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We report on the quantitative nondestructive analysis of the natural isotopic abundances of hafnium and tungsten elements using nuclear resonance fluorescence. Metallic samples of hafnium and tungsten were irradiated to six quasi-monochromatic γ -ray beams generated by laser Compton scattering in the energy range of 2.4–3.2 MeV. Multiple nuclei were simultaneously excited at each of the six γ -ray beam energies. A high-purity germanium detector array detected deexcitations of the nuclei. In total, 51 transitions were unprecedentedly employed to estimate the isotopic abundances of heavy elements nondestructively. The estimated abundances of three hafnium isotopes and three tungsten isotopes are consistent with standard known natural abundances within the experimental uncertainties. The deviation from the standard values ranges from 0.18 % to 1.36 %. [ABSTRACT FROM AUTHOR]

Zircon trace element geochemistry has become an increasingly popular tool to track crustal evolution through time. This has been especially important in early-Earth settings where most of the crust has been lost, but in some fortuitous instances detrital zircons derived from that lost crust have been preserved in younger sediments. To study the formation and geochemical evolution of continental crust from the Hadean to the Paleoarchean, the 3.6 to 3.2 Ga Barberton Greenstone Belt in southern Africa is an excellent target due to its outstanding preservation and presence of detrital zircons that span almost a billion years. Here, we use trace elements, in combination with hafnium and oxygen isotopes, of 3.65 to 3.22 Ga detrital and tuffaceous zircons of the Moodies and Fig Tree groups and compare their geochemistry to previously studied 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed of the Onverwacht Group. The major detrital zircon age clusters in the former at 3.55 Ga, 3.46 Ga, and 3.26–3.23 Ga overlap with episodes of TTG emplacement and felsic volcanism in the Barberton area, suggesting a local provenance. In contrast, age clusters at 3.65 Ga and 3.29 Ga of the Moodies and Fig Tree groups as well as 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed do not have known intrusive sources and were likely derived from outside the present-day Barberton belt. This indicates that more than half of the felsic igneous events in the detrital zircon record do not have a whole-rock representation that can be directly studied. The similar compositions and inferred crustal evolution histories recorded in zircons from the Fig Tree and Moodies groups, as well as from the Green Sandstone Bed, suggest that they were derived from connected terranes experiencing similar crustal processes diachronously. Together, they show three phases of felsic continent formation, reflecting different crustal processes: (1) long-lived protocrust formed in the Hadean from undepleted mantle sources. These zircons are vastly different from younger zircons and, hence, Barberton TTGs are not good analogues of Hadean crust formation. (2) At 3.8 Ga, onset of significant crustal growth though cyclic juvenile additions and hydrous melting, possibly within a volcanic plateau setting but an arc-like setting cannot be excluded based on this data. (3) Between 3.4 and 3.3 Ga, felsic crust is generated through a previously unrecognized episode of crustal growth by shallow melting of mafic, mantle-derived sources. This is immediately followed by the onset of crustal thickening through the transport of surface-altered, hydrated materials to deep crustal levels. Since there is geological evidence for extension and shortening at that time this may reflect the onset of horizontal movement. Whether this last geodynamic setting reflects modern-style plate tectonics or not, continent formation and the onset of plate tectonics in the Barberton area occurred through complex multi-stage processes spanning almost a billion years, most of which is only accessible through the detrital zircon record. [ABSTRACT FROM AUTHOR]

The hafnium isotopes of zircon have recently been identified as a new means of detecting magmatic cyclicity. A hierarchy of possible magmatic cycles has been verified, ranging from the ∼600 Myr supercontinent cycle to 10–20 Myr plutonic cycles. While the supercontinent cycle is well-established, the shorter mantle convective and plutonic cycles require further work to understand how magmatic dynamics may give rise to such cyclicity. In this study, we analyze in detail the long-lived orogenic system of the Australian Tasmanides, and a hierarchy of cycles is visible, including a ∼ 290 Myr accretionary/orogenic cycle, a ∼ 65 Myr upper mantle cycle, and 20–10 Myr magmatic and plutonic cycles. These cycles correlate with the evolution of the orogens and episodic continental growth of eastern Gondwana. The surprising similarity in period length between the opening/closing of a typical Atlantic-like Wilson cycle to the case here of the lifecycle of a West Pacific-type arc system may imply that the two distinct tectonic settings have a similar mantle convective effect, which requires further empirical testing and theoretical study. Similar to magmatic cycles identified in the Hadean and in modern arc settings, the hierarchy of cycles in the Tasmanides implies a uniformitarian similarity in arc magmatism throughout Earth history. • A hierarchy of magmatic cyclicity has been identified in the Australian Tasmanides. • Geological processes of the cycles have been systematically addressed. • Mechanisms of cyclicity and episodic continental growth have been proposed. • Geological significance of modulation of the cycles has been proposed. • The hierarchy of cycles displays a uniformitarian affinity throughout Earth history. [ABSTRACT FROM AUTHOR]