Bizarre geochemistry of komatiites from the Crixás greenstone belt,Brazil

Komatiite lava flows in the Crixás greenstone belt, Goiás, Brazil, have textures and volcanic structures typical of Archean komatiites, but are geochemically most unusual. The flows are porphyritic and massive, or layered with spinifex upper parts and olivine cumulate lower parts. MgO contents range from 18 to 40%. In such lavas, only olivine (and minor chromite) can have crystallized, but neither major nor trace elements fall on olivine control lines. In MgO variation diagrams, CaO and Sr fall on lines with slopes steeper than olivine control lines; SiO₂, FeO, Na₂O, K₂O and Y show little systematic variation; Zr shows a large variation that does not correlate with MgO; and Al₂O₃ decreases markedly with decreasing MgO. The aberrant behaviour is highlighted by the REE (rare earth elements) in spinifex and olivine cumulate layers from three flows: in the spinifex layers, chondrite-normalized REE patterns are hump-shaped with maxima at Nd or Sm ((La/Sm)_N=0.6, (Gd/Yb)_N=1.6–2.3), whereas cumulate zones in the same flows have steadily sloping patterns, with LREE enriched relative to HREE ((La/Sm)_N=1.3, (Gd/Yb)_N=1.4). Neither normal magmatic processes acting during emplacement of the komatiites, nor thermal erosion and wall-rock assimilation can explain these effects, and we speculate that elements commonly thought of as “immobile” (e.g. Al, Zr, REE) migrated during hydrothermal alteration or metamorphism. A Pb-Pb whole rock isochron gave an age of 2,728±140 Ma and selected Sm-Nd analyses an apparent isochron age of 2,825±98 Ma (ɛ_Nd≈0). The Pb-Pb age is believed to be the approximate time of emplacement. Interpretation of the Sm-Nd data is complicated by the evidence of mobility of REE.     

Intraplate deformation and 3D rheological structure of the Rhine Rift System and adjacent areas of the northern Alpine foreland

The lithosphere of the Northern Alpine foreland has undergone a polyphase evolution during which interacting stress-induced intraplate deformation and upper mantle thermal perturbations controlled folding of the thermally weakened lithosphere. In this paper we address relationships among deeper lithospheric processes, neotectonics and surface processes in the Northern Alpine foreland with special emphasis on tectonically induced topography. We focus on lithosphere memory and neotectonics, paying special attention to the thermo-mechanical structure of the Rhine Graben System and adjacent areas of the northern Alpine foreland lithosphere. We discuss implications for mechanisms of large-scale intraplate deformation and links with surface processes and topography evolution.     

Time-temperature-transformation (TTT) analysis of cation disordering in omphacite

The kinetics of cation disordering in a natural ordered (P2/n) omphacite have been followed at P=18 and 30 kb, T= 750–1,260° C, for times of between 1.5 min and 16 days in a piston-cylinder apparatus. Time-temperature-transformation (TTT) analysis of the experimental data, using the presence or absence of the 11¯1 reflection in single crystal X-ray precession photographs to indicate the extent of reaction, yields an equilibrium order/disorder temperature (T _ord) of 865±10° C, an activation enthalpy (1 bar) of 71±6 kcal mole^–1 and an activation volume of 9±4 cm³ mole^–1 (plus and minus figures represent the precision of a best fit between experimental data and TTT theory rather than absolute errors). The activation volume is consistent with a vacancy mechanism of cation diffusion. H₂O, added in the form of oxalic acid, appears to speed the process up slightly. The overall transformation mechanism is continuous, involving neither the nucleation of a disordered phase nor a change in antiphase-domain distribution. This is consistent with both first- and non-first-order character for the C2/cP2/n transformation, though a range of ordered states below T _ord is indicated by the weakening of h+k=odd reflections. A simple extrapolation of the disordering rates to geological conditions leads to the first estimate of how long disordered omphacites would take to order in nature, ranging from less than one year at T800° C to more than 10⁷ years at T<350° C.     

Homology of solar flare-associated radio events

The concept of homology, introduced by Ellison, Mc Kenna, and Rceid (1960) for optical flares, can be extended to flare-associated radio events. Successive flares within the same centre of activity sometimes produce radio events that are remarkably similar. The similarity amounts to the fact that they extend over about the same range(s) in frequency, producing at each frequency responses of comparable intensity and duration. On some occasions the intensity curves at individual frequencies show even detailed resemblance. The occurrence of homologous radio events is commonly restricted to periods of less than 48 hours.Without being homologous, radio events that occur in the same centre of activity may present a common characteristic that is typical for that centre. Two such characteristics are (1) the production of radio responses at centimetric, decimetric and metric frequencies, and (2) the impulsiveness of microwave outbursts. The distribution of time intervals during which such bursts occur is compared with the same distribution for homologous events (Figure 3).     

Energetic protons from the solar flare of March 24, 1966

Ten to 100 meV protons from the solar flare of March 24, 1966 were observed on the University of California scintillation counter on OGO-I. The short rise and decay times observed in the count rates of the 32 channels of pulse-height analysis show that scattering of the protons by the interplanetary field was much less important in this event than in previously observed proton flares. A diffusion theory in which D = M _r is found to be inadequate to account for the time behavior of the count rates of this event. Small fluctuations of the otherwise smooth decay phase may be due to flare protons reflected from the back of a shock front, which passed the earth on March 23.     

Riometer observations in the polar caps of solar cosmic ray events during the IQSY

During the period of the IQSY, January 1964 through December 1965, the sun remained quiet, accelerating few energetic particles. There were many instances during the IQSY when lowenergy detectors on satellites and space probes registered small intensity increases. However, few of these events were associated with protons of energies exceeding 10 MeV. Moreover, the maximum intensities (E _p > 500 keV) were typically 1–8/cm²sec ster. Most of these events were below the threshold of riometer detection.The largest solar cosmic ray event observed in 1964 by polar-based riometers was that of March 16. This event was observed by 30 and 50 Mc/s riometers at McMurdo Sound, Antarctica, and Shepherd Bay, N.W.T., Canada.The largest event in 1965 occurred on February 5 and was the largest during the IQSY. It was associated with a class 2 flare at about 1750 UT, February 5. The propagation time between the sun and Earth was about one hour. This event was well observed by satellites, space probes, and riometers.This paper discusses primarily the 5 February 1965 event. Some discussion is also given to the 16 March 1964 event, other small events during the IQSY, and the recent event in March 1966.     

Quasi-stationary striations in basin-scale oceanic circulation: vorticity balance from observations and eddy-resolving model

Recently, prominent jet-like features of the ocean circulation, called striations, with a meridional scale of O(300–500 ;km) and extending for thousands of kilometers in length, have been detected in satellite and in situ observations and in high-resolution numerical models. In this paper, we study quasi-stationary striations, which are best seen in the multi-year time-averaged velocity fields. Using 1993–2002 mean dynamic ocean topography, satellite altimeter observations, and output of the Ocean General Circulation Model for the Earth Simulator, the dynamics of the quasi-stationary striations in the eastern parts of the subtropical North and South Pacific are examined by assessing individual terms in the time-averaged equations of relative and potential vorticity. While non-linear effects are found to be essential in the dynamics of the striations, rejecting some linear hypotheses forwarded in the earlier studies, the relevance of the Rhines mechanism is not confirmed. Eddy flux does not act as a relative vorticity source for the striations. Using the potential vorticity (PV) diagnostics, we show that the time-mean PV is not conserved along the time-mean streamlines, and on the scale of the striations these changes in PV are largely induced by the eddy flux of layer thickness. The fact that eddy fluxes contribute to the striations’ time-mean PV budget suggests that the striations are not a kinematical artifact of time-averaging of westward-propagating eddies.     

Regional Problems in Water Use in Siberian Industrial Agglomerations

Water Resources - The basic indicators of regional water use have been identified and assessed as a component of a single social–natural–technogenic system. These indicators have been...