Exsolution textures in orthopyroxene in aluminous granulites as indicators of UHT metamorphism: New evidence from the Eastern Ghats Belt, India

Highly aluminous orthopyroxene, coexisting with sapphirine, cordierite, sillimanite, quartz and garnet in various combinations, constitute granoblastic mosaic peak metamorphic assemblages in aluminous granulites from three localities in the Eastern Ghats Belt, India. Orthopyroxene contains four types of intergrowths: (a) involving sapphirine with or without cordierite, (b) involving spinel, but without sapphirine, (c) involving cordierite, but without sapphirine and spinel, and (d) involving garnet, without sapphirine, spinel or cordierite. On the basis of textural and compositional data, origin of the intergrowths is ascribed to breakdown of Mg-Tschermak component, locally also involving Fe- and Ti-Tschermak. An attempt is made to compute the “pre-breakdown” compositions of orthopyroxene by image analysis, which shows maximum Al₂O₃ content of 13.4 wt.% in the pristine orthopyroxene. Geothermometry, phase equilibria consideration and application of existing experimental data on alumina solubility in orthopyroxene coexisting with sapphirine and quartz, collectively indicate extreme thermal conditions of metamorphism (> 1000 °C) for the studied assemblages. This re-affirms the notion that Al₂O₃ solubility in orthopyroxene is the most powerful indicator of UHT metamorphism (Harley, S.L., 2004. Extending our understanding of ultrahigh temperature crustal metamorphism. J. Mineral. Petrol. Sci. 99, 140–158). The intergrowths are considered to have formed due to cooling from the thermal peak spanning a temperature range of approximately 150 °C. Appearance of diverse types of intergrowths is probably related to subtle differences in bulk composition, particularly Fe:Mg ratios.     

Exsolution textures of orthopyroxene and clinopyroxene in high-grade BIF of the Voronezh Crystalline Massif: evidence of ultrahigh-temperature metamorphism

Archaean banded iron formation (BIF) of the Voronezh Crystalline Massif (VCM) contains coexisting clino‐ and orthopyroxenes with exsolution textures. The pyroxene in the VCM BIF is found in two generations, with only the first generation containing such textures. Clinopyroxene contains large (up to 5–10 μm) (0 0 1) orthopyroxene (Opx1L) lamellae in a host clinopyroxene (Cpx1H). This host Cpx, in turn, exsolves into thin (～1 μm) (1 0 0) lamellae of orthopyroxene (Opx2L) and clinopyroxene (Cpx2H). Orthopyroxene exhibits similar exsolution textures with large (up to 50 μm) (0 0 1) clinopyroxene (Cpx1L) lamellae developed in a host orthopyroxene (Opx1H). This clinopyroxene Cpx1L shows further exsolution of thin (1 0 0) Opx2LL lamellae and clinopyroxene (Cpx2LH). Point microprobe analysis, raster‐mode microprobe analysis, and microprobe element mapping of mineral grains with a large number of point analysis were used to determine the composition of the exsolution products and the primary chemistry of the coexisting clinopyroxene (CaO = 14.86–17.26 wt%) and pigeonite (CaO = 4.45–6.23 wt%). These pyroxenes crystallized during the peak of metamorphism, and application of the Lindsley geothermometer suggested that they formed at extremely high temperatures of about 1000 °C. Primary very dense CO₂‐rich fluid inclusions (ρ = 1.152 g cm^?3, T_h = ?49.2 °C) were discovered for the first time in these BIF. With these data, the metamorphic pressure was estimated as 10–11 kbar (depth 36–40 km). Such ultrahigh temperature–high pressure (UHT–HP) conditions for the regional metamorphism of the Precambrian BIF have previously been reported only for Archaean meta‐ironstone from the Napier Complex (Enderby Land, Antarctica). They give an insight into the peak metamorphic conditions of the BIF of the VCM, their burial under thickened continental crust during this period of Earth evolution and suggest a more complicated multistage metamorphic and tectono‐thermal history for the region than has previously been postulated.     

