Single crystal X-ray and Mössbauer study of shocked ilmenite to 80 GPa

Shock loading experiments on single crystal ilmenite (FeTiO₃) are carried out up to peak pressures of 80 GPa using a newly built two-stage light gas gun. Shock effects are investigated by means of X-ray precission technique and Mössbauer spectroscopy. Shock effects are largely controlled by the anisotropic nature of the ilmenite structure. Considerable deformations are observed even in a pressure level of 30 GPa in the shocked crystal when the shock propagation direction is parallel to the c axis, whereas little effects are seen up to 55 GPa when the crystal is shocked parallel to the c plane (cleavage plane). The greatest deformation is introduced in the planes containing the c axis, while less remarkable effects are seen in the plane perpendicular to the c axis. Residual effects are favorably compared with the compression anomalies found in the Hugoniot measurements by King and Ahrens (1976). Mössbauer measurements also reveal that a fraction of highly disturbed regions increases with increasing shock loading pressure. These observations are explained in terms of current heterogeneous yielding model of brittle substances under shock loading, where internal fragmentation is preferentially formed so as to give c-platelet domains that are mutually misoriented with each other.     

High temperature solvent growth of anorthite on the join CaAl2Si2O8-SiO2

Stoichiometric anorthite, CaAl₂Si₂O₈, Pˉ1, with sharp a, b, c, and d diffractions was grown, using a CaV₂O₆ solvent, by cooling at 2 ° C per hour from 1450 to 750 ° C in air. Euhedral crystals up to 5 × 3× 0.5 mm, with prominent {010} and well-developed {110} and {001}, were obtained by spontaneous nucleation. Nonstoichiometric anorthite with excess SiO₂ (CaAl₂Si₂O₈+Si₂Si₂O₈) was grown on the join CaAl₂Si₂O₈-SiO₂. Chemical analysis of the synthetic anorthite, having the highest SiO₂ content, with maximum vacancies on M-site gave □_0.110 Na_0.006Ca_0.884Al_1.80Si_2.20O₈, and X-ray diffraction showed a trend from stoichiometric Pˉ1 through diffuse Pˉ1 to body-centered Iˉ1 probably reflecting increasing disorder caused by a higher Si/Al ratio in the T-sites and the positional disorder accompanied by M-site vancancies. Annealing of the nonstoichiometric diffuse Iˉ1 anorthite in air at 1050 ° C for 14 days resulted in exsolution of minute SiO₂ inclusions due to probable ordering in the T-sites and filling of M-sites by Ca. Stoichiometric Pˉ1 anorthite was not obtained by annealing at 1050 ° due to appreciable solubility of SiO₂ in CaAl₂Si₂O₈. Metastable hexagonal CaAl₂Si₂O₈ was found to be a twinned monoclinic crystal with cell dimensions: a = 10.24 (2), b = 17.74 (3), c= 14.99 (5) ?, β = 92.05 (5) °, space group C2.     

Equatorial field-aligned irregularities deduced from the echo-train and hybrid whistlers

Summary Equatorial field-aligned irregularities have been studied by using low-latitude echotrain and hybrid whistlers observed at Sakushima Station (geomag. lat. 24°). The structure is discussed in relation to the propagation mode trapped by field-aligned irregularities. We then find that the field-aligned irregularities responsible for the trapping of low latitude whistlers are of such small dimension as the HF ducts.     

准噶尔板块东北缘富铌玄武岩的发现及其地质意义

富铌玄武岩是硅过饱和,富Ｎａ2Ｏ,ＴｉＯ2,Ｐ2Ｏ5,同时富集高场强元素的一类岛弧玄武岩,这类玄武岩的ｗ(Ｎｂ)>7×10-6,(ｗ(Ｌａ)/ｗ(Ｎｂ))ＭＮ<2。它是由来源于70100ｋｍ深处、受ａｄａｋｉｔｅ熔体交代过的地幔楔橄榄岩部分熔融形成的,是大洋板块俯冲作用的产物。目前发现的富铌玄武岩大多分布在环太平洋新生代岛弧环境,与洋壳俯冲作用密切相关[1 5]。本研究在准噶尔板块东北缘富蕴县的索尔库都克附近首次发现了富铌玄武岩。它产于下泥盆统托让格库都克组中、上部。该地层自下而上的岩石组合为:玄武岩 火山凝灰角砾岩 火山集块岩 含…     

