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

permian triassic red stained albitized profiles in the granitic basement of ne spain evidence for deep alteration related to the triassic palaeosurface extensive areas of the variscan granitic basement in ne spain display profiles of red stained albitized facies characterized by albitization of ca plagioclase chloritization of biotite and microclinization of orthoclase along with the alteration of igneous quartz to secondary cl dark quartz these profiles have a geopetal structure beneath the triassic unconformity with a very intense and pervasive alteration in the upper part that progressively decreases with depth to 150 200 m where the alteration is restricted to the walls of fractures the red albitized facies contains secondary maghemite and hematite that indicate oxidizing conditions dating of microclinized orthoclase and secondary monazite that have formed in the red stained albitized facies yielded k ar and u th pbtotal ages of 240 and 250 ma respectively suggesting that the alteration developed during the permian triassic period the geopetal disposition of the red albitized profile with respect to the triassic unconformity its large regional extent and the fracture controlled alteration in the lower part of the profile indicate groundwater interaction the 18o values of albitized plagioclase 11 microclinized orthoclase 13 and secondary cl dark quartz 12 suggest that the alteration temperature was about 55 c this low temperature suggests that the alteration occurred during interaction of the granitic rocks with na rich fluids below a surficial weathering mantle on the permian triassic palaeosurface the latter is possibly related to triassic evaporitic environments in long lasting stable landscapes in which na rich solutions infiltrated deep regional groundwaters

from mixed flow reactor to column experiments and modeling upscaling of calcite dissolution rate abstract the objective of this work was to assess the extent to which calcite dissolution rate rcalcite measured at controlled saturation state in mixed flow set ups could be upscaled to column experiments where the solution composition was variable in space and time the dissolution rate of the 104 calcite face was investigated in mixed flow reactor set ups at room temperature and ph 8 various saturation conditions were studied by changing the composition of the inlet solution enabling the determination of an empirical relation between rcalcite and the gibbs free energy of calcite dissolution g by measuring the surface topography with vertical scanning interferometry vsi the prevalent dissolution mode i e etch pit nucleation or homogeneous surface retreat was assessed with the same method a dramatic decrease of the dissolution rate was observed for 8

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

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

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

a high resolution record of multiple diagenetic events ultraviolet laser microprobe ar ar analysis of zoned k feldspar overgrowths abstract high spatial resolution uv laser ar ar analyses were performed in situ on k feldspar overgrowths from sandstone hosted pb zn bodies from central sweden two different populations of ages were detected precambrian cambrian 528 10 to 567 8 ma and late silurian 403 6 to 453 8 ma these were both detected in single pore spaces with the older population adjacent to the detrital feldspar whilst the younger population was always distal to the detrital grain this younging of authigenic k feldspar ages with distance from the detrital grain margin is discussed in terms of disturbance to the argon system within an authigenic overgrowth that is of a single age a the overgrowths are precambrian cambrian in age and the late silurian population might be an artefact of argon loss due to partial resetting b the overgrowths are late silurian and the precambrian cambrian population might have resulted from excess argon uptake and c the overgrowths are late silurian but the precambrian cambrian population is an experimental artefact arising from the analysis of overlapping detrital authigenic feldspars diffusion modelling shows that the partial resetting scenario is a very unlikely cause for the observed age variation because it requires a very particular arrangement of sub grain sizes excess argon was not detected in either the k feldspar or quartz the data are best interpreted as the ages of two discrete diagenetic events an early burial diagenesis and a later tectonically induced fluid flow event relating to the collapse of the caledonian orogen further the use of 38arcl 39ark as a proxy for cl k ratio highlights the physical boundaries between feldspar populations where all other petrographic sem and cl analyses have failed to do so this study shows that high spatial resolution uv laserprobe ar ar analysis is a powerful tool for extracting age information from complex hydrothermal minerals as a means to investigating fluid flow events

characteristics of lightning vhf radiation near the time of return strokes by using a crossed base line interferometer lightning vhf source positions correlated in time with electric field change measurements have been obtained we present data in this paper showing azimuth and elevation pictures with high time resolution of the vhf 34 3 mhz radiation for events near the time of return strokes from their common characteristics we infer spatial and temporal features of these processes no radiation was seen that could be attributed to the return stroke wave front itself radiation from stepped leaders continued up to and in some cases after initiation of the first return stroke a strong burst of radiation often followed the first return stroke strong radiation began before subsequent return strokes coinciding with the electric field change usually associated with the dart leader the end of the radiation was independent of the return stroke the source of this radiation was a well defined region within the cloud a second burst often followed the return stroke located in the same region as that prior to the stroke sources from stroke to stroke showed horizontal displacement with little apparent change in elevation

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

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

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

origins of the surface roughness of unweathered alkali feldspar grains abstract the origins of surface roughness of powdered feldspar grains the ratio of the total surface area quantified by gas adsorption to the surface area estimated by assuming an average grain geometry has been investigated by a detailed study of alkali feldspars from a range of igneous rocks and from pegmatites the of 125 63 m powders of ten different feldspar samples ranges from 5 16 22 00 and correlates well with mineral microtexture powders of feldspars which are homogeneous or have straight lamellar exsolution microtextures yield mean values of 5 59 whereas powders of vein macroperthites give a mean of 6 64 and powders of braid macroperthites yield mean values of 12 15 microtexture influences by partly controlling the manner in which feldspar grains break during grinding and this affects the frequency of steps on grain surfaces feldspars which are homogeneous or have straight lamellar exsolution microtextures break to produce a relatively high proportion of grains with smooth step poor external surfaces whereas vein macroperthites and braid microperthites break to produce grains with much rougher step rich surfaces microtexture is able to influence style of breakage because the presence size and crystallographic orientation of exsolution lamellae has a significant influence on the trajectories of fractures propagating through grains as they fragment during grinding we found little correlation between internal surface area due to micropores expressed as either microporosity or mean perimeter area of micropores and indicating that microporosity is a significantly less important contributor to than external surface area due to steps this conclusion is supported by an analysis of the of a number of different size fractions of two of the feldspar powders the origin of is only one part of the more important question of how closely measured surface areas are related to the proportion of grain surfaces which are reactive during experimental dissolution and natural weathering

