The origin of carbonate mud in isolated carbonate platforms of Belize,Central America

Carbonate mud from three small isolated carbonate platforms of Belize (Central America) is largely a product of the breakdown of skeletal grains. The composition of the 63-20- and 20-4-µm fractions of 37 samples was determined by point counting under SEM; composition of the <4-µm fraction was assessed qualitatively under SEM. The 63-4-µm fractions are dominated by fragments of mollusks, the codiacean alga Halimeda, and other skeletal types. About one-third of the particles in the 63-4-µm fractions remained unidentified, probably due to obliteration of diagnostic features by early recrystallization processes such as micritization. Nanograins (<1 µm) and short (3-5 µm) aragonite needles are most common in the <4-µm fraction. These grains are interpreted to be largely fragments of codiacean algae (Halimeda, Penicillus). The 20-4- and <4-µm sediment fractions are composed of 80 and 75% aragonite on average, respectively. Trace element composition of strontium averages 7,900 ppm in the 20-4-µm fraction and in codiacean algae samples, and 5,600 ppm in the <4-µm fraction. Geochemical (trace element) data also argue against inorganic aragonite precipitation within the water column, and favor a skeletal origin. The lower strontium contents in the <4-µm fraction as compared with the 20-4-µm fraction may also be a consequence of early recrystallization processes.     

Zr contents of glaucophane-bearing meta-basalts of Western Crete,Greece

Chemical analyses of hornblendes from different regional metamorphic terrains and from rocks of different metamorphic grade have been compared. Hornblendes of lowpressure type are distinguished from hornblendes of high-pressure type by their Al^VI and Si contents. The Ti content of hornblende is shown to increase with metamorphic temperature from the greenschist-amphibolite transition facies to the hornblende-granulite facies.     

Late Quaternary erosion events in lowland and mid-altitude Tasmania in relation to climate change and first human arrival

The establishment of a chronology of landscape-forming events in lowland and mid-altitude Tasmania, essential for assessing the relative importance of climatic and human influences on erosion, and for assessing present erosion risk, has been limited by the small number of ages obtained and limitations of dating methods. In this paper we critically assess previous Tasmanian studies, list published radiocarbon ages considered to be dependable, present new radiocarbon and thermoluminescence (TL) ages for 25 sites around Tasmania, and consider the evidence for the hypotheses that erosion processes at low and mid altitudes have been: (1) purely climatically controlled; and (2) influenced both by climatic and anthropogenic (increased fire frequency) effects. A total of 94 dependable finite ages (calibrated for radiocarbon and ‘as measured’ for TL and optically stimulated luminescence (OSL) determinations) are listed for deposits comprising dunes, colluvium, alluvium and loess-like aeolian deposits. Two fall in the >100 ka period, 15 fall in the period 65–35 ka, and 77 fall in the period 35–0.3 ka. There was a sustained increase in erosion recorded in the period 35–15 ka, as reflected by a greater number of dated aeolian deposits during this period.We considered three possible biases that may have affected the age distribution obtained: the limitations of radiocarbon dating, sampling bias, and preservation bias. Sampling bias may have favoured more recent dune strata, but radiocarbon dating and preservation biases are unlikely to have significantly distorted the age distribution obtained.Long but intermittent aeolian deposition is recorded at two sites (Southwood B; c. 59–28 ka and Dunlin Dune; c. 29–14 ka) but there is no evidence of regional loess deposits such as found in New Zealand. The timing of increased erosion in Tasmania between 35 and 30 ka approximately coincides with the intermittent ten-fold increase of dust accumulation between 33 and 30 ka in the Antarctic Dome C ice core. The absence of widespread erosion before 35 ka, the abrupt increase of erosion around this time, the frequent association of erosion products with charcoal, the arrival of people in Tasmania at c. 40 cal ka, and the known use of fires by Aborigines to maintain areas of non-climax vegetation suggest that ecosystem disturbance by anthropogenic fires, in a drier climate than that presently prevailing, may have contributed to erosion in lowland and mid-altitude Tasmania after 35 ka. Thus the Tasmanian erosion record provides circumstantial support for the proposition that human dispersal in southeast Australia was accompanied by significant ecological change.     

