U–Th–Pb zircon geochronology by ID-TIMS, SIMS, and laser ablation ICP-MS: recipes, interpretations, and opportunities

The chronologic record encoded in accessory minerals, based on the radioactive decay of U and Th, is indispensable to extract quantitative process rates over timescales encompassing Earth's evolution from the Hadean to the Holocene, and extending from terrestrial to extra-terrestrial realms. We have essentially three different U–Pb dating tools at hand, a high-precision, whole-grain bulk technique (isotope-dilution thermal ionization mass spectrometry, ID-TIMS), and two high-spatial resolution but less precise in-situ techniques (secondary ion mass spectrometry, SIMS, and laser ablation inductively-coupled plasma mass spectrometry, LA-ICP-MS), all of which are predominantly applied to the mineral zircon. All three have reached a technological and methodological maturity in data quality and quantity, but interpretational differences, which are often common (albeit at different temporal and spatial scales) to all isotopic dating techniques, remain largely unresolved. The choice to use one of these techniques should be governed by the scientific question posed, such as (1) the duration of the geological process to be resolved; (2) the size and abundance of the material to be analyzed; (3) the complexity of the sample material and of the geological history to be resolved; and (4) the number of dates needed to address the question. Our compilation demonstrates that, ultimately, the highest confidence geochronological data will not only result from the optimal choice of appropriate analysis technique and the accurate treatment of analytical and interpretational complexities, but also require comprehensive sample characterization that employs the full gamut of textural (e.g., cathodoluminescence, charge contrast imaging, electron backscatter diffraction) and compositional (e.g., trace element, stable and radiogenic isotope) analysis

Synchronous N-S and E-W extension at the Tibet-to-Himalaya transition in NW Bhutan

Despite ~50 Myr of continuous continent-continent collision, contractional structures in the Himalayan-Tibetan orogen are today limited to the northern and southern margins of the system, while extension dominates much of the interior. On the Tibetan Plateau, Cenozoic E-W extension has been accommodated by strike-slip faults and extensional grabens, while N-S extension at the Tibet-to-Himalaya transition has been accommodated by the South Tibetan fault system (STFS). The genetic relationship between N-S and E-W extension is disputed, although age constraints indicate temporal overlap of at least 7 Myr. In NW Bhutan the two intersect where the STFS basal detachment is cut by the Yadong cross structure (YCS), an extensional half graben that provides a rare opportunity to constrain relative timings. We report U-Pb zircon dates from four STFS footwall leucogranites consistent with episodic magmatism during the middle-late Miocene and in situ U(-Th)-Pb monazite and xenotime dates from three metasedimentary rocks ranging from late Oligocene to middle Miocene. We suggest that amphibolite facies footwall metamorphism was ongoing at the time the basal STFS detachment initiated as a ductile structure in the middle-late Miocene. Late-stage granitic intrusions may reflect footwall melting during extensional exhumation along the STFS, but post-metamorphic and post-intrusion fabrics suggest that most displacement occurred after emplacement of the youngest granites. Some of the oldest YCS-related fabrics are found in a deformed 14 Ma leucogranite, implying middle Miocene ductile deformation. This observation, along with evidence for subsequent brittle YCS deformation, suggests that N-S and E-W extensional structures in the area had protracted and overlapping deformation histories

