Ian Parsons

Origins of Igneous Layering

1. Layering in the Ilimaussaq Alkaline Intrusion, South Greenland.- 2. Layering, Compaction and Post-Magmatic Processes in the Klokken Intrusion.- 3. Gabbroic, Syenogabbroic and Syenitic Cumulates of the Tugtutoq Younger Giant Dyke Complex, South Greenland.- 4. A Large Soft-Sediment Fold in the Lilloise Intrusion, East Greenland.- 5. The Southern Part of the Fongen-Hyllingen Layered Mafic Complex, Norway: Emplacement and Crystallization of Compositionally Stratified Magma.- 6. Layering and Related Structures in the Duke Island and Skaergapard Intrusions: Similarities , Differences, and Origins.- 7. Rhythmic Layering of the Skaergaard Intrusion.- 8. The Rhum Layered Complex, Inner Hebrides, Scotland.- 9. The Organization and Internal Structure of Cyclic Units in the Honningsvag Intrusive Suite, North Norway: Implications for Intrusive Mechanisms, Dole-Diffusive Convection and Pore-Magma Infiltration.- 10. The Formation of Stratiform Pge Deposits in Layered Intrusions.- 11. The Development of Compositional and Textural Layering in Archaean Komatiites and in Prcterozoic Kcmatiitic Basalts from Cape Smith, Quebec, Canada.- 12. Some Illustrations of Igneous Layering.- 13. Constitutional Zone Refining of Layered Intrusions.- 14. Pattern Formation During Crystallization and the Formation of Fine-Scale Layering.- 15. Textural Equilibrium in Layered Igneous Rocks.- 16. Solidification Contraction: Another Approach to Cumulus Processes and the Origin of Igneous Layering.- 17. Laboratory Experiments with Aqueous Solutions Modelling Magma Chamber Processes. I. Discussion of Their Validity and Geological Application.- 18. Laboratory Experiments with Aqueous Solutions Modelling Magma Chamber Processes II. Cooling and Crystallization Along Inclined Planes.- 19. Experimental Modelling of Interstitial Melt Convection in Cumulus Piles.- 20. The Modelling of Formation of Apatite Deposits of the Khibina Massif (Kola Peninsula).- 21. Characteristic Dimensions and Tats for Dynamic Crystallization.- 22. Appendices.

Development of microporosity, diffusion channels and deuteric coarsening in perthitic alkali feldspars

Turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the Klokken syenite using SEM and TEM. It is caused by the presence of myriads of tubular micro-inclusions, either fluid-filled micropores or sites of previous fluid inclusions, and is associated with coarsening of microperthite and development of sub-grains. Micropores are abundant in coarsened areas, in which porosities may reach 4.5%, but are almost absent from uncoarsened, pristine braind-microperthite areas. The coarsening is patchy, and involves a scale increase of up to 103 without change in the composition of the phases, low albite and low microcline, or in the bulk composition of the crystal. It occurs abruptly along an irregular front within individual crystals, which retain their original shapes. The coherent braid microperthite gives way across the front to an irregular semi-coherent film perthite over a few μm and then to a highly coarsened irregular patch perthite containing numerous small sub-grains on scales of a few hundred nm, in both phases. The coarsening and micropore formation occured at a T≤400°–450° C and it is inferred to have been driven by the release of coherent strain energy, low-angle grain-boundary migration being favoured by a fluid. The patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin, possibly arising in part through exsolution of water from the feldspar. The sub-grain texture and microporosity modify profoundly the permeability of the rock, and greatly enhance the subsequent reactivity of the feldspars.

Argon-loss by alkali feldspars

Abstract Ar-loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the K-Ar method, and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins. We have done a 40 Ar- 39 Ar step-heating study of a suite of alkali feldspars from the Klokken syenite intrusion (U-Pb age 1166 ± 4 Ma ) which have been fully characterized by transmission electron microscopy. All are low albite-low microcline intergrowths, but some are non-turbid, ‘strain-controlled’ cryptoperthites, while others are turbid, coarse microperthites. Geological and textural evidence suggests that the turbidity developed by fluid-rock interactions during the initial cooling of the pluton, in the stability field of low microcline ( 5 a after crystallization. The finest cryptoperthites have numerous fully coherent perthite lamellar boundaries (lamellar periodicity ~ 40 nm) and twin composition planes. The have retained radiogenic 40 Ar and yield total degassing ages as high as 1125 ± 16 Ma and maximum ages on the age spectra of 1162 ± 16 Ma. The coarse turbid microperthites, with patchy intergrowths and twins on the mm-scale, gave total degassing ages as low as 650 Ma (maximum age 750 Ma). Argon leakage occurs least in feldspars with the most lamellar interfaces, but in proportion to the degree of turbidity, which is thought to be caused by large numbers of micropores. Strain-controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids. These retain argon and can give reliable K-Ar ages in old rocks. Saddle-shaped age spectra appear to result from the preferential degassing, at 1000°C, of part of the feldspar structure which has lost more of its argon in nature. The laboratory degassing does not occur in a manner analogous to that which has occurred naturally.

