钴硅锌矿—一个富钴的硅锌矿变种

钴硅锌矿是一种具有特征深蓝紫色、油脂光泽和玻璃光泽的钴锌硅酸盐矿物,属三方晶系。空间群:C_(3j)~2—R3、a=13.9559A,c=9.3364A,c/a=0.6690,V=1574.8104A~3,Z=18。该矿物理论式为(Zn,Co)_2SiO_4,实验式为:(Zn_(1.26),Co_(O.74))SiO_4。据红外光谱以及X射线粉晶分析,其结构与硅锌矿(Zn_2SiO_4)类似。     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

热红外遥感定量反演地表岩石的SiO2含量

硅酸盐矿物热红外发射率光谱特征与其SiO2含量有很好的对应关系.本文以ASU(Arizona State University)、JHU(Johns Hopkins University)光谱库为数据源,研究了硅酸盐矿物发射率光谱克里斯琴森特征(CF-Christensen Features)与SiO2含量、SCFM(SiO2/(SiO2 FeO MgO CaO))指数的定量关系,并与Copper对火成岩的研究结果进行了对比,二者吻合,可作为热红外发射率光谱线性混合的佐证.建立了表征硅酸盐矿物SiO2含量的SiO2指数(ε12/ε13)与SiO2含量的定量关系,在此基础上,利用ASTER热红外数据对东天山黄山东地区SiO2进行了定量反演,反演结果与地质事实相符.利用反演结果可以对基性超基性岩体进行圈定,对于寻找与基性超基性岩体有关的铜镍矿床有重要的应用价值.     

新疆北部的富镁火成岩

本文重点讨论的富镁安山岩(MAs)是指 SiO_2>53%,Mg~#≥55的安山岩,也包括富镁英安岩和富镁闪长岩。阿尔泰南的富锾安山岩形成于中泥盆世,东、西天山和阿拉套山的富镁安山岩形成于石炭纪。本区富镁安山岩 SiO_2 53%～65%,TiO_2含量低于1%(0.21%～1.08%),平均0.72%;MgO 平均5.90%,与国外玻安岩平均值相比,Mg 含量较低,Ti、K、Na 含量较高。稀土元素总合量低(<100×10~(-6),范围15～95×10~(-6)),相对富重稀土(La/Yb)_N0.98～6.4,多数在4±;Eu 弱亏损到无亏损(Eu/Eu~*0.65～1.15)。相容微量元素 Cr、Ni 含量高,分别为72×10~(-6)～790×10~(-6)和29×10~(-6)～276×10~(-6);高场强元素 Nb、Ta、Ti 明显亏损;相对富集流体中富集的大离子亲石元素 Rb、K、Pb;Y 含量较高(>15×10~(-6));Sr/Y 比值较低(4.4～6.2)。(~(143)Nd/~(144)Nd)_i 值变化范围较大0.51221～0.51255,ε_(Nd)(t) 0.28- 7.2,Nd 模式年龄 t_(DM)485～1220Ma;(~(87)Sr/~(86)Sr)_i 值变化范围较大0.7029～0.7065,在ε_(Nd)(t)-(~(87)Sr/~(86)Sr)_i 图解中位于第一象限。上述特点表明,本区富镁安山岩源区为复合源,包括有蚀变洋壳、弧前增生棱柱的不成熟、新生地壳物质及地幔楔橄榄岩。成岩作用具复合性,既有俯冲洋壳板片的部分熔融,又有俯冲板片熔体对地幔楔橄榄岩的熔体交代作用。形成于特殊构造环境:高地热梯度和富挥发分;导致弧前增生棱柱俯冲的俯冲剥蚀作用;由俯冲板片撕裂、断离所形成的软流圈窗或洋脊俯冲。     

Intrusion of ultramafic magmatic bodies into the continental crust: Numerical simulation

