The causes and petrological significance of cathodoluminescence emissions from alkali feldspars

The cathodoluminescence (CL) of a variety of alkali feldspars from South Greenland has been examined in an attempt to understand the causes of the CL and its petrological significance. Analytical methods have included CL spectroscopy, secondary ion mass spectrometry (SIMS) and electron paramagnetic resonance (EPR) to correlate the presence of certain CL emissions to the presence of certain trace element and point defects. Where possible, blue and red luminescent fractions of the same rock samples have been separated and analysed separately. Blue CL appears to relate to the presence of electron holes on bridging oxygens, particularly on the Al-O-Al bridge, as determined from EPR studies. No correlation with other proposed activators for blue CL such as Eu²⁺, Ga³⁺ or Ti⁴⁺ was observed. Some blue luminescent feldspars also have an emission in the infra-red (IR), invisible during normal visible CL petrography. The red and IR CL emissions correspond to features in EPR spectra attributed to Fe³⁺ and support previous suggestions that Fe³⁺ is related to this emission. However, our studies indicate that the visible red CL relates specifically to Fe³⁺ on the T1 site, whereas the equivalent CL from disordered feldspars lies in the IR. The difference between red and IR CL emissions therefore relates to the state of Fe³⁺ order across the tetrahedral sites. These data allow more meaningful interpretations of CL as a petrographic tool in alkali feldspar-bearing rocks. Received: 5 March 1998 / Accepted: 23 November 1998     

The distribution of uranium in some basic igneous cumulates and its petrological significance

The distribution of uranium in the minerals and rocks of the Skaergaard, Rhum and Bushveld layered igneous intrusions has been investigated by the delayed neutron analytical technique. The uranium concentration in the minerals depends, in part, upon the nature of the cumulate from which they were separated. In those minerals from adcumulates, pyroxene contains the most uranium and olivine the least. The uranium content of a basic igneous cumulate is shown to be useful as an indicator of the relative amount of mesostasis in the rock, provided uranium-bearing cumulus minerals such as apatite are absent. The amount of mesostasis sometimes changes rapidly from one layer to another.     

Crystal-liquid relationships in some ultrabasic dykes and their petrological significance

Phase relations have been determined at one atmosphere and an oxygen fugacity of 10^–8 atmospheres for a number of rocks from two differentiated ultrabasic dykes. It is demonstrated that these are consistent with other petrological evidence and that they support the view that the dykes were differentiated mechanically during intrusion. Crystal-liquid relations during the equilibrium crystallization of one of these rocks were determined by electron probe microanalysis. Over part of the temperature range where the three phase assemblage olivine+spinel+liquid exists an anomalous relationship was found. Immediately below the liquidus crystallization is normal with the olivine and liquid both becoming relatively enriched in iron as temperature decreases. When spinel starts to crystallize the iron enrichment in the liquid is suppressed and the forsterite content of the olivine actually increases until at lower temperatures a normal trend of iron enrichment is resumed. This phenomenon is tentatively attributed to the crystallization of the spinel under conditions of constant oxygen fugacity.     

The formation and significance of irregularly shaped quartz grains in till

Thin sections and scanning electron microscopy of quartz sand grains from till show clearly that a number of grains have been modified in place. The irregular grains result from the action of circulating alkaline groundwater, episodically concentrated by evaporation on fracture surfaces within grains and on concave surfaces produced by fracture during glacial transport. Evaporation increases the pH of the pore water and localizes its effect in small pores with a high capillary potential. Subsequent dilution by rainwater or snow meltwater flushes the system so that evaporation can repeat the process again. Solution rounding in terrigenous sediments has a number of ramifications for any interpretation based on textural maturity of the sediment.     

Gravity studies of the Fen complex,Norway, and their petrological significance

The Fen alkaline complex, which is about 2 km in diameter, is an example of the carbonatite-ijolite-pyroxenite rock association. A Bouguer anomaly map reveals a sharp and almost circular gravity high centered above the complex, the residual peak value being about + 23 mgal. The gravity interpretation supports the ideas that the carbonatites represent only the volatile end fraction accumulated in the upper part of a long vertical magma chamber such as could form in an abyssal fissure, and that the major part of the pipe consists of an ultramafic parental or cogenetic rock originating from great depth (deep crust or upper mantle).Contribution No. 68 in the Norwegian Geotraverse Project.On leave to Dept. of Geology, University of Wyoming, Laramie, Wyoming 82070.     

