Microbarograph

The microbarograph of a new design has been offered, and the general principle of its operation is considered. The microbarograph is distinguished from well-known ones; it incorporates a compensatory spring eliminating a zero-point drift of the measuring spring, two additional springs that upgrade the angular displacement sensitivity of the microbarograph, and a control and calibration block that makes it possible to perform the remote calibration of the microbarograph and to control it.     

The microstructure of minerals in coarse-grained Ca-Al-rich inclusions from the Allende meteorite

The microstructure and microchemistry of minerals in Ca-Al-rich, coarse-grained inclusions (CAI) from the Allende meteorite have been investigated with transmission electron microscopy (TEM). Spinels contain only low to moderate dislocation densities and are characterized by a ubiquitous, fine black spotty texture believed to originate from a slightly non-stoichiometric composition. Ti-Al-pyroxenes are relatively featureless, but contain veins of secondary phases apparently deposited in unhealed cracks. Chromite has been identified in the veins, suggesting transport of oxidized iron during alteration. Melilites exhibit the greatest variety of microstructures and are the most heavily altered phase in CAI. High dislocation densities are common and crystals exhibit considerable internal strain, indicating that they have not been annealed. Alteration occurs both as veins along cracks and in fronts extending across several grains. Plagioclase is commonly twinned, but dislocations are rare. The size and morphology of antiphase domains suggest a high temperature of formation with significant low-temperature annealing. Submicron pyroxene precipitates are a ubiquitous and unusual feature of Allende plagioclase whose properties are most consistent with prolonged slow cooling and equilibration after plagioclase crystallization. The precipitates appear to be sufficiently abundant to contain the majority of Mg present in plagioclase but do not easily account for Na and Ti abundances. Wollastonite needles from a cavity in a “fluffy” Type A inclusion exhibit the growth habits and relatively perfect external surfaces indicative of direct condensation from a vapor. Alteration products are predominantly crystalline and alteration of melilite appears to have proceeded primarily by solid-state diffusion at a temperature of approximately 920°K. Overall, the TEM observations suggest that CAI formed under near equilibrium conditions characterized by slow cooling and that solid-state bulk diffusion was the major process affecting their post-crystallization history.     

Results from the Big Spring basin water quality monitoring and demonstration projects,Iowa, USA

Agricultural practices, hydrology, and water quality of the 267-km² Big Spring groundwater drainage basin in Clayton County, Iowa, have been monitored since 1981. Land use is agricultural; nitrate-nitrogen (-N) and herbicides are the resulting contaminants in groundwater and surface water. Ordovician Galena Group carbonate rocks comprise the main aquifer in the basin. Recharge to this karstic aquifer is by infiltration, augmented by sinkhole-captured runoff. Groundwater is discharged at Big Spring, where quantity and quality of the discharge are monitored. Monitoring has shown a threefold increase in groundwater nitrate-N concentrations from the 1960s to the early 1980s. The nitrate-N discharged from the basin typically is equivalent to over one-third of the nitrogen fertilizer applied, with larger losses during wetter years. Atrazine is present in groundwater all year; however, contaminant concentrations in the groundwater respond directly to recharge events, and unique chemical signatures of infiltration versus runoff recharge are detectable in the discharge from Big Spring. Education and demonstration efforts have reduced nitrogen fertilizer application rates by one-third since 1981. Relating declines in nitrate and pesticide concentrations to inputs of nitrogen fertilizer and pesticides at Big Spring is problematic. Annual recharge has varied five-fold during monitoring, overshadowing any water-quality improvements resulting from incrementally decreased inputs. Electronic Publication     

Hot dry rocks: An inexhaustible and renewable source of energy

Geological, methodological, and economic aspects of the production and utilization of petrother mal resources (heat of “dry” rocks) for electric energy production and heat supply are considered. Possibility of the utilization of natural fracture zones or artificial fissure zones, which are created by hydraulic fracturing of underground rock massifs, is noted. Application of new tools for drilling deep wells makes the petrochemical energy competitive relative to both other types of renewable resources and traditional organic fuel.     

Early Quaternary Age of the Final Stage of Volcanism in the Central Part of the Sredinny Range of Kamchatka (K–Ar Data)

Doklady Earth Sciences - A first set of K–Ar isotopic ages obtained, which allowed to estimate the age of the largest volcanoes of the Anaunsky Dol (3.2, 2.2 and 1.9 Ma) and eruptive centers...     

Mineralogy and chemistry of bentonite (?) deposits at Minjingu, Lake Manyara, North Tanzania

