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

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.

Evaluation of the 40Ar/39Ar technique for kimberlite geochronology: Three case studies from Finland

Dalton, Hayden1, Giuliani, Andrea 1,2, Hergt, Janet1, Maas, Roland1, Matchan, Erin1, O’Brien, Hugh3, Phillips, Professor David1, Woodhead, Jon1

1School of Earth Sciences, The University of Melbourne, Parkville, Australia, 2Institute of Geochemistry and Petrology, Department of Earth Sciences, Zurich, Switzerland, 3Geological Survey of Finland, Espoo, Finland

Kimberlites are enigmatic, volcanic rocks of both great economic and scientific importance due to acting as the primary host-rock to diamonds and being the deepest-derived continental magmas on Earth. Despite this significance, there remains debate concerning the sources of kimberlites and what triggers mantle melting to form these rocks. Robust determination of the timing of kimberlite eruption is a crucial prerequisite if we are to unravel the presence of any spatiotemporal relationships between kimberlite emplacement and large-scale tectonic processes, super-continental cycles or mantle plumes.

Despite the benefit of the remarkably high precision achieved with modern 40Ar/39Ar analytical techniques, and the presence of K-bearing groundmass phlogopite in many kimberlites, this technique has seldom been applied to kimberlites and related rocks. Early utilisation of this method revealed issues related to the presence of extraneous argon in mica macrocrysts and phenocrysts which yields anomalously old or maximum emplacement ages. Nonetheless, apparently reliable age results have been obtained on magmatic mica from kimberlites and related rocks. In this study we compare new, precise 40Ar/39Ar ages with other independent age constraints (e.g., Rb/Sr, U/Pb) on three clusters of kimberlites and related rocks from Finland to rigorously assess the instances where 40Ar/39Ar dating produces older apparent ages.

Our results indicate that sample selection and groundmass mica (phlogopite or kinoshitalite) separation needs to be extremely judicious prior to analysis. Where fresh mica phenocrysts are available for 40Ar/39Ar analyses we recommend that plateau results are interpreted with caution. Age spectra which are entirely flat, such that an age plateau includes 100% of the gas are likely the most accurate and precise reflection of the emplacement age of a kimberlite. In contrast, aliquots that yield younger apparent ages for heating steps preceding the plateau may reflect argon recoil redistribution resulting in anomalously older high-temperature/plateau ages when compared with independent age constraints. In cases where such discordance exists, we recommend that total-gas ages give a better approximation of the emplacement age and one which agrees more closely with ages from other geochronometers.


Biography

Hayden Dalton is presenting on behalf of the greater AuScope Geochemistry Laboratory Network. Hayden is a PhD researcher in the School of Earth Sciences at the University of Melbourne, his research focuses on the geochronology and geochemistry of kimberlites.

Hydrous polymetamorphic crustal rocks in an eclogite-bearing terrane record post-peak recrystallisation during arc-continent collision

Brown, Dillon1, Hand, Martin1, Morrissey, Laura2,1

1Department of Earth Sciences, University of Adelaide, Adelaide, SA, Australia. 2Future Industries Institute, University of South Australia, Mawson Lakes, SA, Australia

