anna@conferencedesign.com.au | 2021 AESC

Equilibrium Shapes and Geology of Transneptunian Objects and Centaurs

Rapid Fire Presentation, SESSION 7.4 | RAPID FIRE

Branco, Hely¹

¹former MSc. Student, Utfpr

The equilibrium shapes of transneptunian objects, centaurs and other icy small bodies of the Solar System are directly related to their physical characteristics, such as spin velocity and global composition. Through the correlation of the aforementioned properties, it is possible to make inferences regarding their internal structures and geology through the use of analogs such as Pluto and Charon, deepening our understanding of these objects. Some of these inferences are presented in this work.

Biography to come

When the dust settles

Rapid Fire Presentation, SESSION 7.4 | RAPID FIRE

Isley, Dr Cynthia Faye¹

¹Postdoctoral Research Fellow, Macquarie University

Driving, mining, bushfires – they all make the dust fly. We can’t see the tiny particles, but given enough of them, dust turns the sky brown and makes surfaces in our homes look dirty. It permeates our existence: we breathe it, eat it, touch it. It can affect our health.
Dust has a story to tell. Outdoors, it reflects city life, local industry and transport. Indoors, it can tell me how old your house is, where and with whom you live. Dust from bushfires unlocks pollution secrets from decades ago.
Come and hear the hidden secrets of dust.

Biography to come

Petrographically constrained in situ sulfur isotopes: why the “SEDEX” can’t be used as model for sediment-hosted sulfide deposits in the 1.6 Ga Edmund Basin, Australia

Hydrothermal-magmatic ore-forming processes, ORAL, SESSION 9.1 | ENERGY & RESOURCES

Lampinen, Dr Heta¹, LaFlamme, Dr Crystal², Occhipinti, Dr Sandra¹, Fiorentini, Dr Marco³, Spinks, Dr Sam¹

¹CSIRO, Kensigton, Australia, ²Université Laval, , Canada, ³Centre for Exploration Targeting, School of Earth Sciences, University of Western Australia, Crawley, Australia

A common foundation for sediment-hosted massive sulfide (SHMS) deposit sulfur isotope data interpretation is the assumption of sedimentary exhalative “SEDEX” model. The model presumes synsedimentary sulfide precipitation and the sulfur mainly sourced from the contemporaneous ocean via bacterial sulfate reduction, which can be further interpreted to reflect the evolution of ancient hydrosphere. However, synsedimentary SEDEX model has been challenged or disproven for many SHMS deposits, including ones in the McArthur Basin, Australia. Many SHMS deposits also contain multiple coexisting sulfide generations and/or express geospatial associations between the isotope signature and distance from the hydrothermal vent. Due to the internal complexity of SHMS systems, unravelling their sulfur isotope architecture requires both a robust paragenetic framework and a well-known geological context for the data. In situ secondary ion mass spectrometry (SIMS) sulfur isotope analysis has this capability.

Petrographically constrained in situ sulfur isotope SIMS analysis was applied to pyrite and chalcopyrite (n=135) to investigate the spatial and temporal sulfur isotope architecture of replacement and synsedimentary-style SHMS deposits at four sites (including the Abra deposit) in the ca. 1680-1455 Ma Edmund Basin, Western Australia. From this data, the sulfur isotope fractionation associated with the hydrothermal mineral systems, and representativeness for the secular evolution interpretations of the seawater sulfate through the Proterozoic Eon was evaluated.

The epigenetic replacement-style SHMS systems in the Edmund Basin yield δ³⁴S from +24 to +54‰ from pyrite and chalcopyrite. The relatively ³⁴S depleted pyrite were associated with ore fluid composition in main hydrothermal channels. The bulk isotopic composition of the ore fluid can be used as proxy for sulfate in the underlying sediments. The extremely ³⁴S enriched were found in pyrite in distal parts of the deposit hydrothermal footprint. This ³⁴S enrichment was possibly caused by deficiency of iron relative to sulfur in low permeability rocks, which decelerates the formation of pyrite allows the mass-dependent Rayleigh distillation of sulfur isotopes to reach extreme residual fraction. The systems with syn-sedimentary sulfide precipitation yield δ³⁴S from +1 to +22‰, which can be associated with seawater sulfate and bacterial activity in the basin.

