accessibilities of reactive minerals in consolidated sedimentary rock an imaging study of three sandstones abstract widespread implementation of geological storage of co 2 requires an understanding of dissolution reactions with formation minerals this will be aided by reactive transport modeling which relies on accurate estimates of the accessible surface areas of reactive minerals in consolidated sedimentary rocks for three viking sandstones alberta sedimentary basin canada we have employed backscattered electron microscopy and energy dispersive x ray spectroscopy to examine mineral content and to statistically characterize mineral contact with pore space porosities range from 20 in a lightly cemented sandstone with grains on the order of 100 m to 8 in a highly cemented shaly sandstone with a mix of primary pore space and fractures to 7 in a lightly cemented conglomerate sandstone with grain sizes between 500 m and 1 mm in all three specimens kaolinite is the primary authigenic clay mineral cementing quartz grains it accounts for only 5 to 31 of mineral content but 65 to 86 of pore mineral contact boundaries the sandstone specimen has 6 minerals in the reactive category which in this study includes minerals other than kaolinite and quartz such as k feldspar apatite and pyrite for this specimen only one third of the reactive minerals are accessible to pore fluids due to clay mineral grain coatings for the shaly sandstone only one fifth of its 5 reactive minerals are accessible to pore fluids due to regions of cementation of fine detrital matrix thus if a mineral volume fraction is used in reactive transport modeling as a proportional measure of accessible surface area in consolidated sandstones the reaction rates are likely to be overestimated by three to five times the conglomerate sandstone has only 1 of its mineral matter in this category and these are often found as inclusions rather than grains

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

structural architecture of the southern and middle urals foreland from reflection seismic profiles structural architecture of the southern and middle urals foreland from reflection seismic profiles

shallow water anoxia in the mesoproterozoic ocean evidence from the bashkir meganticlinorium southern urals abstract the apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the proterozoic eon 2 5 0 542 ga it is generally accepted that from 1 85 ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous fe ii rich with episodic euxinia confined to productive continental margins and intracontinental basins the geochemical record that informs this picture however is currently sparse and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time here we report fe s c systematics alongside major al mn and trace metal mo u data for 1 6 0 8 ga marine sediments from the bashkir meganticlinorium bma southern urals russia our fe speciation data reveal a water column dominated by ferruginous conditions no evidence for euxinia and oxygenated waters confined to only the shallowest near coastal settings trace metal data support these findings with mo and u enrichment factors in anoxic sediments implying the operation of a particulate fe oxyhydr oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic mo enrichments sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the bma succession which in combination with a low organic carbon loading promoted the development of ferruginous rather than euxinic water column conditions the restriction of oxic conditions to the shallowest near coastal depositional settings within the bashkir basin contrasts to other redox studies from this period such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the mesoproterozoic era and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period

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

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

early cambrian palaeogeography and the probable iberia siberia connection abstract the end of the proterozoic beginning of the cambrian is marked by some of the most dramatic events in the history of earth the fall of the ediacaran biota followed by the cambrian explosion of skeletonised bilaterians a pronounced shift in oceanic and atmospheric chemistry and rapid climatic change from snowball earth to greenhouse conditions all happened within a rather geologically short period of time these events took place against a background of the rearrangement of the prevailing supercontinent some authors view this as a sequence of individual supercontinents such as mesoproterozoic midgardia neoproterozoic rodinia and early cambrian pannotia assembled in the mesoproterozoic this supercontinent appears to have existed through the neoproterozoic into the early cambrian with periodic changes in configuration the final rearrangement took place during the precambrian cambrian transition with the cadomian and related phases of the pan african orogeny the distribution of early cambrian molluscs and other small shelly fossils ssf across all continents indicates a close geographic proximity of all major cratonic basins that is consistent with the continued existence of the supercontinent at that time subsequently rodinia experienced breakup that led to the amalgamation of gondwana separation of laurentia baltica siberia and some small terranes and the emergence of oceanic basins between them spreading oceanic basins caused a gradual geographic isolation of the faunal assemblages that were united during the vendian early cambrian

