There are currently 31 confirmed structures of impact origin in Australia. More than 49 additional structures have been proposed to have formed due to asteroid impact but await confirmation. Many discoveries have been made in Australia in the time since the last comprehensive review of the Australian impact cratering record was published in a peer-reviewed journal in 2005. These include further expanding the record of confirmed craters, and providing new insights into a variety of impact-related processes, such as shock deformation, phase transitions in accessory minerals, new impact age determinations, studies of oblique impacts, and more. This update is a review that focuses principally on summarizing discoveries made since 2005. Highlights since then include confirmation of five new Australian impact structures, identification of Earth’s oldest recognized impact structure, recognition of shock deformation in accessory minerals, discovery of the high-pressure phase reidite in Australia, determination of the links between impact craters and some ore deposits, and publication of the first generation of numerical hydrocode models for some Australian craters.
Constraining the age of many types of ore deposits remains challenging due to the lack of radiogenic isotopes incorporated into common ore-forming minerals. The timing of pre-Caledonian-hosted Cu mineralization along the entire ∼1200 km-long East Greenland Caledonides remains virtually unknown, hampering our knowledge of ore deposit timing and genesis in a frontier exploration region. Here, automated mineral analysis of a series of nodular, disseminated and vein-hosted Cu- ± Pb-mineralized metasedimentary rocks in central East Greenland reveals detrital zircon and hydrothermal xenotime, both amenable to U-Pb geochronology. Detrital zircon geochronology of a co-deposited quartzite reveals an age distribution highly similar to the Cryogenian (∼700 Ma) upper Eleanore Bay Supergroup. Hydrothermal xenotime U-Pb analyses adjacent to nodular and disseminated chalcocite across three proximal samples have variable amounts of common Pb that together yield a well-defined single discordia with a lower concordia intercept of 438 ± 13 Ma (2σ). This age is within uncertainty of the onset of Caledonian regional metamorphism and granitoid production and clearly post-dates deposition of the upper Eleanore Bay Supergroup by several hundred Myr. Considering a published chalcocite Pb-Pb isochron age of 680 ± 65 Ma, the hydrothermal xenotime U-Pb ages imply that Caledonian-driven fluid activity, sourced from metamorphic reactions or from granitoids, remobilized diagenetic Cu and Pb mineralization. Chalcocite Pb-Pb isotopes show that dissolved and reprecipitated portions are volumetrically minor, radiogenic and Pb-poor, implying that fluids stripped most of the Pb from the system. Thus, it is likely that remobilization was localized on the grain scale, although some Cu and Pb was transported away from diagenetic sites, perhaps into veins. Whilst Caledonian metamorphism and granitoid emplacement is widespread in central East Greenland, the full extent of their roles in upgrading Cu mineralization remains to be ascertained. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle Supplementary material: https://doi.org/10.6084/m9.figshare.c.6675384
<p>Constraining precise ages for impact events is crucial in establishing Earth&#8217;s history, and several geochronometers have been developed to date impacts. We present electron backscatter diffraction (EBSD), sensitive high-resolution ion microprobe (SHRIMP) and atom probe tomography (APT) data from shocked xenotime [(Y,HREE)PO<sub>4</sub>] collected from two impact sites to investigate the potential of xenotime as an impact geochronometer. A detrital xenotime grain from the Vredefort dome (South Africa) contains planar fractures, planar deformation bands and {112} twinning, the latter of which are diagnostic shock microstructures. However, APT analysis from the twin domains and also from the host yielded no evidence of Pb mobility at the nanometer scale during the impact. SHRIMP analysis (n=24) on the grain yielded a discordia with an upper intercept of 3136 &#177; 110 Ma and an imprecise lower intercept of 1793 &#177; 280 Ma. These correspond, respectively, to the bedrock age and a post-impact, cryptic terrane-wide fluid infiltration event. Three neoblastic grains from the Araguainha dome (Brazil) experienced partial to complete recrystallisation. The least recrystallised grain yields the oldest <sup>238</sup>U/<sup>206</sup>Pb age of 479 &#177; 26 Ma, whereas a completely recrystallised neoblastic grain gave an age of 257 &#177; 11 Ma. &#160;APT analysis on the latter grain showed different nanoscale features that shed light on Pb mobility during shock deformation and recrystallisation. &#160;Based on observations of nanoscale Pb mobility and the correlation between recrystallisation and isotopic resetting, and prior published ages, we interpret 257 &#177; 11 Ma to date the impact event. These data confirm that recrystallised neoblastic xenotime is a useful impact geochronometer.&#160;</p>
The 119 Ma Dinkey Dome pluton in the central Sierra Nevada Batholith is a peraluminous granite and contains magmatic garnet and zircon that are complexly zoned with respect to oxygen isotope ratios. Intracrystalline SIMS analysis tests the relative importance of magmatic differentiation processes vs. partial melting of metasedimentary rocks. Whereas δ 18 O values of bulk zircon concentrates are uniform across the entire pluton (7.7‰ VSMOW), zircon crystals are zoned in δ 18 O by up to 1.8‰, and when compared to late garnet, show evidence of changing magma chemistry during multiple interactions of the magma with wall rock during crustal transit. The evolution from an early high-δ 18 O magma [δ 18 O(WR) = 9.8‰] towards lower values is shown by high δ 18 O zircon cores (7.8‰) and lower δ 18 O rims (6.8‰). Garnets from the northwest side of the pluton show a final increase in δ 18 O with rims reaching 8.1‰. In situ REE measurements show zircon is magmatic and grew before garnets. Additionally, δ 18 O in garnets from the western side of the pluton are consistently higher (Ave = 7.3‰) relative to the west (Ave = 5.9‰). These δ 18 O variations in zircon and garnet record different stages of assimilation and fractional crystallization whereby an initially high δ 18 O magma partially melted low δ 18 O wallrock and was subsequently contaminated near the current level of emplacement by higher δ 18 O melts. Collectively, the comparison of δ 18 O zoning in garnet and zircon shows how a peraluminous pluton can be constructed from multiple batches of variably contaminated melts, especially in early stages of arc magmatism where magmas encounter significant heterogeneity of wall-rock assemblages. Collectively, peraluminous magmas in the Sierran arc are limited to small < 100 km 2 plutons that are intimately associated with metasedimentary wallrocks and often surrounded by later and larger, metaluminous tonalite and granodiorite plutons. The general This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.
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