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

Many rare metals used today are derived from granitic pegmatites, but debate continues about the origin of these rocks. It is clear that some pegmatites represent the most highly fractionated products of a parental granite body, whilst others have formed by anatexis of local crust. However, the importance of these two processes in the formation of rare-metal pegmatites is not always evident. The Lewisian Gneiss Complex of NW Scotland comprises Archaean meta-igneous gneisses which were highly reworked during accretional and collisional events in the Palaeoproterozoic (Laxfordian orogeny). Crustal thickening and subsequent decompression led to melting and the formation of abundant granitic and pegmatitic sheets in many parts of the Lewisian Gneiss Complex. This paper presents new petrological, geochemical and age data for those pegmatites and shows that, whilst the majority are barren biotite-magnetite granitic pegmatites, a few muscovite-garnet (rare-metal) pegmatites are present. These are mainly intruded into a belt of Palaeoproterozoic metasedimentary and meta-igneous rocks known as the Harris Granulite Belt. The rare-metal pegmatites are distinct in their mineralogy, containing garnet and muscovite, with local tourmaline and a range of accessory minerals including columbite and tantalite. In contrast, the biotite-magnetite pegmatites have biotite and magnetite as their main mafic components. The rare-metal pegmatites are also distinguished by their bulk-rock and mineral chemistry, including a more peraluminous character and enrichments in Rb, Li, Cs, Be, Nb and Ta. New U-Pb ages (c. 1690–1710 Ma) suggest that these rare-metal pegmatites are within the age range of nearby biotite-magnetite pegmatites, indicating that similar genetic processes could have been responsible for their formation. The peraluminous nature of the rare-metal pegmatites strongly points towards a metasedimentary source. Notably, within the Lewisian Gneiss Complex, such pegmatites are only found in areas where a metasedimentary source is available. The evidence thus points towards all the Laxfordian pegmatites being formed by a process of crustal anatexis, with the formation of rare-metal pegmatites being largely controlled by source composition rather than solely by genetic process. This is in keeping with previous studies that have also challenged the widely accepted model that all rare-metal pegmatites are formed by fractionation from a parental granite, and raises questions about the origin of other mineralised pegmatites worldwide

Development of a correlated Fe‐Mn Crust stratigraphy using Pb and Nd isotopes and its application to paleoceanographic reconstruction in the Atlantic

Eight ferromanganese crust samples spanning the complete depth range of Tropic Seamount in the north‐east Atlantic were analysed for Pb and Nd isotopes to reconstruct water mass origin and mixing over the last 75 Ma. Pb isotopes were determined by LA‐MC‐ICP‐MS, which enables the rapid production of large, high spatial‐resolution datasets. This makes it possible to precisely correlate stratigraphy between different samples, compare contemporaneous layers, and create a composite record given the abundance of hiatuses in crusts. Pb and Nd isotope data show the influence of various oceanic and continental end‐members in the north‐east Atlantic Ocean. This reflects its evolution from a restricted, isolated basins in the Late Cretaceous with influxes from the Tethys Ocean, to an increasingly well‐mixed, large‐scale basin, with a dominant Southern Ocean signature until the Miocene. Less‐radiogenic Nd isotope signatures suggest Labrador Sea Water influenced the north‐east Atlantic basin as early as 17‐15 Ma, flowing through a northern route such as the Charlie‐Gibbs Fracture Zone. Pb and Nd isotopes highlight the increasing influence of Saharan aeolian dust input about 7 Ma, imparting a less‐radiogenic excursion to the binary mixing between North Atlantic water masses and riverine discharge from West and Central Africa. This highlights the influence of aeolian dust input on the open ocean Pb and Nd budget, and supports an early stage of North African aridification in the Late Miocene. This signature is overprinted about 3 Ma to the present by a strong North Atlantic Deep Water signature following the onset of Northern Hemisphere glaciation

Modelling silicon supply during the Last Interglacial (MIS 5e) at Lake Baikal

Limnological reconstructions of primary productivity have demonstrated its response over Quaternary timescales to drivers such as climate change, landscape evolution and lake ontogeny. In particular, sediments from Lake Baikal, Siberia, provide a valuable uninterrupted and continuous sequence of biogenic silica (BSi) records, which document orbital and sub-orbital frequencies of regional climate change. We here extend these records via the application of stable isotope analysis of silica in diatom opal (δ30Sidiatom) from sediments covering the Last Interglacial cycle (Marine Isotope Stage [MIS] 5e; c. 130 to 115 ka BP) as a means to test the hypothesis that it was more productive than the Holocene. δ30Sidiatom data for the Last Interglacial range between +1.29 to +1.78‰, with highest values between c. 127 to 124 ka BP (+1.57 to +1.78‰). Results show that diatom dissolved silicon (DSi) utilisation, was significantly higher (p=0.001) during MIS 5e than the current interglacial, which reflects increased diatom productivity over this time (concomitant with high diatom biovolume accumulation rates [BVAR] and warmer pollen-inferred vegetation reconstructions). Diatom BVAR are used, in tandem with δ30Sidiatom data, to model DSi supply to Lake Baikal surface waters, which shows that highest delivery was between c. 123 to 120 ka BP (reaching peak supply at c. 120 ka BP). When constrained by sedimentary mineralogical archives of catchment weathering indices (e.g. the Hydrolysis Index), data highlight the small degree of weathering intensity and therefore representation that catchment-weathering DSi sources had, over the duration of MIS 5e. Changes to DSi supply are therefore attributed to variations in within-lake conditions (e.g. turbulent mixing) over the period, where periods of both high productivity and modelled-DSi supply (e.g. strong convective mixing) account for the decreasing trend in δ30Sidiatom compositions (after c. 124 ka BP)

