Interfacial energy effects and the evolution of pore size distributions during quartz precipitation in sandstone

Pore size is usually thought to control the rate of crystal growth in porous geological media by determining the ratio of mineral surface area to fluid volume. However, theory suggests that in micron-scale to nanometer scale pores, interfacial energy (surface energy) effects can also become important. Interfacial energy typically increases the solubility of very small crystals growing in tiny pores, and when the fluid is close to equilibrium - as is often the case in geological systems - mineral precipitation could occur in relatively large pores, while in very small adjacent pores crystal growth might be suppressed. Such a mechanism would effectively restrict the reactive surface area of the porous medium, thereby reducing the bulk reaction rate. We investigated the pore size distributions in naturally cemented sandstone adjacent to an isolated stylolite and found that quartz precipitation was inhibited in pores smaller than 10 μm in diameter. Furthermore, we demonstrate that kinetic formulations which assume constant solubility cannot reproduce the observed pore size patterns in mineralized samples; by contrast, excellent fits with the data are obtained when interfacial energy effects are taken into account. Reaction rates in geological media determined in field studies can be orders of magnitude lower than those measured in laboratory experiments, and we propose that reduced reaction rates in porous media with micron and submicron-scale porosity could account for much of the apparent paradox.     

Nuclear spin temperature of ammonia in Comet 9P/Tempel 1 before and after the Deep Impact event

The Deep Impact mission succeeded in excavating inner materials from the nucleus of Comet 9P/Tempel 1 on 2005 July 04 (at 05:52 UT). Comet 9P/Tempel 1 is one of Jupiter family short period comets, which might originate in the Kuiper belt region in the solar nebula. In order to characterize the comet and to support the mission from the ground-based observatory, optical high-dispersion spectroscopic observations were carried out with the echelle spectrograph (UVES) mounted on the 8-m telescope VLT (UT2) before and after the Deep Impact event. Ortho-to-para abundance ratios (OPRs) of cometary ammonia were determined from the NH₂ emission spectra. The OPRs of ammonia on July 3.996 UT and 4.997 UT were derived to be 1.28±0.07 (nuclear spin temperature: Tspin=24±2 K) and 1.26±0.08 (Tspin=25±2 K), respectively. There is no significant change between before and after the impact. Actually, most materials ejected from the impact site could have moved away from the nucleus on July 4.997 UT, about 17 h after the impact. However, a small fraction of the ejected materials might remain in the slit of UVES instrument at that time because an excess of about 20% in the NH₂ emission flux is observed above the normal activity level was found [Manfroid, J., Hutsemékers, D., Jehin, E., Cochran, A.L., Arpigny, C., Jackson, W.M., Meech, K.J., Schulz, R., Zucconi, J.-M., 2007. Icarus. This issue]. If the excess of NH₂ on July 04.997 UT was produced from icy materials excavated by the Deep Impact, then an upper-limit of the ammonia OPR would be 1.75 (Tspin>17 K) for those materials. On the other hand, the OPR of ammonia produced from the quiescent sources was similar to that of the Oort cloud comets observed so far. This fact may imply that physical conditions where cometary ices formed were similar between Comet 9P/Tempel 1 and the Oort cloud comets.     

High-temperature properties of geikielite (MgTiO3-ilmenite) from high-temperature high-pressure Raman spectroscopy — Some implications for MgSiO3-ilmenite

The Raman spectra of geikielite (MgTiO₃-ilmenite) have been recorded at high pressure (up to 27 GPa) and at high temperature (up to 1820 K). No phase transitions could be evidenced in both cases. In particular, no cation disordering can be evidenced from the high temperature spectra. The observed Raman wavenumber shifts with pressure and with temperature are used to calculate the intrinsic mode anharmonic parameters. The low absolute values of these parameters indicate that geikielite has a nearly quasi-harmonic behaviour, at least to moderate temperatures. However, systematics and the temperature evolution of Raman linewidths suggest that the absolute values of the anharmonic parameters increase at high temperatures. Anharmonic corrections are applied to Kieffer modelling of the constant volume heat capacity of geikielite. They amount to +4 J · mol^-1 · K^-1 at 1800 K, i.e. are much lower than those inferred for, for instance, olivine and garnet structures. These results are used to discuss some implications on the phase relations of the high-pressure MgSiO₃-ilmenite, and the factors controlling the occurence of order-disorder transitions in ilmenite structures.     

An example of very consistent brittle deformation over a wide intracontinental area: The late Pan-African fracture system of the Tuareg and Nigerian shield

Brittle deformation during the latter part of the Pan-African orogeny in the Hoggar and Nigeria was characterized by a conjugate strike slip fault system, consisting of NE—SW dextral and NW—SE sinistral trending sets of faults. This system is considered to be the result of a horizontal maximum stress axis $\text{σ}{}_1$ orientated E—W.In the crystalline basement, the faults have offset the N—S trending Pan-African mylonitic shear zones, and have cut the late orogenic granites (Taourirt) and ring complexes of Adrar des Iforas.In the western Hoggar, folding of the post-orogenic molassic “série pourprée” considered as Cambrian in age (Caby, 1970), is probably associated with the same stress regime. A similar fault system also affects the Phanerozoic sedimentary cover of the North-Saharian platform, the Ténéré desert, and the Djado basin. Later reactivation of this fault system by phanerozoic epeirogenic events, and recent uplift is demonstrated.The geometry of this late Pan-African brittle deformation is very consistent over a large area and may reflect the stress field operative during indentation of a “plastic” Pan-African shield, by a more rigid West-African craton.     

