differential stress control on the growth and orientation of flame perthite a palaeostress direction indicator abstract albite flames result from the replacement of k feldspar by albite through alkali exchange in flame perthites from deformed granites within the grenville front tectonic zone replacement is driven by retrograde breakdown of plagioclase and hydration of k feldspar within the strain fabric of the host rock albite flames preferentially form subparallel to the inferred maximum compression direction and also develop at high stress points such as grain to grain contacts flame tips are generally parallel to the normal perthite crystallographic direction i e the murchison plane which is the orientation of the plane of minimum crystallographic lattice misfit between the albite and k feldspar we propose that the morphology of an albite flame is controlled by the coherent propagation of its tip into the k feldspar crystal lattice and that the orientation of the flames with respect to the rock fabric is controlled by the differential stress imposed during metamorphism this type of replacement requires dry conditions in which there would be inefficient dissolution reprecipitation along completely incoherent interfaces normal replacement and increased strength of feldspar under these conditions coherency is maintained to avoid a very large kinetic barrier the differences in lattice misfit along the murchison plane between k feldspar and albite requires either high elastic strain energy if the fit is coherent or lattice dislocation energy if the fit is semi coherent such energy might be expected to inhibit the replacement growth of albite lamellae in a host feldspar however compression parallel to the murchison plane reduces the lattice misfit along the compression direction this results 1 in an increase in the helmholtz free energy of the k feldspar and 2 in a reduction in the helmholtz energy of the albite lamellae or of the semi coherent interface thus albite replacement allows a decrease in net free energy without the destruction of the alumino silicate framework and is therefore favoured under conditions of high differential stress since flame growth is controlled by the imposed differential stress and not by strain in the host rock flame perthite has potential as a palaeostress direction indicator

petrological modifications in granitic rocks from the siljan impact structure evidence from cathodoluminescence abstract cathodoluminescence cl and microprobe analyses were performed on quartz plagioclase and k feldspar from granitic rocks in the siljan impact area central sweden in the upper 2200 m and in the vicinity of the gravberg 1 well secondary reddish luminescing quartz and brownish luminescing albite ab 98 and k feldspar or 95 are the dominant minerals primary magmatic quartz plagioclases oligoclase andesine and k feldspar show blue yellowish and bluish luminescence colours respectively the reddish luminescence colour of quartz is interpreted to have been induced by the shock heating episode of the impact and subsequent elevated temperature in the granites the impact shock pressure effects as shown by shatter cones and planar features were retained within granite dominated by blue luminescing quartz most alteration of the feldspar took place during the cooling episode that followed the impact heating post impact fracturing facilitated the infiltration of meteoric water which has further enhanced alteration of feldspars

the microstructures of perthitic alkali feldspars revealed by hydrofluoric acid etching etching of alkali feldspar cleavage fragments with hydrofluoric acid vapor followed by study of the surfaces by scanning electron microscopy sem is a simple and rapid technique for characterizing the microtextures of crypto and microperthites this technique has a number of advantages over conventional transmission electron microscopy tem including ease of sample preparation and the large areas of crystals which can be imaged alkali feldspars studied by the method can yield important information on the cooling history of igneous and metamorphic rocks fluid feldspar interactions and the morphology and microstructures of albite exsolution lamellae some of these applications are illustrated by examples of etched crypto and micro perthites from the klokken layered syenite south greenland and the shap granite north west england

exceptional reservoir quality in hpht reservoir settings examples from the skagerrak formation of the heron cluster north sea uk abstract as the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important the fluvial channel sandstone reservoirs of the skagerrak formation in the central north sea exhibit anomalously high porosities and permeabilities considering their depth of burial 3500 m below sea floor despite the complex depositional setting diagenetic history high overpressure and temperatures encountered in the skagerrak formation hydrocarbons are currently being produced the skagerrak formation reservoirs used in this study have encountered overpressures of 40 mpa and temperatures up to 185 c at present day maximum burial to identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic fluid inclusion and burial history modelling studies has been adopted our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation fluid inclusions found in the quartz overgrowths indicate late stage development with temperatures of formation in the range 130 to 170 c coincident with late stage deeper burial hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation the formation of well developed authigenic chlorite 70 surface coating and to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite rich sandstones a schematic porosity and quartz cement evolution model has been developed which allows for pre drill prediction of reservoir quality in the heron cluster and provide valuable insights for other complex high pressure high temperature reservoirs

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

hydrothermal alteration of tertiary igneous rocks from the isle of skye northwest scotland hydrothermal alteration of tertiary granites from skye involed the reaction of igneous amphibole biotite ferroaugite fayalite alkali feldspar plagioclase feldspar magnetite and ilmenite with aqueous fluid primarily to combinations of secondary alkali feldspar chlorite and montmorillonite lesser amounts of muscovite calcite epidote prehnite zeolite and pyrite also developed during mineralogical alteration of the granites there was a net addition to rock of h and a net loss of ca fe and probably na there is no positive evidence for a significant change in their al si mg or k contents hydrothermal alteration of skye granite involved not only large scale migration of 18o 16o d and h but also of ca fe and probably na mineral thermometry combined with textural observations suggests that chemical isotopic and most mineralogical alteration were synchronous and occurred at 350 450 c textural observations further suggest that the mechanism of isotopic exchange between granite and hydrothermal fluid primarily was solution reprecipitation

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

chemostratigraphy of upper jurassic reservoir sandstones danish central graben north sea abstract a chemostratigraphic study of upper jurassic sandstones in the northern danish central graben has been undertaken within the framework of a well defined stratigraphic sedimentological model based particularly on cored well sections two reservoir sandstone units are recognised the transgressive marginal marine to shoreface sandstone of the gert member and the regressive to transgressive shoreface sandstone of the ravn member both members belong to the heno formation which is equivalent to the fulmar formation uk and the ula formation norway multivariate analysis of geochemical data from 264 core samples from 8 wells reveals the distinction between the two reservoir sandstones gert and ravn members and the two offshore claystones farsund and lola formations specific elements have proven to be important for this separation and these elements demonstrate differences even in 2 dimensional cross plots the farsund formation is characterised by higher v u and p 2 o 5 and lower mgo and k 2 o when compared with the lola formation the gert member typically has higher maximum amounts of cr and tio 2 than the ravn member the high cr and tio 2 content probably from chrome spinel and ti minerals might be related to a source of exposed carboniferous sediments in the gert ridge area the ravn member is characterised by higher na 2 o p 2 o 5 and th contents than the gert member which may reflect a higher content of plagioclase na 2 o and a changed heavy mineral assemblage the mid north sea high is a likely source for the heavy mineral suite that characterises the ravn member the rita 1 well situated closest to the mid north sea high seems to have been more influenced by this source than the other wells in the hejre area a volcanic source supplying k feldspar may be responsible for the relatively high k 2 o al 2 o 3 observed in both the gert and ravn members hence in addition to differentiation between the two reservoir sands and between the two offshore claystones this study also illustrates the use of geochemical data for evaluation of source characteristics and dominance of different sediment source areas

retrograde metamorphic reactions in deforming granites and the origin of flame perthite microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram orthoclase or92 97 albite and oligoclase andesine the albite occurs as grains in the matrix as flame shaped lamellae in orthoclase and in patches within plagioclase grains we present a metamorphic model for albite flame growth in the k feldspar in these rocks that is related to reactions in plagioclase rather than alkali feldspar exsolution flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions reaction 1 is a continuous or discontinuous across the peristerite solvus reaction in plagioclase in which the an component forms epidote or zoisite most of the albite component liberated by reaction 1 stays to form albite in the host plagioclase but some na migrates to form the flames within the k feldspar reaction 2 is the exchange of k for na in k feldspar reaction 3 is the retrograde formation of muscovite as sericite and has all of the chemical components of a hydration reaction of k feldspar the si and al made available in the plagioclase from reaction 1 are combined with the k liberated from the k feldspar to produce muscovite in reaction 3 the muscovite forms in the plagioclase rather than the k feldspar as a result of the greater mobility of k relative to al the composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction using an initial plagioclase composition of an20 the total reaction can be summarized as 20 oligoclase 1 k feldspar 2 h2o 2 zoisite muscovite 2 quartz 15 albiteplagioclase 1 albiteflame this model does not require that any additional feldspar framework be accreted at replacement sites na and k are the only components that must migrate a significant distance e g from one grain to the next allowing al to remain within the altering plagioclase grain the resulting saussuritization is isovolumetric the temperature and extent of replacement depends on when and how much water infiltrates the rock the fugacity of the water and therefore the pressure of the fluid may have been significantly lower than lithostatic during flame growth