高温高压实验产物中出现的流动构造及其岩石学意义

在高温高压实验产物中发现了长石和云母定向排列以及球粒被拉长并定向排列的现象。这种与野外常见的地质事实相一致的现象说明，某些花岗岩体边部钾长石的定向排列可能是岩浆流动造成的。     

新疆软玉、岫岩软玉的岩石矿物学对比研究

对新疆、岫岩软玉的典型品种进行了偏光显微镜、LA-ICP-MS、SEM等测试分析,证明两产地软玉的物理性质相似,但岫岩软玉光泽和透明度不如新疆软玉,其抛光的光洁度不如新疆软玉。黄白玉和绿玉在新疆软玉中基本上没有,而新疆软玉中的黄玉在岫岩软玉中基本上没有,这是两地软玉在颜色上的明显差别。微量元素均以青玉中的微量元素的总含量最高,Be、Ni、Cu元素对软玉中的绿色调有一定的影响。REE配分曲线显示了成矿流体的多来源和多期次叠加成矿的稀土元素特征。扫描电镜下观察,新疆软玉以毡状纤维交织变晶结构最为典型和常见;岫岩软玉以显微纤维结构和显微叶片状结构居多,颗粒较新疆软玉粗大,致密程度差,这也是其质地不如新疆软玉的重要原因。     

The Malvern howardite: a petrological and chemical discussion

The Malvern howardite contains glass, probably formed by impact on the surface of a parent body. This achondrite also contains a large xenolith, a breccia in breccia. A mineralogical and chemical study of the stone has been carried out by means of electron microprobe and neutron activation analysis.Individual mineral and bulk compositions show the eucritic character of the xenolith. Most of the stone results from mechanical mixing of possibly related rocks, with glass derived by total or partial melting. Foreign components seem to be present as well.     

Platinum-group minerals and other solid inclusions in chromite of ophiolitic complexes: Occurrence and petrological significance

Summary Platinum-group mineral, silicate and other solid and fluid inclusions occur in disseminated and massive chromite in a variety of lithologies from ophiolitic and other mafic-ultramafic complex-types. The inclusions are small (<250 microns) and randomly distributed throughout their host. Silicate inclusions are modally more abundant than the other inclusion types. Platinum-group mineral phases are ruthenium-rich sulphides and PGE alloys are osmium-rich. Mafic silicates (olivine, pyroxenes, pargasitic-amphiboles, micas) are magnesium-, titanium-, and alkali-rich and felsic silicates are sodium-rich (albite, nepheline).The intimate association of these inclusions with chromite suggests that their origin must be considered within a chromite crystallization model. A hypothesis of origin is suggested wherein the platinum-group minerals and silicates are trapped as discrete, crystalline euhedral phases and silicate liquid during the precipitation of chromite. The similarity of physical characteristics, modal mineralogy and chemical compositions indicates that this model may be applicable to all mafic-ultramafic complex-types.

---

Minerale der Platin-Gruppe und andere feste Einschlüsse in Chromiten aus Ophiolit-Komplexen: Vorkommen und petrologische Bedeutung

Zusammenfassung Minerale der Platin-Gruppe, Silikate und andere feste und flüssige Einschlüsse kommen in disseminierten und massiven Chromiten in einer Vielzahl von Gesteinen in ophiolitischen und anderen mafisch-ultramafischen Komplexen vor. Die Einschlüsse sind klein (<250 microns) und unregelmäßig im Chromit verteilt. Silikat-Einschlüsse sind modal weiter verbreitet als andere Arten von Einschlüssen. Minerale der Platin-Gruppe sind durch Ruthenium-reiche Sulfide und Osmium-reiche Legierungen vertreten. Mafische Silikate (Olivin, Pyroxen, pargasitische Amphibole, Glimmer) sind Magnesium-, Titan- und Alkali-reich; felsische Silikate sind Natrium-reich (Albit, Nephelin).Die ausgeprägte Assoziation dieser Einschlüsse mit Chromit weist darauf hin, daß ihre Herkunft im Zusammenhang mit einem Kristallisations-Modell für Chromit zu sehen ist. Ein genetisches Konzept wird vorgelegt, wobei die Platin-Gruppen-Minerale als gut ausgebildete, idiomorphe kristalline Phasen, und die Silikate als Silikatschmelze während des Ausfallens des Chromites eingeschlossen werden. Die Ähnlichkeit der physikalischen Eigenschaften, der modalen mineralogischen Zusammensetzung und der chemischen Zusammensetzung weisen darauf hin, daß dieses Modell auf alle mafisch-ultramafischen Komplexe anzuwenden ist.