Dislocation formation and albitization in alkali feldspars from the Shap granite

Abstract Orthoclase-rich alkali feldspars in the Lower Devonian Shap granite, northwest England, contain two generations of albite-rich feldspar. These have partially replaced earlier exsolution microtextures consisting of albite lamellae (coarse semicoherent albite films and fine coherent albite platelets) in tweed orthoclase. The earlier generation of replacive albiterich feldspar (∼Ab10 An9OrI) occurs together with orthoclase-rich feldspar (∼Ab10Or90) in veins that crosscut exsolution microtextures throughout grain interiors. This episode of recrystallization was mediated by magmatic fluids at ∼410 8C (estimated from two-feldspar geothermometry) and was driven by stored elastic strain energy, which was relatively homogeneously distributed throughout the microtextures. The later generation of replacive albite-rich feldspar, which is restricted to grain margins and is compositionally pure (Ab>99), was produced by magmatic-hydrothermal fluids at ∼370 °C. This generation of albite-rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semicoherent albite films. Marked differences in controls of the localization of the two generations of replacive albite-rich feldspar by pre-existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid-feldspar interaction and associated albitization but before the second phase. This relation indicates that edge dislocations formed between 410 and 370 °C. These observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis.

The role of intragranular microtextures and microstructures in chemical and mechanical weathering: Direct comparisons of experimentally and naturally weathered alkali feldspars

Abstract Electron microscopic observations of alkali feldspars from soils show that intragranular microtextures, such as exsolution lamellae, and microstructures, primarily dislocations, are both highly significant determinants of the weathering behaviour of these minerals. In particular, strained structure around intersecting edge dislocations in the plane of exsolution lamellae, ∼( 6 01), dissolves at a rate which is orders of magnitude greater than unstrained feldspar, producing a mesh of intersecting etch tubes extending >5 × 10−3 cm into the crystal. As a result, dissolution at dislocations is the major source of solutes during initial stages of chemical weathering in the field. With progressive chemical weathering, the most highly reactive feldspar is consumed by growth and coalescence of etch tubes, but outer parts of the grain are physically weakened, leading to mechanical flaking that increases available surface area and exposes further reactive sites. In contrast, previous dissolution experiments, and microscopy of reacted surfaces, have shown little or no correlation between dissolution rate and dislocation density and few visible signs of dissolution at particularly reactive sites. To resolve the apparent discrepancy between field and laboratory behaviour we have carried out flow-through dissolution experiments using pH 2 HCl at 25°C on three alkali feldspars with carefully characterized intragranular microtextures and microstructures. These alkali feldspars were: (1) Eifel sanidine, an alkali feldspar that has no microtextures at the TEM scale and a low dislocation density ( 2–3 × 108 cm−2), and (3) naturally weathered alkali feldspars, also from the Shap Granite, which have the same microtextures as unweathered Shap Granite alkali feldspars but, because they have been weathered, have a lower density of dissolution reactive dislocations exposed on grain surfaces (

Towards a more practical two-feldspar geothermometer

The thermodynamic basis of several recent attempts to formulate a simple two-feldspar geothermometer is discussed, together with a review of earlier empirical geothermometers and ones based on experimental studies in the ternary feldspar system. It is shown that double-binary thermometers which involve the combination of regular solution mixing models for the binary alkali feldspar system with ideal mixing in plagioclases do not give a satisfactory representation of two-feldspar relations, especially for albite-rich compositions where a critical point exists. Thermometers based on mixing parameters for ordered alkali feldspar frameworks are even more unjustified both because low-plagioclases are certainly non-ideal, and because of uncertainty in knowing the degree of Al-Si order in the alkali feldspar when exchange equilibrium was achieved. A ‘thermodynamic’ thermometer requires knowledge of ternary activities which are at present unknown.Experimental determinations of relationships in the ternary feldspar system are reviewed and the correct general form of the thermometer constructed using mainly the experimental data of Seck (1971a) and Smith and Parsons (1974). Chemographic tests for equilibrium between feldspar pairs are suggested and petrographie features discussed.In an appendix new values are given of Margules parameters calculated for binary disordered alkali feldspars from recent solvus data up to 15 kbars, and their physico-chemical basis examined. We suggest that accurate representations of the mixing properties of disordered alkali feldspars using Margules parameters are at present premature.

