Mantle Pb paradoxes: the sulfide solution

There is growing evidence that the budget of Pb in mantle peridotites is largely contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt) are very fast. Given the possibility that sulfide melt “wets” sub-solidus mantle silicates, and has very low viscosity, the implications for Pb behavior during mantle melting are profound. There is only sparse experimental data relating to Pb partitioning between sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of Pb behavior in sulfide may hold the key to several long-standing and important Pb paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all known mantle reservoirs lie to the right of the Geochron, with no consensus as to the identity of the “balancing” reservoir. We propose that long-term segregation of sulfide (containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB is greater than bulk earth, and constant at a value of 25. The constancy of this “canonical ratio” implies similar partition coefficients for Ce and Pb during magmatic processes (Hofmann et al. in Earth Planet Sci Lett 79:33–45, 1986), whereas most experimental studies show that Pb is more incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the sulfide melt/silicate melt partition coefficient for Pb has a value of ∼ 14. Modeling shows that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle sources for these basalts. Sulfide may play other important roles during magmagenesis: (1) advective/diffusive sulfide networks may form potent metasomatic agents (in both introducing and obliterating Pb isotopic heterogeneities in the mantle); (2) silicate melt networks may easily exchange Pb with ambient mantle sulfides (by diffusion or assimilation), thus “sampling” Pb in isotopically heterogeneous mantle domains differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an apparent “de-coupling” of these systems.     

阿德朗达克-西魁北克地震带地壳三维速度结构成像

根据在阿德朗达克-西魁北克地震带及其邻近地区的59台短周期数字地震仪记录的直达P波走时观测值,用地震层析成像方法反演了该地区地壳三维速度结构.结果表明,地壳上层（0-5km）在阿德朗达克穹隆山的中南部地区出现正速度异常;第二层（5-10km）及第三层（10-15km）的速度横向变化较小,介质相对比较均匀;第四层（15-25km）出现显著的速度异常,一个是位于地震带中部的正速度异常（+4％）,它与布格重力正异常一致;另一个是位于阿德朗达克穹隆山下的负速度异常（-4％）.结合已有的地质及地球物理资料进行对比,可以认为阿德朗达克是一个正在发展的穹隆上升山,但其热源前锋尚未到达地面的大陆热点.     

A multifractal model of crack coalescence in rocks

Geometric characteristics of fractal sets of cracks are investigated from the standpoint of their coalescence. A 1D computer model, in which a realistic character of forms of crack sets is achieved through the inducing of sets with the use of the multiplicative cascade procedure, is considered. The investigation is aimed at the determination of conditions of coalescence of cracks organized as a superposition of fractal subsets of a unified set of cracks. The process of crack coalescence is investigated for three different values of the coalescence criterion. The geometric characteristics of the sets of cracks that are necessary for the transition of the crack coalescence process into an avalanche-like stage are estimated.     

Structure and collapse of three-dimensional magnetic neutral points

Abstract

The structure and collapse of linear three-dimensional magnetic neutral points is studied by varying the four parameters (p, q,j_|,j_⊥ ) that define, in general, the linear field of a neutral point. The effect of these parameters on both the skeleton structure (i.e. the fan and spine) and the actual field line structure of the null is considered. It is found that one current component (j_⊥ ) causes the skeleton structure of the null to fold up from its potential state, whereas the other current component (j _|;) causes the field lines to bend. The two other parameters (p,q) determine the potential structure of the null and cause the null to transform from a three-dimensional null to a two-dimensional null and from a positive (type B) null to a negative (type A) null.

To investigate the collapse of three-dimensional nulls, solutions to the linear, low-β ideal magnetohydrodynamic equations are found. It is found that three-dimensional null points can collapse if the field line foot-points are free and energy can propagate into the system.     

Dynamics of a moored barge under regular and random waves

Developments in the study of wave forces and construction techniques in deep water by the offshore oil industry have increased the use of marine terminals at deep water locations. A thorough understanding of moored ship dynamics when subjected to waves, wind and current combined with the use of flexible mooring lines would help to design berthing terminals for exposed areas. In this paper, the three dimensional problem of wave interactions with a barge moored to a single point is dealt with, based on the finite element method. The effect of flexibility of the mooring line and the point of mooring on the response of the barge as well as the mooring line tension is investigated. The paper compares the numerical results with model tests carried out on a barge moored to a fixed support under regular and random waves in head sea. The effect of stiffness of the mooring line on the barge response for different mooring points is discussed, which would be useful for the designers. The effect of viscous damping is also considered. The analytical results are in good agreement with the experimental results in both regular and random waves.     

Hydro-mechanical evaluation of stabilized mine tailings

. In this study, mine tailings waste was stabilized using a combination of lime, fly ash type "C", and aluminum. Treated samples were subjected to mineral identification for evaluating the formation of ettringite and gypsum. Also, unconfined compression, hydraulic conductivity, and cyclic freeze and thaw tests were performed to evaluate the hydro-mechanical properties of the stabilized samples. Experimental results have shown that the application of lime and fly ash type "C" to high sulfate content tailings has improved its plasticity, workability, and volume stability. Moreover, upon addition of aluminum to lime and fly ash in a sulfate-rich environment, ettringite and calcium sulfo-aluminate hydrate are formed in these samples. Application of 5% lime, 10% fly ash type "C", in combination with 110 ppm aluminum, resulted in the formation of a solid monolith capable of producing more than 1,000 kPa of unconfined compressive strength, and reduced tailings permeability to 1.96᎒^–6 cm s^–1, which is less than the recommended permeability of 10^–5 cm s^–1 by most environmental protection agencies for reusability of solidified/stabilized samples. The permeability of the treated tailings samples remained below the recommended permeability, even after exposing the treated samples to 12 freeze and thaw cycles. Therefore, based on the experimental results, it is concluded that treatment of high sulfate-content tailings with lime and fly ash, combined with the availability of aluminum for reactions, is a successful method of solidifying highly reactive mine tailings.     

Oxygen isotope and geochemical variations in the Missouri River

Oxygen isotope ratios in streamflow of the Missouri River basin vary geographically due to differences in source precipitation and the integration of waters from upstream regions. Average '¹⁸O values in the Missouri River main stem systematically increase from less than -17‰ in the headwaters to about -9‰ in the lower basin. Seasonal variations at a given location result from fluctuations in meteoric precipitation, residence time in reservoirs and groundwater systems, evaporation, and snowmelt. Average water chemistry values are successfully predicted for the upstream stations of two reaches on the lower Missouri River based on changes in discharge along each reach and water quality measurements collected at the downstream stations. Source regions for some dissolved ions found in the lower Missouri River are also identified. Sodium and sulfate originate predominantly from the basin above Sioux City, Iowa, while nitrate is largely derived from agricultural regions below Sioux City.     

Determination of crustal movements in Turkey by terrestrial geodetic methods

In Turkey, neotectonic activity originated from the collision of the Arabian and Anatolian land masses during the Middle Miocene. As a result of the collision, westward escape of the Anatolian block introduced E-W compression in Western Turkey which began to be relieved by N-S extension. The North Anatolian Fault (NAF) is the major active strike-slip fault that was formed under the neotectonic regime. The rates of the motion along this fault estimated by several authors are in the range of 0.4–2.9 cm/a according to kinematic data. In Turkey, the first studies of crustal movements by geodetic methods were started in the west section of the NAF in 1972. So far, individual activities and studies coordinated by multidisciplinary projects have been realized in this region. The results obtained from available geodetic data indicate the motion of the Anatolian block relative to Eurasia.