Ar,N2, and CO2 in the structural cavities of cordierite,an optical and X-ray single-crystal study

Ar, N₂ and CO₂ were introduced into the structural cavities of channel-evacuated single-crystals of White Well cordierite with the composition: K_0.01Na_0.03(Mg_1.91Fe_0.09Mn_0.01)Al_3.98Si_5.01O₁₈. The gas refilling experiments were carried out in conventional hydrothermal bombs at 6–7 kbar and 600–700°C. The increase in the mean refractive indices for gas-treated crystals, as determined with a spindle-stage equipped microscope, was used along with point-dipole calculations to estimate the percentage of occupied structural cavities. The steep increase of the electronic polarizability parallel to the a-axis, which can be derived from the increase of the refractive index n _γ (Z∥a) upon introduction of volatiles, indicates that N₂ and CO₂ are preferentially aligned parallel to the a-axis of cordierite. Single-crystal structure refinements at room temperature confirm these predictions. Additionally, decreased C–O and N–N bond lengths suggest a librational motion with a mean rotary oscillation angle of 35° (N₂) and 25° (CO₂) about a, where c is the rotation axis. Mean libration angles of 40° (N₂) and 28° (CO₂) were estimated from the electronic polarizability tensors of CO₂ and N₂. Site occupancy refinements of the channel position are in good agreement with the optically derived values for the volatile concentrations, both indicating about 70% and 60% filled cavities for Ar- and N₂-cordierite, respectively. Chemical analyses and point-dipole calculations confirm that about 45% of the cavities are occupied in the CO₂-treated crystal. The structural framework of cordierite is slightly but specifically altered by the various channel occupants.     

Late Pleistocene woodlands in the Bolson de Mapimi: A refugium for the Chihuahuan Desert Biota?

Packrat middens radiocarbon dated at 12,280 ± 345 and 12,700 ± 165 yr B.P. record expansions of junipers and papershell pinyon (Pinus remota) into the desert lowlands of Durango and Coahuila, Mexico (26° N). Extralocal trees and shrubs presently occur 24–580 km in nearly all directions including more subtropical areas to the northeast and southeast. An equable Late Wisconsin climate marked by mild winters with increased precipitation and by cool summers with reduced summer monsoons is proposed. The extensive playas of the Bolson de Mapimi probably held water at that time. The Bolson de Mapimi was not a geographical refugium unaffected by glacial climates, although many Chihuahuan Desert plants and animals probably remain in situ as members of equable woodlands. Equable climates, low extinction rates, and repeated, rapid glacial/interglacial climatic fluctuations may have been important in the evolution and accumulation of species at lower latitudes.     

Fluid ascent through the solid lithosphere and its relation to earthquakes

The Earth is continuously expelling gases and liquids from great depths—juvenile volatiles from the mantle and recycled metamorphic products. Some of these fluids ascend through liquid rock in volcanic processes, but others utilize fractures and faults as conduits through the solid lithosphere. The latter process may have a major influence on earthquakes, since fluids at near lithostatic pressures appear to be required to activate deep faults that would otherwise remain locked.Fluids can be driven upward through solid rock by buoyancy, but only if present in sufficient concentration to form large-scale domains occupying interconnected fracture porosity. A growing fluid domain becomes so mobilized only when it attains the critical vertical dimension required for hydrostatic instability. This dimension, depending on the ultimate compressive yield strength of the rock, may be as much as several kilometers.Any column of fluid ascending through fractures in the solid lithosphere from a prolific deep source must become organized into a vertical sequence of discrete domains, separated by fluid-pressure discontinuities. This is required because a continuous hydrostatic-fluid-pressure profile extending from an arbitrarily deep source to the surface cannot be permitted by the finite strength of rock. A vertically stacked sequence of domains allows the internal fluid-pressure profile to approximate the external rock-stress profile in a stepwise fashion. The pressure discontinuity below the base of the uppermost hydrostatic domain may be responsible for some occurrences of so-called anomalous geopressures. An ascending stream of fluid that percolates upward from a deep source through a column of domains must encounter a sequence of abrupt pressure decreases at the transitions between successive domains. If supercritical gases act as solvents, the dissolved substances may drop out of solution at such pressure discontinuities, resulting in a local concentration of minerals and other substances.At great depths, brittle fracture would normally be prevented by high pressure and temperature, with all excessive stress discharged by ductile flow. Rock strata invaded by an ascending fluid domain are weakened, however, because cracks generated or reactivated by the high-pressure fluid can support the overburden, greatly reducing internal friction. This reduction of strength may cause a previously stressed rock to fail, resulting in hydraulic shear fracture. Thus, earthquakes may be triggered by the buoyant migration of deep-source fluids.The actual timing of the failure that leads to such an earthquake may be determined by the relatively rapid inflation of a fluid domain and not by any significant increase in the probably much slower rate of regional tectonic strain. Many earthquake precursory phenomena may be secondary symptoms of an increase in pore-fluid pressure, and certain coseismic phenomena may result from the venting of high-pressure fluids when faults break the surface. Instabilities in the migration of such fluid domains may also contribute to or cause the eruption of mud volcanoes, magma volcanoes, and kimberlite pipes.     

