The “laser ablation split stream” (LASS) technique is a powerful tool for mineral‐scale isotope analyses and in particular, for concurrent determination of age and Hf isotope composition of zircon. ...Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus, the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a single‐collector (for U‐Pb age determination) and a multicollector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC‐ICPMS). The ability of the LASS for concurrent Hf + age technique to extract meaningful Hf isotope compositions in isotopically zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components can be a significant cause of excess scatter in Hf isotope analyses and demonstrate that the LASS approach offers a robust solution to these issues. The veracity of the approach is demonstrated by accurate analyses of 10 reference zircons with well‐characterized age and Hf isotopic composition, using laser spot diameters of 30 and 40 µm. In order to expand the database of high‐precision Lu‐Hf isotope analyses of reference zircons, we present 27 new isotope dilution‐MC‐ICPMS Lu‐Hf isotope measurements of five U‐Pb zircon standards: FC1, Temora, R33, QGNG, and 91500.
Key Points
Separate analyses of age+Hf isotopes leads to spurious results in zoned zircons
The LASS method is ideal for in situ analysis of complex zircons
Twenty-seven new Lu‐Hf isotope dilution analyses of reference zircons are given
The giant Salobo copper-gold deposit is located in the Carajás Province, Amazon Craton. Detailed drill core description, petrographical studies, and U-Pb SHRIMP IIe and LA-ICP-MS geochronology ...unravel its evolution regarding the host rocks, hydrothermal alteration and mineralization. Within the Cinzento Shear Zone, the deposit is hosted by orthogneisses of the Mesoarchean Xingu Complex (2950 ± 25 and 2857 ± 6.7 Ma) and of the Neoarchean Igarapé Gelado suite (2763 ± 4.4 Ma), which are crosscut by the Old Salobo granite. Remnants of the Igarapé Salobo metavolcanic-sedimentary sequence are represented by a quartz mylonite with detrital zircon populations (ca. 3.1–3.0, 2.95, 2.86, and 2.74 Ga). High-temperature calcic-sodic hydrothermal alteration (hastingsite-actinolite) was followed by silicification, iron-enrichment (almandine-grunerite-magnetite), tourmaline formation, potassic alteration with biotite, copper-gold ore formation, and later Fe-rich hydrated silicate alteration. Myrmekitic bornite-chalcocite and magnetite comprise the bulk of copper-gold ore. All these alteration assemblages have been overprinted by post-ore hematite-bearing potassic and propylitic alteration, which is also recognized in the Old Salobo granite. In the central zone of the deposit the mylonitized Igarapé Gelado suite rocks yield an age of 2701 ± 30 Ma. Zircon ages of 2547 ± 5.3 and 2535 ± 8.4 Ma were obtained for the Old Salobo granite and for the high-grade copper ore, respectively. A U-Pb LA-ICP-MS monazite age (2452 ± 14 Ma) from the copper-gold ore indicates hydrothermal activity and overprinting in the Siderian. Therefore, a protracted tectono-thermal event due to the reactivation of the Cinzento Shear Zone is proposed for the evolution of the Salobo deposit.
New U-Pb zircon ages are reported for granulite facies crustal xenoliths brought to the surface by mafic lavas in the Snake River Plain. All samples yield Meso-to-Neoarchean ages (2.4–3.6 Ga) that ...significantly expand the known extent of the Archean Wyoming Craton at least as far west as the west-central Snake River Plain. Most zircon populations indicate multiple growth episodes with complexity increasing eastward, but they bear no record of major Phanerozoic magmatic episodes in the region. To extrapolate this work further west to the inferred craton boundary, zircons from southwestern Idaho batholith granodiorites were also analyzed. Although most batholith zircons record Cretaceous formation ages, all samples have zircons with inherited cores—with some recording Proterozoic ages (approaching 2 Ga). These data enhance our perspectives regarding lithosphere architecture beneath southern Idaho and adjacent areas and its possible influence on Cenozoic magmatism associated with the Snake River Plain–Yellowstone “melting anomaly”.
The Omineca Belt between Nelson and Creston in southeastern British Columbia was affected by overlapping pulses of Mesozoic magmatism, metamorphism, and deformation. U–Pb geochronological data from ...zircon and monazite were collected by laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) to constrain the timing of these events. The Porcupine Creek stock (162.3 ± 1.3 Ma) intruded across folds and fabrics associated with the earliest phase of regional deformation and metamorphism (D
1
M
1
), restricting it to the Early–Middle Jurassic. The Jurassic structures are overprinted northwards by Early Cretaceous deformation and metamorphism (D
2
M
2
). The Baldy pluton (117.8 ± 1.2 Ma) crosscuts the regional 144–134 Ma M
2
isograds, yet was pervasively affected by the D
2
deformation, indicating that D
2
deformation outlasted M
2
metamorphism but had ceased by 111 Ma, the age of an undeformed pluton. Monazite dates from a kyanite-bearing rock in the contact aureole of the Middle Jurassic Wall stock overlap with the age of the intrusion (167 Ma), indicating a contact rather than regional origin for the kyanite. In the southeast part of the study area, three samples from the regional sillimanite zone contain monazite intergrown with sillimanite that yield dates between 80 and 69 Ma, indicating an episode of Late Cretaceous (M
3
) Barrovian metamorphism and deformation (D
3
). To the north of this domain, in an area characterized by the older D
2
M
2
deformation, a sillimanite zone schist contains two main monazite age populations, suggestive of overlapping effects of Early Cretaceous and Late Cretaceous metamorphic episodes.
