Geology and ore genesis of the manganese ore deposits of the Postmasburg manganese-field,South Africa

The Postmasburg Mn/Fe-ores occur exclusively in dolomitic Precambrian sinkhole structures with siliceous breccias and shales as hostrocks. The main manganese minerals are braunite and bixbyite, apart from secondary alteration products of the psilomelane-manganomelane family. Various generations of ore minerals could be identified. The ore mineralization is subdivided into three different genetic types. They are classified either as pure karst deposits or as combined formations of karst origin and shallow marine sedimentation due to the transgression of the Banded Iron Formation (BIF) sea. Post-sedimentary metamorphism is identified as very low grade. The development of the different ore types is illustrated schematically.     

Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation

We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of <200, <340, ∼500, <550, and <740°C respectively. δ⁵⁶Fe of bulk layers ranges from −0.8 to +0.8‰; in some samples the layers may differ by >1‰ on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which δ⁵⁶Fe of magnetite > silicate ≥ carbonate. The inter-mineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperature-dependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record. An erratum to this article can be found at     

Matrix-independent Fe isotope ratio determination in silicates using UV femtosecond laser ablation

UV femtosecond laser ablation coupled to MC-ICP-MS provides a promising in situ tool to investigate elemental and isotope ratios by non-matrix-matched calibration. In this study, we investigate Fe isotope composition in siliceous matrices including biotite, hornblende, garnet, fayalite and forsterite (San Carlos Olivine), and an oceanic Fe–Mn crust using the iron reference material IRMM-014 for calibration. To test the accuracy of the laser ablation data, Fe isotope compositions were obtained independently by solution ICP-MS after chromatographic separation of Fe. Sample materials with low Cr content, i.e. biotite, hornblende, fayalite and the Fe–Mn crust, reveal δ^56/54Fe and δ^57/54Fe values that agree with those from solution ICP-MS data within the measured precision. For high Cr concentration (⁵⁴Cr/⁵⁴Fe >0.0001), i.e. in the garnet and forsterite sample, δ^56/54Fe and δ^57/54Fe values were derived from ⁵⁷Fe/⁵⁶Fe ratios as correction of the isobaric interference of ⁵⁴Cr on ⁵⁴Fe is unsatisfactory. This approach provides accurate results for both minerals. Moreover, the garnet crystal exhibits isotopic zonation with differences of 0.3‰ in δ^56/54Fe showing that substantial heterogeneities exist in high-temperature metamorphic minerals. Multiple analyses of homogeneous sample materials reveal a repeatability of 0.1‰ (2 SD) for δ^56/54Fe and 0.2‰ (2 SD) for δ^57/54Fe, respectively. This study adds to the observations of Horn et al. (2006) who have shown that the determination of Fe isotope ratios in various matrices including iron alloys, iron oxides and hydroxides, iron sulfide and iron carbonates can be performed with high accuracy and precision at high spatial resolution using UV femtosecond laser ablation ICP-MS. These results demonstrate that femtosecond laser ablation ICP-MS is a largely matrix-independent method, which provides a substantial advantage over commonly employed nanosecond laser ablation systems.     

Magmatic and tectonic history of Jurassic ophiolites and associated granitoids from the South Apuseni Mountains (Romania)

The Jurassic ophiolites in the South Apuseni Mountains represent remnants of the Neotethys Ocean and belong to the East Vardar ophiolites that contain ophiolite fragments as well as granitoids and volcanics with island-arc affinity. New U–Pb zircon ages, and Sr and Nd isotope ratios give insights into their tectono-magmatic history. The ophiolite lithologies show tholeiitic MOR-type affinities, but are occasionally slightly enriched in Th and U, and depleted in Nb, which indicates that they probably formed in a marginal or back-arc basin. These ophiolites are associated with calc-alkaline granitoids and volcanics, which show trace element signatures characteristic for subduction-enrichment (high LILE, low HFSE). Low ⁸⁷Sr/⁸⁶Sr ratios (0.703836–0.704550) and high ¹⁴³Nd/¹⁴⁴Nd ratios (0.512599–0.512616) of the calc-alkaline series overlap with the ratios measured in the ophiolitic rocks (0.703863–0.704303 and 0.512496–0.512673), and hence show no contamination with continental crust. This excludes a collisional to post-collisional origin of the granitoids and is consistent with the previously proposed intra-oceanic island arc setting. The new U–Pb ages of the ophiolite lithologies (158.9–155.9 Ma, Oxfordian to Early Kimmeridgian) and granitoids (158.6–152.9 Ma, latest Oxfordian to Late Kimmeridgian) indicate that the two distinct magmatic series evolved within a narrow time range. It is proposed that the ophiolites and island arc granitoids formed above a long-lived NE-dipping subduction zone. A sudden flip in subduction polarity led to collision between island arc and continental margin, immediately followed by obduction of the ophiolites and granitoids on top of the continental margin of the Dacia Mega-Unit. Since the granitoids lack crustal input, they must have intruded the Apuseni ophiolites before both magmatic sequences were obducted onto the continental margin. The age of the youngest granitoid (~153 Ma, Late Kimmeridgian) yields an estimate for the maximum age of emplacement of the South Apuseni ophiolites and associated granitoids onto the Dacia Mega-Unit.     

