层状出溶结构导致拉长石晕彩形成——来自透射电镜的证据

从拉长石出溶结构的形态特征、成分差异和出溶结构的取向研究出发,采用TEM、STEM 以及SAED等手段对其进行测试,发现拉长石出溶结构层的厚度符合理论计算值,结构层的成分由富钙和贫钙两相长石组成,具备了产生干涉作用的必要条件.研究结果表明,拉长石的晕彩色是因其内部的层状出溶结构对光的干涉所致,结构层的厚度主要受富钙相长石的控制.随着钙(An)含量的增加,结构层相应地增厚,且富钙层的增大幅度大于贫钙层,从而导致晕彩色波长也随之增大.出溶结构层的取向为(153),近于平行晶体的(010)面,但不同产地的拉长石,尽管晕彩色相同,其出溶条纹中的钙含量和条纹的具体取向也不尽相同.     

西藏冈底斯斑岩铜矿带厅宫铜矿床流体包裹体研究

厅宫斑岩铜矿床是西藏冈底斯斑岩铜矿带上重要的矿床之一。为了探明该矿床成矿流体的成分及温压条件等物理化学性质,文章对厅宫铜矿各蚀变阶段石英脉及石英斑晶中的流体包裹体进行了显微测温、四极杆质谱、离子色谱和激光拉曼探针分析,结果表明：厅宫铜矿成矿流体为高温、高盐度岩浆热液,成矿事件主要发生在340～380℃,成矿过程中流体发生了沸腾;成矿流体中气相成分以H2O为主,还含有一定量的CO2及少量的CH4、H2S、C2H6等气体;液相中离子以K^＋、Na^＋、Cl^-、SO4^2-、F^-等为主,还含有少量的Ca^2＋、Mg^2＋等;另外,出现大量气泡先消失、子矿物后熔融的Ⅲa类包裹体,表明有些成矿流体可能直接来源于深部岩浆的出溶作用。     

Symplectitic augite: evidence for discontinuous precipitation as an exsolution mechanism in Ca-rich clinopyroxene

Abstract Part of the augite in the Artfjället gabbro consists of symplectitic intergrowths between augite and blebs or lamellae of orthopyroxene. Mineral compositions are consistent with formation of these symplectites by exsolution of orthopyroxene from magmatic augite at a temperature of ca . 900–1000°C. The microstructures indicate that the exsolution mechanism is discontinuous precipitation, whereby the boundary of an augite grain sweeps through a neighbouring augite, leaving the symplectite in its wake. The formation of this symplectitic augite is catalysed by the presence of an intergranular water-rich fluid phase, which promotes grain boundary mobility.     

A comment on diffusion and the displacement of atoms in metamorphic transformations

Experimental data on diffusion in olivine , are used to define certain terms – diffusion coefficient, jump frequency, characteristic distance, random walk – that are useful in a discussion of atom displacements under natural conditions. Examples of atom displacements in two metamorphic terranes of the Canadian Precambrian Shield are then examined, as follows. (i) In a high‐grade metamorphic terrane in the Mid‐Proterozoic Grenville Province (Otter Lake Area), Mg concentration gradients about dolomite microcrystals in calcite and Na gradients about albite microcrystals in K‐feldspar are viewed as stranded Mg–Ca and Na–K interdiffusion gradients, formed by exsolution during slow cooling from ~700 to ~400 °C. (ii) In the Archean Slave Province (Yellowknife area), the crystallization of sillimanite, near andalusite but within crystals of quartz, possibly occurred by coupled Al–Si and oxygen–vacancy interdiffusion in quartz at ~550 °C. And the crystallization of garnet from chlorite occurred by the two‐way crystal‐boundary diffusion of several kinds of atoms across distances ranging to 3 mm. (iii) In the Otter Lake area, the crystallization of orthopyroxene–hornblende–spinel reaction zones at boundaries between crystals of olivine and plagioclase in metagabbro, evidently occurred by the mechanism of interstitial diffusion, that transported Mg, Fe, Mn and O atoms across the reaction zone from olivine to the plagioclase–(hornblende+spinel) boundary, and Si, Al, Ca and Na atoms from plagioclase to the olivine–orthopyroxene boundary, accompanied by NaSi–CaAl interdiffusion in plagioclase, and the addition of hydrogen and minor Ti, Zn, F, Cl and K from beyond the reaction zone. Also, centimetric reaction zones, with abundant biotite and plagioclase, at boundaries between K‐feldspar gneiss and deformed amphibolite dykes, evidently formed by the reaction, strained hornblende (in amphibolite) + K‐feldspar (in gneiss)→biotite (in amphibolite) + plagioclase (in gneiss), with crystal‐boundary diffusion of (Na + Ca) atoms and of K atoms across the reaction zone.     