In situ sulfur isotope analysis offered the capacity to link isotopic data to a comprehensive spatially and temporally constrained framework representative of the stratigraphic and geodynamic context. The results of this study also highlight the importance of using tailored geological constraints and a mineral system model as a framework for isotope chemistry – not a generic SEDEX. Tailored geological constraints and deposit model are particularly important for the data are intended for evaluation of hydrosphere over time.

¹ CSIRO Mineral Resources, 26 Dick Perry Avenue, Kensington, WA 6151, Australia

² Département de géologie et de génie géologique, Université Laval, Pavillon Adrien-Pouliot 1065, av. de la Médecine, Québec, QC G1V 0A6, Canada.

³ Centre for Exploration Targeting, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.

Corresponding author: heta.lampinen@csiro.au

Co-authors: crystal.laflamme@ggl.ulaval.ca; sandra.occhipinti@csiro.au; marco.fiorentini@uwa.edu.au; sam.spinks@csiro.au

Biography

Heta hails from Kuortane, Finland and has a MSc from University of Turku and a PhD from University of Western Australia. Her research focuses on delineation of multi-scale hydrothermal mineral footprints of undercover ore deposits using integrated geological, hyperspectral, geochemical and geophysical data.

Regional scale folding in the Arthur Metamorphic Complex: structural constraints for the Keith River – Lyons River area, NW Tasmania

ORAL, Recent advances in the geology and mineral potential of the eastern Australian Tasmanides, SESSION 14.2 | EARTH STRUCTURE

Cumming, G¹, Jackman C.¹, Everard, J. L.¹ and Gray, D.²

¹ Geological Survey Branch – Mineral Resources Tasmania – Geological Survey Branch, Rosny Park, Australia, ² Consultant Structural Geologist for Mineral Resources Tasmania – Geological Survey Branch, Rosny Park, Australia

New geological mapping in the Keith River-Lyons River area in NW Tasmania has provided insight into the structural framework of the northern Arthur Metamorphic Complex (AMC) in NW Tasmania. The AMC is flanked to the east by the Oonah Formation and to the west by the Rocky Cape Group. The main lithological units of the AMC share transitional metamorphic, interpreted low angle fault, and both conformable and unconformable contacts. At a regional scale a significant north-plunging synform, or a large north-tilted block, is contained within the high-strain core of the AMC. Five deformation episodes can be observed throughout the area at outcrop-scale. Three early deformational episodes are likely related to the Middle Cambrian Tyennan orogeny, manifested as early, high strain events which caused isoclinal folding and development of schistose axial planar fabric. A rotational shear component, apparent as shear bands, suggests north over south or sinistral transport. Subsequent D3 deformation within the AMC occurred during the later stage of the Tyennan Orogeny. This event folded and tightened the various stacked lithostratigraphic units to form non-cylindrical asymmetric folds. These were later subject to generally northwest-directed, potentially Devonian compression (D4) and tilting. At a regional scale, late-stage north-plunging folds are inferred along the highest strain zone of the northern AMC. This area is a locus for Mesozoic or early Cainozoic faults, and a half-graben also extends along this zone, which is also in the core of the AMC. A late stage (D5) folding event may be partly related to Devonian compression, although the timing and nature of this folding event is largely unclear.

Biography

Grace currently works as a geologist for the Geological Survey Branch at Mineral Resources Tasmania and has spent the last 8 years undertaking mapping work to compile numerous 1:25,000 geological maps of North West Tasmania.

