Now showing 1 - 5 of 5
  • Publication
    Diffusion and Fluid-Interaction in Itrongay Pegmatite (Madagascar): The Results of in situ Ar-Ar Dating of Gem-Quality Alkali Feldspar and U-Pb Dating of Apatite Inclusions within it
    Constraining how the temperature of rocks changes with time is an important aspect of many geological studies. Geoscientists commonly address this problem by interpreting step-heating Ar-Ar data obtained from feldspars [e.g. 1 and therein] and increasingly more often by interpreting U-Pb data obtained from apatite [e.g. 2 and therein]. Reconstruction of thermal histories using these approaches is underpinned by the assumption that the redistribution of radiogenic Ar in feldspars and Pb in apatite over geological timescales is controlled by volume diffusion. However, is this assumption always valid? Here we revisit the mechanisms of Ar redistribution in famous gem-quality alkali feldpsar from Itrongay pegmatite by combining in situ Ar-Ar dating with cathodoluminescence imaging. Previous in situ Ar-Ar studies of Itrongay feldspar suggested that it has partially lost radiogenic Ar by diffusion [3, 4], supporting the underlying assumption of feldspar ArAr thermochronology. However, our results indicate that this feldspar records a protracted history of interaction with fluids between ~475 Ma (dates in the core) and ~180 Ma (dates at the rim), casting doubt on previous interpretations. Alongside, we have obtained in situ U-Pb dates of three apparently protogenetic apatite inclusions within the studied feldspar crystal. These yield older dates than feldpsar (~490- 535 Ma), and in contrast to feldspar seem to have been partially reset by diffusion, possibly prior to their entrapment. [1] Harrison and Lovera (2013) GSL Spec. Pub., 378, 91- 106; [2] Paul et al. (2018) GCA, 288, 275-300 [3] Flude et al. (2014) Geol. Soc. London Spec. Pub., 378, 265–275; [4] Arnaud and Kelley (1997) GCA, 61, 3227–3255.
  • Publication
    Drainage reorganization during breakup of Pangea revealed by in-situ Pb isotopic analysis of detrital K-feldspar
    (The Geological Society of America, 2007-11) ; ;
    The Pb isotopic composition of detrital K-feldspar grains can be rapidly measured using laser ablation MC-ICPMS. The feldspar Pb signal can survive weathering, transport and diagenesis, and careful targeting avoids problems with inclusions and alteration. As common Pb isotopic compositions show broad (100s km scale) variation across the continents, the method provides a powerful provenance tracer for feldspathic sandstones. Here we combine a new Pb domain map for the circum-North Atlantic with detrital feldspar Pb isotopic data for Triassic and Jurassic sandstones from basins on the Irish Atlantic margin. The Pb compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern. Triassic sandstones were sourced from distant Archean and Paleoproterozoic rocks, probably in Greenland, Labrador and Rockall Bank to the NW, implying long (>500 km) transport across a nascent rift system. Later Jurassic sandstones had a composite Paleo- and Mesoproterozoic source in more proximal sources to the north (<150 km away). Little or no feldspar was recycled from Triassic into Jurassic sandstones, and the change in provenance is consistent with distributed, low relief Triassic extension in a wide rift, followed by narrower Jurassic rifting with more localised fault-controlled sediment sources and sinks.
  • Publication
    K-feldspar sand-grain provenance in the Triassic, west of Shetland : distinguishing first-cycle and recycled sediment sources?
