Diaplectic labradorite glass from the manicouagan impact crater: II. X-ray diffraction studies and structural model

The structural state of diaplectic labradorite glass (≈An₅₈) from the Manicouagan impact crater and of its fusion-formed glass analog have been investigated by X-ray diffraction studies. The experimental X-ray intensity distribution patterns indicate that the diaplectic and fusion-formed glasses are structurally rather similar, the former being apparently slightly less disordered. Theoretical X-ray distribution curves have been calculated using the structure of high albite as a quasi-crystalline model of the glass structure. The experimental and theoretical curves show fair similarity when the calculations are based on the complete unit cell. It is inferred therefore, that the structures of both kinds of glasses possess an average short range order comparable to that in high albite and extending to about the dimensions of the unit cell. In addition, the experimental X-ray scattering pattern and X-ray Debye-Scherrer transmission photographs of the diaplectic glass reveal the presence of relics up to about 8 nm in size of the previous crystalline lattice of the primary labradorite. The present results support Grady's shear band model according to which diaplectic glass may represent the quench product of a shock-generated high-density melt frozen in prior to total pressure release.     

Enthalpy of diaplectic labradorite glass

Enthalpies of solution in molten 2PbO·B₂O₃ at ～988 K have been measured for diaplectic labradorite glass from the Manicouagan impact crater and a fused glass formed from the same material. The enthalpies of solution of the diaplectic and fusion-formed glasses are 4,347 and 2,023 cal mol^?1, respectively. The more endothermic enthalpy of solution of the diaplectic glass indicates a greater relative energetic stability of about 2.3 kcal mol^?1. The data are consistent with Diemann and Arndt's (1984) structural model that suggests the diaplectic glass is more ordered than fusion-formed glass and with the presence of crystallites. Comparison of data to enthalpies of solution of crystalline labradorite (Carpenter et al. 1985) indicates a maximum percentage of crystalline relics of ～15–18%, also consistent with Diemann and Arndt's (1984) estimate of <17%. Thus the diaplectic glass is intermediate in thermochemical properties between normal glass and crystal (much closer to glass) and does not represent any state more unstable than normal fusion-formed glass.     

Raman microprobe study of synthetic diaplectic plagioclase feldspars

Raman microprobe spectra were made on three post shock, diaplectic plagioclase feldspars. Optical and X-ray diffraction studies indicated that feldspars maintained a partially or totally crystalline state after having passed through the mixed phase zone of Hugoniot response to shock waves (15–38 GPa). The appearance of uniquely glass-type spectra occurs at different shock pressures for each specimen according to its atomic structural arrangement, below 38 GPa for mosaic structured labradorite, near 40 GPa for anorthite and above 50 GPa for the highly ordered low albite. The diaplectic anorthite and labradorite glasses give spectra which indicate the presence of two glass types. Shifts in the band envelope frequencies compared to spectra of fused glass and statically pressure densified glass suggest that these glasses have specific structural arrangements. These differences suggest that the shock and fusion glass-forming processes are not exactly identical. The results from material shocked in the mixed phase region of Hugoniot response show that the phase transitions are effected at different pressures depending upon the feldspar structural type.     

Stoßwellenmetamorphose des Anorthosits vom Manicouagan Krater,Québec,Canada

The central anorthosite peak of the Maniconagan crater displays characteristic shock deformation and transition phenomena in plagioclase feldspars, scapolithe, apatite and other maphitic minerals. The optical orientation of plagioclases is determined. With increasing shock, a trend to a highly disordered structure of the plagioclase lattice can be observed. Rock fracturing occurs at low pressures. At higher pressures different kinds of isotropisation features and planar deformation structures in plagioclase, scapolithe and apatite can be distinguished. These planar elements can be interpreted as glide planes of low crystallographic indices, set in motion during shock compression by plastic deformation. Their optical orientation is measured. At very high pressures a completely isotropic phase, the s. c. diaplectic glass is formed. The physical properties of diaplectic plagioclase crystals and diaplectic glass are determined which are different from those of an unshocked crystal and its molten plagioclase glass of the same chemical composition. The diaplectic plagioclase phases are apparently mixed phases of molten glass and normal crystal. This can be proved by x-ray and infrared absorption studies. These results are correlated to shock recovery experiments and hugoniot states of plagioclase. All shock effects of plagioclases are classified into three groups according to the low pressure regime, mixed phase regime and high pressure regime. The low pressure regime is characterised by strong fracturing, the mixed phase regime by the development of planar elements, which were transformed during shock compression into the high pressure plagioclase phase with hollandite structure. The latter is converted into diaplectic glass after pressure release. The high pressure regime is characterised by complete transformation of plagioclase into the high pressure phase, which is unstable and reverts completely to the amorphous phase (diaplectic glass or maskelynite) at zero pressure density.