Authigenic calcium carbonate flux in groundwater-controlled lakes: Implications for lacustrine paleoclimate records

Groundwater dominated lakes are an important feature of many landscapes. Their sediments are a particularly valuable source of paleoenvironmental information in semiarid regions where perennial lakes may otherwise be scarce. Where groundwater and lake composition are favorable, carbonate mineral precipitation, evaporative concentration of lake water, and microbial processes can combine to strongly deplete dissolved Ca relative to influent groundwaters. The authigenic carbonate flux (ACF) can then become limited by water column cation availability and thereby be coupled to groundwater inflow rates and aquifer recharge. Here we analyze sedimentary records from two marl-producing, groundwater-controlled lakes and demonstrate a link between one-dimensional ACF and the Palmer Drought Severity Index (PDSI), a measure of land surface wetness. In a restricted outflow lake with high-carbonate alkalinity, ACF is enhanced during historically wet climatic periods in response to increased aquifer recharge rates. ACF in this lake declines during droughts. A neighboring dilute lake with a high rate of groundwater outflow shows comparatively weak coupling between ACF and PDSI history. Ionic chemistry, carbonate mineral equilibria, and δ¹³C patterns of dissolved inorganic carbon show that the sensitivity of the ACF signal depends on the degree of evaporative evolution of lake water and the mineral saturation state of the water column under conditions of stratification and ice cover.     

Analysis of tubular steel bridge piers

A one‐dimensional model is proposed for the static and dynamic analysis of tubular steel bridge piers subjected to strong ground motions. The present formulation does not require experimental results nor shell analysis to obtain the constitutive equation of the model, which shows strength deterioration. The material properties and dimensions of bridge piers are required for static and dynamic analysis of the present model. The present analysis consists of two steps. The first step is to obtain the stress and strain relationship of the base plastic‐hinge region, where local inelastic buckling is observed. The modified Shanley's model and fiber elements are used to establish the compressive skeleton curve. The strength deterioration is taken into account in the resulting constitutive model. The second step is to analyze static and dynamic responses of tubular steel bridge piers. For overall analysis, the base plastic‐hinge region is discretized in the circumferential direction by using fiber elements whose constitutive equation was obtained in the first step. The validity of the present model has been confirmed through comparisons with existing experimental results. Copyright © 2005 John Wiley & Sons, Ltd.     

Planetary Boundary Layer over a Small Island as Observed by a Doppler Sodar

Observations from a new five-beam phasedarray Doppler sodar located over a small islandrevealed the development of an internal boundary layer (IBL)from the edge of a cliff in strong windcases, and the presence of systematic convective motions in unstableconditions. The top of the IBL ischaracterized by a small wind maximum trailing fromthe strong wind area at the edge of the cliff, anddevelops with distance from the edge. In the presence ofcumulus clouds, an upwardvelocity region, surrounded by downward motion, wasobserved in the subcloud layer. Anupward convective pattern of organized motion following the passage of cumulus clouds may be young thermalsrising from a height of about 100 m.     

Holocene Climate in the Northern Great Plains Inferred from Sediment Stratigraphy, Stable Isotopes, Carbonate Geochemistry, Diatoms, and Pollen at Moon Lake, North Dakota

Seismic stratigraphy, sedimentary facies, pollen stratigraphy, diatom-inferred salinity, stable isotope (δ¹⁸O and δ¹³C), and chemical composition (Sr/Ca and Mg/Ca) of authigenic carbonates from Moon Lake cores provide a congruent Holocene record of effective moisture for the eastern Northern Great Plains. Between 11,700 and 9500¹⁴C yr B.P., the climate was cool and moist. A gradual decrease in effective moisture occurred between 9500 and 7100¹⁴C yr B.P. A change at about 7100¹⁴C yr B.P. inaugurated the most arid period during the Holocene. Between 7100 and 4000¹⁴C yr B.P., three arid phases occurred at 6600–6200¹⁴C yr B.P., 5400–5200¹⁴C yr B.P., and 4800–4600¹⁴C yr B.P. Effective moisture generally increased after 4000¹⁴C yr B.P., but periods of low effective moisture occurred between 2900–2800¹⁴C yr B.P. and 1200–800¹⁴C yr B.P. The data also suggest high climatic variability during the last few centuries. Despite the overall congruence, the biological (diatom), sedimentological, isotopic, and chemical proxies were occassionally out of phase. At these times the evaporative process was not the only control of lake-water chemical and isotopic composition.