The use of ground rocks in laterite systems: An improvement to the use of conventional soluble fertilizers?

The geochemistry of lateritic terrains is discussed in terms of nutrient demand in agriculture. The data presented show a dramatic depletion of virtually all nutrients when compared to crustal levels and other soil systems. Increasing world demand for food production, particularly in tropical countries where alcohol production is also needed, has forced the agriculture frontier deep into lateritic areas. As cultivation is intensified the laterite becomes more barren and a neutral recipient for the massive doses of nutrients that are to be constantly added if production is to continue. Conventional soluble fertilizers such as NPK do not answer basic requirements. Designed for the high-nutrient and high-exchange-capacity soils of the northern hemisphere they are not held by the Fe₂O₃---Al₂O₃---SiO₂---H₂O laterite systems and, under condition of excess rain, they can be wasted and consequently polluting. Furthermore, they do not supply a variety of both macro- and micronutrient elements that are normally present in soils in equilibrium with rock debris, but that are lacking in laterites. The replenishment of the laterite to the nutrient levels of natural fertile soils can be basically attained by adding into the system ground rocks of varied composition. The thermodynamical and geochemical background of this proposition is discussed and the state of the art is presented.     

Epidotisation of diorites at Al Hadah,Saudi Arabia: Fluid influx into cooling plutons

Diorite plutons at Al Hadah Saudi Arabia, which constitute part of the pan-African magmatic sequence (ca. 600 Ma), exhibit extensive development of epidote. The epidote alteration is concentrated along veins and dyke margins, and is characterised by transformation of plagioclase (Ab 67)+hornblende+biotite+quartz to epidote+hornblende+tremolite−actinolite+plagioclase (Ab 99)±quartz. The reactions involve addition of CaO and total Fe₂O₃, depletion of MgO, Na₂O and K₂O, with variable gains or losses of SiO₂. Epidotised alteration products are hydrated and oxidised relative to the diorite precursor. The whole rock δ¹⁸O of fresh diorite is + 8.2‰ to + 8.8‰, whereas epidote domains have δ¹⁸O whole rock of +9.8‰ to +10.48‰ and negative Δ¹⁸O_{quartz-plagioclase}, implying oxygen isotope exchange with fluids at low temperatures. Epidotisation is considered to have accompanied influx of fluids into plutons during cooling and contraction. The fluids were probably deep formation waters with relatively high Ca²⁺/Na⁺ ratios, moving up thermal gradient.     

O18 enriched ophiolitic metabasic rocks from E. Liguria (Italy), Pindos (Greece), and Troodos (Cyprus)

Low grade hydrothermally metamorphosed ophiolitic basic rocks from E. Liguria (Italy), Pindos (Greece) and Troodos (Cyprus) are enriched in O¹⁸ relative to the oxygen isotope ratio of fresh basalt (6.0±0.5‰). The maximum observed δO¹⁸ value of +13.22‰ corresponds to a positive isotope shift of 7‰ Enrichments in Sr⁸⁷ relative to Sr⁸⁶ correlate with hydrothermal alteration. The δC¹³ values of secondary calcite from E. Liguria are positive, and fall in the range from +0.2% to +3.6‰ Since ophiolitic rocks are considered to be fragments of the oceanic crust and upper mantle, and since the secondary metamorphic assemblages were produced before mechanical emplacement, it is considered that the hydrothermal metamorphism which affected these rocks occurred in the sub-sea-floor environment. The isotope data are directly consistent with the hypothesis that the alteration was produced by interaction of the basaltic material with introduced sea water. Water: rock ratios were sufficiently large to produce the observed isotope shifts. In the Troodos ophiolite sequence δO¹⁸ values decrease steadily downwards and change to progressively larger depletions in the Sheeted Intrusive Complex. The trend of δO¹⁸ decrease correlates with the original direction of increasing temperature. The O¹⁸ depletions, which have also been observed for oceanic “greenstones” (Muehlenbachs and Clayton, 1972b), resulted from water/rock interaction at temperatures greater than the particular temperature range above which whole rock-water fractionations became less than the isotopic difference between fresh basalt and sea water. Since this isotope geochemistry indicates that the water responsible for hydrothermal metamorphism was of sea water origin, the data support the more general hypothesis that convection of sea water within the upper 4–5 kms of the oceanic crust is a massive and active process at oceanic ridges. This process may be completely or partially responsible for (a.i.), the local scatter and low mean value of the conductive heat flux measured near ridges, (a.ii), the transfer of considerable quantities of heat from spreading oceanic ridges, (b) hydrothermal metamorphism, metasomatism and mineralization of oceanic crust, (c), the production of metal enriched, relatively reduced brines during sea water/basalt interaction, d), the high degree of scatter and low mean value of the compressional wave velocities of oceanic basement layer 2 and (e), the low natural remanent magnetization (NRM) intensity of the lower part of layer 2 and upper part of layer 3 of oceanic crust.     

