David S. Thiede

Paraná flood basalts: Rapid extrusion hypothesis confirmed by new 40Ar/39Ar results

The duration of volcanism in the Parana continental flood basalt (CFB) province, essential information to assess models of melt generation, eruption rates, continental breakup, and volcanism-driven extinction events, remains unresolved due to conflicting sets of 40Ar/39Ar geochronology data. Some results suggest that the Parana CFB volcanism began at 134.7 ± 1 Ma and lasted <1 m.y. Another set of results reveals an extrusion interval of 11 m.y., from ca. 140 Ma to ca. 129 Ma. To resolve this controversy, we reanalyzed three of the exact hand specimens that previously yielded the oldest and youngest ages in the protracted 11 m.y. range. Our new ages are statistically indistinguishable from each other and the previous 134.7 ± 1 Ma result and indicate a short eruption event. A rapid extrusion for the Parana CFB and the lack of a major cotemporal extinction event challenge proposed direct links between CFB volcanism and mass extinctions and refute petrological models that rely on a protracted extrusion of the Parana CFB.

New single crystal 40Ar/39Ar ages improve time scale for deposition of the Omo Group, Omo-Turkana Basin, East Africa

Six tuffaceous beds within the Omo Group of the Omo–Turkana Basin have been dated using the 40Ar/39Ar single crystal total fusion method on anorthoclase, yielding eruption ages. The Omo Group constitutes up to 800 m of subaerially exposed sediments surrounding Lake Turkana within the East African Rift system in northern Kenya and southern Ethiopia. Rhyolitic explosive eruptions produced tuffs and pumice clasts that are considered to have been deposited shortly after eruption. The new age data on feldspars from the pumice clasts range from 4.02 ± 0.04 Ma for the Naibar Tuff of the Koobi Fora Formation to 1.53 ± 0.02 Ma for Tuff K of the Shungura Formation. The Orange Tuff in the KBS Member of the Koobi Fora Formation was dated at 1.76 ± 0.03 Ma, providing good control in this part of the sequence where formerly there was a >200 ka gap. Data are consistent with earlier measurements and significantly improve age resolution within the Omo Group, which has yielded many vertebrate fossils, including hominin fossils comprising a number of species. We suggest new age estimates for a limited number of hominin specimens. Supplementary material: Eleven tables and nine figures are available at www.geolsoc.org.uk/SUP18506.

Rapid change in drift of the Australian plate records collision with Ontong Java plateau

The subduction of oceanic plateaux, which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood-basalt provinces, is an important factor in many current models of plate motion and provides a potential mechanism for triggering plate reorganization. To evaluate such models, it is essential to decipher the history of the collision between the largest and thickest of the world’s oceanic plateaux, the Ontong Java plateau, and the Australian plate, but this has been hindered by poor constraints for the arrival of the plateau at the Melanesian trench. Here we present 40Ar–39Ar geochronological data on hotspot volcanoes in eastern Australian that reveal a strong link between collision of the Greenland-sized Ontong Java plateau with the Melanesian arc and motion of the Australian plate. The new ages define a short-lived period of reduced northward plate motion between 26 and 23 Myr ago, coincident with an eastward offset in the contemporaneous tracks of seamount chains in the Tasman Sea east of Australia. These features record a brief westward deflection of the Australian plate as the plateau entered and choked the Melanesian trench 26 Myr ago. From 23 Myr ago, Australia returned to a rapid northerly trajectory at roughly the same time that southwest-directed subduction began along the Trobriand trough. The timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the Pacific plate, including the archetypal Hawaiian chain, and thus provide strong evidence that immense oceanic plateaux, like the Ontong Java, can contribute to initiating rapid change in plate boundaries and motions on a global scale.

K-Ar and Rb-Sr dating of authigenic illite-smectite in Late Permian coal measures, Queensland, Australia: Implication for thermal history

Abstract K–Ar and Rb–Sr isotopic studies were carried out on authigenic illitic clay minerals in Late Permian coal measures from the Bowen Basin (Australia), in order to determine the timing of maximum paleotemperatures, which were responsible for coal maturation and coal seam gas generation. The results indicate two major thermal events affected the Bowen Basin region, at 205–215 and 140–155 Ma. The narrow range of age data from different size fractions, lithologies and stratigraphic depths indicates episodic, short-lived thermal events, rather than gradual temperature increase due to progressive burial as previously believed. The earlier, thermal event in the latest Triassic postdates maximum burial of the Bowen Basin strata, which occurred during the Middle to Late Triassic. The ages of 205–215 Ma correspond with the published evidence for regional Late Triassic extension in eastern Australia. The younger (140–155 Ma), thermal phase during the Late Jurassic–Early Cretaceous is related to the initial rifting and associated widespread igneous activities prior to break-up of Gondwana. The K–Ar dates in conjunction with vitrinite reflectance data as paleotemperature indicators indicate that the younger thermal event occurred at lower temperatures than the earlier one, except in the shallow part of the southern Bowen Basin. This event is recorded exclusively in less illitic, R=0 illite–smectite (I–S) mixed-layer samples mainly in the shallow part of the southern Bowen Basin (Baralaba Coal Measures).

40Ar/39Ar constraints on the timing and origin of Miocene leucitite volcanism in southeastern Australia

Laser incremental-heating 40Ar/39Ar geochronology of seven leucitites from southeastern Australia indicates that leucite-bearing lavas in individual geographic clusters were erupted in one million years or less. The eruption ages range from 17.9 ± 0.3 Ma (2σ) at El Capitan in northern-central New South Wales to 8.9 ± 0.2 Ma (2σ) at Cosgrove in northern Victoria. The 40Ar/39Ar results demonstrate that the southward migration of leucite-bearing lavas was near-contemporaneous with age-progressive central-volcano magmatism in southeastern Australia. As such, the 40Ar/39Ar results are consistent with a hotspot-related origin for the leucitites. However, the question of whether single or multiple hotspots are required to explain these volcanic chains, which are separated by a distance of about 300 km, awaits a more complete geochronological picture of the onset, duration and migration of leucitite and central-volcano magmatism in eastern Australia.

Coal composition and mode of maturation, a determining factor in quantifying hydrocarbon species generated

Products from various temperatures and heating rates of vitrinite and extractable bitumen from coals of different rank were studied by pyrolysis gas chromatography — mass spectrometry (py-gc-ms) and flash pyrolysis, and compared to ‘naturally’ matured Bowen basin coals. Generation temperatures and quantities of hydrocarbon species from vitrinite and bitumen were shown to be dependant on initial rank of the coal as well, as H/C of vitrinite. A significant amount of bitumen formation characterises rapid heating of vitrinite as established by py-gc-ms, as well as microscopy of residues. Similarly significant bitumen input is noted for Bowen Basin coals, supporting maturation by rapid heating. Using flash pyrolysis, bitumen has been confirmed to be a major source of methane. The light oil during pyrolysis is readily expelled from the coal leaving behind bitumen in vitrinite micro-cleats and char (inertinite) cavities. The ratio of solvent extractable to non-extractable bitumen may be used as indicator of methane generation in these coals.