclude inclusion-based diamond dating is established in detail in Smit et al (2022).…”