Abstract
To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.
Highlights
Mineral inclusions in diamond are potentially fundamental to understanding the formation conditions of diamond and Earth’sEditorial handling: T
Visual observation showed that of the 24 analysed inclusions 14 have some planar crystal faces and of these 6 have partial development of cubo-octahedral morphology (JW078B and C; JW204B and D and JW356D and F) that could have been imposed by the diamond
Similar diamond growth relationships are defined by garnet C with the notable observation that there are triangular growth layers that are parallel to the triangular termination of the inclusion
Summary
Mineral inclusions in diamond are potentially fundamental to understanding the formation conditions of diamond and Earth’sEditorial handling: T. The same group (Bruno et al 2016) reported that there is no thermodynamic benefit for olivine and diamond to form epitaxially as there is no difference in adhesion energies between diamond and forsterite even during synchronous growth there is no reason to develop preferential orientations. This conclusion appears to contrast with reports of preferred orientation and epitaxial growth of olivine inclusions in diamond by Neuser et al (2015). This conclusion appears to contrast with reports of preferred orientation and epitaxial growth of olivine inclusions in diamond by Neuser et al (2015). Milani et al (2016), re-interpreted these data and concluded that there was no epitaxy between olivine and the host diamonds
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