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

red staining of the wall rock and its influence on the reducing capacity around water conducting fractures abstract red staining and alteration of wall rock is common around water conducting fractures in the laxemar simpevarp area se sweden which is currently being investigated by the swedish nuclear fuel and waste management co skb in common with many other places red staining is often interpreted as a clear sign of oxidation but relevant analyses are seldom performed the area is dominated by palaeoproterozoic crystalline rocks ranging in composition from quartz monzodiorite to granite in this study wall rock samples have been compared with reference samples from within 0 1 to 1 m of the red stained rock in order to describe mineralogical and geochemical changes but also changes in redox conditions a methodology for tracing changes in mineralogy mineral and whole rock chemistry and fe3 fetot ratio in silicates and oxides in the red stained wall rock and the reference rock is reported the results show that the red stained rock adjacent to the fractures displays major changes in mineralogy biotite plagioclase and magnetite have been altered and chlorite k feldspar albite sericite prehnite epidote and hematite have been formed the changes in chemistry are however moderate k enrichment ca depletion and constant fetot are documented the fe3 fetot ratio in the oxide phase is higher in the red stained samples whereas the fe3 fetot ratio in the silicate phase is largely similar in the wall rock and the reference samples because most of the fe is hosted in the silicate phase the decrease in reducing capacity fe2 if any in the red stained wall rock is very small and not as high as macroscopic observations might suggest instead the mineralogical changes in combination with the modest oxidation and formation of minute hematite grains in porous secondary minerals in pseudomorphs after plagioclase have produced the red staining increased porosity is also characteristic for the red stained rock moderate alteration in the macroscopically fresh reference rock shows that the hydrothermal alteration reaches further from the fracture than the red staining the extent of the red staining can therefore not be used in the same way as the extent of the alteration adjacent to a fracture the increase in porosity in the red stained rock may result in enhanced retention of radio nuclides due to an increased sorptivity and diffusion close to the fracture the hydrothermal alteration causing the red staining is thought to have occurred at temperatures of about 250 400 c based on the secondary mineralogy the major part of this alteration in the area is assumed to be related to fluid circulation associated with the intrusion of the mesoproterozoic gotemar and uthammar granites nearby

biomechanical and biochemical weathering of lichen encrusted granite textural controls on organic mineral interactions and deposition of silica rich layers the crustose lichen rhizocarpon geographicum weathers the lower devonian shap granite by both biomechanical and biochemical means biomechanical weathering is mediated by fungal hyphae that penetrate into the rock via intergranular boundaries at 0 002 0 003 mm year 1 once inside the granite hyphae exploit intragranular pores along cleavage and fracture planes in biotite alkali and plagioclase feldspar grains of biotite exposed at the lichen granite interface have been fragmented by biomechanical action in 122 years after an extended period of biomechanical weathering of 10 kyr outcrop surfaces sub mm sized fragments of biotite and plagioclase feldspar abound in lower parts of the lichen s thallus grains of biomechanically weathered biotite show the clearest evidence for biochemical weathering typically k and fe are leached from the biotite but other cations may also be removed leaving a silica dominated relic the silica rich remains of biotite and possibly also feldspar have been redistributed along the lichen granite interface forming a silica rich layer this silica rich material also cements fractures and pores beneath the interface alkali feldspars from outcrop surfaces have been weathered chemically as indicated by etch pits on cleavage surfaces and fractures but it is not obvious that lichen was involved in the dissolution the etch pits were probably formed during earlier phases of non biochemical weathering that widened intergranular and intragranular pores facilitating subsequent access of fungal hyphae into the granite despite evidence for biomechanical and biochemical weathering it is unlikely that colonisation of granite surfaces by r geographicum at shap significantly enhances their weathering rate relative to bare rock surfaces the lichen may actually retard weathering by protecting the surface from frost action binding fragmented mineral grains and depositing protective silica rich layers

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

radio interferometric observations of cloud to ground lightning phenomena in florida radio frequency observations of cloud to ground lightning in florida have been analyzed to document a number of features of the lightning the observations have been made using an interferometer system which determined the direction to the lightning radiation as a function of time during close lightning discharges observations are presented for about 50 radiating events during five multiple stroke normal polarity flashes to ground the results confirm and extend results of a similar study of new mexico lightning by rhodes et al 1994 dart leaders in cloud k events and attempted leaders are found to be the same phenomena namely a negative polarity streamer that propagates horizontally and or vertically to ground at estimated speeds of about 10 7 ms 1 down to 10 6 ms 1 the step or impulsive fast electric field change produced by some k events is found to be caused by positive breakdown back at the starting point of the negative streamer after the streamer has progressed over some or all of its distance this breakdown appears to initiate a forward stroke along the streamer channel which can renew the breakdown at the front end of the channel k breakdown which happens to occur during a continuing current discharge to ground produces an m event channel brightening when it connects with the conducting channel to ground the step fast field change of the m events is found to be produced at the time of connection inside the cloud no fast field change is detected when this breakdown reaches ground that would be associated with a return stroke m events can also be initiated by fast 10 7 ms 1 positive streamers which often propagate away from the leader source region within a few milliseconds of the arrival of a return stroke back in the source region these streamers often generate an even faster negative recoil event back along their extent which propagates down the channel to ground as the m event in all cases the current increase or channel brightening produced by k and m events is predicted to occur in a forward direction that is toward or down the channel to ground in the same direction as the initiating negative streamer positive breakdown is detected only immediately following return strokes or during some k streamers in both cases the electric stresses are expected to be very large suggesting that positive breakdown which produces a conducting channel is difficult to initiate results for one flash suggest that negative charge was left at low altitude by the initial stroke to ground this forced the subsequent leaders to develop a new path to ground first as series of attempted leaders and then as a dart stepped leader