experimental development of patch perthite from synthetic cryptoperthite microstructural evolution and chemical re equilibration in this study deuteric coarsening of a lamellar cryptoperthite to a patch perthite has been experimentally induced for the first time an homogeneous alkali feldspar bulk composition ab60or40 was produced by molten salt ion exchange with a gem quality orthoclase from madagascar this was then annealed isothermally for 32 days at 550 c this resulted in a coherent lamellar cryptoperthite periodicity 28 nm probably by spinodal decomposition and subsequent coarsening the cryptoperthite was then reacted with 20 wt h2o or 0 1 m hcl at 400 and 500 c at 200 400 and 1000 mpa for 96 h and for 192 h at 1500 mpa at 1000 and 1500 mpa and 500 c the cryptoperthitic crystal fragments were partially replaced by a mosaic of albite and sanidine subgrains in the form of a patch perthite other than the gibb s free energy of reaction as derived from the chemical potentials of the reactant and product phases the second principle mechanism responsible for driving this reaction is interface coupled dissolution reprecipitation driven by a reduction in the coherency strain energy stored within the initial cryptoperthite the main indicator of this is a change in the microtexture due to the destruction of the coherent lamellar cryptoperthite and its replacement by an assemblage of incoherent subgrains detailed mass balancing based on xrd and epma data indicate that conversion of cryptoperthite to patch perthite is not isochemical experimental formation of patch perthite appears to take place in two steps between the original cryptoperthite and the patch perthite tem investigation identified a transition zone characterized by a significant coarsening of the preexisting lamellar microstructure without any sign of a loss in lattice coherency this zone is probably the result of hydrous species diffusing into the cryptoperthite causing the h2o enhanced diffusion of na and k and hence lamellar coarsening in the second step this coarsened microstructure is replaced by the patch perthite the polycrystalline patch perthite is characterized by a very complex distribution of albite and sanidine sub grains even the larger albite or sanidine patches are polycrystalline this inevitably results in a three dimensional network of grain boundaries characterized by high defect densities resulting from the structural misfit between the albite and sanidine sub grains an increase in average grain size with distance from the interface indicates that secondary coarsening of the initially very fine grained intergrowth occurred this was driven by the reduction of interface and surface energy similar to the coarsening observed in multiphase ceramics in addition the newly formed patch perthite is highly porous despite the fact that the results obtained in this study show that the replacement of cryptoperthite by patch perthite is nearly isovolumetric v 0 6 the observed reaction induced porosity results from differences in relative solubilities between the cryptoperthite and the patch perthite porosity abundance has been estimated at approximately 10 to 11 vol based on the amount of quartz formed during the experiments three dimensional investigations via fib serial sectioning indicate almost no interconnectivity between pores a consequence of albite and sanidine sub grain coarsening is the constant redistribution of the porosity and the sub grain boundaries within the patch perthite mosaic over time this suggests that a dynamically evolving porosity could provide an important transport mechanism for fluid transport through rocks even when the abundance and interconnectivity of the reaction induced porosity is low

routine characterization and interpretation of complex alkali feldspar intergrowths almost all alkali feldspar crystals contain a rich inventory of exsolution twin and domain microtextures that form subsequent to crystal growth and provide a record of the thermal history of the crystal and often of its involvement in replacement reactions sometimes multiple microtextures strongly influence the subsequent behavior of feldspars at low temperatures during diagenesis and weathering they are central to the retention or exchange of trace elements and of radiogenic and stable isotopes this review is aimed at petrologists and geochemists who wish to use alkali feldspar microtextures to solve geological problems or who need to understand how microtextures influence a particular process we suggest a systematic approach that employs methods available in most well founded laboratories the crystallographic relationships of complex feldspar intergrowths were established by the 1970s mainly using single crystal x ray diffraction but such methods give limited information on the spatial relationships of the different elements of the microtexture or of the mode and chronology of their formation which require the use of microscopy we suggest a combination of techniques with a range of spatial resolution and strongly recommend the use of orientated sections sections cut parallel to the perfect 001 and 010 cleavages are the easiest to locate and most informative techniques described are light microscopy scanning electron microscopy using both backscattered and secondary electrons including the use of surfaces etched in the laboratory electron probe microanalysis and analysis by energy dispersive spectrometry in a scanning electron microscope transmission electron microscopy we discuss the use of cathodoluminescence as an auxiliary technique but do not recommend electron backscattered diffraction for feldspar work we review recent publications that provide examples of the need for great care and attention to pre existing work in microtextural studies and suggest several topics for future work

secondary porosity generation during deep burial associated with overpressure leak off fulmar formation united kingdom central graben the overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known from the resulting effective stress the maximum potential porosity mpp can be calculated the mpp is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure measured rock porosities should be at or below the mpp we have determined the mpp history for the fulmar formation sandstones upper jurassic of the central graben north sea and have compared the predictions to measured core data we conclude that for the majority of the fulmar formation sandstones the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation however in wells sited close to regional overpressure leak off points the porosity has been significantly increased from an end of oligocene low mean 21 to the present day values mean 31 this porosity increase occurred by feldspar dissolution with the reaction products being removed from the sandstones secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured although still part of an open hydrogeological system the generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted

nanotunnels and pull aparts defects of exsolution lamellae in alkali feldspars we have studied defects associated with flat lens shaped perthitic albite lamellae in alkali feldspars using sem and tem in orthoclase phenocrysts from the shap granite cumbria nw england bulk composition or 70 20 ab 29 05 an 0 85 no dislocations were found even in optically fresh parts of grains instead dissolution inferred to be localized on edge dislocation loops has created tiny nanotunnels typically t effect of the 1 an in the feldspars is taken into account for commencement of coherent exsolution the orientation of the pericline twins was fixed shortly after coherent exsolution began previous work has indicated that edge dislocations would start to form during cooling at 400 c so that nanotunnels form at still lower t dry heating experiments were carried out to establish the stability of the defects and the homogenization behavior of the exsolution lamellae na k exchange is rapid on heating above the coherent solvus and chemical homogenization of lamellae is complete after 24 h at 700 c in contrast nanotunnels persist for 148 h at 1000 c and 5748 h at 700 c below the coherent solvus exsolution lamellae thin on heating leaving nanotunnels stranded in the orthoclase matrix microtextures related to si al ordering patterns in the framework such as albite twins are not eliminated forming ghost like lamellar strain patterns in chemically homogeneous feldspar the presence of nanotunnels in optically fresh alkali feldspars shows that not only granites but also granulite facies rocks have been pervasively affected by fluids at low t both nanotunnels and pull aparts have important implications for feldspar reactivity in the upper crust for 18 o exchange and for transport of 40 ar both in nature and in laboratory step heating

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 m in length normal to 010 and 0 2 20 m 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

