Slump debris flows in the Akovolwo Mountains area of Benue State,Nigeria

In September 1979, rain-induced complex landslides occurred in two separate localities along the slopes of the Akovolwo Mountains near Jato-Aka in the Kwande Local Government area of Benue State, Nigeria.The two movements developed on bedrock slopes covered by a veneer of colluvium and (or) residuum no more than 1.5 to 2m deep. Each started as a slump and graduated into a debris flow. The slump, extending downhill a short distance away from the head scarp, had been reconstituted into a debris flow as the initial movement (sliding movement) of the soil mass caused remoulding of the saturated moving mass into viscous debris-laden mud. The resulting mass moved partly along a pre-existing mountain stream channel and partly along a fresh channel it cleared in the savanna forest. At the break of slope, the mass of boulders was dumped and a trail of smaller fragments littered the channels towards the Katsina-Ala River.     

Les minéralisations stratiformes à F-Ba de Hammam Zriba,Jebel Guébli (Tunisie nord orientale): l'apport des études d'inclusions fluides à la modélisation génétique

Résumé Le gîte de H. Zriba-J. Guebli (réserves: 5 millions de tonnes à 15–35% CaF₂) est constitué pour l'essentiel d'amas stratoïdes encaissés dans des formations phosphatifères argileuses et carbonatées, d'âge Campanien inférieur. Quatre formations minéralisées (Mo, Fr, F₂ et F₃) ont été distinguées. La plus ancienne (formation Mo) ne contient pas de fluorine; elle est à dominante quartzeuse ou barytique avec quelques sulfures. Le minerai à fluorine rubanée (formation F₁) est le plus important sous l'angle économique. Trois types d'inclusions fluides ont été observés dans la fluorine: 1) d'abondantes inclusions aqueuses, 2) de rares inclusions gazeuses et 3) de fréquentes inclusions à hydrocarbures liquides jaunes, fluorescentes en éclairage U.V. Les études microthermométriques montrent une augmentation des températures d'homogénéisation (100 à 130°C dans la fluorine F₁, 150–185°C dans la fluorine F₃) avec augmentation parallèle de la salinité des solutions piégées. On propose un modèle de mise en place de la fluorine par voie épigénétique à partir de solutions semblables aux eaux de formation des champs pétrolifères, suivi par des remaniements liés à des circulations hydrothermales récentes.

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In the H. Zriba-J. Guebli mine (5 millions tons of ore with 15%–35% CaF₂) the ore deposits are mostly stratiform bodies hosted by lower-Campanian clay-carbonate sediments with significant phosphate contents. Four successive ore formations are apparently fluorite-free and either quartz-rich or barite-rich with minor amounts of sulfides. The fluorine rubannée (formation F₁) is the main economic ore. Three major fluid inclusions types have been observed in fluorite: (1) abundant aqueous inclusions, (2) rare gas inclusions, and (3) usually yellow oil-bearing inclusions with intense UV fluorescence. In microthermometric investigations on primary aqueous inclusions a general increase in the homogenization temperature from about 100°C to 130°C in fluorite F₁ up to 150°–185°C in fluorite F₃ with a simultaneous increase in salinity was observed. Finally, fluorite crystallized epigenetically in formation-water-type brines; the last geodic filling by fluorite F₃ is tentatively related to Miocene hydrothermal circulation.

     

Experimental Petrology of Melilite Nephelinites

Experimental study of natural melilite nephelinite lavas ofintermediate K/Na ratio at low pressure (fo₂ reveals the presenceof a peritectic ‘point’ of distributary type (1090?C)for liquids saturated with leucite, nepheline, and spinel. Withdecreasing temperature on the olivine + melilite cotectic, botholivine and melilite react with such liquids to produce high-calciumpyroxene at the peritectic. Both the olivine + high-calciumpyroxene and melilite + high-calcium pyroxene cotectics arestable at temperatures below the peritectic. Olivines coexistingwith such liquids are much more magnesian than those in comparabletholeiitic liquids. The olivine-liquid Fe-Mg distribution coefficient is a monotonically increasing function of silica activity over the composition range spannedby melilite nephelinite, ugandite, alkali basalt, and tholeiitebasalt liquids. The analogous Fe-Mg distribution coefficientfor melilite and liquid is effectively constant , while that for high-calcium pyroxene and liquidis highly dependent on the chemistry of high-calcium pyroxene(cf., Sack & Carmichael, 1984). Pseudoternary liquidus projectionsof multiply saturated liquids coexisting with nepheline, leucite,and spinel (?olivine?high-calcium pyroxene?melilite) have beenprepared to facilitate graphical analysis of the evolution oflava compositions during hypabyssal cooling. Major element chemicalanalyses and petrographic features of lavas from Mt. Nyiragongo,East Africa and Oahu, Hawaii (e.g., Denaeyer et al., 1965; Wilkinson& Stolz, 1983) confirm the validity of these diagrams andthe systematics established from the experimental data. ^*Reprint requests to R.O. Sack     