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

thermochronology of the yidun arc central eastern tibetan plateau constraints from 40ar 39ar k feldspar and apatite fission track data abstract four k feldspar samples from the yidun arc eastern tibetan plateau were analysed by the 40ar 39ar method with the aim of recovering information on their thermal history using multiple diffusion domain mdd theory arrhenius plots for each of the samples reveal low retentivity early in the heating experiments a property that is attributed to their recrystallised nature this low argon retentivity appears to violate the mdd assumption that volume diffusion is the only mechanism for argon transport within the crystals thus the thermal histories derived from these analyses are considered suspect nevertheless the age spectra themselves suggest that the majority of samples had cooled below 200 c prior to the eocene collision of india with asia thermal history modelling from apatite fission track analyses from the same and nearby samples shows slow cooling through the apatite fission track partial annealing zone during the cenozoic in samples from the high elevation low relief areas of the yidun arc while samples from the major jinsha river valley show rapid cooling through the partial annealing zone beginning in the miocene these results suggest that significant cenozoic denudation has been localised and that most parts of the yidun arc have experienced very little denudation during the cenozoic

zircon of the no 782 deposit from the great xing an range in ne china implications for nb ree zr mineralization during magmatic hydrothermal evolution abstract niobium rare earth elements ree lanthanides y and zirconium are widely used in high tech applications the nb ree zr mineralization in the no 782 deposit of ne china developed in a fine grained porphyry porphyry that occurs as a ring shaped stock the ore forming porphyry is hosted in a coarse grained granite granite that dominates the study area zircon is the only zr bearing mineral in both the porphyry and the granite in contrast to the granite that hosts only magmatic zircon both magmatic type i and metasomatic type ii zircon crystals or aggregates are present in the porphyry most type i zircon occurs as individual crystals and are hosted mainly in interstitial quartz with evident oscillatory zoning most type ii zircon occurs as aggregates rather than individual crystals and exhibits vague zoning distribution of the type ii zircon in the porphyry is heterogeneous with great abundance in places where there are substantial albitization and snowball quartz crystals textural and chemical features of the type ii zircon and of the associated albite and snowball quartz crystals suggest that all these minerals are metasomatic nb is hosted mainly by fergusonite y and ree by fergusonite y and to a lesser extent by bastnasite ce and parisite ce where parisite ce occurs as a replacement of bastnasite ce the nb ree mineralization occurs either as a replacement of the type ii zircon or in chlorite quartz dominant veinlets that postdated both the type i and the type ii zircon most type i zircon crystals in the porphyry were intact from the replacement or alteration by the postdated nb ree minerals possibly because the type i crystals were encased earlier by other magmatic minerals the chronological and oxygen isotopic data of zircon suggest that the porphyry 453 8 3 1 ma is younger than the granite 487 0 4 0 ma and that the two granitic phases derived from different magma chambers the subtle age difference between magmatic type i and metasomatic type ii zircon cannot be shown by shrimp u pb data of zircon the mineral assemblages associated with type ii zircon and the chronological data suggest that the medium from which type ii zircon precipitated is likely an internally derived hydrosilicate liquid hsl that lies compositionally in between a silicate melt and a hydrothermal fluid during the magmatic hydrothermal transition textural features of the nb ree minerals combined with the fact that nb ree mineralization developed only inside the porphyry but not in surrounding country rocks i e the granite suggest that the nb ree mineralization originated from an aqueous fluid that was constrained to the porphyry

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

an evolving magma chamber within extending lithosphere an integrated geochemical isotopic and zircon u pb geochronological study of the gushan granite eastern north china craton the gushan granite located in the jiaobei terrane of the eastern north china craton carries mafic micro granular enclaves mmes and zoned k feldspar phenocrysts and is a typical example of magma gener ation and evolution in an extensional setting here we present whole rock major and trace element geochemistry sr nd isotopes and zircon u pb and hf isotope data on the granite and its enclaves the granite was emplaced at 120 ma it is high in sio2 k 2 o n a 2o lile and lree but low in mgo and hfse the granite are also marked by high initial 87 sr 86 sr 0 7103 and low end t 18 5 to 18 9 and zircon ehf t 16 6 to 22 0 values these features indicate that the host granite was mainly derived from ancient north china craton lower crust field petrographic and compositional studies indicate that mmes are products of mafic felsic magma interactions in the generation of the granite in the relatively late magmatic history available data suggest that enriched lithospheric mantle was a suitable candidate for the mafic magma end member involved in the magma interaction process internal sr nd isotopic variations within k feldspar phenocrysts are also evaluated to decipher the magmatic history in spite of a protracted cooling history and evidence of subsolidus alteration the k feldspar phenocrysts preserve marked primary sr nd isotopic variations initial 87 sr 86 sr ratios increase from the cores 0 7100 0 7102 to rims 0 7105 0 7110 whereas end t values show decrease from the cores 17 7 to 20 5 to rims 21 1 to 27 0 the overall isotopic profiles across the k feldspar phenocrysts are consistent with a continuous magma contamination process highly negative nd isotopic values of some k feldspar rims suggest that the precambrian basement rocks in the upper crust are potential candidates for the contam inant these results indicate that isotopic heterogeneity could be preserved within single crystal from much older slower cooled plutonic rocks and those isotopically zoned minerals are effective recorders of open magma system based on petrographic geochemical and isotopic studies we envisage multiple magmatic pulses mafic felsic magma interaction and continuous assimilation during the formation of the gushan granite a model involving lithospheric thinning and mantle crust interaction in an exten sional setting is proposed to account for the generation of the gushan granite and its enclaves