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

contrasting provenance signals in riphean and vendian sandstones in the sw urals russia constraints for a change from passive to active continental margin conditions in the neoproterozoic abstract two contrasting provenance areas for neoproterozoic sandstones can be distinguished in the southwestern urals by light and heavy mineral analyses as well as by mineral chemistry these reflect a dramatic change of geotectonic conditions at the eastern border of the baltica protocontinent at around 610 620 ma detritus from riphean and lower vendian sandstones representing about 1 ga of continuous sedimentation is characterised by a continental platform provenance reflecting its derivation from the proterozoic basement of the east european platform source rocks include mainly minerals and few lithic clasts from granitoids and high grade metamorphic rocks as well as reworked clastic sediments and silicic volcanic rocks the provenance signal changes abruptly with upper vendian detritus representing a recycled orogenic provenance this includes mineral and lithic clasts of low grade siliciclastic metasediments and mylonites containing phengites with a high pressure signature as well as clasts of bimodal volcanic rocks and reworked siliciclastic sediments the composition of phengites contrasts strongly with those of detrital white mica in the riphean rocks also the composition of tourmalines derived from a metapsammopelitic source indicates a mainly al poor metasediment provenance whereas riphean tourmalines were derived from mainly al rich metapsammopelites zircon morphology tourmaline zoning and a reduced heavy mineral spectrum provide evidence for polycyclic sedimentation during riphean and vendian upper vendian sedimentary rocks were deposited in a foreland basin derived from a proximal uplift to the east provenance characteristics of the upper vendian detritus are consistent with areas affected by a high pressure low temperature metamorphism during a pre uralian orogenic event most likely from the metamorphic complex of beloretsk which was emplaced and exhumed during the upper vendian the change of geotectonic conditions in the upper vendian reflects a change from a passive continental margin in the south urals throughout the entire riphean since at least 1350 ma to a convergent continental margin within a presumed transpressional setting

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

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

crustal architecture of the southern uralides from true amplitude processing of the urals seismic experiment and integrated studies urseis vibroseis profile true amplitude processing of the urals seismic experiment and integrated studies urseis vibroseis deep reflection seismic data acquired by the urseis consortium shows the southern uralide crust to be composed of four major blocks with distinctive reflection characteristics these blocks are juxtaposed along crustal scale boundaries the foreland thrust and fold belt developed from the east european craton crust is imaged as subhorizontal to east dipping reflectivity that can be related to its paleozoic and older tectonic history the moho beneath the foreland thrust and fold belt is not imaged in the vibroseis data set the main uralian fault the major arc continent suture is unreflective but its subsurface location can be inferred by the truncation of the reflection pattern of the east european craton and its contrast with that of the magnitogorsk arc the magnitogorsk arc reflectivity is characterized by patchy noncoherent to coherent reflections in the upper 10 15 km that are interpreted to be related to the arc volcanic rocks below this reflectivity is diffuse or the arc crust is transparent and the moho is not imaged the east magnitogorsk fault zone which juxtaposes the arc against the east uralian zone is not imaged by the data the upper 5 to 6 km of the east uralian zone corresponding to the dzhabyk granite is transparent below the granite the crust is characterised by east dipping patches of moderately coherent high amplitude reflections that in the east become shallowly west dipping a 10 km thick west dipping band of coherent high amplitude reflections between 12 and 35 km depth corresponding to the kartaly reflection sequence extends beneath almost the entire east uralian zone the crust beneath the easternmost east uralian zone reaches 53 km in thickness the upper and middle crust of the trans uralian zone is characterized by a series of east and west dipping concave upward moderately coherent high amplitude reflections the lowermost middle and lower crust displays thin bands of west dipping coherent low amplitude reflectivity the moho is imaged as a sharp transition from reflective lower crust to transparent upper mantle at 49 km depth and the lower crustal reflections appear to merge with it the urseis vibroseis data are integrated with results from the explosion source reflection data the wide angle reflection data and the surface geology to place constraints on the crustal architecture of the orogen and on the timing of its assemblage

