Thermische Energieflüsse an der Wasseroberfläche: Beispiel Sempachersee

Data collected every 20 minutes for 18 months by a meteorological buoy moored on Lake Sempach in Switzerland (maximum depth 86 m, surface area 14.1 km²) are used to calculate different processes contributing to the net heat flux between water and atmosphere. The processes considered are shortwave and longwave radiation, evaporation/condensation and sensible heat transfer. The temporal resolution of the measurements allows the evaluation of the processes occurring on three different time scales: diurnal variations, weather events of a few days and yearly cycles. The heat content of the lake is calculated from quasi-continuous measurements of water temperature at different depths. The yearly amplitude of the heat content is 1100·10⁶ J/m². Short-time variations of the heat flux determined from water temperature analysis agree well with the flux variations modeled using meteorological data. However, the latter generally underestimates the measured net heat flux in the long term. Wind measurements, together with the net heat flux, are used to calculate the Flux Richardson Number and the Monin-Obukov Length. Examples are given to show the predominant influence of the wind on the stratification of the upper water column and thus on the surface water temperature.      

A method to determine crystallization temperatures of hydrated crystals by intracrystalline oxygen isotope effects

The fractionation of the oxygen isotopes between water molecules from different sites in CuSO₄ · 5H₂O has been measured for crystallization temperatures of 25, 40 and 50°C. The temperature dependence found provides a basis for the determination of crystallization temperatures from such intracrystalline oxygen isotope effects; also for hydrated crystals of mineralogical and geological interest. Necessary suppositions for the application of this method are discussed.     

Mesozoic (Late Jurassic) palynomorphs and charcoal from a sandstone pebble from Quaternary glacial deposits of the North Sea

The palynological contents as well as macroscopic charcoal fragments from a calcareous sandstone pebble from Quaternary glacial deposits of the North Sea (80 km SW of the Dogger Bank) are described. The taxonomic composition of the palynoflora points to a Late Jurassic (or Late Jurassic/Early Cretaceous) age of the sandstone. The charcoal is interpreted as direct evidence of palaeo‐wildfire occurring during this period in the source area of the sediments of this particular sandstone. This finding, together with already published data on Mesozoic deposits, allows to conclude that wildfires were obviously widespread during this period in the area of the modern day North Sea and adjacent areas. Copyright © 2012 John Wiley & Sons, Ltd.     

Uranium–lead ages of apatite from iron oxide ores of the Bafq District,East-Central Iran

Iron oxide–apatite (IOA) deposits, often referred to as Kiruna-type iron ore deposits, are known to have formed from the Proterozoic to the Tertiary. They are commonly associated with calc–alkaline volcanic rocks and regional- to deposit-scale metasomatic alteration. In the Bafq District in east Central Iran, economic iron oxide–apatite deposits occur within felsic volcanic tuffs and volcanosedimentary sequences of Early Cambrian age. In order to constrain the age of formation of these ores and their relationship with the Early Cambrian magmatic event, we have determined the U–Pb apatite age for five occurrences in the Bafq District. In a ²⁰⁶Pb/²³⁸U vs. ²⁰⁷Pb/²³⁵U diagram, apatite free of or poor in inclusions of other minerals plots along the Concordia between 539 and 527 Ma with four out of five samples from one deposit clustering at the upper end of this range. For this deposit, we interpret this cluster to represent the age of apatite formation, whereas the spread towards younger ages may reflect either minor Pb loss or several events of IOA formation. Apatite with inclusions of monazite (±xenotime) yields disturbed systems with inclusions having developed after formation of the iron ore–apatite deposits, possibly as late as 130–140 Ma ago. Obtained apatite ages confirms that (IOA) and the apatite-rich rocks (apatites) of the Bafq district formed coevally with the Early Cambrian magmatic (-metasomatic) events.     

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Gondwana breakup: no evidence for a Davie Fracture Zone offshore northern Mozambique,Tanzania and Kenya

Plate tectonic reconstructions assume a major inactive transform fault, the Davie Fracture Zone, in the West Somali Basin, along which Madagascar is thought to have migrated southwards following Gondwana breakup in the Mesozoic. Based on the interpretation of reflection seismic data, we show that the Walu Ridge offshore Kenya and the Kerimbas Basin offshore northern Mozambique are tectonically unrelated to the southward motion of Madagascar and correlate with Late Cretaceous volcanism and inversion in Kenya and the evolution of the East African Rift System respectively. Offshore Tanzania, geophysical data do not show basement structures indicating the presence of a major transform fault. These results challenge the commonly supported transform margin concept and imply a more southerly pre‐breakup position of Madagascar within Gondwana. Opening of the West Somali Basin by SW‐propagating oblique rifting and seafloor spreading is proposed.     