The Application of Television Borehole Logging to Ground Water Monitoring Programs

Borehole television has been successfully utilized to gather in situ information on boreholes and wells in several ground water monitoring programs. Borehole television surveys are proposed as a viable alternative to other downhole instruments in the subsurface investigation stages of a ground water monitoring program.
The borehole television camera used by the authors was originally developed for use in the examination of nuclear reactor cores; the camera has since been modified for use in borehole investigations. The lens attachments are capable of looking sideward or downward and include built-in lighting assemblies. Use of the camera, lenses and various support equipment are discussed.
The in situ characterization of fractures that can provide pathways for contaminant migration poses a significant challenge. Borehole television inspection can provide information on the frequency, size and orientation of these fractures. Vertical correlations of rock cores in areas where voids are present (i.e. deep mining or karst topography) can also be simplified by this technique. In addition, borehole television can also be used to check monitoring well integrity. Casing inspections are especially useful where construction details are not known. Well screens may be inspected in place to determine if rusting has enlarged the screen openings or if screens have been damaged during emplacement or well development operations (i.e. surge block, air jetting, etc.). This information may prove to be very valuable in the decision to decommission a well. Examples of these successful applications in ground water monitoring programs at several Superfund hazardous waste sites are presented.     

Innovative Sampling Techniques for Ground Water Monitoring at Hazardous Waste Sites

The authors have recently used several innovative sampling techniques for ground water monitoring at hazardous waste sites. Two of these techniques were used for the first time on the Biscayne Aquifer Super-fund Project in Miami, Florida. This is the largest sampling program conducted so far under the U.S. Environmental Protection Agency (EPA) Superfund Program.
One sampling technique involved the use of the new ISCO Model 2600 submersible portable well sampling pump. A compressed air source forces water from the well into the pump casing and then delivers it to the surface (through a pulsating action). This pump was used in wells that could not be sampled with surface lift devices.
Another sampling technique involved the use of a Teflon manifold sampling device. The manifold is inserted into the top of the sampling bottle and a peristaltic pump creates a vacuum to draw the water sample from the well into the bottle. The major advantage of using this sampling technique for ground water monitoring at hazardous waste sites is the direct delivery of the water sample into the collection container. In this manner, the potential for contamination is reduced because, prior to delivery to the sample container, the sample contacts only the Teflon, which is well-known for its inert properties.
Quality assurance results from the Superfund project indicate that these sampling techniques are successful in reducing cross-contamination between monitoring wells. Analysis of field blanks using organic-free water in contact with these sampling devices did not show any concentration at or above the method detection limit for each priority pollutant.     

Land Subsidence Near Oil and Gas Fields,Houston, Texasa

Subsidence profiles across 29 oil and gas fields in the 12,200-km² Houston, Texas, regional subsidence area, which is caused by decline of ground-water level, suggest that the contribution of petroleum withdrawal to local land subsidence is small. Despite large volumes of petroleum production, subsidence at most fields was not increased by oil and gas withdrawal. Local increases of subsidence were detected at only six fields—Alco-Mag, Chocolate Bayou, Goose Creek, Hastings, Mykawa, and South Houston. With the exception of the 1-m subsidence from 1917 to 1925 at Goose Creek, differential subsidence across oil and gas fields was smaller by a factor of two or more than subsidence caused by aquifer compaction. At four fields—Barbers Hill, Cedar Bayou, Humble, and Pierce Junction—subsidence was substantially less than in the surrounding area. Except for Cedar Bayou, these fields are associated with shallow salt domes that partly occupy the aquifer system; for the three fields, subsidence during the periods of record came to less than half the subsidence in the surrounding area. In addition to land subsidence, faults with an aggregate length of more than 240 km (150 mi) have offset the land surface in historical time. Natural geologic deformation, ground-water pumping, and petroleum withdrawal have all been considered as potential causes of the historical offset across these faults. The minor amount of localized land subsidence associated with oil and gas fields, however, suggests that petroleum withdrawal is not a major cause of the historical faulting, at least by a differential compaction mechanism.     