     

The development of subgrain misorientations with strain in dry synthetic NaCl measured using EBSD

The development of subgrain boundary misorientations with strain in dry, synthetic NaCl polycrystals, deformed at elevated temperature, has been investigated using electron backscattered diffraction (EBSD). At low natural strains, up to 0.5, average misorientations of subgrain boundaries increase with strain and a power law relationship exists between strain and average misorientations. The average misorientations are strongly influenced by grain orientation, suggesting that the misorientation–strain relationship may also be texture dependent in materials with high plastic anisotropy, like NaCl. A slight grain size dependency of the average misorientations was observed. The results indicate that with suitable calibration, average subgrain boundary misorientations may offer a method for estimating the strain accommodated by dislocation creep in NaCl and thus perhaps in other geological materials, although current theories for polycrystalline plasticity imply that misorientations may also depend on stress in some situations.     

Early Cretaceous Sung Valley ultramafic-alkaline-carbonatite complex, Shillong Plateau, Northeastern India: petrological and genetic significance

Summary The Shillong Plateau of northeastern India hosts four Early Cretaceous (105–107Ma) ultramafic-alkaline-carbonatite complexes (UACC), which have been associated with the Kerguelen plume igneous activity. Petrological and geochemical characteristics of one of these UACC, the Sung Valley, are presented. The Sung Valley UACC was emplaced in to the Proterozoic Shillong Group of rocks and consists of ultramafics (serpentinized peridotite, pyroxenite, and melilitolite), alkaline rocks (ijolite and nepheline syenite), and carbonatites. Serpentinized peridotite, pyroxenite, and ijolitic rocks form the major part of the complex, the others constitute less than 5% of the total volume. Ijolite and melilitolite intrude peridotite and pyroxenite, while nepheline syenite and carbonatite intrude the ultramafic rocks as well as ijolite. Mineralogically, the carbonatites are classified as calcite carbonatite with minor apatite, phlogopite, pyrochlore and ilmenite. The serpentinized peridotites are wehrlitic. Chemical compositions of the silicate rocks do not show a distinct co-genetic relationship amongst them, nor do they show any geochemical relationships with the carbonatites. No noticeable fractionation trend is observed on the chemical variation diagrams of these rocks. It is difficult to establish the genetic evolution of the Sung Valley UACC through fractional crystallization of nephelinitic magma or through immiscible liquids. On the basis of petrological and geochemical data and previously published isotopic results from these rocks, it is suggested that they have been derived from a primary carbonate magma generated by the low-degree melting of a metasomatized mantle peridotite.     

西藏羌塘裂谷带假玄武玻璃的发现及其岩石学特征与构造意义

假玄武玻璃是地震过程中断层滑动产生摩擦熔融并快速冷却的产物,被誉为"地震化石",记录了断层带内地震发生时的物理-化学过程的相关信息,是认识断裂活动历史的重要物质组成。本文以青藏高原腹地羌塘地块SN向正断层——东依布茶卡断层中的假玄武玻璃和碎裂岩等为研究对象,借助偏光显微镜、扫描电镜、粉末X射线衍射分析、原位XRF分析等多种方法对其进行了详细的结构、构造、矿物成分及化学元素分布的分析。野外调查表明该断裂带中假玄武玻璃呈深灰-褐色,与超碎裂岩和碎裂岩相伴生,以断层脉、注入脉和网状脉的形式产出,厚度由几毫米至一厘米不等。显微结构观察显示,假玄武玻璃中残留有摩擦熔融形成的特征构造,如石英的港湾状熔蚀边、蜂窝状气孔构造以及局部发育的长柱状微晶、束状微晶集合体等,指示其为断层滑动摩擦熔融的产物。此外可见假玄武玻璃作为碎屑出现在碎裂岩和角砾岩中,且后期的碎裂岩中包含早期形成的碎裂岩的角砾等,这些特征表明在该断裂带内大地震事件多次重复发生。以该地区正断层初始活动时限13.5Ma、碎裂岩和假玄武玻璃形成深度为10km以下计算,这些断裂岩石从地下深部折返的速率至少为~0.74mm/yr。目前国际上鲜有报导正断层形成的假玄武玻璃,这是我们首次在羌塘地块SN向裂谷正断层中发现熔融成因的假玄武玻璃,对认识区域地震活动及断裂带演化过程具有重要意义。     

Ferric iron partitioning between plagioclase and silicate liquid: thermodynamics and petrological applications

