Stability of phengite and biotite in eclogites and characteristics of biotite- or orthopyroxene-bearing eclogites

Stability of phengite and biotite in eclogite is discussed using petrological data of natural eclogites, and the observational data are examined by thermodynamic calculations. Generally, phengite is a major K phase in natural eclogite and is stable in wide range of bulk composition. However, in eclogites from several localities of the Caledonides, biotite occurs as a stable eclogite-facies mineral, and is often associated with orthopyroxene. Bulk compositions of such biotite- or orthopyroxene-bearing eclogites are compared with those of eclogites from the Dabie–Sulu region, China, where phengite is a major K phase in eclogite. The biotite- or orthopyroxene-bearing eclogites from the Western Gneiss Region of the Caledonides are rich in MgO (10–15 wt%) and relatively poor in CaO (7–8 wt%) and Al₂O₃ (12–16 wt%). The CaO/MgO ratios of the biotite- or orthopyroxene-bearing eclogites are clearly lower than those of eclogites from the Dabie–Sulu region, indicating that MgO-rich and CaO-poor environments should be important for stabilizing of biotite and orthopyroxene in eclogite. Biotite-bearing eclogite from the North-East Greenland Eclogite Province is rich in MgO (≈16 wt%) and CaO (≈15.5 wt%) and extremely poor in Al₂O₃ (≈8 wt%). To stabilize biotite in eclogite, Al₂O₃-poor environments are also important. Bulk compositions of these biotite- or orthopyroxene-bearing eclogites are similar to picrite basaltic compositions. To examine these observational data, thermodynamic calculations were carried out in a seven-component system KH₂O_1.5–Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂, which includes garnet, kyanite, phengite, biotite, quartz, omphacite, orthopyroxene and olivine in conjunction with mass-balance calculations. Firstly, calculations were performed on the average bulk composition of eclogites from the Dabie–Sulu region to lherzolite (KLB-1). The calculation results confirmed that phengite should be stable in eclogite with 'ordinary' basaltic composition, whereas biotite and orthopyroxene should be stable in picrite basaltic compositions (e.g. MgO >11.0 wt%, CaO <9.8 wt%, Al₂O₃ <15.2 wt% at 700 °C, 2.5 GPa). Further calculations in basaltic system confirmed that increase of MgO content and decrease of CaO and Al₂O₃ contents were important to stabilize biotite and orthopyroxene in eclogite. Thus, mineral assemblage in picrite basalt system should be completely different from that in normal basaltic system.     

Persistence and metallic composition of paint particles in sediments from a tidal inlet

Concentrations of Cu, Pb, Sn and Zn have been determined in sediment (<500 μm) and macroscopic paint particles (>500 μm) retrieved from sections of two cores collected from a tidal inlet of the Plym estuary, southwest England. Paint particles contributed up to about 0.2% of the total mass retrieved from each section and were most abundant towards the base of the cores where, according to (210)Pb dating, deposition took place about a decade prior to sampling. Metal concentrations in the paint particles pooled from the sections were highly variable, typically spanning two orders of magnitude in each core, and were greatest for Cu and Zn (up to 460,000 and 170,000 μg g(-1), respectively) due to their use in contemporary antifouling formulations applied to boat hulls. Concentrations of metals in the sediment were, however, relatively invariant, an effect attributed to the abundance and dispersion of microscopic paint particles throughout the cores.     

达拉布特蛇绿岩带镁铁质岩的地球化学

达拉布特蛇绿岩带的镁铁质岩属亚碱性和碱性系列,可能是弧后盆地火册活动的产物。根据镁铁质岩的层位关系、岩浆来源和微量元素特征,可将该盆地的地质演化概括为扩张阶段和封闭阶段。盆地扩张阶段早、中期喷发的镁铁质岩具有低的稀土和不相容元素丰度,而在盆地封闭阶段晚期喷发的碱性玄武岩,其特点是稀土和大多数不相容元素丰度极大增高,其稀土元素分布模式为轻稀土元素强烈富集型。     

Growth of quartz crystals twinned after Japan Law

The flattened or elongated morphology often observed in contact twinned crystals has been referred to the so-called re-entrant corner effect at twin junctions. To re-investigate the validity of this mechanism, natural quartz crystals twinned after Japan Law have been subjected to re-growth in a commercial synthetic quartz autoclave, and the change in their morphology and surface microtopography by re-growth studied. It is found that although twin re-entrant corners may play a role of preferential growth sites, this role can be weakened considerably and in fact can become almost negligible when crystals grow under high supersaturation condition. Preferential growth at the re-entrant corner of a twin junction is probably due to clusters of dislocations concentrated in the composition plane, and not due to the so-called re-entrant corner effect in its original sense.     

Non-ENSO interannual rainfall variability in central Chile during austral winter

The first principal component (PC1) of seasonal rainfall anomalies in central Chile during winter (June–August) is used to analyze the circulation anomalies related to wet and dry conditions, when near-normal or neutral SST anomalies are observed in the equatorial Pacific, i.e., during non-ENSO conditions. Eight wet and eight dry winter seasons were defined as the upper and lower terciles of PC1 for 24 non-ENSO winters in the period 1958–2000. Unlike the single process attributed to ENSO, during non-ENSO winter seasons, there are several sources triggering or modifying the propagation of the stationary waves that impact the rainfall regime in central Chile. Unfortunately, the multiple processes that seem to be involved in the modulation of the interannual rainfall variability in central Chile, as seen in this work, limit the predictability of rainfall during non-ENSO conditions.     