Late Cambrian – Middle Ordovician extension of the northern Tasmanides: thinned crust that facilitated intense Silurian (Benambran) shortening deformation

ORAL, Recent advances in the geology and mineral potential of the eastern Australian Tasmanides, SESSION 14.2 | EARTH STRUCTURE

Henderson, Robert¹, Fergusson, Chris² and Withnall, Ian³

¹Department of Earth and Environmental Sciences, James Cook University, Townsville, Australia, ²School of Earth, Atmospheric and Life Sciences, University of Wollongong, Australia, ³Geological Survey of Queensland, Brisbane, Australia

The Charters Towers and Greenvale Provinces of the Thomson Orogen provide the most extensive exposure of early Paleozoic rocks in the northern Tasmanides. Upper crust represented by the Charters Towers Province developed a thick, Proterozoic passive margin, sedimentary assemblage with a minor contribution from mafic igneous rocks. This assemblage was subsequently overprinted by active margin tectonism commencing in the mid Cambrian (~510 Ma), reflecting a broad scale Tasmanide regime change of that age. The overprinting regime generated thick (>15 km) basinal infill, inclusive of a substantial intermediate to silicic volcanic complement, of the upper Cambrian – Middle Ordovician Seventy Mile Range Group and other upper Cambrian basinal relicts identified for the Charters Towers and Argentine Metamorphics. Magmatic arc plutonism is widely developed as the Ravenswood Batholith and Fat Hen Creek Complex. Regional metamorphism and penetrative fabric development of Early Ordovician age overprinted all of the Proterozoic passive margin assemblage and part of the Cambrian basin fill, with structural analysis favouring its association with extensional strain. Coeval basin formation, plutonism and metamorphism across the province reflects an enduring episode of crustal extension.

The Greenvale Province consists largely of metavolcanic and metasedimentary tracts with protoliths of Early Ordovician age. Lower Ordovician granites are also represented and in broad aspect, rock units of the province both resemble, and are correlative with, the active margin assemblages of the Charters Towers Province. Structural analysis shows a deformation history matching that of the Charters Towers Province with the dominant foliation similarly attributed to extensional strain. Development of the province represents thick basinal infill developed on thinned crust of a continental margin. Contrary to earlier publications, no rocks older than Ordovician are known for the province. Amphibolite and ultramafic units interleaved with metasediments on its eastern (outboard) side suggest it may have developed on oceanic crust.

For both provinces thin crust had developed by the Middle Ordovician, weakening its resistance to subsequent Benambran contraction. Cambro-Ordovician rock assemblages of both provinces were affected by structural/metamorphic overprint with local generation of mylonite in wide shear zones. Contraction is dated as Silurian by Ar–Ar mineral ages, by fabric development in deformed granites of known age and by the age of overlap strata. Tight upright folds and accompanying foliation, generated by shortening, deformed tracts affected by Ordovician extensional strain, steepening previously formed fabrics. Basin fill represented by the Seventy Mile Range Group was inverted. For the Greenvale Province, widespread amphibolite grade metamorphism indicates that much of it has been exhumed from considerable crustal depth (>15km). In contrast exhumation of the Charters Towers Province was heterogeneous, ranging from <5 km (unmetamorphosed Seventy Mile Range Group) to >15 km (amphibolite facies metamorphism and local migmatite). Silurian structural trends, considered by some authors as registering an orocline, are attributed to strain partitioning consequential on oblique convergence.

Biography

Based in Townsville, the presenter has actively pursued research interests in the Tasmanides for over 40 years, with a focus in particular on the Mossman, Thomson and New England Orogens which are extensively developed in Queensland. Mush of this activity has involved collaboration with Chris Fergusson and Ian Withnall.

Early Tasmanides evolution: Passive to convergent margin history in New South Wales, Australia

ORAL, Recent advances in the geology and mineral potential of the eastern Australian Tasmanides, SESSION 13.2 | EARTH STRUCTURE

Greenfield, John¹; Gilmore, Phil¹; Musgrave, Robert¹

¹Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia.

