Rapid XRD determination of the chrysotile/lizardite ratios in asbestos-bearing serpentinites

Summary The quantitative XRD determination of the most common serpentinite minerals, e.g. lizardite and chrysotile, is hampered by strongly overlapping reflections. Reconnaissance investigations indicated that the reflections 204 of lizardite and 008 of chrysotile are best suited for quantitative XRD. These lines are not interferred by other minerals such as brucite, magnesite, chlorite or talc, which are common in serpentinites. A calibration curve for the determination of the chrysotile/lizardite ratios in natural serpentinites has been constructed by means of synthetically prepared chrysotile/lizardite standards. Using this method serpentinites of the Msauli Chrysotile Asbestos Mine, South Africa, were investigated for their relative chrysotile contents. It was found, that the total amount of chrysotile in the ore zone is considerably higher than the amount of extractable chrysotile asbestos fibre.

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Die röntgenographische Bestimmung der Chrysotil/Lizardit-Verhältnisse in asbesthaltigen Serpentiniten

Zusammenfassung Die quantitative röntgenographische Bestimmung der beiden häufigsten Serpentinminerale, Lizardit und Chrysotil, ist wegen der Überlagerung ihrer stärksten Reflexe erschwert. Aufgrund von Voruntersuchungen konnte jedoch festgestellt werden, daß die Reflexe 204 von Lizardit und 008 von Chrysotil für die quantitative Bestimmung geeignet sind. Diese Reflexe werden nicht überlagert von denen anderer häufig in Serpentiniten vorkommender Minerale, wie z.B. Brucit, Magnesit, Chlorit oder Talk. Eine Eichkurve zur Bestimmung der Chrysotil/Lizardit-Verhältnisse in natürlichen Serpentiniten wurde mit Hilfe synthetisch hergestellter Standardmischungen aufgestellt. Serpentinite der Msauli Chrysotilasbest Mine, Südafrika, wurden aufgrund der hier vorgestellten Methoden auf ihren relativen Chrysotilanteil untersucht. Es ergab sich, daß der totale Gehalt an Chrysotil in der erzführenden Zone deutlich größer ist als der Gehalt an ausbringbaren Chrysotilasbestfasern.

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With 4 Figures     

Radioactive anomalies associated with the graphite-bearing metasediments,Wadi Umm Gheig area,Central Eastern Desert,Egypt

The study area is located in the central part of the Eastern Desert of Egypt and is mainly covered by different varieties of Precambrian basement rocks represented mainly by younger and older granites, metadiorite, metavolcanics, and metasediments. The analysis and interpretation of airborne gamma-ray spectrometric survey data are essentially based on the computation of the three radioelements (U, Th, and K) favorability indices, estimation of uranium migration rate percentage, variation of eU with eTh and eU/eTh ratio, and the construction of the contour map of the (eU-eTh/3.5) in the different rock units. The highest percent of uranium migration- out or leaching rate is connected with the red and pink granites of Gebel Kadabora El-hamra, metadiorite, and red and pink granites of G. Umm Rakham (??20.8%, ??18.57%, and ??8.45% respectively), which indicates that they could represent a major U-source bodies in the area. It was observed that the metasediments and associated graphite-bearing schists west and northwest of G. Kadabora El-hamra reflect more precipitation than the other locations around the pluton (the uranium migration rates varies between 2.59 and 30%) which mean that the graphite may have acted as a reducing agent for uranium carried in oxidizing fluids (surface meteoric water) and resulted in its precipitation. In the light of the availability of uranium source, its mobility, and graphite-bearing metasediments, the area has a good potential for the possible occurrence of uranium mineralization.     

The role of convection in surface property and velocity fluctuations

Time series of temperature, humidity and horizontal wind speed are presented from data collected at two tower sites during the 1968 Barbados Experiment. Marked and periodic fluctuations on the convective scale are evident in the traces. It is postulated that these fluctuations are due to moderate cumulus convection. Estimates show that departures in temperature and humidity at the surface must be due to downward mixing from near or above the convective cloud base. Such cloud layer-subcloud layer mixing must change the thermodynamic character of the subcloud layer. This in turn suggests that cloud convection may play an important part in governing the transport of energy across the air-sea interface.     