Implementation of a high-order combined compact difference scheme in problems of thermally driven convection and dynamo in rotating spherical shells

We present an improved solution method for modeling thermally driven convection and dynamo in a rotating spherical shell. In this method, we introduce a high-order three-point combined compact difference scheme (CCDS) on non-uniform grid points in radius, while spherical harmonic expansion is conventionally performed in the angular direction. The governing equations in the spectral form are time-stepped together with the implicit CCDS up to the second derivative. To improve stability of the scheme, a boundary closure scheme is developed on non-uniform mesh. Numerical comparison with a published benchmark solution at moderate Ekman and Rayleigh numbers demonstrates that accuracy and convergence of the CCDS is fairly good and superior to the existing finite difference scheme using more stencil. With this scheme, we could more accurately solve problems of convection and also dynamo action in planetary core with less grid points.     

Dark inclusions in CO3 chondrites: new indicators of parent-body processes

A petrographic and scanning electron microscopic study of the four CO3 chondrites Kainsaz, Ornans, Lancé, and Warrenton reveals for the first time that dark inclusions (DIs) occur in all the meteorites. DIs are mostly smaller in size than those reported from CV3 chondrites. They show evidence suggesting that they were formed by aqueous alteration and subsequent dehydration of a chondritic precursor and so probably have a formation history similar to that of DIs in CV3 chondrites. DIs in the CO3 chondrites consist mostly of fine-grained, Fe-rich olivine and can be divided into two types on the basis of texture. Type I DIs contain rounded, porous aggregates of fine grains in a fine-grained matrix and have textures suggesting that they are fragments of chondrule pseudomorphs. Veins filled with Fe-rich olivine are common in type I DIs, providing evidence that they experienced aqueous alteration on the parent body. Type II DIs lack rounded porous aggregates and have a matrix-like, featureless texture. Bulk chemical compositions of DIs and mineralogical characteristics of olivine grains in DIs suggest that these two types of DIs have a close genetic relationship.The DIs are probably clasts that have undergone aqueous alteration and subsequent dehydration at a location different from the present location in the meteorites. The major element compositions, the mineralogy of metallic phases, and the widely dispersed nature of the DIs suggest that their precursor was CO chondrite material. The CO parent body has been commonly regarded to have been dry, homogeneous, and unprocessed. However, the DIs suggest that the CO parent body was a heterogeneous conglomerate consisting of water-bearing regions and water-free regions and that during asteroidal heating, the water-bearing regions were aqueously altered and subsequently dehydrated. Brecciation may also have been active in the parent body.The DIs and the matrices are similarly affected by thermal metamorphism in their own host CO3 chondrites (petrologic subtypes 3.1 to 3.6), but the degree of the secondary processing (aqueous alteration and subsequent dehydration) of the DIs has no apparent correlation with the petrologic grades of the host chondrites. These observations suggest that the DIs had been incorporated into the host chondrites before the thermal metamorphism took place and that the secondary processes that affected the DIs largely occurred before the thermal metamorphism.     

1991年江淮特大暴雨的降水性质与对流活动

从计算大尺度热源和分析ＴＢＢ资料两个方面，阐述了１９９１年江淮梅雨期间降水性质与对流活动的季节性演变特征。结果表明：梅雨期间江淮上空３个强上升运动时段分别形成了３场暴雨，由暴雨释放的热量使江淮大气出现了３个时段的强加热；３场暴雨的降水性质呈显著的季节性演变，由第１场暴雨以锋面性降水为主发展到第３场暴雨异常强的对流性降水。文中详细分析了热源和水汽汇的时空分布特征，并从大气运动场和热力结构讨论了盛夏强对流降水期间积云对流以涡动形式对热量和水汽的强铅直输送作用。江淮地区ＴＢＢ值能很好地反映降水状况，雨期一致地对应于ＴＢＢ低于２５０Ｋ的时段。梅雨中后期东亚地区对流活动季节性地增强，带状对流区（特别是ＴＢＢ高负距平区）与雨带位置相符。对流带位置及对流活动强弱与西南暖湿气流活动密切相关，它很好地表征了东亚地区的低空急流（给积云输送热带对流大气）。梅雨期间对流带主要出现在江淮流域，但可在东亚范围内飘移，它落在江淮与否则决定了江淮暴雨的维持与中断     

What dynamics drive future wind scenarios for coastal upwelling off Peru and Chile?

The dynamics of the Peru–Chile upwelling system (PCUS) are primarily driven by alongshore wind stress and curl, like in other eastern boundary upwelling systems. Previous studies have suggested that upwelling-favorable winds would increase under climate change, due to an enhancement of the thermally-driven cross-shore pressure gradient. Using an atmospheric model on a stretched grid with increased horizontal resolution in the PCUS, a dynamical downscaling of climate scenarios from a global coupled general circulation model (CGCM) is performed to investigate the processes leading to sea-surface wind changes. Downscaled winds associated with present climate show reasonably good agreement with climatological observations. Downscaled winds under climate change show a strengthening off central Chile south of 35°S (at 30°S–35°S) in austral summer (winter) and a weakening elsewhere. An alongshore momentum balance shows that the wind slowdown (strengthening) off Peru and northern Chile (off central Chile) is associated with a decrease (an increase) in the alongshore pressure gradient. Whereas the strengthening off Chile is likely due to the poleward displacement and intensification of the South Pacific Anticyclone, the slowdown off Peru may be associated with increased precipitation over the tropics and associated convective anomalies, as suggested by a vorticity budget analysis. On the other hand, an increase in the land–sea temperature difference is not found to drive similar changes in the cross-shore pressure gradient. Results from another atmospheric model with distinct CGCM forcing and climate scenarios suggest that projected wind changes off Peru are sensitive to concurrent changes in sea surface temperature and rainfall.