Volcaniclastic ash deposits (tuffs) within the Turkana Basin in Kenya act as temporal markers to constrain the extensive fossil-rich sediments of the Koobi Fora Formation. By using geochemical fingerprinting and precise geochronology this study improves the temporal and spatial resolution of the Koobi Fora, Lower Koobi Fora and Karari Blue tuffs within the Koobi Fora Tuff Complex. Utilising a Bayesian approach to single-grain anorthoclase ages this study presents ages of 1.49257 +/- 0.00186 (2 sigma) Ma for the Karari Blue Tuff, 1.49430 +/- 0.00142 (2 sigma) Ma for the Koobi Fora Tuff and 1.51561 +/- 0.00090 (2 sigma) Ma for the Lower Koobi Fora Tuff. These results provide highly precise (<0.1 % uncertainties) anchoring ages for the stratigraphy of the Okote Member, implying an older age for the base of the Koobi Fora Tuff Complex (>1.51561 +/- 0.00090 Ma). The implications of the similar to 30 ka revision of the Lower Koobi Fora Tuff indicates older dates for significant hominin fossil sites in the Ileret (Area 1A) and Koobi Fora (Area 103) regions. The ages of the three tuffs provide the highest precision ages of the Koobi Fora Tuff Complex thus far and have important implications in the Koobi Fora Formation and correlates across the Turkana Basin.

The sub-cratonic lithospheric mantle (SCLM) is known for its chemical buoyancy and mechanical strength, which have facilitated its preservation for billions of years. However, the thinning of lithospheric mantle beneath some cratons (e.g., North China, Sao Francisco) indicates that some cratons are not permanently stable. We have assembled a new compilation of SCLM thickness data based on previously published, robustly estimated palaeo-geotherms that are defined by mantle xenocrysts/xenoliths in kimberlites and lamproites globally. These data support a localised and episodic decrease in lithospheric thickness (e.g., Siberian craton) with thin lithosphere (< 200 km) only underlying some kimberlites/lamproites younger than 200 Ma. Some late-Phanerozoic thinning of the SCLM in areas of North America and southern Africa is supported by comparison of the xenocryst/xenolith-based estimates with present-day constraints from geophysical methods. To investigate the effect of lithospheric thinning on the compositional evolution of the SCLM and its implication for magmatism in cratonic settings, we have examined the compositions of olivine xenocrysts sampled by kimberlites and lamproites globally. The average composition of entrained lithospheric mantle olivine exhibits no correlation with kimberlite and lamproite eruption ages, nor with SCLM thickness, suggesting no simple relationship between lithospheric thinning and lithospheric composition over time. Our results further reveal a broad relationship between locally thinner cratonic lithosphere in the last 200 Ma and the frequency of kimberlite eruptions, which hints at more favourable conditions for the formation of kimberlite melts in the upper asthenosphere beneath thinned lithosphere (150–200 km) since the Mesozoic period.

Diamonds entrained by kimberlites and olivine lamproites formed predominantly in peridotite and eclogite substrates within the lithospheric mantle. The main growth forms of monocrystalline diamonds are octahedral and cubic. However, many diamonds also exhibit a range of surface features derived during mantle residence and/or entrainment to surface. In pioneering research, Derek Robinson developed an interpretative catalogue of diamond morphologies and surface features, improving understanding of diamond growth, plastic deformation, oxidative etching and resorption processes, and the impacts of sedimentary transport and diamond recovery practices. He also established the sequence of events reflected in diamond physical characteristics. The etching and resorption surface features developed on diamonds provide important constraints on their exposure to oxidizing fluids (mainly CO2, H2O) in the mantle and/or kimberlite melt. There is broad consensus that common resorption features such as tetrahexahedroid (THH) forms result from interaction with H2O-bearing kimberlite fluids. However, other surface features on trigonal octahedral faces (e.g., deep hexagonal pits, triangular plates) have been attributed to either pre-kimberlite mantle metasomatism or variations in kimberlite melt/fluid conditions. Evidence supporting mantle resorption includes cathodoluminescence (CL) imaging of internal diamond growth layers and rounded diamonds in some mantle xenoliths. As most diamonds in mantle xenoliths are typically sharp-edged with few etch features, the formation of specific surface etch features by pre- (or syn-) kimberlite mantle metasomatism is equivocal. Alternative explanations include limited ingress of kimberlitic fluids into host xenoliths during entrainment, ascent and/or emplacement, sampling of multiple diamond resorption groups from different pulses of kimberlite magma with distinct volatile compositions eruption/degassing histories.