华中东部榴辉岩中绿辉石的反相畴、出溶及其成因意义

反相畴及出溶片晶是华中东部高压变质带中绿辉石的主要超微结构，出港片晶的大小受扩散速率和时间的控制，而反相畴的大小可作为变质温度和年龄的标志。通过对Alps、Cali-fornia等高压变质带中绿辉石的反相畴的统计计算，修正了Carpenter所得出的统计公式：δ8=8×（6×1027）e－Ｑ/RT·t。根据作者的成果，绿辉石反相略大小与变质温度及地质年代按如下规律变化：δ9＝9×（6×1026）e－Ｑ／RT·t。由此得出所研究地区曾发生两次地质热事件，其地质年代分别为700Ma（晚元古）及200Ma（印支期）。以上得出的函数关系适用于高压变质带中不同地区的相对地质年代的推算及绝对地质年代的半定量估算。     

蠕状石成因研究的新进展

重点介绍了蠕状石成因的研究现状和目前流行的几种成因假说，并对其进行了讨论和评价，尤其对“出溶硅氧泵模式”假说的蠕状石出溶的原理、过程和“硅氧泵”的实验机理、数据等、做了较为详尽的阐述。     

Raman Spectrum Study on Quartz Exsolution in Omphacite of Eclogite and Its Tectonic Significances

The studies on ultra-microstructue characteristics of quartz exsolution in eclogite and coesite in UHP eclogite of several localities are done with the appliance of laser Raman spectroscopy and Ustage. Research results show that the phase transformation of coesite-quartz in garnet and/or omphacite is a continuous process. Topological relationship is present between quartz exsolution in omphacite and its host mineral which shows orientations of two long axes of quartz exsolution parallel to (100) and (-101) of omphacite. At present, some scholars suggest that thequartz exsolution in omphacite of eclogite is the evidence of UHP metamorphism. However, temperature and pressure condition and the exsolution mechanism of oriented needlelike quartz in omphacite still remain undear. Tnerefore, further study should be enhanced on experimental research on exsolution mechanism of super-silicate clinopyroxene, which could provide experimental quantitative constraint on quartz exsolution as UHP indicator.     

矿石成因研究中若干问题的探讨

根据某些研究成果和实际经验,结合一些新资料,对固溶体的出溶结构、矿物晶体成因类型,尤其对矿物结晶粒度的概率统计特征、矿石中元素组合及其演化规律的定量分析,首次提出新见解。在实际应用上,这些新见识对矿床成因研究乃至矿床的评价将起积极的作用。     

Mineralogical Characteristics of Exsolved Spinel in the Panzhihua V-Ti Magnetite Deposit, Sichuan: Implications for the Mineralization Process

Spinel exsolution is widespread in titanomagnetite from the Fe-Ti oxide gabbro of the Panzhihua intrusion, Emeishan Large Igneous Province, SW China. However, little research has been conducted into the implications of patterns in the mineralogical characteristics of the spinel for spatial variation in the controls on the exsolution mechanism and, hence, the formation process of the ore deposit. This study selected the Lanjiahuoshan Ore Block in the Panzhihua V-Ti magnetite deposit to explore this issue, systematically studying exsolution textures in the titanomagnetite through petrographic observation and the integrated use of in-situ microanalysis. The results show that the exsolved spinel gradually becomes finer-grained and less abundant from the center to edge and the bottom to top of the ore bodies. Compositionally, there is an inverse correlation between the size of exsolved spinel grains and their Mg# value. In addition, there is compositional zonation in the spinel interiors, with a gradual increase in the Mg content and decrease in Fe content from the core to the rim. The analysis suggests that fractional crystallization of ferrotitanium magma with a high oxygen fugacity in a shallow magma chamber caused compositional differences in the primary magnetite solid solution in different parts of the Panzhihua intrusion. Additionally, the thermal evolution of the magnetite solid solution differed in different parts of orebody, bringing about variations in spinel development. Together, these effects resulted in spatial variation in the abundance, grain size, and morphology of spinel in different parts of the orebody and intrusion that follows an identifiable distribution law. Furthermore, the compositional zonation of exsolved spinels reflects the rapid growth of exsolution features in a high-temperature environment. Thus, the size, morphology, abundance, and composition of spinel exsolution features in titanomagnetite provide a valuable petrogenetic tool for estimating the maturity and formational environment of the deposit.     

Vesiculation and crystallization under instantaneous decompression: Numerical study and comparison with laboratory experiments

Vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior, and they interplay each other through the water exsolution process. We conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions (decompression history is unknown) and in laboratory experiments (the amount of decompression is constant with time). The numerical results, which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water, viscosity, and the amount of decompression, provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression. The bubble nucleation in the homogeneous nucleation can be divided into two regimes – the diffusion control regime and viscosity control regime – depending on the modified Peclet number and the effective supersaturation. In the cases of both homogeneous and heterogeneous nucleations, the bubble growth is controlled by diffusion or viscosity, depending on the modified Peclet number and bubble number density. The water exsolution rate, which is controlled by the modified Peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime, acts as an effective cooling rate in a decompression-induced crystallization process. A comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion-limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles, and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion-limited growth of crystal. The several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate.