On Boussinesq's problem for Maxwell continua subject to an external gravity field

Summary. Boussinesq's problem is solved for a uniform and incompressible Maxwell half-space subject to an external gravity field. The solution is based on momentum equations which account for stress advection in the hydrostatically pre-stressed continuum during its deformation. The analysis shows that disregarding the pre-stress term renders the theoretical stress distribution incorrect and the deformation singular in the inviscid limit of the Maxwell continuum. Our solution is contrasted with a recently published alternative solution of the same problem, where regularity in the inviscid limit was forced by modified boundary conditions.     

Accurate U-Pb Age and Trace Element Determinations of Zircon by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

Various zircons of Proterozoic to Oligocene ages (1060-31 Ma) were analysed by laser ablation-inductively coupled plasma-mass spectrometry. Calibration was performed using Harvard reference zircon 91500 or Australian National University reference zircon TEMORA 1 as external calibrant. The results agree with those obtained by SIMS within 2s error. Twenty-four trace and rare earth elements (P, Ti, Cr, Y, Nb, fourteen REE, Hf, Ta, Pb, Th and U) were analysed on four fragments of zircon 91500. NIST SRM 610 was used as the reference material and ²⁹Si was used as internal calibrant. Based on determinations of four fragments, this zircon shows significant intra-and inter-fragment variations in the range from 10% to 85% on a scale of 120 μm, with the variation of REE concentrations up to 38.7%, although the chondrite-normalised REE distributions are very similar. In contrast, the determined age values for zircon 91500 agree with TIMS data and are homogeneous within 8.7 Ma (2 s ). A two-stage ablation strategy was developed for optimising U-Pb age determinations with satisfactory trace element and REE results. The first cycle of ablation was used to collect data for age determination only, which was followed by continuous ablation on the same spot to determine REE and trace element concentrations. Based on this procedure, it was possible to measure zircon ages as low as 30.37 0.39 Ma (MSWD = 1.4; 2 s ). Other examples for older zircons are also given.     

Preliminary Characterisation of New Glass Reference Materials (GSA-1G, GSC-1G, GSD-1G and GSE-1G) by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Using 193 nm, 213 nm and 266 nm Wavelengths

New glass reference materials GSA-1G, GSC-1G, GSD-1G and GSE-1G have been characterised using a prototype solid state laser ablation system capable of producing wavelengths of 193 nm, 213 nm and 266 nm. This system allowed comparison of the effects of different laser wavelengths under nearly identical ablation and ICP operating conditions. The wavelengths 213 nm and 266 nm were also used at higher energy densities to evaluate the influence of energy density on quantitative analysis. In addition, the glass reference materials were analysed using commercially available 266 nm Nd:YAG and 193 nm ArF excimer lasers. Laser ablation analysis was carried out using both single spot and scanning mode ablation. Using laser ablation ICP-MS, concentrations of fifty-eight elements were determined with external calibration to the NIST SRM 610 glass reference material. Instead of applying the more common internal standardisation procedure, the total concentration of all element oxide concentrations was normalised to 100%. Major element concentrations were compared with those determined by electron microprobe. In addition to NIST SRM 610 for external calibration, USGS BCR-2G was used as a more closely matrix-matched reference material in order to compare the effect of matrix-matched and non matrix-matched calibration on quantitative analysis. The results show that the various laser wavelengths and energy densities applied produced similar results, with the exception of scanning mode ablation at 266 nm without matrix-matched calibration where deviations up to 60% from the average were found. However, results acquired using a scanning mode with a matrix-matched calibration agreed with results obtained by spot analysis. The increased abundance of large particles produced when using a scanning ablation mode with NIST SRM 610, is responsible for elemental fractionation effects caused by incomplete vaporisation of large particles in the ICP.     

Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones

Mineral/melt trace element partition coefficients were determined for rutile (TiO₂) for a large number of trace elements (Zr, Hf, Nb, Ta, V, Co, Cu, Zn, Sr, REE, Cr, Sb, W, U, Th). Whilst the high field strength elements (Zr, Hf, Nb, Ta) are compatible in rutile, other studied trace elements are incompatible (Sr, Th, REE). In all experiments we found D_Ta > D_Nb, D_Hf > D_Zr and D_U > D_Th. Partition coefficients for some polyvalent elements (Sb, W, and Co) were sensitive to oxygen fugacity. Melt composition exerts a strong influence on HFSE partition coefficients. With increasing polymerization of the melt, rutile/melt partition coefficients for the high field strength elements Zr, Hf, Nb and Ta increase about an order of magnitude. However, D_Nb/D_Ta and D_Hf/D_Zr are not significantly affected by melt composition. Because D_U ? D_Th, partial melting of rutile-bearing eclogite in subducted lithosphere may cause excesses of ²³⁰Th over ²³⁸U in some island arc lavas, whereas dehydration of subducted lithosphere may cause excesses of ²³⁸U over ²³⁰Th. From our partitioning results we infer partition coefficients for protactinium (Pa) which we predict to be much lower than previously anticipated. Contrary to previous studies, our data imply that rutile should not significantly influence observed ²³¹Pa-²³⁵U disequilibria in certain volcanic rocks.     