New insights into the size and timing of the Lawn Hill impact structure: relationship to the Century Zn – Pb deposit

The Lawn Hill circular structure in northwest Queensland contains unambiguous evidence of an extraterrestrial impact, including planar deformation features in quartz, impact diamonds, widespread shatter cone formation and impact melt breccia in the Mesoproterozoic basement. The question of its relevance to ore genesis is investigated because the world-class Century Zn – Pb deposit is situated at the conjunction of the 100+ km Termite Range Fault and the previously defined margin of the impact structure. The impact structure is considered to be a 19.5 km wide feature, this constrained in part by the outer margin of an annulus of brecciated and highly contorted limestone. New evidence is presented indicating impact into this Cambrian limestone, including: (i) ‘dykes’ of brecciated Cambrian limestone extending hundreds of metres into the Mesoproterozoic basement; (ii) highly contorted bedding in the limestone annulus compared with essentially undeformed limestone away from the impact site; as well as (iii) a 1 Mt megaclast of Mesoproterozoic Century-like ore suspended in the limestone. Through aerial photograph analysis, large-scale convoluted flow structures within the limestone are identified, and these are interpreted to indicate that parts of the Cambrian sequence may have been soft or only semi-consolidated at the time of impact. This highly contorted limestone bedding is suggested to represent slump-filling of an annular trough in response to impact-induced partial liquefaction of a sediment veneer. The age of impact is therefore considered to be concurrent with limestone formation during the Ordian to early Templetonian, at 520 – 510 Ma. Formation of the Century deposit is found to be unrelated to impact-generated hydrothermal activity, although some minor hydrothermal remobilisation of metals occurred. However, there was macro-scale remobilisation of gigantic ore fragments driven by impact-induced lateral and vertical injection of limestone into the Proterozoic sediments. The limestone-filled annular trough surrounds a 7.8 km diameter central uplift, consistent with formation of a complex crater morphology.     

Zircon geochronology and geochemistry to constrain the youngest eruption events and magma evolution of the Mid-Miocene ignimbrite flare-up in the Pannonian Basin,eastern central Europe

A silicic ignimbrite flare-up episode occurred in the Pannonian Basin during the Miocene, coeval with the syn-extensional period in the region. It produced important correlation horizons in the regional stratigraphy; however, they lacked precise and accurate geochronology. Here, we used U–Pb (LA-ICP-MS and ID-TIMS) and (U–Th)/He dating of zircons to determine the eruption ages of the youngest stage of this volcanic activity and constrain the longevity of the magma storage in crustal reservoirs. Reliability of the U–Pb data is supported by (U–Th)/He zircon dating and magnetostratigraphic constraints. We distinguish four eruptive phases from 15.9 ± 0.3 to 14.1 ± 0.3 Ma, each of which possibly includes multiple eruptive events. Among these, at least two large volume eruptions (>10 km³) occurred at 14.8 ± 0.3 Ma (Demjén ignimbrite) and 14.1 ± 0.3 Ma (Harsány ignimbrite). The in situ U–Pb zircon dating shows wide age ranges (up to 700 kyr) in most of the crystal-poor pyroclastic units, containing few to no xenocrysts, which implies efficient recycling of antecrysts. We propose that long-lived silicic magma reservoirs, mostly kept as high-crystallinity mushes, have existed in the Pannonian Basin during the 16–14 Ma period. Small but significant differences in zircon, bulk rock and glass shard composition among units suggest the presence of spatially separated reservoirs, sometimes existing contemporaneously. Our results also better constrain the time frame of the main tectonic events that occurred in the Northern Pannonian Basin: We refined the upper temporal boundary (15 Ma) of the youngest counterclockwise block rotation and the beginning of a new deformation phase, which structurally characterized the onset of the youngest volcanic and sedimentary phase.     

A simple approximation forJ v (σ v ,κ v ,τ v,B v ), and its application for temperature-correction procedures

For the case of isotropic coherent scattering plus absorption a simple expression is given (Equation 12) to compute the mean intensity of the radiationJ _v (as a function of optical depth _v ) if the scattering coefficient _v , the absorption coefficientK _v and the Planck functionB _v are given as a function of depth. In general the accuracy of this approximation is of the order of a few percent.A fairly simple temperature-correction procedure for the case when scattering is important is described.     

Diatoms grow faster using ammonium in rapidly flushed eutrophic Dokai Bay,Japan

The importance of the nitrogen source for phytoplankton growth in a highly eutrophic embayment, Dokai Bay, was investigated. The DIN concentration often exceeded 100 μM of which 40–70% was NH₄ ⁺. During two incubation experiments, the natural assemblage of mainly diatoms took up NH₄ ⁺ instead of NO₃ ⁻. The growth of two Skeletonema species isolated in Dokai Bay were significantly faster on NH₄ ⁺ (1.86 and 1.27 div. d⁻¹ respectively) than on NO₃ ⁻ (1.55 and 1.04 div. d⁻¹ respectively). Our results indicated that these diatoms could grow faster by using NH₄ ⁺ compared to NO₃ ⁻ in this eutrophic bay.