In-situ zircon U–Pb, oxygen and hafnium isotopic evidence for magma mixing and mantle metasomatism in the Tuscan Magmatic Province, Italy

In this study, we have used in-situ U–Pb, Hf and O isotopic analyses of zircon grains to gain insights into both magmatic processes and duration of magmatism in igneous rocks from the Tuscan Magmatic Province (0.1–9 Ma), Italy. Three plutonic centres have been investigated (Monte Capanne and Porto-Azzuro monzogranites in Elba and the Giglio monzogranite) as well as Capraia, the only volcanic centre in the Tuscan Archipelago. New ion microprobe zircon U–Pb data reveal a continuum of plutonic activity in Elba over 2 Ma (8.3–6.3 Ma), with coeval volcanic activity in Capraia (7.1–7.6 Ma), and plutonic activity resuming in Giglio (5.5 Ma) after a gap of 1 Ma. From these zircon data we also show that construction of the Monte Capanne pluton (Elba) may have occurred over a period of c. 0.5 Ma. A significant range of both 176Hf/177Hf (determined by LA–MC-ICPMS) and δ18O (determined by ion microprobe) in zircon (~ 7 epsilon Hf units and ~ 5‰, respectively) is present, which, together with zircon morphology and trace element data (Gagnevin et al., 2010), emphasises the importance of mixing and replenishment involving magma batches with both metaluminous and peraluminous affinities. Inherited and xenocrystic zircons also occur, but are scarce. These have a wide range of 176Hf/177Hf and δ18O values, further emphasising that a variety of crustal components has contributed to the genesis of the Tuscan magmas, either as contaminants or magma sources. While mixing undoubtedly occurred between mafic (metaluminous) and felsic (peraluminous) magmas, the range of Hf and O isotopic data suggests a diversity within the peraluminous component. The unradiogenic Hf composition (εHf(t) 6‰) of the inferred mantle-derived component (represented by Capraia volcanism, and at least in part, lamproitic in composition) strongly supports the idea that the mantle source involved in Tuscan magmatism was severely modified by subduction-related, crustal-derived metasomatic fluids

U–Pb dating and Sm–Nd isotopic analysis of granitic rocks from the Tiris Complex : new constaints on key events in the evolution of the Reguibat Shield, Mauritania

The Reguibat Shield of N Mauritania and W Algeria represents the northern exposure of the West African Craton. As with its counterpart in equatorial West Africa, the Leo Shield, it comprises a western Archaean Domain and an eastern Palaeoproterozoic Domain. Much of the southern part of the Archaean Domain is underlain by the Tasiast-Tijirit Terrane and Amsaga Complex which, along with the Ghallaman Complex in the northeast, preserve a history of Mesoarchaean crustal growth, reworking and terrane assembly. This study presents new U–Pb and Sm–Nd data from the Tiris Complex, a granite–migmatite–supracrustal belt, that intervenes between these units and the Palaeoproterozoic Domain to the northeast. New U–Pb geochronology indicates that the main intrusive events, broadly associated with formation of dome-shaped structures, occurred at around 2.95–2.87 Ga and 2.69–2.65 Ga. This study also recognises younger regional metamorphism and intrusion of syn-tectonic granites located within major shear zones at around 2.56–2.48 Ga. Sm–Nd depleted mantle model ages indicate that magmatism involved recycling of crustal source components older than at least 3.25 Ga in age. Comparison with other Archaean units in the Reguibat Shield and in the Leo Shield illustrate the importance of deformation and tectonism of a regional greenstone-sedimentary province prior to around 3.00 Ga as well as subsequent magmatic episodes broadly equivalent in age to those in the Tiris Complex

Polymetamorphism in the NE Shackleton Range, Antarctica : constraints from petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS dating

