Stephen R. Noble

238U/235U Systematics in Terrestrial Uranium-Bearing Minerals

A Better Date Uranium-lead (U-Pb) dating, which is one of the most commonly used methods of radiometric dating for old terrestrial materials, operates by comparing the ratio of trace levels of U with its nuclear decay product Pb. This dating method, and the similar Pb-Pb method, assumes that the ratio between the two most common U isotopes (238U and 235U) is constant. By accurately measuring the 238U/235U ratio in a suite of minerals representing a range of tectonic environments, Hiess et al. (p. 1610; see the Perspective by Stirling) demonstrate that this ratio is more variable than was previously thought. The variability does not reflect any systematic bias with time, location, or temperature—suggesting that ideally 238U/235U should be determined for every sample to calculate ages. In the absence of such data, a revised 238U/235U ratio for zircon minerals could significantly modify previous age estimates using U-Pb and Pb-Pb dating techniques. Highly variable uranium isotope ratios highlight the need for a revised approach to radiometric dating. The present-day 238U/235U ratio has fundamental implications for uranium-lead geochronology and cosmochronology. A value of 137.88 has previously been considered invariant and has been used without uncertainty to calculate terrestrial mineral ages. We report high-precision 238U/235U measurements for a suite of uranium-bearing minerals from 58 samples representing a diverse range of lithologies. This data set exhibits a range in 238U/235U values of >5 per mil, with no clear relation to any petrogenetic, secular, or regional trends. Variation between comagmatic minerals suggests that 238U/235U fractionation processes operate at magmatic temperatures. A mean 238U/235U value of 137.818 ± 0.045 (2σ) in zircon samples reflects the average uranium isotopic composition and variability of terrestrial zircon. This distribution is broadly representative of the average crustal and “bulk Earth” 238U/235U composition.

Genesis of the southern Abitibi greenstone belt, Superior Province, Canada: Evidence from zircon Hf isotope analyses using a single filament technique

Hafnium isotopic analyses, together with U-Pb geochronology, have been performed on zircons from the main units of the Late Archean southern Abitibi greenstone belt in the Superior Province. A new analytical method in which Hf is loaded and run with a mixture of Ir-Mo and C on single filaments was used. The results indicate a relatively homogeneous isotopic composition of the greenstone belt, with ϵHf values of 6.3–4.2 for the 2730-2700 Ma preorogenic assemblages and 4.7-3.6 for the 2690-2675 Ma synorogenic intrusions. Hafnium isotopes and U-Pb ages indicate that the greenstone belt formed in a relatively short time interval within an ensimatic setting, with minimal interaction with older sialic crust. Primary magmas were derived from mantle sources with a depleted isotopic signature of ϵHf ≈ 5.5 ± 0.5, corresponding to published ϵNd ≈ 2.5, 87Sr86Sr ≈ 7011, 207Pb204Pb ≈ 14.4, and 206Pb204Pb ≈ 13.2. Slightly lower ϵHf values in some rocks may record moderate degrees of source heterogeneity; in the case of the alkalic plutons, the lower ϵHf can be interpreted in terms of mantle wedge contamination by subducted components isotopically similar to sedimentary rocks in the adjacent Pontiac Subprovince. The consistent, positive ϵHf signature confirms that the Archean mantle was already depleted prior to formation of 3.0–2.7 Ga Superior Province crust.

Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains

U-Pb dating of metamorphic and igneous rocks from the Hunza Valley and Baltoro regions of the Karakoram Mountains in northern Pakistan addresses the thermal and magmatic evolution of the thickened Asian plate crust before, during, and after the collision of the Kohistan arc and the Indian plate. Crustal thickening and high- temperature, sillimanite-grade metamorphism in the southern Karakoram Mountains followed the collision and accretion of the Kohistan arc during the Late Cretaceous. U-Pb ages of metamorphic monazites from sillimanite gneisses in the Hunza Valley are 63.3 ± 0.4 Ma, ca. 50–52 Ma, and 44.0 ± 2.0 Ma, and monazites from a kyanite-grade schist from the Baltoro region are 28.0 ± 0.5 Ma. Metamorphic monazites from a highly graphitic garnet + staurolite schist from the Hunza Valley yield a crystallization age of 16.0 ± 1.0 Ma. Sillimanite gneisses from the Dassu dome have magmatic zircons of 1855 ± 11 Ma, reflecting a Proterozoic continental crustal source, and metamorphic monazites of 5.4 ± 0.2 Ma. Magmatism was also sporadic; early granodiorite, monzogranite, and leucogranite dikes yield zircon, monazite, and uraninite ages of 50–52 Ma and 35.0 ± 1.0 Ma. Widespread lower crustal melting during the latest Oligocene–early Miocene culminated with emplacement of the Baltoro Plutonic Unit in the Karakoram batholith that cuts deformation fabrics in the high-grade gneisses to the south. The youngest magmatic phase dated is the 9.3 ± 0.2 Ma Sumayar leucogranite pluton. On the basis of detailed structural field studies combined with U-Pb geochronology, sillimanite-grade metamorphism was either a protracted event lasting as long as 20 m.y. (64–44 Ma) or peaked at different times within the lower crust following collision of first, the Kohistan arc, and later, the Indian plate. We also present evidence for southward propagation of peak metamorphism and postmetamorphic thrusting and folding of isograds within the past 5 m.y. Detailed geochronology shows that deformation, metamorphism, and magmatism in the middle and lower crust of the south Asian margin has been occurring within the Karakoram metamorphic complex for more than 60 m.y. Similar processes may also have affected the unplumbed depths of the south Tibetan crust.

