Search

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]

In modern plate tectonic regimes, continental crust is generated above subduction zones in magmatic island arcs. Models for continental growth-largely based on the modeling of geochemical processes that can transform mantle melts into the intermediate composition of bulk continental crust-have been hampered by a lack of definitive geochronology, which could clarify the temporal emplacement of igneous rocks at the base of island arc crust. The Kohistan Arc Complex (KAC) of Pakistan is a rare window into a nearly complete section of lower arc crust, revealing a sequence of igneous rocks representing discrete batches of magma underplated along the base of the arc crust. We present high-precision isotope dilution-thermal ionization mass spectrometry U-Pb geochronological data from zircon, and Lu-Hf and Sm-Nd isochron data from several mafic lower crustal cumulates within the KAC. These data establish a clear downward-younging age trend throughout this succession of cumulate complexes, demonstrating a total magmatic duration of ~20 m.y., corresponding to a rate of lower crust formation of up to 200-260 km³ km-1 Ma-1. These results provide the first direct evidence of a sequential process of underplating, revealing a fundamental mechanism responsible for building the root of island arc crust. [ABSTRACT FROM AUTHOR]

Atom probe tomography 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 constrain the average nuclear recoil distance of U-series daughter nuclei and thereby 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 recoil loss could be observed, but both surfaces showed no adjacent variations in Pb concentrations. However, the Great Dyke sample shows U zoning and the associated 206Pb zoning is affected by alpha recoil. A forward modelling approach was used where 206Pb redistribution functions were determined for a range of possible alpha recoil distances and synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions. These can be compared to the observed 206Pb/238U profile. A complication is that the 400 nm range of sampling is lower than the range of possible alpha recoil redistribution effects. In order to get a realistic match to the observed 206Pb/238U profile, it was necessary to extrapolate the observed zoning as an oscillatory pattern. This gives a best estimate for the average alpha recoil distance of about 40 nm. [ABSTRACT FROM AUTHOR]

The Butedale pluton, a ca. 100 km long compositionally zoned batholith, is part of the Coast Plutonic Complex that extends the length of the Canadian Pacific coast. Its age and thermal history are relevant to the Butedale pluton's role as a test case of the Baja-BC hypothesis, as paleomagnetic evidence suggests that it may have formed thousands of kilometres to the south and moved northward along what is now the Coast Shear Zone. High-resolution U-Pb (chemical abrasion --thermal ionization mass spectrometry (CA-TIMS), zircon) analysis of rocks across the width of the Butedale pluton indicates that it is actually made up of at least two distinct magmatic events that formed the West Butedale pluton (ca. 95 Ma) and the East Butedale pluton (ca. 85 Ma). The East Butedale pluton was reheated by the emplacement of a younger adjacent pluton, which may have caused partial Pb loss and resulting excess scatter of [sup.206]Pb/[sup.238]U zircon ages within individual samples. The West Butedale pluton may be the same age as, and part of, the nearby Ecstall pluton, thereby doubling the length of the Ecstall pluton to ca. 200 km. Single-grain, high-precision U-Pb analysis of zircon reveals previously unknown complexity and detail of emplacement and thermal history in the Butedale plutons. Le pluton de Butedale, un batholite a composition zonee, d'une centaine de kilometres de long, fait partie du Complexe Plutonique Ccitier qui s'etend sur toute la longueur de la cote Pacifique du Canada. Son age et son historique thermique sont pertinents au role du pluton de Butedale en tant que cas type de l'hypothese Baja-BC, puisque, selon des evidences paleomagnetiques, il pourrait avoir ete forme a des milliers de kilometres au sud et avoir migre vers le nord le long de ce qui est maintenant la zone de cisaillement de la Cote. Des analyses U-Pb a haute resolution (CA-TIMS, zircon) de roches a travers toute la largeur du pluton de Butedale indiquent qu'il est en realite compose d'au moins deux eveenements magmatiques distincts qui ont forme; le pluton de Butedale Ouest (~95 Ma) et le pluton de Butedale Est (~ 85 Ma). Le pluton de Butedale Est a ete rechauffe par la mise en place d'un pluton adjacent plus jeune, ce qui a possiblement cause une perte partielle du Pb et resulte en une dispersion exceedentaire des ages 206Pb/[sup.238]U sur zircons a l'inteerieur d'eechantillons individuels. Le pluton de Butedale Ouest pourrait avoir le meme age que le pluton Ecstall avoisinant et pourrait meme en faire partie, doublant ainsi la longueur du pluton Ecstall a environ 200 km. Des analyses U-Pb de haute precision sur des grains uniques de zircon revelent une complexite jusqu'alors inconnue et des details sur la mise en place et l'historique des plutons de Butedale., Introduction The Butedale pluton is located in western British Columbia, Canada, and is part of the Coast Plutonic Complex of metamorphic and plutonic rocks (also known as the Coast Mountains [...]

