Fluid inclusion and cathodoluminescence studies on fluorite from the Kerio valley, Kenya

The Kerio valley lies between the Elgeyo escarpment and the Tugen hills which mark the western margin of the Kenya rift valley. The main fluorite deposits are located in the southern part of the valley at Kimwarer, Choff and Kamnaon.Three types of inclusion fillings were identified: Liquid+Vapour, Liquid+Daughter Minerals and Liquid. The L+V type is dominant. Inclusions occur as clusters, trails along the crystal growth zones and as isolated ones. Low salinities, apparently lower than the 5% wt. NaCl equivalent, were established. Homogenization temperatures suggest that fluorite mineralization took place at different stages and at temperatures between 120 and 180 °C. Isolated readings above 180°C may be referring to the original inclusions in limestone. These measurements and the absence of CO₂ in the inclusions, as well as the occurrence of vugs and crustifications with fluorite, suggest that mineralization took place at relatively shallow depths.Emission spectrum lines representing Eu²⁺, Dy³⁺, Tb³⁺ and Sm³⁺ in fluorite were identified. Sm³⁺ was detected only in the pinkish luminescence of veined fluorite, whereas the pinkish zone in banded fluorite contains Tb³⁺. Eu²⁺ which gives the strongest emission lines in the blue part of the visible spectrum, apparently is responsible for the strong blue cathodoluminescence (CL) in fluorite. The dominance of Eu²⁺ peaks further points to the fact that fluorite mineralization in the Kerio valley took place in an environment that was enriched in Lanthanide Rare Earth Elements (LREE). The presence of rare earths and radioactive elements in fluorite points towards their enrichment in the environment of fluorite mineralization. A juvenile origin of mineral forming solutions is proposed.Two generations of fluorite were established: allotriomorphic fluorite, forming the matrix, and the idiomorphic variety, occurring either in barite or in druzes in early fluorite. Barite in turn forms idiomorphic crystals in allotriomorphic fluorite. Relics of calcite occur in both K-feldspars and in early fluorite. Oxides and hydroxides of Fe, Mn, Ti and Al commonly occur in open spaces in fluorite. Of significance is the presence of gold in fluorite. Fluorite mineralization is of hydrothermal origin in the post-Miocene era and was formed as a result of metasomatic replacement of marble and open space fillings.     

Deep convective cloud scales and direct adjustment of upper troposphere moisture in TWP environment

Summary A conceptually simple model based on cloud geometry is proposed to explain direct adjustment of moisture by convective clouds. The model is tested using deep convective cloud geometry and changes in upper tropospheric humidity in the Tropical Western Pacific (TWP) during TOGACOARE. The model and the data emphasize the importance of clouds larger than a General Circulation Model (GCM) grid box in drying the upper topospheric environment and in sub grid scale clouds in moistening the upper topospheric environment. The convective cloud sizes and changes in moisture are shown to be linked to precipitation rates. The change from clouds that moisten to clouds that dry the environment occurs when the average cloud size in 6.8×10⁴ km² or rain rate of 2.1 mm hr^–1. In a study of moisture change in the sub cloud layer due to convection, Barnes and Garstang (1982) demonstrated that precipitation rates greater than 2 mm hr^–1 resulted in drying. The critical rain rates above which environmental drying occurs is similar for both upper tropospheric regions and the sub cloud. The similarity of the rain rates indicates that the model concepts maybe used to explain direct adjustment of moisture under a variety of conditions.With 12 Figures     

Evidence for delayed poleward expansion of North Atlantic surface waters during the last interglacial (MIS 5e)

The Last Interglacial (Marine Isotopic Stage or MIS 5e) surface ocean heat flux from the Rockall Basin (NE Atlantic) towards the Arctic Ocean was reconstructed by analysing dinoflagellate cyst (dinocyst) assemblages in four sediment cores. Together with records of stable isotopes and ice-rafted detritus, the assemblage data reflect the northward retreat of ice(berg)-laden waters and the gradual development towards interglacial conditions at the transition from the Saalian deglaciation (Termination II) into MIS 5e. At the Rockall Basin, this onset of the Last Interglacial is soon followed by the appearance of the thermophilic dinocyst species Spiniferites mirabilis, with relative abundances higher than those observed at present in the area. North of the Iceland-Scotland Ridge, however, S. mirabilis only appears in significant numbers during late MIS 5e, between ～118 and 116.5 ka. Hence, fully marine Last Interglacial conditions with most intense Atlantic surface water influence occurred during late MIS 5e in the Nordic seas, and consequently also farther north in the Arctic Ocean, and at times when northern hemisphere summer insolation was already significantly decreased. The stratigraphic position of this Late Interglacial optimum is supported by planktic foraminifers and contrasts with the timing of the early Holocene climatic optimum in this area. We interpret the delayed northward expansion of Atlantic waters towards the polar latitudes as a result of the Saalian ice sheet deglaciation and its specific impact on the subsequent water mass evolution in this region.     