New constraints on the auriferous Witwatersrand sediment provenance from combined detrital zircon U–Pb and Lu–Hf isotope data for the Eldorado Reef (Central Rand Group,South Africa)

Combined U–Pb and Lu–Hf isotope analyses of detrital zircon grains from the auriferous Eldorado Reef conglomerate, upper Central Rand Group, reveal new insights into the provenance of the sediments and thus, by implication, possibly also into that of the gold. Most of the detrital zircon grains, which are of magmatic origin, yielded Mesoarchaean ages clustering around 2.94 and 3.06 Ga. A subordinate zircon population gave ages with maxima at 3.28 and 3.44 Ga. The Mesoarchaean zircon grains mostly show super-chondritic ?Hf_t of up to +5.2, whereas the Palaeoarchaean zircon grains have nearly chondritic composition with ?Hf_t between −1.3 and +2.0. The new dataset of the Mesoarchaean zircon populations provides the first unambiguous evidence of the formation of juvenile crust not only at 3.06 but also at 2.94 Ga. As the analysed zircon grains are from the ruditic fraction, they must be derived from a comparatively proximal source in close vicinity to the Central Rand Basin. Based on currently available data, this source was most likely a magmatic arc that existed at the northern edge of the Witwatersrand Block at 3.06 Ga. An additional source might be the 2.94 Ga magmatic rocks of the Kraaipan Greenstone Belt that occurs to the west of the Witwatersrand Block. The minor fraction of Palaeoarchaean zircon grains in the Eldorado Reef perhaps stem from sources that are isotopically similar to the Barberton Greenstone Belt and the Limpopo Belt but were more proximal to the Central Rand Basin.     

Temporal and geochemical evolution of the Cenozoic intraplate volcanism of Zealandia

In order to constrain better the distribution, age, geochemistry and origin of widespread Cenozoic intraplate volcanism on Zealandia, the New Zealand micro-continent, we report new ⁴⁰Ar/³⁹Ar and geochemical (major and trace element and Sr–Nd–Hf–Pb isotope) data from offshore (Chatham Rise, Campbell and Challenger Plateaus) and onland (North, South, Auckland, Campbell, Chatham and Antipodes Islands of New Zealand) volcanism on Zealandia. The samples include nephelinite, basanite through phonolite, alkali basalt through trachyte/rhyolite, and minor tholeiite and basaltic andesite, all of which have ocean island basalt (OIB)-type trace element signatures and which range in age from 64.8 to 0.17 Ma. Isotope ratios show a wide range in composition (⁸⁷Sr/⁸⁶Sr = 0.7027–0.7050, ¹⁴³Nd/¹⁴⁴Nd = 0.5128–0.5131, ¹⁷⁷Hf/¹⁷⁶Hf = 0.2829–0.2831, ²⁰⁶Pb/²⁰⁴Pb = 18.62–20.67, ²⁰⁷Pb/²⁰⁴Pb = 15.54–15.72 and ²⁰⁸Pb/²⁰⁴Pb = 38.27–40.34) with samples plotting between mid-ocean-ridge basalts (MORB) and Cretaceous New Zealand intraplate volcanic rocks.Major characteristics of Zealandia's Cenozoic volcanism include longevity, irregular distribution and lack of age progressions in the direction of plate motion, or indeed any systematic temporal or spatial geochemical variations. We believe that these characteristics can be best explained in the context of lithospheric detachment, which causes upwelling and melting of the upper asthenospheric mantle and portions of the removed lithosphere. We propose that a large-scale seismic low-velocity anomaly, that stretches from beneath West Antarctica to Zealandia at a depth of > 600 km may represent a geochemical reservoir that has been in existence since the Cretaceous, and has been supplying the upper mantle beneath Zealandia with HIMU-type plume material throughout the Cenozoic. In addition, the sources of the Cenozoic intraplate volcanism may be at least partially derived through melting of locally detached Zealandia lower lithosphere.