Olive green clays likely to be bentonitic in composition have been mineralogically and chemically studied. They occur in association with other lacustrine sediments at Lake Manyara. Radiocarbon dates from four diatom horizons indicate ages ranging from 12 Ka to 135 Ka suggesting a Mid-Holocene age. Middle Pleistocene age have been assigned to the ridged oncolites of Lake Manyara. The olive green coloured clays in the Manyara basin are known to occur in association with other lake beds including phosphorite deposits, stromatolites, bioturbated silty clays, partly silicified marls, conglomerates and olive green coloured opal beds. The results presented herein are from the olive green coloured clays. The olive green clays (bentonite?) are a result of devitrification or alteration of volcanic ashes and/or pyroclasts. The green clays occur in different forms as they are separated from each other by other lacustrine sediments. The alteration might have taken place in slightly different environments in terms of salinity and alkalinity. One of the top layer is friable and shows conchoidal fractures when dry. The other beds below in the lacustrine sequence are cemented with calcite and some dolomite as well as zeolites. The lowermost layer in the sequence is friable and shows cracks filled with coarse crystalline calcite. Mineralogically the bentonite is composed of the clay minerals illite, illite-smectite mixed layer clays, and chlorite. Other authigenic minerals include various zeolites (analcime, clinoptilolite, erionite and some traces of mordenite), opal, and fluorapatite. The clays have magnesium contents varying from 3.01% to 7.43%. The calcium contents vary widely due to presence or absence of one of the two minerals calcite or apatite. Trace elements like Ba, Ce, Sr, Zr are equally attributed to the presence of calcite and apatite. The formation of the illite-smectite mixed layer clays in an alternating manner with other lake sediments depicts different episodes of volcanic eruptions in the area. The mineralogical composition of smectites, zeolites, and opal in the green clays suggests a deposition of pyroclasts and volcanic ashes in a closed lake system with fluctuating levels. Due to evaporation alkalinity and salinity levels were fluctuating. The clays might have been bentonite which have undergone illitisation, a phenomena noted in other neighbouring rift basins.     

Ion microprobe baddeleyite and zircon ages for Late Archaean mafic dykes of the Pilbara Craton,Western Australia

Ion microprobe U–Th–Pb analyses of baddeleyite and zircon yield precise ages for several mafic intrusions in the Pilbara Craton of Western Australia. Baddeleyite was dated from four dolerite dykes of the north‐northeast‐trending Black Range swarm intruded into granitoid‐greenstone basement in the northern part of the craton. The mean ²⁰⁷Pb*/²⁰⁶Pb* age of 2772 ± 2 Ma, interpreted as an unambiguous age of emplacement for the dykes, is within error of previous ion microprobe U–Pb zircon ages for the Mt Roe flood basalts and confirms that the dykes acted as feeders to the volcanic rocks. The Sylvania Inlier, in the southeastern Pilbara Craton, also contains north‐northeast‐trending dykes that were correlated previously with the Black Range swarm. Based on concordant and discordant zircon analyses from samples of two dykes, the best estimate of the age of the Sylvania dykes is 2747 ± 4 Ma. The Sylvania dykes thus appear to be significantly younger than, and hence unrelated to, the Black Range swarm, but may have acted as feeders to younger volcanic units in the Fortescue Group such as the Kylena Formation.     

Grain-size composition of the suspended particulate matter in the marginal filter of the Severnaya Dvina River

The quantitative distribution and grain-size composition of the suspended particulate matter (SPM) in the marginal filter of the Severnaya Dvina River during the summer low-water periods of 2001–2005 were first analyzed in seawater on board of the vessel immediately after its sampling (without preliminary treatment) using a Coulter counter. This analysis revealed the main regularities in the transformation of the grain-size spectra at successive salinity steps of the marginal filter as well as the boundaries between these steps based on the data obtained by direct complex studies of the SPM dispersion. It is established that the water salinity is the main factor that controls the changes in the grain-size distribution and the composition of the particulate matter in the marginal filter. The concentrations of the pelitic fraction and the salinity demonstrate negative correlations between each other. It is shown that the areas characterized by the mass development of phytoplankton are located along the outer boundary of the marginal filter (at the biological step), where the salinity amounts to 23–24 psu. The content of the suspended forms of some chemical (lithogenic) elements and the C_org indicating the SPM’s genetic composition and their relations with the grain-size composition of the latter and the environments are studied.     

The southern cape conductive belt (South Africa): Its composition, origin and tectonic significance

Magnetometer array studies have led to the discovery and mapping of the Southern Cape Conductive Belt (SCCB) crossing the southern tip of Africa from west to southeast coasts. The SCCB lies just south of the Namaqua-Natal Belt of cratonic rocks remobilized about 1000 m.y. B.P. It is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m.y. Relationships between the SCCB and the basement geochronology, geology and tectonics are considered in detail. These relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a Proterozoic subduction which stopped about 1000 to 800 m.y. B.P. Associated with this subduction we propose a Proterozoic range of Andean mountains, whose roots are now exposed in the Namaqua-Natal Belt. Later subduction further south, near the present south coast, is proposed to account for the intrusion, between the south coast and the SCCB, of the Cape Granites in the time interval 600-500 m.y. B.P. There is some evidence for a third, yet more distant, subduction episode off Permian Gondwanaland. After outlining this tectonic history, the paper turns to a closer examination of the hypothesis that the Southern Cape Conductive Belt consists of partly serpentinized basalt accumulated at the top of a Proterozoic subduction. A large static magnetic anomaly, which correlates with the SCCB over most of its length, is well fitted by a model which strongly supports this hypothesis. Bouguer gravity anomalies along western and central profiles likewise support the hypothesis. A discussion follows of the process of formation of the proposed block of serpentinized marine rocks, beginning with serpentinization of the crust near oceanic ridges by reaction of warm, porous, newly-extruded basalt with seawater convecting through it. The serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction, where it accumulates because of its low density. Examples of such accumulations are cited. Finally, it is shown that any iron in the olivine and pyroxene in the original basalt precipitates, when these silicates are hydrated to serpentine, as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites. In particular the Beattie static magnetic anomaly requires, in our model calculation, a very high susceptibility readily attainable in basalt at 15–20% serpentinization. The authors know of no other rock able to provide this high susceptibility. A similar percentage of serpentinization provides the density required to model the gravity anomalies.