http://www.researchonline.mq.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:24788 25 page(s) 2013-04-09T06:40:17.314Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:21476 Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly after the formation of the Earth. However, much of the evidence is cryptic and not apparent in the mapped geology. New Lu-Hf isotopic results, combined with existing Lu-Hf and Sm-Nd isotopic data, indicate five model-age probability peaks in the central and eastern Yilgarn Craton: at ca 4200, ca 3500, ca 3100, ca 2800 Ma and ca 2700 Ma. The ca 3100 Ma, ca 2800 Ma and ca 2700 Ma model-age peaks likely correspond to crust formation events. Evidence of the earlier peaks is not seen directly in the rock record, although zircon crystals in rocks of the Southern Cross Domain of the Youanmi Terrane show a long history of reworking pointing back to mantle extraction more than 4200 million years ago. The earliest peak is not recorded in the Eastern Goldfields Superterrane, indicating that crust formation in this region post-dated the earliest development of the Yilgarn Craton. Subsequent, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets for both the Eastern Goldfields Superterrane and the Southern Cross Domain. Magmas in the Eastern Goldfields Superterrane had a substantial juvenile input whereas those in the Southern Cross Domain recorded major reworking of older crust. The rock records for both the Eastern Goldfields Superterrane and the Southern Cross Domain share common elements of history after ca 2960 Ma. Both regions appear to have been subjected to major heating at ca 3100 Ma and ca 2800 Ma that resulted in the generation of juvenile crust in the east and reworking of older crust in the west. The ca 3500 Ma event is not readily evident in the rock record and may reflect a mixed age. However, the ca 3100 Ma and ca 2800 Ma events are recorded by both granite suites and greenstone successions across the craton. The ca 2700 Ma event is most evident in rocks from the Eastern Goldfields Superterrane. 2012-09-17T06:20:20.056Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13202 We report structural and electronic properties of epitaxial hafnium nitride films grown on MgO by plasma-assisted pulsed laser deposition. The electronic structure measured using soft x-ray absorption and emission spectroscopy is in excellent agreement with the results of a band structure calculation. We show that by varying the growth conditions we can extend the films’ reflectance further toward the UV, and we relate this observation to the electronic structure. 2011-05-25T21:44:01.151Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2126 Two populations of kimberlitic zircon are present in the Timber Creek kimberlites, Northern Territory. Laser-ablation ICP-MS U–Pb dating yields an age of 1483 ± 15 (2σ) Ma for the main group and an age of 179 ± 2 Ma for the other group. This distinction of two age groups is strongly supported by Hf isotope data on the same zircons. Although the trace-element patterns of both populations are typical of mantle-derived zircons, the ‘young’ population has slightly higher concentrations of most trace elements, but has lower Hf, Nb, Ta and Pb contents. The distinct differences in trace-element contents and Hf isotopic composition of the two zircon populations indicate that they were derived from different magma sources. The dating results indicate that the emplacement age of the Timber Creek kimberlites cannot be older than the age of the ‘young’ zircon population (i.e. 179 ± 2 Ma). This clarifies the inconsistency between the previously reported SHRIMP age of the Timber Creek zircons (1462 ± 53 Ma) and the much younger age (1200 Ma) of the sediments of the Victoria River Basin into which these kimberlites have intruded. The Timber Creek kimberlites are a newly recognised extension of the widespread Jurassic kimberlite activity known in Western Australia and South Australia (Wandagee, Orroroo, Cleve and Eurelia kimberlites). 