The molecular composition of ambers

Bulk (elemental composition, IR, CP/MAS ¹³C NMR) and molecular (GC-MS) analyses have been performed on a series of ambers and resins derived from different locations (Dominican Republic, Philippines, Canada, Israel, New Zealand, Chile) having diverse botanical affinities (Araucariaceae, Hymenaea) and variable age (from Holocene to Early Cretaceous). No major differences have been observed from the elemental composition and the spectroscopic data; however, the molecular analyses of the solvent extractable fraction show that a specific mixture of components is present in each sample. These are mainly diterpenoid products that in general are also found abundantly in the higher plants from which the ambers and resins originate. Nevertheless, a direct relationship between major terpenoid constituents in fossil resins and precursor plant materials can only be established for the younger samples.Irrespective of the geographical or botanical origin of the ambers and resins, several common age-dependent molecular transformation trends can be recognized: (1) progressive loss of olefinic bonds (especially those located in exocyclic positions), (2) decrease of functionalized products, and (3) increasing proportion of aromatized components. However, even in the samples of older age (Cretaceous) the degree of aromatization is very low when compared with that of other higher-plant related materials such as fossilized woods or low rank coals. This indicates that maturation must involve essentially olefin polymerization processes instead of extensive aromatization.     

High temperature calorimetry of sulfide systems

The standard enthalpies of formation of FeS (troilite), FeS₂ (pyrite), Co_0.9342S, Co₃S₄ (linnaeite), Co₉S₈ (cobalt pentlandite), CoS₂ (cattierite), CuS (covellite), and Cu₂S (chalcocite) have been determined by high temperature direct reaction calorimetry at temperatures between 700 K and 1021 K. The following results are reported: $$\Delta {\rm H}_{f,FeS}^{tr} = - 102.59 \pm 0.20kJ mol^{ - 1} ,$$ $$\Delta {\rm H}_{f,FeS}^{py} = - 171.64 \pm 0.93kJ mol^{ - 1} ,$$ $$\Delta {\rm H}_{f,Co_{0.934} S} = - 99.42 \pm 1.52kJ mol^{ - 1} ,$$ $$\Delta {\rm H}_{f,Co_9 S_8 }^{ptl} = - 885.66 \pm 16.83kJ mol^{ - 1} ,$$ $$\Delta {\rm H}_{f,Co_3 S_4 }^{In} = - 347.47 \pm 7.27kJ mol^{ - 1} ,$$ $$\Delta {\rm H}_{f,CoS_2 }^{ct} = - 150.94 \pm 4.85kJ mol^{ - 1} ,$$ $$\Delta {\rm H}_{f,Cu_2 S}^{cc} = - 80.21 \pm 1.51kJ mol^{ - 1} ,$$ and $$\Delta {\rm H}_{f,CuS}^{cv} = - 53.14 \pm 2.28kJ mol^{ - 1} ,$$ The enthalpy of formation of CuFeS₂ (chalcopyrite) from (CuS+FeS) and from (Cu+FeS₂) was determined by solution calorimetry in a liquid Ni_0.60S_0.40 melt at 1100 K. The results of these measurements were combined with the standard enthalpies of formation of CuS, FeS, and FeS₂, to calculate the standard enthalpy of formation of CuFeS₂. We found \(\Delta {\rm H}_{f,CuFeS_2 }^{ccp} = - 194.93 \pm 4.84kJ mol^{ - 1}\) . Our results are compared with earlier data given in the literature; generally the agreement is good and our values agree with previous estimates within the uncertainties present in both.     

