Weed species diversity on arable land of the dryland areas of central Tanzania: impacts of continuous application of traditional tillage practices

This paper presents findings from a study that assessed influence of continuous application of a particular traditional tillage practice on weed species richness, diversity and composition and identifies weed species with positive benefits to the communities in semi-arid areas of Mpwapwa district, central Tanzania. In this area farmers apply three different traditional tillage practices which are no-till (NT), shallow tillage (ST) and Ridging System (RT). A total of 36 farm fields were surveyed in 2006/2007 cropping season where 63 weed species from 26 families were identified. Analysis of variance indicated significant differences between practices (p < 0.05), with NT practice having highest weed species richness and diversity. Among the five more prevalent weed species appearing, Bidens lineariloba was observed to exist in all the three practices. Community representatives during focus group discussions indicated 9 weed species out of 63 identified to have beneficial uses. These species are Cleome hirta, Amaranthus graecizans, Bidens lineoriloba, Bidens pilosa, Dactyloctenium aegyptium, Launaea cornuta, Heteropogon contortus, Tragus berteronianus and Trichodesma zeylanicum. Their main uses include leaf-vegetable, medicines, fodder and materials for thatching. From this study NT has highest weed species richness and diversity which therefore suggests that much more time is needed for weeding in this practice compared to other practice which was the farmers’ concern. It was also noted that although weed species have negative effects in crop production and production costs, they still play a vital role in food security and for the health of different people in marginal areas as well as for the complete ecosystem including micro and macrofauna.     

High‐Resolution Photography for Soil Micromorphology Slide Documentation

It is common practice today in soil micromorphology to scan slides with a flatbed scanner for slide documentation as well as for mesoscopic scale observation. However, the imagery produced by flatbed scanners often results in boundaries becoming diffuse when zooming in, a side effect of the continuously changing refraction of light caused by the moving scan head. This can be restricting or even unsatisfactory to specialists who rely on such imagery and while alternatives exist, their availability or suitability is not always guaranteed. This paper describes two variations on a static high‐resolution image acquisition method using a professional camera and common attributes of a standard photography studio. Minor postphotography processing too can be done with commonly used software packages. The presented method results in pictures with a resolution of 36 million pixels per image, providing high enough quality and resolution (approximately 4200 dpi) to allow the soil micromorphology practitioner to navigate the entire mesoscopic spectrum and as such offers a continuum of observation from the macroscopic scale to the optical microscopic observation at low (40×) magnification.     

Geology and Metallogenesis of the Sawayaerdun Gold Deposit in the Southwestern Tianshan Mountains, Xinjiang, China

The Sawayaerdun gold deposit, located in Wuqia County, Southwest Tianshan, China, occurs in Upper Silurian and Lower Devonian low‐grade metamorphic carbonaceous turbidites. The orebodies are controlled by a series of NE‐NNE‐trending, brittle–ductile shear zones. Twenty‐four gold mineralized zones have been recognized in the Sawayaerdun ore deposit. Among these, the up to 4‐km‐long and 200‐m wide No. IV mineralized zone is economically the most important. The average gold grade is 1–6 g/t. Gold reserves of the Sawayaerdun deposit have been identified at approximately 37 tonnes and an inferred resource of 123 tonnes. Hydrothermal alteration is characterized by silicification, pyritization, arsenopyritization, sericitization, carbonatization and chloritization. On the basis of field evidence and petrographic analysis, five stages of vein emplacement and hydrothermal mineralization can be distinguished: stage 1, early quartz stage, characterized by the occurrence of quartz veins; stage 2, arsenopyrite–pyrite–quartz stage, characterized by the formation of auriferous quartz veinlets and stockworks; stage 3, polymetallic sulfide quartz stage, characterized by the presence of auriferous polymetallic sulfide quartz veinlets and stockworks; stage 4, antimony–quartz stage, characterized by the formation of stibnite–jamesonite quartz veins; and stage 5, quartz–carbonate vein stage. Stages 2 and 3 represent the main gold mineralization, with stage 4 representing a major antimony mineralization episode in the Sawayaerdun deposit. Two types of fluid inclusion, namely H₂O–NaCl and H₂O–CO₂–NaCl types, have been recognized in quartz and calcite. Aqueous inclusions show a wide range of homogenization temperatures from 125 to 340°C, and can be correlated with the mineralization stage during which the inclusions formed. Similarly, salinities and densities of these fluids range for each stage of mineralization from 2.57 to 22 equivalent wt% NaCl and 0.76 to 1.05 g/cm³, respectively. The ore‐forming fluids thus are representative of a medium‐ to low‐temperature, low‐ to medium‐salinity H₂O–NaCl–CO₂–CH₄–N₂ system. The δ³⁴S_CDT values of sulfides associated with mineralization fall into a narrow range of ?3.0 to +2.6‰ with a mean of +0.1‰. The δ¹³C_PDB values of dolomite and siderite from the Sawayaerdun gold deposit range from ?5.4 to ?0.6‰, possibly reflecting derivation of the carbonate carbon from a mixed magmatic/sedimentary source. Changes in physico‐chemical conditions and composition of the hydrothermal fluids, water–rock exchange and immiscibility of hydrothermal fluids are inferred to have played important roles in the ore‐forming process of the Sawayaerdun gold–antimony deposit.     

