A reappraisal of low-temperature thermochronology of the eastern Fennoscandia Shield and radiation-enhanced apatite fission-track annealing

Abstract

We assess the proposal of Hendriks & Redfield (Earth and Planetary Science Letters, 236, 443-458, 2005) that cross-over of the predicted apatite fission track (AFT) . (U-Th-Sm)/ He (AHe) age relationship in the southeastern Fennoscandian shield in southern Finland reflects a-radiation-enhanced annealing (REA) of fission tracks at low temperatures and that more robust estimates of the denudation history are recorded through reproducible AHe data. New AHe results from southern Finland showing variable dispersion of single-grain ages may be biased by different factors operating within grains, which tend to give a greater weighting towards older age outliers. AHe ages from mafic rocks show the least dispersion and tend to be consistently lower than their coexisting AFT ages. In general, it is at the younger end of the single-grain variation range from such lithologies where most meaningful AHe ages can be found. AHe data from multigrain aliquots are, therefore, of limited value for evaluating thermal histories in southern Finland, especially when compared against coexisting AFT data as supporting evidence for REA.New, large datasets from the southern Canadian and Western Australian shields show the relationship between AFT age, single-grain age or mean track length as a function of U content (determined by the external detector method). These do not display the moderately strong inverse correlations previously reported from southern Finland in support of REA. Rather, the trends are inconsistent and generally exhibit weak positive or negative correlations. This is also the case for plots from both shields, as well as those from southern Finland, where AFT parameters are plotted against effective U concentration [eU] [based on U and Th content determined by inductively coupled plasma-mass spectroscopy (ICP-MS)], which weights decay of the parents more accurately in terms of their a-productivity. Further, samples from southern Finland yield values of chi-square x 2 .5%, indicating that there is no significant effect of the range of uranium content between grains within samples on the AFT ages, and that they are all consistent with a single population.The oldest AFT ages in southern Finland apatites (amongst the oldest recorded from anywhere) are found in gabbros, which also have the highest Cl content of all samples studied. We suggest, that it is Cl content rather than REA that has influenced the annealing history of the apatites, which have experienced a history including reburial into the partial annealing zone by Caledonian Foreland basin sedimentation. The study of apatite from low U and Th rocks, with relatively low levels of a-radiation damage may provide the most practical approach for producing reliable results for AFT and AHe thermochronometry studies in cratonic environments.

Phanerozoic denudation across the Kola Peninsula, Northwest Russia: implications for long-term stability of Precambrian shield margins

“…Specifically, identification of reheating in AFT studies has led to appeals for burial by thick sedimentary covers of which few, if any traces remain (Gunnell, 2000). The constraints provided by long-term denudation indicators in areas such as the Kola Peninsula are a neglected component in the vigorous debates over the systematics and meaning of AFT analyses on the Fennoscandian Shield (Murrell & Andriessen, 2004;Hendriks & Redfield, 2005;Bonow et al, 2006;Larson et al, 2006;Kohn et al, 2009). …”

“…A more recent and sharply contrasting view, grounded firmly in apatite fission track (AFT) thermochronology (Osadetz et al, 2002;Belton et al, 2004;Kohn et al, 2009), is that even shield interiors are tectoni cally dynamic, with km-scale phases of burial and exhumation that require large vertical movements of deep-rooted continental crust and long periods of erosion during the Phanerozoic. Resolving this debate is important not only for assessing the stability of shields but also for reconstructing erosion histories on shields and quantifying sediment supply to intra-cratonic and marginal basins.…”

Interpreting data dispersion and “inverted” dates in apatite (U–Th)/He and fission-track datasets: An example from the US midcontinent

“…These techniques have been jointly applied in numerous studies to better restrict timetemperature (tT) paths. Several datasets, however, most notably from cratonic settings, have highlighted two aspects of low temperature thermochronometry data that require additional examination: some cratonic AHe data are characterized by significant dispersion, and a number of reported cratonic AHe datasets conflict with corresponding AFT dates when using conventional models for apatite He retentivity (e.g., Crowley et al, 2002;Spotila et al, 2004;Hendriks and Redfield, 2005;Hansen and Reiners, 2006;Danisik et al, 2008;Kohn et al, 2009). Cratonic AHe and AFT data have therefore become the focal point of a controversy over the cause(s) of inconsistency between the techniques (e.g., .…”

Linking an Early Triassic delta to antecedent topography: Source-to-sink study of the southwestern Barents Sea margin

“…2A; Roberts and Lippard, 2005). From the Ordovician through the Devonian, the Fennoscandian Shield was buried by foreland basin sediments related to the Caledonian orogeny (490-390 Ma), which were later eroded (Larson et al, 1999(Larson et al, , 2006Kohn et al, 2009). Several NE-SW-oriented rift zones were formed in the Barents Sea Basin during the middle Carboniferous (Gudlaugsson et al, 1998).…”