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 m 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 m 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 ab2 5or97 4an0 1 associated with very pure porous albite ab97 0or1 6an1 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

young high temperature granulites from the base of the crust in central mexico characterization of the lower continental crust is essential to understanding processes of crustal formation and evolution many workers have concluded that the lower crust is composed of granulite facies rocks see for example refs 1 2 but many exposed granulite facies terranes seem to have mid crustal rather than lower crustal origins3 5 here we present new data from direct samples of the lower continental crust in the form of metapelitic rock fragments brought to the surface by quaternary volcanism in central mexico these xenoliths contain exsolved ternary feldspars that record extremely high minimum metamorphic temperatures of 950 1 100 c the highest yet known to be preserved in deep seated metamorphic rocks the metamorphism seems to be the result of underplating by basaltic magma and to be regional in extent our barometry on these samples demonstrates that granulite facies metamorphic rocks are present in the lower crust and provides important constraints on tectonic models and interpretations of geophysical data chemical and text lira i evidence suggest that the xenoliths were not able to cool to below 850 900 c before being entrained in their host magmas indicating that the granulite facies metamorphism occurred no more than 30 myr ago

reservoir quality and rock properties modeling triassic and jurassic sandstones greater shearwater area uk central north sea abstract the complex burial and diagenetic histories of the jurassic fulmar and triassic skagerrak sandstones in the uk central north sea present significant challenges with regard to reservoir quality and rock property prediction commercial reservoir quality is retained despite deep burial and associated high temperatures and pressures shallow marine fulmar sands are normally compacted mean igv 26 3 yet have porosities of 21 33 porosity was preserved through inhibition of quartz cementation by clay and microquartz coatings and enhanced by dissolution of framework grains 5 skagerrak fluvial sands are more compacted mean igv 23 2 exhibit minor feldspar dissolution we modeled reservoir quality evolution using the forward diagenetic model touchstone which simulates porosity loss due to compaction and quartz cementation quantitative petrographic analyses and burial history data were used to calibrate touchstone model parameters the results were applied to deeper prospects for pre drill prediction of porosity and permeability in parallel petrophysical data were used to characterize the elastic properties of the sandstones to provide a basis for quantitative seismic forward modeling experimental data and core calibrated petrophysical results reflecting variable in situ fluids and saturations were used to build an elastic properties model the model is robust and was used to generate fluid filled sandstone properties incorporating touchstone results for prospect specific seismic attribute modeling well results from exploration wells are in good agreement with pre drill touchstone and elastic properties model predictions

aluminium loss during sandstone diagenesis petrographical observations from a sandstone buried to between 3 2 and 5 7 km in the north sea central graben suggest a significant change in bulk chemistry during burial the sand contained 35 or more feldspar upon deposition but due to extensive dissolution only 5 remains in the deep sandstones this has released much more al than is currently contained as authigenic minerals up to 1460 mol cm 3 of al have been exported this deep secondary porosity is economically important but its recognition by petrographic textures is unreliable as are bulk chemical provenance studies of deeply buried sedimentary and metamorphic strata

diagenetic k feldspar textures a tem study and model for diagenetic feldspar growth abstract diagenetic k feldspar overgrowths from the permo triassic continental clastic sediments of the elgin area in scotland were examined using conventional petrographic techniques and transmission electron microscopy to investigate the mechanisms of feldspar authigenesis the detrital k feldspar in these rocks is tartan twinned whereas the diagenetic k feldspar is untwinned the overgrowths are optically homogeneous but are not in optical continuity with the detrital grains overgrowths are up to 50 m thick and comprise numerous rhombs indicating that nucleation was a relatively easy process most overgrowths consist of a mosaic of micron sized rhombs nucleation on different twin planes on the detrital grain surface resulted in different orientations of the diagenetic nuclei the different orientations were propagated into the overgrowth as new nuclei formed on the surface of pre existing diagenetic crystals the detrital k feldspar is triclinic maximum microcline diagenetic k feldspar deviates only slightly from monoclinic orthoclase even though the triclinic polymorph is the stable k feldspar under the low temperature conditions of growth disordered crystals involve a simpler nucleation growth mechanism than ordered crystals consequently disordered crystals grow faster than ordered crystals monoclinic feldspar has disordered al si and triclinic feldspar has ordered al si diagenetic feldspar has metastable low triclinicity because this crystal structure could grow faster than the stable feldspar nanometer scale growth zones occur within individual rhomb representing variations in al si disorder such variations probably result from subtle variations in the state of supersaturation of the pore fluids and therefore in the rate of growth the interface between the detrital grain and the overgrowth represents no profound break in the feldspar crystal lattice despite the change in crystallography but is marked by the termination of twin planes in the detrital grain and the presence of dislocations in the overgrowth the termination of twins resulted in strain in the overgrowth which was relieved by the formation of dislocations small pores representing fluid inclusions and rare clay grains also occur at the interface

1984 sepm presidential address diagenetic albitization of potassium feldspar in arkosic sandstones abstract petrographic studies of subsurface cores of the fountain formation pennsylvanian in the denver region colorado show that detrital grains of potassium feldspar have been pervasively and complexly albitized during burial diagenesis in some strata all detrital grains of potassium feldspar have been convened to albite in some instances the conversion has taken place directly in others it has involved prior replacement of the potassium feldspar by other minerals particularly anhydrite calcite and dolomite which in turn are replaced by albite the albitized grains display fabrics such as perthitic intergrowths and chessboard twinning that are strikingly similar to replacement fabrics produced in feldspars by igneous and metamorphic processes if successive stages of the convers on to albite were not preserved the diagenetically albitized grains might easily be interpreted erroneously as detrital grains of plagioclase perthite or antiperthite derived from igneous or metamorphic source rocks the sodium required for the albitization is presumed to have been derived from intraformational sources particularly from diagenetically altered detrital grains of sodium rich plagioclase which have been extensively replaced by sodium free clay minerals ions released in the course of the albitization have provided important intrastratal sources of potassium aluminum and silicon which have been reprecipitated locally in authigenic minerals particularly albite illitic clays and adularia

origin of cryptoperthites in the bishop tuff and their bearing in its thermal history the bishop tuff is a large rhyolitic ash flow which erupted from long valley caldera in eastern california and contains abundant sanidine cryptoperthite phenocrysts which can be used to constrain the time between the two major cooling units the cryptoperthites bulk composition or66ab33an1 have exsolved in two principal stages followed by a third minor stage the earliest exsolution which produced patches of relatively wide lamellae 100 800 nm without wedge shaped ends wse occurred by coherent nucleation and was probably related to a high flux of water associated with fumarolic activity in this area the spacing of these lamellae shows no correlation with depth or thermal history the second major stage of exsolution occurred by spinodal decomposition which produced lamellae with wse having spa ein g s of from 80 to 170 nm and the spacing of these lamellae varies with depth in a pattern consistent with the two known cooling units heat flow calculations indicate that this pattern of lamellar spacings is consistent with an interval of about 1 0 2 0 years between the eruption of these two cooling units

dislocation formation and albitization in alkali feldspars from the shap granite 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 ab90an9ori occurs together with orthoclase rich feldspar ab10or90 i n 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 8c 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 8c 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

denticles on chain silicate grain surfaces and their utility as indicators of weathering conditions on earth and mars abstract denticulated margins also known in older literature as sawtooth cockscomb or hacksaw terminations are a common feature of pyroxenes and amphiboles visible in optical and electron microscopes denticles are remnants of undissolved material that formerly constituted the walls between elongate etch pits the characteristic aqueous dissolution form of chain silicate minerals denticles occur widely in terrestrial near surface materials that have experienced low temperature aqueous alteration including chemically weathered regoliths soils in a variety of climatic and geomorphic settings sediments and sedimentary rocks ranges of dissolution forms and dimensions commonly tens of microns in length are identical on both pyroxenes and amphiboles in these materials denticles on terrestrial chain silicates are formed by low temperature aqueous alteration denticles on a soil mineral grain of unknown composition at the phoenix mars lander site and on terrestrial pyroxenes naturally weathered in terrestrial low temperature aqueous environments are broadly similar in size shape and orientation relationships between denticles however quantitative morphometric parameters vary among different terrestrially weathered pyroxenes and even between different images of compositionally identical pyroxenes from the same naturally weathered sample such variations are too large to allow simple identification of minerals of unknown composition from morphology alone nevertheless the overall morphological similarities are consistent with a low temperature aqueous corrosion origin of denticles at the phoenix mars lander site

numerical assessment of dissolution versus replacement in the subsurface destruction of detrital feldspars oligocene frio formation south texas abstract dissolution is the main process responsible for modifying detrital feldspar assemblages in the oligocene frio formation of south texas accounting for up to 80 of feldspar modification in completely altered assemblages this finding is supported through two independent calculations one based on an estimate of initial feldspar content of the sandstones the other on estimated initial composition of the feldspars themselves volumetric loss of detrital feldspar on the observed scale over such a short time span has major implications for provenance interpretations and for understanding the basin wide mass balance of several major diagenetic components the wide variation of feldspar content and composition at any given depth temperature attests to the premier significance of fluid co position and flux in driving the reactions that bring about dissolution of feldspars in these rocks

feldspar thermometry in ultrahigh temperature metamorphic rocks evidence of crustal metamorphism attaining 1100 c in the archean napier complex east antarctica ultrahigh metamorphic temperatures attained in the mid to lower crust have been assessed by examining the mineral chemistry of ternary feldspars with relatively coarse exsolution lamellae from the archean napier complex east antarctica chemical compositions of re integrated perthitic mesoperthitic and antiperthitic feldspars are calculated from the modal proportions and the chemical analyses of host and lamellar domains formed through exsolution based on ternary feldspar solvus models re integrated compositions of feldspars from a variety of rock types yield the minimum equilibrium temperatures ranging from 1000 to 1110 c 0 8 gpa these data confirm the suggestion that the regional thermal conditions of the napier complex reached or exceeded 1100 c as feldspar is one of the common constituents of the crustal rocks this approach could be applicable to a wide variety of rocks in which feldspar represents exsolution textures