Exsolution in augite from the Skaergaard Intrusion

The exsolution phenomena of augite from Ferrogabbro 4430 of the Skaergaard Intrusion were examined in detail by single crystal X-ray diffraction and heating experiments to study the stepwise exsolution process. In the augite crystals, five different phases were detected: pigeonite (001), pigeonite (100), orthopyroxene (a), orthopyroxene (p) and a small amount of clinoamphibole. The two different pigeonites nearly share the corresponding (001) and (100) planes with the host. Orthopyroxene (a) and orthopyroxene (p) have (100) in common with the host and with exsolved pigeonite (001), respectively. Clinoamphibole was observed in the form of rather weak reflections in many crystals. It has (010) in common with the host.A large number of augite crystals exhibited a pigeonite (001) phase with curved, rotated reflections and diffuse streaks along the a* direction in (h0l) precession photographs. It appears that these streaks are related to orthopyroxene (p). Orthopyroxene (p) seems to be crystallized from pigeonite (001) by nucleation at (100) stacking fault planes (inverted pigeonite). Pigeonite (100) may be formed at growth ledges between augite host and exsolved orthopyroxene (a) at a later stage of exsolution to stabilize the boundaries.From the X-ray diffraction profiles and the results of the heating experiments, a possible exsolution sequence is suggested. Clinoamphibole appears to be a product of alteration at the latest stage of the exsolution process. It seems to be related to particular conditions of partial water pressure.     

The effect of aluminum and water on the development of deformation fabrics of orthopyroxene

The effect of alumina and water solubility on the development of fabric in orthopyroxene in response to simple shear deformation has been investigated at a pressure of 1.5 GPa and a temperature of 1,100 °C using the D-DIA apparatus. The microstructure observations at these conditions indicate that dislocation glide is the dominant deformation mechanism. In MgSiO₃ enstatite and hydrous aluminous enstatite, partial dislocations bounding the stacking faults in [001] glide parallel to the (100) (or) the (100) [001] slip system. Electron backscattered diffraction analysis of anhydrous aluminous enstatite, however, indicates operation of the (010) [001] slip system, and microstructure analysis indicates dislocation movement involving [001] on both (100) and {210} planes. The strong covalent bonding induced by the occupation of M1 and T2 sites by Al could have restricted the glide on (100), activating slip on {210}. The resulting seismic anisotropies (~2 %) in orthopyroxene are weaker compared to olivine (~9.5 %), and reduced anisotropy can be expected if orthopyroxene coexists with olivine. Weak anisotropy observed in stable cratonic regions can be explained by the relatively high abundance of orthopyroxene in these rocks.     

Kinetics and Mechanism of Pyrite Exsolution from Pyrrhotite

Pyrite exsolution from pyrrhotite at 325 °C occurs by heterogeneousnucleation of pyrite and subsequent growth by volume diffusionof iron away from the nuclei. Sulfur atoms are required to moveonly short distances and although their diffusivity is muchlower than iron, their movement is not the rate determiningfactor. The exsolution rate is primarily dependent on the nucleationrate, which increases with the degree of supersaturation. Inaddition, the impurities in National Bureau of Standards 55diron or 500 ppm of As, Sb, or Bi retard the exsolution rateby two or more orders of magnitude at 325 °C. This reductionis primarily the result of a lower nucleation rate and is believedto be due to a decreased vacancy mobility caused by a high bindingenergy of vacancies with the slow diffusing impurities. Thusthe strain associated with the nucleation cannot be as easilyrelieved. The impurities may also reduce the growth rate bycausing a decrease in mobility of the pyrite interface. The most important general aspect of this study is the effectof a few hundred parts per million of an impurity on the exsolutionrate of this reaction. This suggests that the rate of othermineral reactions may also be dependent on impurities; however,the importance of this factor in any particular mineral reactionrequires experimental verification.     

