Uranium-lead dating of perovskite from the Afrikanda plutonic complex (Kola Peninsula, Russia) using LA-ICP-MS.
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Abstract:
Perovskite (CaTiO3) is a common early crystallizing accessory phase in a variety of alkaline rocks, and has been shown to contain enough U and Th for U-Pb dating. U and Pb analysis of perovskite has been primarily carried out using the SHRIMP or ID-TIMS techniques, and the resulting U-Pb dates commonly yield the emplacement age of the host rock. To our knowledge, only one U-Pb study of perovskite has been done using the LA-ICP-MS (Cox and Wilton, 2006). Some of the advantages of this method over the SHRIMP and ID-TIMS techniques include greater speed and lower cost of analysis.Keywords:
Kola peninsula
Radiometric dating
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LA ‐ ICP ‐ MS U–Pb detrital zircon studies typically analyse 50–200 grains per sample, with the consequent risk that minor but geologically important age components (e.g., the youngest detrital zircon population) are not detected, and higher abundance age components are misrepresented, rendering quantitative comparisons between samples impossible. This study undertook rapid U–Pb LA ‐ ICP ‐ MS analyses (8 s per 18–47 μm diameter spot including baseline and ablation) of zircon, apatite, rutile and titanite using an aerosol rapid introduction system ( ARIS ). As the ARIS resolves individual single pulses at fast sampling rates, spot analyses require a high repetition rate (> 50 Hz) so the signal does not return to baseline and mass sweep times (> 80 ms) that span several laser pulses (i.e., major undersampling of the signal). All rapid U–Pb spot analyses employed 250–300 pulses, repetition rates of 53–65 Hz (total ablation times of 4.1–5.7 s) and low fluence (1.75–2.5 J cm −2 ), resulting in pit depths of ca . 15 μm. Zircon, apatite, rutile and titanite reference material data yield an accuracy and precision (2 s ) of < 1% for pre‐Cenozoic reference materials and < 2% for younger reference materials. We present a detrital zircon data set from a Neoproterozoic tillite where > 1000 grains were analysed in < 3 h with a precision and accuracy comparable to conventional LA ‐ ICP ‐ MS analytical protocols, demonstrating the rapid acquisition of huge detrital data sets.
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The U-Pb geochronologic analysis of accessory minerals has played an important role in Earth and solar system science in constraining the ages of a wide variety of rocks and minerals. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is one of the most popular techniques for U-Pb geochronologic analysis. Currently, the significant matrix effects observed between different accessory minerals and the lack of high-quality standards for many minerals of interest are the major limitations of its geochronological applications. In this study, we investigated the effects of the addition of oxygen, nitrogen, and water vapor before and after the ablation cell on the accuracy of the U-Pb dating of different minerals (e.g., zircon, monazite, titanite, and xenotime) by LA-ICP-MS. We found that the addition of water vapor, unlike that of oxygen and nitrogen, before the ablation cell can significantly suppress the matrix effects on U-Pb dating. The deviations of the measured 206Pb/238U ratios in these accessory minerals were significantly reduced from 10 to 24% to less than 1-2% when using NIST 610 glass as an external standard. This can be attributed to the suppression of elemental fractionation in both the laser ablation and ICP ionization processes by the presence of water vapor. The developed water vapor-assisted LA-ICPMS U-Pb dating method has been successfully applied to the analysis of zircon, monazite, xenotime, and titanite with NIST 610 glass as a reference material in both the 193 nm excimer laser and 213 nm Nd:YAG laser ablation systems.
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We report the first U–Pb geochronological investigation of schorlomite garnet from carbonatite and alkaline complexes and demonstrate its applicability for U–Pb age determination using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) due to its relatively high U and Th abundances and negligible common Pb content.
Carbonatite
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U-(Th)-Pb techniques is widely applied for dating of accessory minerals in magmatic, metamorphic and sedimentary rocks. Among the common accessory minerals the zircon is the favourite phase for dating because of its distribution in a wide variety of rocks, the relatively high U content combined with general lack of common lead and the remarkable resistance to high-temperature diffusive re-equilibration. The well-studied relation of the trace-element chemistry, Hf-isotope composition and morphology of the zircon with the composition and temperature of the parental magma made him a reliable tracer of magmatic processes. The new LA-ICP-MS (Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry) equipment at the Geological institute of the Bulgarian academy of Science (GI–BAS), owned by a consortium with the Sofia University (Faculty of geology and geography), the National archaeological institute with museum and the Institute of mineralogy and crystallography (BAS) offers an excellent opportunity for U-Pb zircon (also monazite/titanite) dating. It arises a question when is this techniques applicable for timing and tracing of geological processes and when we should prefer the “conventional” ID-TIMS (Isotope Dilution-Thermal Ionisation Mass Spectrometry). To answer this general question we present here some examples for zircons and titanites from granitoid rocks of the Lutzkan magmatic complex (Lutzkan and Ruy plutons) in Tran region, Western Bulgaria (Belev, 1960). These plutons consist of coarseto medium-grained, porphyroid K-feldspar-bearing, amphibole-biotite monzogranite to granodiorite (i) and leucocratic, mediumto fine-grained, equigranular granites (Dyulgerov et al., 2006). Accessory phases are zircon, apatite, titanite, allanite, Fe-Ti oxides.
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