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 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

effect of water regime and vegetation on initial granite weathering in a glacier forefield evidences from cl sem and nomarski dic microscopy glacier forefields are characterised by high spatial variability in water availability solar radiation and vegetation as well as weathering processes the aim of this study is to improve the understanding of initial weathering of granite material with respect to the water regime availability and residence time of the water and the vegetation cover samples of granite powder were buried in four selected sites at the damma glacier forefield for one year a combined application of cathodoluminescence cl microscopy and spectroscopy scanning electron microscopy sem and nomarski differential interference contrast nomarski dic microscopy was used to investigate the initial stage of weathering the examination of the initial weathering stage revealed that the exposed mineral grains were strongly characterised by intra variations a clear change in particle shape and surface morphology could not be observed within the relatively short exposure period of one year however the grains from the lake sediment exhibited a clearly rounded shape caused by mechanical abrasion during transport processes the combination of several microscopic methods revealed i a strong influence of vegetation in the case of plagioclase oligoclase alteration ii a mutual effect of water and vegetation in the case of biotite delamination and iii alterations of potassium feldspar independent of local environments investigations with cathodoluminescence microscopy showed the suppression of blue luminescence and developed brown luminescence and confirmed a new insight in potassium feldspar weathering

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

peristeritic plagioclase in north sea hydrocarbon reservoir rocks implications for diagenesis provenance and stratigraphic correlation reservoir sandstones from well 30 16 7 in the upper jurassic humber group uk north sea contain plagioclase feldspars with a range of chemical compositions microtextures and origins the chemical composition of feldspar grains in samples from three different depths in the core ranges from 0 to 18 mol an two of the samples have a well defined gap in the compositional range between an 4 and an 9 this gap reflects the absence of plagioclase feldspars with those compositions in the ultimate source area of the humber group sandstones examination of an bearing feldspars on either side of the gap by tem shows that they have lamellar and or tweed exsolution microtextures that are characteristic of plagioclase feldspars having bulk compositions within the peristerite miscibility gap this is the first time that peristerites have been found in a sedimentary rock and they suggest a low grade metamorphic provenance for the plagioclase population compositional gaps have been commonly observed in ab rich plagioclase feldspars from metamorphic rocks although the precise reasons why feldspars do not form within the gap remain poorly understood in addition to the an bearing peristerites 30 16 7 contains a varied suite of an poor feldspars the microtexture of some of these grains is consistent with a low grade metamorphic provenance whereas others are inferred to be diagenetic in origin and to have formed by albitization of detrital alkali or plagioclase feldspar stratigraphic differences in the presence and size of the compositional gap within the humber group suggests that plagioclase feldspars may be a powerful new tool for high resolution stratigraphic correlation and may also be very sensitive indicators of changes in provenance

low temperature coherent exsolution in alkali feldspars from high grade metamorphic rocks of sri lanka in the alkali feldspars of the amphibolite and granulite facies rocks of sri lanka a late stage final exsolution event is observed which produced film lamellae and fine scale spindles these were investigated by optical microprobe single crystal transmission electron microscopy and atomic resolution microscopy techniques the lamellae and spindles exsolved below the coherent solvus at temperatures as low as 300 to 350 c precession photographs and arm micrographs show that the intergrowth is perfectly coherent in sections 010 the rhombic section of the pericline twins corresponds to analbite or high albite the albite lamellae and spindles nucleated and grew at low temperatures in a metastable disordered structural state within a tweed orthoclase matrix and became periodically twinned analbite or high albite which subsequently developed only a slight increase in al si order the relationship between twin periodicity and lamellar width predicted for coherent intergrowths by willaime and gandais 1972 is obeyed in or rich grains in which coherent exsolution is the only exsolution event the film lamellae tend to be restricted to the rim the fine scale spindles to the centre of the grains the films nucleated heterogeneously at grain boundaries and grew towards the grain centres fine scale spindles probably nucleated homogeneously in the interior part of grains heterogeneous nucleation and coherent growth are not mutually exclusive

microtextural controls of diagenetic alteration of detrital alkali feldspars a case study of the shap conglomerate lower carboniferous northwest england abstract the diagenetic alteration of detrital alkali feldspar grains in the lower carboniferous shap conglomerate which were derived from the nearby lower devonian shap granite was strongly influenced by their pre diagenetic microtextures alkali feldspars collected from a working quarry in the shap granite are perthites composed of two types of microtextures primary igneous and deuterically altered primary igneous microtextures which formed during early cooling of the pluton are composed of albite exsolution lamellae in tweed orthoclase small lamellae platelets form cryptoperthites whereas larger lamellae films form microperthites albite platelets have coherent interfaces with orthoclase i e the crystal structure is strained but continuous across the interface of the exsolution lamellae whereas larger films are semicoherent i e dislocations have formed to relieve strain at the exsolution interface deuteric alteration leads to the development of micropore rich veins composed of discrete subgrains of albite and microcline referred to as patch perthites which cut the cryptoperthites and microperthites these veins formed during fluid feldspar interaction within the granite in the later stages of igneous cooling subgrains within these veins are semicoherent or are incoherent i e there is no continuity in crystal structure across the subgrain boundary during diagenesis of shap granite derived alkali feldspars in the shap conglomerate the semicoherent and incoherent albite in microperthites and patch perthites was selectively dissolved or replaced by microcline whereas coherent albite platelets in cryptoperthites remained largely unaltered this diagenetic history reflects an interplay between two main determinants of alteration i the nature of the pore fluids which resulted in albite rich feldspar being more heavily altered than orthoclase rich feldspar and ii the coherency of interfaces which determined the specific albite rich feldspar microtextures that were affected fluid feldspar interaction was favored at semicoherent and incoherent interfaces owing to their high surface and strain energies release of energy stored at such interfaces is also the major intragranular driving force of deuteric alteration of alkali feldspars in cooling igneous rocks in general as a result petrographically and geochemically similar microtextures may result from both deuteric and diagenetic alteration and care must be taken to discriminate between them results of this study underscore the importance of understanding the nature of alkali feldspars in igneous and metamorphic rocks for an accurate interpretation of their subsequent diagenetic history

alkali feldspar microtextures as provenance indicators in siliciclastic rocks and their role in feldspar dissolution during transport and diagenesis abstract alkali feldspars usually exhibit optical or sub optical intergrowths of albite and k feldspar known generically as perthite which have morphologies and crystallographic characteristics that are distinctive of the igneous or metamorphic environment in which they grew and cooled to surface temperatures we review the current terminology and understanding of how the microtextures form and discuss their role in feldspar dissolution during weathering transport and diagenesis we show how microtextures can be used as provenance indicators in arkosic siliciclastic rocks using examples from the fulmar formation a reservoir rock in the upper jurassic humber group in the central north sea in which feldspar dissolution is a major source of secondary porosity we describe the most effective techniques for routine characterization of which the most practical is back scattered electron imaging in a scanning electron microscope often coupled with cathodoluminescence we provide an atlas of the main types of perthitic intergrowth likely to be encountered in igneous and metamorphic rocks worldwide and show how many of the microtextures can be matched in clastic grains in the fulmar formation grains which are preferentially preserved have microtextures that make them relatively unreactive in aqueous fluids at low temperatures and we conclude that provenance has a major impact on reservoir quality detrital grains in the fulmar exhibit authigenic albitization k feldspar overgrowths and replacement and we discuss how diagenetic features can be distinguished from replacive features developed in the protolith

