A geomicrobiological study of soils collected from auriferous areas of Argentina

Geomicrobial and geochemical studies were carried out in Argentina (Patagonia, Chubut Province) on four Au and polymetallic sulfide vein-type deposits. A horizon soils were analyzed for Bacillus reacting to lecithin [Bacillus L.(+)], Au and 12 additional elements. In two of the four sampling sites, exhibiting known and relatively simple mineralized structures, Bacillus L.(+) populations are clearly related to Au, As, Pb, Zn, Cu-sulfide mineralization. In areas containing more complex mineralized structures, the spatial relationship between Bacillus L.(+) and metals in the A horizon is more difficult to interpret. Results of a factor analysis performed on all analytical data (n = 130) suggest a partial relationship between Bacillus L.(+) and Au-As-Y pedochemical associations located above known Au mineralization. Bacillus L.(+) was first analyzed in Argentina in December 1994 and re-analyzed in Belgium five to seven months later. Most of the Bacillus contents (85%) of the Belgian tests are higher than those determined in Argentina. The present results and data of a previous study in Mexico (Melchior et al., 1994a; Melchior et al., 1994b) suggest that this may be the result of temperature variations during sample storage between periods of microbial analysis. From a strictly analytical point of view, the geomicrobial method is not an accurate, reproducible technique. However, Bacillus L.(+) can be used as a microbiological indicator of Au and polymetallic mineralization at a reconnaissance-level regional survey. At a local scale, this microbiological tool should be combined with classical exploration techniques such as soil geochemistry. It is recommended that the collection of all A horizon samples (for microbial study) should be accompanied by B or C horizon soils (for potential geochemical study, after prioritizing targets) so that a second field sampling program does not have to be undertaken.     

Tidal loading along a profile Europe-East Africa-South Asia-Australia and the Pacific Ocean

A knowledge of the vertical component of the oceanic tidal load to a precision of at least one microgal is essential for the geophysical exploitation of the high-precision absolute and differential gravity measurements which are being made at ground level and in deep boreholes. On the other hand the ocean load and attraction signal contained in Earth tide gravity measurements can be extracted with a precision which is sufficient to characterize the behaviour of the oceanic tides in different basins and this provides a check of the validity of the presently proposed cotidal maps. The tidal gravity profiles made since 1971 from Europe to Polynesia, through East Africa, Asia and Australia, with correctly intercalibrated gravimeters, comprise information from 91 tidal gravity stations which is used in this paper with this goal in mind.A discussion of all possible sources of error is presented which shows that at the level of 0.5 μgal the observed effects cannot be ascribed to computational or instrumental errors. Cotidal maps which generate computed loads in agreement with the Earth tide gravity measurements over a sufficiently broad area can be used with confidence as a working standard to apply tidal corrections to high-precision measurements made by using new techniques in geodesy, geophysics and geodynamics, satellite altimetry, very long baseline interferometry, Moon and satellite laser ranging and absolute gravity. The recent cotidal maps calculated by Schwiderski for satellite altimetry reductions agree very well with land-based gravimeter observations of the diurnal components of the tides (O₁, K₁ and P₁ waves) but his semi-diurnal component maps (M₂, S₂ and N₂ waves) strangely appear less satisfactory in some large areas. The maps of Hendershott and Parke give good results in several large areas but not everywhere. More detailed investigations are needed not only for several coastal stations but mainly in the Himalayas.     

Aenigmetites from the Ilímaussaq intrusion, south Greenland: Chemistry and petrological implications

Aenigmatite in the Ilímaussaq intrusion shows a variety of textural relations to the other mafic minerals and an unusual range in chemical compositions. The saturated and oversaturated rocks contain zoned aenigmatities with Ca, Al, Fe²⁺-rich cores coexisting with katophorite, and near-ideal Ti-aenigmatite rims coexisting with arfvedsonite and aegirine. The aenigmatite substitutions are outlined, and the varying chemistry discussed. A no-oxide field seems to have existed in the (log f_O2, T) space for the undersaturated magma, and an arfvedsonite-aenigmatite oxygen buffer equilibrium is suggested for the coexisting Fe³⁺-rich aenigmatite and katophorite. This buffer was later invalidated by the stabilisation of aegirine, whereby the Fe³⁺-aenigmatite component broke down, causing extensive recrystallisation into near-ideal Ti-aenigmatite.     

