Conditions of pocket formation in the Oktyabrskaya tourmaline-rich gem pegmatite (the Malkhan field, Central Transbaikalia, Russia)

Coexisting melt (MI), fluid-melt (FMI) and fluid (FI) inclusions in quartz from the Oktaybrskaya pegmatite, central Transbaikalia, have been studied and the thermodynamic modeling of PVTX-properties of aqueous orthoboric-acid fluids has been carried out to define the conditions of pocket formation. At room temperature, FMI in early pocket quartz and in quartz from the coarse-grained quartz–oligoclase host pegmatite contain crystalline aggregates and an orthoboric-acid fluid. The portion of FMI in inclusion assemblages decreases and the volume of fluid in inclusions increases from the early to the late growth zones in the pocket quartz. No FMI have been found in the late growth zones. Significant variations of solid/fluid ratios in the neighboring FMI result from heterogeneous entrapment of coexisting melts and fluids by a host mineral. Raman spectroscopy, SEM EDS and EMPA indicate that the crystalline aggregates in FMI are dominated by mica minerals of the boron-rich muscovite–nanpingite CsAl₂[AlSi₃O₁₀](OH,F)₂ series as well as lepidolite. Topaz, quartz, potassium feldspar and several unidentified minerals occur in much lower amounts. Fluid isolations in FMI and FI have similar total salinity (4–8 wt.% NaCl eq.) and H₃BO₃ contents (12–16 wt.%). The melt inclusions in host-pegmatite quartz homogenize at 570–600 °C. The silicate crystalline aggregates in large inclusions in pocket quartz completely melt at 615 °C. However, even after those inclusions were significantly overheated at 650±10 °C and 2.5 kbar during 24 h they remained non-homogeneous and displayed two types: (i) glass+unmelted crystals and (ii) fluid+glass. The FMI glasses contain 1.94–2.73 wt.% F, 2.51 wt.% B₂O₃, 3.64–5.20 wt.% Cs₂O, 0.54 wt.% Li₂O, 0.57 wt.% Ta₂O₅, 0.10 wt.% Nb₂O₅, 0.12 wt.% BeO. The H₂O content of the glass could exceed 12 wt.%. Such compositions suggest that the residual melts of the latest magmatic stage were strongly enriched in H₂O, B, F, Cs and contained elevated concentrations of Li, Be, Ta, and Nb. FMI microthermometry showed that those melts could have crystallized at 615–550 °C.

Crystallization of quartz–feldspar pegmatite matrix leads to the formation of H₂O-, B- and F-enriched residual melts and associated fluids (prototypes of pockets). Fluids of different compositions and residual melts of different liquidus–solidus P–T-conditions would form pockets with various internal fluid pressures. During crystallization, those melts release more aqueous fluids resulting in a further increase of the fluid pressure in pockets. A significant overpressure and a possible pressure gradient between the neighboring pockets would induce fracturing of pockets and “fluid explosions”. The fracturing commonly results in the crushing of pocket walls, formation of new fractures connecting adjacent pockets, heterogenization and mixing of pocket fluids. Such newly formed fluids would interact with a primary pegmatite matrix along the fractures and cause autometasomatic alteration, recrystallization, leaching and formation of “primary–secondary” pockets.     

The study of metal contamination in urban topsoils of Mexico City using GIS

This research presents and discusses information concerning the spatial distribution of heavy metals (Pb, Cu, Zn, Ba, Co, Cr, Ni and V) in the urban environments of Mexico City using geographical information system and statistical analysis. Superficial soil samples (n = 146) were analyzed. The highest contamination indices were found in the north and center zone of the metropolitan area. In contrast, the surrounding rural fields show a lower impact grade. The higher concentrations of Pb, Cu, Zn and Ba were observed as being related to high vehicular traffic, nevertheless other elements such as Co, Cr, Ni and V do not show anthropogenic influence and their content can be attributed to the parental rock. The results are compared with previous surveys carried out in 2003 in order to evaluate temporal deposition trends. No changes were found on reported concentrations except for Cu and Zn, whose concentration has increased in later years. The results suggest that spatial distribution analysis and results in comparison with previous studies could be useful for the management and sustainable development of the metropolitan area of Mexico City.     

Geology and geochemistry of Iberian roofing slates

This article analyzes the geochemical and mineralogical composition of the roofing slate deposits of the Iberian Peninsula. Most roofing slate deposits are found on Middle and Upper Ordovician terrains, although there are also deposits in the Ediacaran, Cambrian and Devonian terrains. Samples of slate were taken in most of the active quarries of the Iberian Peninsula, and being analyzed by X-ray Diffraction and Fluorescence. Each lithotect or slate productive formation has its own specific proportions of major and trace elements, being able to differentiate between them by using these relationships. This fact is particularly interesting in geological exploration and prospecting, as it allows knowing exactly the slate lithotect, and also for restoration work in the architectural heritage, due to the ability of identifying the source of the original slate tiles.     

