Allende(CV3)陨石中一种特殊蠕虫状橄榄石集合体的矿物岩石学特征及其成因探讨

在通过对Allende(CV3)碳质球粒陨石的4个光薄片中发现的5个特殊蠕虫状橄榄石集合体(AOA)的岩石学和矿物化学特征研究,证实它们的矿物组合以富橄榄石和霞石为特征,可见少量的金属硫化物。橄榄石颗粒Fa(Fe/(Fe Mg)原子百分比)值范围在34.1 mol%～42.2 mol%,百分标准平均方差(PMD)值为6.1,表明这些颗粒达到了一定的热力学平衡。AOA可能属于星云直接凝聚形成,AOA和细粒CAI(FTA和富尖晶石-辉石型CAI)可能代表了太阳星云从高温到低温连续凝聚的产物。认为AOA的蚀变作用发生在太阳星云中,霞石和铁橄榄石可能是后期水化蚀变的产物,霞石可能替代了AOA中原始的矿物——钙长石和黄长石等。AOA可能在星云中还经历了热蚀变作用的过程,橄榄石和霞石颗粒均具有高的FeO质量分数(29.1%～34.4%,4.04%～9.70%),表明蚀变反应发生在高逸氧度的星云环境下。     

古特提斯多岛洋洋-陆俯冲:木孜塔格蛇绿岩的矿物学证据

前期研究认识到新疆东昆仑木孜塔格蛇绿岩形成在俯冲带环境.为了进一步研究该俯冲带的类型,对新疆东昆仑木孜塔格蛇绿岩变质橄榄岩的岩石学和矿物学特征进行了分析.薄片观察发现变质橄榄岩的橄榄石以变质残余、变质重结晶和斜方辉石熔融结晶3种结构存在,但探针分析发现它们具有稳定且低Fo值(87.8～89.5);斜方辉石发育变质残余和熔融残余结构,En较低(88～90),Al2O3含量变化大(2.90%～5.13%);尖晶石为他形-半自形结构,其Cr#(=Cr/(Cr Al))集中分布在0.508~0.723和0.100~0.118两个范围内.根据这些来自岩石学和矿物学的证据,并结合该蛇绿岩的构造背景与时代,认识到该蛇绿岩形成在大洋向具有厚陆壳的大陆弧俯冲的俯冲带环境,为新疆东昆仑地区古特提斯多岛洋洋-陆俯冲的结果.     

The large volume multi-anvil press as a highP-T deformation apparatus

The rheological properties of mantle materials are being investigated up to pressures of 16 GPa and temperatures of 1600°C for times up to 24 h, using a new sample assembly for the 6–8 multi-anvil apparatus. Al₂O₃ pistons, together with a liquid confining medium, are used to generate deviatoric stress in the specimen. Strain rates are estimated by monitoring the relative displacement of the guide blocks of the multi-anvil apparatus, scaled to the total axial strain of the sample. The applied stress on the sample is estimated using grain size piezometry. Strain rates and flow stresses of approximately 10^–4 to 10^–6 s^–1 and 50 to 250 MPa respectively, are presently attainable.Preliminary results on San Carlos olivine single crystals, partially dynamically recrystallized to a grain size of 10 to 300 m, indicate that the effective viscosity of polycrystalline olivine is consistent with values obtained from olivine single crystal creep laws. Assuming a dislocation creep mechanism (n3.5) with (010)[001] as the dominant slip system, the data are best fit using a creep activation volume of 5 to 10×10^–6 m³ mol^–1.     

The electrical conductivity of olivine under highly reducing conditions

The electrical conductivity of San Carlos olivine has been measured at 1100 °C under reducing conditions at controlled oxygen fugacity, inside and outside the olivine stability field, in order to study the kinetics of olivine destabilization. Electrical conductivity increases along the direction [010] and decreases along [001]. as oxygen fugacity decreases. To understand these dependences, electrical conductivity transitory regimes were studied. In response to decreases in oxygen fugacity, two transient regimes with different time scales have been observed. A fast (≈1–2 min) increase of electrical conductivity is first observed, followed by a slower decrease (1–10 h, depending on the crystal orientation). After a few hours of annealing, precipitation of metallic iron and nickel and formation of amorphous silica can be observed at the crystal surface. The fast conductivity increase in the first transient regime is ascribed to an increase in the population of electrons at the olivine surface. Two effects: (1) equilibration of surface defects with the bulk of the crystal, and (2) iron loss from the olivine due to metal precipitation, could explain the subsequent decrease of electrical conductivity. Anisotropic diffusion of surface defects to the bulk of the crystal, by a process faster than atomic diffusion is the most likely. Received: 3 September 1997 / Revised, accepted: 16 April 1998     

Experimental investigation of the α-β transformation of San Carlos olivine single crystal

