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A new approach to revealing the composition of kimberlite melts and their deep mantle source

Periodic Reporting for period 1 - KimberliteNewApproach (A new approach to revealing the composition of kimberlite melts and their deep mantle source)

Reporting period: 2016-07-01 to 2018-06-30

The overarching aim of this project is to provide novel constraints on the composition of the Earth’s deep mantle, particularly its volatile content, by undertaking an innovative geochemical and isotopic study of the deepest formed melts on Earth: kimberlites. Kimberlite melts are derived from depths in excess of 150-200 km. They are important as the major host of diamonds and because entrain xenoliths (i.e. fragments) of upper mantle and deep crustal rocks during ascent to the surface, providing a major source of information about the geochemistry of the deep Earth. Despite their importance, the composition of primary kimberlite melts and their exact mantle source are hotly debated issues.

Diamonds are not generally produced by the crystallisation of kimberlite melts, but are xenocrysts entrained from mantle wall-rocks during ascent of kimberlite magmas. The project investigates barren kimberlites and diamondiferous kimberlites showing different grades. This project could have a major impact on diamond industry if it identifies significant differences in the composition of the kimberlite melts that generated the different diamond deposits.
During the 7 months of this project, a range of kimberlite samples have been characterised for their texture and mineralogical compositions including details analysis of olivine grains. Olivine is the most abundant phase in kimberlites and the main repository of fluid/melt inclusions, both being the major target of this project. Abundant major element and trace element data for olivine grains were acquired combined with detailed observations of complex zoning using a range of micro-analytical techniques (e.g. scanning electron microscopy; electron microprobe measurements).

Some of these results have been already published in the highly regarded international peer-reviewed journal Chemical Geology (Giuliani et al., in press). This article reports details of the mineralogy and petrology, including olivine and fluid/melt inclusion compositions, of the Bultfontein kimberlite in the Kimberley area (i.e. the kimberlite type locality). The results of this research will be included in the keynote presentation that Dr Giuliani will deliver at the prestigious International Kimberlite Conference (Botswana; September 2017), and subsequently published in the conference proceedings.
"The detailed investigation of olivine in kimberlites undertaken as part of this project has revealed the complexity of kimberlite magma evolution to an unprecedented detail. This project has indeed shown that olivine composition and zoning provide information on the evolution of kimberlite magmas at various stages: 1) early in deep mantle kimberlite magmas assimilate mantle material including wall-rock olivine; 2) entrained crystals of wall-rock olivine after initial resorption operates as nuclei for the overgrowth of magmatic olivine later during kimberlite ascent; 3) these olivine overgrowths present complex zoning which reflects the composition of the progressive of the kimberlite magma including the influence of various processes (e.g. wall rock assimilation; crystallisation of other minerals).

The major breakthrough of this project is that, for the first time, we have identified a direct relationship between the composition of olivine xenocrystic cores (i.e. derived from mantle wall-rocks) and that of magmatic olivine rims in kimberlites worldwide. This evidence provides the ""smoking gun"" that the composition of kimberlite magmas is a direct function of the characteristics of the mantle the magma ascends through. This is a radically new inference of possible wider applicability to the field of geochemistry and igneous petrology because it is commonly assumed that the composition of most of mantle-derived magmas reflects of the mantle source."
complexly zoned olivine grain in kimberlite from Russia