Reclassification of Villalbeto de la Peña—Occurrence of a winonaite‐related fragment in a hydrothermally metamorphosed polymict L‐chondritic breccia

The Villalbeto de la Peña meteorite that fell in 2004 in Spain was originally classified as a moderately shocked L6 ordinary chondrite. The recognition of fragments within the Villalbeto de la Peña meteorite clearly bears consequences for the previous classification of the rock. The oxygen isotope data clearly show that an exotic eye‐catching, black, and plagioclase‐(maskelynite)‐rich clast is not of L chondrite heritage. Villalbeto de la Peña is, consequently, reclassified as a polymict chondritic breccia. The oxygen isotope data of the clast are more closely related to data for the winonaite Tierra Blanca and the anomalous silicate‐bearing iron meteorite LEW 86211 than to the ordinary chondrite groups. The REE‐pattern of the bulk inclusion indicates genetic similarities to those of differentiated rocks and their minerals (e.g., lunar anorthosites, eucritic, and winonaitic plagioclases) and points to an igneous origin. The An‐content of the plagioclase within the inclusion is increasing from the fragment/host meteorite boundary (approximately An₁₀) toward the interior of the clast (approximately An₅₂). This is accompanied by a successive compositionally controlled transformation of plagioclase into maskelynite by shock. As found for plagioclase, compositions of individual spinels enclosed in plagioclase (maskelynite) also vary from the border toward the interior of the inclusion. In addition, huge variations in oxygen isotope composition were found correlating with distance into the object. The chemical and isotopical profiles observed in the fragment indicate postaccretionary metamorphism under the presence of a volatile phase.     

Measurement of the lunar neutron density profile

Anin situ measurement of the lunar neutron density from 20 to 400 g cm^?2 depth below the lunar surface was made by the Apollo 17 Lunar Neutron Probe Experiment (LNPE) using particle tracks produced by the¹⁰B (n,α)⁷Li reaction. Both the absolute magnitude and the depth profile of the neutron density are in good agreement with theoretical calculations by Lingenfelter, Canfield, and Hampel. However, relatively small deviations between experiment and theory in the effect of Cd absorption on the neutron density and in the relative¹⁴⁹Sm to¹⁵⁷Gd capture rates reported previously (Russet al., 1972) imply that the true lunar¹⁵⁷Gd capture rate is about one half of that calculated theoretically.     

Bosumtwi impact structure,Ghana: Evidence for fluidized emplacement of the ejecta

The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.     

Textural evolution and partial melting of arkose in a contact aureole: a case study and implications

Recognition of partial melting in metamorphic rocks is a difficult task, as leucosomes can have a variety of origins. By comparing the observed values of the solid-solid dihedral angles with the known equilibrium values, and close examination of the shapes and compositions of feldspar grains, it is possible to unequivocally identify melt textures. Textural relations in a series of meta-arkose samples from the contact aureole of the Ballachulish Igneous Complex in the Scottish Highlands demonstrate that, when former melt pockets are not highly deformed, their presence can be recognized petrographically, by detailed examination of textures on the grain scale. Identification of melt textures and their distribution in the Ballachulish aureole has led to appreciation of the fundamental role of magmatically derived H₂O in producing the partial melting. It has also allowed calculation of the H₂O flux involved, and recognition that fractures were the major fluid pathways during metamorphism.     

Axially-symmetric Velocities in the 15 May 2000 Eruptive Prominence

Large Doppler velocities with unique, almost regular elliptical features were observed in the H spectra of the May 15, 2000 eruptive prominence. These features were interpreted in the frame of axially symmetric models of the eruptive prominence. The rotational (7–60 km s^–1), expansion (30–44 km s^–1), axial (3–19 km s^–1), and global (66–160 km s^–1) prominence plasma velocities were derived. The plasma velocity patterns were compared with the observed helical structures of the H prominence. The velocities of selected H blobs in the image plane were determined. The axially symmetric detwisting process of the magnetic flux rope of the eruptive prominence was recognized.     

The significance of channel recharge rates for estimating debris‐flow magnitude and frequency

The quantification of debris‐flow hazard requires estimates of debris‐flow frequency and magnitude. Several methods have been proposed to determine the probable volume of future debris flows from a given basin, but most have neglected to account for debris recharge rates over time, which may lead to underestimation of debris‐flow volumes in basins with rare debris flows. This paper deals with the determination of debris recharge rates in debris‐flow channels based on knowledge of debris storage and the elapsed time since the last debris flow. Data are obtained from coastal British Columbia and a relation is obtained across a sample of basins with similar terrain and climatic conditions. For Rennell Sound on the west coast of the Queen Charlotte Islands, the power‐law relation for area‐normalized recharge rate, R_t, versus elapsed time, t_e was R_t = 0·23t_e^?0·58 with an explained variance of 75 per cent. A difference in recharge rates may exist between creeks in logged and unlogged forested terrain. The power function for undisturbed terrain was R_t = 0·20t_e^?0·49, while the function for logged areas was R_t = 0·30t_e^?0·77. This result suggests that for the same elapsed time since the last debris flow, clearcut gullies tend to recharge at a slower rate than creeks in old growth forest. This finding requires verification, particularly for longer elapsed times since debris flow, but would have important implications for forest resource management in steep coastal terrain. This study demonstrates that commonly used encounter probability equations are inappropriate for recharge‐limited debris flow channels. Copyright © 2005 John Wiley & Sons, Ltd.     

Wüstite in the fusion crust of Almahata Sitta sulfide‐metal assemblage MS‐166: Evidence for oxygen in metallic melts

Meteorite fusion crusts form during the passage of a meteoroid through the Earth's atmosphere and are highly oxidized intergrowths as documented by the presence of e.g., oxides. The porous and irregular fusion crust surrounding the Almahata Sitta sulfide‐metal assemblage MS‐166 was found highly enriched in wüstite (Fe_1‐xO). Frictional heating of the outer portions of the assemblage caused partial melting of predominantly the Fe‐sulfide and minor amounts of the outer Ni‐rich portions of the originally zoned metal in MS‐166. Along with melting significant amounts of oxygen were incorporated into the molten fusion crust and mainly FeS was oxidized and desulfurized to form wüstite. Considerable amounts of FeS were lost due to ablation, whereas the cores of the large metal grains appear largely unmelted leaving behind metal grains and surrounding wüstite‐rich material (matte). Metal grains along with the surrounding matte typically form an often highly porous framework of globules interconnected with the matte. Although textures and chemical composition suggest that melting of Fe,Ni metal occurred only partially (Ni‐rich rims), there is a trace elemental imprint of siderophile element partitioning influenced by oxygen in the metallic melt as indicated by the behavior of W and Ga, the two elements significantly affected by oxygen in a metallic melt. It is remarkable that MS‐166 survived the atmospheric passage as troilite inclusions in iron meteorites are preferentially destroyed.     

Projection of auroral intensity contours into the magnetosphere

Isointensity contours of 630 nm auroral emission are traced into the magnetosphere, using two different empirical magnetic field models, the Mead-Fairfield model, and the Hedgecock-Thomas model. The auroral data are for a specific ISIS-II satellite pass, and so the starting points are expressed in geographic latitude and longitude coordinates, at a specific universal time. The magnetic field models are constructed from satellite magnetometer measurements, and those used correspond to magnetically quiet times. The projections are found to agree reasonably well with direct plasma measurements of the plasma sheet. The projections of the dayside contour connect to widely different regions of the magnetosphere, providing an interpretation that is consistent with observations of the dayside aurora. It is concluded that field line projections of the aurora into the magnetosphere using these models is a valid procedure, but only under quiet-time conditions.