Crystal fractionation and partial melting in the petrogenesis of a Proterozoic high-MgO volcanic suite,Ungava, Québec

There is little concensus on the relative importance of crystal fractionation and differential partial melting to the chemical diversity observed within most types of volcanic suites. A resolution to this controversy is best sought in suites containing high MgO lavas such as the Chukotat volcanics of the Proterozoic Cape Smith foldbelt, Ungava, Quebec. The succession of this volcanic suite consists of repetitive sequences, each beginning with olivine-phyric basalt (19-12 wt% MgO), grading upwards to pyroxene-phyric basalt (12-8 wt% MgO) and then, in later sequences, to plagioclase-phyric basalt (7-4 wt% MgO). Only the olivine-phyric basalts have compositions capable of equilibrating with the upper mantle and are believed to represent parental magmas for the suite. The pyroxene-phyric and plagioclase-phyric basalts represent magmas derived from these parents by the crystal fractionation of olivine, with minor chromite, clinopyroxene and plagioclase. The order of extrusion in each volcanic sequence is interpreted to reflect a density effect in which successively lighter, more evolved magmas are erupted as hydrostatic pressure wanes. The pyroxene-phyric basalts appear to have evolved at high levels in the active part of the conduit system as the eruption of their parents was in progress. The plagioclase-phyric basalts may represent residual liquids expelled from isolated reservoirs along the crust-mantle interface during the late stages of volcanic activity.A positive correlation between FeO and MgO in the early, most basic olivine-phyric basalts is interpreted to reflect progressive adiabatic partial melting in the upper mantle. Although this complicates the chemistry, it is not a significant factor in the compositional diversification of the volcanic suite. The preservation of such compositional melting effects, however, suggests that the most basic olivine-phyric basalts represent primitive magmas. The trace element characteristics of these magmas, and their derivatives, indicate that the mantle source for the Chukotat volcanics had experienced a previous melting event.     

Velocity Oscillations in Active Sunspot Groups

Time series of two-dimensional spectra were taken with the Göttingen 2D spectrometer at the VTT on Tenerife in 1996. They were investigated for Doppler velocities and velocity oscillations in small spots and pores of rapidly evolving sunspot groups. For the present measurements the magnetically insensitive lines Fe i 557.6 nm and Fe i 709.0 nm were selected. Spots with penumbrae exhibit the Evershed effect. Some pores seem to be connected with downflows, but the centres of the downflows are somewhat displaced from their associated pores. The surroundings of the pores show red shifts relative to the whole field of view. The power in the 5-min range is reduced inside the spots and pores as well as in their immediate vicinity. This reduction inside the spots is in agreement with former results. Outside the area of the spot group the 5-min power has a patchy structure with a typical size of 5 arc sec. For periods below 3.3 min, the behaviour of individual spots and pores is different. Some spots show clearly enhanced power for these periods, and it remains high down to the Nyquist period at 1.5 min. The small pores do not show enhanced three-minute oscillations compared with their vicinity. Inside one spot we find a ring of enhanced power in the period range between 8 and 20 min corresponding to the time scales of granular variations. This result could be an indication of a relation between spots and convection, but magneto-accoustic waves are also possible. The same ring exhibits also enhanced power for short periods.     

A six-step practical approach to semivariogram modeling

Geostatistical prediction and simulation are being increasingly used in the earth sciences and engineering to address the imperfect knowledge of attributes that fluctuate over large areas or volumes—pollutant concentration, electromagnetic fields, porosity, thickness of a geological formation. Central to the application of such techniques is the need to know the spatial continuity, knowledge that is commonly condensed in the form of covariance or semivariogram models. Their preparation is subdivided here into the following steps: (1) Data editing, (2) Exploratory data analysis, (3) Semivariogram estimation, (4) Directional investigation, (5) Simple modeling, (6) Nested modeling. I illustrate these stages practically with a real data set from a geophysical survey from Elk County, Kansas, USA. The applicability of the approach is not limited by the physical nature of the attribute of interest.     

Diurnal variations of temperature as a cause of periodic variations of high-frequency geoacoustic emission

Investigated is the periodic component of geoacoustic emission within the frequency band from 2.0 to 6.5 kHz registered by the piezoceramic hydrophone installed near the bottom of Lake Mikizha in the Kamchatka Peninsula. It is revealed that the variation period is 24 hours and the maximum variation is observed in summer, when the average daily air temperature is above 10°C. In that period the close connection is revealed between the series of air temperature and geoacoustic emission. Taking into account the similar results of measurements of the ground surface slope at the observation site, the most probable reason for the diurnal variations of high-frequency geoacoustic emission is the deformation of near-surface sedimentary rocks caused by diurnal variations of temperature.     

Global climate changes and moisture conditions in the intracontinental arid zones

In order to estimate a transient response of the local hydrological cycle and vegetation cover in the African monsoon area to global climate changes, a simple two-dimensional water vapor transport model coupled with a carbon cycle model for the soil was used. The key difference from other models is that we take into account a positive feedback between the precipitation and development of the vegetation root system in the underlying surface. As our calculation shows, this feedback is responsible for a long-term transient response of local hydrological cycles to the global temperature changes. In the case of a four component vegetation system - tropical forests, savannah, semi desert and desert, (and 2 °C ocean surface water warming), a new steady-state is reached in about 1500 years.In previous works of other authors, the increase of summer precipitations during Holocene or Last Interglacial could be explained only as a result of the surface temperature increase in the intracontinental parts of Africa. However, from paleodata indicates, the temperature in the intracontinental regions of Africa rather decreased during warm epochs of geological past: Holocene optimum, Last Interglacial and middle Pliocene climatic optimum. Our simple model simulations agree with both paleoprecipitation and paleotemperature data.