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Meinem verehrten Lehrer, Herrn Prof. Dr. W. v. Engelhardt, danke ich für die Unterstützung bei der Bearbeitung des Themas. Herrn Dr. D. Stöffler danke ich für klärende Diskussionen und Ratschläge. Dem Ministère des Richesses Naturelles, Québec, Canada, sowie Herrn M.S. J. Murtaugh sei für die großzügige Unterstützung bei den Geländearbeiten im Manicouagan-Krater gedankt. Die Deutsche Forschungsgemeinschaft hat die Arbeit finanziell unterstützt.     

Effects of shock pressures on calcic plagioclase

Samples of single crystal calcic plagioclase (labradorite, An63, from Chihuahua, Mexico) have been shock-loaded to pressures up to 496 kbar. Optical and electron microscopic studies of the recovered samples show the effects of increasing shock pressures on this mineral. At pressures up to 287 kbar, the recovered specimens are still essentially crystalline, with only a trace amount of optically unresolvable glass present at 287 kbar. Samples recovered after shock-loading to pressures between 300 and 400 kbar are almost 100% diaplectic glasses; that is formed by shock transformation presumably in the solid-state. Above about 400 kbar, glasses with refractive indices similar to thermally fused glass were produced. The general behavior of the index of refraction with shock pressures agrees closely with previous work, however, the absence of planar features is striking. At pressures less than 300 kbar, the most prominent physical feature is the pervasive irregular fracturing caused by the shock crushing, although some (001) and (010) cleavages are observed. No fine-scale shock deformation structures, i.e. planar features, were noted in any of the specimens. We conclude, in contrast to previous studies of shocked rocks that planar features are not necessarily definitive shock indicators, in contrast to diaplectic glass (e.g., maskelynite) and high-pressure phases, but are rather likely indicative of the local heterogeneous dynamic stress experienced by plagioclase grains within shocked rocks.     

Shock-induced amorphous textures in plagioclase,Manicouagan, Quebec,Canada

The textural relationships and structural states of optically isotropic labradorite from the Manicouagan, Quebec, impact structure have been examined by light (optical) and transmission electron (TEM) microscopy. Two distinct diaplectic glasses have been recognized based on their contrasting morphology, timing and the inferred modes of formation. The earliest isotropic bands and grain-scale isotropism (maskelynite) optically exhibit a gradational,in situ transformation from crystalline plagioclase with preservation of relict textures (twins, grain boundaries). The same transformation from crystalline to amorphous structure is observed in TEM to occur heterogeneously at scales on the order of the unit cell. The progressive transformation of optical properties reflects an increase in the volume fraction and eventual coalescence of these amorphous units. This maskelynite-type diaplectic glass is interpreted to form in the solid-state directly from crystalline material during the compressional phase of the shock wave. The other isotropic material occurs in spatially discrete tensiongashes and planar deformation features (PDFs) that overprint the maskelynite-type glass. This second type of diaplectic glass (PDF-type) is developed homogeneously within a given glass band and exhibits sharp crystal-glass boundaries, in contrast to the gradational boundaries of the maskelynite-type glass. PDF-type glass is interpreted to form by melting in tensional release zones during passage of the rarefaction wave. These observations emphasize the ability of naturally shocked rocks to preserve subtle evidence of variations in the shock process from highly transient events.     

