Due to developments in reaction cell technologies, it is now possible to collect large 87 Rb/ 86 Sr and 87 Sr/ 86 Sr data sets using LA-ICP-MS/MS. Until now, LA-ICP-MS/MS Rb-Sr ages were constrained using conventional isochrons on a contiguous set of Rb-Sr data. In this contribution, we present a straightforward procedure on how to measure, calculate and validate Rb-Sr ages from individual laser spots even from detrital grains devoid of context. This approach does not only allow provenance studies of detrital micas, but also furthermore opens the opportunity to produce age maps with unprecedented spatial resolution, both for zoned single crystals and targeting different textural domains. The main challenge for calculating single-spot Rb-Sr ages is the estimation of the initial 87 Sr/ 86 Sr composition, if it cannot be constrained through analysis of a paragenetic phase. In this case, we propose to use a range of geologically relevant initial 87 Sr/ 86 Sr compositions rather than one fixed value: (1) 0.703 AE 0.003 (for mantle derived magmatic rocks), (2) 0.715 AE 0.015 (for evolved magmatic rocks) and (3) 0.730 AE 0.030 (for crustal rocks). Modelling shows that the 87 Sr/ 86 Sr ratio of the targeted material delineates the accuracy and precision of single-spot Rb-Sr ages. If the measured Sr isotopic composition is sufficiently radiogenic [ 87 Sr/ 86 Sr of > 1.0 for scenario (1) to > 4.5 for scenario (3)], the calculated age and its uncertainty are not significantly influenced by the initial 87 Sr/ 86 Sr composition. Using Mica-Mg as the primary reference material, single-spot Rb-Sr dating was tested on biotite (Mount Dromedary, La Posta, McClure Mountain), muscovite (H ögsbo) and a nano-powder tablet (Mica-Fe) that we further propose using as secondary reference materials for Rb-Sr geochronology.
Provenance studies from Cambro‐Ordovician sediments of the North Gondwana passive margin typically ascribe a North African source, a conclusion that cannot be reconciled with all observations. We present new U‐Pb ages from detrital rutile and zircon from Late Ordovician sediments from Saxo‐Thuringia, Germany. Detrital zircons yield age populations of 500–800 Ma, 900–1050 Ma and 1800–2600 Ma. The detrital rutile age spectra are unimodal with ages between 500 and 650 Ma and likely represent, together with the 500–800 Ma and 1800–2600 Ma zircon populations, detritus sourced predominantly from North Africa. In contrast, the c. 950 Ma zircons, which are persistently found in Cambro‐Ordovician sediments of North Gondwana, have no obvious African source. We propose that these zircons are sourced from the Rayner Complex–Eastern Ghats regions of Antarctica and India. An Indo‐Antarctic source indicates either continental‐scale sedimentary transport from central Gondwana to its peripheries or multiple cycles of sediment reworking and redeposition.
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