Siri Lene Simonsen

Granitic magmatism by melting of juvenile continental crust: new constraints on the source of Palaeoproterozoic granitoids in Fennoscandia from Hf isotopes in zircon

Abstract: Zircons from nine Palaeoproterozoic granitoid intrusions within the southern part of the Fennoscandian Shield have been studied by laser ablation inductively coupled plasma source mass spectrometry to obtain U–Pb ages (in the range 1.88–1.68 Ga) and Hf isotope compositions. Six granitoids are from the 1.85–1.65 Ga Transscandinavian Igneous Belt; during that period more than 106 km3 of granitoid magma intruded the pre-existing crust. The large majority of magmatic zircons from the nine granitoids have a limited initial range, 176Hf/177Hf = 0.2816–0.2818, and define an evolutionary trend given by an initial value of εHf(1.88Ga) ≈ +2 ± 3 at an average 176Lu/177Hf = 0.015. These data show that a geographically extensive, long-lived, relatively homogeneous, and dominant magma source resided within 2.1–1.86 Ga Svecofennian juvenile crust between 1.88 and 1.68 Ga. Zircon xenocrysts (1.91–1.98 Ga) with initial εHf = +0 to +2.5 from one of the intrusions provide additional evidence for such a long-lived crustal source of granitic magmas in central Fennoscandia. The granitoids were emplaced during a period of active mafic underplating that supplied heat to the anatectic zone in the lower–middle crust, but little or no mantle-derived Hf to the granitic magmas, in contrast to many mixed intermediate rocks. Supplementary material: U–Pb analyses, age calculations and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18330.

Carbonatite melt–CO2 fluid inclusions in mantle xenoliths from Tenerife, Canary Islands: a story of trapping, immiscibility and fluid–rock interaction in the upper mantle

Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO2 (Type A); carbonate-rich CO2–SO2 mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a “coating” (a few microns thick) consisting of an aggregate of a platy, hydrous Mg–Fe–Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the “coating”, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO2–H2O–NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO2–SO2 fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO2–H2O–NaCl fluid inside the Type A inclusions were separated into CO2-rich fluid and H2O–NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or “coating” along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO2 fluid. The homogenization temperatures of +2 to +25 °C obtained from the Type A CO2 inclusions reflect the densities of the residual CO2 after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000–1200 °C, and the spinel lherzolite phase assemblage of the xenolith, which is 0.7–1.7 GPa.

Use of lead isotopic ratios to discriminate glass samples in forensic science

Even though a variety of well-established methods exists for forensic scientists, evaluating the potential of presently un-adopted techniques within the forensic society will always be of high importance. Demand for reduced turnover time is always requested and so are methods that show high ability to discriminate samples that originate from different sources. Glass is common as construction material and in consumer goods. Since glass easily breaks, traces of glass often occur in criminal cases. An obvious assignment for the forensic scientist will be to establish the link between the glass fragments collected on a suspect and the glass from the broken window at the scene of crime. Determination of the refractive index of the seized glass by the oil immersion method is a common method in forensic laboratories. During the last decades, it has been observed that the refractive index has become more similar for glass from different sources. Hence, the evidential value of glass may decrease when refractive index is the single parameter used to determine the origin of glass from the suspect. Consequently, new methods are necessary, and analysis of lead isotope ratios is relevant in this perspective. This work establishes a novel method for analysis of lead isotope ratios in order to differentiate glass from different sources. To perform such analysis by Multi-Collector Inductive Coupled Plasma Mass Spectrometry, an analytical protocol is developed. The statistical method of Tukeys Honestly Significant Difference Test for multiple comparisons of samples appeared helpful in a forensic context.

Lead isotope ratios for bullets, a descriptive approach for investigative purposes and a new method for sampling of bullet lead

To establish a link between a bullet fired from a suspected firearm, investigation of striation marks are one of the corner stones in the forensic laboratory. Nevertheless, on some occasions, the bullet may be deformed to such extent that traditional investigation of striation marks will be impossible. Fragments of lead can be investigated by lead isotope ratio determination in order to distinguish between bullets with different origin. This approach initially seems reasonable, since the abundance of lead isotopes varies significantly in nature. To make a method valid for forensic purposes, it is important to have a fundamental understanding of the variation within a box of lead bullets and the expected variation between boxes. Studies of variability within and between boxes of ammunition are imperative to perform any type of forensic interpretation, both in an investigative and evaluative context. This work presents an extensive study of variability within and between boxes of ammunition by use of multicollector inductive coupled mass spectrometry. As a first approximation to classify bullets to any given source, a simple and robust graphical method is presented. In addition, an easy-to-use sampling procedure of lead is presented.

Studies of SRM NIST glasses by laser ablation multicollector inductively coupled plasma source mass spectrometry (LA-ICP-MS)

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.

Hafnium isotope characteristics of late Palaeoproterozoic magmatic rocks from Blekinge, southeast Sweden: possible correlation of small-scale Hf and Nd isotope variations in zircon and whole rocks

The isotopic composition of hafnium has been measured in zircons from seven magmatic rocks from the Blekinge region, southeast Sweden, previously dated by the U–Pb zircon method to 1.74–1.76 Ga, with one rock at 1.81 Ga. The results from all samples show limited variation, with a total range in initial ϵHf between − 1.5 and +3.2, only slightly more than analytical uncertainty ( ± 1.5 epsilon units). However, taken together with previously published initial ϵNd whole-rock values of − 0.3 to +1.0 from the same rock samples, a trend with a positive correlation between average initial ϵHf (zr) for each sample and initial ϵNd (WR) can be discerned. This trend could be taken to indicate mixing between a “mildly depleted” component derived from the lithospheric mantle or lower crust, and a slightly more enriched crustal component, either by mixing of separate magmas or by contamination of a mantle-derived magma with crustal material. Such mixing may have occurred both on a regional scale, as indicated by the overall trend, and locally within each intrusion and each rock sample with small-scale variations in isotope composition of the magma during crystallization, indicated by the variation in initial ϵHf between the zircons in each sample. The overall results support the presence of a “mildly depleted” lithospheric mantle reservoir, both in terms of Nd and Hf isotopes, also beneath southern Fennoscandia in Proterozoic time.