Techniques and instrumentation for study of natural electromagnetic induction at sea

Electromagnetic fluctuations in the ocean have external sources above (ionospheric) and below (secular variation of the earth's magnetic field), and internal, purely oceanic sources associated with interaction between water velocity fields and the earth's field. Energy diagrams indicative of the electromagnetic activity in the sea are presented. From the latter, estimates of the resolution required in electromagnetic research at sea can be made. Absolute minima of 1 γ and 0.05 μV/m are necessary for magnetic and electric fields, respectively. Because the ocean shields overhead sources at frequencies above a few hundred c/h and because motional fields have weak signatures, a resolution at least 10 times higher would considerably enhance the scope of such research.The response of electric field instruments to motionally induced fields depends upon whether they are fixed or drifting, but both types respond similarly to fields of external origin.The most stringent limitation to electric field sampling in the sea is the difficulty in achieving low-noise electrical continuity between measuring circuits and sea water. Even the best matched silver—silver chloride electrodes introduce variable electrochemical signals hard to maintain below a millivolt. These mask very low frequency signals unless sophisticated techniques such as electrode switching are used.     

On the presence of leucite in basaltic rocks from central France

The existence of interstitial leucite is demonstrated in a number of alkali-basalts and basanites from the tertiary and quaternary volcanic province of central France. This leucite occurs in nepheline-normative, K₂O-rich basaltic rocks. Available experimental data show that the assemblage nepheline_ss+leucite_ss+ alkali feldspar_ss would be a frequent one, and our observations tend to support the view that it is indeed common in basaltic rocks of appropriate composition.     

Glacial geological evidence for the medieval warm period

It is hypothesised that the Medieval Warm Period was preceded and followed by periods of moraine deposition associated with glacier expansion. Improvements in the methodology of radiocarbon calibration make it possible to convert radiocarbon ages to calendar dates with greater precision than was previously possible. Dating of organic material closely associated with moraines in many montane regions has reached the point where it is possible to survey available information concerning the timing of the medieval warm period. The results suggest that it was a global event occurring between about 900 and 1250 A.D., possibly interrupted by a minor readvance of ice between about 1050 and 1150 A.D.     

Axial depth anomalies from 10 to 50° north along the Mid-Atlantic Ridge: correlation with other mantle properties

We present an analysis of intermediate- to long-wavelength (a few hundred to a few thousand kilometers) axial depth anomalies along the Mid-Atlantic Ridge between 10 and 50°N. The maximum depth of the rift valley is chosen as the elevation datum for oceanic crust of zero-age. The large depth anomalies are correlated, for short wavelengths (less than a few hundred kilometers), with some fracture zones irrespective of their offset and, for intermediate to long wavelengths, with mantle properties expressed in (1) excess elevation near triple junctions, (2) trace element and isotope geochemistry of the basaltic liquids emplaced at the ridge axis, and (3) anomalies in the Earth's gravity field. We suggest that the correlations may be explained in two ways: the depth anomalies of intermediate to long wavelength may represent the sites of upwelling and downwelling of the deep mantle; alternatively, the depth anomalies could be due to a regionalization in temperature or mineralogy in the asthenosphere.     

Contribution de l'aeromagnetisme a l'etude du golfe de Valence (Mediterranee occidentale)

We present an aeromagnetic survey of the Gulf of Valencia and the Balearic Islands (western Mediterranean). A total field anomaly map and a map of the anomalies reduced to the pole have been obtained. From these maps, it is apparent that there are two regions of opposing magnetic style: the Balearic archipelago which is magnetically very smooth, and the north Balearic basin (or Gulf of Valencia) where anomalies are in places very intense. From a comparison of these two domains, we conclude that the Valencian basin was created during an extensional tectonic phase.     

Transcrystalline shear fracturing and pseudotachylite generation in a meta-anorthosite (Harris, Scotland)

The meta-anorthosite is locally deformed by brittle shear fracturing, which progressively increases from isolated fractures with little cataclasite to many generations of closely spaced fractures, the intervening rock being highly deformed, in both a plastic and brittle way. In most cases an E-W compression on gently dipping to steep reverse shear planes occurs, which we relate to a Caledonian thrust zone.In places, the highly deformed rock is cut by pseudotachylite veins, which locally form networks. The pseudotachylite is generally intrusive, but does not appear to be related to movement on major slip surfaces. Very locally it may have formed in situ. Pseudotachylite only occurs in highly deformed rock, is only very occasionally deformed itself and, thus, generally represents at each locality the last stage of a complex deformation history, as if its presence welded the rock and prevented further deformation. These striking differences from the country-rock gneisses (in which pseudotachylite occurs on well developed fractures in very slightly deformed rock) are considered to be due to the low anisotropy of the meta-anorthosite, to its lower shear strength and to the easy propagation and branching of the shear fractures in plagioclase. The source of the heat necessary to generate the pseudotachylite melt is not clear—it may come from crack propagation as well as frictional sliding.     

Two examples of uranium associated with organic matter

Precambrian carbonaceous material from Cluff, Saskatchewan (Canada), and samples of Late Carboniferous material from Arlit (250 km NNW of Agadès, Niger), were studied by high resolution transmission electron microscopy and energy dispersive X-ray analysis. The Cluff sample is very rich in uranium, whereas those from Arlit are poorer. An intact organo-uranium complex is the major component of the Cluff sample, the carbon and oxygen content of which are high. Uraniferous fragments show very small molecular orientation, whereas those that are uranium-free present a statistical long range preferred orientation. The carbonaceous material of Arlit is entirely free of uranium and behaves as any other humic poorly evolved material; it acquires a small molecular orientation on heat treatment. Uranium was found associated with titaniferous illite in the form of poorly crystallized UTi₂O₆ (Brannerite). The formation of brannerite is attributed to a topotactic transformation of clay particles containing adsorbed organic matter. Clay yields TiO₂; organic matter is the source of UO₂. Destruction of clay leads to release of organic matter, free of uranium, which undergoes diagenesis.     

UPb and RbSr geochronology of the eastern part of the south Rogaland igneous complex,southern Norway

The successive intrusive events in the eastern part of the south Rogaland anorthosito-mangeritic complex have been dated by the UPb method applied to zircon, sphene, monazite, uraninite and accessorily by the RbSr whole rock isochron method. The whole magmatic activity in that part of the complex takes place in a short time interval: between 955 m.y., the intrusion age of the Bjerkrem-Sogndal layered norites, and 910 m.y., the emplacement age of a pergmatite in the Lyngdal granodiorite. The UPb geochronological data are consistent with field and petrological data on origin and mutual relations of the intrusives. The RbSr data give incoherent results probably due to open system behaviour in some cases.     

Correlated seasonal and climatic variations of trace constituents in the stratosphere

Summary Spectrometric experiments performed, in November 1976, within the framework of the Latitude Survey Mission on board the NASA Convair 990 from Ames Research Center are briefly deseribed. The results presented concern odd nitrogen molecules, HCl and water vapor. In terms of vertical column density, HNO₃ is predominant over NO+NO₂ at all latitudes higher than 40 degrees. A seasonal variation of NO₂ abundance is observed, with larger values in the summer hemisphere at high latitude. The mean zenith column density of HCl above 11 km is 1.5×10¹⁵ mol.cm^–2, with no evidence for any seasonal or climatic variation. Local number densities as high as 1.4×10¹⁰ mol.cm^–3 for HNO₃ and 5.4×10¹⁴ mol.cm^–3 for water vapor have been measured during the same flight near 11 km.