Intrusions of ultramafic bodies into the lower density continental crust are documented for a large variety of tectonic settings spanning continental shields, rift systems, collision orogens and magmatic arcs. The intriguing point is that these intrusive bodies have a density higher by 300-500 kg m⁻³ than host rocks. Resolving this paradox requires an understanding of the emplacement mechanism. We have employed finite differences and marker-in-cell techniques to carry out a 2D modeling study of intrusion of partly crystallized ultramafic magma from sublithospheric depth to the crust through a pre-existing magmatic channel. By systematically varying the model parameters we document variations in intrusion dynamics and geometry that range from funnel- and finger-shaped bodies (pipes, dikes) to deep seated balloon-shaped intrusions and flattened shallow magmatic sills. Emplacement of ultramafic bodies in the crust lasts from a few kyr to several hundreds kyr depending mainly on the viscosity of the intruding, partly crystallized magma. The positive buoyancy of the sublithospheric magma compared to the overriding, colder mantle lithosphere drives intrusion while the crustal rheology controls the final location and the shape of the ultramafic body. Relatively cold elasto-plastic crust (T_Moho = 400 °C) promotes a strong upward propagation of magma due to the significant decrease of plastic strength of the crust with decreasing confining pressure. Emplacement in this case is controlled by crustal faulting and subsequent block displacements. Warmer crust (T_Moho = 600 °C) triggers lateral spreading of magma above the Moho, with emplacement being accommodated by coeval viscous deformation of the lower crust and fault tectonics in the upper crust. Strong effects of magma emplacement on surface topography are also documented. Emplacement of high-density, ultramafic magma into low-density rocks is a stable mechanism for a wide range of model parameters that match geological settings in which partially molten mafic-ultramafic rocks are generated below the lithosphere. We expect this process to be particularly active beneath subduction-related magmatic arcs where huge volumes of partially molten rocks produced from hydrous cold plume activity accumulate below the overriding lithosphere.     

The GeoPT Proficiency Testing Scheme for Laboratories Routinely Analysing Silicate Rocks: A Review of the Operating Protocol and Proposals for its Modification

The GeoPT proficiency testing programme for geochemical laboratories has been operating for nearly 20 years. This review, based on an analysis of accumulated data and extensive experience, examines whether the protocol followed during this period continues to be appropriate. Possible changes that might be considered for future implementation are examined, in particular the procedures for calculating assigned values, target values, z‐scores and the applicability of dual fitness‐for‐purpose criteria. We conclude that the existing protocol remains largely appropriate and effective, but propose that a single fitness‐for‐purpose criterion be adopted, with an option for laboratories to recalculate their z‐scores if the criterion is inappropriate for their requirements.     

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 1. Melting relations and major element compositions of melts

Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite + K-richterite, some including 5% of accessory phases, have been conducted at 15 and 50 kbar. The assemblages represent probable source components that contribute to melts in cratonic regions, but whose melt compositions are poorly known. A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene (CPX), phlogopite (PHL) and K-richterite (KR) with or without 5% ilmenite, rutile or apatite. Additional experiments were run without KR and with higher proportions of accessory phases. Melt traps were used at near-solidus temperatures to facilitate accurate analysis of well-quenched melts, for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50 °C of the solidus, so that melts reflect the composition of the amphibole and its melting reaction. Melts have high SiO₂ and especially K₂O but low CaO and Al₂O₃ relative to basaltic melts produced from peridotites at similar pressures. They have no counterparts amongst natural rocks, but most closely resemble leucite lamproites at 15 kbar. KR and PHL melt incongruently to form olivine (OL) and CPX at 15 kbar, promoting SiO₂ contents of the melt, whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures, leading to an increase in MgO and decrease in SiO₂ in melts, which resemble olivine lamproites. Melts of mica pyroxenites without KR are richer in CaO and Al₂O₃ and do not resemble lamproites. These experiments show that low CaO and Al₂O₃ in igneous rocks is not necessarily a sign of a depleted peridotite source. Accessory phases produce melts exceptionally rich in P₂O₅ or TiO₂ depending on the phases present and are unlike any melts seen at the Earth’s surface, but may be important agents of metasomatism seen in xenoliths. The addition of the 5% accessory phases ilmenite, rutile or apatite result in melting temperatures a few ten of degrees lower; at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX + PHL + KR mixtures are close to cratonic geotherms at depths > 130 km: minor perturbations of the stable geotherm at >150 km will rapidly lead to 20% melting. Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth, but will change if reaction with wall-rocks occurs, leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite. At higher temperatures, extensive melting of peridotite will dilute the initial alkaline melts: this is recognizable as alkaline components in basalts and, in extreme cases, alkali picrites. Hydrous pyroxenites are, therefore, components of most mantle-derived igneous rocks: basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.     

