Petrogenesis of some Ligurian peridotites—II. Rare earth element chemistry

REE compositions of nine partially serpentinized Iherzolitic rocks from Liguria have been investigated: eight samples are from the Erro-Tobbio thrust sheet of the Gruppo di Voltri (western Liguria); one sample is from an allochthonous eastern Liguria mass within the External Ligurides. As previously described (Ernst and Piccardo, 1979, Geochim. Cosmochim. Acta43, 219–237), all specimens are spinel (±plagioclase) Iherzolites. They possess major element bulk rock chemistries which are more or less ‘primitive’ or only slightly depleted compared to estimated upper mantle compositions. Mineral chemistries, varying insignificantly from rock to rock, evidently reflect mantle equilibration in the range 1000–1150°C and 14–16 kbar; pervasive retrograde effects testify to later decompression at shallower levels characterized by temperatures exceeding a typical intraplate oceanic geothermal gradient.The investigated lherzolitic samples show concordant REE patterns, LREE-depleted with respect to chondrites. A wide range in values is observed among the LREE: the observed depletion of LREE in most of the samples is accompanied by a Ce-negative anomaly. Inasmuch as the analyzed samples have been affected to a variable extent by serpentinization, REE mobility during hydrous alteration is treated employing both a theoretical approach and natural evidence.Effects of partial melting prior to serpentinization (resulting chiefly in depletion in LREE) can be rationalized by postulating a two-times chondritic REE abundance in the original unaltered peridotitic material and by considering the HREE contents to have been unaffected by hydrous alteration. When comparison is made with REE compositions for residual material after different degrees of partial melting (computed using an equilibrium, non-modal partial melting model and selected REE partition values), the analyzed Ligurian samples show effects of incipient melting on only a small scale, i.e. 1–5%. Judging from their major and REE compositions, the investigated lherzolites cannot have produced substantial amounts of tholeiitic melt, hence they cannot have provided the primary magmas for the oceanic crustal sequences so widespread within the Northern Apennine and Gruppo di Voltri ophiolitic terranes.     

Strain and rotation in a multilayered fold

Strain was estimated in a fold of Cambrian interlayered siltstones and pelites by determining the preferred orientation of chlorite grains with an X-ray goniometer. Strains so obtained and the postulate that continuity be preserved allowed unfolding of the fold and the determination of rigid body rotations that accompanied the strain. Petrologic investigation showed no sign of major differential volume changes in the siltstones, and this in conjunction with measured strains led to the conclusion that one of the silty layers making up the fold was not, originally, a bed of uniform thickness but a lenticular body, probably representing a single ripple on a ripple-marked tidal flat.Unfolding by piecemeal fitting of unstrained domains shows that none of the principal axes of strain lie consistently parallel to or at right angles to the fold axis. Rock material was displaced with components orthogonal to the profile plane as well as parallel to it. Strain due to compaction during an early history of increasing sediment overburden cannot be separated from strain during tectonic deformation. Its influence is most clearly seen in differential volume change between siltstones and pelites. Additional differential volume changes within pelite beds could have occurred at any time of the deformational history.A comparison of the orientation of strain and rotation axes in the two limbs of the fold, and also comparison of the same orientations in Eulerian coordinates (Cartesian coordinates in the observed fold) and in Lagrangean coordinates (Cartesian in the unfolded fold) make it probable that episodes of relatively uniform strain both preceded and followed the buckling episode that produced the sharp hinge in the competent silt-stone. The siltstone may have been less indurated and thus no more competent than the pelite during early deformation.     

Tritium and90Sr in North Atlantic surface water

Areal distributions and complete time histories since 1952 are presented of the tritium and⁹⁰Sr concentrations in North Atlantic surface water. The distributions are based on a compilation of measured North Atlantic surface water tritium concentrations which is part of this paper and includes hitherto unpublished measurements, and on available⁹⁰Sr compilations. To reconstruct the insufficiently represented early concentrations, a two-box North Atlantic mixing model with tropospheric input is employed, for which the input is specified (on relative scales), and which is fitted to the available surface water observations. This procedure gives a natural tritium concentration of 0.2 TU (±30%), and furthermore suggests that part of the old oceanic tritium and⁹⁰Sr measurements are high. The fit requires characteristic model mixing times of 2.5 years (exchange with an intermediate-depth reservoir about three times the size of the surface box) and 30 years (loss into the deep ocean), and a tritium/⁹⁰Sr input ratio of 310 Ci/Ci. The areal distributions and time histories can serve as boundary value data for evaluations of subsurface tritium and⁹⁰Sr measurements.     

