Mutual replacement reactions in alkali feldspars II: trace element partitioning and geothermometry

Perthitic alkali feldspar primocrysts in layered syenites in the Klokken intrusion in South Greenland, underwent dissolution–reprecipitation reactions in a circulating post-magmatic aqueous fluid at ~450°C, and are to a large degree pseudomorphs. These ‘mutual replacement’ reactions provide a perfect natural experiment with which to study trace element partitioning between sodium and potassium feldspars growing simultaneously. The reactant ‘phase’ was a cryptoperthitic feldspar consisting of low albite and low microcline in a coherent sub-μm ‘braid’ intergrowth and the product phases were ‘strain-free’ incoherent subgrains of low albite and low microcline forming microporous patch perthites on scales up to 200 μm. The driving force for the reaction was reduction of coherency strain energy. The mechanisms of this process are described in Part I. Five mixed braid perthite–patch perthite crystals were analysed for major and trace elements using laser ablation-inductively coupled plasma mass spectrometry with a 19 μm beam diameter. This gave bulk analyses of the braid texture, which were in the range Ab_73–54Or_45–27An_4.3–0.8, but could resolve Ab- and Or-rich patches in patch perthite. The major element bulk compositions of the crystals were retained during the replacement reactions. Major components in patches plot on tielines in the Ab–Or–An ternary system that pass through or very close to the parent braid perthite composition and indicate local equilibrium on the scale of a few tens of mm. Many trace elements, including REE, were lost to the fluid during the deuteric reactions, but the effect is large only for Fe and Ti. Cs, Pb and Sr were added to some crystals. Plots of log distribution coefficient D for Rb, Ba, Pb, Eu²⁺, La and Ce between Or- and Ab-rich patches against ionic radius are straight lines, assuming eightfold coordination, and to a first approximation are independent of ionic charge. K also lies on these lines, and the smaller ions Na and Ca lie close to them. The best linear fits were obtained using ionic radii for ^[8]K and ^[8]Ca, but there is ambiguity as to whether ^[7]Na or ^[5]Na is most appropriate. The linear relationship shows that the listed trace elements are in the feldspar M-site rather than in inclusions. Tl is in M although an exact D could not be obtained. The very large Cs ion partitions strongly into the Or-rich phase but its D value appears to be less than predicted by extrapolation. The near-linearity arises because partitioning is occurring between two solids into sites which have similar Young’s moduli, so that the parabolas that normally represent trace element partitioning between crystals and liquids (which have negligible shear strength) approximately cancel out. Ga and Be are in T-sites, as well as some of the Fe and Ti present, although part is in oxide inclusions. The site of Sc is unclear, but if structural it is likely to be T. Partitioning on M-sites is a potential geothermometer but because the effective size of the irregular M-site is defined by its K and (Na + Ca) contents, which are controlled by ternary solvus relationships, its calibration is not independent of conventional two-feldspar geothermometers. Trace elements may however provide a useful means of confirming that feldspar pairs are in equilibrium, and of recognising feldspar intergrowths produced by non-isochemical replacement rather than exsolution. Two-feldspar geothermometry for the ternary phases in the low-albite microcline patch perthites gives temperatures above the stability range of microcline, markedly so if a correction is made for Si–Al ordering. This is probably because current geothermometers are too sensitive to low concentrations of An in ordered Or-rich feldspars. This interpretation is supported by two-feldspar assemblages growing at known temperatures in geothermal systems and sedimentary basins. This paper and the earlier Part I are dedicated in the memory of J. V. Smith and W. L. Brown, both of whom died in 2007, in acknowledgement of their unrivalled contributions to the study of the feldspar minerals over more than half a century. An erratum to this article can be found at     

Eight-phase alkali feldspars: low-temperature cryptoperthite, peristerite and multiple replacement reactions in the Klokken intrusion

Eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the Klokken intrusion. The processes leading to the formation of the first four phases have been described previously. The feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition, between 750 and 600 °C, depending on bulk composition, to give fully coherent, strain-controlled braid cryptoperthites with sub-μm periodicities. Below ~500 °C, in the microcline field, these underwent a process of partial mutual replacement in a deuteric fluid, producing coarse (up to mm scale), turbid, incoherent patch perthites. We here describe exsolution and replacement processes that occurred after patch perthite formation. Both Or- and Ab-rich patches underwent a new phase of coherent exsolution by volume diffusion. Or-rich patches began to exsolve albite lamellae by coherent nucleation in the range 460–340 °C, depending on patch composition, leading to film perthite with ≤1 μm periodicities. Below ~300 °C, misfit dislocation loops formed, which were subsequently enlarged to nanotunnels. Ab-rich patches (bulk composition ~Ab₉₁Or₁An₈), in one sample, exsolved giving peristerite, with one strong modulation with a periodicity of ~17 nm and a pervasive tweed microtexture. The Ab-rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at ~450 °C. This is the first time peristerite has been imaged using TEM within any perthite, and the first time peristerite has been found in a relatively rapidly cooled geological environment. The lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion. All the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity. We term this material ultra porous late feldspar (UPLF). It is composed predominantly of regions of microporous very Or-rich feldspar (mean Ab_2.5Or_97.4An_0.1) associated with very pure porous albite (Ab_97.0Or_1.6An_1.4) implying replacement below 170–90 °C, depending on degree of order. In TEM, UPLF has complex, irregular diffraction contrast similar to that previously associated with low-temperature albitization and diagenetic overgrowths. Replacement by UPLF seems to have been piecemeal in character. Ghost-like textural pseudomorphs of both braid and film parents occur. Formation of patch perthite, film perthite and peristerite occurred 10⁴–10⁵ year after emplacement, but there are no microtextural constraints on the age of UPLF formation.     

Time–temperature evolution of microtextures and contained fluids in a plutonic alkali feldspar during heating

Microtextural changes brought about by heating alkali feldspar crystals from the Shap granite, northern England, at atmospheric pressure, have been studied using transmission and scanning electron microscopy. A typical unheated phenocryst from Shap is composed of about 70 vol% of tweed orthoclase with strain-controlled coherent or semicoherent micro- and crypto-perthitic albite lamellae, with maximum lamellar thicknesses <1 μm. Semicoherent lamellae are encircled by nanotunnel loops in two orientations and cut by pull-apart cracks. The average bulk composition of this microtexture is Ab_27.6Or_71.8An_0.6. The remaining 30 vol% is deuterically coarsened, microporous patch and vein perthite composed of incoherent subgrains of oligoclase, albite and irregular microcline. The largest subgrains are ~3 μm in diameter. Heating times in the laboratory were 12 to 6,792 h and T from 300°C into the melting interval at 1,100°C. Most samples were annealed at constant T but two were heated to simulate an ⁴⁰Ar/³⁹Ar step-heating schedule. Homogenisation of strain-controlled lamellae by Na↔K inter-diffusion was rapid, so that in all run products at >700°C, and after >48 h at 700°C, all such regions were essentially compositionally homogeneous, as indicated by X-ray analyses at fine scale in the transmission electron microscope. Changes in lamellar thickness with time at different T point to an activation energy of ~350 kJmol⁻¹. A lamella which homogenised after 6,800 h at 600°C, therefore, would have required only 0.6 s to do so in the melting interval at 1,100°C. Subgrains in patch perthite homogenised more slowly than coherent lamellae and chemical gradients in patches persisted for >5,000 h at 700°C. Homogenisation T is in agreement with experimentally determined solvi for coherent ordered intergrowths, when a 50–100°C increase in T for An₁ is applied. Homogenisation of lamellae appears to proceed in an unexpected manner: two smooth interfaces, microstructurally sharp, advance from the original interfaces toward the mid-line of each twinned, semicoherent lamella. In places, the homogenisation interfaces have shapes reflecting the local arrangements of nanotunnels or pull-aparts. Analyses confirm that the change in alkali composition is also relatively sharp at these interfaces. Si–Al disordering is far slower than alkali homogenisation so that tweed texture in orthoclase, tartan twinning in irregular microcline, and Albite twins in albite lamellae and patches persisted in all our experiments, including 5,478 h at 700°C, 148 h at 1,000°C and 5 h at 1,100°C, even though the ensemble in each case was chemically homogeneous. Nanotunnels and pull-aparts were modified after only 50 min at 500°C following the simulated ⁴⁰Ar/³⁹Ar step-heating schedule. New features called ‘slots’ developed away from albite lamellae, often with planar traces linking slots to the closest lamella. Slot arrays were often aligned along ghost-like regions of diffraction contrast which may mark the original edges of lamellae. We suggest that the slot arrays result from healing of pull-aparts containing fluid. At 700°C and above, the dominant defects were subspherical ‘bubbles’, which evolved from slots or from regions of deuteric coarsening. The small degree of partial melting observed after 5 h at 1,100°C was often in the vicinity of bubbles. Larger micropores, which formed at subgrain boundaries in patch perthite during deuteric coarsening, retain their shape up to the melting point, as do the subgrain boundaries themselves. It is clear that modification of defects providing potential fast pathways for diffusion in granitic alkali feldspars begins below 500°C and that defect character progressively changes up to, and beyond, the onset of melting.     

