Two episodes of fluid migration in the Kaapvaal Craton lithospheric mantle associated with Cretaceous kimberlite activity: Evidence from a harzburgite containing a unique assemblage of metasomatic zirconium-phases

In this study we combine textural evidence with mineral chemical, mineral structural and isotopic data in an attempt to reconstruct the history of metasomatic events recorded in a spinel-harzburgite xenolith which was sampled by a Cretaceous kimberlite of the central Kaapvaal Craton, South Africa. Two episodes of interaction between the host rock and silicate-rich high-density fluids can be distinguished which together gave rise to a uniquely complex assemblage of LILE-HFSE-rich phases including zircon together with both monoclinic (baddeleyite) and cubic (tazheranite) zirconium oxide, srilankite and a new Mn-Fe-rich member of the pyrochlore-group of phases. The pre-metasomatic assemblage is olivine + orthopyroxene + Cr-spinel. Subsequent modal metasomatism formed phlogopite + K-richterite + crichtonite-group (lindsleyite-mathiasite) phases + Nb-Cr-rich rutile + srilankite + zircon + Fe-Ni-sulfide. K-richterites are strongly zoned in Ca, Na, Fe and Cr with up to 2.3 wt.% Cr2O3 which is the highest Cr content reported so far for K-richterite. SIMS U-Pb dating of the zircons yields ages in the range from 81 ± 2 to 91 ± 2 (2σ) Ma which are indistinguishable from emplacement ages of Cretaceous kimberlites in the Kimberley area. The age spread is interpreted as a result of minor re-setting of the U-Pb isotopic system. The coincidence between zircon and kimberlite eruption ages further supports a temporal and genetic link between Cretaceous kimberlite activity and hydrous potassic metasomatism in the central Kaapvaal Craton lithosphere and limits the residence time in the mantle of metasomatized peridotites to < ~ 5-10 Ma. Thermobarometry of the harzburgite yields 750-760 °C at 3 GPa with a redox state of + 0.9 to + 1.5 log units relative to FMQ. Infiltration of a hot and alkali-rich (kimberlitic?) high-density fluid with aSiO2 lower than that defined by olivine + orthopyroxene into the cool metasomatized peridotite led to partial breakdown of K-richterite, crichtonite-group phases, zircon and sulfides and formed Al-Ti-Ni-rich spinel + K-Cr priderite + Nb-rich titanite + baddeleyite + tazheranite + Mn-Fe-betafite. In addition, a second generation of Ni-poor and Mn-rich olivine formed along with Ti-rich K-richterite, phlogopite and clinopyroxene. Both tazheranite and Mn-Fe-betafite were unambiguously identified with electron microprobe analysis combined with selected area diffraction using FIB-TEM. Mn-Fe-betafite is new member of the pyrochlore group of phases and the tazheranite from this study represents the first occurrence reported from an upper mantle environment.