Shock-effects on the K-Ar system of plagioclase feldspar and the age of anorthosite inclusions from North-Eastern Minnesota

Artificial shock pressures up to 52.5 GPa have no influence on the K-Ar system of plagioclase feldspar. The ⁴⁰Ar-³⁹Ar analysis of feldspar (labradorite An₆₇ from anorthosite of North-Eastern Minnesota) shocked up to 45 GPa—in vacuo, to prevent massive entrapment of atmospheric argon-shows that the age spectra and the argon diffusion properties remain unaltered. Similar feldspar samples (labradorite An₅₁ from Nain, Labrador), shocked in air up to 52.5 GPa and dated by the conventional K-Ar method, also yield the same age as unshocked samples but with a higher atmospheric argon contribution. The Minnesota anorthosite has an ⁴⁰Ar-³⁹Ar age of 1075 ± 10 m.y. No information on a possible previous history of the anorthosite became apparent.     

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An₆₈-An₆₀), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr_i: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An₆₀-An₃₈). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm³ during early fractionation of troctolites and then decreased continuously to 2.16 g/cm³ with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An₆₉-An₆₀) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An₆₀) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.     

http://dx.doi.org/10.1016/j.gsf.2016.11.007

Lunar anorthosite is a major rock of the lunar highlands,which formed as a result of plagioclasefloatation in the lunar magma ocean(LMO).Constraints on the sufficient conditions that resulted in the formation of a thick pure anorthosite(mode of plagioclase 95 vol.%) is a key to reveal the early magmatic evolution of the terrestrial planets.To form the pure lunar anorthosite,plagioclase should have separated from the magma ocean with low crystal fraction.Crystal networks of plagioclase and mafic minerals develop when the crystal fraction in the magma(φ) is higher than ca.40-60 vol.%,which inhibit the formation of pure anorthosite.In contrast,when φ is small,the magma ocean is highly turbulent,and plagioclase is likely to become entrained in the turbulent magma rather than separated from the melt.To determine the necessary conditions in which anorthosite forms from the LMO,this study adopted the energy criterion formulated by Solomatov.The composition of melt,temperature,and pressure when plagioclase crystallizes are constrained by using MELTS/pMELTS to calculate the density and viscosity of the melt.When plagioclase starts to crystallize,the Mg~# of melt becomes 0.59 at 1291 C.The density of the melt is smaller than that of plagioclase for P 2.1 kbar(ca.50 km deep),and the critical diameter of plagioclase to separate from the melt becomes larger than the typical crystal diameter of plagioclase(1.8-3 cm).This suggests that plagioclase is likely entrained in the LMO just after the plagioclase starts to crystallize.When the Mg~# of melt becomes 0.54 at 1263 C,the density of melt becomes larger than that of plagioclase even for 0 kbar.When the Mg~# of melt decreases down to 0.46 at 1218 C,the critical diameter of plagioclase to separate from the melt becomes 1.5-2.5 cm,which is nearly equal to the typical plagioclase of the lunar anorthosite.This suggests that plagioclase could separate from the melt.One of the differences between the Earth and the Moon is the presence of water.If the terrestrial magma ocean was saturated with H_2O,plagioclase could not crystallize,and anorthosite could not form.     

A HFSE- and REE-enriched ferrodiorite suite from the Bolangir Anorthosite Complex, Eastern Ghats Belt, India

The massif-type anorthosite complex at Bolangir in the northern part of the Eastern Ghats belt occurs in a milieu of predominantly supracrustal granulite-grade rocks. The massif is separated from the host gneisses by coarse-grained garnetiferous granitoid gneisses which are interpreted as coeval crustallyderived melts. Melanocratic ferrodiorite rocks occur at the immediate contact with the anorthosite massif which they intrude in cross-cutting dikes and sheets. The emplacement age of the anorthosite diapir and the associated igneous suites is deemed to be pre-D₂. Recrystallization of the igneous assemblages of the ferrodiorite suite (750–800°C, 7–8kbar, ) during a period of near-isobaric cooling from the igneous crystallization stage to the regionalP-T regime led to extensive development of coronitic garnet at the interface of plagioclase phenocrysts with the mafic matrix assemblage (opx + fay + cpx + ilm ± amph, bio). Abundant accessory phases are zircon, apatite and thorite. The mafic phases have extremely ferrous compositions (X_Fe gar: 0.93-0.87, fay: 0.90-0.87, opx: 0.80-0.60, cpx: 0.70-0.47, amph: 0.81-0.71) reflecting the low Mg-number (16-8) of the rocks. Compared to worldwide occurrences of similar rocks, the Bolangir ferrodiorites (SiO₂ 36–58 wt.%, FeO*: 39-10 wt.%) are characterized by exceptionally high concentrations of HFSE and REE (TiO₂: 4.8-1.0 wt.%, P₂O₅: 1.7-0.5 wt.%, Zr: 5900-1300 ppm, Y: 240-80 ppm, La: 540-100 ppm, Ce: 1100-200 ppm, Yb: 22-10 ppm, Th: 195-65 ppm). Well defined linear variation trends for major and trace elements reflect progressive plagioclase accumulation towards the felsic members of the suite. The ferrodiorites are interpreted to represent residual liquids of anorthosite crystallization which after segregation and extraction from the ascending diapir became enriched in HFSE and REE through selective assimilation of accessory phases (zircon, monazite, apatite) from crustal felsic melts. Ferromonzodioritic rock presumably formed through hybridization between the ferrodiorite and overlying felsic melts.     

