U–Pb apatite geochronology is increasingly recognized as a valuable tool for constraining the age of mid-crustal ductile shear zones. The crustal-scale Outer Hebrides Fault Zone (OHFZ) within the Laurentian foreland of the Scottish Caledonides has long been of uncertain age and tectonic significance. Earliest deformation within the OHFZ was associated with top-to-the-NW ductile thrusting that formed a belt of greenschist facies mylonites within host Archean−Paleoproterozoic basement gneisses. Previous estimates for the timing of thrusting vary between c . 1600 Ma and c . 430 Ma. The mylonitic fabrics are defined by a recrystallized assemblage of quartz + albite/oligoclase + sericite + actinolite + epidote + apatite ± calcite, consistent with deformation temperatures of 400–500°C and within the range of reported closure temperatures for Pb diffusion in apatite. U–Pb (LA−ICP−MS) dating of two texturally distinct apatite grain types within the mylonites has yielded ages mostly in the range c . 1100–900 Ma. The OHFZ is therefore interpreted as a Grenville–Sveconorwegian structure that formed during the tripartite collision of Laurentia, Baltica, and Amazonia and the assembly of Rodinia. Supplementary material : U–Pb isotopic data (Table S1), trace element data (Table S2) and laser ablation spot images are available at https://doi.org/10.6084/m9.figshare.c.7084925
Abstract. Baddeleyite is a powerful chronometer of mafic magmatic and meteorite impact processes. High precision and accuracy U-Pb ages can be measured from single grains by isotope dilution thermal ionisation mass spectrometry (ID-TIMS), but this requires destruction of the host rock for highly challenging grain isolation and dissolution. As a result, the technique is rarely applied to precious samples with very limited availability (such as lunar, Martian and asteroidal meteorites and returned samples) or samples containing small baddeleyite grains that cannot readily be isolated by conventional mineral separation techniques. Here, we use focused ion beam (FIB) techniques, utilising both Xe+ plasma and Ga+ ion sources, to liberate baddeleyite subdomains in-situ, allowing their extraction for ID-TIMS dating. We have analysed the U-Pb isotope systematics of domains ranging between 200 um and 10 um in length and 5 ug to 0.1 ug in mass. In total, seven domains of Phalaborwa baddeleyite extracted using a Xe+-pFIB yield a weighted mean 207 Pb/206 Pb age of 2060.1 ± 2.4 Ma (0.12 %; all uncertainties 2 sigma), within uncertainty of reference values. The smallest extracted domain (ca. 10 × 15 times; 10 um) yields an internal 207 Pb/206 Pb uncertainty of ±0.15 %. Comparable levels of precision are achieved using a Ga+-source FIB instrument (±0.20 %), though the slower cutting speed limits potential application to larger grains. While the U-Pb data are between 0.5 and 13.6 % discordant, the results generate a precise upper intercept age in U-Pb concordia space of 2061.1 × 7.4 Ma; (0.72 %). Importantly, the extent of discordance does not correlate with the ratio of material to ion-milled surface area, showing that the FIB extraction does not induce disturbance of U-Pb systematics even the smallest extracted domains. Instead, we confirm the natural U-Pb variation and discordance within the Phalaborwa baddeleyite population observed with other geochronological techniques. Our results demonstrate the FIB-TIMS technique to be a powerful tool for high-accuracy in-situ U-Pb dating, which makes a wide range of targets and processes newly accessible to geochronology.
Dall'assottigliamento litosferico Permo-Triassico al rifting Giurassico al margine Adriatico: memoria petrologica e geocronologica in Valtournenche (Alpi Occidentali Italiane).Scaglie di metasedimenti polimetamorfici sono amalgamate tettonicamente con il basamento polideformato del sistema tettonico della Dent Blanche (Alpi Occidentali), lungo una zona di shear alpina, di dimensioni chilometriche (Becca di Sale, Valtournenche). L'evoluzione polifasica strutturale e metamorfica di queste scaglie comprende sia il ciclo pre-Alpino sia quello Alpino. Tale evoluzione e stata ricostruita usando un approccio multidisciplinare che combina analisi micro-strutturale e tessiturale, petrologia metamorfica, geochimica e geocronologia. Sono stati stimati il regime termico e l'eta dell'assottigliamento litosferico e del rifting Giurassico, preservati in queste scaglie polimetamorfiche.
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