Initial development of the Tasmanides in southeastern Australia involved Early Neoproterozoic rifting/break-up of the Rodinian supercontinent, expressed as tholeiitic dyke swarms and continental rifting in the Adelaide Rift Complex of eastern South Australia. This left highly-extended, transitional crust between the Gawler Craton and Curnamona Province, which became the depocentre for extensive platform carbonate and shallow marine clastic sedimentation. By ~700 Ma, a passive margin developed east of the Curnamona Province which saw the initiation of the palaeo-Pacific Ocean. A final NE–SW phase of rifting in the Late Neoproterozoic was associated with shallow marine platform sedimentation and alkaline magmatism (Mount Arrowsmith Volcanics), further attenuating the eastern Curnamona Province crust and presenting an angular continental salient towards the nascent ocean to the east.

This crustal configuration profoundly influenced the palaeogeography and tectonism that followed during the Delamerian Cycle, as passive margin clastic deposition in the early Cambrian gave way to west-facing subduction in the mid Cambrian. Elements of the resulting Cambrian volcanic arcs (Mount Wright and Loch Lily–Kars) are now immediately adjacent to the Curnamona Province margin. However, regional geological mapping in the Koonenberry Belt has shown that the Mount Wright Arc developed in a rift zone within the Curnamona Province that probably initiated during the last phase of Rodinia break-up in the Late Neoproterozoic. In contrast, the strike-equivalent Loch Lily–Kars Arc was developed in an intra-oceanic setting. Mapping, drillhole and geophysical data shows that this arc segment, along with forearc volcanic rocks of the Ponto Group, were oroclinally folded clockwise almost 90°, and thrust against the southeastern Curnamona Province margin during the Late Cambrian Delamerian Orogeny. If the original arc segments were part of a linear belt, it would have extended southeast from the oroclinal hinge at the Grasmere Knee Zone. Recently acquired and modelled AusLAMP magnetotelluric data show a strong lower crustal conductive anomaly aligned along this trend.

The Delamerian Orogeny caused strong ductile deformation of rocks deposited in the Delamerian Cycle. Areas that were highly attenuated during break-up suffered tight upright folding and oroclinal bending, which may also have been affected by clockwise rotation of the Curnamona Province. Proterozoic Olarian Cycle metamorphic rocks along the margins of the Curnamona Province and the eastern edge of the Gawler Craton also suffered upright open to tight folding during the Delamerian, with σ₁ perpendicular to the outboard margin. In the Broken Hill Domain, early southwest-directed fold-thrusting switched to northwest-directed fold-thrust and strike-slip deformation at the end of the Delamerian Orogeny in the Early Ordovician.

Clearly the switch to a convergent setting, with arc accretion and terminal deformation in the Delamerian Orogen, caused extensive shortening of a highly attenuated margin that had been in extension for c. 300 Ma. The Curnamona Province, although acting as a salient in the early Delamerian Orogeny, was itself deformed and could not insulate the distal-foreland Flinders Ranges from deformation late in this event. This had cumulative effects on the ensuing cycles of subduction roll-back, extension and contraction that defined the Tasmanides throughout the Palaeozoic.

Biography

John leads the Geoscience Acquisition & Synthesis Unit in the Geological Survey of NSW, which collects and interprets geoscientific data from geological mapping, geophysical surveys, and specialist studies in mineral deposits, palaeontology, and petrography.

Disorientation control on trace element segregation in fluid-affected low-angle boundaries in olivine

ORAL, SESSION 7.2 | CORE TO CRUST, The Australian lithosphere in 2021: What do we know and future challenges

Tacchetto Tommaso^1,2, Reddy Steven^1,2, Saxey David², Fougerouse Denis ^1,2, Rickard William², Clark Chris¹

¹School of Earth and Planetary Sciences, Curtin University, Perth, Australia, ²Geoscience Atom Probe, Curtin University, Perth, Australia