fractional crystallization and zoning in igneous feldspars ideal water buffered liquid fractionation lines and feldspar zoning paths diagrams giving plagioclase and sanidine fractionation paths and liquid fractionation lines under conditions of ideal water buffered fractional crystallization in the ternary feldspar system were constructed graphically using topological reasoning and experimental data and calculated phase relationships from the literature the liquidus lines and solidus or solvus paths are unique at constant p and ah2o the composition of a liquid evolves with time and moves along a fractionation line by removal of successive crystal fractions whereas the compositions of each of the crystal fractions lie on and define a solidus or solvus path most but not all such water buffered lines and paths differ only slightly from those in which water is free to build up during crystallization and ah2o to increase as in many rocks liquid compositions lying along liquidus fractionation lines are not normally preserved unless erupted as aphyric lavas the solidus or solvus paths may be preserved either as overgrowth zones in crystals zoning paths or as a series of crystal fractions in layered intrusions the topologies of the lines and paths depend mainly on the nature of the two feldspar boundary line separating the plagioclase and sanidine fields which is a function of ph2o or ah2o at constant p increases in either progressively lower the liquidi and solidi and cause larger intersections of the solidi with the solvus one feldspar solidus paths at high p and ah2o are simple whereas they are complex and may bend back on themselves at low ph2o or low ah2o at high p two feldspar paths may be simultaneous cotectic or sequential peritectic the former are simple and do not meet at high p and ah2o the critical solution line lying in the gap they are complex and may bend back or overlap at low ph2o or low ah2o at high p the position of the critical solution line being hard to determine liquids which have simultaneously fractionated two feldspars may fractionate only one towards the end crystallization changing from subsolvus to hypersolvus sequential paths may involve overgrowth of an early feldspar by a later one usually sanidine overgrowths on plagioclase but plagioclase overgrowths on sanidine occur these complexities explain in part the difficulties of unravelling the textural and compositional relationships of ternary feldspars in water poor felsic igneous rocks even in the absence of alteration or complex magma dynamics and of trying to deduce phase relationships from natural occurrences of feldspars

petrography and composition of authigenic feldspars oligocene frio formation south texas abstract detrital feldspar assemblages 1 to 4 km burial depth in oligocene sandstones from south texas are extensively altered by dissolution in some samples cementation in secondary pores principally by albite but also by authigenic k feldspar and other minerals occurs subsequent to the dissolution and can lead to wholesale replacement textures solid state diffusion of cations has not been important in the replacement process information from conventional light microscopy sem examination and back scattered electron imaging suggests that relative rates of dissolution versus replacement varied widely among different detrital grains through time even on the scale of a thin section dissolution of detrital feldspars and precipitation of relatively pure albite and k feldspar take place ver a large depth interval and thus can be markedly separated in time vacuolized albite occurring as irregular veins within detrital grains is associated with little visible secondary porosity and forms when precipitation is rapid relative to dissolution of the detrital grain more rapid dissolution results in formation of larger scale intragranular secondary pores commonly filled by subsequent precipitation of clear albite within the former grain volume electron microprobe analyses show that the composition of vacuolized albite and authigenic k feldspar deviate somewhat from ideal compositions presumed typical for authigenic feldspars regardless of their cause these subtle variations in composition have important ramifications for basin scale mass balance calculations requiring accurate estimates of the volume of authigenic feldspar

characterization of mineral surfaces using fib and tem a case study of naturally weathered alkali feldspars using a focused ion beam fib instrument electron transparent samples termed foils have been cut from the naturally weathered surfaces of perthitic alkali feldspars recovered from soils overlying the shap granite northwest england characterization of these foils by transmission electron microscopy tem has enabled determination of the crystallinity and chemical composition of near surface regions of the feldspar and an assessment of the influence of intragranular microtextures on the microtopography of grain surfaces and development of etch pits damage accompanying implantation of the 30 kv ga ions used for imaging and deposition of protective platinum prior to ion milling creates amorphous layers beneath outer grain surfaces but can be overcome by coating grains with 85 nm of gold before fib work the sidewalls of the foil and feldspar surrounding original voids are also partially amorphized during later stages of ion milling no evidence was found for the presence of amorphous or crystalline weathering products or amorphous leached layers immediately beneath outer grain surfaces the absence of a leached layer indicates that chemical weathering of feldspar in the shap soils is stoichiometric or if non stoichiometric either the layer is too thin to resolve by the tem techniques used i e similar to 2 5 nm or an insufficient proportion of ions have been leached from near surface regions so that feldspar crystallinity is maintained no evidence was found for any difference in the mechanisms of weathering where a microbial filament rests on the feldspar surface sub micrometer sized steps on the grain surface have formed where subgrains and exsolution lamellae have influenced the propagation of fractures during physical weathering whereas finer scale corrugations form due to compositional or strain related differences in dissolution rates of albite platelets and enclosing tweed orthoclase with progressive weathering etch pits that initiated at the grain surface extend into grain interiors as etch tubes by exploiting preexisting networks of nanopores that formed during the igneous history of the grain the combination of fib and tem techniques is an especially powerful way of exploring mechanisms of weathering within the internal zone beneath outer grain surfaces but results must be interpreted with caution owing to the ease with which artifacts can be created by the high energy ion and electron beams used in the preparation and characterization of the foils

what classic greywacke litharenite can reveal about feldspar diagenesis an example from permian rotliegend sandstone in hessen germany abstract rotliegend siliciclastic sediments in southern hessen germany are a good example of dissolution of detrital feldspars which is a common feature in many sandstones dissolution occurred after mechanical compaction of the lithic rich sandstone which experienced framework collapse with pores and pore connections filled and obstructed by deformed ductile lithic grains pseudomatrix thereby reducing pore space to microporosity the advanced degree of compaction and reduced porosity caused low permeability and low hydraulic conductivity of the rock mass this is further reduced by the presence of wackes and shales that occur intercalated with the sandstones feldspar dissolution thus took place in low permeable sediments when large scale flow of meteoric or acidic fluids is ruled out as a cause of feldspar dissolution mineral precipitation illite kaolinite and albite took place within pseudomatrix and detrital matrix as well as in secondary pores created by feldspar dissolution feldspar was the source for the authigenesis the system was thus closed during burial after framework collapse and diagenetic reactants in the form of detrital components were already present within the system the original mass was preserved but redistributed and diagenetic minerals were the local sinks for the dissolved reactants precipitating within the system this also suggests that burial diagenesis in general might be more mass conservative than usually assumed rotliegend sandstones thus form a case where despite of the lack of external exchange of mass by fluid flow major diagenetic processes did take place and significantly modified the original mineralogy and texture feldspar diagenesis can take place from other processes than mere large scale flushing of open systems as often supposed it implies that the volumes of rock affected by feldspar diagenesis may be much larger than anticipated based upon the common hold believe that feldspar diagenesis is linked to unconformities and surface weathering or dissolution in near surface aquifers

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 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 sub m to m 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 4 0 ar 3 9 ar 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

numerical simulation of the 9 10 june 1972 black hills storm using csu rams strong easterly flow of low level moist air over the eastern slopes of the black hills on 9 10 june 1972 generated a storm system that produced a flash flood devastating the area based on observations from this storm event and also from the similar big thompson 1976 storm event conceptual models have been developed to explain the unusually high precipitation efficiency in this study the black hills storm is simulated using the colorado state university regional atmospheric modeling system simulations with homogeneous and inhomogeneous initializations and different grid structures are presented the conceptual models of storm structure proposed by previous studies are examined in light of the present simulations both homogeneous and inhomogeneous initialization results capture the intense nature of the storm but the inhomogeneous simulation produced a precipitation pattern closer to the observed pattern the simulations point to stationary tilted updrafts with precipitation falling out to the rear as the preferred storm structure experiments with different grid structures point to the importance of removing the lateral boundaries far from the region of activity overall simulation performance in capturing the observed behavior of the storm system was enhanced by use of inhomogeneous initialization

diagenetic k feldspar pseudomorphs in the triassic buntsandstein sandstones of the iberian range spain early diagenetic k feldspar in the triassic buntsandstein of the iberian range spain occurs as pseudomorphs after detrital k feldspar or 99 untwinned commonly heavily clouded by vacuoles and tiny inclusions dark luminescing and are composed of numerous fine euhedral crystals of k feldspar the latter property suggests that the pseudomorphs form via dissolution of detrital k feldspar and plagioclase and precipitation of authigenic k feldspar x ray diffraction analysis shows that the authigenic k feldspar is intermediate microcline

