Mineral assemblage within secondary crystallized melt inclusions in olivine of mantle xenoliths from the Bultfontein kimberlite pipe (Kaapvaal craton, South Africa)
Authors: Tarasov Alexey , Sharygin Igor , Golovin Alexander , Dymshits Anna , Rezvukhin Dmitriy 
Published: March 2021

&lt;p&gt;For the first time, snapshots of crystallized melts in olivine of sheared garnet peridotite xenoliths from the Bultfontein kimberlite pipe have been studied. This type of xenoliths represents the deepest mantle rocks derived from the base of lithosphere (at depths from 110 to 230 km for various ancient cratons). According to different models, such type of inclusions (secondary) in mantle minerals can be interpreted as relics of the most primitive (i.e., close-to-primary) kimberlite melt that infiltrated into sheared garnet peridotites. In general, these secondary inclusions are directly related to kimberlite magmatism that finally formed the Bultfontein diamond deposits. The primary/primitive composition of kimberlite melt is poorly constrained because kimberlites are ubiquitously contaminated by xenogenic material and altered by syn/post-emplacement hydrothermal processes. Thus, the study of these inclusions helps to significantly advance in solving numerous problems related to the kimberlite petrogenesis.&lt;/p&gt;&lt;p&gt;The unexposed melt inclusions were studied by using a confocal Raman spectroscopy. In total, fifteen daughter minerals within the inclusions were identified by this method. Several more phases give distinct Raman spectra, but their determination is difficult due to the lack of similar spectra in the databases. Various carbonates and carbonates with additional anions, alkali sulphates, phosphates and silicates were determined among daughter minerals in the melt inclusions: 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,K)&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;, 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, 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;), 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;, 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;, apatite Ca&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;(OH,Cl,F) and 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). Note that carbonates are predominant among the daughter minerals in the melt inclusions. Moreover, there are quite a lot of alkali-rich daughter minerals within the inclusions as well. During the last decade, some research groups using different approaches proposed a model of carbonate/alkali&amp;#8209;carbonate composition of kimberlite melts in their source regions. This model contradicts to the generally accepted ultramafic silicate nature of parental kimberlite liquids. This study is a direct support of a new model of carbonatitic composition of kimberlite melts and also shows that alkali contents in kimberlite petrogenesis are usually underestimated.&lt;/p&gt;&lt;p&gt;This work was supported by the Russian Foundation for Basic Research (grant No. 20-35-70058).&lt;/p&gt;