Modeling microclimate environments: A verification study

A numerical model is developed for simulating microclimate of plants and bare soil. The model evaluates heat, mass, momentum, and radiative fluxes in the soil-plant-atmosphere system. Its vertical domain may extend throughout the whole Planetary Boundary Layer (PBL). The model requires, either, temporal meteorological data of solar radiation, wind speed, air temperature and humidity measured over the field, or, when applied to the whole PBL, initial values of the latter three at its top. Vegetation parameters (leaf area index, photometric properties, root distribution and density) as well as soil texture, hydraulic and photometric properties are considered. The model was verified with meteorological data taken from two different climatological regions, above a bare soil and two cotton fields.For all case studies, observed and calculated values of air (except for within-canopy) and soil temperatures, wind speed, net radiation, and soil-, latent-, and sensible heat fluxes, agreed well with measurements.     

MODEL-BASED TRANSFORMATIONS OF COMMON MIDPOINT GATHERS1

A method for transforming Normal Moveout corrected CMP-gathers is proposed. The method is based upon the availability of a model of the CMP-gather. However, the transformation can be performed with any degree of accuracy in the model. Ideally the employed model should be a synthesis of all available a priori information about the particular data set. Mathematically the transformation is performed as follows. The CMP-gather is considered to be a matrix. This matrix is first decomposed into a set of submatrices of the same dimensions. Each submatrix consists of non-zero elements or samples with the same relative amount of noise. By reducing each of these submatrices to a vector (a trace) we get a new set of traces. This set then represents the transformed CMP-gather. The purpose of the transformation is to organize the CMP data in a form which makes it easier both to analyse the noise distribution and to take the necessary steps to improve the signal-to-noise ratio at the stacking stage. In principle the method incorporates the exploitation of multichannel recordings with the use of models. Several examples of transformed gathers and their applications to the improvement of real seismic data are shown.     

Fluid inclusion and stable isotope studies of Pb-Zn-fluorite-barite mineralization in the lower and middle Benue Trough,Nigeria

Lead-zinc-fluorite-barite veins in the lower and middle Benue Trough (Nigeria) are located within the Lower Cretaceous (Albian) carbonaceous shales, limestones, and arkosic sandstones of this intracontinental rift structure. The veins in the lower Benue Through consist of sphalerite + galena+marcasite ± chalcopyrite ± barite in a gangue of siderite and quartz hosted by carbonaceous shales, whereas in the middle Benue Trough, fluorite, barite, quartz, and similar sulfide minerals are hosted by limestone and sandstone. Fluid inclusion temperatures in vein minerals range from 95°C to 200°C (without pressure corrections) and salinities range from 14 to 24 equiv. wt% NaCl. Oxygen isotope compositions of limestone wall rocks (middle Benue) have been lowered from premineralization ¹⁸O values of about 25 per mil to approximately 16 per mil. Fluid in equilibrium with vein calcite has a calculated ¹⁸O of +2.6 per mil at 130°C. The ⁸⁷Sr/⁸⁶Sr ratio of this calcite (0.71497) suggests that strontium and calcium had a considerably more radiogenic source than the Cretaceous limestone or evaporite did (⁸⁷Sr/⁸⁶Sr=0.7073–0.7078). Observed strontium data, lead isotope compositions of galena, and REE patterns in fluorite suggest that the Lower Paleozoic basement rocks in the trough or their weathered equivalents are likely sources for the Benue Trough ore components. Sulfur isotope data suggest that the sulfur was probably contributed from the Cretaceous evaporites in the trough.Our data favor a basinal brine source for the ore-forming fluid. Fluid criculation probably resulted from high geothermal gradients accompanying continental rifting. Brine interaction with the clastic, carbonate, and evaporite rocks led to metal and sulfur leaching and later deposition in fractures accompanying the Cenomanian deformation and uplift in the Benue Trough.     

Partitioning data pertaining to Fe-Mg ordering around trace cations in olivine and low-Ca pyroxene

We have used trace element partitioning data available in the literature to investigate nonideality of the cations of Yb, Sm, Gd, Ca, Mn, Sc, Ni, and Al in silicate melt, olivine, and low-Ca pyroxene. Results are consistent with ordering of Mg and Fe around trace cations in olivine and pyroxene. On the basis of these data, we suggest there is an increasing tendency for Fe to congregate in the vincinity of the trace cation as the size of the trace cation increases. These results are important both in achieving a better understanding of trace element behavior in crystals and in constraining the temperature and compositional dependence of trace element partitioning.