P. Van den haute

Post‐Variscan thermal and tectonic evolution of the KTB site and its surroundings

The post-Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to the pre-Carboniferous record. There are no depth-related differences in K-Ar cooling ages of hornblende and white mica, in petrology or in lithology. All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300°C from the Carboniferous onward. The late to post-Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE-SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission-track ages, the sericite K-Ar ages of fault rocks, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle-ductile transition in post-Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.

Composition of srm and cn u-doped glasses: Significance for their use as thermal neutron fluence monitors in fission track dating

Abstract The I mm thick (NIST) SRM glasses 611, 613, 615 and 617, and the (Corning) CN glasses 1, 2, 5 and 6 have been investigated for their U and Th content and for their concentration of neutron absorbing elements (B, Cd, Sm, Eu, Gd, Dy, It and Hg). The analyses of these elements were performed with reactor neutron activation analysis and, for boron, with charged-particle activation analysis. The significance of the results is discussed in relation to the use of the glasses as thermal neutron fluence monitors in fission track dating.

Thermal neutron fluence determination for fission-track dating with metal activation monitors: A re-investigation

Abstract During the past decade thermal neutron fluence determination with metal activation monitors has gradually become discredited by geochronologists applying the fission-track method. The opinion that the calculated fluences were not accurate was sustained by the fact that the few calibration experiments including more than one metal monitor often yielded inconsistent results. In this work 14 irradiations have been carried out calibrated with Cu and Au monitors while in 10 of them also a Cc monitor was included. The thermal neutron fluences registered by the Cc and Au monitors are very consistent, diverging 1.4% on the average, and 3.3% at the most. The Cu measurements are less consistent yielding fluences that are systematically ∼8.5% lower than those obtained with Au. The difference is ascribed to possible inaccuracies in the nuclear parameters of Cu and to problems inherent to the measurement of the 511-keV annihilation radiation of 64 Cu. The good agreement between the Au and Cc measurements in our opinion sustains the accuracy of the fluence determinations with these monitors. A fission-track analysis with the population method of two apatite reference samples (Fish Canyon tuff and Durango) with the use of combined Au and Cc fluence determination clearly supports a nominal value of 8.5·10 −17 a −1 for γ f Based on the above results some suggestions are made for a standardization of calibration procedures relying on the determination of the absolute thermal neutron fluence.

A New U Doped Glass Certified By the European Commission for the Calibration of Fission-Track Dating

In this paper we present a new series of unirradiated and pre-irradiated uranium doped glass discs that were manufactured and characterized for use in fission-track analysis. They are issued as Nuclear Reference Material IRMM-540. The certified quantities are the uranium mass fraction [=13.9 mgxkg−1], the n(235U)/n(238U) amount ratio [=0.007277] and the thermal Maxwellian neutron fluence [=1.070x1019 m−2]. Additional information is given related to the nominal composition of the glass and its content of thorium and neutron absorbing elements, the uranium homogeneity between and within discs, the neutron fluence monitors, the parameters of the neutron spectrum in the irradiation facility, the thermal stability of the fission tracks in the glass and the properties of the mica that has been used as external detector in contact with the irradiated discs.

Investigations on the origin of the equivalent dose distribution in a Dutch coversand

Abstract In the course of optical dating of a sequence of Dutch coversands, the distribution of equivalent doses was examined by applying the single-aliquot regenerative-dose protocol to small aliquots containing ∼0.5 – 1 mg of quartz grains. Broad Gaussian D e distributions with relative standard deviations of 11–12% were observed. This was not expected, given the windblown nature of the deposits. This paper reports on a study on the causes of the broad dose distribution, using the quartz grains extracted from one of the samples from this coversand sequence. Investigations were made as to whether the spread might be related to the measurement procedures or to partial bleaching, but no evidence for these was found. The possibility of variations in microdosimetry was investigated by performing neutron activation analysis on small amounts ( ∼0.5 – 1 g ) of sediment. A large scatter in U and Th concentrations throughout the profile was found. NAA of etched quartz extracts showed furthermore a non-negligible internal radioactivity. It is concluded that small-scale differences in the annual dose rate are the main cause for the observed distributions, although at this stage of our investigations it could not be established whether external or internal variations in microdosimetry are the dominant factor. The implications of our findings for dating these deposits are discussed in short.

Post-Palaeozoic cooling and uplift of the Brabant Massif as revealed by apatite fission track analysis

A fission track study has been carried out on apatite from the igneous rock belt running along the southern border of the Brabant Massif. The study includes age determinations and a length analysis of both surface tracks and confined tracks. Apatite fission track ages vary between 146 Ma and 209 Ma. Confined track length distributions and the projected length age spectra indicate that the rocks cooled relatively rapidly from above 100-degrees-C to ambient temperatures. The fission track ages therefore date a cooling phase of the Brabant Massif which is interpreted as reflecting an important uplift during the major part of the Jurassic, related to the Cimmerian tectonism which affected the North Sea basin and adjacent areas. Two apatite samples from the southerly Dinant Basin yield fission track ages around 200 Ma, similar to the oldest ages observed in the Brabant Massif, and with comparable track length characteristics. This indicates that the uplift was not limited to the Brabant region but also affected the Hercynian basement to the south.