Feldspars and the Thermal History of Igneous Rocks

Development of alkali feldspars in igneous rocks can be considered in three stages: magmatic, involving crystal growth from the melt; subsolidus or postmagmatic, involving coherent exsolution and development of regular, strain-controlled crypto or microperthites; and deuteric or hydrothermal involving feldspar-fluid interactions which give rise to irregular coarse microperthites. The interplay of crystallization temperature, cooling rate, deformation, bulk composition and deuteric interactions leads to the variety in alkali feldspar textures.

Microtextural controls of weathering of perthitic alkali feldspars

The relationship between the microtexture and dissolution behaviour of fresh, HF acid-etched and naturally weathered alkali feldspar phenocrysts from the Lower Devonian Shap granite has been investigated by SEM and TEM. A novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface. Further electron microscope work suggests that Shap phenocrysts are representative of the alkali feldspar in the protolith of many soils. Fresh and unweathered Shap feldspars have a complex microtexture, comprising areas of pristine cryptoperthite and lamellar microperthite cross-cut by volumes of microporous altered feldspar or “patch perthites.” Cryptoperthites are made up of 75 nm wide albite films. The platelets are coherent, but albite films have numerous edge dislocations along their interface with orthoclase; (001) and (010) cleavage surfaces intersect ∼2–3 edge dislocations/μm2. In three dimensions, these edge dislocations form an orthogonal net in the “Murchison plane” of easy fracture, close to (601). Patch perthites are irregular, semicoherent to incoherent intergrowths of albite and irregular microcline subgrains, with ∼0.65-0.70 sub-μm to μm-sized pores/μm2. Microporous patch perthites form by dissolution-reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime. Dissolution of Shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture, especially by dislocations and exsolution lamellae. The core and strain energy associated with dislocation outcrops on (001) and (010) cleavage surfaces promotes rapid dissolution at those sites and formation of nn-sized etch pits after <30 s of laboratory etching with HF acid vapour. With progressive HF etching, crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite. Naturally weathered feldspars were collected from the glacial erratic boulders, fine gravels surrounding exposed granite surfaces, and from peat soil overlying the granite. During natural weathering, etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate ≥15 μm below the cleavage surface, forming an interconnecting, ladder-like grid of submicrometer wide channels in the Murchison plane. Coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly, but faster than orthoclase. This is probably because the albite lamellae have significant elastic coherency strain, but this is much less, per unit volume of albite, than the core and strain energy associated with edge dislocations. Patch perthites etch in HF vapour and weather rapidly in nature to produce a honeycomb-like texture of interconnecting nanometer- to micrometer-sized pits, which nucleate at preexisting micropores or incoherent subgrain boundaries. The size and density of etch pits on microperthite surfaces, which is determined by rates of growth and coalescence, may be a useful progress variable for natural and experimental dissolution. All alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale. These microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains, the controls on absolute dissolution rates and why they commonly change over time, the nonlinear variation of dissolution rate with grain size, the ratio of alkali ions released into solution, and disparities between laboratory dissolution rates and those observed in the field.

40Ar39Ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the Klokken syenite, South Greenland

Laser probe andin vacuo crushing have been used to apply the40Ar39Ar technique to microtexturally well-characterised pristine and turbid alkali feldspar from the 1166 Ma Klokken syenite. Individual crystals are several millimetres in size, but SEM and TEM studies reveal complex microstructures which define a range of sub-grain sizes over three orders of magnitude, from > 100 μm in pristine areas with fine-scale cryptoperthite, down to < 200 nm in turbid areas with coarsened perthites and numerous micropores. Laser-probe40Ar39Ar measurements of turbid feldspar gave low apparent ages indicating significant40Ar loss (40% average), which can be accounted for by sustained heating of the small sub-grains ( ⩽ 1 μm) for 1166 Ma at low temperature ( < 150°C). Pristine feldspar gives high apparent ages indicating the presence of excess40Ar. This component was readily lost from the feldspar by laboratory heating at low temperature (600°C) and was also released by crushing, which indicates that it was released from fluid inclusions. During crushing, release of excess40Ar is correlated with Cl release and the40Ar/*Cl ratio of the fluid is similar to that of mantle fluids. Turbid feldspar also released a fluid during crushing, but the40Ar/*Cl ratio was lower, probably as a result of evolution of the original fluid during boiling. Anomalously high apparent ages are often a feature of low-temperature Ar release in stepped-heating age spectra of feldspars. The presence of excess40Ar, in fluid inclusions, may substantially affect the lower-temperature release in feldspar stepped-heating spectra. The character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures, whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite.

Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap Granite

Abstract Alkali feldspar phenocrysts (bulk composition Or75.0Ab24.6An0.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ≤ 590°C by heterogeneous nucleation of thin albite films which coarsened to > 1 µm length. Cryptoperthites developed at < 400°C by homogeneous nucleation of sub-µm long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous µm to sub-µm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400°C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto-and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.