甲基硅酸盐抑制粘土水化性能及机理

泥页岩水化分散是引起钻孔孔壁失稳,导致缩径、掉块、坍塌等孔内事故的主要原因,泥页岩抑制剂一直是复杂地层钻进技术研究的热点。通过对甲基硅酸钾、甲基硅酸钠、氯化钾、硅酸钠、硅酸钾5种抑制剂对粘土沉降稳定性、水化膨胀及造浆效果等性能测试,结果表明甲基硅酸盐具有良好抑制性;通过红外光谱、X射线衍射、水接触角和光学显微镜对甲基硅酸钾的抑制性作用机理进行了探讨。明确了钾离子的抑制性与表面疏水膜结构协同是甲基硅酸钾抑制粘土水化和防塌作用的原因。     

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems

Liquidus relations in the four-component system Na₂O–Al₂O₃–SiO₂–F₂O_–1were studied at 0· 1 and 100 MPa to define the locationof fluoride–silicate liquid immiscibility and outlinedifferentiation paths of fluorine-bearing silicic magmas. Thefluoride–silicate liquid immiscibility spans the silica–albite–cryoliteand silica–topaz–cryolite ternaries and the haplogranite-cryolitebinary at greater than 960°C and 0· 1–100 MPa.With increasing Al₂O₃ in the system and increasing aluminum/alkalication ratio, the two-liquid gap contracts and migrates fromthe silica liquidus to the cryolite liquidus. The gap does notextend to subaluminous and peraluminous melt compositions. Forall alkali feldspar–quartz-bearing systems, the miscibilitygap remains located on the cryolite liquidus and is thus inaccessibleto differentiating granitic and rhyolitic melts. In peralkalinesystems, the magmatic differentiation is terminated at the albite–quartz–cryoliteeutectic at 770°C, 100 MPa, 5 wt % F and cation Al/Na =0· 75. The addition of topaz, however, significantlylowers melting temperatures and allows strong fluorine enrichmentin subaluminous compositions. At 100 MPa, the binary topaz–cryoliteeutectic is located at 770°C, 39 wt % F, cation Al/Na 0·95, and the ternary quartz–topaz–cryolite eutecticis found at 740°C, 32 wt % F, 30 wt % SiO₂ and cation Al/Na 0· 95. Such location of both eutectics enables fractionationpaths of subaluminous quartz-saturated systems to produce fluorine-rich,SiO₂-depleted and nepheline-normative residual liquids. KEY WORDS: silicate melt; granite; rhyolite; fluorine; liquid immiscibility     

川西卡拉地区晚古生代硅质岩成因及沉积环境

为了研究川西卡拉地区晚古生代沉积环境特征,以该地区下石炭统邛依组和上二叠统卡翁沟组中广泛分布的硅质岩为研究对象,开展了岩石学、地球化学特征等研究。结果表明: 研究区古生代硅质岩属纯硅质岩范畴,在沉积过程中有持续的陆源物质供给; Al/(Al+Fe+Mn)平均值0.62,结合Al-Fe-Mn三角图解,显示它们为生物成因硅质岩; Al₂O₃/(Al₂O₃+Fe₂O₃)、Th/U、Th/Sc、(La/Ce)_N、(La/Yb)_N等特征比值以及构造环境判别图,反映出硅质岩沉积环境为大陆边缘的浅海-斜坡; Ce_anom值为-0.04~0.07,从石炭纪—二叠纪逐渐增大,指示水体为缺氧环境,且还原性逐渐增强。结合区域地质演化,认为甘孜理塘洋在石炭纪时期已经打开,并在二叠纪时期持续扩张,区内沉积盆地的海水逐渐变深。