Calibrating chemical abrasion: Its effects on zircon crystal structure, chemical composition and U-Pb age

The present sub-permil precision of single zircon chemical abrasion, isotope-dilution, thermal ionisation mass spectrometry (CA-ID-TIMS) UePb dates often reveals age dispersions that are outside of analytical uncertainty. Interpreting these complex age distributions requires the ability to distinguish between protracted crystallization of zircon over a few 100 kyr, age bias due to radiation damage induced Pb-loss, and analytical artefacts. This is a particularly critical issue when a number of these factors occur together. To ensure geologically meaningful results, the complete eradication of Pb-loss is of paramount importance. The impact of Pb-loss can be removed by chemical abrasion (CA) applied prior to the dissolution of zircon. However, CA is an empirical approach that is used without a detailed understanding of how the temperature applied during the annealing step, or the temperature and duration of the partial dissolution step affect the radiation-damaged zones. In addition, the conditions of the CA procedures differ between laboratories making comparisons of age data problematic. This study presents an experimental approach to quantify how chemical abrasion affects the crystal structure and the chemical composition of zircon as well as its UePb age. For this experiment, we have chosen the Plešovice reference zircon, because of its known variation in trace element concentrations and especially the presence of domains rich in actinides. We performed CA experiments under different temperature-time conditions on fragmented Plešovice crystals. These were compared in respect to the changes in trace element concentration, lattice order and UePb date. The most reliable UePb results are obtained by chemically abrading Plešovice fragments at 210 °C for 12 h. Additionally, we demonstrate that the Plešovice zircon cannot be considered homogenous at the current level of precision achieved by CA-ID-TIMS dating due to a natural age variation at the ~900 kyr scale

Evaluating baddeleyite oxygen isotope analysis by secondary ion mass spectrometry (SIMS)

Two baddeleyite megacrysts were evaluated as potential reference materials (RMs) for SIMS oxygen isotope analysis, and utilized to understand and calibrate instrumental mass fractionation (IMF). A baddeleyite crystal (S0045) from the Phalaborwa carbonatite, South Africa has a mean δ18OVSMOW=+4.6 ± 0.3‰(range 0.75‰) measured using laser fluorination gas source mass spectrometry (LF-GMS) and one (S0069) from the Mogok metamorphic belt, Myanmar has δ18OVSMOW=+22.2 ± 0.4‰ (range 0.89‰). SIMS standardization utilizing these inherently heterogeneous RMs is possible by analyzing a number of crystal fragments and utilizing one of them lying at the median of the range. Metamictization, lattice orientation, and chemical composition do not appear to be significant (< 0.5‰) variables in matrix matching of RMs and unknowns. Propagation of errors while utilizing the imperfect RMs results in 10 μm diameter spot uncertainties of about±0.3‰ (2σ). SIMS oxygen isotope analysis of co-crystalline zircon and baddeleyite from the 2.2 Ga Duck Lake sill (DLS) in the Northwest Territories, Canada, yield predominant δ18OVSMOW modes of +6.0‰ and +3.2‰, respectively. This difference is consistent with preserving high-temperature isotopic equilibrium between zircon and baddeleyite. DLS baddeleyite δ18O data as a whole are negatively skewed (to 0.0‰), and interpreted to reflect low temperature, open-system behaviour. Zircon δ18O are less affected, but also show hints of the same influences of secondary alteration and oxygen isotope exchange

Enriched lithospheric mantle keel below the Scottish margin of the North Atlantic Craton: Evidence from the Palaeoproterozoic Scourie Dyke Swarm and mantle xenoliths

The Lewisian Gneiss Complex of NW Scotland represents the eastern margin of the North Atlantic Craton. It comprises mid-late Archaean tonalite-trondhjemite-granodiorite gneisses that were metamorphosed and deformed during the Late-Archaean and Palaeoproterozoic. A major swarm of mafic-ultramafic dykes, the Scourie Dyke Swarm, was intruded at ca. 2.4–2.3 Ga during a period of extension that can be correlated across the North Atlantic Craton. The majority of dykes are doleritic, with volumetrically minor picrite and olivine gabbro suites. New major and trace element geochemical data and Re-Os isotopes indicate that the Scourie Dyke Swarm was not solely derived from a ‘typical’ asthenospheric mantle source region. The geochemical signatures of the dykes show significant negative Nb, Ta and Ti anomalies, coupled with enrichment in Th, Light Rare Earth Elements and other large ion lithophile elements. These features cannot be reproduced by simple contamination of asthenospheric sources with Lewisian granulite-facies crust. Instead they are a feature of the mantle source that produced the Scourie Dykes and may have developed during Archaean subduction episodes. Spinel lherzolite mantle xenoliths from the Isle of Lewis offer direct insight into the lithospheric mantle below this region. They display similar geochemical ‘enrichments’ and ‘depletions’ observed in the Scourie Dykes and the magma source is thus considered to reside primarily in the sub-continental lithospheric mantle (SCLM), with some potential contribution from asthenospheric melts. Platinum Group Element geochemistry and trace element modelling indicate that the dolerite dykes were formed by moderate (<15%) partial melting of the source, whilst higher degrees of partial melting led to the formation of picritic and olivine gabbro suites. Magma production was triggered by significant crustal and lithospheric extension, causing both asthenospheric and substantial lithospheric melting