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 m2 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 m to m sized pores m2 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 m 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

dissolution of olivine during natural weathering abstract naturally weathered olivine occurring as phenocrysts in hawai ian volcanic rocks from several volcanic centers and regolith outcrop settings and as tectonized olivines from several metadunite bodies in the southern appalachian blue ridge are all similarly corroded by natural weathering conical funnel shaped etch pits occur as individual pits base to base pairs of cone shaped pits or en echelon arrays etch pit shapes and orientations in the smallest etch pit arrays visible in conventional scanning electron microscopy resemble even smaller features previously reported from transmission electron microscope investigations of olivine weathering etch pits occur in samples with chemical and or mineralogical evidence of weathering and or are associated with or proximal or directly connected to fractures or exposed outcrop surface and therefore are formed by weathering and not inherited from pre weathering aqueous alteration e g serpentinization iddingsitization of these parent rocks many etch pits are devoid of weathering products natural weathering of olivine is surface reaction limited similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization recrystallization history eruption weathering exposure ages of the olivine s host rock and the local regolith history

40 ar 39 ar thermochronology using alkali feldspars real thermal history or mathematical mirage of microtexture the multiple diffusion domain mdd theory of ar loss based on the 40ar 39ar method has been used to obtain thermal histories over tens of millions of years in the range 500 c to near surface temperatures of alkali feldspars from slowly cooled geological environments and recovered in situ from sedimentary basins and geothermal systems it assumes that ar diffuses in nature over geological time from the same domains and by the same processes that control its diffusion during laboratory step heating experiments mathematically the model has many stringent requirements the domains must form at t above those being modelled and must remain unchanged during geological time and during step heating must not interact must occur in discrete size ranges sharing one simple geometrical shape contain no compositional gradients in 40k and release ar into an infinite reservoir we describe the nature of possible diffusion domains and the fast pathways for diffusion between them known to exist in alkali feldspars from direct electron microscopic observations and explore their time temperature evolution based on established phase behaviour both over geological time and during step heating strain controlled microtextures whose morphology is controlled by the minimization of coherency strain energy do not provide fast pathways for ar loss and behave as crystallographically complex but single domains for ar subregular dislocations may form over a range of t in some strain controlled intergrowths but in nature the majority form closed loops and do not provide pathways to the crystal surface deuteric microtextures cut across strain controlled microtextures and are composed of microporous subgrain and grain mosaics with many pathways and a wide spectrum of possible domain sizes and shapes the deuterically altered zones provide pathways for ar diffusion out of the crystal although some ar is retained in fluid inclusions all microtextures likely to define diffusion domains form below 500 c over a range of t and in slowly cooled rocks of times the most important involve fluid feldspar reactions which can reset ar and continue to surface t feldspars in geothermal systems and young sedimentary basins are subject to ongoing chemical and microtextural changes because the microtextures are metastable during step heating experiments in the t range over which most ar is released domains and pathways for rapid diffusion of ar between domains will be significantly modified during step heating particularly above 750 c when departures from a simple arrhenius diffusion law are most marked pre melting is likely to be an important cause of complexity in ar loss behaviour few if any of the requirements of the mdd model are met and we conclude that thermal histories cannot be obtained from alkali feldspars using this method

composition of modern stream sand derived from a mixture of sedimentary and metamorphic source rocks henares river central spain the henares river central spain flows westward from the iberian range mesozoic sedimentary rocks under semiarid climatic conditions in the middle and lower reaches the henares river receives sediment from three tributaries canamares bornova and sorbe rivers that drain the central system paleozoic crystalline rocks low grade metamorphic rocks and minor amounts of mesozoic sedimentary rocks modern sands from the henares river and its tributaries offer an excellent opportunity to evaluate the importance of lithology and physiography in determining detrital modes from mixed metamorphic and sedimentary source terrain sand modes from the henares river and its tributaries are quartzolithic they plot in a restricted area on a qmflt diagram with low contents of feldspar and variable amounts of quartz and carbonate metamorphic and minor siliciclastic lithic fragments higher feldspar abundances in the canamares and bornova stream sands are related to the erosion of gneissic rocks by contrast sands from sorbe river are low in feldspar reflecting the absence of coarse metamorphic sources thus proportions of bedrock lithologies in the drainage sub basins are the main control on detrital modes of the tributaries in addition slope acts on the sand productivity of source rocks sedimentary source rocks in the upper reaches of the three tributaries have a poor productivity in the carbonate lithic fragments lsc compared with the lsc productivity of sedimentary sources located downstream with higher relief the percentage of bedrock types in the source area versus petrographic indices lm l lss l and lsc l provide a useful contrast between source and sand composition in the tributaries thus the high content of metamorphic lithic grains in the lithic grain population over represents this lithology slate plus schist at the source terrain lithic grains from sedimentary clastic sources generate few recognizable grains lss in the lithic grain population local supplies from carbonate sources may produce important increases of lsc in short reaches of the channels sand compositions in the henares river seem to be very homogeneous in a qmflt diagram with only minor differences caused by the supplies from the tributaries these differences can be modeled in terms of end member mixing processes between henares detritus and detritus from the relevant tributary these mixing processes are more evident when lithic grain contents lmlvls and lmlsslsc diagrams are compared changes produced by inputs from the canamares bornova and sorbe rivers are not permanent along the henares course an important homogenization of henares sand composition takes place by the mixing of tributary deposits with previously deposited henares river terraces abrasion during transport does not appear to cause significant changes in the sand composition along the henares river

exsolution and coarsening mechanisms and kinetics in an ordered cryptoperthite series cryptoperthites from the klokken layered syenite intrusion were examined by tem to determine the role of exsolution ordering and twinning in the development of the coherent microtextures during slow cooling the stratigraphic position of the samples in the layered series giving an independent variable in determining their evolution both periodicity primary and secondary and morphology change with distance from the top of the series most samples contain low microcline in the diagonal association

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

zoned ternary feldspars in the klokken intrusion exsolution microtextures and mechanisms the microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the klokken intrusion described in the preceding paper were determined by tem and their origin and evolution deduced the feldspars normally have a plagioclase core and an alkali feldspar rim cores become smaller and rims larger and the an content of both decrease with distance from the contact of the intrusion the following microtextural sequence was observed the inner plagioclase cores are homogeneous oligoclase andesine with albite growth twins only but are crypto antiperthitic towards the outer core at first small platelets of low sanidine a few nanometres thick and up to 10 nm long occur sporadically only on albite twin composition planes with further increase in bulk or they are homogeneously distributed in the plagioclase thicker through going plates in platelet free areas are found which induce albite twins in the surrounding plagioclase the microtextures in the rims are regular cryptomesoperthitic with 601 lenses or lamellae depending on the bulk or content of low sanidine in albite twinned low oligoclase andesine albite and pericline twins in plagioclase in an m twin relationship together with lenticular low sanidine were found in only one small area the overall diffraction symmetry of the mesoperthites is monoclinic showing that exsolution started in a monoclinic feldspar whereas that of the antiperthites is triclinic the intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres the cryptomesoperthites are very regular where or rich and probably arose by spinodal decomposition the platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere near the intermediate zone they coarsened to give larger plates which induced albite twins in the plagioclase because of the zoning microtextures that were initiated in areas of given composition can propagate laterally into zones of different composition a diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly cooled rocks