Braid perthite in nepheline syenite pegmatite, Langesundsfjorden, Oslo Region (Norway)

Three occurrences of braid perthite (lamellae parallel to (110) and ( )) and micro braid perthite (lamellae parallel to ( ) and ( )) from Permian nepheline syenite pegmatites in monzonite (larvikite) of the Oslo Graben are described. All the crystals studied show a zoned arrangement with either alternating mmacro- and microperthitic bands parallel to (010) or with microperthitic cores surrounded by macroperthite and a marginal zone of plagioclase. Both types of perthite are mesoperthitic and composed of maximum microcline and low albite. Chemical analyses suggest that the alkali feldspar is stoichiometric, while precession exposures imply unusual cell parameters in the host K-feldspar phase. The macroperthite seems to have crystallized at the expense of the microperthite. The braided texture of exsolved albite lamellae may be due to some kind of strain causing cracking along the prism and pyramidal planes.     

Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging

Charge contrast imaging in the scanning electron microscope can provide new insights into the scale and composition of alkali feldspar microtextures, and such information helps considerably with the interpretation of their geological histories and results of argon isotope thermochronological analyses. The effectiveness of this technique has been illustrated using potassium-rich alkali feldspars from the Dartmoor granite (UK). These feldspars contain strain-controlled lamellar crypto- and microperthites that are cross-cut by strain-free deuteric microperthites. The constituent albite- and orthoclase-rich phases of both microperthite generations can be readily distinguished by atomic number contrast imaging. The charge contrast results additionally show that sub-micrometre-sized albite ‘platelets’ are commonplace between coarser exsolution lamellae and occur together to make cryptoperthites. Furthermore, charge contrast imaging reveals that the orthoclase-rich feldspar is an intergrowth of two phases, one that is featureless with uniform contrast and another that occurs as cross-cutting veins and grains with the {110} adularia habit. Transmission electron microscopy shows that the featureless feldspar is tweed orthoclase, whereas the veins and euhedral grains are composed of irregular microcline that has formed from orthoclase by ‘unzipping’ during deuteric or hydrothermal alteration. The charge contrast imaging results are especially important in demonstrating that deuteric perthites are far more abundant in alkali feldspars than would be concluded from investigations using conventional microscopy techniques. The unexpected presence of such a high volume of replacement products has significant implications for understanding the origins and geological histories of crustal rocks and the use of alkali feldspars in geo- and thermochronology. Whilst the precise properties of feldspars that generate contrast remain unclear, the similarity between charge contrast images and corresponding cathodoluminescence images of deuteric microperthites indicates that trace element chemistry and possibly also elastic strain within the crystal play a major role.     

Nucleation and growth of myrmekite during ductile shear deformation in metagranites

Myrmekite is extensively developed along strain gradients of continuous, lower amphibolite facies shear zones in metagranites of the Gran Paradiso unit (Western Alps). To evaluate the role of stress, strain energy and fluid phase in the formation of myrmekite, we studied a sample suite consisting of weakly deformed porphyric granites (WDGs), foliated granites (FGs) representative of intermediate strains, and mylonitic granites (MGs). In the protolith, most K‐feldspar is microcline with different sets of perthite lamellae and fractures. In the WDGs, abundant quartz‐oligoclase myrmekite developed inside K‐feldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction. In the WDGs, stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between K‐feldspar and albite. In the FGs and MGs, K‐feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine‐grained recrystallized myrmekite and perthite aggregates. This was accompanied by a more pervasive fluid influx into the reaction surfaces, and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone. In the MGs, myrmekite forms complete rims along the outer boundary of the small K‐feldspar porphyroclasts, which are almost completely free of internal reaction interfaces. Therefore, we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress. Mass balance calculations indicate that, in Al–Si‐conservative conditions, myrmekite growth was associated with a volume loss of 8.5%. This resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite.     