Compositional mapping of planetary moons by mass spectrometry of dust ejecta

Classical methods to analyze the surface composition of atmosphereless planetary objects from an orbiter are IR and gamma ray spectroscopy and neutron backscatter measurements. The idea to analyze surface properties with an in-situ instrument has been proposed by Johnson et al. (1998). There, it was suggested to analyze Europa's thin atmosphere with an ion and neutral gas spectrometer. Since the atmospheric components are released by sputtering of the moon's surface, they provide a link to surface composition. Here we present an improved, complementary method to analyze rocky or icy dust particles as samples of planetary objects from which they were ejected. Such particles, generated by the ambient meteoroid bombardment that erodes the surface, are naturally present on all atmosphereless moons and planets. The planetary bodies are enshrouded in clouds of ballistic dust particles, which are characteristic samples of their surfaces. In situ mass spectroscopic analysis of these dust particles impacting onto a detector of an orbiting spacecraft reveals their composition. Recent instrumental developments and tests allow the chemical characterization of ice and dust particles encountered at speeds as low as 1 km/s and an accurate reconstruction of their trajectories. Depending on the sampling altitude, a dust trajectory sensor can trace back the origin of each analyzed grain with about 10 km accuracy at the surface. Since the detection rates are of the order of thousand per orbit, a spatially resolved mapping of the surface composition can be achieved. Certain bodies (e.g., Europa) with particularly dense dust clouds, could provide impact statistics that allow for compositional mapping even on single flybys. Dust impact velocities are in general sufficiently high at orbiters about planetary objects with a radius >1000 km and with only a thin or no atmosphere. In this work we focus on the scientific benefit of a dust spectrometer on a spacecraft orbiting Earth's Moon as well as Jupiter's Galilean satellites. This ‘dust spectrometer' approach provides key chemical and isotopic constraints for varying provinces or geological formations on the surfaces, leading to better understanding of the body's geological evolution.     

Der Basaltvulkanismus des Djebel Haroudj Ostfezzan/Libyen

A geological map of Djebel Haroudj is presented which shows the distribution of volcanoes and six major basalt flows over an area of approximately 45,000 square kilometres. Differences between the flows are based on age rather than petrographic composition. The oldest flows most probably erupted in the late Pliocene, though it is possible that locally flows of Miocene or even Oligocene age are present. The middle flows most probably occurred during the Pleistocene. The absence of erosional forms and drainage indicates that the youngest flows are of post-pluvial, Holocene age. The zones of volcanic activity at Djebel Haroudj are related to three major tectonic elements of very different ages (see alsoKlitzsch 1966 a): the early Paleozoic southern Haroudj uplift, the western edge of the Mesozoic Tibesti-Sirte uplift, and the south-eastern prolongation of the Hon graben fault system of late Cretaceous to Tertiary age.     

Sur la paléoflore post-carbonifère de la bordure est du Bassin de Mourzouk (Libye)

Until recently the age of the continental sediments overlaying marine strata of Carboniferous age at the Murzuk Basin was unknown and long distance correlation was not possible because of lack of correlation criteria. The flora now described allows, for the first time, to identify the age of parts of this strata. Together with new sedimentological criteria of subdivision worked out by R.Glaeser, H.Kallenbach and I.Zöller which will be published later, the paleobotanical investigations contribute towards a final subdivision of these continental sediments. The terms “Nubian Series”, and “Posttassilien” will be abandoned, because they are useless within the area concerned. The creation of correlatable stratigraphical data is an important prerequisite for groundwater and mineral exploration within the large areas covered by these strata. Along the eastern Murzuk Basin these sediments containFilicophytae, Pteridophyllae,Pteridospermaphytae, Cycadophytae andConiferophytae. At Jebel Ben Ghnema the upper part of the lower third of the continental sediments contains plants which are known from triassic strata in Europe, India, North America, South America, South Africa, and Asia. At Jebel Ben Ghnema the top of the strata contains Jurassic plants known from Europe, North America, and Asia. Almost 300 kilometres further South at Jebel Ati near the top of these continental sediments plants of Jurassic age were found, too. It seems that the development of this flora has taken place under a generally warm and dry climate with wet seasons. From the paleophytographical point of view the flora described is of great interest, because it contains coexisting species of the Northern and Southern hemisphere. The Jurassic flora is comparable to that known from adjacent basins in Algeria and Egypt, and to flora known from Virginia.     