U-Pb baddeleyite ages and paleomagnetic poles obtained for dykes on Devon Island and Ellesmere Island in the Canadian Arctic and the Thule region of Greenland show that they are associated with the Franklin magmatic event. This study is the only one devoted to Franklin igneous rocks where a primary paleomagnetic remanente and U-Pb age have been obtained from the same rocks. Ages from this study range from 721 to 712 Ma, but paleomagnetic directional data show no clear age progression. The paleomagnetic poles from each of the two regional subsets are significantly different at the 95% confidence level from paleomagnetic results previously published for the Franklin event in the CanadianShield. The difference in the pole locations can be accounted for, to first approximation, by a simple model of early Cenozoic block rotations among the North American plate, Greenland, and a hypothesized ancient microplate comprising Ellesmere, Devon, Cornwallis, and perhaps Somerset islands. A new grand-mean paleopole for the Franklin event, including restoration of Greenland and the proposed 'Ellesmere microplate' to North America, is located at (8.4°N, 163.8°E, [A.sub.95] = 2.8°, N = 78 sites) and is a key pole for Neoproterozoic supercontinent reconstructions. Des ages U-Pb, determines sur de la baddeleyite, et des poles paleomagnetiques obtenus de dykes des files de Devon et d'Ellesmere, dans fArctique canadien, et de la region de Thule au Groenland, montrent que ces dykes sont associes a fevenement magmatique de Franklin. La presente etude est la seine consacree aux roches ignees de Franklin ou une remanente paleomagnetique primaire et des ages U-Pb ont ete obtenus des memes roches. Les ages dans cette etude varient de 721 a 712 Ma, mais les donnees sur les directions paleomagnetiques ne montrent aucune progression Claire de l'age. Les poles paleomagnetiques de chacun des deux sous-ensembles regionaux different de maniere significative, au niveau de confdance 95 %, des resultats paleomagnetiques publies anterieurement pour l'evenement de Franklin dans le Bouclier canadien. La difference d'emplacement des poles peut etre expliquee, en une premiere approximation, par un modele simple de rotations de blocs, au Cenozoique precoce, entre la plaque Nord-americaine, le Groenland et une ancienne plaque hypothetique qui comprenait les files d'Ellesmere, de Devon, de Cornwallis et peut-etre de Somerset. Un nouveau paleopole de grande moyenne pour l'evenement Franklin, incluant la restoration du Groenland et de la << microplaque d'Ellesmere >> proposee a l'Amerique du Nord, est situe a (8,4[degre]N, 163,8[degre]E, [A.sub.95] = 2,8[degre], N = 78 sites) et constitue le pole cle pour les reconstructions du super continent au Neoproterozoique. [Traduit par la Redaction], Introduction Precisely dated rocks with well-defined paleomagnetic poles are scarce in the Proterozoic record, yet are critical for reconstructing the past positions of continents and correlating what are today the [...]