Metapelitic rock samples from the NE Shackleton Range, Antarctica, include garnet with contrasting zonation patterns and two age spectra. Garnet porphyroblasts in K-rich kyanite-sillimanite- staurolite-garnet-muscovite-biotite schists from Lord Nunatak show prograde growth zonation, and give Sm-Nd garnet, U-Pb monazite and Rb-Sr muscovite ages of 518 +/- 5, 514 +/- 1 and 499 +/- 12 Ma, respectively. Geothermobarometry and P-T pseudo-section calculations in the model system CaO-Na2O-K2O- TiO2-MnO-FeO-MgO-Al2O3-SiO2-H2O are consistent with garnet growth during prograde heating from 540degreesC/7 kbar to 650degreesC/7.5 kbar, and partial resorption during a subsequent P-T decrease to <650degreesC at <6 kbar. All data indicate that rocks from Lord Nunatak were affected by a single orogenic cycle. In contrast, garnet porphyroblasts in K-poor kyanite-sillimanite- staurolite-garnet-cordierite-biotite-schists from Meade Nunatak show two growth stages and diffusion-controlled zonation. Two distinct age groups were obtained. Laser ablation plasma ionization multicollector mass spectrometry in situ analyses of monazite, completely enclosed by a first garnet generation, yield ages of c. 1700 Ma, whereas monazite grains in open garnet fractures and in most matrix domains give c. 500 Ma. Both age groups are also obtained by U-Pb thermal ionization mass spectrometry analyses of matrix monazite and zircon, which fall on a discordia with lower and upper intercepts at 502 +/- 1 and 1686 +/- 2 Ma, respectively. Sm-Nd garnet dating yields an age of 1571 +/- 40 Ma and Rb-Sr biotite analyses give an age of 504 +/- 1 Ma. Integrated geochronological and petrological data provide evidence that rocks from Meade Nunatak underwent a polymetamorphic Barrovian-type metamorphism: (1) garnet 1 growth and subsequent diffusive garnet annealing between 1700 and 1570 Ma; (2) garnet 2 growth during the Ross Orogeny at c. 500 Ma. During the final orogenic event the rocks experienced peak P-T conditions of about 650degreesC/7.0 kbar and a retrograde stage at c. 575degreesC/4.0 kbar

Determination of Sr isotopes in calcium phosphates using laser ablation inductively coupled plasma mass spectrometry and their application to archaeological tooth enamel

The determination of accurate Sr isotope ratios in calcium phosphate matrices by laser ablation multi-collector ICP-MS is demonstrated as possible even with low Sr concentration archaeological material. Multiple on-line interference correction routines for doubly-charged REE, Ca dimers and Rb with additional calibration against TIMS-characterised materials are required to achieve this. The calibration strategy proposed uses both inorganic and biogenic apatite matrices to monitor and correct for a 40Ca–31P–16O polyatomic present at levels of 0.3–1% of the non-oxide peak, which interferes on 87Sr causing inaccuracies of 0.03–0.4% in the 87Sr/86Sr isotope ratio. The possibility also exists for synthetic materials to be used in this calibration. After correction for interferences total combined uncertainties of 0.04–0.15% (2SD) are achieved for analyses of 13–24 μg of archaeological tooth enamel with Sr concentrations of ca. 100–500 ppm using MC-ICP-MS. In particular, for samples containing >300 ppm Sr, total uncertainties of 0.05% are possible utilising 7–12 ng Sr. Data quality is monitored by determination of 84Sr/86Sr ratios. When applied to an archaeological cattle tooth this approach shows Sr-isotope variations along the length of the tooth in agreement with independent TIMS data. The 40Ca–31P–16O polyatomic interference is the root cause of the bias at mass 87 during laser ablation ICP-MS analysis of inorganic and biogenic calcium phosphate (apatite) matrices. This results in inaccurate 87Sr/86Sr ratios even after correction of Ca dimers and doubly charged rare earth elements. This interference is essentially constant at specific ablation conditions and therefore the effect on 87Sr/86Sr data varies in proportion to changes in the Sr concentration of the ablated material. Complete elimination of this interference is unlikely through normal analytical mechanisms and therefore represents a limitation on the achievable accuracy of LA-(MC-)ICP-MS 87Sr/86Sr data without rigorous calibration to known reference materials