2010-09-30T05:45:28.658Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:982 The integrated in situ microanalysis of Pb and Hf isotopic compositions of zircon (TerraneChron® methodology) has been applied to detrital zircons from locally derived stream sediments in the South Para River, which samples the Houghton Inlier, South Australia. The zircons record two major events: an igneous event at 1718±8 Ma and a metamorphic event at 1625±9 Ma. The igneous event is of broadly similar age to syndepositional magmatism associated with the lower Willyama Supergroup of the Curnamona Province and syn-Kimban Orogeny intrusives of the eastern Gawler Craton. The later metamorphic event is of comparable age to the Olarian Orogeny in the Curnamona Province and to magmatic and metamorphic events in the central to western Gawler Craton. The combination of U - Pb dating and Hf-isotope data provides an additional tool to overcome ambiguities in correlating terranes with similar U - Pb ages. The Hf-isotopic compositions of the Houghton Inlier zircons and their model ages of 2.5 - 2.4 Ga suggest a closer match with the rocks of the Gawler Craton. 2010-01-27T23:21:46.928Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:1917 Zircon megacrysts from the Mir kimberlite, Yakutia, contain inclusions of chromite, chrome diopside, magnesian olivine, Ni-rich monosulfide solid solution and phlogopite. The mineral chemistry of the inclusion suite suggests that the zircons grew in a metasomatized peridotite matrix. Twenty-three zircons were chosen for U–Pb dating, Hf isotope and trace element determinations. The trace element data are typical of kimberlitic zircons worldwide. LAM-ICPMS U–Pb dating yields a weighted mean ²⁰⁶Pb/²³⁶U age of 353.6±2.5 Ma. Hf isotope measurements by LAM-MC-ICPMS yield ℇ Hf values of 3.0–9.2, and model ages (T DM) of 600–800 Ma. These data constrain the crystallization of the zircons to between 350 and 600 Ma. However, LAM-MC-ICPMS microanalysis of Os isotopes in sulfides included in three zircons yields T RD model ages of 2.37–2.92 Ga (T MA=2.39–3.19 Ga). To explain the discordance between the ages of the zircons and their sulfide inclusions, we suggest that these zircons grew in a metasomatized peridotite, which contained sulfides that were residual from ancient melting events. These sulfides, together with other peridotite phases, were trapped in the metasomatic zircon with little modification of their elemental or isotopic composition. This model has important implications for the interpretation of Re–Os model ages of sulfide inclusions in diamonds. Diamonds also could capture and preserve older sulfides during their growth or regrowth in mantle rocks, and the inclusions therefore do not necessarily date the formation of the diamond. 2010-01-27T23:10:34.126Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:3647 The results of a laser ablation microprobe–inductively coupled plasma mass spectrometry Lu–Hf isotope study of zircons in 0·93–1·67 Ga rocks from south Norway indicate that early Proterozoic protoliths of the Baltic Shield have present-day ¹⁷⁶Hf/¹⁷⁷Hf ≤ 0·28190 [εHf(t) = 5–6], whereas 1·52–1·60 Ga juvenile additions to the continental margin have ¹⁷⁶Hf/¹⁷⁷Hf = 0·2810 [εHf(t) = 12–13]. Mid- to late Proterozoic felsic igneous rocks in the region are characterized by a range of Hf isotopic compositions suggesting mixing of material derived from Palaeoproterozoic crust from the Baltic Shield and/or mid-Proterozoic juvenile crust. New mantle-derived magmas were added to the crust at ~1·48 Ga and in Sveconorwegian time. Late Sveconorwegian granites from the area west of the Oslo Rift have inherited zircons with low ¹⁷⁶Hf/¹⁷⁷Hf (<0·28180), suggesting that a pre-1·7 Ga crustal source contributed to the magmas. The evolution of the continental crust in this region is thus a result of repeated interaction between mantle-derived magmas and mid- to early Proterozoic crustal rocks. The results of this study confirm the presence of early Proterozoic rocks in the deep crust west of the Oslo Rift, and support tectonic models in which the protolith of the western part of south Norway has been part of the Baltic Shield since the early Proterozoic. 2010-01-27T22:50:42.775Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5203 The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe–multicollector–inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The ¹⁷⁶Hf/¹⁷⁷Hf ratio shows a wide range, from 0.2811 to 0.2858 (εHf = –55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low ¹⁷⁶Hf/¹⁷⁷Hf ratios (<0.2812) and Lu/Hf ratios of ~0.04, that dominated their Hf budget. The MARID rocks were later metasomatized by a fluid and/or melt that had caused the breakdown of eclogitic or lherzolitic garnet with Lu/Hf ratios of ~0.6, providing a source of highly radiogenic Hf. The low ¹⁷⁶Hf/¹⁷⁷Hf ratios preserved in some MARID rutiles show that the Nd-Hf isotope systematics of kimberlites and lamproites can be explained by mixing between an asthenospheric melt and the ancient subcontinental lithospheric mantle. 2010-01-27T22:33:07.235Z ]]>