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 m 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 T400°–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.     

The nature of potassium feldspar,exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars

Transmission electron microscope data on the morphology of exsolution lamellae, the nature of the potassium feldspar and the development of dislocations at lamellar interfaces in coherent cryptoperthites and fine microperthites are reviewed. Dislocations have been reported previously in only two crystals, and periodic dislocations noted in only one, an Or-rich microperthite. Periodic dislocations (spacing 100–150 nm) are here described from a ternary mesoperthite (Or₂₆ Ab₅₂ An₂₂). Small crystallites (<30 nm) of other phases have sometimes nucleated on the dislocations. The 020 lattice fringes of the feldspar phases have been imaged; the difference in 020 spacings can be almost entirely accommodated by the regular dislocations, so that the boundaries may be termed nearlyperfectly semicoherent.Dislocations have been found so far only in cryptoperthites with lens-shaped or straight lamellae, either in Or-rich feldspars or in Ab-rich ternary ones. In intermediate compositions with wavy or zig-zag albite lamellae, or lozengeshaped albite areas (braid microperthites) dislocations have not been observed. Strain reduction in intermediate compositions occurs by migration of lamellar interfaces from (¯601) to near (¯6¯61) as microcline forms in the diagonal association. In Ab-rich ternary feldspars the relatively high Ancontent blocks interface migration, and strain reduction occurs by nucleation of dislocations; the Or-rich feldspar phase is tweed orthoclase. In Or-rich bulk compositions the low volume of albite exerts insufficient stress to promote microcline formation, and tweed orthoclase develops. Interfaces do not migrate, and dislocations again develop. Fields in which different potassium feldspar polymorphs occur and in which the different exsolution textures are developed are summarized on a ternary diagram.     

碱性长石在次固相下的微组构重组织:碱性长石流体相互作用

辫状微纹长石通过粗化、微孔隙和亚颗粒的形成 ,最终发展为脉状微纹长石和条纹长石 ,粗化沿着不规则的前锋从晶体边缘往内部推移和扩展。亚颗粒和微孔隙的形成极大地提高了碱性长石的反应性和岩石的渗透性。通过沿着晶体边缘的拱状褶边、平行褶边以及褶头的过渡带往整个晶体内的推移和“繁殖” ,辫状微纹长石最终改造为脉状微纹长石和条纹长石。水从褶边向晶体内部的扩散促进了褶边的粗化以及过渡带的发展。流体长石相互作用机制包括 :体积扩散、管道扩散、溶解再沉淀。碱性长石流体的氧同位素交换机制主要是溶解再沉淀。碱性长石在次固相下的微组构重组织发生于约 4 75～ 4 0 0℃的温度下 ,区块性条纹长石的形成温度更低。碱性长石的微组构重组织导致放射成因氩的局部和部分丢失 ,从而给出年轻的表面年龄。     

Feldspar–fluid interactions in braid microperthites: pleated rims and vein microperthites