Cratonic lamproites are rare ultrapotassic rocks that occur in every continent and were emplaced over the last 2 billion years. Owing to their highly radiogenic Sr and unradiogenic Nd-Hf isotopic compositions along with enrichment in K and incompatible trace elements (e.g., Ba, Sr, Ti, Zr), lamproites are believed to be formed by melting geochemically enriched regions in the sub-continental lithospheric mantle (SCLM). In contrast, olivine compositions indistinguishable from those of kimberlites and the occurrence of diamonds containing inclusions of majorite-bearing garnet in some cratonic lamproites, suggest that primary lamproitic (or proto-lamproitic) melts may originate in the convecting mantle. Here we re-evaluate the different source component(s) responsible for the genesis of cratonic lamproites worldwide using major-, trace-element and radiogenic isotope geochemistry. We report new major-, trace-element and Sr, Nd and Hf isotope compositions for 61 cratonic lamproite samples from sixteen cratons, with an emphasis on localities lacking Hf isotope data. These data, combined with published results, reveal three discernible end-member compositions in Sr-Nd-Hf isotope space. The first end- member includes lamproites with the least geochemically-enriched isotopic signatures (i.e. less radiogenic Sr, less negative and locally positive epsilon Nd and epsilon Hf values) (e.g., Wajrakarur and Bunder in India, Melville in Canada), similar to the Bulk Silicate Earth (BSE), which overlap with those of global ocean island basalts (OIBs) and archetypal kimberlites. The second end-member is represented by the Cenozoic West Kimberley lamproites (Australia), which exhibit the highest epsilon Sr values with low epsilon Nd and epsilon Hf compositions and resemble EM II OIBs from Samoa. The third end-member, defined by lamproites from Leucite Hills and Smoky Butte (USA), exhibits strongly negative epsilon Nd and epsilon Hf values, associated with moderately negative epsilon Sr compositions similar to, although more extreme than, EM I OIBs. The remaining lamproites included in this study have intermediate isotopic compositions that fall between these three end-members. The similar Sr-Nd-Hf compositions of the least geochemically enriched lamproites, OIBs and kimberlites suggest a predominant input from the convective mantle for these lamproites. Conversely, the lamproites with more geochemically enriched Sr-Nd-Hf isotopic compositions (i.e. more radiogenic Sr, more negative epsilon Nd and epsilon Hf values) necessitate extreme source compositions that, locally, cannot be accounted for by commonly observed mantle components, such as lithospheric mantle xenoliths dominated by mica (e.g., MARID) or subducted crust/sediment. Theoretical components generated by long-term enrichment of the lithospheric mantle mediated by subduction-related fluids appear to be required to produce mica-bearing peridotites that, with time, develop isotopic compositions similar to those observed in the most geochemically enriched lamproites. The spread in isotopic compositions observed in lamproites from some regions (e.g., Cenozoic West Kimberley) is best explained via assimilation of anciently metasomatised lithospheric mantle peridotites by melts derived from moderately depleted convective mantle sources rather than exclusive melting of lithospheric mantle sources. Differences in the proportions of various end-members, combined with the diverse isotopic compositions of metasomatized SCLM across different regions and at different times are crucial in creating the unique mineralogical and isotopic characteristics documented in lamproites from each province. Thus, cratonic lamproites offer insights into the cratonic lithosphere and its metasomatic evolution.

The Turkana Basin is a renowned paleoanthropological region in Kenya and Ethiopia and is famous for discoveries of numerous hominin fossils and their associated cultural technologies. The Plio-Pleistocene sedimentary sequences hosting these important remains are interbedded with volcanic ash (tuff) beds that provide crucial bracketing age constraints. The Nariokotome Tuff Complex, comprising the Upper, Middle, and Lower Nariokotome Tuffs, preserves deposits that cover an important time interval during the late Early Pleistocene that saw milestone events for Homo erectus and associated technological development. Unfortunately, characterization of these tuffs has been hampered by a) overlapping published eruption ages and b) indistinguishable major element compositions. In addition, fluvial reworking of feldspar-bearing pumice clasts (the target rock for age determinations) from older volcanic deposits into younger tuff layers complicates correct age assignments. Here, we use multiple tephrochronological correlation tools, including high-resolution 40Ar/39Ar geochronology and grain-specific major- and trace-element geochemistry, to establish a well-characterized geochemical and geochronological framework for the Nariokotome tuffs. Utilizing a modern-generation mass-spectrometer, we report distinct ages for the Upper Nariokotome Tuff at 1,233.1 ± 1.3 ka (± 1.9 ka, 2σ; including external uncertainties), the Middle Nariokotome Tuff at 1,263.4 ± 1.2 ka (± 1.9 ka) and the Lower Nariokotome Tuff at 1,285.8 ± 1.0 ka (± 2.1 ka). In addition, high-spatial resolution Laser Ablation Inductively Coupled Mass Spectrometry trace element compositions provide distinct characterization of each tuff, aiding intrabasin correlation of these units. This combined methodology demonstrates the potential to resolve the stratigraphic complexities associated with assigning ages to key paleoanthropological sites.

