Evaluating the use of clay mineralogy, Sr-Nd isotopes and zircon U-Pb ages in tracking dust provenance: An example from loess of the Carpathian Basin
by Gábor Újvári
Gábor Újvári, Andrea Varga, Frank C. Ramos, János Kovács, Tibor Németh, Thomas Stevens (2012)
Chemical Geology 304-305, 83-96, doi: 10.1016/j.chemgeo.2012.02.007
Multiple competing hypotheses have been proposed to explain the source of loess dust in the Carpathian Basin yet none... more Multiple competing hypotheses have been proposed to explain the source of loess dust in the Carpathian Basin yet none has been demonstrated. Here bulk and grain-size differentiated Nd and Sr isotopic and bulk and clay mineralogical compositions, together with detrital zircon ages and morphologies from loess along the Danube in the Carpathian Basin are used to demonstrate that no single method is capable of unequivocally isolating sources, yet combined, they allow for discriminating likely provenance. Zircon ages indicate multiple sources for Carpathian Basin loess, restricting the use of bulk or grain-size differentiation methods. Ambiguities are further highlighted by the fact that the Sr-Nd isotopic composition of Greenland dust is very similar to Carpathian loess. This match demonstrates that such methods are not always capable of unequivocally excluding unlikely potential source areas and implies that current hypotheses regarding the origin of Greenland dust require further evaluation. Sr isotopes are also limited by high dolomite contents of loess, while palygorskite is of questionable utility as a Saharan dust indicator due to its rapid weathering under typical Carpathian climates. In terms of specific Carpathian loess sources, alluvial fans of the Danube River likely contribute, but detrital zircon ages and morphologies suggest that a proportion directly originates from eroded uplands and local rocks throughout the basin, while detrital dolomite implies local, primary sources such as the Transdanubian Range. An upper limit of 5-10% is placed on the North African contribution to fine dust in loess of the mid-Carpathian Basin based on mineralogy and grain size. Overall, results suggest that the single grain (zircon) approach is likely the most diagnostic method to identify sources in loess studies, although to solve complex provenance issues, simultaneous consideration of the isotopic characteristics of more than one heavy (or light) mineral is required.
Coupling mineral analysis with conceptual groundwater flow modelling: The source and fate of iron, aluminium and manganese in a back-barrier island
Reference:
Hodgkinson, J., Cox, M.E. & McLoughlin, S., 2008. Coupling mineral analysis with conceptual groundwater flow modelling: The source and fate of iron, aluminium and manganese in a back-barrier island. Chemical Geology 251: 77-98.
Mineral and aqueous geochemical data are combined with a conceptual groundwater flow model, to establish the origin... more Mineral and aqueous geochemical data are combined with a conceptual groundwater flow model, to establish the origin and fate of iron, aluminium and manganese in the groundwater system of a small backbarrier island. The flow model domain consists of an unconfined island fresh groundwater lens overlying a semi-confined hypersaline aquifer. The two aquifers are separated by a discontinuous, clay-rich aquitard and both contain diffusion governed variable density flow fields. High concentrations of dissolved iron and manganese are associated with brackish to hypersaline groundwater, although there is no systematic relationship with salinity. Calculation of S2_/SO4 2_ and Fe2+/Fe3+ redox couples and the results of thermodynamic modelling show that redox disequilibrium in the groundwater is widespread. Groundwater samples containing aqueous sulphide and ferric iron complexes are supersaturated with respect to pyrite, goethite and haematite but the prevailing state of redox disequilibrium controls mineral dissolution and precipitation. Aqueous iron in the deeper regions of both aquifers is derived from the dissolution of iron oxide–hydroxides in lateritic palaeosols controlled by seasonal fluctuations in groundwater redox state. Aqueous manganese is potentially derived from the dissolution of ilmenite and amorphous oxide– hydroxides. The oxidation of iron sulphides contributes to the aqueous iron concentration and sulphuric acid production in the shallow groundwater. The solubility of aluminium is also limited by this process, governed by acidity regulation. A significant proportion of aqueous iron is transmitted from the semi-confined to the overlying unconfined aquifer through discontinuities in the aquitard layer. Movement of metals in solution outside the island groundwater system is restricted by the presence of diffusion boundaries within variable density transition zones.
Download (.pdf) Geochronology of the Tardree Rhyolite Complex, Northern Ireland: Implications for zircon fission track studies, the North Atlantic Igneous Province and the age of the Fish Canyon sanidine standard
by Fiona Meade
Ganerød, M., Chew, D.M., Smethurst, M.A., Troll, V.R. Corfu, F., Meade, F. and Prestvik, T. (in press) Geochronology of the Tardree Rhyolite Complex, Northern Ireland: Implications for zircon fission track studies, the North Atlantic Igneous Province and the age of the Fish Canyon sanidine standard. Chemical Geology.