Braid microperthitic alkali feldspars in the Klokken, South Greenland and Coldwell, Ontario syenite intrusions have bulk-compositional variations along grain boundaries called pleated rims. These, together with vein microperthites in aplites which cross-cut the syenites, have been investigated by SEM and TEM. We distinguish two main types of pleated rims, “arched ” and “parallel-sided ”, consisting of alternating Ab- and Or-rich areas on (001), which are 0.5–300 μm in length normal to (010) and 0.2–20 μm in width along (010). The smallest pleats, which occur on intracrystalline boundaries in Klokken feldspars, are fully coherent and composed of low albite and low microcline. Above the heads of some of the coarser pleats, braid microperthite grades into a film crypto- and micro-perthite and antiperthite microtexture called a “transitional zone” containing roughly planar lamellae of low albite and tweed orthoclase. During pleat development, local alternating volumes form in which the proportions of the phases differ ( phase separation) and the morphology of the intergrowths changes from braided to straight in response to this change in local bulk composition. Straightening is also accompanied by transformation of low microcline to tweed orthoclase. The coarsest pleats, which occur along grain boundaries in feldspars from the Coldwell syenite, are semi- or in-coherent and have a thick coherent and semicoherent transitional zone. Coarsening of pleats and development of the transitional zone has been facilitated by diffusion of “water” into grain interiors. In many cases, pleated rims have suffered deuteric alteration, by dissolution–reprecipitation processes, through the action of a water-rich fluid from the grain boundary, in which tweed orthoclase was transformed into irregular microcline and micropores developed. Vein microperthites in aplites from Klokken, and by extension the vein microperthites almost universal in most alkali granites, are interpreted to have formed by propagation of pleat heads across entire crystals during pervasive interaction with water. Received: 10 June 1996 / Accepted: 12 December 1996     

Strain-driven disordering of low microcline to low sanidine during partial phase separation in microperthites

Microperthitic feldspar crystals containing low microcline in a braid intergrowth often have distinctive microtextures including coarse semi- to in-coherent grain-boundary pleated rims and fine coherent intracrystalline Ab- and Or-rich pleats (Lee et al. 1997). The coarser pleated rims are generally separated from the braid microtexture in the crystal interior by a coherent to semi-coherent transitional zone. Partial phase separation has occurred in the transitional zone in step with that in the Ab- and Or-rich pleats at the grain boundaries, such that Ab-rich lamellar film micro-antiperthite alternates along (010) with more Or-rich lamellar film microperthite; the microtextures and phases are those expected for the respective local bulk compositions. Lamellar microtextures contain tweed orthoclase, whereas low microcline is the only K-feldspar in the fine coherent pleats and braid microperthite. We propose that the small coherent pleats developed from the braid microtexture by interaction of the spontaneous coherency strains with discontinuities within or at the surface of the crystal, and that their initial spacing is guided by that of the braid microperthite. We infer that the transitional zone formed by straightening of the zig-zag braid microtexture above the pleat heads during coarsening and partial phase separation. We further infer that the resulting coherency shear strains induced a reversal of the K-feldspar phase transformation, involving Si, Al disordering of low microcline into low sanidine, now tweed orthoclase, although the crystal was at a T within the hydrostatic T-stability of microcline. Received: 10 June 1996 / Accepted: 12 December 1996     

P-T-X-controlled element transport through granulite-facies ternary feldspar from Lofoten,Norway

Fluid transport on the grain-scale controls many rock properties and governs chemical exchange. Charnockites from Lofoten indicate fluid penetration into ternary alkali feldspars controlled by their microtextures. In a process of fluid infiltration at granulite-facies conditions (∼600°C and 8–11 kbar), tiny pyroxenes enclosed in alkali feldspar reacted to amphiboles, which are always spatially connected to perthitic albite. Investigation of these microtextures by TEM imaging of Focused Ion Beam (FIB) prepared foils revealed that pyroxenes in contact with albite lamellae show dissolution features. An amorphous Fe- and Cl-bearing material interpreted to be a residuum of the percolating fluid was found within albite lamellae. Textures and mineral compositions indicate that a Cl-rich aqueous fluid attacked the lamellae, which then provided pathways for further fluid flow. A correspondence was found between feldspar compositions, their microtexture and their degree of alteration as a result of their permeability for fluids at specific temperatures. Hence, in addition to pressure and temperature, small variations of feldspar composition can strongly influence the fluid permeability of feldspathic rocks under lower crustal conditions.     