This dataset contains geochemical results on the magmatic component and mantle xenoliths in a diatreme at Bokhara River in eastern Australia. The magmatic componet is bulk-rock analysis and Ar-Ar dating on the groundmass. The mantle xenoliths have both bulk-rock and mineral analyses.

Single grain, laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) trace element analysis of volcanic glass shards has emerged as a valuable tool in constructing regional tephrostratigraphic frameworks for paleoanthropological sites. The limiting factors in analysing single shards for trace element compositions are a) analytical precision (% RSD), which is dependent on the size and homogeneity of the area available for analysis, and b) accuracy (% bias), which is dependent on compositional differences between natural rhyolitic samples and available reference materials. The limited vertical thickness of the tephra glasses, the small surface areas accessible for ablation and the presence of inclusions requires increased spatial resolution, ideally utilizing laser spot diameters <= 20 mu m, to achieve effective 'fingerprinting' of tephra layers. Typically, LA-ICP-MS, 'spot' analyses at such high spatial resolution yield degraded precision (>10% RSDs) and accuracy (> +/- 5% bias) for natural tephra samples compared to reference materials. Here we present a novel approach for LA-ICP-MS trace element analysis of tephra glass and apply the method to two Plio-Pleistocene tephra layers, namely the Upper and Lower Nariokotome tuffs, and their enclosed pumice clasts, from the Turkana Basin, NW Kenya. These tuffs were chosen as they are characterised by homogeneous yet distinct major element glass composition and are examples of tephra layers of significant paleoanthropological importance. Our approach involves the use of ablation 'traverses' across individual glass fragments and we utilize the Lower Nariokotome Tuff, as a matrix-matched secondary reference material for our analytically 'unknown' samples. This method yields significant improvement in both analytical precision (1-5% RSD) and accuracy (<+/- 5% bias) for 25 trace elements, using both 10 mu m and 20 mu m beam diameters. These results allow us to identify trace element discriminators for these tuffs and successfully correlate pumices to their respective eruption events. This study contributes to the developing field of single-grain LA-ICP-MS trace element geochemistry as a tephra correlation tool by enhancing the data quality acquired for natural samples.

Eastern Australia is covered by extensive, thick regolith, which obfuscates much of its basement geology, making geological sampling difficult. The Bokhara River diatremes erupted through the Thomson Orogen in eastern Australia and are covered by similar to 300 m of Cretaceous regolith cover. Hitherto, these diatremes have only been characterised by magnetic anomaly surveys and private exploration drilling. The melts were assumed to be leucititic, owing to their proximity to Miocene leucitite centres to the south, while their position along the Cosgrove hotspot track led to the assumption that the diatremes erupted at ca 20 Ma. However, whole-rock chemistry shows that the diatremes are basanites and, according to 40Ar/39Ar dating, erupted during the mid-Jurassic (ca 180 Ma; estimated by reproducible slightly discordant ages), substantially older than the assumed age and coincident with widespread Mesozoic flood basalts across Gondwana. The basanites entrained abundant mantle material during accent, which contaminated the basanite with similar to 12.8 wt% xenocrystic material. The xenoliths are all fertile spinel lherzolites with Mg# 87-89, CaO 2.64-5.90 wt% and Al2O3 2.87-3.83 wt% that have been cryptically metasomatised by a mafic silicate melt, which resulted in chromatographic rare-earth element patterns. This demonstrates that mantle metasomatism occurred during the Mesozoic in this part of eastern Australia, revealing another expression of intraplate volcanism prior to Australia rifting away from Gondwana. The identification and characterisation of this Jurassic volcanism hosting evidence for mantle metasomatism suggest that intraplate volcanism and mantle metasomatism are synonymous within orogenic eastern Australia, and not restricted to the Cenozoic.