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

pathways for 39ar loss during step heating of alkali feldspar megacrysts from the shap granite uk combined evidence from diffusion experiments and characterisation of heating induced texture modifications abstract this study aims to identify the dominant fast pathways for 39ar loss during step heating 40ar 39ar analysis of alkali feldspar from the shap granite uk in our analysis we combined step heating 40ar 39ar data acquired from variably sized feldspar fragments 0 4 to 2 2 mm in the shortest dimension and optical and electron microscopy observations of microtextural modifications caused by heating in a muffle furnace in air our optical and electron microscopy results suggest that laboratory heating of the shap feldspar causes it to fracture the resulting cracks form an interconnected network which may link with some of previously documented nanoscale defects such as nanotunnels slots pull aparts and bubbles our step heating 40ar 39ar analyses yielded non linear arrhenius trajectories of 39ar release the non linearity is best explained by heating induced reduction of the effective diffusion length due to fracturing likely with some contribution from nanotunnels pull aparts slots and bubbles incoherent grain boundaries apparently played a minor role in providing fast pathways for 39ar loss if any importantly unambiguous evidence for a reduction of the diffusion length during step heating was only observed in experiments with large fragments 1 mm in the shortest dimension the arrhenius trajectory obtained from the smaller fragments 0 5 mm in the shortest dimension appears to be consistent with the presence of variably sized non interacting intra grain diffusion domains that existed prior to mineral separation heating induced fracturing must be taken into account when interpreting 40ar 39ar dates of alkali feldspar with particular focus on the following two points first this process is clearly inconsistent with the underlying assumptions of commonly utilised multi diffusion domain mdd theory although it can result in 39ar release patterns that appear consistent with them therefore it is important to identify on a case by case basis the dominant fast pathways for 39ar loss from alkali feldspar during step heating before applying mdd theory second if heating induced fracturing is ubiquitous in alkali feldspar and affects even gem quality samples then presently available diffusion parameters of ar in alkali feldspar structure may be inaccurate to some degree since all of them were obtained without considering the presence of cracks

constraints regarding gold deposition in episyenites the permian episyenites associated with the villalcampo shear zone central western spain the villalcampo shear zone around 307 ma shows second order shear zones 420 390 c with gold ore bodies hosted by episyenites which consist of albite episyenites albitites and albite kfs episyenites both types with different contents in sulphides and gold mass transfer modelling supports higher fluid rock ratios in albitites than in albite kfs episyenites the study of worldwide barren and gold bearing episyenites reveals abundant sulphides in the latter as a distinguishing feature the electrochemical processes at the surface of sulphide would have enhanced gold precipitation sulphides working as a gold trap a complex fluid history occurred in gold ore bodies hosted by episyenites although in essence it was similar to quartz sealed faults hosting late variscan gold deposits a an early fluid equilibrated with the metamorphic pile with sulphides or with a metamorphic fingerprint resulted in a sulphide deposition and b a shallower fluid reservoir of meteoric origin provided gold deposition in contrast to earlier claims regarding episyenite fluid flow a down temperature and probably an upwards fluid flow are proposed for the episyenitization process also in keeping with the early stages of fluid flow in quartz sealed faults fluid inclusions in albite confirm that the striking coupled quartz leaching albitization processes occurred around 400 c and 60 mpa crosscutting the retrograde solubility field of silica and yielding a vuggy rock initially albite and later quartz and sulphide filled the vugs from the same or a very similar fluid uraninites deposited during the albitization and probably the onset of the sulphide deposition afforded the same age 270 12 ma as other spanish episyenites confirming a synchronous and a regional scale process and ruling out a relationship with the granite cooling history 324 11 ma

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

crystallization of the shap granite evidence from zoned k feldspar megacrysts textural chemical and sr isotopic studies of feldspars from the subsolvus shap granite northern england demonstrate that a number of magma mixing events have dominated the evolution of this pluton k feldspar megacrysts are phenocrysts formed in the magma chamber they contain a number of ba rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas diorite enclaves represent the relicts of these magmas and these also contain k feldspar megacrysts which show evidence of major dissolution they are xenocrysts picked up from the host granite and incorporated in the basic magma increasing h 2 o contents during fractional crystallization caused a late switch from growth of megacrysts to finer grained k feldspars in the matrix the chemically and isotopically zoned k feldspar megacrysts preserve an exceptional record of the evolution of the magma and the zones also had a significant influence on the development of exsolution microtextures during cooling

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

hematite in porous red clouded feldspars evidence of large scale crustal fluid rock interaction abstract transmission electron microscopy shows that red clouded alkali feldspars from granitic rocks contain numerous open pores up to several hundred nanometres in cross section and that almost every pore contains rosettes or needles of crystalline hematite in the samples described here the hematite forms at the reaction interfaces at which plagioclase feldspar is replaced by alkali feldspar these observations together with the fact that the origin of the porosity can be explained by the mechanism of sub solidus mineral fluid replacement processes indicate that hematite is a direct product of fluid rock interaction and is not a solid state exsolution product from the feldspar

ion microprobe study of intragrain micropermeability in alkali feldspars alkali feldspar cleavage fragments from the klokken layered syenite south greenland were heated to 700 c at 0 1 gpa in 99 h218o for 75 h these samples were then polished and imaged by ion microprobe for 18o the feldspars were known to contain areas of pristine braid micro perthite which were not turbid and areas of deuteric patch perthite which were turbid turbidity is related to the presence of micropores in the feldspars on imaging the grain it was found that the 18o had penetrated into the parts of the grain which were microporous and not into the pristine areas micropores are therefore responsible for rendering the feldspars permeable as well as porous the implications of micropermeable feldspars in several areas of geology are discussed