A series of Fe and Mg partition experiments between plagioclase and silicate liquid were performed in the system SiO₂-Al₂O₃-Fe₂O₃-FeO-MgO-CaO-Na₂O under oxygen fugacities from below the IW buffer up to that of air. A thermodynamic model of plagioclase solid solution for the (CaAl,NaSi,KSi)(Fe³⁺,Al³⁺)Si₂O₈-Ca(Fe²⁺,Mg)Si₃O₈ system is proposed and is calibrated by regression analysis based on new and previously reported experimental data of Fe and Mg partitioning between plagioclase and silicate liquid, and reported thermodynamic properties of end members, ternary feldspar and silicate liquid. Using the derived thermodynamic model, FeO^t, MgO content and Mg/(Fe^t+Mg) in plagioclase can be predicted from liquid composition with standard deviations of ǂ.34 wt% (relative error =9%) and ǂ.08 wt% (14%) and ǂ.7 (8%) respectively. Calculated Fe³⁺-Al exchange chemical potentials of plagioclase, m_{Fe^{3 +} ( Al )- 1} ^Pl{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Pl}} agree with those calculated using reported thermodynamic models for multicomponent spinel, m_{Fe^{3 +} ( Al )- 1} ^Sp{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Sp}} and clinopyroxene, m_{Fe^{3 +} ( Al )- 1} ^Cpx{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Cpx}} . The FeO^t content of plagioclase coexisting with spinel or clinopyroxene is affected by Fe³⁺/(Fe³⁺+Al) and Mg/(Fe+Mg) of spinel or clinopyroxene and temperature, while it is independent of the anorthite content of plagioclase. Three oxygen barometers based on the proposed model are investigated. Although the oxygen fugacities predicted by the plagioclase-liquid oxygen barometer are scattered, this study found that plagioclase-spinel-clinopyroxene-oxygen and plagioclase-olivine-oxygen equilibria can be used as practical oxygen barometers. As a petrological application, prediction of plagioclase composition and f_O2 are carried out for the Upper Zone of the Skaergaard intrusion. The estimated oxygen fugacities are well below QFM buffer and consistent with the estimation of oxidization states in previous studies.     

大兴安岭中段二叠系大石寨组细碧岩的岩石学地球化学特征及其成因探讨

大兴安岭中段林西地区二叠纪大石寨组下部的细碧角斑岩建造记录了大石寨裂陷槽早期的发育历史，因而对其成因和构造环境的研究具有一定的意义。本文在详细地野外工作基础上，对二叠系大石组的细碧岩进行了较为系统的岩石学、元素地球化学及同位素地球化学研究。结果表明，细碧岩的岩石类型为玄武岩、玄武安山岩、安山岩、橄榄组安岩和粗安岩（或歪长粗面岩），属于拉斑玄武岩岩石系列；细碧岩的主量元素质量分数及变化范围类型似于N－MORB玄武岩；与正常洋中脊玄武岩相比较，细碧岩的LILE元素明显富集，HFSE元素的质量分数与大洋中脊玄武岩（N－MORB）相接近，Nb,Ta明显亏损；细碧岩的稀土元素分配模式具平坦型；（^87Sr/^86Sr)t为0．7013－0．7056。细碧岩形成于大石寨裂陷槽早期裂陷阶段弧后盆地局部扩张的构造环境；细碧岩是由拉斑玄武质岩浆在上升过程中热卤水的严重混染后喷出地表结晶而形成的。     

The petrological and geological significance of theb 0 values of potassic white micas in low-grade metamorphic rocks. An application to the Eastern Alps

Summary In the Eastern Alps theb ₀ values of potassic white micas of Hercynian low-grade schists differ sharply from those of Alpine isograde and isochemical rocks. These differences, which indicate variations in the phengite content present as solid solution in the white micas, are to be referred to differences in pressure. In this way it is possible to recognize, in the low-grade metamorphic rocks of different tectonic units of the Eastern Alps, the lowpressure character of the Hercynian metamorphism, and the higher-pressure character of the Alpidic thermic event; this recognition is not possible using traditional methods, owing to the lack of characteristic minerals in the lowgrade metamorphic rocks. The results of the present report allow us to state that generally, when studying low-grade metamorphic complexes, theb ₀ measurements can supply valuable data which are not obtainable in any other way at the present time. These data are useful both for estimating recrystallization pressure and for recognizing the age of metamorphism.

