In this paper we present the fundamentals for a protocol for optimizing isotopic determination of lead for comparison of man-made glass by use of laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Comparison of objects in order to establish a possible common origin is relevant within different disciplines, such as forensic sciences and archaeometry. Measurement of isotope ratios (IR) of lead is a method widely used by geochemists in order to establish the origin and evolution of minerals and rocks. In archaeometry, lead isotopes are used to determine the provenance of artefacts. Lead isotope ratios have shown potential not only for provenance determination, but also for comparison of different objects. Laser ablation as a sample introduction method to inductively coupled plasma mass spectrometry (LA-ICP-MS) is an established method in the geosciences, where it is used both for concentration and isotope ratio analysis of solids, but has still not obtained the same position in forensic science and archaeometry. The elegance of laser ablation as a sample introduction system is due to the simplicity of sample preparation and high spatial resolution. This spatial resolution provides advantages over methods where the sample has to be dissolved or melted in larger amounts. In order to outline a suitable protocol for analysis of lead isotope ratios in glass by LA-ICP-MS, we have followed a threefold approach. Firstly, we describe the influence of laser conditions on the Pb-isotope ratios obtained for low-lead glasses such as SRM NIST 610–614. Secondly, we evaluate the influence of the selection of detectors (ion counters vs. Faraday detectors) on the reliability of the final result. Thirdly, we discuss the phenomenon of fractionation and instrumental mass discrimination for lead during analysis.
A precise U-Pb radiometric age of 1171±5 Ma has been obtained from zircons from a pegmatite in the Nunarsuit (previously spelt Nunarssuit) complex, Gardar Province, South Greenland. This age is slightly older than a corresponding Rb-Sr isochron determination. Since Nunarsuit is believed to be among the youngest Gardar centres, this radiometric age date more closely delimits the end of magmatism in the Gardar rift province. A comparison of our data with other published isotopic work may suggest that Gardar magmatism was a continuous rather than a punctuated process.
Abstract In the Fen complex (Telemark, S.E. Norway), carbonatites of different compositions have penetrated feldspathic fenites (alkali feldspar(s) + aegirine augite ± alkali amphibole) or older carbonatites, inducing different types of contact metasomatic alterations in their wall-rocks. (1) Pyroxene søvite has induced alkali metasomatism (i.e. fenitization s.s. ), with alkali feldspars remaining stable and aegirine-augite transformed to nearly pure aegirine. (2) Søvite and dolomite carbonatite with phlogopite and/or alkali or alkali-calcic amphibole have caused replacement of feldspathic fenite by phlogopite, i.e. magnesium metasomatism. (3) Granular (dyke facies) ferrocarbonatite has increased the ferromagnesian components in calcite in wall-rock søvite. (4) Heterogeneous (pyroclastic) ferrocarbonatite induced pseudomorphic replacement of phlogopite by chlorite (leaching of alkalis). The different contact metasomatic processes reflect contrasts in compositional character among carbonatite magmas in the Fen complex, which may be evaluated in terms of differences in alkali and magnesium carbonate activities. The different types of carbonatite magma represent the products of local evolutionary trends, and are genetically related to spatially associated silicate rocks, rather than to a single ‘primitive’ carbonatite parent magma.
The Proterozoic Bamble Sector, South Norway, is one of the world's classic amphibolite- to granulite-facies transition zones. It is characterized by a well-developed isograd sequence, with isolated 'granulite-facies islands' in the amphibolite-facies portion of the transition zone. The area is notable for the discovery of CO2-dominated fluid inclusions in the granulite-facies rocks by Jacques Touret in the late 1960's, which triggered discussion of the role of carbonic fluids during granulite genesis. The aim of this review is to provide an overview of the current state of knowledge of the Bamble Sector, with an emphasis on the Arendal-Froland-Nelaug-Tvedestrand area and off shore islands (most prominantly Tromøy and Hisøy) where the transition zone is best developed. After a brief overview of the history of geological research and mining in the area, aspects of sedimentary, metamorphic and magmatic petrology of the Bamble Sector are discussed, including the role of fluids. Issues relevant to current geotectonic models for SW Scandinavia, directly related to the Bamble Sector, are discussed at the end of the review.