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

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

pegmatitic pods in the mealy mountains intrusive suite canada clues to the origin of the olivine orthopyroxene dichotomy in proterozoic anorthosites the anorthositic members of the mealy mountains intrusive suite mmis labrador canada are host to 0 5 5 m diameter pegmatitic pod like segregations originally described as graphic granite pods u pb zircon geochronology confirms that the pods are coeval with the 1650 1630 ma emplacement age range for the mmis yielding ages of 1654 6 8 to 1628 6 3 5 ma petrographic and geochemical analysis of five pods from anorthositic rocks of the mmis reveals that the pods have a diverse compositional range from monzodiorite to granite varying from fe rich and si poor to fepoor and si rich compositions fe rich si poor pods in the mmis and other massifs e g laramie anorthosite complex tend to be hosted by olivine bearing anorthosites whereas si rich fe poor pods are hosted by pyroxene bearing anorthosites each pod shows a range of graphic myrmekitic and symplectitic textures along with distinctive mineral assemblages e g apatite and zircon and highly enriched trace element compositions evolved mineral assemblages high concentrations of fe ti and p and in some cases sio2 and 10 1000 chondrite enrichment in light rare earth elements u th and rb indicate that many of the pods are highly fractionated the array of textural intergrowths provides clues about the final stages of crystallization in the pods and by extension the anorthosites macroscopic quartz k feldspar graphic intergrowths indicate high viscosity fluid bearing and significantly undercooled magmatic conditions whereas microscopic myrmekitic plagioclase quartz and symplectitic plagioclase orthopyroxene intergrowths on primary grain boundaries indicate replacement of phases in the presence of reactive fluids in assessing the nature of these pegmatitic pods based on field petrographic and geochemical evidence we conclude that they represent the fluid bearing late stage crystallization products of a residual liquid in the massif anorthosite system the fe and si compositional variations observed in these late stage pods can be linked to a fundamental olivine pyroxene dichotomy observed in most proterozoic anorthosite massifs suggesting that pulses of magma experience variable contamination in amount and or composition leading to varying differentiation paths a range of lithologies monzonites monzonorites ferrodiorites and jotunites observed in similar pod like structures as well as dykes and plutons has been observed in other proterozoic anorthosite massifs and all have at one time or another been interpreted as the residual liquids of anorthosite crystallization our observation of in situ pods with similar compositions to all of the aforementioned lithologies and displaying textures indicative of late stage crystallization supports the notion that all of these associated lithologies can be interpreted as comagmatic with but variably contaminated and isolated residual

the geology and mineralogy of the loulo mining district mali west africa evidence for two distinct styles of orogenic gold mineralization loulo is a world class orogenic gold mining district in the birimian terrane of western mali orebodies are located along second or higher order shears associated with the senegal mali shear zone with gold mineralization largely linked to a transtensional event the mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics one group is typified by the gara deposit whereas the other by the yalea deposit both deposit styles are hosted by similar rock types calcareous graywackes and calciticdolomitic marbles gara style orebodies occur as sulfide disseminations or ankerite rich shear vein stockworks hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the senegal mali shear zone these deposits are characterized by intense multi stage albitization and tourmalinization pre syn and postmineralization gold lodes are fe rich dominated by nickeloan pyrite contain cu ni co minor and trace sulfides e g chalcopyrite gersdorffite pentlandite cobaltite millerite and show consistently high levels of p ree w bearing phases apatite monazite xenotime and scheelite base metal concentrations show a marked increase in marble host rocks with the formation of nickeloan pyrite cobaltite clausthalite ores in contrast yalea style deposits are associated with quartz ankerite vein lodes and disseminated sulfide stringer zones mineralization occurs along highly altered tourmaline absent brittle ductile shears up to 8 km away from the senegal mali shear zone wall rock alteration is characterized by addition of k 2 o cao co 2 h 2 o and sio 2 with mineral assemblages consisting of chlorite sericite carbonate quartz albite ore paragenesis is enriched in as mainly as multistage growth of arsenopyrite and arsenian pyrite base metal sulfides scheelite and ree phosphates are extremely rare the diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region the as rich yalea style deposits have characteristics typical of birimian gold mineralization in ghana and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism on the other hand the polymetallic fe b rich gara style orebodies show atypical features for birimian gold mineralization instead field relations and the mineralogy and geochemistry of the gara style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks with possible links between gold mineralization and iron oxide skarn development in the region the data collected at loulo highlight the diverse nature of orogenic gold deposits especially in west africa this style of ore deposit can form from a variety of fluid sources both metamorphic and magmatic

imprints of multiple fluid regimes on episyenites in the bohus granite sweden abstract a number of episyenite occurrences within the neoproterozoic bohus granite southwestern sweden were investigated in order to elucidate their genesis and past associated fluid regimes fluid inclusion microthermometry and o h isotope systematics integrated with information from detailed mineralogy reveal a paragenetic succession within individual episyenites including 1 albitization of plagioclase accompanied by dissolution of quartz with a vuggy feldspathic reservoir rock as a result 2 resumed infilling and replacement of igneous fe ti phases by various phyllosilicates muscovite chlorite and or illite hematite assemblages and quartz due to rift related reactivation and 3 sporadic argillitic alteration and precipitation of fracture calcite although sparse owing to later overprinting there are a few remnants that provide arguments for the involvement of meteoric water shifted to a more 18 o rich composition during the initial stage of dissolution and albitization the main vug filling phase is proposed to be associated with the ingress of surface derived fluids with negative 18 o values during the regional thermal anomaly accompanying the development of the nearby permo carboniferous oslo rift at 250 ma the fluids involved during this event show consistently a marked but variable 18 o depletion and resemble the modern day shield brines with modal salinities of 11 24 wt nacl eq low nacl nacl cacl 2 weight ratios and no discernible co 2 even though no p t path can be defined for this phase the fluid pressure can be restricted to 0 3 0 8 kbar and temperature to 160 280 c assuming a sedimentary overburden of 3 km an overall variability in the 18 o and d data can mainly be ascribed to 1 precipitation from fluids affected by isotopic exchange with the host rock and or sedimentary overburden under highly variable water rock ratios and to some extent with 2 varying degrees of partial re equilibration during subsequent hydrothermal interaction at lower temperatures as emphasized by the presence of fluid inclusions of suspected secondary origin in quartz