Age of crustal melting and leucogranite formation from U-Pb zircon and monazite dating in the western Himalaya, Zanskar, India

New U-Pb data from leucogranites in the High Himalayan slab in the western Himalaya of Zanskar reveal that crustal melting occurred at 21–19.5 Ma and that anatexis occurred along the Himalaya at least from Kashmir-Zanskar to eastern Nepal at (Approx.)24–19.5 Ma. High-U monazites from leucogranitic melt pods in migmatites within the 35-km-wide sillimanite + K-feldspar zone at the deepest structural levels exposed (Umasi-la) have U-Pb ages of 20.6–19.5 Ma. Higher-level migmatitic melt pods from Shafat in the Suru valley also contain magmatic monazites with U-Pb ages of 20.8 ± 0.3 Ma. Zircons from the leucogranites contain an (Approx.)460 Ma inherited component, as do some Umasi-la monazites, showing that the protoliths were at least Ordovician in age.

Zircon U-Th-Pb geochronology by isotope dilution: Thermal ionization mass spectrometry (ID-TIMS)

ID-TIMS is the acronym for Isotope Dilution Thermal Ionization Mass Spectrometry. This refers to the addition of an isotope tracer to a dissolved sample to make a homogeneous isotopic mixture, and the measurement of isotopic composition of the mixture using a thermal ionization mass spectrometer. The method is one of the most accurate and precise methods of isotopic techniques because it is relatively insensitive to chemical yields or mass spectrometric sensitivity. It is a method very widely applied both in earth and many other areas of science involving the measurement of element or isotope concentrations and isotopic ratios. The ID-TIMS technique was first applied to the U-Th-Pb dating of zircon in the 1950s (Tilton et al. 1955, Wetherill 1956, Tilton et al. 1957), exploiting the general availability to academia of enriched uranium isotopes developed in the 1940s and 1950s related to nuclear energy research. ID-TIMS has remained the main foundation to zircon geochronology ever since, in spite of the proliferation of other analytical methodologies. In the past 50 years, many improvements have been made and they have contributed to the maturity and reliability of the method. As a method, it was effectively unchallenged until the 1980s when secondary ionization mass spectrometry (Anderson and Hinthorne 1972) was further developed and applied to zircon geochronology by W. Compston and colleagues at the Australian National University (Compston et al. 1984). The instrument developed by the ANU group (SHRIMP, or Sensitive High Resolution Ion Microprobe) and the associated measurement protocols facilitated measurement of Pb/U isotopic ratios within a small region of a single zircon grain, and it proved to be a powerful tool to address complex age structure of multi-component zircons. In the 1990s, laser ablation quadrupole ICP-MS methods came on stream and offered an alternate way to make intra-grain U-Th-Pb isotopic …

Old origin for an active mountain range: Geology and geochronology of the eastern Hindu Kush, Pakistan

Prior to accretion of the Kohistan island arc during the Late Cretaceous and final suturing of India with Asia at ca. 50 Ma, the Hindu Kush mountains along the border of Afghanistan and northwest Pakistan were situated on the active southern margin of Asia. Geology and geochronology of the eastern Hindu Kush range in Pakistan demonstrate that localized crustal melting and leucogranite intrusion took place in the Gharam Chasma area at ca. 24 Ma. More regionally developed and widespread deformation, metamorphism, and magmatism took place before collision between both India and the Kohistan island arc with Asia. Ca. 195 Ma U-Pb monazite ages on a deformed leucogranite dike from the upper Lutkho valley indicate an Early Jurassic phase of crustal melting. U-Pb monazite ages of 135–126 Ma on a staurolite schist from near Gharam Chasma are interpreted as a minimum age for staurolite-grade metamorphism. Within the Tirich Mir fault zone, pegmatite dikes crosscut the staurolite schists. U-Pb dating of uraninites from one of these pegmatite dikes reveals an age of 114 ± 2 Ma. Monazites from the same rock give ages of 125–121 Ma, possibly due to inheritance of older cores. These Jurassic–Early Cretaceous constraints on metamorphism and magmatism relate to subduction and accretion processes, perhaps including the suturing of the Karakoram and Hindu Kush terranes along the Tirich Mir fault. In general, high-temperature, low-pressure metamorphism and subduction-related granitoid magmatism in the eastern Hindu Kush suggest a high thermal gradient in an active-margin setting from Early Jurassic to Cretaceous time.