Crystal bending,subgrain boundary development,and recrystallization in orthopyroxene during granulite-facies deformation

A prominent feature of a granulite-facies shear zone from the Hidaka Main Zone (Japan) is the folding of orthopyroxene (opx) porphyroclasts. Dislocation density estimated by transmission electron microscope (TEM) and chemical etching in homogeneously folded domains is too low to account for the amplitude of crystallographic bending, leading us to propose a model similar to “flexural slip” folding, where folded layers are micrometer-wide opx layers between thin planar clinopyroxene (cpx) exsolutions. Extension (compression) in the extrados (intrados) of the folded layer is accommodated by dislocations at the cpx–opx interfaces. Alternatively to distributed deformation, crystal bending also localizes in grain boundaries (GBs), mostly oriented close to the (001) plane and with various misorientation angles but misorientation axes consistently close to the b-axis. For misorientation up to a few degrees, GBs were imaged as tilt walls composed of regularly spaced (100)[001] dislocations. For misorientation angles of 7°, individual dislocations are no longer visible, but high-resolution TEM (HRTEM) observation showed the partial continuity of opx tetrahedral chains through the boundary. For 21° misorientation, the two adjacent crystals are completely separated by an incoherent boundary. In spite of these atomic-scale variations, all GBs share orientation and rotation axis, suggesting a continuous process of misorientation by symmetric incorporation of (100)[001] dislocations. In addition to the dominant GBs perpendicular to the (100) plane, boundaries at low angle with (100) planes are also present, incorporating dislocations with a component of Burgers vector along the a-axis. The two kinds of boundaries combine to delimit subgrains, which progressively rotate with respect to host grains around the b-axis, eventually leading to recrystallization of large porphyroclasts.     

冀东地区变质岩中斜方辉石的研究

通过对冀东地区３４个斜方辉石榈的研究得出结论：斜方辉石由早、晚两期变质作用形成。斜方辉石的化学成分变化主要与变质作用的条件及岩中共存矿物种类有关，同时也受原岩成分的控制。据温压计的估算结果，早期变质作用的温度为８１７－９３７℃、压力为０．９９－１．１１ＧＰａ；晚期变质作用的温度为７４７－８３４℃、压力为０．９５ＧＰａ。     

COSAD: A computer aided system for automatization of deformation measurements

The COSAD measurement system detects deformations in structures, rock faces and extended earth-surface areas. The instrument is semi-automatic with electronic data-acquisition parts. In this paper, the theory and the accuracy, the measurement method and examples of applications are given. Results from structural deformation and a computer simulation of deformation in large extended areas are also included.     

Exsolution in terrestrial and lunar plagioclases revealed by chemical etching

A new chemical etching technique reveals fine scale exsolution structures on polished surfaces of plagioclases in the bytownite and labradorite range. A very obvious blue and locally brown stain is produced (by the action of citric acid) in growth zones of a restricted range of compositions on etched surfaces. The blue-stained zones contain very fine grooves, corresponding to the traces of thin exsolution lamellae. Exsolution features have been revealed in this way in plagioclases in a terrestrial diabase, lunar basalts and an amphibolite. Compositions of the exsolved areas found by microprobe analysis represent averages of the matrix and exsolution lamellae. Analytical methods with higher spatial resolution are required to determine the composition of the lamellae. This etching method permits easy optical identification of compositional zoning and of fine scale exsolution over relatively large areas and is therefore complementary to other techniques.     

An Experimental Study of Nepheline-Kalsilite Exsolution

The nepheline-kalsilite exsolution reaction was studied isothermallybetween 400 and 700°C. Under nonaqueous conditions the mechanisminvolves nucleation of kalsilite and growth by diffusion ofthe alkalis. As predicted by simple nucleation theory, the nucleationrate and hence the over-all exsolution rate are strongly dependenton the supersaturation of the nepheline. A decrease in temperatureat constant composition increases the supersaturation and therebythe nucleation rate. This increased nucleation rate is opposedby the decrease in the growth rate due to slower volume diffusion.At a supersaturation of more than 8–10 mole per cent thenumber of nuclei is large and the over-all exsolution rate isdetermined primarily by the growth rate. The activation energyfor growth is 28 kcal/mole. An increase of two kilobars in thehydrostatic pressure has little effect on the kinetics of thereaction. Under nonhydrostatic conditions the exsolution rateincreases significantly because the nucleation rate is faster. Under hydrothermal conditions the ‘exsolution’ rateis approximately two orders of magnitude faster due to a modificationin the mechanism. Partial dissolution of the original solidsolution in distilled water creates a condition of nonequilibriumin which the fluid is sodium-rich. Rapid alkali exchange eliminatesthis condition but produces the equilibrium compositions ofthe solids because kalsilite nucleates and grows in contactwith the fluid. The experimental evidence for this mechanismincludes X-ray diffraction data showing a gradual change inthe composition of the initial supersaturated solid, essentiallyidentical activation energies for growth under aqueous and nonaqueousconditions, and a lower percentage of oxygen isotope exchangethan ‘exsolution’ in the same experiment.     