The north-south-trending Clarence Head dyke swarm, located on Devon and Ellesmere Islands in the Canadian High Arctic, has a trend orthogonal to that of the Neoproterozoic Franklin swarm that surrounds it. The Clarence Head dykes are dated by the U-Pb method on baddeleyite to between 716 ± 1 and 713 ± 1 Ma, ages apparently younger than, but within the published age range of, the Franklin dykes. Alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages, but a comparison of the U-Pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the Clarence Head dykes are indeed a distinctly younger event within the period of Franklin magmatism. The Clarence Head dykes represent a large swarm tangential to, and cogenetic with, a giant radiating dyke swarm ~800 km from the indicated source. The preferred mechanism for the emplacement of the Clarence Head dykes is the exploitation of concentric zones of extension around a depleting and collapsing plume source. While the paleomagnetism of most Clarence Head dykes agrees with that of the Franklin dykes, two dykes have anomalous remanence directions, interpreted to be a chemical remanent magnetization carried by pyrrhotite. The pyrrhotite was likely deposited from fluids mobilized southward from the Devonian Ellesmerian Orogeny to the north that used the interiors of the dykes as conduits and precipitated pyrrhotite en route. L'essaim de dykes Clarence Head, a tendance nord-sud, situe sur les iles Devon et Ellesmere dans le Grand Nord canadien, a une tendance orthogonale a l'essaim Franklin (Neoproterozoique) qui l'entoure. Les dykes Clarence Head ont ete dates par methode U-Pb sur de la baddeleyite et ils seraient ages entre 716 ± 1 et 713 ± 1 Ma, des ages apparemment plus jeunes que la plage d'age des dykes Franklin, mais a l'interieur des dates publiees pour ces derniers. Le recul alpha dans la baddeleyite pourrait constituer une explication possible pour les differences d'age, mais une comparaison des ages U-Pb sur des grains de taille semblable des deux essaims suggere que les distances de recul dans la baddeleyite soient inferieures a celles dans le zircon et que les dykes de Clarence Head constituent en effet un evenement plus jeune a l'interieur de la periode de magnetisme Franklin. Les dykes Clarence Head representent un vaste essaim qui est coge netique avec, et tangent a, un gigantesque essaim de dykes qui rayonne sur une distance d'environ 800 km de la source indiquee. Le mecanisme preferentiel pour la mise en place des dykes Clarence Head est l'exploitation de zones concentriques d'extension autour d'un panache source qui s'appauvrit et qui s'effondre. Bien que le paleomagnetisme de la plupart des dykes Clarence Head concorde avec celui des dykes Franklin, deux dykes ont des directions remanentes anomales, interpre tees comme une magnetisation chimique remanente portee par la pyrrhotite. La pyrrhotite a probablement ete depose e a partir de fluides mobilises vers le sud a partir de l'orogene ellesmerien (Devonien) au nord; ces fluides auraient utilise l'interieur des dykes comme conduits et auraient precipite la pyrrhotite en chemin. [Traduit par la Redaction], Introduction Giant mafic dyke swarms are common worldwide (Ernst et al. 1995), and many have radiating patterns of emplacement that appear to indicate their source at the focus, often considered [...]

The lower crustal domain of the Ivrea‐Verbano Zone (NW Italy) hosts five ~300‐m‐wide pipe‐like ultramafic intrusions that are metasomatized and exhibit Ni‐Cu‐PGE sulphide mineralization. To better constrain the role of metasomatism in the ore genesis, we studied the best‐preserved pipe at Valmaggia which was emplaced 249 Myrs ago. Phlogopite 40Ar/39Ar analyses show that the pipe was infiltrated by metasomatic fluids derived from the subcontinental lithospheric mantle (SCLM) in two pulses at ~208 Ma and ~189 Ma which introduced sulphides into the pipe. Consequently, the pipe repeatedly acted as a preferred path for mass transfer from the SCLM into the lower crust over >60 Myrs (i.e., emplacement to second metasomatic pulse). Uplifted block margins, such as the Ivrea‐Verbano Zone, are potentially important exploration targets for magmatic sulphides. We argue that exploration strategies should focus on structures such as pipes that can focus metasomatic agents during ascent through the lithosphere. [ABSTRACT FROM AUTHOR]