Upper mantle anisotropy: evidence from free oscillations

Summary Isotropic earth models are unable to provide uniform fits to the gross Earth normal mode data set or, in many cases, to regional Love-and Rayleigh-wave data. Anisotropic inversion provides a good fit to the data and indicates that the upper 200km of the mantle is anisotropic. The nature and magnitude of the required anisotropy, moreover, is similar to that found in body wave studies and in studies of ultramafic samples from the upper mantle. Pronounced upper mantle low-velocity zones are characteristic of models resulting from isotropic inversion of global or regional data sets. Anisotropic models have more nearly constant velocities in the upper mantle.
Normal mode partial (Frediét) derivatives are calculated for a transversely isotropic earth model with a radial axis of symmetry. For this type of anisotropy there are five elastic constant. The two shear-type moduli can be determined from the toroidal modes. Spheroidal and Rayleigh modes are sensitive to all five elastic constants but are mainly controlled by the two compressional-type moduli, one of the shear-type moduli and the remaining, mixed-mode, modulus. The lack of sensitivity of Rayleigh waves to compressional wave velocities is a characteristic only of the isotropic case. The partial derivatives of the horizontal and vertical components of the compressional velocity are nearly equal and opposite in the region of the mantle where the shear velocity sensitivity is the greatest. The net compressional wave partial derivative, at depth, is therefore very small for isotropic perturbations. Compressional wave anisotropy, however, has a significant effect on Rayleigh-wave dispersion. Once it has been established that transverse anisotropy is important it is necessary to invert for all five elastic constants. If the azimuthal effect has not been averaged out a more general anisotropy may have to be allowed for.     

The use of wetland water balances to link hydrogeological processes to ecological effects

A methodology for modelling wetland water levels using the water balance approach was developed for Pulborough Brooks Site of Special Scientific Interest in West Sussex, in the UK. The methodology was applied to assess potential groundwater abstraction impacts on communities and species of ecological importance in the wetland. The modelling methodology links regional hydrogeological processes to wetland hydrology and ecology that is typically influenced by processes that take place at a more reduced spatial scale. To assess impacts, it is necessary to understand the hydrological sources of water that are affected by abstraction as well as to consider how the magnitude of these processes relates to other processes that influence wetland water levels. Water levels are commonly the most important factor determining the presence or absence of species or communities in a wetland and the model predicts wetland water levels and assesses how suitable these water levels are for species of ecological significance. Model results show that typical historic rates of abstraction have not resulted in an adverse effect on the integrity of the Pulborough Brooks relative to a ‘natural’ (no abstraction) scenario.     

北美西部陆相白垩系-古近系界线孢粉学特征简介―兼评中国东部白垩系-古近系界线研究

已经报道的北美西部陆相白垩系-古近系界线剖面已达近百处,这些剖面分布在阿拉斯加南部向南经加拿大直到美国南部的新墨西哥州北部的广大北美西部内陆地区,其中很多属迄今所知全球很好的陆相白垩系-古近系界线剖面.这些剖面常有标志性的界线黏土,并伴有其他地球化学、生物地层学特征.因为这些剖面所在地当时都在湿润或半干旱地区内,有利于生物化石,特别是孢粉化石的保存,易于开展孢粉学研究.经过近四十年不懈努力,孢粉学已经成为北美精确确定陆相白垩系-古近系界线最重要的、也是迄今为止所知最为有效的古生物学工具.孢粉植物群在界线上下特征差别显著,容易区分.界线之下含有大量主要繁盛于白垩纪的被子植物花粉,而在临近界线时则突然消失或仅零星出现,界线附近则以一层孢子异常丰富层为古近系底部特征.进入古近纪之后,虽然被子植物花粉有所恢复,但是繁盛于白垩纪的成分却很少出现.北美和新西兰、日本白要系-古近系界线附近在沉积岩和孢粉学方面的相似性表明这一事件可能是全球性事件.简要回顾和讨论了我国与孢粉学相关的几个热点地区陆相白垩系-古近系界线的研究概况,我国东北及江苏北部和北美西部大部分地区白垩系-古近系界线附近的孢粉植物群及其沉积环境比较类似,在苏北到东北地区范围内首次发现陆相白垩系-古近系界线地层的可能性比其他地区大.     

Method to characterize aquitards above leaky aquifers with water supply wells

Leaky aquifers provide protected drinking water since the aquifer is overlain by an aquitard, and this study develops a method to estimate hydraulic properties of the latter deposit. Steady pumping, supply well shutdowns, and slug tests generate data in adjacent monitoring well clusters that characterize the aquitard. An existing steady model estimates a site-averaged value of the aquitard permeability k if its thickness is known, and this site-averaged estimate may be compared with local k estimates from conventional and extended slug tests. A shutdown attenuation model estimates a local value of the consolidation coefficient C _V, which combines with the local k value to specify the compressibility α of the aquitard. The method is illustrated for the Fowl Meadow Aquifer, a stratified drift deposit used as a drinking water supply in eastern Massachusetts (USA), with an overlying silt aquitard of 10 m thickness. Steady data and theory suggest a site-averaged k of 2.3?×?10^–17?m² for the aquitard, while the shutdown attenuation model generates local C _V values that vary from 10^–5 to 10^–3?m²/s. The slug tests yield a local k variation (10^–17–10^–13?m²) that brackets the site-averaged value, and an α range of 10^–9–10^–7?Pa^–1.     

