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Composition and emplacement of the Benfontein Kimberlite Sill Complex (Kimberley, South Africa): Textural, petrographic and melt inclusion constraints

Citation

Abersteiner, A and Kamenetsky, VS and Goemann, K and Giuliani, A and Howarth, GH and Castillo-Oliver, M and Thompson, J and Kamenetsky, M and Cherry, A, Composition and emplacement of the Benfontein Kimberlite Sill Complex (Kimberley, South Africa): Textural, petrographic and melt inclusion constraints, Lithos, 324-325 pp. 297-314. ISSN 0024-4937 (2019) [Refereed Article]

Copyright Statement

Crown Copyright 2018

DOI: doi:10.1016/j.lithos.2018.11.017

Abstract

The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5 m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7 ± 4.4 Ma and 86.5 ± 2.6 Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster.

The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections.

The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel - magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration.

Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali‑carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn-/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.

Item Details

Item Type:Refereed Article
Keywords:benfontein, kimberlite, sill, magma differentiation, atoll-spinel, melt inclusion
Research Division:Earth Sciences
Research Group:Geology
Research Field:Igneous and Metamorphic Petrology
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Earth Sciences
UTAS Author:Abersteiner, A (Mr Adam Abersteiner)
UTAS Author:Kamenetsky, VS (Professor Vadim Kamenetsky)
UTAS Author:Goemann, K (Dr Karsten Goemann)
UTAS Author:Thompson, J (Mrs Jennifer Thompson)
UTAS Author:Kamenetsky, M (Dr Maya Kamenetsky)
UTAS Author:Cherry, A (Mr Alex Cherry)
ID Code:129288
Year Published:2019 (online first 2018)
Funding Support:Australian Research Council (DP1092823)
Web of Science® Times Cited:7
Deposited By:CODES ARC
Deposited On:2018-11-21
Last Modified:2019-03-18
Downloads:0

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