Ensemble empirical mode decomposition for tree-ring climate reconstructions

A novel data adaptive method named ensemble empirical mode decomposition (EEMD) was used to reconstruct past temperature and precipitation variability in two 2,328- and 1,837-year tree-ring chronologies from the Dulan region, northeastern Qinghai–Tibetan Plateau. Our results show that EEMD can be used to extract low-frequency signals from the Dulan tree-ring data. The extracted low-frequency temperature trends in the two chronologies correlate significantly with Northern Hemisphere temperatures over the past two millennia. In addition, the newly reconstructed precipitation data have a higher standard deviation than that of data reconstructed with the conventional ordinary least squares and variance matching methods and yield the best amplitude match to the instrumental data. This study shows that EEMD is a powerful tool for extracting the full spectrum of climate information in tree-ring chronologies.     

Ollo de Sapo Cambro‐Ordovician volcanics from the Central Iberian basement—A multiphase evolution

The Cambro‐Ordovician rhyodacitic to dacitic volcanics from the Central Iberian basement, currently known as Ollo de Sapo (toads eye), have been reported as a specific group of felsic porphyritic rocks with blue quartz and large phenocrysts of K‐feldspar, in a partly vitreous or fine‐grained matrix. Interpreted to form Cambro‐Ordovician volcanic domes, they are accompanied by tuffs, ignimbrites and products of reworking in a near‐surface environment. The coarse‐ to fine‐grained rocks exhibit rather large K‐feldspar phenocrysts, plagioclase and rounded blue quartz, representing former corroded phenocrysts. Their colouration indicates unmixing of TiO₂ at around 900°C during cooling from relatively high crystallisation temperatures, indicating their origin at hot lower crustal conditions. We propose at least a two‐step evolution (1) starting around 495 Ma in the lower crust of a collapsing cordillera, generating a phenocryst‐rich mush and adiabatic melting of the lower crustal protolith to produce the spectacular Ollo de Sapo porphyrites, before (2) magma ascent and crustal extension leading to a different thermal regime around 483 Ma.     

GREGOR solar telescope: Design and status

The integration and verification phase of the GREGOR telescope reached an important milestone with the installation of the interim 1 m SolarLite primary mirror. This was the first time that the entire light path had seen sunlight. Since then extensive testing of the telescope and its subsystems has been carried out. The integration and verification phase will culminate with the delivery and installation of the final 1.5 m Zerodur primary mirror in the summer of 2010. Observatory level tests and science verification will commence in the second half of 2010 and in 2011. This phase includes testing of the main optics, adaptive optics, cooling and pointing systems. In addition, assuming the viewpoint of a typical user, various observational modes of the GREGOR Fabry‐Pérot Interferometer (GFPI), the Grating Infrared Spectrograph (GRIS), and high‐speed camera systems will be tested to evaluate if they match the expectations and science requirements. This ensures that GREGOR will provide high‐quality observations with its combination of (multi‐conjugate) adaptive optics and advanced post‐focus instruments. Routine observations are expected for 2012 (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diurnal Habitability of Frozen Worlds

In this work we discuss effects allowing local habitability of some extraterrestrial planets of low average surface temperatures. We analyze the problem of diurnal and seasonal changes of temperature and biological productivity at different locations on a hypothetical Earth-like planet. We have found, that under some circumstances the temperature may locally rise well above the average value, allowing periods of enhanced biological activity. In this way, bioproductivity can become periodically possible on a planet that has an average temperature clearly below 0°C. Such thermal conditions are encountered on Mars (Smith et al. in Science 306:1750–1753, 2004) generally considered as inhabitable. In reality, an appropriate temperature is not sufficient for habitability. The presence of liquid water at the considered location is also necessary. We discuss how temperature oscillations affect habitability in the framework of a conceptual model. We find that the considered effect of diurnal and seasonal temperature oscillations can extend the outer boundary of the habitable zone up to 2 AU, while global average temperatures are below 0°C for heliocentric distances R _h > 1.12 AU (dry atmosphere, low CO₂ pressure), or R _h > 1.66 AU (humid atmosphere, high CO₂ pressure).     

Mass transfer and loss of the massive semi-detached binary AI Crucis

AI Crucis is a short-period semi-detached massive close binary (P = 1.41771d, Sp.=B1.5) in the open cluster NGC 4103. It is a good astrophysical laboratory for investigating the formation and evolution of massive close binary stars via case A mass transfer. Orbital period variations of the system were analyzed based on one newly determined eclipse time and the others compiled from the literature. It is discovered that the orbital period of the binary is continuously increasing at a rate of dP/dt = +1.00(±0....     

The simulation of medicanes in a high-resolution regional climate model

Medicanes, strong mesoscale cyclones with tropical-like features, develop occasionally over the Mediterranean Sea. Due to the scarcity of observations over sea and the coarse resolution of the long-term reanalysis datasets, it is difficult to study systematically the multidecadal statistics of sub-synoptic medicanes. Our goal is to assess the long-term variability and trends of medicanes, obtaining a long-term climatology through dynamical downscaling of the NCEP/NCAR reanalysis data. In this paper, we examine the robustness of this method and investigate the value added for the study of medicanes. To do so, we performed several climate mode simulations with a high resolution regional atmospheric model (CCLM) for a number of test cases described in the literature. We find that the medicanes are formed in the simulations, with deeper pressures and stronger winds than in the driving global NCEP reanalysis. The tracks are adequately reproduced. We conclude that our methodology is suitable for constructing multi-decadal statistics and scenarios of current and possible future medicane activities.