The interfaces between minerals (grain boundaries s.l.) play a critical role in controlling the rheological behaviour of rocks and the ability of fluids to penetrate them in the deep crust. However, the role of chemical segregation in controlling the behaviour of mineral interfaces remains largely unexplored. In this work, we combined electron backscattered diffraction (EBSD) and atom probe tomography (APT) to assess the relationship between deformation-related low-angle boundaries in naturally-deformed olivine and the degree of trace element segregation to those boundaries. The sample studied comes from the Bergen Arcs (Norway), where high-grade, dry, metamorphic rocks of the lower crust have been overprinted by fluid-present high-pressure metamorphism during the Caledonian tectonic subduction between 430 and 410 Ma. EBSD orientation mapping of deformed olivine is used to characterise the slip systems associated with deformation and the misorientation relationships within different parts of the microstructure. APT has then been used to systematically target grain boundaries of different misorientation angle (up to 8°).

The analysed boundaries formed by sub-grain rotation recrystallisation associated with {100}<001> slip system developed during the fluid-catalysed metamorphism. APT data show that olivine trace elements segregated to the low-angle boundaries during this process. Boundaries with < 2° degrees show marked enrichment associated with the presence of multiple, non-parallel dislocation types. However, at increased misorientation (> 2°), the interface becomes more ordered with dislocation geometries defined by linear concentrations of trace elements, and which are consistent with the EBSD data. These boundaries show a systematic correlation of increasing trace element segregation with misorientation angle. In particular, elements are generally more enriched at higher degrees of distortion, where variations are mostly significant for Ca (from 0.07 up to 0.6 at%) and Cl (up to 0.3 at%). Elements that are segregated to the low-angle boundaries (Ca, Al, Ti, P, Mn, Fe, Na, Mg and Co) are here interpreted to be captured and accumulated by dislocations as they migrate to the sub-grain boundary interfaces. However, the exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect a small but significant contribution of an external fluid source during the fluid-related deformation. In particular, since the occurrence of H in olivine is strongly attributed to Ti defects, the segregation of Ti to grain boundaries is consistent with the detected enrichment of hydrogen in the low-angle interfaces.

The observed compositional segregation of trace elements to low-angle boundaries have significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. Furthermore, the nanoscale correlation between heterogeneous distribution of elements like Ti and the diffusion of H along boundary interfaces within olivine might have the potential to yield important implication for the understanding of the hydrogen distribution in the upper mantle and its consequences for the rheological properties of mantle-rocks during deformation.

Biography

Tommaso completed his MSc in Geology at the Univesity of Padova (Italy) in 2017 and awarded in 2017 of a Temporary Research Fellowship. He is now completing his 3rd year of PhD at Curtin University focused on the investigation of metamorphic processes in the precence of fluids at the nanoscale.

Lapstone Structural Complex and uplift of the Blue Mountains, tectonic backdrop to western Sydney, Australia

ORAL, SESSION 7.2 | CORE TO CRUST, The Australian lithosphere in 2021: What do we know and future challenges

Fergusson, Chris¹ and Hatherly, Peter²

¹School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, ²Lavender Bay, Sydney, Australia