Syn-emplacement recrystallization and deformation microstructures in the Poe Mountain anorthosite,Wyoming

 Plagioclase recrystallization microstructures and petrofabrics in the unmetamorphosed, 1.43 Ga Poe Mountain anorthosite, Wyoming, are indicative of very high-temperature deformation and recrystallization during the emplacement of the anorthosite body. The Poe Mountain anorthosite consists of a core of recrystallized, massive anorthosite transitional with a series of layered anorthositic cumulates at the margin of the intrusion. Irregular grain boundaries and dissected grain microstructures in the massive core and transitional anorthosites suggest that the anorthositic rocks recrystallized by “fast” grain boundary migration and possibly subgrain rotation recrystallization, at very high temperatures (≈1050°C) during emplacement of the intrusion in the mid-crust (3 kbar). The deformation and recrystallization of the Poe Mountain anorthosite was continuous from subliquidus to subsolidus temperature conditions during the emplacement of the intrusion. Anorthosites with the lowest modal percentages of ferromagnesian minerals and Fe-Ti oxides are always the most recrystallized. This suggests that melt interstitial to the plagioclase-crystal framework was removed during deformation and recrystallization of the intrusion. Bulging of plagioclase grain boundaries around Fe-Ti oxides together with deformed oikocrystic ferromagnesian minerals and plagioclase chadacrysts indicate that the deformation and recrystallization of the intrusion continued after the crystallization of the interstitial melt minerals. Received: 28 February 1995/Accepted: 20 July 1995     

CO2-rich fluid inclusions in the Whitestone Anorthosite: implications for the retrograde history of the Parry Sound Shear Zone, Grenville Province, Canada

The Whitestone Anorthosite (WSA), located in southern Ontario, underwent granulite facies metamorphism during the Grenville orogeny at 1.16 Ga. During the waning stages of metamorphism fluids infiltrated the outer portions of the anorthosite and promoted the formation of an envelope comprised of upper amphibolite facies mineral assemblages. Also, this envelope corresponds to portions of the anorthosite that underwent deformation related to movement along a high-grade ductile shear zone. Samples from this portion of the anorthosite (the margin) contain CO₂-rich inclusions in plagioclase porphyroclasts (relict igneous phenocrysts), matrix plagioclase and garnet. These inclusions have features which normally are interpreted as indicating that they are texturally primary, but they have relatively low CO₂ densities (0.61–0.95 g cm^-3). Plagioclase from the anorthosite interior contains texturally secondary inclusions with relatively high CO₂ densities (generally from 0.99 to 1.10 g cm^-3). The high CO₂ densities suggest that the inclusions in the plagioclase of the anorthosite core formed prior to inclusions in porphyroclast minerals of the outer portions of the anorthosite, an interpretation that is apparently inconsistent with inclusion textures. This apparent paradox indicates that most fluid inclusions from the anorthosite margin were formed during, or were modified by, the dynamic recrystallization that affected this portion of the WSA. In either case, late formation or modification, the texturally primary fluid inclusions do not contain pristine samples of the peak metamorphic fluid. Furthermore, because shear-related deformation is apparently associated with entrapment of the lowest fluid densities, some strain localization persisted to relatively low temperatures (e.g. less than approximately 500° C). These results constrain a part of the retrograde P–T path for this portion of the Grenville Orogen to temperatures of approximately 400–500° C at pressures of approximately 1–2 kbar.     