Distribution of REE and other trace elements between phenocrysts and peralkaline undersaturated magmas, exemplified by rocks from the Gardar igneous province, south Greenland

Partition coefficients for iron-rich olivine and pyroxene, sanidine, nepheline and apatite are reported from peralkaline trachytic to phonolitic dyke rocks and the agpaitic Ilímaussaq intrusion. Partition coefficients for many elements in olivine and pyroxene decrease with increasing peralkalinity and undersaturation of the magma, i.e. with decreasing polymerisation. The REE partition coefficients for olivine and pyroxene also show dependence on the mineral chemistry, i.e. the iron content. Probably due to the larger lattice sites in the iron end-members the heavy REEs enter the small six-coordinated lattice sites with increasing ease as the iron content of the mineral increases. La and Ce partition coefficients for apatite increase with increasing peralkalinity; this condition seems to stabilise a Na-REE-phosphate component in the mineral.     

Alkali loss and retention in an iron-rich peralkaline phonolite dyke from the Gardar province,south Greenland

An unusually iron-rich, strongly peralkaline dyke occurring near the Ilimaussaq intrusion, south Greenland, is described. The dyke has developed two facies characterised respectively by the mafic assemblages pyroxene+amphibole+aemigmatite, and pyroxene+magnetite. The differences can be explained as an effect of loss of up to 4% Na₁O from the magnetite-bearing facies (relative to the magnetite-free facies). The effect is in accordance with the phase relations at 1 atm in the synthetic system Na₂OAl₂O₂Fe₂O₂SiO₂. Continued alkali loss and magnetic precipitation will change the differentiation trend from agpaitic with iron enrichment towards miaskitic with iron depletion. It is suggested that the extreme degree of iron enrichment in the dyke resulted from the coincidence of low oxygen fugacity and strong peralkalinity tending to stabilise both ferrous and ferric iron in the liquid.     

Earth tides

The main geometrical characteristics and mechanical properties of bodily tides are described, using the convenient elastic parameters of Love. The problem of the Earth's deformation is a problem of spherical elasticity of the sixth order. The importance of Earth tides in astronomy and geophysics is emphasized by their relation to the precession-nutation and tesseral tidal problems, the secular retardation of the Earth's speed of rotation due to the dissipation of energy in sectorial tides, the periodic variations of the speed of rotation due to zonal tides, the satellite orbit perturbations due to the Earth's potential variation, and the radial deformations in laser distance measurements. The possibility that dynamical effects would be produced in the Earth's liquid core was pointed out by Poincaré and developed by Jeffreys, Vicente, and Molodensky. An experimental confirmation is presented here. The role of the Earth tide phenomenon in gravimetry and oceanography is also described, as are the perturbing effects due to regional tectonic features. Instrumental developments are critical in the acquisition of precise data; the calibration problem is fundamental for a correct comparison with Earth models.     

Water Table Depth Estimates over the Contiguous United States Using a Random Forest Model

Water table depth (WTD) has a substantial impact on the connection between groundwater dynamics and land surface processes. Due to the scarcity of WTD observations, physically-based groundwater models are growing in their ability to map WTD at large scales; however, they are still challenged to represent simulated WTD compared to well observations. In this study, we develop a purely data-driven approach to estimating WTD at continental scale. We apply a random forest (RF) model to estimate WTD over most of the contiguous United States (CONUS) based on available WTD observations. The estimated WTD are in good agreement with well observations, with a Pearson correlation coefficient (r) of 0.96 (0.81 during testing), a Nash-Sutcliffe efficiency (NSE) of 0.93 (0.65 during testing), and a root mean square error (RMSE) of 6.87 m (15.31 m during testing). The location of each grid cell is rated as the most important feature in estimating WTD over most of the CONUS, which might be a surrogate for spatial information. In addition, the uncertainty of the RF model is quantified using quantile regression forests. High uncertainties are generally associated with locations having a shallow WTD. Our study demonstrates that the RF model can produce reasonable WTD estimates over most of the CONUS, providing an alternative to physics-based modeling for modeling large-scale freshwater resources. Since the CONUS covers many different hydrologic regimes, the RF model trained for the CONUS may be transferrable to other regions with a similar hydrologic regime and limited observations.     

High-pressure structural studies of Li x La1/3NbO3 (x = 1/6, 1/3, 1/2, 2/3)

The high-pressure behavior of Li _x La_1/3NbO₃ (x = 1/6, 1/3, 1/2, 2/3) perovskites where Li cations were substituted for the existing vacancies was studied using synchrotron X-ray diffraction. It was shown that all these materials undergo irreversible pressure-induced amorphization around 14.5 GPa regardless of the Li concentration. The Li-inserted materials were found to exhibit a standard pressure response (bulk modulus pressure derivative B ₀′ ~4) when in the crystalline phase, whereas La_1/3NbO₃ shows a linear volume contraction versus pressure, i.e., B ₀′ ~(?1). These results suggest that the structural collapse is not a consequence of cation disorder resulting from the Nb atoms (B-site) migrating to the A-site vacancies. The observed pressure response can be understood by increased occupancy of the A-sites opposing the tilting of the NbO₆ octahedra. The pressure evolution of the Nb oxidation state is discussed.