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This article presents a history of the development of U-Ph geochronology with emphasis on the role of Canadian researchers and some of its applications to Canadian geology. Modern U-Pb dating is the result of work by many individuals over the past 60 years, but the most important was Tom Krogh, who established methods that allowed determination of precise ages ( Key words: U-Pb, geochronology, zircon, history, Canadian geoscience, Introduction During public tours of the Jack Satterly Geochronology Laboratory (JSGL), which were frequent when it was at the Royal Ontario Museum (ROM), a question often posed was 'Why would [...]

The Hamburg meteorite fell on January 16, 2018, near Hamburg, Michigan, after a fireball event widely observed in the U.S. Midwest and in Ontario, Canada. Several fragments fell onto frozen surfaces of lakes and, thanks to weather radar data, were recovered days after the fall. The studied rock fragments show no or little signs of terrestrial weathering. Here, we present the initial results from an international consortium study to describe the fall, characterize the meteorite, and probe the collision history of Hamburg. About 1 kg of recovered meteorites was initially reported. Petrology, mineral chemistry, trace element and organic chemistry, and O and Cr isotopic compositions are characteristic of H4 chondrites. Cosmic ray exposure ages based on cosmogenic 3He, 21Ne, and 38Ar are ~12 Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic 10Be concentration point to a small 40-60 cm diameter meteoroid. An 40Ar-39Ar age of 4532 ± 24 Ma indicates no major impact event occurring later in its evolutionary history, consistent with data of other H4 chondrites. Microanalyses of phosphates with LA-ICPMS give an average Pb-Pb age of 4549 ± 36 Ma. This is in good agreement with the average SIMS Pb-Pb phosphate age of 4535.3 ± 9.5 Ma and U-Pb Concordia age of 4535 ± 10 Ma. The weighted average age of 4541.6 ± 9.5 Ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system.

Zircon grains from metasiltstone beds in the Precambrian Ashburn Formation were dated to constrain the age of the stromatolite Archaeozoon acadiense. The largest group of analyzed grains consists of mostly well-rounded detrital zircon no younger than 1060 Ma with significant peaks at about 1200-1100, 1500, 1800, 2050, and 2700 Ma, as well as a few Mesoarchean ages. A second population of apparently euhedral grains gives ages in the range 680-400 Ma. The distribution of detrital zircon ages along with field relationships suggest that the Ashburn and Martinon formations are proximal and distal parts of the Green Head Group, respectively, and hence a previously reported maximum depositional age of ca. 602 [+ or -] 8 Ma for the Martinon Formation also applies to stromatolite in the Ashburn Formation. Paleozoic zircon recovered from the Ashburn Formation cannot be detrital because the rocks were intruded and contact metamorphosed by the Golden Grove Plutonic Suite at ca. 550-525 Ma. The Paleozoic zircon grains were found in four independently processed samples and have also been seen in other studies in the region, showing that they are not due to laboratory contamination. Zircon formed during younger magmatic activity may have been transported in hydrothermal fluids and emplaced into these metasedimentary rocks in veins. A single euhedral zircon recovered from an atypically thick vein gave an age of 659 [+ or -] 33 Ma. Hydrothermally transported zircon might provide a window into younger igneous processes for which evidence is not otherwise preserved in this and other areas. Key words: U--Pb geochronology, zircon, hydrothermal transport, detrital ages, Ashburn Formation. Des grains de zircon provenant de lits de metasiltstone dans la Formation precambrienne d'Ashburn ont ete dates afin de circonscrire l'age du stromatolite Archaeozoon acadiense. Le plus grand groupe de grains analyses est principalement constitue de zircons detritiques arrondis d'un age minimum de 1060 Ma avec des pics significatifs a environ 1200-1100, 1500, 1800, 2050 et 2700 Ma, ainsi que quelques ages mesoarcheens. Une deuxieme population de grains en apparence idiomorphes donne des ages dans la fourchette de 680-400 Ma. La distribution des ages de zircons detritiques combinee aux relations de terrain donne a penser que les Formations dAshburn et de Martinon sont les parties proximale et distale du Groupe de Green Head, respectivement, et qu'un age de depot maximum deja publie d'environ 602 [+ or -] 8 Ma pour la Formation de Martinon s'applique egalement aux stromatolites dans la Formation dAshburn. Des zircons paleozo'iques provenant de la Formation dAshburn ne peuvent etre detritiques puisque les roches ont ete recoupees par la serie plutonique de Golden Grove vers 550-525 Ma et en ont subi un meta-morphisme de contact. Les grains de zircon paleozo'iques ont ete trouves dans quatre echantillons traites de maniere independante et ont egalement ete releves dans d'autres etudes dans la region, ce qui demontre qu'ils ne sont pas dus a une contamination en laboratoire. Des zircons formes durant des episodes magmatiques plus recents pourraient avoir ete transported dans des fluides hydrothermaux et mis en place dans des veines dans ces roches metasedimentaires. Un unique grain de zircon idiomorphe provenant d'une veine inhabituellement epaisse a donne un age de 659 [+ or -] 33 Ma. Des zircons transported par des fluides hydrothermaux pourraient ouvrir une fenetre sur des processus ignes plus jeunes dont d'autres preuves ne sont pas preservees dans cette region et ailleurs. [Traduit par la Redaction] Mots-cles: geochronologie U--Pb, zircon, transport hydrothermal, ages de zircons detritiques, Formation d'Ashburn., Introduction The Saint John area of southern New Brunswick, Canada (Fig. 1), has been a centre of geological investigation since the mid part of the 19th century, when the Steinhammer [...]