The eastern margin of the Blue Mountains uplift is well defined by the Lapstone Structural Complex (LSC) with elevations typically of 150 to 200 m but locally up to 600 m at Kurrajong Heights. While it has been suggested that the LSC is related to normal faulting associated with passive margin development, this suggestion is inconsistent with a west-dipping reverse fault with minimal fault damage development (the Bargo Fault) we have found in the southern part of the LSC. We prefer the alternative view that the LSC consists of east-facing monoclines and reverse faults that dip both east and west. The underlying controlling structure could be a west-dipping thrust and the surface expression is due to the formation of a triangle zone. Further support for our interpretation is found in the northern part of the LSC where there are gravels (Rickabys Creek Gravel) and clay (Londonderry Clay) which are draped along the front of the monocline. Their position indicates that they were folded during development of the LSC. Thus the gravels do not occur in a series of terraces as was previously considered. Unfortunately no direct age of the Rickabys Creek Gravel and the Londonderry Clay has been determined but their unconsolidated state is consistent with a Neogene age thereby constraining formation of the LSC to the Neogene. Ongoing seismic activity associated with the LSC indicates neotectonic activity along the structure. The higher parts of the Blue Mountains are at elevations of over 1000 m and overall rock units of the Sydney Basin slope to the east at an angle of ca 1.5°. We consider that the latest phase of uplift in the Blue Mountains was associated with formation of the LSC and other structures including the Mt Tomah Monocline. Given earlier phases of uplift associated with formation of the eastern highlands it is problematic to identify the limits of this younger phase of Blue Mountains uplift particularly in the west where it apparently merges with the Central Tablelands. The enduring debate about the cause of the uplift of the Great Dividing Range of eastern Australia has become increasingly centred on the importance of mantle upwelling. We consider that the younger phase of uplift of the Blue Mountains associated with formation of the LSC was related to Australia’s setting west of the Southwest Pacific convergent margin rather than mantle upwelling.

Biography

Peter Hatherly is a retired geophysicist who has taken an interest in the structural and geomorphological evolution of the Blue Mountains. He has published relevant papers on the results of high resolution seismic reflection surveys, analysis of longitudinal stream profiles and mapping of semi-consolidated gravels above river systems.

Provenance of Late Cambrian-Ordovician sedimentary rocks in western, north-eastern Tasmania and southern Victoria: Constraints from U/Pb dating, zircon geochemistry and εHf isotope

ORAL, SESSION 7.2 | CORE TO CRUST, The Australian lithosphere in 2021: What do we know and future challenges

Habib, Umer¹, Meffre, Sebastien¹, Kultaksayos, Sitthinon¹, Berry, Ron¹

¹Centre of Ore-Deposit Geology and Earth Sciences, University of Tasmania, Hobart, Australia

Email address. umer.habib@utas.edu.au

The sedimentary sequences in western, north-eastern Tasmania and southern Victoria preserves an excellent record of distinctive Cambrian to Ordovician deposition environments and sediment provenance. Here we integrate new and already published U/Pb detrital age data, field observations, zircon geochemistry, and Hf isotope data to establish the sediment source and tectonostratigraphic history of these rocks. Overall, the detrital age spans from 2.6 Ga to 0.476 Ga, which includes some major Precambrian and Cambrian age peaks. The 1.8-1.2 Ga peaks for Western Tasmania are consistent with derivation from the Tyenna region which incorporate sediments derived from granitoids in Laurentia (North America) and Baltica. A small population of detrital ages from 1.5-1.1 Ga from north-eastern Tasmania and southern Victoria is suggested to have Grenville provenance, possibly from central Australia or other parts of the Grenvillian Orogen. The 850-545 Ma detrital ages occur in all sedimentary rocks and are from the widely represented Pacific-Gondwana Phanerozoic sediments of south eastern Australia. These 850-545 Ma detrital ages are rare in the Owen Group, relative to the overlying Middle-Late Ordovician Pioneer Sandstone, implying a sharp shift in provenance in western Tasmania in the Early Ordovician. The Pioneer Sandstone is very similar to Early Ordovician sandstones from Waratah Bay in southern Victoria and Ordovician sandstone from eastern Tasmania. Also present in the samples are 480-520 Ma zircons. However, the proportions of these in the sedimentary rocks are variable. The Th/U ratios from the Cambrian zircons in western Tasmania support a proximal source from the Mt Read Volcanics. The Cambrian zircons in eastern Tasmania and southern Victoria have much lower Th/U ratios, implying a different provenance. The εHf isotope signatures and statistical analysis, along with sedimentological and paleocurrent data from previous studies, support a local derivation from Precambrian and Cambrian detrital sources for the western Tasmanian rock units, indicating deposition in a segmented basin-margin fault system. The eastern Tasmanian and southern Victorian sandstone were deposited in a basin offshore from the Tyennan-Delamerian Orogen during the Ordovician.

