Contribution à la surveillance de l'activité de l'Etna à partir de l'étude des zones fumerolliennes

Resume La convection de la vapeur d'eau formée en profondeur dans une zone fumerollienne crée en surface une anomalie hydrique, thermique et électrique, cette dernière pouvant être la plus sensible à une variation de l'activité thermique profonde. L'amplitude des anomalies dépend de la position du site par rapport à la configuration de l'activité magmatique, mais aussi des conditions hydrogéologiques locales. La surveillance spatiale et temporelle des éruptions à partir du phénomène convectif hydrique doit tenir compte de ces conditions.

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The water steam convection existing below a fumarolic area creates hydraulic, thermal and electric anomalies, the electric anomaly being probably the most correlated to a change of the deep thermal activity. The anomaly range depends on the position of the site connected with the configuration of the magmatic activity, but also on the local hydrogeologic conditions. The spatial and temporal surveillance of the eruptions based on these convective data should take into consideration these conditions.

     

Fe3+ in shocked olivine crystals of the ALHA 77005 meteorite

ALHA 77005, a shocked achondrite of the shergottite group, is unique in containing cumulus olivine crystals which display a brownish color in thin section. Mössbauer, EPR, and optical spectroscopic studies show the presence of Fe³⁺ in the olivine. Approximately 4.5 wt.% of the total iron in the olivine (24 wt.% as FeO) is in the trivalent state. The Fe³⁺ ions preferentially occupy M2 lattice positions. Charge transfer between Fe³⁺ and Fe²⁺ as well as between Fe³⁺ and oxygen ligands causes a broad absorption band in the optical spectrum at high energies which is the reason for the olivine color. It is concluded that the color of the ALHA 77005 olivine is preterrestrial and may be the result of shock-induced oxidation of the olivine crystals on the meteorite parent body.     

The trapped fluid phase in upper mantle xenoliths from Victoria,Australia: implications for mantle metasomatism

Mantle-derived xenoliths of spinel lherzolite, spinel pyroxenite, garnet pyroxenite and wehrlite from Bullenmerri and Gnotuk maars, southwestern Victoria, Australia contain up to 3 vol.% of fluids trapped at high pressures. The fluid-filled cavities range in size from fluid inclusions (1–100 m) up to vugs 11/2 cm across, lined with euhedral high-pressure phases. The larger cavities form an integral part of the mosaic microstructure. Microthermometry and Raman laser microprobe analysis show that the fluids are dominantly CO₂. Small isolated inclusions may have densities 1.19 g/cm³, but most inclusions show microstructural evidence of partial decrepitation during eruption, and these have lower fluid densities. Mass-spectrometric analysis of gases released by crushing or heating shows the presence of He, N₂, Ar, H₂S, CO_s and SO₂ in small quantities; these may explain the small freezing-point depressions observed in some inclusions. Petrographic, SEM and microprobe studies show that the trapped fluids have reacted with the cavity walls (in clinopyroxene grains) to produce secondary amphiboles and carbonates. The trapped CO₂ thus represents only a small residual proportion of an original volatile phase, which has undergone at least two stages of modification — first by equilibration with spinel lherzolite to form amphibole (±mica±apatite), then by limited reaction with the walls of the fluid inclusions. The inferred original fluid was a CO₂-H₂O mixture, with significant contents of (at least) Cl and sulfur species. Generation of this fluid phase in the garnet-peridotite stability field, followed by its migration to the spinel peridotite stability field, would provide an efficient mechanism for metasomatic enrichment of the upper mantle in LIL elements. This migration could involve either a volatile flux or transport in small volumes of silicate melt that crystallize in the spinel peridotite field. These observations suggest that some portions of the subcontinental upper mantle contain large reservoirs of free fluid CO₂, which may be liberated during episodes of rifting or magmatism, to induce granulite-facies metamorphism of the lower crust.     

Numerical and experimental models on the formation mechanism of collapse basins during the Green Tuff orogenesis of Japan

The geological model about volcanism of the Green Tuff geosyncline deduced from the field observations consists of the following processes:

1. Dome-shaped uplift with a mean diameter of 30 km.
2. Collapse of the central part of the domes forming basins with a mean diameter of 10 km.
3. Volcanic activity inside the collapse basins. It is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust.