Experimental investigation of large-ion-lithophile-element-, high-field-strength-element- and rare-earth-element-partitioning between calcic amphibole and basaltic melt: the effects of pressure and oxygen fugacity

The effects of pressure and oxygen fugacity (fO₂) on trace element partitioning between pargasitic amphibole and alkali-basaltic melts have been determined at pressures from 1.5 to 2.5 GPa and oxygen fugacities at 2 log units above and below the nickel–nickel oxide buffer. Amphibole crystallization experiments were performed in a piston cylinder apparatus and partition coefficients between amphibole and quenched melt of large-ion-lithophile elements (LILE: Rb, Sr, Ba), high-field-strength elements (HFSE: Zr, Nb, Ta, Hf, U, Th) and rare-earth elements (REE: La to Lu; +Y) were measured with a LASER ablation inductively coupled plasma – mass spectrometer. Increasing pressure from 1.5 to 2.5 GPa at similar temperatures and approximately constant fO₂ increases D _Rb but decreases D _Zr and D _Hf and D _REE (D _La, D _Ce, D _Pr). An empirical relationship was observed between D _Zr and (Ti/Al)_M2 in the amphibole, which can be described by:

Origins of compositional heterogeneity in olivine-hosted melt inclusions from the Baffin Island picrites

The Baffin Island picrites are highly magnesian (22 wt% MgO) olivine tholeiites, erupted through felsic continental crust. Plots of most major and minor element oxides against MgO for the lavas define very tight trends consistent with modification of melts parental to the erupted suite by olivine fractionation or accumulation. However, melt inclusions trapped in primitive olivine phenocrysts in these lavas have much more diverse compositions. After correction for post-entrapment modification, the inclusions are systematically slightly lower in Al₂O₃, and significantly higher in SiO₂, K₂O and P₂O₅ than the lavas fractionation trends. CaO, Na₂O and TiO₂ contents lie within the lavas fractionation trends. Similarly, most inclusions are higher in Sr/Nd, K/Nb, Rb/Ba, Rb/Sr, U/Nb and Ba/Th than the lavas. These characteristics resulted from up to 15% contamination of evolving picritic-basaltic liquids by locally-derived, broadly granitic partial melts of the quartz + feldspar-rich crust through which the picrites erupted. Contamination was minor in the bulk lavas (<1%), suggesting that the inclusions compositions partly reflect a link between wall rock reaction and precipitation of liquidus olivine. Rapid crystallisation of liquidus olivine from the picrites, along with melting of felsic crustal wall rocks of magma chambers or conduits, were likely during emplacement of hot picritic magmas into cooler felsic crust. Inclusion compositions may thus reflect mixing trends or may be constrained to phase boundaries between olivine and a phase being resorbed, for example, an olivine-plagioclase cotectic. The extent of contamination was probably a complex function of diffusion rates of components in the magmas, and phenocryst growth rates and proximity to wall rock. These results bear on the common observation that melt inclusions compositions are frequently more heterogeneous than those of the lavas that host them.     

Bubble growth in slightly supersaturated albite melt at constant pressure

Bubble growth experiments were performed in a piston-cylinder by hydrating albite melt with ∼11 wt.% H₂O at 550 MPa followed by rapid decompression at 1 MPa s⁻¹ to pressures of 450 or 400 MPa. At these conditions the melt was supersaturated with ∼0.5 or ∼1.5 wt.% H₂O, respectively, which caused rapid exsolution and bubble growth. Results at 1200 °C demonstrate that portions of the initial cumulative bubble-area distributions may be characterized by a power law with an exponent near 1, but they rapidly evolve to exponential distributions and approach a unimodal distribution after 32 h of growth. This evolution occurs by the growth of larger bubbles at the expense of smaller ones. The growth rate of the average bubble radius in these experiments is described by a power law whose exponent is 0.35, close to the theoretical exponent of 1/3 for phase growth in which coalescence is dominated by Ostwald ripening of the bubbles. Over the range of pressures and water contents investigated at 1200 °C, the bubble-size distributions and growth rate are not significantly affected by changes in the amount of exsolved water or by splitting the decompression path into two steps. Similar decompression experiments at 800 °C are dominated by smaller bubbles than in the 1200 °C experiments and also demonstrate exponential cumulative size distributions, but consistently contain a small fraction of larger bubbles. The growth rate of these bubble radii cannot be fit with a power law, but a logarithmic dependence of the bubble radii on time is possible, suggesting a difference in the growth mechanisms at low and high temperatures. This difference is attributed to the orders of magnitude changes in melt viscosity and water diffusion in the melt as the temperature varies from 800 to 1200 °C. At 1200 °C the transport properties of albite melt resemble those of natural basaltic melts whereas at 800 °C the properties are similar to those of andesitic to dacitic melts. The decompression rate used in this study exceeds natural rates by one to two orders of magnitude. Thus, these results indicate that natural mafic-to-intermediate magmas supersaturated with only a small excess of water should easily nucleate bubbles during ascent and that bubble growth in mafic magmas will proceed much more rapidly than in andesitic to dacitic magmas. Intermediate composition magmas also may be capable of forming bimodal bubble-size distributions even in the case when only one nucleation event occurred. The rapid evolution of the bubble-size distribution from a power law to an exponential may be useful in constraining the time duration between bubble nucleation and the quenching of natural samples.