Raman-spectroscopy and X-ray diffractometer studies of experimentally produced diaplectic feldspar glass

Raman vibrational spectra and X-ray diffractometer scans were obtained from experimentally shocked samples of oligoclase (An₁₉) and andesine (An₄₉). Some 11 oligoclase and 15 andesine targets were shocked between 24 and 40 GPa to address the transition from crystalline to diaplectic states and to explore differences in the structural state of diaplectic feldspar glasses (maskelynite) as a function of peak shock stress. Thy symmetrical VS (T-O-T) (T=Si or Al) stretch bands are the most persistent. They disappear, however, in the noise of an unusually strong luminescent spectrum at > 32 GPa in the oligoclase and at > 30 GPa in the andesine; i.e., at pressures where transition to diaplectic glass is complete. The Raman investigations yield a maskelynite structure that is probably one of a multitude of very small domains with some order, but with a large range of local properties on the scale of small domains, either in heterogeneous size-distribution of domains or in their detailed order, if not both. This results in a very large number of Raman photon-phonon frequencies unlike glasses derived from quenched melts. Our study corroborates conclusions by others, that diaplectic glasses may be the quench products of very dense, disordered phases that exist during shock compression and that subsequently relax to these unusually dense glasses that are only known from shock processes. An origin by relaxation of highly ordered, genuine high pressure polymorphs possessing the structure of hollandite is unlikely, as no evidence for any six-fold Si-coordination was found. Detailed luminescent emission spectra were taken of the oligoclase samples and they show disappearance of the IR band and a strengthening of the green band (the blue band could not be detected with a primary radiation of wavelength 448 nm). This supports previous views that the disappearance of IR emission is most likely caused by shock-induced changes of the crystal field near Fe³⁺ sites, rather than due to quenching by Fe²⁺. The X-ray studies were primarily intended to explore whether differences in structural states of maskelynite occur on sufficiently large scales to be detected by standard diffractometry methods. This is not the case. X-ray diffractometer patterns are grossly similar, if not identical, in samples shocked between 30 and 40 GPa and may not be used to fine-tune the shock histories of naturally produced diaplectic glasses.     

元古宙岩体型斜长岩的特征及研究现状

斜长岩是指斜长石含量>90%的岩浆岩,可分为6类。其中,岩体型斜长岩仅赋存于前寒武纪变质地体中,形成时代主要为元古宙（2.1~ 0.9 Ga）,代表地球演化史上很重要的构造-热事件。岩体呈穹隆状或层状产出,具典型堆晶结构,有含钾长石和斜长石出溶片晶的巨晶斜长石和富铝辉石。巨晶的出溶指示了岩浆由高压至低压的变压结晶过程,体现了斜长岩体深成、浅侵位的特点。关于斜长岩的源区,之前普遍认为源于幔源玄武质岩浆,而近10年来更趋向于源区为下地壳,母岩浆的成分为纹长苏长岩和铁闪长岩等新认识;其成因模式以底侵模式和地壳舌状物熔融模式最具代表性。岩体型斜长岩时空上常与奥长环斑花岗岩共生,构成AMCG（Anorthosite Mangerite Charnockite Granite）岩石组合,被认为属非造山岩浆作用的产物,可能代表大陆裂谷环境。然而,新近一些年龄结果显示,它们形成于造山作用的后期阶段,暗示岩体产出于碰撞后环境。斜长岩体中常赋存有Fe Ti V氧化物矿床,有的富含P及Cu,Ni硫化物等,属典型的岩浆矿床。对此,目前主要有结晶分异过程、早期堆晶过程及不混熔分离3种成因机制解释。由此对今后研究中值得关注的问题提出了一些看法。     

Mantle and crustal sources of Archean anorthosite: a combined in situ isotopic study of Pb–Pb in plagioclase and Lu–Hf in zircon

Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth’s mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase (An_75–89) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenæsset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U–Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenæsset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ε_Hf units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenæsset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenæsset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the surface to date, could still be present within the Fiskenæsset and Nunataarsuk regions.     