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

feldspar microtextures and multistage thermal history of syenites from the coldwell complex ontario optical and tem transmission electron microscopy observations of perthites from augite syenites in the coldwell complex ontario reveal a complex set of microtextures that outline a multistage thermal history regular microtextures linear or braid texture straincontrolled coherent intergrowths show a progressive evolution from the margin of the intrusion inwards with lamellar spacings in the range 40 100 nm the textures evolve in a manner similar to those for the klokken intrusion and reflect differences in cooling rates and bulk composition superimposed upon the regular microtexture are 10 m scale compositional fluctuations which we call ripples the boundary relationships and bulk composition of ripples which are themselves ab rich and or rich linear coherent cryptoperthites suggest that they formed by coarsening during a phase of high temperature 530 c fluid feldspar interaction this was followed by a return to coherent exsolution in which fluid was not involved coarse irregular patch microperthite cross cuts all other microtextures these final deuteric intergrowths are believed to result from a further low temperature 380 c fluid feldspar interaction and are associated with subgrain formation and the presence of micropores the outermost syenite sample against a gabbro ring structure has distinctive modified microtextures indicating that the gabbro is at least in part a later intrusion our findings show that tem work on alkali feldspar microtextures can identify discrete thermal events in the cooling history of igneous plutons and illustrates the potential of such microtextures for establishing the relative ages of intrusive rocks

albite of secondary origin in charny sandstones quebec a re evaluation abstract the contested origin of pink untwinned sodic plagioclase grains in the cambrian charny sandstones of the northern appalachians near quebec first studied by middleton and discussed by lajoie has been re evaluated by detailed petrographic x ray and electron microprobe analyses the majority of the grains exhibit a perthire like lamellar structure the coexisting feldspars are an8 12 host and an0 lamellae we contend that these represent antiperthitic feldspars derived from grenville anorthositic massifs and transformed in situ during diagenesis the k feldspar lamellae were efficiently albitized during earlier diagenesis whereas the original andesine host composition failed even at higher grade conditions to reach the equilibrium composition and degree f si al order of albite this is a reflection of the profound changes required in the proportion of tetrahedrally coordinated cations in the plagioclase and the low solubilities of these constituents in the pore fluids twinned grey sodic plagioclase also an8 12 but free of albite lamellae may represent albitized detritus from the ubiquitous grey gneisses that surround the grenville anorthositic complexes

granulite hosted gold tectonic setting and lithogeochemistry of the tropicana deposit western australia the tropicana gold deposit 7 9 moz is hosted by the neoarchean amphibolite to granulite grade tropicana gneiss these rocks form part of the plumridge terrane a tectonic unit that separates the in situ eastern margin of the yilgarn craton from proterozoic elements of the albany fraser orogen western australia the gneissic host package to the orebody originally constituted a suite of medium to high k calc alkaline and adakitic basaltic to rhyolitic lavas emplaced in a relatively thick crustal continental margin arc amphibolitized mafic intrusions have back arc basin basalt like compositions consistent with their having formed during extension and incipient rifting of the arc gold mineralization occurs mainly within a sheet like syenitic gneiss that ranges compositionally from basaltic andesite to rhyolite and derived through metasomatism of compositionally equivalent lavas within an interpreted shear zone coarse perthite textures in microcline ruffle exsolution in brown biotite and occasional pyrite inclusions away from fractures in microcline porphyroblasts indicate that this shear zone predated peak metamorphism relative to the unmineralized host gneisses the mineralized syenitic gneisses show significant enrichments in s and the ore elements mo te tl ag au w and occasionally bi and sb but not as cu or zn in the k group elements and also in p light rare earth elements lree th u zr and hf depletions in most major elements si ti fe mg and ca as well as v cr ni and co are considered to reflect dissolution of relatively low volume mafic phases hornblende augite fe ti oxides and plagioclase in the protoliths some of the distinctive enrichments p lree zr hf are linked to volume loss in excess of 50 associated with dissolution of silica and concentration of insoluble apatite and zircon in the mineralized unit but the ore elements and strongly enriched similar to x10 th and u were added from the hydrothermal fluids involved exhumation of the tropicana gneiss into the upper crust was accompanied by fluid ingress and retrogression this produced mm to cm scale wavy biotite pyrite shears solution seams and disseminations throughout the syenitic gneiss which were further retrogressed to sericite chlorite assemblages in places both of which host most of the gold in the deposit thus although the present deportment of gold is unambiguously associated with the postpeak metamorphic greenschist grade retrogression at ca 2520 ma there is compelling evidence that the paucity of quartz and carbonate veining atypical element enrichments and sheet like nature of the tropicana deposit in part reflect metasomatism associated with a prepeak metamorphic relatively deep shear zone at present we have no direct evidence for the timing of introduction of gold and a protracted history of gold precipitation redistribution and enrichment is implied

lateral variations in sediment composition and bedding in middle triassic interplatform basins buchenstein formation southern alps italy hemipelagic deposits are widespread in triassic sequences of the alpine belt and provide important data for stratigraphy and the study of bedding rhythms the area of the western and central dolomites of northern italy escaped strong alpidic deformation and preserves such deposits in their original palaeogeographic setting the buchenstein formation the object of this study was deposited in up to 1000 m deep middle triassic interplatform basins which extend over an area of 500 km 2 within the dolomites excellent outcrops and volcaniclastic markers allow a detailed correlation of the formation in both laminated anoxic and bioturbated facies down to a bed scale and show its relationship to coeval carbonate platforms correlation of lithostratigraphically well constrained intervals in the bioturbated facies reveal that lateral thickening and thinning of the deposits depends on the amount of shallow water debris in the succession and is a function of the distance to coeval carbonate platform sources in the laminated background deposit thickness variations in limestones and marls parallel the thickness variations in ash layers and were caused by local redeposition of sediment on the basin floor lateral persistence of laminae indicates that bottom currents were weak in the buchenstein basin but were able to redeposit mud in a significant way in the area around seceda and geisler western dolomites a lateral transition from dark grey laminated to grey bioturbated to red bioturbated facies is observed which is probably linked to local relief with different oxygenation conditions on the sea floor a comparison shows that decimetre scale bedding is preserved in all facies types and that the bedding rhythm is partly the result of different cementation of the sediment during early diagenesis correlation of individual layers in the bioturbated facies reveals that beds are thicker and enriched in lime mud in the western part of the basin and decrease in thickness and contain less micrite towards the east further away from the main shallow water areas nearslope calciturbidites change gradually from distinct layers into lateral arrays of micrite nodules and bands further out in the basin these observations point to a platform source of lime mud in the buchenstein basin

sidewall crystallization in the klokken intrusion zoned ternary feldspars and coexisting minerals the syenitic layered series in the klokken intrusion is surrounded by a zone 500 m thick of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro 200 m thick at the outer rim the unlaminated syenite is intrusive into the gabbro and develops a thin 2 m transition zone of syenodiorite at the contact a traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill mafic silicates olivine clinopyroxene biotite show inward evolution in fe fe mg across the syenodiorite unlaminated syenite zones feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase in some cases together with two alkali feldspars one a mesoperthite or cryptomesoperthite the other a cryptoperthite to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals plagioclase is absent from the layered series

the non glacial and non cratonic origin of an early archean felsic volcaniclastic unit barberton greenstone belt south africa abstract recent studies have suggested that 3 445 ga felsic volcaniclastic rocks of member h6 of the hooggeneoeg formation in the barberton greenstone belt south africa show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older granitic continental crust these conclusions represent fundamental re interpretations of this sequence its crustal setting and processes of deposition we here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits storm influenced shallow marine units and turbidity current like storm driven submarine flows all of which lack any evidence of a glacial influence during sedimentation they accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface the debris consists entirely of volcanic derived materials no clasts of plutonic rock are found in the conglomerates low temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline previously thought to be characteristic of granitic plutonic rocks this potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the greenstone belt this succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center

