Evidence for olivine deformation in kimberlites and other mantle-derived magmas during crustal emplacement

Abstract

This paper highlights published and new field and petrographic observations for late-stage (crustal 16 level) deformation associated with the emplacement of kimberlites and other mantle-derived 29 17 magmas. Thus, radial and tangential joint sets in the competent 183 Ma Karoo basalt wall rocks to 32 18 the 5 ha. Lemphane kimberlite blow in northern Lesotho have been ascribed to stresses linked to 34 19 eruption of the kimberlite magma. Further examples of emplacement-related stresses in 35 36 20 kimberlites are brittle fractures and close-spaced parallel shears which disrupt olivine macrocrysts. 37 21 In each of these examples, there is no evidence of post-kimberlite regional tectonism which might 38 39 22 explain these features, indicating that they reflect auto-deformation in the kimberlite during or 40 41 23 immediately post-emplacement. On a microscopic scale, these inferred late-stage stresses are 42 43 24 reflected by fractures and domains of undulose extinction which traverse core and margins of some 44 25 euhedral and anhedral olivines in kimberlites and olivine melilitites. Undulose extinction and kink 45 46 26 bands have also been documented in olivines in cumulates from layered igneous intrusions. Our 47 48 27 observations thus indicate that these deformation features can form at shallow levels (crustal 49 50 28 pressures), which is supported by experimental evidence. Undulose extinction and kink bands have 51 52 29 previously been presented as conclusive evidence for a mantle provenance of the olivines -i.e. that 53 30 they are xenocrysts. The observation that these deformation textures can form in both mantle and 54 31 crustal environments implies that they do not provide reliable constraints on the provenance of the 32 olivines. An understanding of the processes responsible for crustal deformation of kimberlites could 33 potentially refine our understanding of kimberlite emplacement processes. 34 35 31 50 2017) argued that the majority of kimberlitic olivines are cognate phenocrysts. Skinner (1989) took 51 an intermediate view, concluding that the euhedral olivine phenocrysts and microphenocrysts are 52 cognate to the kimberlite, but that the majority of macrocrysts are xenocrysts. 37 53 54 These contrasting interpretations have important implications for the composition and generation of 40 41 55 kimberlite magmas. If a majority of olivines are cognate, this would point to a highly Mg-rich 43 56 primitive kimberlitic liquid, whereas the xenocrystal model implies a carbonatitic, relatively Mg-poor 57 primary magma. It is therefore clearly critical to determine the origin of kimberlite olivines in order 10 346 Frieder Reichardt for providing the Murowa dyke sample illustrated in Fig. 3a&b. The core sample 12 13 347 illustrated in Fig. 3d was kindly provided by Petra Diamonds Ltd. Tsodilo Resources (a Canadian-14 348 listed diamond exploration company), kindly facilitated a field visit to the company's BK16 15 16 349 kimberlite, located in the mid-Cretaceous (~90 Ma) Orapa pipe cluster in central Bots...

Origins of olivine in Earth’s youngest kimberlite: Igwisi Hills volcanoes, Tanzania craton

“…Olivine deformation features, such as kink banding and undulose extinction, are often ascribed to strain within the lithospheric mantle, and their identification is typically used as evidence for a xenocrystic origin of olivine in mantlederived magmatic rocks (Skinner 1989;Tappe et al 2009;Cordier et al 2015). This concept has been contested by Moore et al (2020), who proposed that olivine grains in kimberlites may have been deformed at crustal levels, with the implication that deformation features alone do not provide unequivocal evidence for a xenocrystic origin from the cratonic mantle. A similar line of evidence was developed earlier by Kresten (1973), Moore (1988Moore ( , 2012 and Shaikh et al (2018), in which deformation of olivine phenocrysts was ascribed to torsional forces applied to the kimberlite magma during ascent.…”

“…Macrocrysts dominate among the olivine populations and their cores typically show evidence of deformation such as kink bands and undulose extinction. Together with evidence from mineral inclusions, the deformation features have been interpreted in light of lithospheric mantle origins of the olivine macrocryst cores (Kamenetsky et al 2008;Brett et al 2009;Bussweiler et al 2015;Sobolev et al 2015;Giuliani 2018), although Moore et al (2020) considered this line of evidence as ambiguous and ascribed some of the olivine deformation features to the kimberlite magma ascent mechanism at crustal depths. In contrast, undeformed euhedral olivine phenocrysts often contain inclusions of other near-liquidus or even groundmass phases such as spinel, phlogopite and rutile (Kamenetsky et al 2008;Bussweiler et al 2015;Soltys et al 2018).…”