Previous studies have shown that radiometric dating of fossils can be complicated by post-fossilisation alteration. A key challenge to obtaining geologically meaningful fossilisation dates in ...bioapatite is identifying regions that have remained relatively undisturbed since the time of fossilisation. In order to determine whether a geochemical indicator can be used to identify least-disturbed regions, we combine in situ trace element analysis and U-Pb geochronology of fossil teeth from the Arroyo Chijuillita Member, New Mexico, and the Dinosaur Park Formation, Alberta. The reliability of these U-Pb dates is supported by good agreement between the isotope dilution thermal ionization mass spectrometry (61.5 ± 1.6 Ma) and laser ablation ICP-MS (64.2 ± 2.4 Ma) dates for a targeted tooth region. Of fourteen teeth analyzed from these units, two record dates within uncertainty of the depositional age: one has a “diffusion” concave upward cross-sectional yttrium profile, the other a concave downward profile reflective of trace element loss. It was found for teeth from both units that smaller yttrium concentration ranges are correlated with younger U-Pb dates, likely the result of post-fossilisation alteration. A yttrium screen based on this relationship is proposed, and is found to be a better indicator of post-fossilisation open system behaviour in the samples analyzed than other geochemical features thought to be produced during fossilisation. This screen provides a new tool for identifying fossil material least disturbed since fossilisation, facilitating the use of bioapatite to date the time of diagenesis.
•Mesoproterozoic, Cryogenian and Ediacaran strata identified at the Igarka Uplift.•Age of the Ediacaran strata confirmed by Pb-Pb and C-isotope data from limestones.•Intraplate rifting at >715 Ma ...correlates to coeval LIP in Laurentia and Siberia.•U-Pb detrital zircon ages evidence for proximal Neoproterozoic source.
New geochronological, geochemical and carbon isotope data from the Igarka Uplift constrain the age and reveal geodynamic evolution of the northwestern Siberian Platform in the latest Mesoproterozoic and Neoproterozoic. Dolostones of the Medvezhy Formation, the oldest part of the sedimentary succession, correspond to the latest Mesoproterozoic (∼1000 Ma) according to stable isotopic composition. Overlaying volcano-sedimentary Ludy, Gubinskaya and Igarka formations are no older than 715 Ma according to detrital zircon ages and carbon isotope data. Chemical composition of the Igarka Formation and synchronous mafic intrusions of the Gubinskaya Formation suggest a single magma source developed in an intraplate rifting environment. The Ediacaran Chernaya Rechka and Izluchina formations unconformably cover both meso- and mid-Neoproterozoic sequences. The age of the Ediacaran units are confirmed by the Pb-Pb isochron age of 610 ± 50 Ma and large positive C-isotope anomaly in the Chernaya Rechka limestones. Abundant Neoproterozoic (950–800 Ma) detrital zircons in the studied Neoproterozoic sedimentary rock units points to the existence of a hypothetic large Neoproterozoic orogenic belt on the northeastern periphery of the Siberian Platform unexposed nowadays. The pre-Ediacaran rift trough of the Igarka Uplift and other similar structures, mark extensional environments along the present western margin of the Siberian platform in the mid-Neoproterozoic. These structures are coeval to a large igneous province (ca. 720 Ma) at the northern Laurentia associated with the Rodinia breakup.
The Acasta Gneiss Complex (AGC) is a ∼2,400 km2 Hadean‐Mesoarchean terrane that contains the oldest known zircon‐bearing rocks on Earth. Despite its importance for early Earth geology, only a small ...fraction (∼50 km2) of the AGC has been mapped in detail. We use detrital zircon grains from late Pleistocene eskers that transect the Complex to approximate the lateral extent and relative proportions of diverse‐aged ancient rock units within the vast, little‐studied parts of the AGC. The esker sediment was derived from glacially eroded bedrock and therefore zircon grains can serve as a proxy for the ages of exposed bedrock in the study area. U‐Pb dates on ∼2400 detrital zircons from coarse and fine grain‐size fractions along the esker transect yield age distributions that coincide with ages of regionally mapped AGC bedrock, the adjacent Wopmay Orogen, and granitoids of the Slave craton. Based on detrital zircon age distributions and new reconnaissance‐scale mapping, we infer that 3.37 Ga granitoids are a volumetrically significant component of the unmapped AGC. Esker zircons older than 3.7 Ga are present in most esker samples but at low abundance, which suggests that Eoarchean and Hadean rocks are a volumetrically subordinate component of the AGC. However, the data also suggest that unmapped rocks at least as old as 3.95 Ga are present toward the inferred eastern limit of the AGC, a location where Eoarchean rocks have not been recognized previously.
Plain Language Summary
The Acasta Gneiss Complex is a small region of bedrock in the Northwest Territories, Canada, which contains the oldest precisely dated rocks currently known on Earth (4.02 billion years old). The study of this area is crucial to understanding how terrestrial crust first formed on our planet. Despite its significance, the region is poorly mapped with the exception of the initial discovery site. To overcome these difficulties, we sampled glacial sediment (sand and gravel) derived from local bedrock during glaciation, then extracted and dated zircon crystals to estimate relative proportions of diversely‐aged bedrock in the area. Our findings indicate the majority of unmapped bedrock is roughly 3.37 billion years old. Our findings also indicate that small vestiges of bedrock as old as 3.95 billion years old remain yet to be discovered in the unmapped area east of the region.
Key Points
U‐Pb dates from detrital zircons indicate that a large area of the Acasta Gneiss Complex (AGC) is underlain by 3.37 billion years old granite
The provenance of glacially derived sediment in the western Slave craton indicate a transport distance of less than 50 km
Detrital zircons older than 3.9 billion years indicate presence of ancient crust up‐ice from the few mapped AGC outcrops
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