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.     

Deformation and recrystallisation of orthopyroxene from the giles complex, Central Australia

The microstructures and fabrics of naturally deformed orthopyroxenites from the Giles Complex, central Australia are described in some detail. Coarse grained enstatite is deformed and recrystallised where it is incorporated in planar gneissic (mylonite) zones which show a gradation in strain from their margins inwards. Deformation takes place by slip on (100) [001] to produce regular lattice bending and kinking, and recrystallisation takes place preferentially along grain boundaries and kink band boundaries (KBB's). The microstructures and preferred orientation of recrystallised grains along KBB's are interpreted in terms of possible nucleation mechanisms, and both bulge nucleation (Bailey and Hirsch, 1962) and subgrain coalescence (Hu, 1963) are likely contributors. Electron microprobe analyses have indicated a small compositional difference between new (recrystallised) and host (deformed) grains, which is related to the nucleation mechanism. The total preferred orientation patterns for host and new grains are discussed with special reference to previous measurements and interpretations.     

Some kinetics of bronzite orthopyroxene dissolution

The kinetics of bronzite orthopyroxene dissolution were investigated in HClKCl solutions having a total concentration of 0.1 M, over the pH range 1–4.5, at temperatures between 42 and 1°C. Dissolution of the pyroxene was incongruent and followed a parabolic rate law. The activation energy of the reaction is 10.5 ± 0.6 kcal mole^?1. The rate dependence on hydrogen ion activity is one-half order. The rate of dissolution is unaffected by substitution of sodium for potassium, or sulfate or nitrate for chloride anion, or by addition of citrate or acetate ions. However, traces of fluoride increase the dissolution rate. The rates observed are one to two orders of magnitude slower than those previously reported by Luceet al. (1972) for dissolution of enstatite.     

Electron Petrography of Exsolution Textures in an Iron-Rich Clinopyroxene

Ion-thinned samples of an iron-rich augite in an inninmoritepitchstone from Ardnamurchan, Scotland, have been examined bytransmission electron microscopy at 100 and 1000 kV. Eutecticcrystallization of augite and pigeonite occurred and subsequentlysemi-coherent irregularly-shaped regions and coherent (001)lamellae of pigeonite were exsolved in the augite. Nucleationoccurred heterogeneously at grain boundaries and subsequentgrowth of the lamellae proceeded by the migration of ledgesalong the (001) interface. The analytical electron microscope EMMA-4 has been used to analysethe exsolved pigeonite lamellae. Their composition was foundto be similar to that of the co-existing primary pigeonite. It is concluded that the augite cooled through the solvus slowlyenough for heterogeneous nucleation and growth to occur whereaspyroxenes which cool rapidly unmix by the mechanism of spinodaldecomposition.     

攀枝花钒钛磁铁矿中纳-微米尖晶石的出溶序次及其成因机制

钛磁铁矿内部尖晶石出溶体的成分组成和形成机制对估算磁铁矿固溶体的成分以计算铁钛氧化物的氧逸度-温度具有重要意义.为了探究攀枝花钒钛磁铁矿中尖晶石的成因及形成机制,运用岩相观察和各种微区原位观测手段,系统研究了各类尖晶石的矿物学特征.尖晶石的粒度在纳微米之间,有3种类型:第1种为钛磁铁矿晶界处不规则的大颗粒尖晶石,Mg#为60~70;第2种为钛磁铁矿颗粒内部的粒状尖晶石,Mg#为71~77;第3种为沿钛磁铁矿（100）方向定向分布的尖晶石片晶,Mg#为75~77.3类尖晶石分别与磁铁矿主晶具有相同的取向关系:{111}_Mag//{111}_Spl,{110}_Mag//{110}_Spl和{100}_Mag//{100}_Spl.3类尖晶石均是磁铁矿主晶的出溶体,钛磁铁矿晶界处及其内部的粒状尖晶石的形成与某些晶体缺陷关系密切,是在降温过程中较早出溶的产物,尖晶石片晶在两者之后以旋节分解出溶形成.      