the microstructures of microcline from some granitic rocks and pegmatites optical microscopy transmission electron microscopy tem and electron diffraction have been used to study the microstructures in a series of microclines from granitic rocks and pegmatites optical microscopy revealed exsolution of albite in all but one specimen and in general the k phase exhibited cross hatched patterns the tem and optical observations confirm earlier ideas that the cross hatching is associated with intersecting sets of albite and pericline twin lamellae which were formed at the transformation from monoclinic to triclinic symmetry however significant changes subsequently occur which produce microstructures involving essentially albite twinning only although the cross hatched morphology is retained and features associated with the original pericline twins are observed in the tem images and in the associated diffraction patterns the four spot diffraction patterns characteristic of m twinning are observed only from volumes of specimen containing discrete domains of albite twinning and domains of pericline twinning streaking of the diffraction spots is associated with the complex structure of the domain boundaries the influence of exsolution of albite on the initiation of the transformation twinning has also been investigated domains of orthoclase within a matrix of twinned microcline were observed in two specimens but the observations do not appear to provide any direct information about the mechanism of the transformation between these two structures

experimental na k exchange between alkali feldspar and an nacl kcl salt melt chemically induced fracturing and element partitioning the exchange of na and k between alkali feldspar and a nacl kcl salt melt has been investigated experimentally run conditions were at ambient pressure and 850 c as well as 1 000 c cation exchange occurred by interdiffusion of na and k on the feldspar sub lattice while the si al framework remained unaffected due to the compositional dependence of the lattice parameters compositional heterogeneities resulting from na k interdiffusion induced coherency stress and associated fracturing depending on the sense of chemical shift different crack patterns developed for the geometrically most regular case that developed when potassic alkali feldspar was shifted toward more sodium rich compositions a prominent set of cracks corresponding to tension cracks opened perpendicular to the direction of maximum tensile stress and did not follow any of the feldspar cleavage planes the critical stress needed to initiate fracturing in a general direction of the feldspar lattice was estimated at 0 35 gpa fracturing provided fast pathways for penetration of salt melt or vapor into grain interiors enhancing overall cation exchange the na k partitioning between feldspar and the salt melt attained equilibrium values in the exchanged portions of the grains allowing for extraction of the alkali feldspar mixing properties

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

diagenetic albitization of detrital k feldspar in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway i textures and origin abstract the origin of albitized grains in sandstones has been the topic of considerable discussion in recent years some workers believe that the albitization of feldspar can occur in situ during burial diagenesis but others argue that albitization is a feature inherited from source clastic grains our petrological and chemical studies of subsurface samples of the jurassic lower cretaceous and tertiary reservoir sandstones from offshore norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis criteria suggesting that the albitization has occurred during burial diagenesis include 1 euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces 2 generally untwinned albitized grains mostly riddled with minute br wnish inclusions 3 lack of cathodoluminescence in albite 4 homogeneous and pure albite composition 99 ab 5 absence of albitized grains in carbonate cemented zones and 6 increase in the percentage and degree partial to complete of albitized k feldspar with depth under sem albitized grains display two types of fabrics type i albitized grains are composed of numerous tiny albite crystals 1 30 m growing along cleavage planes in parent grains and they have abundant dissolution porosity while type ii albitized grains show blocky albite crystals 5 80 m forming pseudomorphs of detrital k feldspar and they lack any dissolution porosity type i albitized grains are found commonly at shallower depths between 2 2 and 3 km and temperatures varying from 65 to 90 c whereas type ii albitized grains dominate at greater depths 3 5 km and at temperatures 90 c albitization proceeds by a dissolution precipitation mechanism at shallower depth dissolution of the parent k feldspar is more rapid than the precipitation of albite and rates of albite precipitation increase with depth favoring pseudomorphic replacement of k feldspars other factors that influence albitization processes are ana ak permeability reduction and surface dissolution kinetics of the parent grain

feldspar evolution in the roxby downs granite host to fe oxide cu au u mineralisation at olympic dam south australia abstract the textural relationships and geochemistry of feldspars from least altered to sericite hematite altered and mineralised 1 595 ga roxby downs granite rdg at olympic dam south australia were examined the sample suite is representative of rdg both distal 5 km and proximal 27 34 is recognised along with a more abundant less calcic plagioclase an 12 20 both displaying rapakivi and anti rapakivi textures with alkali feldspar alkali feldspars or 55 ab 43 an 2 record post magmatic evolution from cryptoperthite to patch perthite subsequent patch perthite is overprinted by highly porous near end member albite and k feldspar while plagioclase undergoes replacement by albite sericite ba rich k feldspar in sericite hematite altered and mineralised rdg along the margin of the odbc sericite replaces all plagioclase whereas red stained fe rich k feldspar persists sulphide uranium rare earth element mineralisation is observed in association with hydrothermal feldspars and increases in abundance with proximity to the orebody petrographic observations and whole rock geochemistry illustrate the transformation of plagioclase and alkali feldspar from igneous to hydrothermal processes and indicate that hydrothermal albite and k feldspar formed within the rdg without the need for an external source of alkalis feldspar geothermometry indicates a minimum crystallisation temperature of 765 c at 2 2 kbar for alkali feldspar pressure estimate obtained using plagioclase amphibole geobarometry followed by a range of lower temperature transformations late stage magma mixing contamination is postulated from supportive temperature and pressure estimates along with feldspar and mafic mineral relationships

compositional and microtextural zoning in alkali feldspars from the shap granite and its geochemical implications alkali feldspar phenocrysts in the lower devonian shap granite are compositionally and microtexturally zoned most of each phenocryst has exsolved to form microperthite with an approximate bulk composition of or 72 ab 28 an but sub millimetre wide zones within the phenocrysts with an approximate bulk composition or 69 ab 30 an 1 have exsolved to cryptoperthite the sub optical scale of exsolution microtextures in the narrow an rich zones is due to ca al coupling which slows rates of interdiffusion during exsolution relative to an poor volumes of feldspar this form of coupled compositional microtextural zoning has not been described previously from or rich alkali feldspars using electron microscopy but we have seen other examples and predict that it is common cryptoperthite zones are of particular importance because they will have trace element and isotopic compositions least altered subsequent to growth in the magma of any microtextures in alkali feldspars and so will be the most rewarding targets for chemical analysis

exsolution by spinodal decomposition ii perthite formation during slow cooling of anatexites from ngoronghoro tanzania perthites in slowly cooled granulite facies rocks from the ngoronghoro structure tanzania show complex microstructures reflecting several stages of exsolution and coarsening mesoperthites with an integrated bulk composition of or26 5ab71an2 5 are comprised of lamellar intergrowth of 5 to 10 m wide orthoclase rich and albite rich lamellae spindle perthites with a bulk composition of or67ab31 5an1 5 are comprised of 2 to 10 m wide and several 109s of m long albite rich spindles in an orthoclase rich host in the mesoperthite primary exsolution occurred by spinodal decomposition whereas the spindle perthite formed by a nucleation and growth mechanism the interfaces between the exsolved phases are generally incoherent unmixing and coarsening of the mesoperthite is discussed in the light of non linear uphill diffusion the microstructure evolution and successive chemical separation of the exsolved phases during cooling are modeled based on cahn hilliard theory we find that coarsening primarily occurs during the first 20 c to 30 c of cooling after the homogeneous precursor feldspar has entered the coherent spinodal the extent of coarsening and the characteristic size of the exsolved phases depend on the cooling rate over this temperature interval for geologically relevant cooling rates on the order of 5 c ma to 50 c ma the compositions of the exsolved phases develop along the solvus down to about 480 c to 530 c and are frozen in only at lower temperatures the compositions that are finally preserved in the orthoclase rich and in the albite rich phases depend on cooling rate over this temperature interval both the characteristic lamella width of the mesoperthite as well as the compositions of the exsolved phases indicate slow cooling at 5 c ma during the coarsening stage and down to about 450 c during later stages of the cooling history the albite rich phase of the mesoperthite exsolved about the peristerite gap and the orthoclase rich host of the spindle perthite exsolved a second generation of coherent albite rich spindles the incoherent phase boundaries of the primary perthite microstructures served as passageways for fluids during late stage deuteric alteration which lead to secondary coarsening in the mesoperthite and to alteration of the albite rich precipitates in the spindle perthite finally both types of perthites were replaced by coarse grained patch perthite during deuteric alteration