Ian McDougall was a renowned Earth scientist who gained worldwide distinction for his research in the fields of K-Ar and 40Ar/39Ar geochronology. He was born in Hobart, Tasmania and obtained a BSc (Hons) at the University of Tasmania and a PhD at the Australian National University in Canberra. Following his PhD, he was introduced to the emerging field of K-Ar geochronology during a postdoctoral year at the University of Berkeley in 1961. On his return to Australia, Ian accepted a position in the newly established K-Ar laboratory at the Australian National University. He pioneered the application of the K-Ar dating method to young volcanic rocks and played a pivotal role in developing the geomagnetic timescale. These findings provided crucial evidence in support of the ‘hot spot’ (mantle plume) model and the emerging theory of plate tectonics. He subsequently established the 40Ar/39Ar geochronology method at the Australian National University and gained an international reputation for meticulous experimental work. In the 1980s, he resolved a significant controversy with regard to the age of hominin fossils and artefacts in the Turkana Basin, Kenya. Over the following four decades, he developed a comprehensive geochronological framework for volcanism across the Turkana Basin, providing the basis for current interpretations of early human evolution in the region. Other notable collaborative accomplishments include the application of the 40Ar/39Ar method to thermal history studies and noble gas geochemistry studies of volcanic glasses and mantle material, which provided insights into the evolution of Earth’s atmosphere and interior. Ian’s scientific contributions were recognised with numerous honours and awards, including being elected a Fellow of the American Geophysical Union, a Fellow of the Australian Academy of Science, and receiving the Jaeger Medal from the Australian Academy of Science and the Centenary Medal for ‘service to Australian society and science in geochronology’.

The diamond potential of kimberlites is difficult to assess due to several mantle and magmatic processes affecting diamond content. Traditionally, initial evaluations are based on the compositions of mantle-derived minerals (garnet, chromite, clinopyroxene), which allow an assessment of pressure-temperature conditions and lithologies suitable for diamond formation. Here we explore a complementary approach that considers the conditions of diamonds destruction by interaction with melts/fluids (metasomatism). We test the hypothesis that carbonate-rich metasomatism related to kimberlite melt infiltration into the deep lithosphere is detrimental to diamond preservation. Our results show that high diamond grades in kimberlites worldwide are exclusively associated with high-Mg/Fe olivine, which corresponds to mantle lithosphere minimally affected by kimberlite-related metasomatism. Diamond dissolution in strongly metasomatised lithosphere containing low-Mg/Fe olivine provides a causal link to the empirical associations between low diamond grades, abundant Ti-Zr-rich garnets and kimberlites with high Ti and low Mg contents. This finding show-cases olivine geochemistry as a viable tool in diamond exploration.

The Turkana Basin in NW Kenya and SW Ethiopia hosts remarkable fossil-rich sediments that are central to our understanding of early hominin evolution, with interbedded volcanic tuffs providing critical time markers. However, the resolution of existing Early Pleistocene–Pliocene ages is limited to c. 20–60 kyr, inhibiting the evaluation of climatic and environmental drivers of evolution. We present high-precision, single-feldspar 40 Ar/ 39 Ar age and elemental data for four stratigraphically significant tuffs. These samples exhibit variably dispersed age distributions correlated with feldspar compositional trends, interpreted to indicate the partial retention of inherited 40 Ar related to crustal ‘cold storage’ and rapid melt infiltration preceding eruption. We evaluated various statistical methods and calculated astronomically calibrated Bayesian age estimates of 1879.1 ± 0.6 ka (±2.4 ka including external errors) for the Kay Behrensmeyer Site (KBS)/H2 Tuff, 1837.4 ± 0.9 ka (±2.4 ka) for the Malbe/H4 Tuff, 1357.5 ± 1.8 ka (±2.5 ka) for the Chari/L Tuff and 1315.4 ± 1.9 ka (±2.5 ka) for the Gele Tuff. Our results permit refined age constraints for important early Homo fossils, including the cranium KNM-ER1813 ( Homo habilis ) and various Homo erectus fossils. The KBS Tuff age also provides an important calibration locus for orbital tuning of palaeoclimate proxy records, revealing the complex interplays between palaeoclimate and geological drivers of sedimentation. Supplementary material : The supplementary tables and figures include previously published ages (Table S1), sample and irradiation information (Table S2), electron probe microanalytical results for tuff glasses (Table S3, Figs S2 and S3) and feldspars (Table S4), 40 Ar/ 39 Ar analytical data (Tables S5 and S6, Fig. S4), orbital tuning model parameters (Table S7), a summary of previous and revised hominin ages (Table S8) and a worked Bayesian estimation example (Fig. S1) and are available at https://doi.org/10.6084/m9.figshare.c.6602994