Active oceanic spreading in the northern North Fiji Basin: Results of the NOFI cruise of R/V L'Atalante (newstarmer project)

The South Pandora and the Tripartite Ridges are active spreading centers located in the northern part of the North Fiji Basin. These spreading centers were surveyed over a distance of 750 km during the NOFI cruise of R/V L'Atalante (August–September 1994) which was conducted in the frame of the french-japanese Newstarmer cooperation project. SIMRAD EM12-dual full coverage swath bathymetric and imagery data as well as airgun 6-channel seismic, magnetics and gravity profiles were recorded along and offaxis from 170°40 E to 178° E. Dredging and piston coring were also performed along and off-axis. The axial domain of the South Pandora Ridge is divided into 5 first-order segments characterized by contrasted morphologies. The average width of the active domain is 20 km and corresponds either to bathymetric highs or to deep elongated grabens. The bathymetric highs are volcanic constructions, locally faulted and rifted, which can obstruct totally the axial valley. The grabens show the typical morphology of slow spreading axes, with two steep walls flanking a deep axial valley. Elongated lateral ridges may be present on both sides of the grabens. Numerous volcanoes, up to several kilometers in diameter, occur on both flanks of the South Pandora Ridge. The Tripartite Ridge consists of three main segments showing a sigmoid shape. Major changes in the direction of the active zones are observed at the segment discontinuities. These discontinuities show various geometrical patterns which suggest complex transform relay zones. Preliminary analysis of seismic reflection profiles suggest that the Tripartite Ridge is a very young feature which propagates into an older oceanic domain characterized by a significant sedimentary cover. By contrast, a very thin to absent sedimentary cover is observed about 100 km on both flanks of the South Pandora Ridge active axis. The magnetic anomaly profiles give evidence of long and continuous lineations, parallel to the South Pandora Ridge spreading axis. According to our preliminary interpretation, the spreading rate would have been very low (8 km/m.y. half rate) during the last 7 Ma. The South Pandora and Tripartite Ridges exhibit characteristics typical of active oceanic ridges: (1) a segmented pattern, with segments ranging from 80 to 100 km in length; (2) an axial tectonic and volcanic zone, 10 to 20 km wide; (3) well-organized magnetic lineations, parallel to the active axis; (4) clear signature on the free-air gravity anomaly map. However, no typical transform fault is observed; instead, complex relay zones are separating first-order segments.     

Surface modes in the infrared spectrum of hydrous minerals: the OH stretching modes of bayerite

The theoretical infrared (IR) and Raman spectra of bayerite (β-Al(OH)₃) are computed in the density functional theory framework, using the linear response theory. The results are consistent with the occurrence of six non-equivalent OH groups in a bayerite structure with space group P2₁/n. Similar to gibbsite, the transmission powder IR spectrum of bayerite in the region of the OH stretching bands is found to depend on the shape of particles. In particular, the broadening of the strong band observed at about 3,460 cm⁻¹ in the spectrum of Al hydroxides is related to the electrostatic charges occurring at the surface of the polarized dielectric particles. The experimental correlation observed between the shape of this band and morphological parameters has therefore a physical, instead of chemical, origin.     

大陆岩石圈的流变学性质和矿物中的水

评述了近些年来岩石圈(尤其是大陆岩石圈)流变学研究中的主要进展。这些研究中最重要的一个发现是水的存在可以显著地增强岩石的变形,从而对其流变性质产生明显影响。大陆岩石圈的流变性质比大洋岩石圈要复杂得多,尤其是较深处的下地壳和岩石圈地幔之间流变性质的对比和差异成为近些年来人们争执较大的问题。大陆岩石圈的流变性质可能具有显著的不均一性,不仅体现在垂向上,也体现在横向上。根据流变学实验研究的进展和对深部壳幔捕虏体中主要构成矿物结构水含量的测定,对华北克拉通深部岩石圈的流变性质进行了定量计算。结果表明华北克拉通在重力梯度带两侧的岩石圈有着截然不同的流变特征,这种差异可能对两侧不同的岩石圈动力学过程有重要的影响。     

Experimental melting of phlogopite-peridotite in the garnet stability field

Melting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (? = 0.008–0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150–1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide.     

