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In-situ assimilation of mantle minerals by kimberlitic magmas - Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa)


Soltys, A and Giuliani, A and Phillips, D and Kamenetsky, VS and Maas, R and Woodhead, J and Rodemann, T, In-situ assimilation of mantle minerals by kimberlitic magmas - Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa), Lithos, 256-257 pp. 182-196. ISSN 0024-4937 (2016) [Refereed Article]

Copyright Statement

2016 Elsevier

DOI: doi:10.1016/j.lithos.2016.04.011


The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult.

In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~ 1060 C and 43 kbar (~ 140145 km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by CaMg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., NaK bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are interpreted to represent immiscibility between carbonate and silicate melts.

The preservation of reaction mantles, immiscibility textures and disequilibrium in the vein assemblage, suggests quenching, probably triggered by entrainment and rapid transport toward the Earth's surface in the host kimberlite magma. Based on the Sr-isotope systematics of vein carbonate minerals, and the close temporal relationship between carbonate-rich metasomatism and kimberlite magmatism, we suggest that the carbonate-rich vein assemblage was produced by the interaction between a melt genetically related to the Bultfontein kimberlite and wehrlitic mantle wall rock. If correct, this unique xenolith sample provides a rare snapshot of the assimilation processes that might characterise parental kimberlite melts during their ascent through the lithospheric mantle.

Item Details

Item Type:Refereed Article
Keywords:kimberlite, melt evolution, assimilation, carbonate metasomatism, lithospheric mantle, Kimberley
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:Kamenetsky, VS (Professor Vadim Kamenetsky)
UTAS Author:Rodemann, T (Dr Thomas Rodemann)
ID Code:108981
Year Published:2016
Funding Support:Australian Research Council (DP130100257)
Web of Science® Times Cited:49
Deposited By:Earth Sciences
Deposited On:2016-05-15
Last Modified:2017-10-30

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