Reconstruction of the composition of the kimberlite melt of the Bultfontein pipe, Kaapvaal craton
Authors: Tarasov Alexey , Golovin Alexander , Sharygin Igor 
Published: March 2022

&lt;p&gt;Information about the compositions of primitive kimberlite melts is important for understanding the petrogenesis of kimberlites. Reconstruction of the composition of these melts is very difficult because the melts greatly changed their compositions via assimilation of mantle and crust xenogenic materials and degassing during ascent.&lt;/p&gt;&lt;p&gt;To reconstruct the composition of the kimberlite melt of the Bultfontein pipe (Kaapvaal craton, South Africa), the mineral assemblage of secondary melt inclusions in olivines of mantle peridotite xenoliths from the pipe has been studied. The depths of equilibrium of the studied peridotites range from 120 to 150 km.&lt;/p&gt;&lt;p&gt;The inclusions occur along the healed cracks in the olivine grains. Twenty-five daughter minerals were found in the inclusions by Raman spectroscopy and scanning electron microscopy. Liquids and gases were not detected. The inclusions are mainly made up of carbonates (calcite CaCO&lt;sub&gt;3&lt;/sub&gt;, magnesite MgCO&lt;sub&gt;3&lt;/sub&gt;, dolomite CaMg(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, eitelite Na&lt;sub&gt;2&lt;/sub&gt;Mg(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, nyerereite Na&lt;sub&gt;2&lt;/sub&gt;Ca(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, gregoryite (Na,K,Ca)&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt;, K-Na-Ca-carbonate (K,Na)&lt;sub&gt;2&lt;/sub&gt;Ca(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, shortite Na&lt;sub&gt;2&lt;/sub&gt;Ca&lt;sub&gt;2&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;) or carbonates with additional anions (nahcolite NaHCO&lt;sub&gt;3&lt;/sub&gt;, bradleyite Na&lt;sub&gt;3&lt;/sub&gt;Mg(PO&lt;sub&gt;4&lt;/sub&gt;)(CO&lt;sub&gt;3&lt;/sub&gt;), northupite Na&lt;sub&gt;3&lt;/sub&gt;Mg(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;Cl, burkeite Na&lt;sub&gt;6&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt;(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;,&amp;#160;tychite Na&lt;sub&gt;6&lt;/sub&gt;Mg&lt;sub&gt;2&lt;/sub&gt;(CO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;(SO&lt;sub&gt;4&lt;/sub&gt;)). Halides (halite NaCl, sylvite KCl), sulfates (glauberite Na&lt;sub&gt;2&lt;/sub&gt;Ca(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, thenardite Na&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;, aphthitalite K&lt;sub&gt;3&lt;/sub&gt;Na(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;), phosphate (apatite Ca&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;(F,Cl,OH)), oxides (rutile TiO&lt;sub&gt;2&lt;/sub&gt;, magnetite FeFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;), sulfide (heazlewoodite Ni&lt;sub&gt;3&lt;/sub&gt;S&lt;sub&gt;2&lt;/sub&gt;) and silicates (phlogopite KMg&lt;sub&gt;3&lt;/sub&gt;AlSi&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;10&lt;/sub&gt;(F,Cl,OH), tetraferriphlogopite KMg&lt;sub&gt;3&lt;/sub&gt;FeSi&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;10&lt;/sub&gt;(F,Cl,OH), richterite Na&lt;sub&gt;2&lt;/sub&gt;Ca(Mg,Fe,Mn,Al)&lt;sub&gt;5&lt;/sub&gt;[Si&lt;sub&gt;4&lt;/sub&gt;O&lt;sub&gt;11&lt;/sub&gt;](OH,F)&lt;sub&gt;2&lt;/sub&gt;) are also present in the inclusions.&lt;/p&gt;&lt;p&gt;These inclusions are considered to be relics of a near&amp;#8208;primary or primitive kimberlitic melt that later formed the Bultfontein pipe. The observed mineral assemblage indicates that the captured melt had an alkali-carbonatitic composition and was rich in Cl and S.&lt;/p&gt;&lt;p&gt;&lt;em&gt;This work was supported by the Russian Foundation for Basic Research (grant No. 20-35-70058).&lt;/em&gt;&lt;/p&gt;