In finite element analyses performed as plane strain problems, earth's crust is assumed to be an elasto-plastic homogeneous layer and to undergo sinusoidal vertical displacement at the base of the layer due to an ascending magma reservoir. These analyses reveal that the diameter of the dome is proportional to the depth of the magma reservoir rather than to its size. The magma reservoir is estimated at 12 ～ 24 km in depth. Scale model experiments using powdered material were performed in order to reproduce a collapse basin. These three-dimensional models are reduced to a scale of 1:200,000 th of the natural size. The results of experiments show that radial and concentric cracks are produced on top of the dome and a central part encircled by concentric cracks collapses to form a basin. The boundary of the collapsed portion forms a steep cliff with a height of about 2 mm. This is equivalent to 400 m in natural size and is nearly similar to field observations.     

14C content of ten meteorites measured by tandem accelerator mass spectrometry

¹⁴C has been measured in three North American and seven Antarctic meteorites with the Chalk River MP tandem accelerator. In most cases cosmogenic¹⁴C, which is tightly bound, was separated from absorbed atmospheric radiocarbon by stepwise heating extractions. Terrestrial ages obtained by comparing cosmogenic¹⁴C in the meteorite to that in Bruderheim are (7.2 ± 0.6) × 10³ years for Yamato 7304, (11.6 ± 0.4) × 10³ years for Estacado, and range from (32.7 ± 0.5) × 10³ to (41.0 ± 0.8) × 10³ years for six meteorites recovered at Allan Hills and its vicinity. The present upper limit to age determination by the accelerator method varies from 50 × 10³ to 70 × 10³ years depending upon mass and carbon content of the sample. The natural limit caused by cosmic ray production of¹⁴C in silicate rocks at 2000 m elevation is estimated to be (55 ± 5) × 10³ years. “Weathering ages” were estimated for the Antarctic meteorites from the specific activity of loosely-bound CO₂ considered to be absorbed from the terrestrial atmosphere on weathering. The accelerator measurements are in accordance with previous low-level counting measurements but have higher precision and sensitivity.     

Roof bolting in underground mining: a state-of-the-art review

Conclusions With continuing investigations and developments, roof bolting today can in most cases successfully reinforce the mine roof in underground mining. In order to cope with the increasing use of roof bolts, efforts should be made to maximize the safety and minimize the cost. With regard to mechanical bolting, two important parameters need more advanced improvements and study, namely, optimum design of the shape and type of expansion shell and optimum bolt tension for a specific bolt pattern. For the fully grouted resin bolt, the most critical requirement is to develop a fast-setting, low-cost, intoxic, inflammable grouting material that can be used in the high speed mining cycle. It should be noted that geological conditions such as the strata type, rock properties,in situ stress, and planes of weakness play an important role in the successful application of any roof bolting system. These factors should be specified as accurately and quantitatively as possible in the design of any roof bolting system. Finally, proper and careful installation and continuous monitoring are imperative for the success of any roof bolting system.     

A statistical analysis applied to the Rf/φ method

A statistical analysis for the R_f/φ method is developed in the paper. The technique provides estimates of R_s, R_i and θ according to R_f and φ. Moreover, we can restore the pattern of initial orientations of marker objects by means of their frequency histogram, provided the strain is homogeneous. It is unnecessary to postulate random initial orientations if the principal directions of strain ellipsoid are known.     

Sulphur saturation in the lower and critical zones of the Eastern Bushveld Complex

A suite of ultramafic and mafic rocks from the lower, critical and lower portion of the main zones of the Bushveld Complex has been analysed for Th, Cs, Zr, Ni, Cr and Au by INAA and XRF spectrometry. The incompatible elements Th, Cs, and Zr correlate positively, and show a gradual upward increase in abundance. Assuming constant average proportion of intercumulus material, this upward increase implies that the zones of the Complex studied represent crystallization of a single magma type some 3600 m thick. Pyroxenites dominate the lower portion of the section studied and their Ni content shows an initial rapid decrease from 850 ppm in the lowermost rocks, to around 500 ppm, with considerable scatter. This distribution is most likely to have resulted from bottom crystallization with superimposed convective overturn near the transient floor of the chamber. Gold abundances are generally higher in chromitites, and correlate positively with Ni, indicating the presence of significant amounts of cumulus immiscible sulphide. In the silicate rocks, Au does not correlate with any of the analysed elements, and it is concluded that Au was trapped in small quantities of immiscible sulphide which precipitated continuously during crystallization. There is an upward increase in the amount of cumulus immiscible sulphide, indicating a progressive increase in sulphur solubility in the magma.