cretaceous albitization and dequartzification of hercynian peraluminous granite in the salvezines massif french pyrenees abstract the salvezines massif in northern french pyrenees has undergone extensive albitization the massif consists of gneisses and a leucogranite hosted in paleozoic sediments schists and carbonates the leucogranite was emplaced at the end of the hercynian orogeny the granite underwent monazite fractionation during magmatic stages which induced lowering of ree and th contents in the most evolved rocks hydrothermal alteration during late magmatic stages is also identified through the development of a tetrad effect in the ree patterns of the granite and through the fractionation of ratios like zr hf out of the charac charge radius controlled range the granite underwent partial to complete sub solidus albitization progressive metasomatic overprint induced metasomatic replacement of feldspars into albite and dissolution of quartz episyenitization together with new albite formation hydrothermal muscovite with a sheaf like structure sometimes crystallised in albitites inherited muscovites from magmatic stage and newly formed muscovites from albitites have distinct chemical compositions with the latter being much more phengitic the initial peraluminous chemistry of the leucogranite was lost during albitization and composition evolved towards a pure albite one a cnk 1 albitization induced gains in na and al and losses in si and k leaching of ree and u is also identified during albitization as well as the non charac behaviour of some elements 18o values of quartz and muscovite in albitized granites are in the magmatic range about 12 and 9 5 respectively feldspars acquired high 18o values during albitization up to 15 muscovite grains from albitites have higher 18o values than inherited ones and tend towards isotopic equilibrium with albite the albitizing fluids have interacted with high 18o rocks probably local carbonates prior to alter granite muscovites inherited from magmatic stages display very characteristic disturbed 39ar 40ar age spectra with saddle shapes when a muscovite associated with albitization provided a plateau age at 117 5 ma for this alteration event albitization in the salvezines massif just preceded the regional talc chlorite hydrothermal mineralization both events might then represent two outward signs of a huge hydrothermal system at the time of the rotation of the iberian plate around europe the north pyrenean metamorphism is identified by 39ar 40ar analyses at ca or younger than 100 ma

textural features and chemical evolution in ta nb oxides implications for deuteric rare metal mineralization in the yichun granite marginal pegmatite southeastern china

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

the geochemistry metasomatism and petrogenesis of the granites of the english lake district new geochemical data for the carrock threlkeld ennerdale shap skiddaw and eskdale granites of the lake district are presented and discussed with particular reference to the metasomatism and petrogenesis of the intrusions the caledonian granites of the lake district have more associated hydrothermal activity and mineralization than their equivalents north of the iapetus suture in scotland pervasive high temperature metasomatism which affects the shap skiddaw and eskdale intrusions is accompanied by remobilization of the large ionic lithophile elements k rb sr u and li and b although high field strength elements including the rare earth elements remain unaffected except near greisen and mineral veins the threlkeld intrusion appears to have suffered loss of a ba and sr during a low temperature event however there is no consistent relationship between granite composition hydrothermal activity and mineralization which could be used to support the granite cupola model for mineralization in the province rather the episodic mineralization which affects the lake district may be related more to the broad geothermal field consequent upon the emplacement of the deeper lake district batholith identified from geophysical data despite the variable metasomatic perturbations of their primary chemistry three distinct groups of intrusions can be recognized on geochemical grounds the carrock granophyre is a product of tholeiitic fractionation and the threlkeld and ennerdale bodies are more typical calc alkaline intrusions whereas the eskdale shap and skiddaw plutons show a progressive evolution towards more complex geochemical patterns suggesting decreasing hornblende but increasing plagioclase control of fractionating processes the culmination of this trend may have been the emplacement of a large high heat production granite beneath the lake district at the end of the caledonian orogeny the suggestion made on geophysical grounds that the eskdale intrusion is a cupola of the lake district batholith cannot be simply reconciled with geochemical modelling none of the lake district granites has geochemical or isotopic characteristics which are convincingly s type i e formed by partial melting of a sedimentary protolith and a model of subcrustal magmagenesis beneath an evolving arc or continental margin appears to be more appropriate

time temperature evolution of microtextures and contained fluids in a plutonic alkali feldspar during heating microtextural changes brought about by heating alkali feldspar crystals from the shap granite northern england at atmospheric pressure have been studied using transmission and scanning electron microscopy a typical unheated phenocryst from shap is composed of about 70 vol of tweed orthoclase with strain controlled coherent or semicoherent micro and crypto perthitic albite lamellae with maximum lamellar thicknesses 700 c and after 48 h at 700 c all such regions were essentially compositionally homogeneous as indicated by x ray analyses at fine scale in the transmission electron microscope changes in lamellar thickness with time at different t point to an activation energy of 350 kjmol 1 a lamella which homogenised after 6 800 h at 600 c therefore would have required only 0 6 s to do so in the melting interval at 1 100 c subgrains in patch perthite homogenised more slowly than coherent lamellae and chemical gradients in patches persisted for 5 000 h at 700 c homogenisation t is in agreement with experimentally determined solvi for coherent ordered intergrowths when a 50 100 c increase in t for an1 is applied homogenisation of lamellae appears to proceed in an unexpected manner two smooth interfaces microstructurally sharp advance from the original interfaces toward the mid line of each twinned semicoherent lamella in places the homogenisation interfaces have shapes reflecting the local arrangements of nanotunnels or pull aparts analyses confirm that the change in alkali composition is also relatively sharp at these interfaces si al disordering is far slower than alkali homogenisation so that tweed texture in orthoclase tartan twinning in irregular microcline and albite twins in albite lamellae and patches persisted in all our experiments including 5 478 h at 700 c 148 h at 1 000 c and 5 h at 1 100 c even though the ensemble in each case was chemically homogeneous nanotunnels and pull aparts were modified after only 50 min at 500 c following the simulated 40ar 39ar step heating schedule new features called slots developed away from albite lamellae often with planar traces linking slots to the closest lamella slot arrays were often aligned along ghost like regions of diffraction contrast which may mark the original edges of lamellae we suggest that the slot arrays result from healing of pull aparts containing fluid at 700 c and above the dominant defects were subspherical bubbles which evolved from slots or from regions of deuteric coarsening the small degree of partial melting observed after 5 h at 1 100 c was often in the vicinity of bubbles larger micropores which formed at subgrain boundaries in patch perthite during deuteric coarsening retain their shape up to the melting point as do the subgrain boundaries themselves it is clear that modification of defects providing potential fast pathways for diffusion in granitic alkali feldspars begins below 500 c and that defect character progressively changes up to and beyond the onset of melting