A felsic dyke swarm formed under the sea: the Archean Hunter Mine Group,south-central Abitibi Belt,Quebec, Canada

An extensive rhyolitic dyke swarm has intruded subaqueous pyroclastic deposits, iron-formations, hyaloclastite breccias and lava flows of the 2730 Ma Hunter Mine Group (HMG) in the south-central part of the Archean Abitibi belt, Quebec. The dyke swarm has a minimum width of 500 m and can be traced perpendicular to the section for 2.4 km. Based on crosscutting relationships, chilled margins, quartz content and colour, five distinct dyke generations have been established. Each dyke generation has several magmatic pulses as indicated by parallel rows of columnar joints. Absence of brecciation between parallel rows suggests extremely brief intervals between magma pulses. The central parts of most dykes display inverted V-shaped patterns of columnar-joint convergence, inferred to indicate differential cooling during the late stages of dyke propagation. The dykes commonly display delicate spherulites suggesting rapid cooling, solidification temperatures between 400 and 600°C and penecontemporaneous devitrification. Quartz-feldspar aggregates in the groundmass have locally developed microgranophyric textures. Large spherulites near the chilled margins probably formed at temperatures below 400°C. Percolation of abundant water throughout the dyke complex is suggested by ubiquitous prominent chilled dyke margins. Development of a chilled margin 500 m along one dyke suggests that water percolated at least 500 m below the water/rock interface. Because the dykes intruded subaqueous pyroclastic deposits of similar composition, dyke emplacement below the sea floor is inferred. Interstratification of pillowed flows and brecciated pillowed flows containing rhyolite fragments at the top of the 4–5-km-thick sequence indicates that the central felsic complex probably never emerged during its evolutionary history, supporting the contention that the felsic dyke complex was emplaced beneath the Archean sea floor.     

Speculations on the causes of crustal rifting and subduction,with applications to the atlantic margin of Brazil

It is suggested that ideal regions for continental separation involve regions of crust dominated by granulite and amphibolite facies rocks. Such rocks are strong and refractory. The formation of thick crust of this type requires, at some stage, that the crust be tectonically thickened to about twice the equilibrium thickness which is itself controlled by melting. It is suggested that subduction is initiated by the basalt-eclogite transition in the upper oceanic lithosphere. A large sediment load may induce this reation and lead to subsidence or subduction. If the rate of sediment supply is small, subduction will be transient, or may not occur at all and sinking slow. Such motions could account for sporadic igneous activity by partial melting of slowly sinking ocean-floor materials.     

Changes in winter cyclone frequencies and strengths simulated in enhanced greenhouse warming experiments: results from the models participating in the IPCC diagnostic exercise

The effect of enhanced greenhouse warming on the behaviour of mid-latitude cyclones is examined for changes in the total number of cyclone events and for changes in the number of intense events using the daily averaged mean sea level pressure simulated by coupled climate models participating in the IPCC AR4 (Fourth Assessment Report) diagnostic exercise. Results are presented for a set of scenarios which were produced using a wide range of increasing levels of greenhouse gases. For the enhanced greenhouse warming experiments, the models simulated a reduction in the total number of events and an increase in the number of intense events. This is a robust result, which essentially all the models exhibit. Comparison of the results for each of the scenarios shows that the magnitude of the changes in the number of simulated events increases with increasing levels greenhouse gas forcing used in the scenarios. Even though the numbers of events change, there is no apparent change seen in the geographical distribution of the events, i.e. there is no obvious change in the positions of the storm tracks seen on hemispheric charts. This was also evident in the results for the filtered variance of the meridional wind which was used as a proxy for cyclone activity. In spite of this, it is possible that small shifts in the storm tracks, which are difficult to resolve with the relatively coarse grid used for analysis, could occur.     