High-precision U–Pb monazite geochronology of the c. 806 Ma Grampian Shear Zone and the implications for the evolution of the Central Highlands of Scotland

Two pegmatites and mylonitic rocks from the Grampian Shear Zone yield U–Pb monazite ages of 806 ± 3 Ma, 808–9+11Ma, and 804–12+13, respectively. These confirm that shearing and pegmatite crystallization was essentially synchronous. Lower intercepts of c. 440 Ma indicate disturbance during Ordovician reworking. These new data question the concept of a single latest Proterozoic–early Palaeozoic orogeny in the Grampian terrane and the significance of the Great Glen Fault.

U-Pb zircon evidence for an extensive early Archean craton in Zimbabwe: A reassessment of the timing of craton formation, stabilization, and growth

U-Pb single-zircon analyses provide direct evidence for an enlarged early Archean craton forming the core to the present Zimbabwe craton. Virtually identical dates from the south-central Tokwe segment (3455 ± 2 Ma) and Midlands (3456 ± 6 Ma) parts of the craton strongly suggest their synchronous formation, during an event that formed a single early cratonic nucleus which we propose to call the “Sebakwe protocraton.” This is considered to underlie most of the current Zimbabwe craton. Parts of the craton are at least 3565 ± 21 Ma, a rock age reported here that represents the oldest rock dated from Zimbabwe. A ca. 3350 Ma relatively undeformed and unmetamorphosed intrusive granitic phase constrains the timing of the high-grade metamorphism and the stabilization of the protocraton. Comparison with published Re-Os data for the Zimbabwe craton strongly indicates a depleted subcontinental lithospheric mantle underlying the entire Sebakwe protocraton. Subsequent intrusive and volcanic activity from 3.0 to 2.6 Ga represents a second major period of magma genesis and crustal formation within which the predominant rocks of the exposed Zimbabwe craton were generated.

U-Pb zircon geochronology of migmatization in the northern Central Highlands: evidence for pre-Caledonian (Neoproterozoic) tectonometamorphism in the Grampian block, Scotland

The age and formation of zircons in Neoproterozoie migmatites from the northern Grampian Highlands are assessed using field observations, imaging and U-Pb isotope dilution techniques. Acicular zircons within monzogranitie leucosomes provide evidence for neocrystalline growth during localized anatexis. U-Pb zircon data show that formation of the regional gneissose fabric, melt generation and new zircon growth occurred at 840 ±11 Ma contemporaneous with peak metamorphism. These data and previously dated shear fabrics at c. 800 Ma, confirm that a Neoproterozoic tectonometamorphic event affected at least part of the metasedimentary sequence southeast of the Great Glen Fault. Data from detrital zircons indicate a Palaeoproterozoic/Grenville province, rather than Archaean, as the most likely source for the sedimentary protolith. In the absence of identifiable orogenic discontinuities within the Scottish Highlands, we question current correlations with Laurentian passive margin sequences, and the inferences that the rocks of the Grampian Block only experienced a simple Caledonian (Palaeozoic) orogenic history.

Anatomy, age and evolution of a collisional mountain belt: the Baltoro granite batholith and Karakoram Metamorphic Complex, Pakistani Karakoram

Abstract: Geological mapping and U–(Th)–Pb geochronology from the Karakoram Metamorphic Complex and Baltoro granite batholith has resulted in more detailed timing constraints on tectonic evolution of Asian crust. During and following collision and accretion of the Kohistan Arc and the Indian plate to the southern margin of Asia, crustal thickening along the Karakoram resulted in polyphase deformation, metamorphism and melting. U–Pb zircon and monazite ages show that kyanite- and sillimanite-grade metamorphism in the Baltoro region peaked during Oligocene–Early Miocene (c. 28–22 Ma) times. At structurally deeper levels sillimanite-grade migmatites of the Dassu dome have Pliocene (c. 5.4–3.5 Ma) ages. New U–Pb ages from crustal melt biotite + muscovite + garnet leucogranites from the Trango Towers, Cathedral peak and Paiyu peak across the Baltoro batholith range from 19.8 ± 0.5 Ma to perhaps as young as 14 Ma. Using data from both the Hunza and Baltoro regions we suggest that regional high-grade metamorphism was diachronous in space and time and could have lasted continuously for at least 37 Ma (c. 50–13 Ma) or as much as 50 Ma (c. 63–13 Ma) along the southern margin of the Asian plate in the Karakoram. Crustal melting along the Baltoro granite batholith spanned at least 13 Ma (26.4–13 Ma). Supplementary material: Analytical methods and description of U–(Th)–Pb data of samples are available at http://www.geolsoc.org.uk/SUP18382.