    Sandstone provenance studies can help constrain palaeogeographic reconstructions and ancient drainage system scales and pathways. However, these insights can be obscured by difficulties in geochemically distinguishing or adequately characterising potential sourcelands, or by failure to identify sedimentary recycling. Triassic basins west of Shetland accumulated ~2.5 km of sand-rich sediment. The Middle-Upper Triassic Foula Formation represents fluvial, aeolian and sabkha facies deposited in the northern interior of the Pangean supercontinent. Published U-Pb zircon geochronology and heavy mineral analysis suggest that these sandstones were derived from East Greenland. They contain significant fresh K-feldspar which is likely to be first-cycle and derived directly from its source. Pb isotopic analyses of individual K-feldspar sand-grains show a single, unradiogenic Pb population, consistent with the provenance indicated by U-Pb zircon geochronology. Archaean and Palaeo-Mesoproterozoic rocks – the Nagssugtoqidian Mobile Belt, the Lewisian Complex or equivalents - are the likely source, with terranes south of the Moine Thrust (Grampian, Caledonian and Variscan) ruled out by both the Pb and U-Pb data. However, it is not possible to distinguish between rift flank sources to the east and west, as both areas have similar crustal affinity and/or share the same tectonic history. It is possible that the sediment was derived from the West Shetland Platform and not from Greenland. The comparison of provenance signals from robust and less stable mineral phases provides a means of recognising sedimentary recycling. Robust zircon populations and less stable feldspar in Foula Formation sandstones concur in indicating the same source, suggesting that they are likely to be first-cycle. The Triassic sand supply can be contrasted with that in Upper Carboniferous (Namurian) basins in the north of England where a significant zircon population has no corresponding K-feldspar component. This zircon population is likely to have been recycled from Lower Palaeozoic greywackes from the Southern Uplands Belt or its along strike extension.
      732Scopus© Citations 37
  • Publication
    Sedimentology, sandstone provenance and palaeodrainage on the eastern Rockall Basin margin : evidence from the Pb isotopic composition of detrital K-feldspar
    The Rockall Basin, west of Ireland, is a frontier area for hydrocarbon exploration but currently the age and location of sand fairways through the basin are poorly known. A recently developed provenance approach based on in-situ Pb isotopic analysis of single K-feldspar grains by laser ablation multi-collector inductively-coupled mass spectrometry (LA-MC-ICPMS) offers advantages over other provenance techniques, particularly when applied to regional palaeodrainage issues. K-feldspar is a relatively common, usually first-cycle framework mineral in sandstones and its origin is typically linked to that of the quartz grains in arkosic and sub-arkosic rocks. Consequently, in contrast to other techniques, the Pb-in-K-feldspar tool characterises a significant proportion of the framework grains. New Pb isotopic data from K-feldspars in putative Permo-Triassic and Middle Jurassic sandstones in Well 12/2-1z (the Dooish gas condensate discovery) on the eastern margin of the Irish Rockall Basin are reported. These data suggest that three isotopically distinct basement sources supplied the bulk of the K-feldspar in the reservoir sandstones and that the relative contribution of these sources varied through time. Archaean and early Proterozoic rocks (including elements of the Lewisian Complex and its offshore equivalents), to the immediate east, north-east and north of the eastern Rockall Margin, are the likely sources. More distal sourcelands to the north-west cannot be ruled out but there was no significant input from southern sources, such as the Irish Massif. These data, together with previously published regional Pb isotopic data, highlight the important role played by old, near and far-field Archaean – Proterozoic basement highs in contributing sediment to NE Atlantic margin basins. The Irish Massif appears to have acted as a significant, but inert, drainage divide from the Permo-Triassic to the Late Jurassic and hence younger, Avalonian and Variscan, sand sources appear to have been less important on the Irish Atlantic Margin.
      1848Scopus© Citations 27
  • Publication
    Large-scale, linked drainage systems in the NW European Triassic: insights from the Pb isotopic composition of detrital K-feldspar
    Pb isotopic data from K-feldspars in Middle Triassic (Anisian) sandstones in the Wessex Basin, onshore southwest UK, and the East Irish Sea Basin, some to the north, show that the same grain populations are present. This indicates that the drainage system (the “Budleighensis” River) feeding these basins originated from the same source/s, most probably the remnant Variscan Uplands to the south. Fluvial and aeolian sandstones have the same provenance, suggesting that if water- and wind-driven sands were originally derived from different sources, this has been obscured through reworking prior to final deposition. Significant recycling of feldspar from arkosic sandstones in earlier sedimentary basins can be ruled out. The provenance data agree with previous depositional models, indicating transport distances in excess of , with a drainage pattern that linked separate basins. This supports the idea that the regional fluvial system was driven by topography and episodic flooding events of sufficient magnitude to overcome evaporation and infiltration over hundred’s of kilometres. Importantly, this drainage system appears to have been isolated and independent from those operating contemporaneously to the northwest of the Irish and Scottish massifs, where the remnant Variscan Uplands apparently exerted no influence on drainage or sand supply.
      1095Scopus© Citations 28