Ablation Characteristic of Ilmenite using UV Nanosecond and Femtosecond Lasers: Implications for Non‐Matrix‐Matched Quantification

Ilmenite (FeTiO₃) is a common accessory mineral and has been used as a powerful petrogenetic indicator in many geological settings. Elemental fractionation and matrix effects in ilmenite (CRN63E‐K) and silicate glass (NIST SRM 610) were investigated using 193 nm ArF excimer nanosecond (ns) laser and 257 nm femtosecond (fs) laser ablation systems coupled to an inductively coupled plasma‐mass spectrometer. The concentration‐normalised ⁵⁷Fe and ⁴⁹Ti responses in ilmenite were higher than those in NIST SRM 610 by a factor of 1.8 using fs‐LA. Compared with the 193 nm excimer laser, smaller elemental fractionation was observed using the 257 nm fs laser. When using 193 nm excimer laser ablation, the selected range of the laser energy density had a significant effect on the elemental fractionation in ilmenite. Scanning electron microscopy images of ablation craters and the morphologies of the deposited aerosol materials showed more melting effects and an enlarged particle deposition area around the ablation site of the ns‐LA‐generated crater when compared with those using fs‐LA. The ejected material around the ns crater predominantly consisted of large droplets of resolidified molten material; however, the ejected material around the fs crater consisted of agglomerates of fine particles with ‘rough' shapes. These observations are a result of the different ablation mechanisms for ns‐ and fs‐LAs. Non‐matrix‐matched calibration was applied for the analysis of ilmenite samples using NIST SRM 610 as a reference material for both 193 nm excimer LA‐ICP‐MS and fs‐LA‐ICP‐MS. Similar analytical results for most elements in ilmenite samples were obtained using both 193 nm excimer LA‐ICP‐MS at a high laser energy density of 12.7 J cm^?2 and fs‐LA‐ICP‐MS.     

Palaeoenvironmental interpretation of fluvial red beds by statistical analysis of palaeocurrent data: Examples from the buntsandstein (lower triassic) of the eifel and bavaria in the German basin (middle Europe)

Palaeocurrent data from parts of the Upper Buntsandstein (Lower Triassic) of both the western (Eifel: Kyllburg-Schichten) and eastern (Bavaria: Plattensandstein) margin of the German Triassic Basin (Middle Europe) have been interpreted by circular directional statistics, eigenvalue and eigenvector analysis, autocorrelation analysis and vector trend analysis. Examination of current roses, histograms of normalized vector magnitudes and plots, of circular skewness of the distribution vs. circular kurtosis, together with the palaeocurrent plots indicate bimodal palaeocurrent distributions with bipolar orientations within long, short and very short sedimentary sequences. Subset I of the directional data is interpreted to represent larger, more continuous flows in a high-energy regime of probably perennial type, whereas subset II may represent smaller, partially episodic flows of probably ephemeral type with larger variability in direction and flow regime; transport and sedimentation rates being significantly smaller than during deposition of subset I. A part of the bimodal palaeocurrent distributions within short sedimentary sequences fits rather well into the general transport pattern. Other bipolarities are primarily interpreted as effects of superimposition of channels of different orientations as well as of main channels and crevasse-splay channels, and condensation of deposition from discontinuous or episodic flows. The main mechanism creating the vertical succession of various directions is stacking of different substratum members to multistorey complexes by primary-depositional restriction of formation and/or secondary-erosional removal of topstratum sediments. The occasional occurrence of herring-bone cross-stratification representing bipolarities within very short depositional sequences, testifies to probably episodic and random changes in water surface slopes allowing currents to reverse locally. Examination of long sedimentary sequences by autocorrelation analysis reveals changes of transport directions with time in a sinusoidal manner, indicating a conformable environmental development of the alluvial watercourses which reflects a time-cyclic trend of sedimentation by spatially continuous and time-concordant shifting of the channel system. The results of the statistical analysis enable an enhanced palaeoenvironmental reconstruction of the alluvial network which fits best to a transitional meandering-thalweg-braided channel pattern. Comparison of the statistical results, especially in the light of vector trend surface analysis, clearly reflects the different palaeogeographic positions of both investigated areas, but rules out only minor influences of regionally different palaeoslopes on the alluvial depositional environment, with the basic principles of fluvial style at the western margin (Eifel) generally matching those at the eastern margin (Bavaria) of the German Basin.     

193nmLA-ICPMS对国际地质标准参考物质中42种主量和微量元素的分析

本文采用配备有 193nm Ar F准分子 (excimer)激光器的 Geo L as2 0 0 M剥蚀系统和 Elan6 10 0 DRC ICP- MS对 4个美国地质调查所 (USGS)玻璃标准参考物质以及 3个美国国家标准技术研究院 (NIST)人工合成硅酸盐玻璃标准参考物质中几乎覆盖整个质量数范围 (从 7L i到 2 38U)的 38个微量和 4个主量 (Na、Mg、Ti和 Mn)元素进行了分析。分析结果表明 ,无论是对 USGS还是 NIST玻璃 ,元素分析的相对标准偏差 RSDs和分析值与参考值之间的相对偏差 (RDs)一般优于 10 % ,RSD和RD较大的元素主要出现在含量很低或不均匀样品中。稀土元素的 RSD显示 ,除 AGV- 2 G可能存在不均匀现象外 ,其它所测样品在 6 0 μm尺度上 ,元素分布是均匀的。本研究证明 ,由于 ICP- MS具有 10 8cps(每秒计数 )的动态线性范围 ,本实验室的L A- ICPMS系统可定量分析含量在百分之几的主量元素及微量元素。分析精密度和准确度可与常规溶液雾化进样 ICP- MS方法相媲美