The generation of batholiths

The formation of the melts which produce intrusions of the granite family are considered to result from the partial fusion of high-grade metamorphic rocks. The melting behaviour of such materials is considered. Such melts will rarely be water-saturated and the degree of water-saturation must set limits on the ability of the melt to rise. The natural residue of fractional fusion will be metamorphic rocks of the granulite facies. The motion of the melts seems reasonable in relation to the theory of Taylor instability and Stokesian rise of the materials. Differences between Archaean batholith patterns and those in modern belts of plate subduction will be considered. It is suggested that such differences could result from fusion processes in the upper mantle occurring at higher levels than at the present time and “ocean ridges” being more closely spaced.     

The ‘suitcase hypothesis’: Can entrainment of meroplankton by eddies provide a pathway for gene flow between Madagascar and KwaZulu-Natal,South Africa?

Similarities in the marine fauna found off the coasts of southern Madagascar and KwaZulu-Natal Province (KZN), South Africa, led to the development of the ‘suitcase project,’ with the aim of establishing whether eddies that form off southern Madagascar may package and transport biological material across the Mozambique Channel, facilitating connectivity and gene flow. Meroplankton (larval stages of fishes and benthic invertebrates) were collected on the Madagascan shelf and along a transect through a cyclonic eddy in the Mozambique Channel. The samples were analysed using microscopy and DNA barcoding, seeking to identify species known to be common to both the southeast coast of Madagascar and the east coast of South Africa and thereby to reveal potential indicators of connectivity between these regions. The greatest zooplankton biovolume in the upper 200 m occurred on the shelf, followed by in the western part of the eddy and in the region outside the eddy to the west, and was lowest in the region outside the eddy to the east. The meroplankton were dominated by taxa of coastal origin and these were also most abundant on the shelf and in the western part of the eddy, with the lowest abundances in the region outside the eddy to the east. The findings show greater zooplankton biovolumes and larval abundances and the presence of reef-associated larval assemblages on the Madagascan shelf and along the transect through the cyclonic eddy, providing support for the suitcase hypothesis that planktonic organisms are entrained within eddies as they propagate south-westwards of the Madagascan shelf.     

Advances in,and applications of,X-ray photoelectron spectroscopy (ESCA) in mineralogy and geochemistry

The important uses of X-ray photoelectron spectroscopy (XPS or ESCA) in mineralogy and geochemistry are described. Crystal chemical, oxidation state and chemical bonding information can be obtained from the chemical shifts of the observed peaks. The high surface sensitivity of ESCA enables detection of 0.01 of a monolayer for many ions on mineral surfaces. ESCA is extremely useful for monitoring adsorption, desorption, dissolution and exchange reactions on mineral surfaces. Future improvements in intensity and resolution will greatly enhance the crystal chemical applications of ESCA.     

Local modification of rock chemistry by deformation

Metabasalts subjected to progressive deformation in large-scale shear zones at Yellowknife display corresponding changes in major element abundances. Deformation, under conditions of greenschist facies metamorphism, has involved grain size reduction from 1200 m to <20 m, depletion of SiO₂ (5%) and Na₂O, together with hydration, and a decrease in specific gravity from 2.97 to 2.80. Chemical redistribution by deformation has been accomplished through a decrease in grain diameter of quartz and albite by intercrystalline diffusive mass transport (pressure solution), with concomitant transfer of material into extension veins. The degree of chemical modification is related to the finite strain. Deformation has involved a redistribution of 7.10¹⁵g of SiO₂ over a volume of about 50km³.The microstructure of an adamellite deformed in a shear zone at higher temperature, under conditions of amphibolite facies metamorphism is indicative of dominant dislocation creep. A low degree of tectonic grain refinement is present. Constant values of major element abundances and specific gravity determined across the shear zone at increasing states of strain imply isochemical and isovolumetric deformation. These results are taken to support the precept that crustal deformation is characterised by a low temperature deformation regime dominated by pressure solution, with local changes of rock chemistry and volume; and a high temperature regime in which strain is accommodated principally by dislocation creep, an isochemical and isovolumetric deformation mechanism.