Lithium in spilites

During spilitisation the abundance of Li increases from about 12 ppm in normal basaltic rocks to an average of 75 ppm, with some spilites containing up to 190 ppm. This is relatively far greater than the increase of <50% in Na. Also the ratio CaO/Al₂O₃ decreases from about 1.0 to <0.1 and the H₂O content increases markedly.Alteration of this kind requires waters rich in both Na and Li, with the material added having a ratio 10⁵ Li/Na of about 750: the ratio for seawater is 1.8 but most subsurface waters have higher ratios and are more likely agents of spilitisation.Volcanic hydrothermal brines have suitable Li and Na abundances and are known to have reacted with fresh basalts to produce spilitic alteration: in cases of spilitisation of volcanogenic sediments, reaction with non-volcanic brines may have occurred. In both cases the waters must initially have high CO₂ and low Ca activities.     

Additional estimates of continental surface Precambrian shield composition in Canada

New trace element analyses have been made on the composite Canadian Precambrian shield samples reported in 1967.The overall mean abundance of Cr has been revised to 35 ppm (from 99). New abundances similar to 1967 values are (in ppm): Ni, 19; Co, 12; Cu, 14; Zr, 300; Sr, 315; Ba, 1070; Rb, 110: individual 1967 Rb values were erroneous.Elements not previously determined have the following overall mean values (in ppm): Zn, 52; Sc, 7.0; Nb, 26; Hf, 6.9; La, 32; Ce, 65; Nd, 26; Sm, 4.5; Eu, 0.94; Gd, 2.8; Tb, 0.48; Ho, 0.62; Yb, 1.5; Lu, 0.23; Y, 21; Pb, 17; values in ppb are: Ir, 0.02; Au, 1.8; Tl, 520.Clear positive correlations among Mg-Cr-Ni-Ir-Au appear for all rock-types, marble and quartzite as well as mafic igneous. Regional differences are apparent for several elements: e.g. higher Au, Ir, Cr, Ni in Baffin Island and Northern Quebec composites, compared with Saskatchewan and Southwestern Quebec; high Ti, Zn, Nb, Zr, Hf, REE, Y, Sr, K/T1 abundances and negative Eu anomalies in Southwestern Quebec.The overall REE abundances (omitting Southwestern Quebec) differ from other surface continental crustal rock estimates.     

Development of the early continental crust. Part III. Depletion of incompatible elements in the mantle

New analyses and critical re-evaluation of older data permit another estimate of the abundances of some 42 elements in the silicate shells of the Earth, excluding the core.These data and estimations of volcanic rock production lead to a series of estimates of the mass-fractions of depleted and undepleted mantle. Most likely estimates of the latter lie in the range 0.2–0.4. The best estimates are obtained for the most incompatible elements K, Rb, Sr, Ba, Nb, Th, U.The development of the continental crust and the change in tectonic processes over time is consistent with these interpretations.     

Preliminary results obtained with a vertical strainmeter at the underground laboratory of geodynamics at walferdange (grand-duchy of Luxembourg)

A new vertical strainmeter is described. An harmonic analysis of earth tides measured with that instrument has been performed. It yields a numerical estimation of the first Love number derivative which seems to be compatible with the Poisson coefficient of the local rocks of approximately 0.4.     

Quantitative Imaging of Stable Isotopes by Ion Microprobe

A new analytical procedure has been developed to measure oxygen isotopic ratios and, more generally, stable isotopic ratios in microparticles with a few % precision. This procedure uses ion microprobe quantitative imaging with a scanning microbeam. In this mode, ion images are acquired on the electron multiplier. Image processing allows extraction of areas of interest from the whole image and computation of isotopic ratios in the selected areas. The accuracy of this method has been tested for the analysis of the 18O/16O ratio in quartz and magnetite reference samples powdered to a grain size of 2-5 μm. A detailed study of the instrumental mass fractionation allowed an understanding of differences between classical spot analysis and scanning ion imaging, and showed that fractionation effects were related to grain topography for microparticle analysis, and to image processing or to secondary optical settings for ion imaging. The precision obtained for the measurement of ä18O was 3% for grains having a diameter of 2 μm.     

The origin of laminated and massive anorthosite,Sept Iles layered intrusion,Québec,Canada

Well-laminated plagioclase-rich rocks (‘anorthosite’) occur as layers in mineralogically similar but massive rocks in the Sept Iles intrusion. Several phases of crystallization of poikilitic clinopyroxene have preserved the intermediate stages in the production of these rocks. Plagioclase nucleated and initially grew in random orientations in the stationary part of the boundary layer. There, in the absence of other events, crystallization produced the massive anorthosite. The laminated anorthosite may have been produced by episodic increases in the velocity of the magma adjacent to the boundarylayer induced by magmatic density-currents. Such currents would have caused the zone of simple shear of the boundary-layer to migrate outwards into the previously stationary part. This simple shear would then have rotated the existing plagioclase crystals, as well as crystals that nucleated at that time, towards the shear-plane. Some rocks show evidence of several periods of enhanced flow-velocities. The production of the lamination may have weakened the crystal-mush and enabled slumping to occur, producing folds and contorted layers. Finally, compaction liberated large quantities of intercumulus magma of which some was trapped as granophyric segregations and some escaped by overturing and churning the crystal-pile. Earlier-formed granophyric segregations and laminated layers were disrupted during this process to produce a range of more-complex rocks including igneous breccias.     

Variations in geometric measures of topographic surfaces underlain by fractured granitic plutons

Silicate melts impose systematic fracture patterns on their hosts and their own subsolidus portions as they crystallize and cool to ambient conditions. Weathering of these plutonic bodies accentuates the fractures and produces fragmented topographic surfaces whose geometric measures exceed those of a Euclidean surface. Because geometric measures of these surfaces are potentially indicative of the percolation properties of the fracture network, contours and vertical sections were digitized from 1250000 and 1240000 scale maps, and their statistical fractal dimensions,D, were computed. Regions underlain by granitic plutons with similar fragmentation patterns were found to haveD values ranging from 1.15 to 1.28 within a single pluton. This range of values is caused by noise and several geologic factors. The values contain noise introduced by the map-making and digitizing procedures that amounts toD1.05. However all values lie well above this threshold, and they correlate closely with local differences in weathering. SmallerD values derive from contours where accumulation of screen or glacial erosion has smoothed the surface, whereas largerD values derive from contours along aretes and in unglaciated terrains. In each of these regions, largerD values occur locally where fractures are most frequent and/or continuous. LargeD values were also derived from regions underlain by host rocks that lie in the near-field region of the stress field caused by the pluton. The fractral values derived are apparently related to the magma-hydrothermal event, they can potentially be used to derive properties of the percolation networks that were active during the hydrothermal events.     

Lunar magnetic variations

Displayed daygraphs of magnetic observatory hourly mean values and of lunar magnetic variations reconstructed from spherical harmonic coefficients are used to illustrate the difficulties that arise in separating lunar magnetic effects from those associated with the 27 day recurrence tendency in magnetic activity.