Zircon petrochronology reveals the timescale and mechanism of anatectic magma formation

Igneous rocks of intermediate to acidic composition commonly exhibit considerable degrees of isotope variability preserved at the crystal and sub-crystal scale, as well as a significant U\u2013Pb age range, reflecting protracted timescales of zircon crystallization and long magma residence times. The association of high-precision U\u2013Pb zircon dates with stable and radiogenic isotope data represents a powerful tool to unravel the petrological evolution of granitic rocks, hence allowing a better understanding of the processes that led to the formation and reworking of the continental crust. In this case study, we combine high-precision U\u2013Pb dates with stable and radiogenic isotope data from zircon crystals in the Larderello\u2013Travale (Italy) shallow-level granites. These rocks are peraluminous two-mica, cordierite-bearing granites and represent pure crustal anatectic magmas, generated in a post-collisional extensional setting. As such, they are ideal candidates to investigate the timing, rates and mechanisms of melt production during anatectic magma formation, giving insights into the process of intracrustal differentiation. Magmatic zircon crystals from the Larderello\u2013Travale granites contain \u3b418O values ranging from 8.6 to 13.5\u2030 and crystals from individual samples exhibit inter- and intra-grain oxygen isotope variability exceeding 3\u2030. The analysed crystals have \u3b5Hf values ranging between 127.4 and 1212.4, with moderate, intra-sample \u3b5Hf isotope variability. All CA-ID-TIMS (chemical abration isotope-dilution thermal ionization mass spectrometry) 206Pb/238U zircon ages range from 4.5 to 1.6 Ma and suggest four pulses of magmatic activity at 3c3.6, 3.2, 2.7 and 1.6 Ma. More importantly, zircon crystals from individual samples typically exhibit an age spread as large as 300\u2013500 ka. This age dispersion suggests that most of the zircon did not crystallize at the emplacement level but in the middle crust and were subsequently recycled and juxtaposed during ascent and emplaced at shallow level. When plotted against age, stable and radiogenic isotope data suggest the co-existence of multiple and isotopically distinct magma batches produced by partial melting of different crustal domains. This requires coeval magma batches that are physically separated and evolve independently for hundreds of thousands of years before coalescing during ascent and emplacement. The involvement of multiple sources in the production of crustal anatectic magmas reflects the inherent heterogeneous nature of the continental crust and result from the interplay between the rise and evolution of the geotherms through the crust and the composition of the fertile source rocks. Finally, the isotopically diverse zircon-bearing magma batches mixed and assembled into shallow-level intrusions generated during the four major magma pulses

New evidence for the presence of the Inner Tauride Ocean: Lithological, geochronological and P-T correlations with the Tavşanlı and Afyon zones of Central Anatolia (Türkiye)

The Tavs¸anlı and Afyon zones, which are well defined in western Türkiye, represent high pressure-low temperature (HP-LT) metamorphic rocks formed in a subduction zone. The Korumaz and Hınzır mountains (northeast of Kayseri) between the Central Anatolian Crystalline Complex and the Eastern Tauride Belt are represented by a HP-LT metamorphic sequence, which is thought to have formed between the Carboniferous and the Cretaceous. Here we present 40Ar/39Ar data from white micas within the Lower-Middle Triassic calc-phyllites from Korumaz Mountain, which yielded an age of 87 ± 2 Ma. Therefore, based on new mineralogical and geochronological data we suggest that this low-grade (HP-LT) metasedimentary sequence correlates with the Afyon Zone. Tectonically emplaced on-top of the HP-LT metasedimentary units of the Korumaz and Hınzır mountains is a meta-accretionary complex consisting of serpentinite, listvenite, marble, phyllite, calc-phyllite, metabasite, and meta-plagiogranite. Petrographic and mineral chemical studies show that the metaaccretionary complex was subjected to metamorphism under the HP-LT conditions that are similar to those of the Tavs¸anlı Zone in western Türkiye. Zircon Hf isotopic compositions from the meta-plagiogranite in the Korumaz and Hınzır mountains exhibit a depleted mantle signature with positive epsilon Hf(t) values of +5.6 to +13.1, and yielded a U-Pb crystallization age of 101.14 ± 0.84 Ma, which is the oldest age determined in oceanic crustal rocks of the ophiolites from the Tauride Belt. Geochemically, the metabasites and meta-plagiogranites from the meta-accretionary complex show subduction-related characteristics. White micas from phyllites and listvenite, which dominate the matrix of the meta-accretionary complex, yielded 40Ar/39Ar metamorphic ages ranging from 78 to 60 Ma. Our new geological and geochronological data suggest that metamorphic units of the Korumaz and Hınzır mountains are the eastern continuation of the Afyon and Tavs¸anlı zones. This indicates that the Inner Tauride Ocean, the existence of which has been controversial in the literature, was indeed present between the Central Anatolian Crystalline Complex and the Eastern Tauride Belt

Diffusion and fluid-interaction in Itrongay pegmatite (Madagascar): the results of in situ Ar-Ar dating of gem-quality alkali feldspar and U-Pb dating of apatite inclusions within it

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