The Feldspathic Syenites of the Loch Ailsh Intrusion, Assynt, Scotland

The highly feldspathic syenites are divided on field evidenceinto three phases, S1, S2, and S3. S3 overlies S1 and S2, andencloses xenoliths and xenocrysts from the underlying and earliersyenites. The feldspars show a range of forms and coarseness of perthite,and show enrichment in potassium from Or₂₀ to Or₄₀, and increasein 2V, in successive syenites. X-ray diffractometer and single-crystalmethods show that SI has a dominantly monoclinic K-phase, S2a microcline with some monoclinic material, and S3 a highlyoblique microcline, exclusively. The earlier feldspars preservethese features even when enclosed by later S3 material. Thereis a suggestion of systematic regional distribution of obliquityvalues. The compositional range of the feldspars is related to the experimentallydetermined minimum in the Ab-Or-An system. The feldspars crystallizedas sodium-rich sanidines. Temperature-pressure limits are deduced,and the unmixing history of the perthites discussed. A mechanismfor the preservation of orthoclase in S1 and S2 is suggested.     

Zonal Distribution of Variations in Structural State of Alkali Feldspar within the Rader Creek Pluton, Boulder Batholith, Montana

The granodioritic Rader Creek pluton of the composite Boulderbatholith contains microperthitic alkali feldspar of bulk compositionOr₆₅ to Or₈₆ with a structurally variable potassic phase. Completecell parameters, 2V measurements, and bulk composition are givenfor 11 feldspar samples. The 131 and 131 reflections for theseand 58 additional samples show the following structural typesin the potassic phase: orthoclase only; orthoclase with subordinatemaximum or near-maximum microcline (obliquity = 0.75–1.00);orthoclase with subordinate intermediate microcline (obliquity= 0.64–0.71); and intermediate microcline (obliquity =0.56–0.77) with subordinate orthoclase. Within the plutondifferent feldspar structural types occur in zones whose boundariesare approximately parallel to contacts with younger intrusiverocks cutting the Rader Creek pluton but are, in places, nearlyperpendicular to zonation within the pluton defined by rockcomposition. In general, the orthoclase zone is closest to thecontact with younger intrusives; the intermediate microclinezone is the most distant. Bulk compositions of alkali feldsparare more potassic in the orthoclase zone than elsewhere. Thedata suggest a complex history for the alkali feldspar, involvingat least two stages: 1. Exsolution and partial inversion oforthoclase to intermediate microcline during cooling of theRader Creek pluton; 2. Transformation of the intermediate-microclineassemblage to orthoclase during reheating of the pluton at thetime of intrusion of younger plutons of the batholith. The transitionalstage in this transformation is characterized by orthoclaseco-existing with subordinate microcline, whose obliquity usuallyapproaches that of maximum microcline.     

Intracrystalline microstructures in alkali feldspars from fluid-deficient felsic granulites: a mineral chemical and TEM study

Samples of essentially “dry” high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) contain up to 2-mm-large perthitic alkali feldspars with several generations of plagioclase precipitates in an orthoclase-rich host. The first generation takes the form of lenses homogeneous in size, whereas the size of a second generation of very thin albite-rich precipitates is more variable with comparatively high aspect ratios. In the vicinity of large kyanite, garnet or quartz inclusions, the first generation of plagioclase precipitates is significantly less abundant, the microstructure is coarser than in the remainder of the perthitic grain and the host is a tweed orthoclase. The first generation of precipitates formed at around 850 °C during the high-pressure stage (16–18 kbar) of metamorphism. Primary exsolution was followed by primary coarsening of the plagioclase precipitates, which still took place at high temperatures (850–700 °C). The coarsening was pronounced due to the access of fluids in the outer portions of the perthitic alkali feldspar and in more internal regions around large inclusions. The second generation of albite-rich precipitates was formed at around 570 °C. TEM investigations revealed that the interfaces between the second-generation plagioclase lamellae and the orthoclase-rich host are coherent or semi-coherent. During late evolutionary stages of the perthite, albite linings were formed at phase boundaries, and the perthitic microstructure was partially replaced by irregularly shaped precipitates of pure albite with incoherent interfaces. The albitization occurred below 400 °C and was linked to fluid infiltration in the course of deuteric alteration. Based on size-distribution analysis, it is inferred that the precipitates of the first generation were most probably formed by spinodal decomposition, whereas the precipitates of the second generation rather were formed by nucleation and growth.     