Exsolution of Ca-amphibole from glaucophane and the miscibility gap between sodic and calcic amphiboles

Transmission and analytical electron microscopy (TEM/AEM) of glaucophane from glaucophane + Ca-amphibole-bearing blueschist and eclogite from two Vermont localities (Ecologite Brook and Tillotson Peak) and one California locality (Cazadero) has revealed the first evidence from exsolution for the miscibility gap between sodie and calcic amphiboles. The Tillotson Peak samples and the Cazadero samples contain coarsegrained glaucophane—actinolite pairs, while the Eclogite Brook samples contain coexisting glaucophane and actinolitic hornblende. Ca-rich glaucophanes contain abundant fine-scale lamellae of Ca-rich amphibole. These lamellae are usually oriented near (100) and . High-resolution TEM (HRTEM) images show them to be coherent. The exsolution lamellae are so narrow, beyond the resolution of AEM, that their true Ca contents are obscured by analytical contributions from the surrounding host. The AEM data suggest that the lamellae are either winchite or actinolite, depending on the true Ca concentration. In most cases, the exsolution lamellae have very curved interfaces and show variable orientation. This is attributed to the close similarity of unit-cell parameters for the two amphiboles. Three-dimensional optimal phase-boundary calculations using EPLAG (Fleet 1982) show that the observed 100 and orientations are consistent with the minimization of area strain between the two lattices along the interface. Some samples show evidence for incipient exsolution in the form of homogeneously distributed, fine-scale precipitates. These results suggest that Ca-rich glaucophanes from other glaucophane—actinolite assemblages may be exsolved at the TEM scale. The coexisting amphiboles from Eclogite Brook also have been studied using the electron microprobe (EMP). The compositional gap defined by the Eclogite Brook pairs is consistent with previously reported results, but shows a wider break along the glaucophane—actinolite pseudobinary join, suggesting very limited solid solution up to temper-atures of about 500–550°C. The glaucophanes are relatively poor in Ca, except for one anomalous grain containing Ca-amphibole lamellae. Ferric iron, estimated by normalization to fixed cation numbers, is strongly partitioned into the actinolitic hornblende and the glaucophanes are very poor in the riebeckite component.     

Strength of single-crystal orthopyroxene under lithospheric conditions

Creep strength of oriented orthopyroxene single crystals was investigated via shear deformation experiments under lithospheric conditions [P (pressure) = 1.3 GPa and T (temperature) = 973–1,373 K]. For the A-orientation (shear direction [001] on (100) plane), the samples have transformed completely to clinoenstatite and much of the deformation occurred after transformation. In contrast, for the B-orientation (shear direction [001] on (010) plane), samples remained orthoenstatite and deformation occurred through dislocation motion in orthoenstatite. The strength of orthopyroxene with these orientations is smaller than for olivine aggregates under all experimental conditions. Flow of the B-orientation samples is described by a power-law, and the pre-exponential constant, the apparent activation energy, and the stress exponent are determined to be A = 10^−9.5 s⁻¹·MPa^−4.2, Q = 114 kJ/mol and n = 4.2. However, for the A-orientation, the results cannot be fit by a single flow law and we obtained the following: A = 10^8.9 s⁻¹·MPa^−3.0, Q = 459 kJ/mol and n = 3.0 at high temperatures (≥1,173 K), and A = 10^−27.4 s⁻¹·MPa^−14.3, Q = 296 kJ/mol and n = 14.3 at low temperatures (<1,173 K). The stress exponent for the low-temperature regime is high, suggesting that deformation involves some processes where the activation energy decreases with stress such as the Peierls mechanism. Our study shows that orthopyroxene with these orientations is significantly weaker than olivine under the lithospheric conditions suggesting that orthopyroxene may reduce the strength of the lithosphere, although the extent to which orthopyroxene weakens the lithosphere depends on its orientation and connectivity.     