Single crystalline San Carlos olivine (1 mm cube) was transformed to (Mg,Fe)₂SiO₄β-phase at 13.5–15 GPa, 1030–1330 °C for 0–600 min using a multi-anvil high pressure apparatus. The α-β transformation occurred by incoherent surface nucleation and interface-controlled growth and recovered partially transformed samples showed sharply defined reaction rim. The growth rate of the β-phase rim significantly decreased with time and the growth eventually ceased. TEM observations revealed that many dislocations were created in both the relict olivine just near the α-β interface and the β-phase in the rim, which show evidence for deformation caused by interfacial stresses associated with the misfit elastic strain of the transformation. The observed tangled dislocation texture in β-phase suggested that the β-phase rim was hardened and relaxation of the interfacial stress was retarded. This probably caused a localized pressure drop in the relict olivine and decreased the growth rate. Time-dependent growth rates of β-phase is possibly controlled by the rheology of β-phase, which must be considered for the prediction of the olivine metastability in the subducting slabs. Received: 24 January 1997 / Revised, accepted: 24 July 1998     

The preparation of silica from the olivine process and its possible use as a catalyst support

The production of silica from the dissolution of olivine in sulphuric acid has been studied. Variations in grainsize of the olivine result in different specific surface areas of the silica. The microporosity of the silica supports an aggregative growth model responsible for the increasing particle size. The silica is used in the preparation of a nickel catalyst. The performance of the produced catalyst is equal to a nickel catalyst based on commercially available silica     

Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine     

济南辉长岩岩浆演化过程:来自熔体包裹体的证据

本研究首次报道了早白垩世济南辉长岩中橄榄石斑晶捕获的熔体包裹体的研究结果。济南辉长岩中橄榄石的Fo(60.3~74.6),Mn(2500~3500μg/g),Ni(70~1349μg/g),Fe/Mn比值(61.2~83.5),与源区母岩为纯的橄榄岩形成的熔体结晶出的橄榄石性质不同,可能与源区存在辉石岩的贡献有关。橄榄石中熔体包裹体主量元素具有较大的变化范围。熔体包裹体成分的标准矿物计算(CIPW)表明,MgO10%的熔体包裹体为含有霞石和橄榄石标准矿物分子的硅不饱和熔体,Mg O10%时为含石英标准矿物分子的硅饱和熔体。橄榄石中包裹有辉石和斜长石,说明岩浆演化过程应该处于开放环境。熔体包裹体的(~208Pb/~206Pb)i和(~207Pb/~206Pb)i与MgO具有良好的负相关关系,与SiO_2具有良好的正相关关系,以及熔体包裹体具有较高的SiO_2特征表明岩浆演化过程中可能有下地壳长英质组分的加入。熔体包裹体的Pb同位素落在EMI附近并向EMII延伸,其源区可能有EMI和EMII的贡献,熔体包裹体的主量元素成分说明其源区母岩可能有橄榄岩和辉石岩的贡献。     

Seismic Properties of Anita Bay Dunite: an Exploratory Study of the Influence of Water

As a pilot study of the role of water in the attenuation ofseismic waves in the Earth's upper mantle, we have performeda series of seismic-frequency torsional forced-oscillation experimentson a natural (Anita Bay) dunite containing accessory hydrousphases, at high temperatures to 1300°C and confining pressure(P_c) of 200 MPa, within a gas-medium high-pressure apparatus.Both oven-dried and pre-fired specimens wrapped in Ni–Fefoil within the (poorly) vented assembly were recovered essentiallydry after 50–100 h of annealing at 1300°C followedby slow staged cooling. The results for those specimens indicatebroadly similar absorption-band viscoelastic behaviour, butwith systematic differences in the frequency dependence of strain-energydissipation Q^–1, attributed to differences in the smallvolume fraction of silicate melt and its spatial distribution.In contrast, it has been demonstrated that a new assembly involvinga welded Pt capsule retains aqueous fluid during prolonged exposureto high temperatures—allowing the first high-temperaturetorsional forced-oscillation measurements under high aqueousfluid pore pressure P_f. At temperatures >1000°C, a markedreduction in shear modulus, without concomitant increase inQ^–1, is attributed to the widespread wetting of grainboundaries resulting from grain-scale hydrofracturing and themaintenance of conditions of low differential pressure P_d =P_c – P_f . Staged cooling from 1000°C is accompaniedby decreasing P_f and progressive restoration of significantlypositive differential pressure resulting in a microstructuralregime in which the fluid on grain boundaries is increasinglyrestricted to arrays of pores. The more pronounced viscoelasticbehaviour observed within this regime for the Pt-encapsulatedspecimen compared with the essentially dry specimens may reflectboth water-enhanced solid-state relaxation and the direct influenceof the fluid phase. The scenario of overpressurized fluids andhydrofracturing in the Pt-encapsulated dunite specimen may havesome relevance to the high Q^–1 and low-velocity zonesobserved in subduction-zone environments. The outcomes of thisexploratory study indicate that the presence of water can havea significant effect on the seismic wave attenuation in theupper mantle and provide the foundation for more detailed studieson the role of water. KEY WORDS: seismic wave attenuation; water; dunite; hydrous mineral; shear modulus; viscoelasticity; olivine; grain-scale hydrofracturing     

Exsolutions of Diopside and Magnetite in Olivine from Mantle Dunite, Luobusa Ophiolite, Tibet, China

The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chromite （1%-4%） ＋ diopside （〈1%）. Two types of olivine are found in thin sections： one （Fo = 94） is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other （Fo = 96） is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system （EDS） with unitary analytical method, the average compositions of the inclusions are： Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f＇dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^＋, Al^3＋, Ca^2＋, Fe^3＋, Cr^3＋. With temperature d