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

the complex hydrothermal history of granitic rocks multiple feldspar replacement reactions under subsolidus conditions recurring subsolidus re equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid rock interactions that affect large volumes of the earth s continental crust this has a direct bearing on the interpretation of the present day granitic rock mineralogy and geochemistry we examine palaeoproterozoic grey and red stained granitoids from the simpevarp and laxemar areas in se sweden particularly focusing on consecutive feldspar replacement reactions to provide an in depth understanding of subsolidus re equilibration of granitic rocks with hydrothermal fluids the apparently most unaltered grey granitoids contain highly porous oligoclase grains that enclose crystallographically continuous microcline relicts this texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline oligoclase is progressively replaced by albite an9 along polysynthetic twinning and intragranular fractures the features of this replacement are characteristic of a dissolution reprecipitation mechanism finegrained mica sericite is closely associated with the albite porosity within micron sized pores observable with scanning electron microscopy as well as in nanopores imaged with transmission electron microscopy the reddening phenomenon in the vicinity of fractures is contemporaneously related to the k feldspathization of sericite which is restricted to the altered oligoclase submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration the complex associations between the fluid feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re equilibration changing the original magmatic mineralogy therefore the effects of large scale re equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered

transient porosity resulting from fluid mineral interaction and its consequences the term porosity is very widely used in geosciences and normally refers to the spaces between the mineral grains or organic material in a rock measured as a fraction of the total volume these spaces may be filled with gas or fluids and so the most common context for a discussion of porosity is in hydrogeology and petroleum geology of sedimentary rocks while porosity is a measure of the ability of a rock to include a fluid phase permeability is a measure of the ability for fluids to flow through the rock and so depends on the extent to which the pore spaces are interconnected the distribution of pores and pore neck size as well as on the pressure driving the flow this chapter will be primarily concerned with how reactive fluids can move through tight rocks which have a very low intrinsic permeability and how secondary porosity is generated by fluid mineral reactions a few words about the meaning of the title will help to explain the scope of the chapter 1 fluid mineral interaction when a mineral is out of equilibrium with a fluid it will tend to dissolve until the fluid is saturated with respect to the solid mineral we will consider fluids to be aqueous solutions although many of the principles described here also apply to melts the generation of porosity by simply dissolving some minerals in a rock is one obvious way to enhance fluid flow dissolution of carbonates by low ph solutions to produce vugs and even caves would be one example however when considering the role of fluid mineral reaction during metamorphism the fluid provides mechanisms that enable re equilibration of the rock i e by replacing one assemblage of minerals by a more stable assemblage this not only involves the dissolution of the parent mineral phases but the

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

the frequency of meteorite falls on the earth photographic observations from a network of 60 cameras in western canada are used to derive the influx rate of meteorites on the earth9s surface the first time instrumental data have been used for this purpose forty three observed events are believed to have dropped between 0 1 and 12 kilograms of meteorites each the flux values are corrected for a minor latitude effect and agree with earlier estimates near 10 kilograms but vary more slowly with mass eight events per year drop at least 1 kilogram of meteorites in an area of 106 square kilometers

microstructural evolution during experimental albitization of k rich alkali feldspar crystals of k feldspar c2 m in contact with highly concentrated aqueous nacl solutions at 500 c and 200 mpa are pseudomorphically replaced by high albite c bar 1 as a result of an interface coupled dissolution reprecipitation process the reaction occurs at an extremely sharp reaction front 10 nm and involves the complete breakdown of the initial framework structure this results in the release of tetrahedrally incorporated elements such as fe3 and ti4 and a significant increase in si al disorder across the reaction interface the evolving microstructure is controlled by crystallographic relations between the phases this leads to highly anisotropic sawtooth shaped intergrowths of albite and initial k feldspar resulting in the least structural misfit between the two framework structures as a result the newly formed interfaces appear to be semicoherent and cracks across the reaction fronts even indicate elastic strain the reaction produces 2 distinctive albite types albite 1 and albite 2 both are polycrystalline with albite 2 showing significantly larger subgrain sizes this indicates a secondary coarsening step driven by the reduction in interfacial energy within the polycrystalline replacement product the reaction also produces a highly porous rim however the porosity is not evenly distributed resulting in a porous albite 1 and a non porous albite 2 that mostly surrounds large euhedral pores despite the substantial volume fraction of porosity in albite 1 no significant 3d interconnectivity could be detected making the presence of a pervasive porosity unlikely however the result of coarsening is the continuous modification of the 3d porosity distribution this could potentially provide a mechanism for fluid transport through the replacement rim until textural and chemical equilibration is achieved

cooling process of a granitic body deduced from the extents of exsolution and deuteric sub solidus reactions case study of the okueyama granitic body kyushu japan abstract the variation in cooling processes with depth in a magma body is evaluated quantitatively by analysis of the extent of exsolution coarsening and deuteric coarsening as sub solidus reactions this method is applied to evaluation of the okueyama granitic body of central kyushu japan exsolution coarsening has produced microperthite textures in this body while deuteric coarsening has resulted in patchperthite myrmekite and reaction rims respectively through measurement of six textural parameters including the width and spacing of lamellae and the thickness of myrmekite and reaction rims the extent of these sub solidus reactions is shown to increase systematically with depth in the granite body indicating that the okueyama cooled gradually from the roof the hornblende plagioclase and ternary feldspar thermometers indicate temperature a range of 710 to 620 c for volume diffusion associated with exsolution coarsening while deuteric coarsening is found to have occurred at temperatures below 500 c on the basis of the ternary feldspar thermometer the cooling period corresponding to exsolution coarsening is estimated using a one dimensional heat transfer model yielding periods of 820 y at the roof and 1390 1890 y at the base of the exposure 1000 m below the roof depending on total depth of the original magma body