High pressure infrared spectra of diaplectic anorthite glass

Samples of synthetic diaplectic anorthite glass (38 GPa shock pressure), thermal glass and synthetic anorthite crystals were investigated using infrared spectral methods at one atmosphere and high pressures (near 4 GPa). Band positions and pressure derivatives for the Si-O asymmetric modes in the region 1,300–900 cm^?1 indicate that the diaplectic glass has more structural similarities with the crystalline material than with thermal glass even though the overall infrared spectral characteristics suggest a glassy state.     

Structure of mineral glasses—I. The feldspar glasses NaAlSi3O8, KAlSi3O8, CaAl2Si2O8

The short range distribution of interatomic distances in three feldspar glasses has been determined by X-ray radial distribution analysis. The resulting radial distribution functions (RDF's) are interpreted by comparison with RDF's calculated for various quasi-crystalline models of the glass structure.The experimental RDF's of the alkali feldspar glasses were found to be inconsistent with the four-membered rings of tetrahedra associated with crystalline feldspars; the structures of these glasses are probably based on interconnected six-membered rings of the type found in tridymite, nepheline, or kalsilite. In contrast, the RDF of calcic feldspar glass is consistent with a four-membered ring structure of the type found in crystalline anorthite. T-O bond lengths (T = Si,Al) increase from 1.60 Å in SiO₂ glass [J. H. Konnert and J. Karle (1973) Acta Cryst.A29, 702–710] to 1.63 Å in the alkali feldspar glasses to 1.66 Å in the calcic feldspar glass due to the substitution of Al for Si in the tetrahedra] sites. The T-O-T bond angles inferred from the RDF peak positions are 151° in SiO₂ glass (see reference above), 146° in the alkali feldspar glasses, and 143° in the calcic feldspar glass. Detail in the RDF at distances greater than 5 Å suggests that the alkali feldspar glasses have a higher degree of long range order than the calcic feldspar glasses.Assuming that the structural details of our feldspar glasses are similar to those of the melts, the observed structural differences between the alkali feldspar and calcic feldspar glasses helps explain the differences in crystallization kinetics of anhydrous feldspar composition melts. Structural interpretations of some thermodynamic and rheologic phenomena associated with feldspar melts are also presented based on these results.     

An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, A?r, Niger

Laminated anorthosite grading outwards into leucogabbro, gabbro,and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion,cut by a quartz alkali syenite plug and concentric syenite andgranite ring-dykes. The anorthosite-gabbro series is laminatedbut not modally or otherwise texturally layered. The lamination,defined by large tabular plagioclase crystals, forms a set ofinwarddipping cones, the dips of which decrease from 60–45?in the central anorthosite to < 25? in the outer gabbros.Rocks close to the outer contact are medium-grained isotropicgabbros. Plagioclase, forming >80% of the series, generallyhas homogeneous labradorite cores (An_62–58 in the wholeseries) and thin strongly zoned rims, which follow progressivelylonger solidus paths from the anorthosites to the gabbros. Allrocks contain a late-magmatic alkali feldspar. Plagioclase isthe main or only cumulus phase, the anorthosites being ad- tomesocumulates and the gabbros orthocumulates. Olivine (FO_49–41)is more abundant than clinopyroxene in most of the series. Dependingon quartz content, the syenites and granites are hypersolvusor subsolvus and the depth of crystallization was calculatedto be 5 ? 2 km. A Rb/Sr isochron for the syenites and granites gave an age of399 ? 10 Ma with an initial strontium isotopic ratio of 0.7084? 0.0005. Ten samples from the anorthosite-gabbro scries havean average calculated initial ratio of 0.70582 ? 0-00004 at– 400 Ma, showing that the two series are not comagmatic.The anorthosite-gabbro series has parallel REE trends (La_N/Yb_N{small tilde} 7–10) with decreasing positive Eu anomaliesand increasing total REE contents from anorthosite to gabbro;two monzogabbros have almost no Eu anomaly. The liquid calculatedto be in equilibrium with the lowest anorthosite has almostno Eu anomaly and its normalized REE pattern lies just abovethose for the monzogabbros. The syenites and granites have complementaryREE patterns with negative Eu anomalies. The inferred parental magma was alkalic and leucotroctoliticwith high TiO₂ P₂O₅, Sr and K/Rb and with low MgO, very similarto parental magmas in the Gardar province, South Greenland.It was probably produced at depth by settling of olivine andclinopyroxene but not of plagioclase, which accumulated by flotation.It is suggested that plagioclase crystals from this lower chamberwere progressively entrained (from 0% in the gabbros to 30–40%in the anorthosites), giving rise to the flow lamination inthe upper chamber. The magma in the lower chamber may have beenlayered, because the plagioclase cores in the anorthosite areconsiderably richer in Or than those in the leucogabbros orgabbros. Overall convection did not occur in the upper chamber,whereas compositional convection occurred in the more slowlycooled central anorthositic adcumulates.     

