Desmond E. Moser

DATING THE SHOCK WAVE AND THERMAL IMPRINT OF THE GIANT VREDEFORT IMPACT, SOUTH AFRICA

U-Pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded Vredefort impact structure of South Africa. Results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 ± 3 Ma. Discovery of a synimpact to postimpact dike of norite that intruded at 2019 ± 2 Ma is the first recognition of mafic igneous activity related to impact. The dike is either derived from a Sudbury-type impact melt layer (since eroded) or is the product of decompression melting of Kaapvaal mantle in response to the ablation of >15 km of crust at the center of the crater. The combined heating effects of the shock wave and impact-triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with Kaapvaal craton, and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study.

UPb geochronological constraints on the geological evolution of the Pinware terrane and adjacent areas, Grenville Province, southeast Labrador, Canada

Abstract UPb geochronological data show that the Pinware terrane and adjacent areas in the Grenville Province in southeast Labrador experienced three orogenic events: Labradorian, Pinwarian and Grenvillian. Labradorian (1710-1600 Ma) rocks in the Pinware terrane, previously only known from a dated felsic volcanic enclave in a younger granite, are now recognized to be widespread and include both supracrustal units and granitoid intrusions. One quartzite was deposited between ∼1805 and 1500 Ma, the ages of the youngest detrital zircon and subsequent metamorphism, respectively, and includes 1878 and 2720 Ma detrital zircon suggesting derivation of source material from pre-Labradorian Laurentia. A volcaniclastic(?) unit is interpreted to have an age of 1637 ± 8 Ma. Two Labradorian quartz monzonite intrusions yielded ages of 1650+18−9 Ma and 1649 ± 7 Ma. Coupled with previous information, these data indicate coeval plutonism, volcanism and sedimentation taking place at the same time as formation of the Trans-Labrador batholith farther north. The Pinwarian Orogeny is now extended to include events between 1510 and 1450 Ma. Granitoid rocks of the Upper Paradise River pluton, a large AMCG suite in the Mealy Mountains terrane, has been dated at 1495 ± 7 and 1501 ± 9 Ma, making it the only suite of this type and age known in North America. Migmatitic quartz monzonite, dated at 1450+15−21 Ma, provides the first proof of high-grade metamorphism in the Pinware terrane, and, in conjunction with recent geochronological data from other parts of the eastern Grenville Province, justify upgrading the Pinwarian to orogenic status. Grenvillian metamorphism throughout the Pinware terrane occurred between 1050 and 985 Ma, as indicated mainly on the basis of zircon lower intercepts, but including some monazite data and constraints imposed by dated younger rocks. This range of ages contrasts with the time span for Grenvillian metamorphism in the Lake Melville terrane, previously dated to between 1080 and 1000 Ma, and with which a newly determined age of 1047 ± 2 Ma from a granodiorite dyke at the Mealy Mountains-Lake Melville terrane boundary conforms. In the Pinware terrane, near the end of Grenvillian orogenesis, widespread alkalic magmatic activity occurred between 990 and 980 Ma. Units investigated include an aegerine-bearing alkali-feldspar syenite emplaced at 991 ± 5 Ma, a clinopyroxene-fayalite alkali-feldspar syenite having a probable age of 985 Ma, and alkalic mafic dykes emplaced at 985 ± 6 Ma. Similar (although amygdaloidal) alkalic mafic dykes in the Lake Melville terrane may be slightly younger, having an age of 974 ± 6 Ma. The short-lived alkalic activity was followed by late-tectonic magmatism in the Pinware terrane at 983 ± 3 Ma, which heralded post-tectonic granitoid plutons between 974 and 956 Ma. Cooling and uplift in the Pinware terrane is documented by titanite ages between 972 ± 5 Ma and 939 ± 5 Ma. It is inferred that cooling occurred 30 million years sooner in the southeast Pinware terrane than near its northwest margin.

Late Jurassic age for the Morokweng impact structure, southern Africa

Abstract A roughly 70 km diameter circular feature buried beneath the Kalahari sands in South Africa is revealed on regional aeromagnetic maps. Boreholes drilled into the centre of the structure intercept a ∼ 250 m thick sheet of quartz norite, interpreted as an impact melt, which overlies brecciated and shock metamorphosed basement granite. Zircons recovered from the quartz norite, yield U-Pb ages of 145 ± 0.8 Ma, and biotites provide Ar-Ar ages of 144 ± 4 Ma. These data provide strong evidence for the occurrence of a Late Jurassic impact crater (the Morokweng impact structure) ∼ 100 m beneath the surface.

Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon

Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U–Pb (uranium–lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.

A record of ancient cataclysm in modern sand: Shock microstructures in detrital minerals from the Vaal River, Vredefort Dome, South Africa

The record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried. Discovery of the missing impact record from Hadean to present may be advanced through identifi cation of residual shocked detritus. To evaluate which shocked minerals survive erosion and sedimentary transport, we investigated modern sands from the Vaal River in South Africa, where it crosses the 2.02 Ga Vredefort Dome, the largest terrestrial impact structure known to date. Shocked minerals were identifi ed in all sediment samples, including from the Vaal channel and tributaries within the structure. In transmitted light, detrital quartz preserves discontinuous decorated planar features previously identifi ed as Brazil twins, which are readily visible as bright, continuous features in cathodoluminescence images. Detrital zircons preserve fi ve orientations of planar fractures (PFs), which can produce dramatically offset growth zoning and apparent rotation of subgrains. Other zircons contain fi lled fractures that may represent a new shock microstructure. Detrital monazite preserves four orientations of PFs, and many grains contain oscillatoryzoned shocked zircon inclusions, which thus represent shocked inclusions within shocked accessory grains. Zircon and monazite with granular texture were also identifi ed. This study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event, and it demonstrates their potential for preserving evidence of ancient impacts. The recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age, and may be particularly relevant to early Earth studies.

Archean age for the granulite facies metamorphism near the center of the Vredefort structure, South Africa

Granulite facies metamorphic assemblages in rocks exposed near the center of the 2.02 Ga Vredefort impact structure previously have been interpreted either as Early Proterozoic, genetically related to the 2060 Ma Bushveld Complex, or as Archean, and representative of lower crust that rebounded to upper crustal levels following an impact event. Zircon and monazite recovered from the granulite facies rocks record high-grade metamorphism at 3107 ± 9 Ma and a primary age of ≥ 3425 Ma for detrital zircon. A shock-deformed, but otherwise pristine, dolerite dike that intrudes the granulite terrane yields a U-Pb zircon age of ≥ 2560 Ma, providing a minimum age for the metamorphism. These isotopic age data are difficult to reconcile with a regional high-grade metamorphic event in the crust beneath Vredefort at 2060 Ma. Instead, the preimpact, high-temperature metamorphic history observed in the Vredefort lower crustal rocks indicates an enigmatic high-temperature event during the stabilization of diamondiferous Archean tectosphere.

Creation of a continent recorded in zircon zoning

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ 18 O values of 5.1‰–7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ 18 O (8.4‰–10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ∼80 m.y. of high-temperature (∼700–650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (∼ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning.

Zircon U-Pb isotope, δ18O and trace element response to 80 m.y. of high temperature metamorphism in the lower crust: Sluggish diffusion and new records of Archean craton formation

Coordinated cathodoluminescence (CL) imaging and ion microprobe (SHRIMP and CAMECA 1280) analysis document micron-scale U-Pb-O isotope and trace element zoning in zircons from deep crust exposed to 80 m.y. of high temperature and pressure metamorphism. Three, along-strike paragneiss samples across the amphibolite to granulite facies transition in the Kapuskasing Uplift crustal cross-section in the Archean Superior province yield detrital, originally igneous zircon cores overgrown by progressively larger volumes of metamorphic zircon with increasing grade. The cores generally retain primary age (2.85±0.03 to 2.67±0.02 Ga), oxygen isotope (5.1 to 7.0‰) and trace element compositions similar to those reported for magmatic arc sources. Dark CL, metamorphic zircon rims record nearly continuous overgrowth events for ∼80 m.y. from 2.66±0.01 to 2.58±0.01 Ga during uppermost amphibolite to granulite facies regional metamorphism. These rims have significantly higher δ18O values (8.4 to 10.4‰) and trace element compositions quite distinct from those of the cores; these differences indicate that their δ18O and trace element compositions were not inherited from the igneous cores, consistent with extensive textural evidence for rim formation as metamorphic overgrowths. Multi-spot traverses record steep oxygen isotope discontinuities (4‰ over <10 μm) at core-rim boundaries, confirming the extremely sluggish rates of volume diffusion of O in non-metamict zircon during extended (∼80 m.y.) granulite-grade metamorphism (peak T=750-800 °C) at substantial f(H2O) but water-undersaturated (fluid-absent) conditions. Likewise no evidence of significant diffusive exchange of δ18O could be detected along deformation microstructures such as annealed fractures in cores infilled with high δ18O zircon. Application of simple diffusion models to detailed δ18O profiles in a large number of zircon grains constrain maximum values of the diffusivity of oxygen in zircon (logDZrcox) to the range −27.5 to −26.4 m2/s. For the estimated 80 m.y. and 700 to 800 °C time-T window of rim formation, these maximum values are similar to or slower than values reported by Page and others (2007, 2010) and the experimentally-determined “dry” diffusivity of oxygen in zircon (Watson and Cherniak, 1997), but are markedly slower than the experimentally-determined “wet” diffusivity of oxygen in zircon (Watson and Cherniak, 1997). Fast diffusion of oxygen in zircon predicted by hydrothermal experiments may, in nature, require the presence of a hydrous fluid rather than a threshold value of f(H2O). Our test demonstrates that unrecrystallized metamorphosed igneous zircons and metamorphic zircons will retain the geochemical (U-Pb age, trace element and δ18O) record of their origin and evolution despite prolonged, high-grade metamorphism at significant f(H2O) but water under-saturated (fluid-absent) conditions. Such zircons, particularly those that exhibit δ18O zoning, are micron-scale records for the T-time-fluid interaction history of deep crustal rocks. Such records will not be preserved in less refractory phases and promise new insights into the processes of continent formation and evolution.