幔源云母巨晶氢同位素组成的微尺度不均一性:离子探针研究

运用离子探针技术对来自安徽女山新生代碧玄岩中的幔源云母巨晶进行了氢同位素组成的测定 ,发现其δD值和水含量存在微尺度不均一性 ,推断是与大气降水发生相互作用的结果 ;云母巨晶的原始δD值为 - 2 3‰ ,暗示其源区曾受到过再循环地壳的影响 ,同时也表明地幔氢同位素组成至少在大尺度上是不均一的 .     

Spectroscopic investigation and theoretical modeling of kaolinite-group minerals and other low-temperature phases

This article summarizes some recent results obtained on the physical properties of environmental minerals, mostly kaolinite-group minerals and Fe- and Al-(hydr)oxides occurring in lateritic soils. The defective structure of these minerals, including impurities, stacking faults and radiation-induced defects, is probed using infrared spectroscopy and electron paramagnetic resonance. Resulting information bears on models of soil formation and transformation mechanisms of minerals in low-temperature environments. We underline the increasing impact of quantum chemical modeling in this field, providing straightforward interpretations of spectroscopic signals and overcoming the limits of fingerprint approaches. Importantly, the first-principles modeling of isotopic fractionation factors provides new links between mineralogical and geochemical investigations of secondary minerals.     

Numerical investigations of extreme winds over Switzerland during 1990–2010 winter storms with the Canadian Regional Climate Model

This study reports on the ability of the Canadian Regional Climate Model to simulate the surface wind gusts of 24 severe mid-latitude storms in Switzerland during the period 1990–2010. A multiple self-nesting approach is used, reaching a final 2-km grid which is centred over Switzerland, a country characterised by complex topography. A physically-based wind gust parameterization scheme is applied to simulate local surface gusts. Model performance is evaluated by comparing simulated wind speeds to time series at weather stations. While a number of simulated variables are reproduced in a realistic manner, the surface wind gusts show differences when compared to observed values. Results indicate that the performance of this parameterization scheme not only depends on the accuracy of the simulated planetary boundary layer, the vertical temperature, wind speed and atmospheric humidity profiles, but also on the accuracy of the reproduction of the surface fields such as temperature and moisture.     

Infrared spectroscopic properties of goethite: anharmonic broadening,long-range electrostatic effects and Al substitution

The infrared spectrum and its temperature dependence (20–320 K) were collected on a synthetic goethite sample (α-FeOOH). In addition, the infrared powder absorption spectrum of goethite and aluminum-substituted goethite was computed using ab initio quantum mechanical calculations based on density functional theory. This combined experimental and theoretical approach allows (1) the unequivocal assignment of absorption bands to the corresponding vibrational modes, (2) separate identification of the effects of the particle shape and of the aluminum substitution on the infrared spectrum, and (3) a discussion of the anharmonic properties and the origin of the line broadening in goethite. In particular, the two well-resolved OH bending absorption bands show different temperature evolution. Their detailed analysis suggests that the broadening of the band at ~800 cm^?1 cannot be described solely by a usual three-phonon process. The strong anharmonic behavior of this mode implies the addition of a four-phonon process, such as a pure dephasing process. In our calculations, the effect of the Hubbard U correction is also investigated and found to be most visible on the OH stretching and bending modes, in relation to the associated structural relaxation. The OH stretching frequencies decrease, leading to a better agreement with experimental frequencies, while the OH bending frequencies increase.     

Potential Orthopyroxene,Clinopyroxene and Olivine Reference Materials for In Situ Lithium Isotope Determination

Over 1400 electron probe and 700 ion probe microanalyses were performed on eleven mineral separates to evaluate their potential as reference materials for in situ Li isotopic determination. Our results suggest the homogenous distributions of major elements, Li and its isotopes for each sample. Hence, these samples are suitable to be used as reference materials for in situ measurements of Li abundance and Li isotopes by secondary ion mass spectrometry (SIMS) or laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). These samples have the advantage of mitigating probable matrix effects during calibration owing to the wide range of compositions. The effect of composition on the δ⁷Li of olivine measured by SIMS is a linear function of composition, with δ⁷Li increasing by 1.0‰ for each mole per cent decrease in forsterite component.