Cryptoperthites and cooling rate in a layered syenite pluton: a chemical and TEM study

Alkali feldspars in the Klokken layered syenite (South Greenland) are optically either coarse, turbid, deuteric patch perthites or clear, unaltered, fine braid microor cryptoperthites. Irregular, clear volumes can be found in most turbid grains. Electron microscopy shows that all clear crystals or volumes are cryptoperthites in which the periodicity of the exsolution lamellae increases systematically with distance from the roof of the intrusion, from 40 nm to 300nm. They are composed of low albite and diagonally associated maximum microcline. A secondary coarsening is responsible for optically visible braid microperthites.The layered series consists of interleaved, granular syenites and coarser grained, laminated syenites with mineral layering. Microprobe analyses show a downward decrease in Ca and Ba in the granular syenite feldspars consistent with chilling from the roof inwards. Most laminated syenite feldspars contain 1% An increasing to 5% in mafic layers.The periodicity of the exsolution lamellae in the laminated syenite feldspars is consistently up to twice that in adjacent granular syenites. Periodicity is not influenced by Ab:Or or An content, but high-An feldspars have continuous zig-zag intergrowths, while others are more lozengeshaped. Lamellar periodicities largely reflect cooling rate in the pluton. Cooling times in the exsolution interval were probably about 10³–10⁴ years at the top of the series, whereas existing experimental data suggest that the periodicities would be reached in a few days or years. The periodicities at the top of the granular series are only slightly greater than experimentally determined initial wavelengths for spinodal decomposition, suggesting that little or no coarsening occurred despite the long cooling times. Coarsening did occur lower in the pile. Framework ordering and twinning must slow or stop coarsening.The difference in periodicity between the granular and laminated syenite series may be caused by differences in amount of water incorporated in the feldspar structure from the magma. The exsolution of this water at low temperatures could be the cause of the deuteric unmixed patch perthites. Small volumes of cryptoperthite which have escaped deuteric unmixing may be common in igneous rocks in general and may be useful relative indicators of cooling rate.     

Microtextures of authigenic Or-rich feldspar in the Upper Jurassic Humber Group, UK North Sea

ABSTRACT Detrital alkali feldspars currently at burial depths of 3·2–3·5 km in the Upper Jurassic Humber Group of the Fulmar oilfield, UK North Sea, are overgrown and have been partially replaced by authigenic Or‐rich feldspar. Intracrystal microtextures suggest several different provenances for the detrital grains. The overgrowths are uniformly non‐cathodoluminescent and have occasional celsian‐rich zones. Transmission electron microscopy shows that they are composed of a microporous mosaic of subµm‐ to µm‐sized sub‐grains associated with barite, illite and pyrite. The subgrains are somewhat rounded but have an approximate {110} Adularia habit and display a faint modulated microtexture on the nanometre scale. They have triclinic symmetry, but the lattice angles depart only slightly from monoclinic symmetry. These features are characteristic of K‐feldspar precipitated relatively rapidly and at low temperature. Authigenic Or‐rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains, often at a suboptical scale. Although, like diagenetic albitization, replacement by K‐feldspar is probably a very common diagenetic reaction, it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers. The permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for K/Ar and ⁴⁰Ar/³⁹Ar dating of diagenesis, and the existence of suboptical, replacive authigenic K‐feldspar within detrital grains may significantly modify the apparent Ar ages of detrital grains. Similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated.     

The replacement of plagioclase feldspars by albite: observations from hydrothermal experiments

Oligoclase and labradorite crystals have been experimentally replaced by albite in an aqueous sodium silicate solution at 600°C and 2 kbars. The replacement is pseudomorphic and is characterised by a sharp chemical interface which progresses through the feldspar while preserving the crystallographic orientation. Reaction rims of albite, up to 50 μm thick, can be readily achieved within 14 days. Re-equilibration of plagioclase in an ¹⁸O-enriched sodium- and silica-bearing solution results in oxygen isotope redistribution within the feldspar framework structure. The observed characteristics of the reaction products are similar to naturally albitised plagioclase and are indicative of an interface-coupled dissolution–reprecipitation mechanism. Chemical analyses demonstrate that the albitisation is accompanied by the mobilisation of major, minor and trace elements also including elements such as Al and Ti which are commonly regarded as immobile during hydrothermal alteration. The results contribute to developing our understanding of the close association between large-scale albitisation and secondary ore mineralisation which is common in nature.     