The lunar dust environment

Each year the Moon is bombarded by about 10⁶ kg of interplanetary micrometeoroids of cometary and asteroidal origin. Most of these projectiles range from 10 nm to about 1 mm in size and impact the Moon at 10–72 km/s speed. They excavate lunar soil about 1000 times their own mass. These impacts leave a crater record on the surface from which the micrometeoroid size distribution has been deciphered. Much of the excavated mass returns to the lunar surface and blankets the lunar crust with a highly pulverized and “impact gardened” regolith of about 10 m thickness. Micron and sub-micron sized secondary particles that are ejected at speeds up to the escape speed of 2300 m/s form a perpetual dust cloud around the Moon and, upon re-impact, leave a record in the microcrater distribution. Such tenuous clouds have been observed by the Galileo spacecraft around all lunar-sized Galilean satellites at Jupiter. The highly sensitive Lunar Dust Experiment (LDEX) onboard the LADEE mission will shed new light on the lunar dust environment. LADEE is expected to be launched in early 2013.Another dust related phenomenon is the possible electrostatic mobilization of lunar dust. Images taken by the television cameras on Surveyors 5, 6, and 7 showed a distinct glow just above the lunar horizon referred to as horizon glow (HG). This light was interpreted to be forward-scattered sunlight from a cloud of dust particles above the surface near the terminator. A photometer onboard the Lunokhod-2 rover also reported excess brightness, most likely due to HG. From the lunar orbit during sunrise the Apollo astronauts reported bright streamers high above the lunar surface, which were interpreted as dust phenomena. The Lunar Ejecta and Meteorites (LEAM) Experiment was deployed on the lunar surface by the Apollo 17 astronauts in order to characterize the lunar dust environment. Instead of the expected low impact rate from interplanetary and interstellar dust, LEAM registered hundreds of signals associated with the passage of the terminator, which swamped any signature of primary impactors of interplanetary origin. It was suggested that the LEAM events are consistent with the sunrise/sunset-triggered levitation and transport of charged lunar dust particles. Currently no theoretical model explains the formation of a dust cloud above the lunar surface but recent laboratory experiments indicate that the interaction of dust on the lunar surface with solar UV and plasma is more complex than previously thought.     

Das System KFeSi3O8−KAlSi3O8−H2O mit Untersuchungen über die Mikroklin-Sanidin-Beziehungen

Zusammenfassung Im System KFeSi₃O₈–KAlSi₃O₈ wird eine Mischungslücke gefunden, welche den Bereich von 10–60 Mol. % K-Fe-Feldspat umfaßt. Die Mischkristalle links und rechts der Mischungslücke verhalten sich ähnlich wie ihre benachbarten Endglieder. Das Fehlen von intermediären Phasen auf der Eisenseite und die Mischungslücke machen es wahrscheinlich, daß das Verhalten des K-Fe-Feldspates nicht auf das Verhalten des K-Al-Feldspates extrapoliert werden darf.

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Summary In the system KFeSi₃O₈–KAlSi₃O₈ a miscibility-gap is found from 10 to 60 Mol.% K-Fe-felspar. The mixed crystals on the right and left side of the miscibility-gap show a behaviour similar to the corresponding end-members. The lack of intermediate phases on the iron-side and the miscibility-gap make probable that one cannot extrapolate the behaviour of the K-Fe-felspar to the behaviour of the K-Al-felspar.

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Mit 3 Textabbildungen

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Herrn Professor Dr.F. Machatschki zum 70. Geburtstag gewidmet.