Reactions involving hydration of orthopyroxene in anorthosite-gabbro

Hydration of orthopyroxene (En₆₀) in contact with plagioclase (An₃₆) leads to oronas of the type orthopyroxene-cummingtonite-edenite + quartz-edenitic hornblende. The compositions of the minerals involved indicate that kyanite needles in the adjacent plagioclases form as part of the reaction: orthopyroxene (En₆₀) + plagioclase (An₃₆) + H₂O→ cummingtonite + edenite/edenitic hornblende + plagioclase (< An₃₆) + kyanite + quartz. The reaction proceedings consumption first of the orthopyroxene and later of the cummingtonite to produce a corona structure consisting of a core of intergrown edenite and quartz surrounded by edenitic hornblende. Corona evolution of this type is controlled by diffusion.     

单斜辉石中石英出溶体的显微结构和成因机制

矿物出溶结构保存有早期母体矿物的物理化学条件信息,对其开展研究不仅有助于了解寄主岩石的来源深度,而且有助于研究减压折返的动力学演化过程.在世界许多高压-超高压带的榴辉岩和石榴辉石岩中,人们普遍发现单斜辉石中有定向排列的针状或棒状SiO₂析出物,其矿物相主要为α石英,有时会伴生钙质角闪石等含水矿物.这些定向针状或棒状体通常平行于单斜辉石c[001]轴方向延伸,石英长轴可以为其c[0001]轴或a[1120]轴.电子背散射衍射(EBSD)测试结果表明,多数石英(96%)析出物与寄主单斜辉石具有结晶学取向关系:(1) 50%的石英c轴平行,并且[0001]_Qz//[001]_Cpx;(2)35%的石英至少有一个a轴平行,并且[1120]_Qz//[001]_Cpx;(3)11%的石英至少有一个s{1121}面平行,并且(1121)_Qz//(100)_Cpx.钙质角闪石析出物与寄主单斜辉石也具有密切结晶学取向关系:(100)_Amp//(100)_Cpx、[010]_Amp//[010]_Cpx、[001]_Amp//[001]_Cpx、[100]_Amp∧[100]_Cpx≈32°.上述定量显微构造证据表明,单斜辉石中定向石英析出物是由出溶作用所形成,并且多数石英出溶体形成于α石英稳定域.已有高温高压实验研究数据表明,单斜辉石中空位的形成和钙埃斯科拉组分(CaEs)的含量均受化学组成、压力、温度等多种因素综合影响:单斜辉石中CaEs含量对化学组成非常敏感,并受到共生矿物体系中自由SiO₂相和蓝晶石的共同缓冲;相同化学组成和等压条件下,CaEs含量总体上随温度升高缓慢降低;相同化学组成和等温条件下,CaEs含量在＜6 GPa区间随压力升高而增加,在＞6 GPa区间随压力升高而降低.单斜辉石定向SiO₂析出物的形成可能涉及多种因素,高压只是其中必要条件之一.榴辉岩质单斜辉石中“石英±角闪石”析出物很可能形成于开放体系,与熔流体活动密切相关,涉及多阶段物质扩散、晶体成核生长、重结晶、退变质反应等复杂作用过程.单斜辉石中定向SiO₂析出物的显微结构特征并非超高压岩石的必要条件,这种特殊显微结构也不能作为证明超高压的充分条件.      

Electron microscopical study of garnet exsolution in orthopyroxene

By means of electron-optical methods the structure and orientation of lamella-like garnet exsolutions in an orthopyroxene from the eclogite of Gilsberg, Saxony Granulite Complex, are investigated. It can be shown that besides the macroscopic and microscopic submicroscopic exsolution lamellae also exist; they have widths up to nearly 5 nm. The intergrowth conditions are such shat (100)_op is parallel to {100}_gr and [001]_op parallel to 〈110〉_gr. The lamellae are coherent, but sometimes extended dislocation arrangements are found in microscopic lamellae which are caused by a deformation. These investigations point to formation of the rock as a result of a solid state reaction under high pressure conditions with subsequent diffusive-dissociative exsolution of garnet from primary pyroxene.