---

La signification pétrologique et géologique des valeurs b₀ des micas blancs potassiques dans les roches de faible metamorphisme

Résumé Dans les Alpes orientales les valeursb ₀ des micas blancs potassiques des schistes épimétamorphiques d'age hercynien diffèrent nettement de celles de roches isogrades et isochimiques d'age alpin. Ges différences, qui indiquent des variations de la quantité de phengite présente en solution solide dans les micas blancs, sont référables à différences de pression. Par cette méthode on a reconnu, dans les schistes épimétamorphiques appartenant à de différentes unités tectoniques des Alpes orientales, le caractère alpinotype du métamorphisme morphisme hercynien, et d'autre part le caractère alpinotype du métamorphisme alpin; cette distinction est impossible à faire à faire à l'aide des méthodes traditionelles, à cause de l'absence de minéraux caractéristiques dans les schistes épimétamorphiques. Les résultats de cette étude ont permis d'établir que, au cours de l'analyse des complexes épimétamorphiques, les mesures deb ₀ peuvent fournir des données précieuses qu'à présent il est impossible d'obtenir autrement. Ces recnées sont utiles soit pour évaluer la pression de recrystallisation, soit pour reconnaître l'age du métamorphisme.

---

---

With 2 Figures     

Partitioning of trace elements between carbonate-silicate melts and mantle minerals: Experiment and petrological consequences

The partitioning of a number of trace elements (Ba, Nb, Zr, Y, REE, etc.) between orthopyroxene, garnet, and carbonate-silicate melt was experimentally studied using a belt apparatus at pressures of 3.5–4.2 GPa and temperatures of 1300–1500°C. The experimental products were investigated by electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The experimental melts varied from carbonatitic (～5 wt % SiO₂) at low temperatures (1300–1350°C) to kimberlitic compositions (30 wt % SiO₂) at high temperatures (1500°C). The partition coefficients of most elements between orthopyroxene and melt (D _i ^Opx/L ) and garnet and melt (D _i ^Grt/L ) were almost independent of melt composition (temperature). The D _i ^Opx/L values ranged from <0.01 for the most incompatible Ba and light REE to 0.02–0.08 for moderately incompatible Zr, Y, and heavy REE. The D _i ^Grt/L values were approximately an order of magnitude higher, ～0.07 for light REE, 0.7 for Y, and 1.5 for Yb. The character of D _i ^Grt/L variations in the systems studied is in general similar to that established for silicate melts without volatile components. However, the differences in the behavior of moderately incompatible and compatible elements (e.g., light and heavy REE) in the experimental systems are less pronounced compared with CO₂-free systems. Considering carbonate-silicate and silicate melts as possible agents of mantle metasomatism, it can be concluded that the former can efficiently transport heavy REE, and the latter have a greater affinity for Nb, Ba, and light REE. A characteristic feature of mantle rocks enriched by carbonate-silicate melts is high Ba/La ratio coupled with relatively weakly fractionated REE distribution patterns. It was shown that the high degrees of enrichment observed in natural kimberlites can be explained by a two-stage scenario, including a preliminary invasion of carbonate-silicate melt into depleted harzburgites in the lower parts of the lithosphere and subsequent very low degree melting.     

The use of linear programming in the analysis of petrological mixing problems

Petrological mixing problems such as modal analysis, magma mixing, and liquid line of descent calculations, can be solved using the methods of linear programming. If estimates of the standard error of the chemical data are introduced as weights into the set of equations, it is possible to assign confidence limits to the solutions which are obtained and to apply formal statistical tests to geological hypotheses based on the mixing model. This approach is applied to petrological data previously analysed by Wright and Doherty (1970) using a combination of linear programming and least squares methods. It is shown that some of the geological inferences which they drew were based on an overoptimistic assessment of the confidence limits on their solutions, and cannot be regarded as proven.     

Factor analysis of sphericity and roundness data of clastic quartz grains: Environmental significance

The first four sample statistics of two-dimensional projection sphericity and those of two-dimensional projection roundness of clastic quartz grains belonging to river, beach and eolian environments and to the three size grades 0.350 mm-0.297 mm, 0.297 mm-0.250 mm and 0.250 mm-0.210 mm were subjected to R-mode factor analysis. The results show that the particles of these size grades are carried largely as a traction carpet in the river and beach environments and are carried largely by saltation in the eolian environment. Sorting and abrasion are the causes of areal variation in the mean sphericity and mean roundness of a population, respectively. Selective sorting causes nonnormality in the sphericity distribution, whereas roundness distribution is originally nonnormal. Selective sorting is most effective in the eolian, less so in the beach and least so in the river environment, and consequently sphericity-nonnormality constitutes a valuable parameter for environmental discrimination. Intensity of rounding is highest in the beach, less in the eolian and least in the river environments. Due to its wide variation from environment to environment the mean roundness constitutes a valuable parameter for differentiating different environments. Sphericity variables contribute more to the discrimination of environments than roundness variables.