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Constraining the processes that drive the disruption and re-healing of cratonic mantle roots is essential in understanding how cratons evolve. A striking example of this “cycle” of disruption and re-healing is provided by the deep mantle root currently observed beneath the ∼2.05 Ga Bushveld-Molopo Farms Complex (BMFC) of the central Kaapvaal Craton. Magmatism that produced the BMFC involved considerable disruption of the craton mantle root, yet the diamondiferous nature of the root, as sampled by Mesoproterozoic kimberlite eruptions, indicates that following Paleoproterozoic disruption, the lithosphere was re-healed and a cool geotherm re-established. A snap-shot of this mantle root provided by 48 peridotite xenoliths from the ∼1.2 Ga Premier (now Cullinan) kimberlite allows us to examine the evolution of the central Kaapvaal mantle lithosphere to constrain this re-healing process. Premier peridotites can be classified into 3 petrographic groups: an olivine-rich group (>80% olivine), with predominantly Archean Re-depletion (TRD) ages (Group I), a more orthopyroxene-rich group (Group II; <80% olivine) with dominantly Paleoproterozoic TRD ages, whose mode at ∼2.1 Ga matches the age of the BMFC, and a deformed, metasomatized group, with TRD ages mostly ∼2.1 Ga. Paleoproterozoic Group II and deformed peridotites represent 70% of the peridotite population and have systematically lower olivine Fo (molar 100×Mg/(Mg+Fe)) contents and higher Al2O3 compared to Archean Group I peridotites. Re-Os and other highly siderophile element (HSE) systematics indicate that the Paleoproterozoic TRD ages reflect the time of melt extraction. The dominant Group II peridotites have Al-Yb systematics indicative of a low-P melt-residue origin. The combined data rule out the suturing of the mantle root by accumulation of melting residues associated with the BMFC plume event. Instead, we invoke a model in which rapid deca

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re-Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present-day Os-187/Os-188 isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re-Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.

Two Fe-Ni sulfides, resembling the chemical composition of a typical mantle base metal sulfide (BMS), were synthesized and used to test micro-analytical procedures (sample digestion and chemical separation) to determine 187Os/188Os and highly siderophile element (HSE) mass fractions in single BMS grains. The bulk 187Os/188Os and HSE mass fractions of the synthetic sulfides were independently determined after high pressure asher (HPA) digestion and conventional HSE separation (Os solvent extraction and Ir, Ru, Pt, Pd, Re separation via anion exchange chromatography), while the homogenous distribution of HSE was assessed via LA-ICP-MS. Tests were performed following the protocol of Pearson et al. (1998), where sulfides are digested in H2SO4/CrO3 and Os is simultaneously extracted as OsO4. Additional tests were performed adding a pre-digestion step in HBr and/or HCl. The duration and the temperature of the pre-digestion and micro-distillation steps, as well as different chromatographic separations of the HSE were also evaluated. While the majority of the tests yielded 187Os/188Os in agreement with that obtained after HPA digestion, HSE mass fractions show large deviations from the reference content, depending on the used procedure. Such variations are interpreted as the result of incomplete sulfide digestion, Os spike loss, and the possible presence of undigested sub-micrometric platinum group minerals (PGM). Overall, the bulk sulfide HSE mass fractions obtained after HPA digestion are best reproduced (mean deviations ≤10%) using a pre-digestion step in HBr + HCl at 120 °C. This study highlights the need for matrix-matched sulfide reference materials for routine use in laboratories determining HSE mass fractions and Os isotope ratios on single BMS grains. Such an approach is fundamental for comparing and compiling Re-Os ages collected on BMS in many different laboratories. This study also demonstrates that there is need for further testing and for methodological develo, SCOPUS: ar.j, info:eu-repo/semantics/published

Late accretion of chondritic material to differentiated planetary bodies is thought to have been common in the early solar system. However, the timing and scale of admixing this material to terrestrial planets are poorly constrained. Using platinum (Pt) stable isotope data in a range of solar system bodies, we show that Earth's post-Archean mantle has chondritic 198Pt/194Pt, consistent with addition of a chondritic late-veneer after core formation. Conversely, terrestrial Archean samples record non-chondritic, heavy, 198Pt/194Pt, indicating preservation of early mantle components that escaped complete mixing with the late-veneer. These data suggest admixing of ≤50 % of the eventual full late-veneer inventory. Such effective mixing within Earth's mantle by 3.85 Ga is most consistent with modern-style plate tectonics., SCOPUS: ar.j, info:eu-repo/semantics/published

Understanding the mineralogical controls on radiogenic chronometers is a fundamental aspect of all geochronological tools. As with other common dating tools, it has become increasingly clear that the Re-Os system can be impacted by multiple mineral formation events. The accessory and micrometric nature of the Re-Os-bearing minerals has made assessing this influence complex. This is especially evident in cratonic peridotites, where long residence times and multiple metasomatic events have created a complex melting and re-enrichment history. Here we investigate a harzburgitic peridotite from the Bultfontein kimberlite (South Africa) which contains sub-micron Pt-Fe-alloy inclusions within base metal sulphides (BMS). Through the combination of the focused ion beam lift-out technique and low blank mass spectrometry we were able to remove and analyse the Pt-Fe-alloy inclusions for their Re-Os composition and highly siderophile element (HSE) systematics. Six repeats of the whole-rock yield 187Os/188Os compositions of 0.10893-0.10965, which correspond to Re depletion model ages (TRD) of 2.69-2.79 Ga. The Os, Ir and Pt concentrations are slightly variable across the different digestions, whilst Pd and Re remain constant. The resulting HSE pattern is typical of cratonic peridotites displaying depleted Pt and Pd. The Pt-Fe-alloys have PUM-like 187Os/188Os compositions of 0.1294±24 (2-s.d.) and 0.1342±38, and exhibit a saw-tooth HSE pattern with enriched Re and Pt. In contrast, their BMS hosts have unradiogenic 187Os/188Os of 0.1084±6 and 0.1066±3, with TRD ages of 2.86 and 3.09 Ga, similar to the whole-rock systematics. The metasomatic origin of the BMS is supported by (i) the highly depleted nature of the mantle peridotite and (ii) their Ni-rich sulphide assemblage. Occurrence of Pt-Fe-alloys as inclusions within BMS grains demonstrates the genetic link between the BMS and Pt-Fe-alloys and argues for formation during a single but continuous event of silicate melt percolation., SCOPUS: ar.j, info:eu-repo/semantics/published

The Re-Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth's dynamics from the mantle perspective. In order to constrain the effects of metasomatic processes on the Re-Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and Os-187/Os-188 signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP-GPP-PP additionally constitute a sequence of increasing modal metasomatism. The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15-40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their Os-187/Os-188 signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5-2.8 Ga, T-RD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root. The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE-Se-Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001-0.05 wt.% metasomatic BMS +/- PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE-Se-Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS +/- PGM did not dist