Günther A. Wagner

Geochronology of high-pressure rocks on Sifnos (Cyclades, Greece)

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M1 phengite.K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M1+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon.K-Ar and Rb-Sr ages on 2M1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.

FISSION TRACK DATING OF APATITES.

Abstract Fission track dating was applied to a large number of apatites for the first time. In apatite it is easy to identify fission tracks on the (0001) face if one uses the oil immersion technique. Fission tracks in apatite survive 1 m.y. only if the temperature does not exceed 100°C. Apatites taken from different types of rocks from the Odenwald (Germany) were studied. The fission track ages obtained have to be interpreted individually. The apatites taken from volcanic rocks yield crystallization ages. In contrast, apatite ages obtained on basement rocks (granites, granodiorites, gabbros) are related to the uplift history of the Odenwald.

Fission-track stability in zircons under geological conditions

Abstract The stability behavior of fission tracks in zircon under natural annealing conditions has been studied. The samples have been obtained from the drill core of basement rocks (1664–3330 m depth) of Urach (Southwest Germany). Using the external detector method, zircon fission-track ages between 158 and 327 Myr were found. The ages decrease steadily downhole. For a cooling rate of 1°C/1 Myr the closure temperature for zircon has been estimated to 210 ± 20°C. A geometry factor of 1.47 ± 4.7% (4π-zircon/2π-muscovite) has been determined relevant to the dating procedure used in this work.

Radiometric dating of the type-site for Homo heidelbergensis at Mauer, Germany

The Mauer mandible, holotype of Homo heidelbergensis, was found in 1907 in fluvial sands deposited by the Neckar River 10 km southeast of Heidelberg, Germany. The fossil is an important key to understanding early human occupation of Europe north of the Alps. Given the associated mammal fauna and the geological context, the find layer has been placed in the early Middle Pleistocene, but confirmatory chronometric evidence has hitherto been missing. Here we show that two independent techniques, the combined electron spin resonance/U-series method used with mammal teeth and infrared radiofluorescence applied to sand grains, date the type-site of Homo heidelbergensis at Mauer to 609 ± 40 ka. This result demonstrates that the mandible is the oldest hominin fossil reported to date from central and northern Europe and raises questions concerning the phyletic relationship of Homo heidelbergensis to more ancient populations documented from southern Europe and in Africa. We address the paleoanthropological significance of the Mauer jaw in light of this dating evidence.

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.

Recognition of insufficient bleaching by small aliquots of quartz for reconstructing soil erosion in Greece

Optical dating of sediments requires sufficient bleaching of the material to be dated. Using single aliquot techniques, the statistical parameter of the coefficient of variation v in combination with the reduction of mineral grains per aliquot enables the detection of insufficiently bleached samples. This was empirically tested on quartz extracts from the Greek study area, which were artificially bleached, differently dosed and then mixed to simulate insufficiently bleached samples. The single aliquot regenerative dose protocol was applied. The test measurements show that for the studied quartz material, the detection of insufficient bleaching is possible if small aliquots of 200–500 grains and a threshold value of v=10% are used. The technique was applied to the age determination of colluvial and alluvial/torrential deposits from the study area, the Basin of Phlious in Greece. The optically stimulated luminescence age data allow reconstruction of the history of Holocene soil erosion, which essentially started in the 7th millennium BC, simultaneous with the onset of agricultural activities in the early Neolithic.

New K-Ar and fission track ages of impact glasses and tektites

Abstract Recently collected Ivory Coast tektites, moldavites and impact glasses have been dated by the K-Ar and fission track methods. The measured ages for moldavites and impact glass from the Nordlinger Ries Kessel agree well with each other as do the Ivory Coast tektites and impact glass from the Bosumtwi crater (Ghana). These facts give further support to a hypothesis of terrestrial origin of tektites.

North American microtektites from the Caribbean Sea and their fission track age

Over 6000 microscopic glass spherules between 125 μm and 1 mm in diameter were found in a sediment core (RC9-58) from the Caribbean Sea. These glassy objects are mostly confined to a zone ∼ 40 cm thick at a depth of ∼ 250 cm. We believe that the microscopic glass objects are microtektites belonging to the North American strewnfield, based on their geographical location, appearance, physical properties, stratigraphic age (middle Upper Eocene), fission track age (∼34.6 my) and major element compositions. The occurrence of North American microtektites in the Caribbean Sea indicates that the North American strewnfield is two to three times larger than previously indicated. An estimate on the abundance of microtektites in core RC9-58 indicates that the North American strewnfield may contain greater than 1017 g of tektite material.

Infrared stimulated luminescence dating of holocene colluvial sediments using the 410 NM emission

Abstract Infrared stimulated luminescence (IRSL) ages are reported for loessic colluvial sediments from Wiesenbach, south Germany. The samples were collected by boring and the polymineral fine-grain fraction t4-1 I μm) was used for IRSL investigations. IRSL-spectra were measured prior to dating. They show two major emissions at approx. 560 and 410 nm. The advantages of the subtraction dating technique (Aitken and Xie. 1992) using the 410 run emission are demonstrated. The ages obtained are used for the reconstruction of the landscape development at the studied site: subsequent to an erosion phase, the deposition of a colluvium started prior to the Roman period and increased to a maximum accumulation rate during the Middle Ages.

Correction and Interpretation of Fission Track Ages

In its early stages, fission track dating was often underrated and regarded as an unreliable age determination method since its data frequently turned out as younger than independently determined ages on the same samples. Obviously, the prerequisite that all fission tracks must be retained since the formation of the sample was not met. It is now known that track retention is a very temperature-sensitive and characteristic property of minerals and natural glasses. The understanding of track retention is the key for the proper interpretation of fission track ages.