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The development and application of geochronological tools suitable for dating Cenozoic rocks and processes have been instrumental to our understanding of the modern history of Australia. Geochronology reveals a dynamic continent that traced a long and rapid trajectory from a position adjacent to Antarctica in the early Cenozoic to its present position near the tropics. The average travel velocity along this path is revealed by the age of hotspot volcanoes, derived by the K-Ar method, and is complemented by measured geomagnetic pole positions on dated igneous rocks and sedimentary deposits. K-Ar dating of volcanic rocks also provided constraints on rates of landscape evolution before and after volcanism and the timing and pattern of dispersion of life—including human inhabitation. K-Ar geochronological results reveal a history of faunal and floral evolution suggestive of a continent undergoing progressive cooling and dehydration with a few brief warm and humid excursions. In contrast, 40Ar/39Ar, SHRIMP U-Pb, fission-track thermochronology, luminescence techniques, and cosmogenic-isotope methods have played relatively minor roles in reconstructing the chronology of Cenozoic volcanism in Australia. Integrated application of these techniques will be critical to providing more precise constraints on the volcanic history of the continent and its climatic and biological evolution. While Cenozoic volcanism, uplift, and denudation were active along the eastern and southeastern margins, a significant part of Australia west of the Tasman Line remained relatively quiescent. The history of this part of the continent is marked by slow and subdued uplift and subsidence, with subtle displacements along major continental structures, and occasional invasion by shallow seas. Despite the general absence of Cenozoic igneous rocks west of the Tasman Line, Australia (east and west) is blanketed by Cenozoic sedimentary covers and weathering profiles. If we consider weathering as a fourth rock-forming process (in addition to igneous, metamorphic and sedimentary), Australia has one of the most complete Cenozoic rock covers of any continent. Retrieving information recorded in these weathering profiles is essential for unravelling its Cenozoic history. Paleomagnetic studies, calibrated δ18O curves, and weathering geochronology by K-Ar, 40Ar/39Ar, and (U-Th)/He provide insights into the imprint of climatic events and tectonic processes and illustrate the importance of erosion and weathering to the formation or enrichment of ore and mineral deposits. Except for diamondiferous lamproites of Western Australia and sapphire-rich volcanic rocks in eastern Australia, all other Cenozoic ore and mineral deposits in Australia are related to weathering and erosion. The widespread weathered blanket in Australia suggests low Cenozoic erosion rates. Numerical constraints on chronology and erosion rates are derived from the cooling and denudation histories retrieved from apatite and zircon fission-track and, more recently, (U-Th-Sm)/He thermochronology and cosmogenic isotope studies. Geochronological studies of veneers of sediments, lake and cave deposits, marine carbonates, organic matter and groundwaters provide information on sediment provenance, subtle tectonic movements, and the Australian Cenozoic climatic history. These studies reveal a continent sensitive to global climatic cycles and subject to active, but subtle, tectonism and erosion. This record shows that Australia suffered periods of extreme aridity during cyclical glaciation at high latitudes and precise dating of carbonate sediments and speleothems reveals the exact timing and duration of these glacial and interglacial periods. Cosmogenic isotopes also provide constraints on the age and migration paths of Australia's limited and finite groundwater resources. Lastly, age information extracted from surficial deposits reveals a protracted history of human occupation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Laser 40Ar/39Ar analyses were undertaken on Oligocene bimodal intraplate volcanic rocks from five volcanic areas spanning 320 km along a north-northeast-trending belt in southeast Queensland. Sixty-one mineral grains and groundmass fragments from 21 samples were analysed by incremental heating, while 99 mineral grains from 10 samples were analysed by total fusion. Fourteen samples are from localities dated previously by the K – Ar method. The 40Ar/39Ar ages are reproducible among different aliquots from the same sample and different samples from the same location, and correlate with latitude, ranging from 30.6 ± 0.3 Ma on Fraser Island in the north (25°S) to 25.8 ± 0.2 Ma at Flinders Peak in the south (27.7°S). This age progression is not evident from K – Ar ages compiled for the same region. The decrease of 40Ar/39Ar ages with latitude is consistent with previous suggestions for a hotspot origin for east Australian central volcanoes and yields an Australian Plate velocity, relative to the postulated fixed hotspot reference frame, of 71 (+7, -4) mm/y at an azimuth of N10°E for the period of ca 31 – 26 Ma. This velocity agrees within error with the values of about 65 ± 3 mm/y obtained from previously reported K – Ar analyses of east Australian central volcano provinces active in the last 35 million years. These results demonstrate that the improved accuracy and precision of the 40Ar/39Ar method permit resolution of age versus latitude relationships for narrower time windows, which may potentially provide constraints on changing plate velocities with time. This improved temporal resolution may also contribute to resolving current debate over the existence and the stationary versus mutable position of postulated mantle plumes and hotspots. [ABSTRACT FROM AUTHOR]

Discusses the idea that manganese oxides in association with paleoweathering can offer insight into the factors affecting the formation and evolution of weathering profiles. Finding from laser probe step-heating analysis of supergene hollandite and cryptomelane samples collected from central Queensland, Australia; Indication the plateaus and consistent isochron ages confirm the feasibility dating of supergene manganese oxides by the 40Ar/39Ar technique; Identification of two distinct structural sites hosting Ar isotopes; Mention of degassing behaviors discovered by incremental heating analyses; Assertion there is no significant Ar gas released at laser power higher than 1.0 W, which suggests the breakdown of tunnel sites hosting the radiogenic and nucleogenic components; Suggestion that some isotopes occupy crystallographic sites; Evidence that 39ArK loss from the tunnel site during neutron irradiation and/or bake-out procedure preceding isotopic analysis does not occur; Assertion that neither the overwhelming atmospheric 40Ar nor the very-fined nature of the supergene manganese oxides will be a problem in extracting significant information from the analysis of supergene manganese oxides minerals.

Abstract: Preliminary results of 53Mn measurements for seven Brazilian haematites are presented. The production rate of 53Mn due to cosmic-ray induced disintegration of iron is estimated to be 103±19 atoms g(Fe)-1 yr−1 at sea level and high latitude. This is consistent with the only previously published measurement. The muon contribution to the total 53Mn production at the surface is estimated to be ∼7%. Cosmogenic-isotope dating employing 53Mn is applicable to any rock/mineral type that contains iron as a major constituent, such as haematite, goethite, pyroxene and olivine. The method has the potential to extend the time scale of cosmogenic exposure dating to >10Ma. [Copyright &y& Elsevier]