ratio of leader to return stroke electric field change for first and subsequent lightning strokes the ratio of leader to return stroke electric field change versus distance is presented as a function of stroke order for 218 first and subsequent strokes in 70 cloud to ground flashes which occurred near tampa florida in 1979 the measured leader to return stroke field ratio tended to be more negative as the stroke order increased implying that the charge sources for successive strokes were on average progressively farther from both the observer and the ground strike point sixteen percent of all flashes analyzed contained at least one leader return stroke sequence with a negative net electric field change that is a ratio that is less than 1 these strokes were observed at distances from 2 5 to 11 6 km with 56 farther than 8 km they exhibited a strong tendency to occur late in a flash with 89 occurring after the fourth stroke the geometic mean initial electric field peak normalized to 100 km for strokes with a ratio less than 1 was found to be 1 1 v m 2 6 times lower than for all subsequent strokes in our data base

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

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

linking nm scale measurements of the anisotropy of silicate surface reactivity to macroscopic dissolution rate laws new insights based on diopside linking nm scale measurements of the anisotropy of silicate surface reactivity to macroscopic dissolution rate laws new insights based on diopside damien daval a b roland hellmann c giuseppe d saldi a richard wirth d kevin g knauss a a earth sciences division lawrence berkeley national laboratory ms 90r 1116 1 cyclotron road berkeley ca 94720 usa laboratoire d hydrologie et de ge ochimie de strasbourg universite de strasbourg eost cnrs umr 7517 1 rue blessig 67084 strasbourg france c institute for earth sciences isterre universite de grenoble 1 cnrs observatoire des sciences de l univers a grenoble 38041 grenoble france d gfz german research centre for geosciences telegrafenberg 14473 potsdam germany b abstract the interfacial zone between a bulk uid and a mineral surface is where all exchange of matter and energy occurs during chemical weathering however our knowledge is still limited with respect to understanding where and how the rate determin ing dissolution reactions take place a complicating factor is the commonplace formation of amorphous si rich surface layers assls which may hinder contact between the uid and the mineral surface to address the role of assl we investigated the dissolution of a common silicate diopside and related the bulk dissolution rate with the nanoscale dissolution rate and surface chemistry of its individual prevalent faces while assl were evidenced on all of the investigated faces only those formed on 1 1 0 and 1 10 were passivating thereby controlling the reactivity of the underlying faces the 1 1 0 and 1 10 faces intersect the highest density of mg o si and fe o si bonds and this speciecity may explain the passivating behavior of the corresponding assl moreover we evidenced an inverse relation between aqueous silica concentration and the bulk dissolution rate of crushed diopside grains which suggest that the 1 1 0 and 1 10 faces are predominant in a powder by considering assl as a separate phase that can control silicate dissolution rates extrapolated laboratory based rates at conditions relevant to the eeld can be lowered by up to several orders of magnitude thereby decreasing the large gap between laboratory and natural rates this has important implications for more accurately modeling chemical weathering reactions so important today for the c cycle and co 2 sequestration and earth science processes whether it determines the rates of denudation and soil formation godderis et al 2010 co 2 uptake and its impact on climate change le hir et al 2008 beaulieu et al 2012 hydrothermal circu lation fritz et al 2010 durability of radioactive waste conenement glasses cailleteau et al 2008 frugier et al 2008 or geological sequestration of co 2 knauss et al 2005 the same strategy is commonly applied for determin ing the long term evolution of uid rock interactions this strategy relies on the experimental determination of the 1 introduction the chemical weathering of silicate materials plays a central role in many major engineering environmental corresponding author at laboratoire d hydrologie et de ge ochimie de strasbourg universite de strasbourg eost cnrs umr 7517 1 rue blessig 67084 strasbourg france tel 33 0 3 68 85 05 56 fax 33 0 3 68 85 04 02 e mail address ddaval unistra fr d daval

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

neoproterozoic crustal growth and reworking of the northwestern yangtze block constraints from the xixiang dioritic intrusion south china abstract neoproterozoic magmatism along the northwestern margin of the yangtze block south china produced voluminous adakitic and normal arc felsic intrusions associated with many mafic ultramafic bodies dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region the xixiang dioritic intrusion at hannan on the northwestern margin of the yangtze block provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the neoproterozoic continental crustal evolution of the region the pluton has a zircon u pb age of 764 9 ma rocks from the pluton have relatively low sio 2 59 3 63 0 wt and k 2 o na 2 o 5 8 6 2 wt and high mgo 2 3 3 1 wt and al 2 o 3 15 8 17 1 wt they have chondrite normalized ree patterns enriched in lree la ybn 7 2 14 7 with negative eu anomalies their primitive mantle normalized trace element patterns are characterized by enrichment of lile and depletion of hfse nb and ta with positive pb and negative ti anomalies they have a narrow range of initial 87 sr 86 sr ratios 0 7033 to 0 7035 with positive end values 1 94 to 3 16 their zircon ehf values range from 5 5 to 9 8 with an average of 7 32 these isotopic data are similar to those of the neoproterozoic mafic ultramafic intrusions in the hannan region suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks the diorites have mg values 50 3 54 4 higher than those of experimental melts of mafic igneous rocks in addition they contain fe rich clinopyroxene suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue based on its arc related geochemical affinity and emplacement preceding voluminous adakitic magmatism the xixiang intrusion is proposed to have been generated in a neoproterozoic arc setting during crustal growth and reworking that lasted from 950 ma to