Biotites and associated minerals as markers of magmatic fractionation and deuteric equilibration in granites

Analyses of 80 biotite, alkali feldspar, oligoclase, hornblende, Fe-Ti oxide separates from the coarse-grained granites of a late-hercynian epizonal diapir, the Ploumanac'h complex, Brittany, show that these minerals display a regular concentric cryptic layering related to fractional crystallization. The Ca, Mg, Ba, Sr, Cr, V content of minerals decreases as the Na, Fe, and Rb content increases. Biotites become more dioctahedral towards the outer residual syenogranite, with a correlative K deficiency. Trioctahedral biotites from the inner accumulative monzogranite are secondarily oxidized with a gain of Fe³⁺ and a loss of OH. This alteration id due to the percolation of exsolved fluids rich in H₂O and containing a small amount of CO₂, F, S, Cl. During this autometamorphic stage, trace elements like Rb, Sr are completely redistributed on the scale of hand specimens, with a restricted range of partition coefficients between biotite, perthite and oligoclase. This equilibration occurred at a temperature about 550 ° C and a fluid pressure about 1,000 bars, with f _H2_O probably less than 500 bars. A later stage of fluid circulation along fractures brings up a slight Li metasomatism. Biotites are a sensitive marker of both magmatic and postmagmatic stages of subsolvus or ‘wet’ plutonites.     

Alteration at the Olympic Dam IOCG–U deposit: insights into distal to proximal feldspar and phyllosilicate chemistry from infrared reflectance spectroscopy

Twenty thousand metres of diamond drill core representing a 14 km cross-section from weakly to intensely altered Roxby Downs Granite through the Olympic Dam Breccia Complex, host to the Olympic Dam iron-oxide–copper–gold–uranium deposit in South Australia, was analysed using the HyLogger-3 spectral scanner. Thermal and shortwave infrared spectroscopy results from 30 drill holes provide insight into the spatial relationships between quartz, orthoclase–microcline, albite–oligoclase and progressively changing sericite and chlorite compositions. The relative proportions of quartz, feldspars and phyllosilicates were mapped with thermal infrared spectroscopy. Variations in the chemistry of sericite and chlorite were extracted by proxy from their shortwave infrared spectral response, together with their relative spatial distribution. HyLogger scanning has revealed four deposit-scale mineralogical trends, progressing from least-altered Roxby Downs Granite into mineralisation where most of the feldspar has been replaced by sericite + hematite + quartz: (1) a progressive Al–OH wavelength shift of 2205 nm to 2210 nm for sericite, followed by a spatially rapid reversal corresponding to lower phengite/muscovite abundance ratios; (2) progressive Mg/Fe–OH wavelength shift of 2248 nm to 2252 nm reflecting an increase in the Fe:Mg ratio of chlorite; (3) increasing ratio of microcline to orthoclase followed by a rapid decrease; and (4) slightly decreasing ratio of albite to oligoclase followed by plagioclase destruction prior to albite replacement by sericite. The HyLogger feldspar results support recent petrographic evidence for hydrothermal albite and K-feldspar at the Olympic Dam deposit, not previously reported. The spectral results from continuous HyLogger scans also show that the microscopic observations and proposed feldspar replacement reactions are not locally isolated phenomena, but are applicable at the deposit and regional-scale. A modified quartz–K-feldspar–plagioclase ternary diagram utilising mineralogy interpreted from HyLogger thermal infrared spectra (QAP_TIR) diagram along with supporting data on the abundance ratios of orthoclase/microcline and albite/plagioclase, and the wavelength shifts in characteristic absorption features for sericite and chlorite, can be used as empirical vectors towards mineralisation within the Olympic Dam mineral system, with potential application to other IOCG ore-forming systems. Intrusion of Gairdner Dyke Swarm dolerite dykes into sericite ± hematite altered Roxby Downs Granite results in retrograde albite–chlorite–magnetite alteration envelopes (up to tens of metres thick) overprinting the original sericite ± hematite alteration zone and needs to be carefully evaluated to ensure that such areas are not falsely downgraded during exploration.