Keywords. Western Tasmania, U-Pb dating, Tectonic configuration, Palaeozoic

Biography

Umer has done his Honors and masters degree in geology from Pakistan and worked for MOL from 2013-2015. He joined Codes in 2018 to persue his PhD.

The KBS Tuff Controversy fifty years on: New ultra-precise ages for the KBS tuff and correlates, Omo-Turkana Basin, Kenya

ORAL, SESSION 6.4 | Ian McDougall Symposium

Phillips, Professor David¹, Matchan, Dr Erin¹

¹School of Earth Sciences, The University of Melbourne, Parkville, d9cec488-596f-4927-97dc-c72b3e7f8ea7

Archaeological expeditions by the National Museum of Kenya to east Lake Turkana (formerly Lake Rudolf) in 1968 and 1969 led to the discovery of remarkable stone artefacts and hominin fossils, associated with volcanic tuffs, including the famous KBS (Kay Behrensmeyer Site) tuff.

Initial attempts to date the KBS tuff proved unsuccessful due to fluvial deposition of most tuffs and contamination by older material. Early ⁴⁰Ar/³⁹Ar dating of pumice feldspar grains from the KBS tuff yielded a reported age of 2.61 ± 0.26 Ma, which was soon disputed on the basis of faunal correlations, thereby precipitating a controversy that played out in Nature publications for the next decade. In 1975, the Berkeley geochronology laboratory published K-Ar ages of 1.60 and 1.8 Ma for two KBS localities, with the former age later attributed to laboratory error. The Cambridge geochronology laboratory then reported a range of ⁴⁰Ar/³⁹Ar ages (0.52 – 2.61 Ma) and revised the age of the KBS tuff to 2.42 ± 0.02 Ma, in accord with preliminary zircon fission track ages. Subsequent geochemical correlations with the H2 (=KBS) tuff in Ethiopia, dated at ca. 1.8 Ma by the K-Ar method, heightened the controversy. Later K-Ar and ⁴⁰Ar/³⁹Ar dating analyses by Ian McDougall at the Australian National University largely resolved the debate, with reported ages of 1.89 ± 0.01 Ma and 1.88 ± 0.02 Ma, coincident with a revised fission track age of 1.87 ± 0.04 Ma. More recent analyses of single feldspar crystals from KBS pumice clasts produced a weighted mean age of 1868 ± 14 ka – but with significant scatter, suggesting the presence of inherited grains or extraneous argon.

New ultra-precise ⁴⁰Ar/³⁹Ar data obtained for single feldspar pumice crystals from several tuff localities across the Omo-Turkana Basin, including the KBS tuff, show variably complex age distribution patterns even within single pumice clasts. Based on co-irradiated A1T and FCT sanidine aliquots and a Bayesian statistical analysis approach, we calculate astronomically calibrated ages for several tuff horizons at precision levels approaching <<0.1%. The KBS and correlated H2 tuff give an astronomically calibrated, weighted mean age of 1879.2 ± 1.3 ka (0.069% 95%CI). The stratigraphically younger Malbe (=H4) tuff, which was originally misidentified as the KBS tuff, gives a Bayesian eruption age of 1837.75 ± 0.86 (0.047%; 95%) ka. The These results enable effective stratigraphic correlations across the Basin, and reveal paleoclimate and paleo-environmental variability at millennial timescale resolution.

This study is dedicated to the memory of Ian McDougall and Frank Brown, who worked tirelessly to unravel the magmatic and geological history of the Omo-Turkana Basin.

Biography

Professor David Phillips is Head of the School of Earth Sciences and Director of the 40Ar/39Ar Laboratory at the University of Melbourne. He is internationally recognised for his research on ultra-precise 40Ar/39Ar dating methods and their application volcanic rocks, including tuffs related to hominin localities in the Turkana Basin, Kenya.