Raman-spectroscopy and X-ray diffractometer studies of experimentally produced diaplectic feldspar glass

Raman vibrational spectra and X-ray diffractometer scans were obtained from experimentally shocked samples of oligoclase (An₁₉) and andesine (An₄₉). Some 11 oligoclase and 15 andesine targets were shocked between 24 and 40 GPa to address the transition from crystalline to diaplectic states and to explore differences in the structural state of diaplectic feldspar glasses (maskelynite) as a function of peak shock stress. Thy symmetrical VS (T-O-T) (T=Si or Al) stretch bands are the most persistent. They disappear, however, in the noise of an unusually strong luminescent spectrum at > 32 GPa in the oligoclase and at > 30 GPa in the andesine; i.e., at pressures where transition to diaplectic glass is complete. The Raman investigations yield a maskelynite structure that is probably one of a multitude of very small domains with some order, but with a large range of local properties on the scale of small domains, either in heterogeneous size-distribution of domains or in their detailed order, if not both. This results in a very large number of Raman photon-phonon frequencies unlike glasses derived from quenched melts. Our study corroborates conclusions by others, that diaplectic glasses may be the quench products of very dense, disordered phases that exist during shock compression and that subsequently relax to these unusually dense glasses that are only known from shock processes. An origin by relaxation of highly ordered, genuine high pressure polymorphs possessing the structure of hollandite is unlikely, as no evidence for any six-fold Si-coordination was found. Detailed luminescent emission spectra were taken of the oligoclase samples and they show disappearance of the IR band and a strengthening of the green band (the blue band could not be detected with a primary radiation of wavelength 448 nm). This supports previous views that the disappearance of IR emission is most likely caused by shock-induced changes of the crystal field near Fe³⁺ sites, rather than due to quenching by Fe²⁺. The X-ray studies were primarily intended to explore whether differences in structural states of maskelynite occur on sufficiently large scales to be detected by standard diffractometry methods. This is not the case. X-ray diffractometer patterns are grossly similar, if not identical, in samples shocked between 30 and 40 GPa and may not be used to fine-tune the shock histories of naturally produced diaplectic glasses.     

Comparison of the trace element composition of Tagish Lake with other primitive carbonaceous chondrites

Abstract— A meteorite fall on 2000 January 18 was detected by U.S. Defense Department satellites which established its pre‐impact orbit. Fresh samples were collected from frozen Tagish Lake in British Columbia a week later and some properties of these samples reveal it to be a unique meteorite. We characterized Tagish Lake and 8 other samples using inductively‐coupled plasma mass spectrometry and radiochemical neutron activation analysis: data for 47 elements reveal that each of 9 carbonaceous chondrites of different type exhibit the Orgueil‐normalized plateaus expected for members of such types. Trends evident in Tagish Lake differ from all other carbonaceous chondrites, including CI and CM. Samples of Tagish Lake collected later show similar patterns affected by weathering.     

SIMS studies of Allende projectiles fired into Stardust‐type aluminum foils at 6 km/sec

Abstract— We have explored the feasibility of C, N, and O isotopic measurements by NanoSIMS and of elemental abundance determinations by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) on residues of Allende projectiles that impacted Stardust‐type aluminum foils in the laboratory at 6 km/sec. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with microcraters that were produced during the encounter of the Stardust space probe with comet 81P/Wild‐2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF‐SIMS. Crater sizes were between 3 and 190 μm. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and C and O isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, which is the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of solar system origin. TOF‐SIMS studies are restricted to materials from the crater rim. The element ratios of the major rock‐forming elements in the Allende projectiles are well‐characterized by the TOF‐SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits chemical information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially C, is crucial. This information could not be obtained in the present study due to unavoidable contamination during impact experiments.     

Properties and origin of small particles in the atmosphere of Central Asia

The properties and originof small C-bearing and mineral particles are thoroughly studied above the Central Asian Region. It is shown that the Aral Sea soils play a significant role in accumulation of the higher concentrations of the metals of the crustal origin and rare-earth elements in small mineral particles of samples     

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single‐domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion‐shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock‐related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.     

Chemical compositions and classifica tion of five thermally altered carbonaceous chondrites

To establish the chemical group provenance of the five thermally altered carbonaceous chondrites Asuka (A‐) 881551, Asuka‐882113, Elephant Moraine (EET) 96026, Mulga (west), and Northwest Africa (NWA) 3133, we quantified 44 trace elements in each of them. We also analyzed Larkman Nunatak (LAR) 04318 (CK4), Miller Range (MIL) 090001 (CR2), Roberts Massif (RBT) 03522 (CK5) as reference samples as their chemical group affinity is already recognized. We conclude that Asuka‐881551, Asuka‐882113, and Mulga (west) are thermally metamorphosed CK chondrites. Compositionally, Elephant Moraine 96026 most resembles the CV chondrites. NWA 3133 is the most significantly thermally altered carbonaceous chondrite in our suite of samples. It is completely recrystallized (no chondrules or matrix remain), but its bulk composition is consistent with a CV–CK clan provenance. The thermally labile element (e.g., Se, Te, Zn, and Bi) depletion in NWA 3133 indicates a chemically open system during the heating episode. It remains unclear if the heat necessary for its thermal alteration of NWA 3133 was due to the decay of ²⁶Al or was impact related. Finally, we infer that MIL 090001, Mulga (west), and NWA 3133 show occasional compositional signatures indicative of terrestrial alteration. The alteration is especially evident within the elements Sr, Ba, La, Ce, Th, U, and possibly Sb. Despite the alteration, we can still confidently place each of the altered chondrites within an established chemical group or clan.     

Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids

We use a combination of 2D and 3D petrographic examination and ⁴⁰Ar‐³⁹Ar analyses to examine the impact histories of a suite of seven ordinary chondrites (Baszkówka, Miller, NWA 2380, Mount Tazerzait, Sahara 98034, Tjerebon, and MIL 99301) that partially preserve their ancient, but postaccretionary, porosity ranging from 10 to 20%. We examine whether materials that seem to be only mildly processed (as their large intergranular pore spaces suggest) may have more complex shock histories. The ages determined for most of the seven OCs studied here indicate closure of the ⁴⁰Ar‐³⁹Ar system after primary accretion, but during (Baszkówka) or shortly after (others) thermal metamorphism, with little subsequent heating. Exceptions include Sahara 98034 and MIL 99301, which were heated to some degree at later stages, but retain some evidence for the timing of thermal metamorphism in the ⁴⁰Ar‐³⁹Ar system. Although each of these chondrites has olivine grains with sharp optical extinction (signaling an apparent shock stage of S1), normally indicative of an extremely mild impact history, all of the samples contain relict shock indicators. Given the high porosity and relatively low degree of compaction coupled with signs of shock and thermal annealing, it seems plausible that impacts into materials that were already hot may have produced the relict shock indicators. Initial heating could have resulted from prior collisions, the decay of ²⁶Al, or both processes.     

Zur Frage der Ausbildung und Schlußprüfung der „praktischen Geologen“

Zusammenfassung Prinzipiell sollte zwischen praktischen Geologen, die eingehende Kenntnisse in Stratigraphie und Paläontologie haben müssen, und solchen mit nur allgemeinen Kenntnissen dieser Disziplinen unterschieden werden. Erstere sollten an der Universität, letztere an den Technischen Hochschulen und Bergakademien studieren, als Schlußprüfung der Dr. phil. bezw. Dr. der technischen Geologie. Der Examenstitel ist vorläufig und schließt Änderungen im Laufe der Zeit nicht aus. Ein weiteres Examen für die angehenden praktischen Geologen wird als überflüssig erachtet.     

Petrogenesis of Miller Range 07273, a new type of anomalous melt breccia: Implications for impact effects on the H chondrite asteroid

Miller Range 07273 is a chondritic melt breccia that contains clasts of equilibrated ordinary chondrite set in a fine‐grained (<5 μm), largely crystalline, igneous matrix. Data indicate that MIL was derived from the H chondrite parent asteroid, although it has an oxygen isotope composition that approaches but falls outside of the established H group. MIL also is distinctive in having low porosity, cone‐like shapes for coarse metal grains, unusual internal textures and compositions for coarse metal, a matrix composed chiefly of clinoenstatite and omphacitic pigeonite, and troilite veining most common in coarse olivine and orthopyroxene. These features can be explained by a model involving impact into a porous target that produced brief but intense heating at high pressure, a sudden pressure drop, and a slower drop in temperature. Olivine and orthopyroxene in chondrule clasts were the least melted and the most deformed, whereas matrix and troilite melted completely and crystallized to nearly strain‐free minerals. Coarse metal was largely but incompletely liquefied, and matrix silicates formed by the breakdown during melting of albitic feldspar and some olivine to form pyroxene at high pressure (>3 GPa, possibly to ~15–19 GPa) and temperature (>1350 °C, possibly to ≥2000 °C). The higher pressures and temperatures would have involved back‐reaction of high‐pressure polymorphs to pyroxene and olivine upon cooling. Silicates outside of melt matrix have compositions that were relatively unchanged owing to brief heating duration.     

Relationship between P- and S-wave velocities and geological properties of near-surface sediments of the continental slope of the Barents Sea

Seismic velocities ( V _p and V _s) of compressional (P-) and shear (S-) waves are important parameters for the characterization of marine sediments with respect to their sedimentological and geotechnical properties. P- and S-wave velocity data of near-surface marine sediments (upper 9 m) of the continental slope of the Barents Sea are analysed and correlated to sedimentological and geotechnical properties. The results show that the S-wave velocity is much more sensitive to changes in lithology and mechanical properties than the P-wave velocity, which is characterized by a narrow range of values. The correlation coefficients between S-wave velocity and silt and clay content, wet bulk density, porosity, water content and shear strength are higher than 0.5 while the correlation coefficients of P-wave velocity and the same parameters are always lower than 0.4. Although the relationship between V _s and clay content has been widely described, the data show that V _s is better correlated with silt content than with clay content for the sediments of the area investigated. However, they show different trends. While V _s increases with increasing clay content, it decreases with increasing silt content.