Dieter F. Mertz

Sr-Nd-Pb isotope evidence against plume-asthenosphere mixing north of Iceland

Iceland straddles the mid-Atlantic spreading axis, between the Kolbeinsey Ridge to the north and the Reykjanes Ridge to the south. Published geochemical data from the Reykjanes Ridge show evidence for mixing between a MORB component and the Iceland plume. Available data from the Kolbeinsey Ridge suggest that similar mixing may not be occurring there. To investigate in detail the relationship between the Iceland plume and MORB along the Kolbeinsey Ridge, we have collected and analysed samples between the Tjo¨rnes and Spar fracture zones (ca. 67°–69°N). The 16 Kolbeinsey Ridge samples show limited isotopic variation and are characterised by relatively unradiogenic Pb (206Pb/204Pb= 17.912 to 18.053, 207Pb/204Pb= 15.404 to 15.453 and 208Pb/204Pb= 37.543 to 37.690, 87Sr/86Sr= 0.70280 to 0.70298, 143Nd/144Nd= 0.51307 to 0.51323). On the basis of their Rb, Sr, Nd, Sm, U, Th and Pb concentrations, the basalts are N-type MORB. Sr and Nd isotope ratios show significant systematic variations with latitude, becoming more enriched (87Sr/86Sr increases, 143Nd/144Nd decreases) towards Iceland, apparently supporting the classical model of plume-asthenosphere mixing. However, the Pb isotopes show no such relationship, and are thus inconsistent with this mixing model. On the basis of Pb and Sr isotope data it is possible to exclude the Iceland source as an end-member in the genesis of the Kolbeinsey Ridge basalts, implying that Iceland plume material does not flow northward along the Kolbeinsey Ridge. The isotopic variations within the Kolbeinsey data set can be attributed to heterogeneities in the MORB source. The boundary between the plume and MORB sources appears to coincide with the Tjo¨rnes Fracture Zone. This fracture zone may, by analogy with the Australia-Antarctic Discordance, overlie a zone of mantle convergence. The topographic anomalies over the Kolbeinsey and Reykjanes Ridges imply that hot, less dense material underlies them both. The absence of an Icelandic plume signature in the Kolbeinsey geochemistry, however, leads us to propose an asymmetrical shape for the plume, generated by a southerly component of flow in the Kolbeinsey MORB source. A similar flow direction has previously been proposed for the whole North Atlantic on the basis of independent mantle mass-balance calculations

Numerical dating of the Eckfeld maar fossil site, Eifel, Germany: a calibration mark for the Eocene time scale.

Abstract Sediments of the Eckfeld maar (Eifel, Germany) bear a well-preserved Eocene fauna and flora. Biostratigraphically, Eckfeld corresponds to the Middle Eocene mammal reference level MP (Mammals Paleogene) 13 of the ELMA (European Land Mammal Age) Geiseltalian. In the maar crater, basalt fragments were drilled, representing explosion crater eruption products. By 40Ar/39Ar dating of the basalt, for the first time a direct numerical calibration mark for an Eocene European mammal locality has been established. The Eckfeld basalt inverse isochron date of 44.3±0.4 Ma suggests an age for the Geiseltalian/Robiacian boundary at 44 Ma and, together with the 1995 time scale of Berggren et al., a time span ranging from 49 to 44 Ma for the Geiseltalian and from 44 to 37 Ma for the Robiacian, respectively. Additional 40Ar/39Ar dating on a genetically related basalt occurrence close to the maar confirms a period of volcanism of ca. 0.6 m.y. in the Eckfeld area, matching the oldest Eocene volcanic activity of the Hocheifel volcanic field.

Geodynamic Setting of the Tertiary Hocheifel Volcanism (Germany), Part I: 40Ar/39Ar geochronology

The Eifel volcanism is part of the Cenozoic Central European Volcanic Province and is located close to the Rhine Graben which has been formed by rifting and subsidence since the Eocene. Whereas the Quaternary volcanism of the Eifel appears to be genetically related to mantle plume activity, the cause of the Tertiary volcanism of the Hocheifel volcanic field is less clear. Here, we present geochronological evidence for the geotectonic setting of the Tertiary Eifel volcanism based on 40Ar/39Ar dating of 27 samples from 25 volcanic occurrences. Included are samples from the northern Upper Rhine Graben in order to evaluate a possible relationship between Hocheifel volcanism and Rhine Graben taphrogenesis.

Geodynamic Setting of the Tertiary Hocheifel Volcanism (Germany), Part II: Geochemistry and Sr, Nd and Pb Isotopic Compositions

Major and trace element as well as isotopic compositions on 26 volcanic rocks from the Tertiary Hocheifel volcanic field and for comparison from Upper Rhine Graben occurrences (western Central European Volcanic Province) were measured in order to provide geochemical evidence for the geodynamic setting of the Tertiary Eifel volcanism. Except for a few differentiated lavas there are mainly basanitic compositions. These rocks were produced by low degree partial melting of a previously metasomatized garnet peridotite source at pressures and temperatures corresponding to depths of about 75 to 90 km. In contrast to the differentiated lavas, most of the basanites are not significantly affected by crustal contamination. In the Hocheifel, the crustally contaminated basanites and the differentiated lavas form an older age group (ca. 44-39 Ma). In contrast, the basanites not affected by crustal contamination belong to a younger age group (ca. 37-35 Ma) indicating a change in the petrogenetic style of evolution with time. Low-radiogenic Sr together with high-radiogenic Nd and Pb of the Hocheifel lavas show isotope characteristics similar to LVC (Low Velocity Composition) or FOZO (Focal Zone) mantle sources. Since FOZO is considered to be a common component in the entire mantle, a contribution from the deep mantle can be neither confirmed nor excluded. Plate reconstruction modelling indicates that the Hocheifel volcanic field at its time of activity ca. 40 m.y. ago was located ∼1000 km southwest relative to its recent position, and therefore a relation to a mantle-stationary Quaternary Eifel plume is not plausible. Integrating evidence from geochronology, geochemistry, isotopic compositions and plate reconstruction, we conclude that the Tertiary Hocheifel volcanism is not genetically related to a mantle plume but is caused by Middle to Upper Eocene pre-rift extensional decompression related to the Rhine Graben taphrogenetic evolution.

Einfaches Verfahren zur Wasserbestimmung in biologischem Material

ZusammenfassungDie Wasserbestimmung beruht auf dem Prinzip der Entmischung eines Xylol-Alkohol-Wasser-Gemisches. Der Wassergehalt läßt sich bei konstanter Temperatur aus der verbrauchten Xylolmenge errechnen, wenn man eine bestimmte Alkoholmenge genau eingestellter Konzentration verwendet. Für eine Bestimmung genügen 0,4 cm3 bzw. 0,4 g Untersuchungsmaterial. Die Methode weist eine Genauigkeit bis 0,3% auf.

Students' field research extends knowledge of origin of a UNESCO World Heritage site in Germany

In 1992, as part of field-based course work with the Earth science department of the Universitat Minz, students began to investigate the structures of oil shale basins located in the Sprendlinger Horst, a horst-type block forming the northeastern shoulder of the Tertiary Upper Rhine Graben in southwestern Germany (figure 1). The Sprendlinger Horst is mainly built up by Hercynian or pre-Hercynian basement, Permian sediments, and volcanic rocks, as well as by several Tertiary alkali basalts and rare Cretaceous trachytes. In 1992, it was unknown whether the oil shale basins were of tectonic, volcanic, or even of impact origin.