Abstract
Research Article| March 01, 2001 The origin of mantle-derived megacrysts and sheared peridotites-evidence from kimberlites in the northern Lesotho - Orange Free State (South Africa) and Botswana pipe clusters A.E. Moore (*); A.E. Moore (*) AMPAL Pty) Ltd., Box Gaborone botswana. Search for other works by this author on: GSW Google Scholar NP. Lock NP. Lock Kalahari Exploration (Pty) Ltd., Private Bag 00351, Gaborone, Botswana., e-mail: nlock@info.bw Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2001) 104 (1): 23–38. https://doi.org/10.2113/104.1.23 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.E. Moore (*), NP. Lock; The origin of mantle-derived megacrysts and sheared peridotites-evidence from kimberlites in the northern Lesotho - Orange Free State (South Africa) and Botswana pipe clusters. South African Journal of Geology 2001;; 104 (1): 23–38. doi: https://doi.org/10.2113/104.1.23 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Megacrysts (clinopyroxene, orthopyroxene, garnet, olivine and ilmenite) from different kimberlites of the northern Lesotho—eastern Orange Free State Cluster are characterised by Closely overlapping Compositional fields. Minerals in sheared peridotites from these pipes define Continuous chemical fields that straddle the compositional gap between Corresponding phases in coarse, relatively low temperature peridotites, and the Mg-rich (high temperature) extreme of the megacryst suite. The highest temperatures recorded for the sheared peridotites are from rocks with minerals closest in composition to the high temperature megactysts. These relationships are consistent with models that interpret sheared peridotites as representing metasomatised mantle samples that have responded chemically and thermally to the magma, here inferred to be the host kimberlite, which was parental to the megacryst suite. The latter crystallised in pegmatitic veins from small volumes of liquid injected into a fracture network in the thermal aureole surrounding the main kimberlite magma reservoir in the mantle prior to eruption. Crustal contamination of the kimberlite magma during ascent may in part explain Sr-isotopic signatures that differ from those of the entrained megacrysts. However, such differences can also be ascribed to melting of a Sr-rich carbonate in the thermal aureole surrounding the kimberlite magma in the mantle prior to eruption. This carbonate melt would mix with the small volumes of kimberlitic liquid injected into the fracture network surrounding the main magma reservoir. An important implication of this model is that sheared xenoliths cannot be used to infer vertical mantle zonation, and thus chemical and isotopic differences between the lithosphere and asthenosphere. The model accounts for the link between ilmenite chemistry and kimberlite diamond grade. It also provides an explanation for the range in zircon ages which are reported in some kimberlites You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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