Abstract The Qassiarsuk (formerly spelled Qagssiarssuk) complex is located in a roughly E–W trending graben structure between Qassiarsuk village and Tasiusaq settlement in the northern part of the Precambrian Gardar rift, South Greenland. The complex comprises a sequence of alkaline silicate tuffs and extrusive carbonatites interlayered with sandstones, and their subvolcanic equivalents, which represent possible feeders for the extrusive rocks. The Rb-Sr, Sm-Nd and Pb isotopic characteristics of 65 samples of extrusive carbonatite- and silicate tuffs and carbonatite diatremes have been determined by mass spectrometry. The Qassiarsuk complex can be dated to c . 1.2 Ga by Rb-Sr and Pb-Pb isochrons on whole-rocks and mineral separates, agreeing with previous isotopic ages for the volcanic rocks of the Eriksfjord formation in the Eriksfjord area of the Gardar rift, but not with previous, indirect age estimates of >1.31 Ga for assumed Eriksfjord equivalents in the Motzfeldt area further east. Recalculated isotopic compositions at 1.2 Ga indicate that the Qassiarsuk carbonatite- and alkaline-silicate magmas were comagmatic and derived from a depleted mantle source (ε Nd >4, ε Sr <−13, time-integrated, single- stage 238 U/ 204 Pb ≤ 7.4). The mantle-derived magmas were contaminated with crustal material, equivalent to the local, pre-Gardar granites and gneisses and sediments derived from these. The crustal component has a depleted mantle Nd model age of 2.1-2.6 Ga; at 1.2 Ga it was characterized by ε Sr = +76, ε Nd = −8.4, time-integrated, single- stage 238 U/ 204 Pb = 8.2−8.3. Strong decoupling of the Pb from the Sr and Nd isotopic systems suggests that the contamination happened only after carbonatitic and alkaline-silicate magmas had evolved from a common parent, by processes such as liquid immisicibility and/or fractional crystallization. Post-magmatic hydrothermal alteration (oxidation, hydration of mafic silicates, carbonatization of melilite) may have contributed further to the contamination of the carbonatite and alkaline silicate rocks of the Qassiarsuk complex.
Abstract U–Pb and Lu–Hf isotope analyses of detrital zircon from the latest Ordovician (Hirnantian) Langøyene Formation, the Late Silurian Ringerike Group and the Late Carboniferous Asker Group in the Oslo Rift were obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Overall the U–Pb dating yielded ages within the range 2861–313 Ma. The U–Pb age and Lu–Hf isotopic signatures correspond to virtually all known events of crustal evolution in Fennoscandia, as well as synorogenic intrusions from the Norwegian Caledonides. Such temporally and geographically diverse source areas likely reflect multiple episodes of sediment recycling in Fennoscandia, and highlights the intrinsic problem of using zircon as a tracer-mineral in ‘source to sink’ sedimentary provenance studies. In addition to its mostly Fennoscandia-derived detritus, the Asker Group also have zircon grains of Late Devonian – Late Carboniferous age. Since no rocks of these ages are known in Fennoscandia, these zircons are inferred to be derived from the Variscan Orogen of central Europe.
The boron isotopic composition of tourmaline and hambergite (Be2BO3[OH,F]) from peraluminous (n = 12), peralkaline (n = 1), and peralkaline nepheline syenite (n = 16) pegmatites has been measured by secondary ion mass spectrometry, for which a new hambergite reference material was developed. The focus of this study is on nepheline syenite pegmatites from the Larvik Plutonic Complex (Norway) and one peralkaline pegmatite related to the nearby Eikeren-Skrim Complex (Norway), where we investigate the source of boron as being from magmatic vs. external fluids. Tourmaline-hambergite mineral pairs were also analysed from peraluminous pegmatite localities (Russia, Tajikistan, and Pakistan) to test for systematic B-isotope fractionation between these two minerals. Tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios (δ11B = −12.9to −1.0‰) associated with S-type granites, whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios (δ11B = −1.7 to 11.8‰), which we interpret is a result of heavy‑boron enrichment from external fluids. Our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite. The results suggest a slight but consistent partitioning of B-isotopes between tourmaline and hambergite, with Δ11B = δ11Btourmaline−δ11Bhambergite in the range of approximately −3‰ to −5‰. Boron is in trigonal coordination with oxygen in both of these mineral phases as verified by NMR. Single crystal XRD analyses of tourmaline and hambergite reveal consistent longer distances of tourmaline relative to hambergite. We attribute the boron isotopic fractionation to the longer bond-lengths in tourmaline compared with hambergite.
Abstract: In situ Lu–Hf (laser ablation microprobe–inductively coupled plasma mass spectrometry (LAM-ICPMS)) and U–Pb (LAM-ICPMS, secondary ionization mass spectrometry (SIMS)) analyses of zircon, and whole-rock Sm–Nd isotope analyses were performed on rocks formed during magmatic events in three Archaean complexes in the Karelian Province of Fennoscandia (Pudasjärvi, Koillismaa and Iisalmi). These complexes have U–Pb ages ranging from 3.5 to 2.6 Ga. In Pudasjärvi, sparse xenocrystic cores give ages of 3.6–3.7 Ga and initial 176 Hf/ 177 Hf suggesting influence of a crustal component T ≥ 4.0 Ga (assuming a CHUR-like mantle source). Ages and Nd and Hf isotope patterns indicate magmatic events at 3.6–3.7 Ga (Siurua, Pudasjärvi with ≥4.0 Ga precursor), 3.2 Ga (Iisalmi, Koillismaa), 2.8 Ga (Pudasjärvi) and 2.7 Ga (Pudasjärvi, Iisalmi). In the Meso- and Palaeoarchaean events, there is no evidence of sources equivalent to present-day depleted mantle; such sources were, however, involved in the 2.8–2.7 Ga events. ε Hf and ε Nd are strongly correlated. Contrasts between the Archaean complexes indicate that they evolved separately until c . 2.7 Ga. The age and ε Hf pattern of ≤2.8 Ga rocks in the Karelian Province is compatible with a scenario in which the Karelia, Superior, Yilgarn and Slave cratons were part of a late Archaean supercontinent, but does not constitute proof of the existence of such a supercontinent. Supplementary material: U–Pb and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18430 .
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