Atom probe tomography (APT) of 238U and 206Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791 ± 1 Ma) and the Great Dyke of Mauritania (2732 ± 2 Ma) in an effort to map U and Pb concentration at the nanometre scale. The purpose was to constrain the average nuclear recoil distance of 238U-series daughter nuclei in order to correct U-Pb ages determined on small baddeleyite crystals for alpha-recoil loss of Pb. Both crystals were thought to expose natural crystal surfaces providing a boundary where maximum effects of recoil loss could be observed. The Hart Dolerite sample showed no variations in Pb concentrations near the edge. The Great Dyke sample shows U zoning and the associated 206Pb zoning is affected by alpha recoil, apparently adjacent to a natural grain surface. The sample also shows 10 nm-scale apparently primary clusters of U atoms that contain about 40 % of the U. These are too small to constrain alpha recoil distance beyond a few nm but are apparently primary and their formation mechanism poses a dilemma. To constrain alpha recoil distance, a forward modelling approach is presented where 206Pb redistribution functions were determined for a range of possible distances and synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions that were compared to the observed 206Pb/238U profile. A complication is the fact that the 40 by 400 nm size of the sample is lower than the range of possible alpha recoil redistribution effects so it was necessary to extrapolate the observed U zoning. An oscillatory pattern gives the best fit to the observed profile but any reasonable extrapolation constrains the average alpha recoil distance to be close to 80–90 nm, which is much larger than previous estimates using other methods. Either recoil distances can be highly anisotropic within small crystal samples or surface roughness was a factor that modified the recoiled Pb distribution. APT is a potentially useful approach to determining average alpha recoil distance but requires sampling of primary, smooth crystal faces. [ABSTRACT FROM AUTHOR]

The Creignish Hills and North Mountain areas of southwestern Cape Breton Island consist mostly of Neoproterozoic rocks typical of the Ganderian Bras d'Or terrane. U-Pb ages presented here for detrital zircon in the Blues Brook Formation of the Creignish Hills confirm a depositional age no greater than about 600 Ma. Although it is possible that some components of the formation are much older, similarities in rock types and field relations suggest that this is not the case. It is likely that the equivalent Malagawatch Formation of the North Mountain area, as well as high-grade metasedimentary rocks of the Melford Formation and Chuggin Road complex in the Creignish Hills and Lime Hill gneiss complex in the North Mountain area, represent the same or stratigraphically equivalent units as the Blues Brook Formation. The minimum ages of all of these units are constrained by cross-cutting syn- and post-tectonic plutons with ages mostly between 565 and 550 Ma, indicating that sediments were deposited, regionally metamorphosed, deformed, and intruded by plutons in less than 40-50 million years. The assemblage of pelitic, psammitic, and carbonate rocks indicates that a passive margin in a tropical climate was quickly changed to an active Andean-type continental margin in which voluminous calcalkaline dioritic to granitic plutons were emplaced. This sedimentary and tectonic history is characteristic of the Bras d'Or terrane and is shared by its likely correlative, the Brookville terrane in southern New Brunswick. Les secteurs des collines Creignish et du mont North dans le sud-ouest de l'ile du Cap-Breton sont principalement constitues de roches neoproterozo'iques typiques du terrane de Ganderian Bras d'Or. Les ages U-Pb sur zircon detritique de la formation de Blues Brook des collines Creignish presentes ici confirment une sedimentation survenue il y a au plus 600 Ma environ. Meme s'il est possible que certains elements de la formation soient beaucoup plus ages, les similarites des lithotypes et les observations sur le terrain laissent supposer que ce n'est pas le cas. Il est probable que la formation equivalente de Malagawatch dans le secteur du mont North de meme que les roches metasedimentaires a forte teneur de la formation de Melford et du complexe du chemin Chuggin dans les collines Creignish et le complexe de gneiss de la colline Lime dans le secteur du mont North representent les memes unites que la formation de Blues Brook ou des unites stratigraphiquement equivalentes. Les ages minimaux de toutes ces unites sont limites par les plutons syntectoniques et post-tectoniques transversaux, ages pour la plupart de 550 a 565 Ma, ce qui revele que les sediments se sont deposes, se sont metamorphises et deformes a lechelle regionale, et ont ete penetres par les plutons au cours dune periode de moins de 40 a 50 millions d'annees. L'assemblage de roches pelitiques, psammitiques et carbonatees signale qu'une marge passive dans un climat tropical s'est rapidement transformee en une marge continentale de type andeen active dans laquelle de volumineux plutons calco-alcalins dioritiques a granitiques se sont mis en place. Un tel passe sedimentaire et tectonique est caracteristique du terrane de Bras d'Or et constitue un trait que le terrane a en commun avec l'unite vraisemblablement correlative du terrane de Brookville dans le sud du Nouveau-Brunswick. [Traduit par la redaction], INTRODUCTION The Creignish Hills and North Mountain areas of western Cape Breton Island (Fig. 1) consist mostly of Neoproterozoic metasedimentary and plutonic rocks typical of the Bras d'Or terrane of [...]

Compares differences in resource exploitation and energy development in Louisiana and western mineral-producing states. Identifies socioeconomic impacts of Louisiana's offshore drilling and western coal, oil, and natural gas mining, noting the boom and bust cycles and "hyperurbanization" that attends both. Stresses the necessity of balancing national energy needs with regional and environmental health. (CH)