Discovery of the largest impact crater field on Earth in the Gilf Kebir region, Egypt

Using orbital imaging radar, we have detected a large number of circular structures in the southwestern Egyptian desert, covering more than 4500 km² close to the Gilf Kebir plateau in sandstones of Upper Cretaceous. Fieldwork confirmed that it is a new impact crater field: 13 craters from 20 m to 1 km in diameter were studied. The impact origin is confirmed by the observation of shock-related structures, such as shatter cones and planar fractures in quartz grains of breccia. Considering the extension of the crater field, it was possibly created by several meteorites that broke up when entering the atmosphere. To cite this article: P. Paillou et al., C. R. Geoscience 336 (2004).

Résumé

À partir d'images satellites issues de systèmes radar, nous avons détecté un grand nombre de structures circulaires dans le Sud-Ouest du désert égyptien, s'étendant sur plus de 4500 km² à l'est du plateau du Gilf Kebir, dans des grès du Crétacé supérieur. Une étude sur le terrain a permis de vérifier qu'il s'agit d'un champ d'impacts météoritiques jusque là inconnu : 13 structures, d'un diamètre compris entre 20 m et 1 km, ont été reconnues comme cratères d'impact. La présence d'un grand nombre de cônes de percussion et de brèches dans lesquelles des quartz choqués à structures planaires ont été observés confirment l'hypothèse de l'impact. Considérant l'extension du champ d'impact, il a probablement été créé par plusieurs météorites qui se sont fragmentées dans l'atmosphère terrestre. Pour citer cet article : P. Paillou et al., C. R. Geoscience 336 (2004).     

Sources of very high heavy metal content in soils of volcanic island (La Réunion)

The objective of this study was to conduct an inventory of heavy metal concentrations (Cd, Cr, Cu, Hg, Ni, Pb and Zn) in the soils of La Réunion. 39 sampling sites were selected to cover the distribution of soils of the island. The results show that soils of La Réunion are rich in heavy metals: most of them exceed the French standard values beyond which sewage sludge spreading is not authorized. To identify the sources of heavy metals, we used: (i) the relationship between the heavy metal content in soils and the origin of the volcanic parent material; (ii) the comparison of heavy metal content between cultivated and uncultivated soils; and (iii) the heavy metal distribution in soil profiles. Cd and Pb evolution in soil profiles indicate an impact of human activities. High Hg concentrations in soils can be explained by the volcanic activity of the island. For Cr, Cu, Ni and Zn, we demonstrate that high concentrations in soils are mainly determined by the natural pedo-geochemical background.     

Optimized Computation of Transfer Coefficients in Surface Layer with Different Momentum and Heat Roughness Lengths

A new method for computing the surface transfer coefficients is proposed, based on state-of-the-art empirical flux-profile relationships. The influence of the roughness length ratio is first demonstrated with the classical iterative calculation method. Then a non-iterative algorithm is developed, taking into account the difference between momentum and heat roughness lengths.The new method is validated by comparison with the reference iterative computation. The large gain-in computer processing time (CPU) time gain for the calculation of surface fluxes in Eulerian grid models is finally assessed.     

Analytical models for the rigid body motions of skew bridges

The rigid body motions of the bridge deck, along with the impact between the bridge deck and the abutments, were the source of extensive damage on skew highway bridges during the 1971 San Fernando earthquake. In this paper, a model for the rigid body motions of skew bridges is presented and analysed. The focus of the model is the appropriate representation of the impact between the bridge deck and the abutments and the explanation of the inducement of in-plane rotational vibrations of the bridge deck as the result of this impact. A simplified model is briefly described first, and the kinematic mechanism of the problem is explained. Then, the analysis of a more detailed and realistic model follows. This model is applied on a short skew bridge located in Riverside, California. The response of this bridge to several earthquake shakings revealed that the planar rigid body rotations of the deck are induced primarily as a result of the skewness of the deck and the impact between the deck and the abutments.     

Local erosion rates versus active tectonics: cosmic ray exposure modelling in Provence (south-east France)

Over the past decade, in situ-produced cosmogenic nuclides have revolutionised the study of landscape evolution. In particular, numerous studies have demonstrated that, in active tectonic settings, cosmic ray exposure dating of deformed or displaced geomorphic features makes it possible to quantify long-term deformation rates. In western European countries, erosion due to climatically driven processes and human activities is probably the factor that most limits the accuracy of exposure ages and landscape modification rates. In this study, we present the results of a depth-profiling technique applied to alluvial terraces located along the Rhône and the Moyenne Durance rivers. The expected decrease with depth of the measured ¹⁰Be concentrations has been modelled using a χ² inversion method in order to constrain the exposure history of the alluvial sediments. The results suggest that: (1) over the Quaternary, the local surface erosion rates including both regional uplift and climatically driven processes acting on landforms are on the order of 30 m/Myr in southeastern France, and (2) providing a fairly good bracketing of the exposure age, the modelled abandonment age of alluvial terraces affected by the Moyenne Durance Fault allows estimating incision rates, comparing the alluvial terrace elevations with topographic river profiles, and a minimum vertical slip rate value of roughly 0.02 mm/yr for the southern segment of the Moyenne Durance Fault.