Carbon dioxide in igneous petrogenesis: II. Fluid dynamics of mantle metasomatism

Petrographic, fluid inclusion, geochemical and isotopic evidence from xenoliths in alkali basalts suggests that low-viscosity fluids rich in O-H-C, dissolved silicates and especially the incompatible elements may ascend, decompress and precipitate crystalline phases and/or induce partial fusion in the upper mantle. Such mantle metasomatic fluids (MMF) may be important in generating isotopic heterogeneity and in transporting and focusing mantle heat. In order to model the movement of MMF, the ordinary differential equations governing the variation ofP, T, ascent velocity and fluid density of a compressible, viscous, single-phase (H₂O or CO₂) non-reacting fluid ascending through a vertical crack of constant width have been solved. A large number of numerical simulations were carried out in which the significant factors affecting flow behavior (thermodynamic and transport fluid properties, roughness and width of cracks, geothermal gradient, initial conditions, etc.) were systematically varied. The calculations show that: (1) MMF tends to move at uniform rates following a short period of rapid initial acceleration, (2) MMF ascends nearly isothermally, (3) MMF acts as an efficient heat transfer agent; numerical experiments show that transport of heat into regions undergoing metasomatism can lead to partial fusion. The heat transported by movement of MMF averaged over the age of the Earth is sufficient to generate about 0.1 km³ of basaltic magma per year, which is approximately equal to the production rate of alkaline magma. If an intense period of mantle degassing occured early in the history of the Earth, the transport of heat and mass (K, U, Rb, LREE) by migrating fluids might have been important.     

The effect of ionic interactions on the oxidation of metals in natural waters

The effect of ionic interactions of the major components of natural waters on the oxidation of Cu(I) and Fe(II) has been examined. The various ion pairs of these metals have been shown to have different rates of oxidation. For Fe(II), the chloride and sulfate ion pairs are not easily oxidized. The measured decrease in the rate constant at a fixed pH in chloride and sulfate solutions agrees very well with the values predicted. The effect of pH (6 to 8) on the oxidation of Fe(II) in water and seawater have been shown to follow the rate equation

$\text{-d in [Fe(II)]/dt = k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{] + k}{}_2\text{β}{}_2\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{]}{}^2$

where k₁ and k₂ are the pseudo first order rate constants, β₁ and β₂ are the hydrolysis constants for Fe(OH)⁺ and Fe(OH)⁰. The value of α_FE is the fraction of free Fe²⁺. The value of k₁ (2.0 ±0.5 min^?1) in water and seawater are similar within experimental error. The value of k₂ (1.2 × 10⁵ min^?1) in seawater is 28% of its value in water in reasonable agreement with predictions using an ion pairing model.For the oxidation of Cu(I) a rate equation of the form

$\text{?d ln [Cu(I)]/dt = k}{}_0\text{α}{}_{\mathrm{Cu}}\text{+ k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Cu}}\text{[Cl]}$

was found where k₀ (14.1 sec^?1) and k₁ (3.9 sec^?1) are the pseudo first order rate constants for the oxidation of Cu⁺ and CuCl⁰, β₁ is the formation constant for CuCl⁰ and α_Cu is the fraction of free Cu⁺. Thus, unlike the results for Fe(II), Cu(I) chloride complexes have measurable rates of oxidation.     

The Gibbs method and Duhem's theorem: The quantitative relationships among P,T, chemical potential,phase composition and reaction progress in igneous and metamorphic systems

The Gibbs method permits simultaneous evaluation of the relationships among all intensive thermodynamic variables of a heterogeneous system in equilibrium. Addition of mass balance constraints permits simultaneous evaluation of both intensive and extensive variables so that changes in phase chemistry and modes may be monitored. Assumption of closed system behavior results in a system of equations with two degrees of freedom, regardless of the thermodynamic variance, as specified by Duhem's theorem. Open system behavior increases the number of degrees of freedom by the number of components to which the system is open. The methodology presented is therefore a formal statement of the constraints among the differential of all of the intensive and extensive variables of a heterogeneous system.Examples of the application of this formalism include contouring pressure-temperature space for mineral composition, modal changes and reaction progress; contouring reaction space with pressure, temperature and mineral composition; and calculation of compositional and modal changes of phases for prescribed changes in pressure and temperature, as, for example, in the calculation of synthetic garnet zoning profiles or liquid lines of descent in crystallizing magmas.     

Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions: experiments, theoretical considerations, and constraints on their thermal evolution

Experiments exposing Type B calcium-, aluminum-rich inclusion (CAI)-like melts at high temperatures to high vacuum or reducing hydrogen-rich gas mixtures were used to determine the rates and consequences of elemental and isotopic fractionation by evaporation. Silicon and magnesium were found to evaporate much faster than calcium and aluminum, and the resulting residual liquid trajectories in composition space are reproduced via a thermodynamic model for the saturation vapor pressure of the evaporating species. Isotopic fractionations associated with evaporation were measured for magnesium. The resulting relationship between fraction of magnesium lost and enrichment of the residue in the heavy isotopes of magnesium follows a Rayleigh fractionation curve with a fractionation factor that is close to, but not exactly, the theoretically expected value. The rate of evaporation is found to be a strong function of temperature, oxygen fugacity, and melt composition, which can be understood and modeled in terms of the dependence of the saturation vapor pressures on these variables. The relationship between evaporation rate, which we measure, and calculated saturation vapor involves empirical evaporation coefficients that we find to be significantly less than one (∼0.1). Analytical and numerical models are used to characterize how diffusion in both the melt and in the surrounding gas affects evaporation rates and the degree of chemical and isotopic fractionation. The experimental data and theoretical considerations are combined to give a parameterization of the rates and consequences of evaporation of Type B CAI-like liquids, which is then used to translate the measured isotopic fractionation of Type B CAIs into constraints on their thermal history. Cooling rates of the order of 10°C per hour are indicated.     

Gold partitioning in melt-vapor-brine systems

We used laser-ablation inductively coupled plasma mass spectrometry to measure the solubility of gold in synthetic sulfur-free vapor and brine fluid inclusions in a vapor + brine + haplogranite + magnetite + gold metal assemblage. Experiments were conducted at 800°C, oxygen fugacity buffered at Ni-NiO (NNO), and pressures ranging from 110 to 145 MPa. The wt% NaCl eq. of vapor increases from 2.3 to 19 and that of brine decreases from 57 to 35 with increasing pressure. The composition of the vapors and brines are dominated by NaCl + KCl + FeCl₂ + H₂O. Gold concentrations in vapor and brine decrease from 36 to 5 and 50 to 28 μg/g, respectively, and the calculated vapor:brine partition coefficients for gold decrease from 0.72 to 0.17 as pressure decreases from 145 to 110 MPa. These data are consistent with the thermodynamic boundary condition that the concentration of gold in the vapor and brine must approach a common value as the critical pressure is approached along the 800°C isotherm in the NaCl-KCl-FeCl₂-HCl-H₂O system.We use the equilibrium constant for gold dissolution as AuOH⁰, extrapolated from lower temperature and overlapping pressure range, to calculate expected concentrations of AuOH⁰ in our experimental vapors. These calculations suggest that a significant quantity of gold in our experimental vapors is present as a non-hydroxide species. Possible chloridogold(I) species are hypothesized based on the positively correlated gold and chloride concentrations in our experimental vapors. The absolute concentration of gold in our synthetic vapor, brine, and melt and calculated mass partition coefficients for gold between these physicochemically distinct magmatic phases suggests that gold solubility in aqueous fluids is a function of aqueous phase salinity, specifically total chloride concentration, at magmatic conditions. However, though we highlight here the effect of salinity, the combination of our data with data sets from lower temperatures evinces a significant decrease in gold solubility as temperature drops from 800°C to 600°C. This decrease in solubility has implications for gold deposition from ascending magmatic fluids.     

Scattered Data Approximation on the Bisphere and Application to Texture Analysis

The paper deals with the approximation and optimal interpolation of functions defined on the bisphere \mathbb S²×\mathbb S²\mathbb {S}^{2}\times \mathbb {S}^{2} from scattered data. We demonstrate how the least square approximation to the function can be computed in a stable and efficient manner. The analysis of this problem is based on Marcinkiewicz–Zygmund inequalities for scattered data which we present here for the bisphere. The complementary problem of optimal interpolation is also solved by using well-localized kernels for our setting. Finally, we discuss the application of the developed methods to problems of texture analysis in material science.