Ultrahigh- and high-pressure terranes are identified based on the occurrence of mafic eclogite-facies mineral assemblages, which effectively record burial and subduction metamorphic conditions. In such terranes however, mafic mineral assemblages are volumetrically minor compared to the continental rocks that host them, which typically preserve anhydrous quartzofeldspathic amphibolite-facies mineral assemblages. Continued debate centres around two hypotheses accounting for the petrology of the continental rocks: (1) the protoliths to the continental rocks did not respond to burial and subduction, and (2) the continental rocks developed high-pressure mineral assemblages which were subsequently overprinted during exhumation. However, less is known about continental rocks that preserve hydrous amphibolite-facies assemblages and schistose fabrics, which are also documented in high-pressure rock systems. In western Tasmania, south-east Australia, eclogite-facies mafic boudins with previously constrained subduction metamorphic conditions of 18–21.5 kbar and 650–700 °C are hosted by weakly foliated to mylonitic metapelitic continental rocks preserving hydrous amphibolite-facies assemblages dominated by siliceous muscovite, quartz, and garnet. Weakly foliated metapelites preserve relict kyanite and two possible textural generations of garnet, moderately foliated metapelites record evidence of partial melting, and mylonitic metapelites contain sillimanite within the rock fabric. Monazite LA–ICP–MS U–Pb geochronology documents two instances of monazite growth in the metapelites: a possible Mesoproterozoic growth event at c. 1385 Ma and a younger, yet poorly constrained growth event in the Cambrian. Rutile LA–ICP–MS U–Pb geochronology more precisely constrains Cambrian-aged metamorphism in the metapelites between 520–505 Ma. Monazite-garnet petrochronology reveals that Mesoproterozoic-aged monazite formed in a system with little or no influence of garnet whereas Cambrian-aged monazite formed in the presence of garnet during subduction. Metamorphic mineral equilibrium modelling of Cambrian subduction indicates that the weakly foliated metapelites best approximate peak metamorphism, recording metamorphic conditions of 13–17 kbar and 600–720 °C. Migmatitic metapelites record lower pressure conditions of 8–13 kbar and 660–740 °C and mylonitic metapelites equilibrated at 3.5–7 kbar and 590–680 °C. We infer that the amphibolite-facies metapelites record different stages of Cambrian-aged exhumation and, unlike their mafic counterparts, do not record burial or subduction. We attribute the inferred eradication of peak subduction mineral assemblages in the metapelites to the influence of fluid and localised deformation.


Biography

Dillon is a PhD student from the University of Adelaide working under the themes of metamorphic petrology, geochemistry, and petrochronology. His research focuses on understanding the geodynamic character and tectonism associated with the Cambro-Ordovician East Gondwana margin.

Sub-solidus replacement of rapakivi textures during high-temperature potassium metasomatism of the Mannum Granite

Forster, Marnie1,2, Goswami, Naina1,2, Lister, Gordon1, Reid, Anthony3

1Research School of Earth Sciences, Australian National University, Canberra, Australia, 2MinEx CRC, Canberra, Australia, 3Geological Survey of South Australia, Adelaide, Australia

The Mannum Granite is a high-temperature A-type granitoid emplaced during the late stages of the Delamerian Orogeny in South Australia. It has attracted considerable research interest over past decades because it is a porphyritic A-type granite that displays well-developed rapakivi structures and has been linked to the process of magma-mingling above deep-seated mafic igneous bodies. It contains many mafic enclaves of varied sizes and textures including potassium feldspar and quartz porphyroblasts with disseminated sulphides. The alkali feldspar cores of the rapakivi textures are lobate and overgrow the plagioclase rims, suggesting alkali feldspars are younger.

We conducted 40Ar/39Ar geochronology in conjunction with ultra-high-vacuum (UHV) step heating 39Ar diffusion experiments on K-feldspar and biotite from the Mannum granite. Conjoint inversion of data from the UHV 39Ar diffusion experiments using the Wunderkind program applied to data from the K-feldspar experiment demonstrates the presence of highly retentive core domains capable of retaining radiogenic argon even to temperatures in excess of 600°C. Inversion of the geochronology data using the MacArgon program produces a temperature-time history that suggests that the K-feldspar in the core of the rapakivi texture formed as the result of sub-solidus solid state replacement and/or metasomatism at ~443 Ma, and then cooled rapidly at ~440 Ma. Since biotite spectra which define a plateau age at ~472 Ma are relatively undisturbed, the temperature excursion associated with the metasomatic event would have to have been limited in its magnitude (<~500°C) and duration (<< 1 Ma). 

The biotite age (~473 Ma) is close to the previously determined early Ordovician crystallisation ages for the granites in this belt. In addition, the biotite appears to be highly retentive, with closure temperature for cooling at 20°C/Ma determined as the result of the UHV 39Ar diffusion experiment at ~485°C. These are thus exceptionally retentive biotites, which correlate with the fact that they are iron-rich and fluorine-rich, and fluorine-rich biotite typically exhibits higher argon retentivity. Overall this short/sharp thermal pulse may have occurred in consequence of fluid movement associated with the Old Teal Flat Shear Zone to the east, which contains fabrics dated in this study at ~445 Ma.