On the efficiency of fission-track counts in an internal and external apatite surface and in a muscovite external detector

Abstract The frequency distributions per unit area of the projected lengths ( p -distributions) of fission tracks intersecting an internal and external apatite surface and the surface of a muscovite external detector have been established by measurement. Deviations from the ideal distributions on a number of points can be tied to the effects of track revelation and observation. The effect of track revelation, in particular, masks the effect of variations of true track length, and precludes temperature–time path modelling based on length measurements of surface tracks. These effects do not prevent calculation of the track counting efficiencies ( ηq ) in the track registration geometries of interest to fission-track analysis: ηq =1.01±0.01 for induced tracks revealed in an external surface of Durango apatite, ηq =0.91±0.01 for both fossil and induced tracks revealed in an internal surface of Durango apatite, and ηq =0.91±0.01 for induced tracks revealed in a muscovite external detector. The fact that the latter are significantly less than unity is not due to an etching effect (critical angle θ c ) but to an observation threshold, best described by a critical depth z c . For tracks revealed in an internal surface, ηq decreases rapidly with decreasing track length. As a result, the apparent age of strongly annealed apatite samples may be underestimated by as much as 5%, irrespective of whether the absolute method, the Z -method, or the ζ -method is used for dating.

A comparison of methods for the annual radiation dose determination in the luminescence dating of loess sediment.

Abstract A study was made of the performance of several methods for annual radiation dose determination in luminescence dating of loess sediment. In a homogeneous loess site at Volkegem (Belgium), NaI(Tl) gamma-ray field measurements were performed, followed by collection of samples which were then analyzed in the laboratory with the following techniques: low-background NaI(Tl) gamma-ray spectrometry; low-background extended-energy range Ge gamma-ray spectrometry; k 0 -standardized reactor neutron activation analysis; thick-source ZnS alpha counting, both in the integral and in the pair counting mode; low-background GM beta counting; and atomic absorption spectrometry. A critical comparison was made of the K, Th and U concentration data, and also of calculated and measured annual radiation doses, as obtained from the different methods investigated. From the Ge gamma-spectrometric results, conclusions could be drawn concerning the equilibrium of the Th and U series.

The Last Interglacial palaeosol in the Belgian loess belt: TL age record

Abstract In the Belgian loess plateau, the Rocourt soil is a well-known luvisol horizon that is regarded as representing the Last Interglacial. The name of the soil comes from the locality of Rocourt in the east of the country, where it has been observed for the first time. Later, exposures of a similar soil were found in other localities. Most typically, a sequence of three horizons is observed: a red-brown illuvial Bt horizon, overlain by a bleached horizon and a compound dark horizon. In this paper, we present the results of TL age determinations obtained on sediments directly bracketing the soil at the type locality of Rocourt and at the locality of Momalle more to the west, together with some supplementary data for the exposures at Kesselt to check a controversial result that was obtained earlier (J. Quaternary Sci. 13 (5) (1998) 487). The TL analyses involved both the (total bleach) additive dose and regeneration methods. Our TL ages confirm the assumed chronostratigraphic position (oxygen isotope stage 5) of the soil exposed at the type locality. The ages obtained at Momalle and Kesselt indicate that we are dealing with the same soil here but the palaeodose data are less consistent. This is probably due to the fact that the loess has been disturbed by post-depositional processes such as solifluction, surface run-off, cryo- and bioturbation. This seems to be a major problem with the sediments in the eastern part of the Belgian loess belt.

CALIBRATION OF THE FISSION-TRACK DATING METHOD : IS CU USEFUL AS AN ABSOLUTE THERMAL NEUTRON FLUENCE MONITOR ?

Thermal neutron fluence measurements have been performed with Au and Co standard monitors at the one side and Cu monitors (foils and wires) at the other. using three reactor channels with different neutron energy spectra and two calibrated Ge detectors. Care was taken to eliminate errors caused by epithermal activation and neutron self-shielding while special attention was also paid to the problem of incomplete annihilation of the 64Cu β+ radiation. Our new measurements reveal consistent fluences between Cu and the Au and Co standard monitors, when using the recommended and/or recently evaluated nuclear parameters for the 63Cu(n,γ)64Cu reaction. However, we do not endorse the usage of Cu as a standard fluence monitor for fission-track dating because from a metrological standpoint too large uncertainties still exist on some of its relevant nuclear parameters such as the thermal neutron activation cross-section (σ0) of 63Cu.