Transport of water in frozen soil: V, Method for measuring the vapor diffusivity when ice is absent

A new experimental method is introduced for determining the relative magnitudes of liquid and vapor diffusion by using a small amount of soluble chemical as a tracer. The theoretical justification of the method is presented for the case where ice is absent. The feasibility of the method is demonstrated by an experiment using marine-deposited clay.     

Magma mixing: the Marsco suite,Isle of Skye,Scotland

Magma mixing has been proposed as a major mechanism for the origin of a wide variety of rock suites. In mid-ocean basalts there are few obstacles to mixing because of the similarities in chemical and physical properties of the proposed end-members. However, in calc-alkaline rocks the proposed end-members have disparate properties and these present obstacles to mixing.The Marsco suite is ideal to evaluate the process of magma mixing because it is considered to be a classic example of magma mixing of diverse magma types and because compositionally diverse, coexisting liquids were present throughout the area.The results of this study are that the chemical data for the Marsco suite fit a mixing model remarkably well and that the mixing did not take place at the present level of exposure. The Marsco mixed suite is similar to the experiments of Kouchi and Sunagawa (1982, 1983) in which mixing was produced by forced convection. Based on the Marsco suite, a similar occurrence in Ardnamurchan, and the experimental data, we conclude that mixing was dominated by mechanical mixing and this mechanical mixing was efficient enough to allow diffusional processes to homogenize the liquids.It is postulated that injection of mafic magma into the base of a silicic magma system leads to mixing and mobilization of the system. The mechanical interaction that produces the mixing also leads to disruption of the system. Liquids involved with the mixing process at depth are mobilized and coexist at shallower levels.     

Palaeoenvironmental interpretation of fluvial red beds by statistical analysis of palaeocurrent data: Examples from the buntsandstein (lower triassic) of the eifel and bavaria in the German basin (middle Europe)

Palaeocurrent data from parts of the Upper Buntsandstein (Lower Triassic) of both the western (Eifel: Kyllburg-Schichten) and eastern (Bavaria: Plattensandstein) margin of the German Triassic Basin (Middle Europe) have been interpreted by circular directional statistics, eigenvalue and eigenvector analysis, autocorrelation analysis and vector trend analysis. Examination of current roses, histograms of normalized vector magnitudes and plots, of circular skewness of the distribution vs. circular kurtosis, together with the palaeocurrent plots indicate bimodal palaeocurrent distributions with bipolar orientations within long, short and very short sedimentary sequences. Subset I of the directional data is interpreted to represent larger, more continuous flows in a high-energy regime of probably perennial type, whereas subset II may represent smaller, partially episodic flows of probably ephemeral type with larger variability in direction and flow regime; transport and sedimentation rates being significantly smaller than during deposition of subset I. A part of the bimodal palaeocurrent distributions within short sedimentary sequences fits rather well into the general transport pattern. Other bipolarities are primarily interpreted as effects of superimposition of channels of different orientations as well as of main channels and crevasse-splay channels, and condensation of deposition from discontinuous or episodic flows. The main mechanism creating the vertical succession of various directions is stacking of different substratum members to multistorey complexes by primary-depositional restriction of formation and/or secondary-erosional removal of topstratum sediments. The occasional occurrence of herring-bone cross-stratification representing bipolarities within very short depositional sequences, testifies to probably episodic and random changes in water surface slopes allowing currents to reverse locally. Examination of long sedimentary sequences by autocorrelation analysis reveals changes of transport directions with time in a sinusoidal manner, indicating a conformable environmental development of the alluvial watercourses which reflects a time-cyclic trend of sedimentation by spatially continuous and time-concordant shifting of the channel system. The results of the statistical analysis enable an enhanced palaeoenvironmental reconstruction of the alluvial network which fits best to a transitional meandering-thalweg-braided channel pattern. Comparison of the statistical results, especially in the light of vector trend surface analysis, clearly reflects the different palaeogeographic positions of both investigated areas, but rules out only minor influences of regionally different palaeoslopes on the alluvial depositional environment, with the basic principles of fluvial style at the western margin (Eifel) generally matching those at the eastern margin (Bavaria) of the German Basin.