Mg---NH4 exchange on humic acid: A radiotracer technique for conditional exchange constants in a seawater medium

A radiotracer technique, employing ²⁷Mg, is used to determine the Mg released by ammonium exchange on undis-solved humic acid in a seawater medium. This new method allows for the measurement of exchangeable Mg on the solid phase surface, which eliminates the problem caused by the high-Mg background in the seawater matrix. The precision calculated from the counting statistics is better than ±2%; the reproducibility among repeated counts ranged from ±1% to ±3%. The higher sensitivity of the method allows for monitoring the Mg---NH₄ exchange at concentrations as low as 30 mM NH₄. This is a major improvement relative to the data obtained with the analytical methods used so far, which allow detection of exchangeable Mg only at NH₄ concentrations higher than 1 M. The lower experimental concentrations are more in accordance with the natural ammonium levels found in anoxic marine sediments. For the undissolved humic acid used in this experiment, the amount of exchangeable Mg in apparent equilibrium with an ammonium-free seawater matrix was found to be 96.6 ± 0.4 meq/100g. The Mg---NH₄ exchange on humic acid in seawater comes to a steady-state value in < 18 min. The conditional equilibrium constant obtained for this reaction, K_cond = 0.039 ± 0.001 M⁻¹. The technique can be expanded to other geochemical solid phases in seawater and it can be modified to study the behavior of the major cations by using ²⁴Na, ⁴²K and ⁴⁹Ca.     

Open problems on the eve of the next millennium

To predict the future of Celestial Mechanics and to investigate unsolved problems for the next millennium contradicts the nature of our field of science which is based on non-integrable and transcendental equations with short time predictability. The reliability and predictability in our field depend on several factors, most of them unpredictable. Numerical predictions depend on the development of computer technology, which is itself not predictable. New analytical approaches call for the genius of Newton. We must realize that between Ptolemaeus (100–178) and Copernicus (1473–1543) our field received little attention. Between 1700 and 1800 the explosion occurred (Newton, Euler, Laplace, Lagrange and Gauss) followed by the period of 1800–1900 associated with Hamilton, Hill, Jacobi, Poincaré and Tisserand. Can we expect anyone to make similar contributions in the next 1000 years? The entrance and emphasis of probability in our field certainly will continue. Speaking about bounds of trajectories instead of individual orbits gains significant popularity. Theory of stability and chaos are expected to receive more and more attention together with Poincare's conjectures concerning the denseness of periodic orbits in the set of bounded orbits. Planetary computations for long time have shown considerable advances, partly due to computer advances as well as the selection of the proper formulation of the problems. The preparation of “exact” short time ephemerides and qualitative long time predictions require initial conditions with small observational errors, the use of proper models (including relativity effects and non-conservative forces) and the use of correct values of astronomical constants. The results of such calculations in the future should include approximate estimations of errors in the outcome. The observations and orbital calculations of Earth-crossing asteroids should receive special attention along with development of various techniques of space dynamics to avoid collisions. Interplanetary orbit calculations of space probes with optimizations included, shall receive additional attention since interplanetary flights expect to become regular travels in the next millennium. Here, again, the use of proper models, constants and planetary ephemerides will be essential. This paper was prepared with the help of many of my colleagues whose suggestions and ideas were incorporated with my many thanks.[/p]     

Derivative of Kaula's inclination function

The derivative of Kaula's inclination function is expressed as a linear combination of two inclination functions themselves.On leave from Institute of Applied Astronomy, 197042 St.-Petersburg, Russia.     

Next-generation robotic planetary reconnaissance missions: A paradigm shift

A fundamentally new scientific mission concept for remote planetary surface and subsurface reconnaissance will soon replace the engineering and safety constrained mission designs of the past, allowing for optimal acquisition of geologic, paleohydrologic, paleoclimatic, and possible astrobiologic information of Mars and other extraterrestrial targets. Traditional missions have performed local ground-level reconnaissance through rovers and immobile landers, or global mapping performed by an orbiter. The former is safety and engineering constrained, affording limited detailed reconnaissance of a single site at the expense of a regional understanding, while the latter returns immense datasets, often overlooking detailed information of local and regional significance. A “tier-scalable” paradigm integrates multi-tier (orbitatmosphereground) and multi-agent (orbiterblimpsrovers/sensorwebs) hierarchical mission architectures, not only introducing mission redundancy and safety, but enabling and optimizing intelligent, unconstrained, and distributed science-driven exploration of prime locations on Mars and elsewhere, allowing for increased science return, and paving the way towards fully autonomous robotic missions.     

A widespread positive δ13Ccarb anomaly at around 2.33–2.06 Ga on the Fennoscandian Shield: a paradox?

Carbon isotope measurements carried out on 201 carbonate samples from the early Proterozoic of the Kola Peninsula, N. Karelia and Norway yield δ¹³C (PDB) spanning - 20.5% to + 11%. A general δ¹³C secular trend shows that prior to 2.33 Ga values are typically 'normal' marine, averaging around - 3%0. Between 2.33 and 2.06 Ga, in Jatulian time, there follows a rapid excursion to positive δ¹³C of around + 6%. Post-Jatulian time is characterized by δ¹³C of sedimentary carbonates fluctuating between - 5% and +3%; also it is remarkable for the first pronounced development of diagenetic carbonates, which have δ¹³C between - 14 % and - 6% . The c. 6% positive δ¹³C shift with a duration of about 270 Myr coincides with a maximum in the diversity and abundance of stromatolites, and with widespread development of 'red beds', but does not coincide with the maximum of buried C_org mass. The Fennoscandian Shield represents the largest isotoically anomalous carbonate province yet reported, and the positive δ¹³C excursion together with a series of major global palaeoenviromental changes seems to be more intense than the Precambrian/Cambrian transition events. However, it is still not clear what kind of mechanism this phenomenon could be attributed to. An increase of the 'Ronov ratio', and/or 'Broecker ratio' and other possible models are discussed as the target for future investigations.