Ion microprobe isotopic measurements of individual interplanetary dust particles

Ion microprobe measurements of D/H ratios in individual fragments of eight stratospheric dust particles give δD values ranging from ?386 to +2534‰ relative to SMOW. The δD values in five particles far exceed those in terrestrial samples and prove that the samples are interplanetary dust particles (IDPs). The hydrogen isotopic composition is heterogeneous on a scale of a few microns demonstrating that the dust is unequilibrated. Measurements of D/H ratios in conjunction with elemental and molecular ion signals in different fragments of individual IDPs show that a carbonaceous phase, not water, is the carrier of the D enrichments. Previous infrared transmission measurements have shown that IDPs fall into three main spectral classes. Particles from two of those three IR classes show large D/H ratios. Two particles studied from the third class do not. However, one of these contains solar flare tracks and is extraterrestrial. Thus, most, but not all, IDPs contain hydrogen with a non-terrestrial isotopic composition.Carbon isotopic measurements on fragments of three IDPs give ratios similar to terrestrial values and show a largely uniform isotopic composition for a given particle. Small, but significant, differences in δ¹³C of ~40‰ between particles are seen. No correlations between the hydrogen and carbon isotopic compositions are observed.The magnesium and silicon isotopic compositions of fragments of three IDPs are found to be normal within measurement errors.     

Global momentum loss in a non-expanding Universe

Applying the basic concepts of general relativity to the global motion of a particle in a mass-filled universe leads to a loss of momentum relative to the rest frame of the Universe. This loss is caused by the different running times of the gravitational interaction quanta exchanged with masses in front and behind the moving particle, if the signal velocity is limited to the speed of light. Due to this gravitational viscosity of space, the energy of photons will be reduced with the time, and thus with the distance of the emitting source. This red shift is superimposed on the Doppler shift in an expanding universe. A discussion of the limiting case of vanishing expansion leads to predictions about mass and radius of the Universe. The value of the mass density in such a steady-state universe must be about three times the closing density discussed in Big-Bang theories. The existence of the gravitational viscosity casts severe doubts on all estimations of the age of the Universe derived from the red-shift data.     

Thixotropie und Fließeigenschaften feinkörniger Böden

Ohne ZusammenfassungVortrag auf der Mitgliederversammlung der Geologischen Vereinigung April 1947. - Manuskript erhalten Frühjahr 1947. Die Schriftleitung.     

Seismicity in the outer rise offshore southern Chile: Indication of fluid effects in crust and mantle

We examine the micro-earthquake seismicity recorded by two temporary arrays of ocean bottom seismometers on the outer rise offshore southern Chile on young oceanic plate of ages 14 Ma and 6 Ma, respectively. The arrays were in operation from December 2004–January 2005 and consisted of 17 instruments and 12 instruments, respectively. Approximately 10 locatable events per day were recorded by each of the arrays. The catalogue, which is complete for magnitudes above 1.2–1.5, is characterized by a high b value, i.e., a high ratio of small to large events, and the data set is remarkable in that a large proportion of the events form clusters whose members show a high degree of waveform similarity. The largest cluster thus identified consisted of 27 similar events (average inter-event correlation coefficient > 0.8 for a 9.5 s window), and waveform similarity persists far into the coda. Inter-event spacing is irregular, but very short waiting times of a few minutes are far more common than expected from a Poisson distribution. Seismicity with these features (high b value, large number of similar events with short waiting times) is typical of swarm activity, which, based on empirical evidence and theoretical considerations, is generally thought to be driven by fluid pressure variations. Because no pronounced outer rise bulge exists on the very young plate in the study region, it is unlikely that melt is accessible from decompression melting or opening of cracks. A fluid source related to processes at the nearby ridge is conceivable for the younger segment but less likely for the older one. We infer that the fluid source could be seawater, which enters through fractures in the crust. Most of the similar-earthquake clusters are within the crust, but some of them locate significantly below the Moho. If our interpretation is correct, this implies that water is present within the mantle. Hydration of the mantle is also indicated by a decrease of P_n velocities below the outer rise seen on a refraction profile through one of the arrays [Contreras-Reyes, E., Grevemeyer, I., Flueh, E.R., Scherwath, M., Heesemann, M., 2007. Alteration of the subducting oceanic lithosphere at the southern central Chile trench-outer rise. Geochem., Geophys. Geosyst. 8, Q07003.]. The deepest events within the array on the 6 Ma old plate occur where the temperature reaches 500–600 °C, consistent with the value observed for large intraplate earthquakes within the mantle (650 °C), suggesting that the maximum temperature at which these fluid-mediated micro-earthquakes can occur is similar or identical to that of large earthquakes.     

A New Ectasian Event of Basitic Magmatism in the Southern Siberian Craton

Doklady Earth Sciences - On the basis of U–Pb dating of zircon and baddeleyite from gabbro–dolerite of the Goloustnaya dyke swarm (southern margin of the Siberian Craton), the age of...