School of Earth Sciences - Theses
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ItemPetrology and geochemistry of mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa): new insights into lithospheric mantle fluids( 2013)In cratonic areas the lithospheric mantle formed and stabilised during the Archean eon and has been modified through time by multiple episodes of enrichment (i.e. metasomatism) driven by fluids and melts of variable composition. The vast majority of metasomatised mantle rocks are thought to have interacted with basic/ultrabasic and carbonate melts/fluids, variably enriched in alkali and other incompatible elements. The common occurrence of volatile-dominated fluids and brines included in diamonds and mantle silicate minerals suggests that such fluids might be widespread in the Earth’s mantle; however the metasomatising impact of these fluids still require further study. This thesis provides new textural, mineralogical and geochemical data for mantle xenoliths entrained by the Bultfontein kimberlite (Kimberley, South Africa). The studied xenoliths include two mantle polymict breccias, a Ni-mineralised spinel harzburgite and a sulphate-rich MARID (mica-amphibole-rutile-ilmenite-diopside) rock. Mantle polymict breccias are complex mixtures of mantle clasts and minerals cemented together by variable amounts of olivine, phlogopite, ilmenite, rutile, orthopyroxene and sulphides. Groundmass olivine and ilmenite host carbonate-rich inclusions dominated by magnesite, dolomite, alkali-carbonates, phlogopite and kalsilite. These inclusions probably represent an alkali-carbonate melt, which was entrapped during olivine and ilmenite crystallisation in the mantle. This is the first evidence for an alkali-carbonate fluid in the lithospheric mantle above the diamond stability field. The heterogeneous mineralogy and geochemistry of polymict breccias suggest these rocks formed immediately prior to entrainment and transport by kimberlite magmas. Polymict breccias are therefore regarded as failed kimberlite intrusions frozen at depth. The alkali-carbonate melt preserved as inclusions in olivine and ilmenite could be parental to the cementing phases of polymict breccias and could either derive from silicate-carbonate liquid immiscibility of a precursor proto-kimberlite melt or represent a pristine example of primitive kimberlite melt. The Ni-rich spinel harzburgite hosts millimeter-sized mineralised areas that include native nickel, heazlewoodite and Ni-rich silicates (e.g., olivine, phlogopite). The presence of several mineral phases enriched in alkali and volatile species (e.g., phlogopite, phosphates, carbonates, chlorides, djerfisherite) indicates that the transition metal cations were introduced during metasomatism by alkali-rich C–O–H fluids or alkali-carbonate melts. The sulphate-rich MARID sample is traversed by veins dominated by Ba-rich celestine and clinopyroxene, with minor phlogopite, pectolite, sphene, apatite, barite and Sr-Ca carbonates. Celestine hosts the other metasomatic vein phases, but also occurs as inclusions in clinopyroxene, suggesting co-precipitation of these minerals. Celestine was partly replaced by serpentine during alteration by hydrous fluids after kimberlite emplacement in the upper crust. The texture and chemical composition of the metasomatic phases indicate that the MARID rock was infiltrated by a sulphate fluid enriched in Sr, Ba, Na and Ca, with lesser P, Ti, LREE, CO2 and F. A mantle origin for the sulphate fluid is supported by: (i) comparisons between the Sr–S isotopic compositions of celestine, the host kimberlite, crustal and mantle lithologies from the area, and (ii) alteration of celestine by late-stage hydrous fluids. The celestine-bearing veins provide the first evidence for the occurrence of sulphate-dominated fluids in the Earth’s mantle. In summary this thesis provides new insights into the compositions of widespread mantle metasomatic agents, namely alkali-carbonate melts, and documents some of the metasomatic processes occurring in the lithospheric mantle during ascent of primitive or precursor kimberlite magmas. My research also provides unexpected evidence for the occurrence of previously unknown fluids in the Earth’s mantle, including Ni-rich C-O-H fluids and sulphate-dominated fluids.