Microstructural analysis shows sericite and calcite precipitated along grain boundaries, fractures and exsolution lamellae. The presence of interpenetrating grain boundaries, sericite, calcite and secondary K-feldspar is attributed to the infiltration of an acidic hydrothermal agent hydrolysing primary K-feldspar to produce secondary sericite and quartz, further reacting with Ca-rich plagioclase and quartz to produce secondary K-feldspar phase(s) and calcite. Microstructures thus confirm the presence of a potassium-rich metasomatic event at Mannum leading to fluid-assisted in-situ replacement of K-feldspars with concomitant precipitation of secondary mineral phases.

Acknowledgements

The work has been supported by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Program. This is MinEx CRC presentation.


Biography

Naina Goswami is a second year PhD candidate at The Australian National University (ANU). She finished her Masters (advanced) from ANU in 2018 and her B.Sc (Hons) Chemistry from University of Delhi 2016. She specialises in 40Ar/39Ar geochronology and geochemistry. Currently she is undertaking her PhD at ANU working on a MinEx-CRC project based in South Australia.

Dating the timing of motion in major ductile shear zones

Forster, Marnie1, Lister, Gordon2

1Research School of Earth Sciences, Australian National University, Canberra, Australia; 2The Virtual Explorer

There is confusion in the argon geochronology world as to what allows movement in a ductile shear zone to be dated. Some assert that all that is necessary is to data mica and to obtain a ‘plateau‘. But this is not at all sufficient to make the argument. 40Ar/39Ar geochronology (like all radiometric dating techniques) does not have the ability to date movement. It is the microstructural modification of existing grains that must be dated, e.g., growth or regrowth during movement. Otherwise it may be that the fabric forming minerals are remnant: namely, relicts of an earlier formed mineralogy, and the ages obtained not at all relevant to the timing of movement in a later shear zone.

An ideal circumstance would be clear and unambiguous demonstration that growth of a particular mica had taken place immediately prior to or during the operation of a ductile shear zone, and that mineral separation (or laser spots) had focussed on those volumes during measurement. An example would be sudden growth of mica porphyroblasts that were then rotated and aligned in a developing shear zone fabric, as occurs in retrograde shear zones in the Cycladic Eclogite-Blueschist Belt. Movement has not been dated, but an estimate has been obtained as to the timing of growth during or immediately preceding movement. Another optimal circumstance would be if the operation of the ductile shear zone had shredded mica, progressively reducing its diffusion dimension. This behaviour leads to staircase spectra characteristic of fractal diffusion. Such age spectra appear to be able to allow the distinction of the start and the end of shear zone operation. In some cases, the age of relict mica microstructures is also evident, e.g., in mica from the Main Central Thrust of the Himalaya.

A more difficult circumstance occurs when age spectra from fabric forming minerals appear to be unrelated to the timing of movement, e.g., for mica from greenschist facies ductile shear zones in the Cap de Creus, Spain. We inverted data from 40Ar/39Ar geochronology step-heating experiments, using potassium feldspar, after conjoint inversion of data from simultaneous ultra-high-vacuum (UHV) 39Ar diffusion experiments. The resultant temperature-time curves imply that these mylonites formed in Eocene to Oligocene time, and therefore that they are not Variscan or Jurassic, as previously argued. Their tectonic significance is likely to be as right-lateral strike-slip shear zones formed in transfer faults accommodating roll-back of the Tethyan subduction zone as it dragged Sardinia and Corsica away from the Palaeo-European margin during opening of the Gulf of Lyon.

These examples suggest caution needs to be exerted in the dating of movement in ductile shear zones. Laser-step heating (or laser spot analysis) is not suited for this purpose, since this method provides no information in respect to Arrhenius data. In consequence the retentivity of relevant minerals in respect to argon diffusion cannot be assessed. In addition, laser methods do not produce consistent detail in age spectra. In contrast, robotic methods applied to resistance-furnace step-heating experiments offer a cheap, efficient, and reliable way to obtain the detailed age spectra that are necessary (in conjunction with Arrhenius data) to characterise the pattern of argon release.


Biography

Gordon Lister and Marnie Forster are structural geologists with a particular interest in the theory and practice of argon geochronology, in particular in the study of the dynamics of the evolution of orogenic architecture and its impact on metallogenesis.

About the GSA

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