Origin of nelsonite and Fe–Ti oxides ore of the Damiao anorthosite complex,NE China: Evidence from trace element geochemistry of apatite,plagioclase, magnetite and ilmenite

Nelsonite and Fe–Ti oxides ore are common in Proterozoic massif-type anorthosites and layered intrusions. Their geneses have long been controversial, with existing hypotheses including liquid immiscibility between Si-rich and Fe–Ti–P-rich melts and gravitational fractionation among apatite, magnetite, ilmenite and silicates. In this paper, we report detailed field geology and mineral geochemical studies of the nelsonite and Fe–Ti oxides ore from the Damiao anorthosite complex, NE China. Geological observations indicate that the nelsonite and Fe–Ti oxides ore occur as irregularly inclined stratiform-like or lensoid or veins, and are in sharp contact with the anorthosite and gabbronorite. The widespread veins and lenses structure of the Damiao nelsonite and Fe–Ti oxides ore in the anorthosite indicates their immiscibility-derived origin. The apatite in the nelsonite and gabbronorite shows evolution trends different from that in the gabbronorite in the diagrams of Sr versus REEs and Eu/Eu*, suggesting that petrogenesis of the nelsonite and gabbronorite is different from the gabbronorite. Compared with the gabbronorite, the nelsonite and Fe–Ti oxides ore have magnetite high in Cr, plagioclase high in Sr and low in An, and apatite high in Sr, low in REEs with negative Eu anomaly. The evidence permits us to propose that the Damiao Fe–Ti oxides ore/nelsonite and gabbronorite were derived from different parental magmas. The gabbronorite was formed by solidification of the interstitial ferrodioritic magma in the anorthosite, which was the residual magma after extensive plagioclase and pyroxene crystallization and was carried upward by the plagioclase crystal mesh. In contrast, the Fe–Ti oxides ore and nelsonites and mangerite were produced by crystallization of the Fe–Ti–P-rich and SiO₂-rich magmas, respectively, due to the liquid immiscibility that occurred when the highly evolved ferrodioritic magma mixed with newly replenished magmas. The variation from Fe–Ti oxides ore to nelsonite and gabbro-nelsonite upwards (as apatite content increases with height) in the steeply inclined Fe–Ti oxides orebodies suggest that gravity fractionation may have played important roles during the crystallization of the Fe–Ti–P-rich magma.     

Cordierite formation during the experimental reaction of plagioclase with Mg-rich aqueous solutions

The reaction between plagioclase (labradorite and oligoclase) and Mg-rich aqueous solutions was studied experimentally at hydrothermal conditions (600–700 °C, 2 kbar). During the experiments, plagioclase grains were readily converted to cordierite and quartz within 4 days. The cordierite crystals had well-developed polyhedral shapes, but showed skeletal internal morphologies suggesting that the initial growth occurred fast under high-driving-force conditions. In pure MgCl₂ solutions (0.5–5 M), plagioclase dissolution and cordierite precipitation were spatially uncoupled indicating that Al was to some extent mobile in the fluid. Cordierite crystals formed at 700 °C showed orthorhombic symmetry, whereas those formed at 600 °C dominantly persisted in the metastable hexagonal form suggesting a strong increase in Al, Si ordering speed between 600 and 700 °C. The thermodynamic evolution of the fluid–solid system ultimately resulted in stabilization of Ca-rich plagioclase as demonstrated by partial anorthitization of unreacted plagioclase grains. Cordierite was also observed to form when Mg was added to a potentially albitizing Na-silicate-bearing solution. In that case, cordierite precipitation appeared to be more closely coupled to plagioclase dissolution, and secondary alteration of remnant plagioclase grains did not occur most likely due to armoring of the plagioclase by the cordierite overgrowth. The fast reaction rates observed in our experimental study have potential implications for Mg-metasomatism as a rock-forming process.