The British-Irish Palaeogene Igneous Province (BIPIP) is part of the larger North Atlantic Igneous Province and... more The British-Irish Palaeogene Igneous Province (BIPIP) is part of the larger North Atlantic Igneous Province and includes the lava fields of Antrim, Mull, and Skye. The Tardree Rhyolite Complex (TRC) in Northern Ireland forms an important stratigraphic unit between the Lower and Upper Basalt Formations of the Antrim Lava Group (ALG). Previous zircon age determinations obtained from the TRC have been used as a standard in zircon fission track studies, but contradict several 40Ar/39Ar sanidine and U-Pb zircon results. We provide new 40Ar/39Ar sanidine and U-Pb CA-TIMS zircon ages which resolve this discrepancy. Two sanidine samples from the Sandy Braes vent and the columnar-jointed dome-forming rhyolites of Tardree Forest yield a weighted mean 40Ar/39Ar age of 61.13 ± 0.42 Ma (2σ, internal error). Ten U-Pb CA-TIMS zircon analyses were undertaken, eight of which employed the CA-TIMS approach on both multi-grain fractions and single grains. Six of the CA-TIMS data yield a disequilibrium-corrected weighted mean 206Pb-238U age of 61.32 ± 0.09 Ma (2σ). The consistency of the 40Ar/39Ar ages with the CA-TIMS U-Pb zircon age, points to a closed system of both K and Ar since eruption. We propose that the crystallization age of the TRC be taken as 61.32 ± 0.09 Ma and that the currently used age of the zircon fission track standard (58.4 ± 0.7 Ma) be changed accordingly. This also places the eruption of the TRC in magnetochron C26r, which is consistent with the reversed polarity magnetic remanence observed in the ALG, and supports the conclusion of Ganerød et al. (2010) that the Lower Basalt Formation is older than the Vaigat Formation in Western Greenland. No resolvable zircon inheritance has been detected by the TIMS analyses, consistent with the fact that the temporal and geographic extent of rhyolitic magmatism within this sector of the BIPIP was very limited, and hence was unlikely to provide inherited magmatic zircons from slightly older magmas (antecrysts). Potentially older zircon xenocrysts would be derived from the underlying Caledonian basement (> 400 Ma) or yet older rocks. These should be easily detectable if the Tardree zircon was to be employed as a U-Pb zircon standard. The paired 40Ar/39Ar and 206Pb-238U results from this study indicate an age of 28.393 ± 0.194 Ma for the widely used Fish Canyon sanidine standard and gives further support to the recent calibrations of Kuiper et al. (2008) and Renne et al. (2010).
Magma ascent along a major terrane boundary: crustal contamination and magma mixing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland.
by Fiona Meade
Meade, F.C., Chew, D.M., Troll, V.R., Ellam, R.M., Page, L.M. (2009). Magma ascent along a major terrane boundary: crustal contamination and magma mixing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland. Journal of Petrology., v.50 (12), 2345-2374. http://petrology.oxfordjournals.org/content/50/12/2345
The composite intrusions of Drumadoon and An Cumhann crop out on the SE coast of the Isle of Arran, Scotland and form... more The composite intrusions of Drumadoon and An Cumhann crop out on the SE coast of the Isle of Arran, Scotland and form part of the larger British and Irish Palaeogene Igneous Province, a subset of the North Atlantic Igneous Province. The intrusions (shallow-level dykes and sills) comprise a central quartz–feldspar-phyric rhyolite flanked by xenocryst-bearing basaltic andesite, with an intermediate zone of dark quartz–feldspar-phyric dacite. New geochemical data provide information on the evolution of the component magmas and their relationships with each other, as well as their interaction with the crust through which they travelled. During shallow-crustal emplacement, the end-member magmas mixed. Isotopic evidence shows that both magmas were contaminated by the crust prior to mixing; the basaltic andesite magma preserves some evidence of contamination within the lower crust, whereas the rhyolite mainly records upper-crustal contamination. The Highland Boundary Fault divides Arran into two distinct terranes, the Neoproterozoic to Early Palaeozoic Grampian Terrane to the north and the Palaeozoic Midland Valley Terrane to the south. The Drumadoon Complex lies within the Midland Valley Terrane but its isotopic signatures indicate almost exclusive involvement of Grampian Terrane crust. Therefore, although the magmas originated at depth on the northern side of the Highland Boundary Fault, they have crossed this boundary during their evolution, probably just prior to emplacement.