A terrestrial perspective on using ex situ shocked zircons to date lunar impacts

Deformed lunar zircons yielding U-Pb ages from 4333 Ma to 1407 Ma have been interpreted as dating discrete impacts on the Moon. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter diffraction and sensitive high-resolution ion microprobe U-Pb analysis of zircons eroded from the ca. 2020 Ma Vredefort impact structure (South Africa) show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 Ma to 2645 Ma. An impact age of 1975 ± 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance, and then targeting specific age-reset microstructures. With the recognition that zircon can deform plastically in both impact and magmatic environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts. Identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon-based impact chronologies for the Moon, Earth, or other planetary bodies.

Correlating planar microstructures in shocked zircon from the Vredefort Dome at multiple scales: Crystallographic modeling, external and internal imaging, and EBSD structural analysis

Abstract Microstructural and geochronological analysis of shocked zircon has greatly advanced understanding the formation and evolution of impact structures. However, fundamental aspects of shock-produced planar microstructures in zircon remain poorly known, such as their deformation mechanisms, crystallographic orientations, and how planar microstructures visible at the grain scale by scanning electron microscopy correlate to microstructures visible at sub-micrometer scales by transmission electron microscopy and electron backscatter diffraction (EBSD). To unify observations of planar microstructures in zircon made at different scales into a consistent framework, we integrate the results of: (1) three-dimensional crystallographic modeling of planar microstructure orientations, with (2) 360° external prism backscattered electron imaging at the grain scale, and (3) polished section cathodoluminescence and EBSD analysis at the sub-micrometer scale for a suite of detrital shocked zircons eroded from the Vredefort Dome in South Africa. Our combined approach resulted in the documentation of seven planar microstructure orientations that can be correlated from grain to sub-micrometer scales of observation: (010), (100), (112), (11̄2), (1̄12), (1̄1̄2), and (011). All orientations of planar microstructures exhibit minor variations in style, however all are considered to be fractures; no amorphous ZrSiO4 lamellae were identified. We therefore favor the usage of “planar fracture” (PF) over “planar deformation feature” (PDF) for describing the observed planar microstructures in zircon based broadly on the nomenclature developed for shocked quartz. Some {112} PFs visible at the grain scale contain impact microtwins detectable by EBSD, and are the first report of polysynthetic twinning in zircon. The microtwins consist of parallel sets of thin lamellae of zircon oriented 65° about <110> and occur in multiple crosscutting {112} orientations within single grains. Curviplanar fractures and injected melt are additional impact-related microstructures associated with PF formation. Crosscutting relations of shock microstructures reveal the following chronology: (1) Early development of c-axis parallel PFs in (010) and (100) orientations; (2) the development of up to four {112} PFs, including some with microtwins; (3) the development of curviplanar fractures and the injection of impact derived melt; (4) the development of (011) PFs associated with compressional deformation; and (5) grain-scale non-discrete crystal plastic deformation. Experimental constraints for the onset of PFs, together with the absence of reidite, suggest formation conditions from 20 to 40 GPa for all of the planar microstructures described here.