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

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

the growth and destruction of continental crust during arc continent collision in the southern urals abstract the southern urals of russia contain a well preserved example of a paleozoic arc continent collision in which the laurussia margin was subducted beneath the magnitogorsk island arc in the devonian providing an ideal field area for studying the possible growth and destruction of the continental crust during this process high pressure rocks derived from the leading edge of the continental margin indicate that it was subducted to a depth of between 70 km eclogite assemblages and 120 km micro diamonds the vast majority of the high pressure rocks have a sedimentary protolith with mafic eclogite having been derived from dikes intruding into the sediments high pressure rocks derived from the mafic ganulite that currently makes up the middle and lower crust of the southern urals do not occur in outcrop suggesting that much of the subducted margin remained in the upper mantle however extensive geological geochemical and geophysical data do not unequivocally clarify its subsequent fate crustal delamination and foundering into the mantle can be ruled out and recycling through the volcanic arc system appears to have been minor while the metamorphic products of subducted continental crust can remain in the mantle for long periods of time in the southern urals it has not been imaged by the available geophysical data sets a possible explanation for this is that the progressive metamorphism of mafic granulite to eclogite assemblages would have altered its physical properties to those of typical mantle lithologies making them difficult to detect by geophysical methods it is estimated that the volume of continental crust that was subducted and lost to the mantle was approximately one third of the volume that was added to the laurussia margin by the accretion of the magnitogorsk arc

geology and geodynamic evolution of the high p low t maksyutov complex southern urals russia the high pressure low temperature maksyutov complex is situated in the southern urals between the silurian devonian magnitogorsk island arc and the east european platform the elongated n s trending complex is made up of two contrasting tectono metamorphic units unit 1 consists of a thick pile of proterozoic clastic sediments suggested to represent the passive margin of the east european platform the overlying unit 2 composed of paleozoic sediments volcanic rocks and a serpentinite melange with rodingites is interpreted as a remnant of the uralian paleo ocean devonian eastward subduction of oceanic crust beneath the magnitogorsk island arc resulted in an incipient blueschist facies metamorphism of unit 2 indicated by lawsonite pseudomorphs in the rodingites while unit 2 was accreted to the upper plate subduction of the continental passive margin caused the high pressure metamorphism of unit 1 buoyancy driven exhumation of unit 1 into the forearc region led to its juxtaposition with unit 2 along a retrograde top to the ene shear zone further exhumation of the maksyutov complex into its present tectonic position was accomplished by later shear zones that were active as normal faults and are exposed along the margins of the complex at the western margin a top to the west shear zone juxtaposed a low grade remnant of a paleozoic accretionary prism suvanyak complex above the maksyutov complex along the eastern margin a top to the east shear zone and the brittle main uralian normal fault emplaced the maksyutov complex against the magnitogorsk island arc in the hanging wall

the main tectonic events depositional history and the palaeogeography of the southern urals during the riphean early palaeozoic abstract investigation of former and new sections in the classic riphean region of the bashkirian anticlinorium ba revealed that this palaeozoic segment of the passive margin of the east european platform eep during its proterozoic history was initially formed as an intracratonic aulacogen basin which developed as an inter cratonic half structure only after the early ordovician opening of the uralian palaeo ocean the originally opposite half structure of this riphean basin may have been located either on the afro european part of gondwana or along the margins of the siberian craton or elsewhere but definitely not along the conjugate permian collision suture with the kazakh block or the mugodzhary microcontinent the ca 1150 ma 1635 to 488 ma depositional history of the bashkirian basin was punctuated several times due to major tectonic events folding downfaulting uplift magmatic intrusions metamorphism which may have caused extended periods of erosion and non deposition furthermore numerous cyclothems of unknown regional extent are incorporated into the enormous time span involved in the evolution of the ca 12 15 km thick riphean sequence the riphean vendian sedimentary assemblages reflect alternating shallow marine to peritidal and continental fluviatile environments the regional distribution of which is illustrated in sets of palaeogeographic maps angular unconformities are seen at the base of the lower riphean ai formation dated at 1635 ma at the base of the middle riphean mashak formation dated at 1340 ma at the base of the upper riphean zilmerdak formation approx 1000 ma and below the middle ordovician quartzites containing shelly fossils which overlie steeply folded and slightly metamorphosed proterozoic suites along the southeastern margin of the bashkirian anticlinorium during the vendian to ordovician interval major eastern parts of the bashkirian anticlinorium as well as regions further north along the european margin of the urals were subjected to strong diastrophism probably related to the cadomian or baikalian orogeny later on during the ordovician the western half part of the bashkirian basin turned into a marginal basin of the east european platform and thus was modified by the early stages of the uralian oceanic development

organic matter preservation on continental slopes importance of mineralogy and surface area abstract theoretical considerations calculations and data reported by keil et al 1994a were used to assess the recent hypotheses that mineral surface roughness controls the specific surface area of continental margin sediments and that the matrix linked organic carbon content of continental margin sediments occurs as a monolayer or monolayer equivalent coating of organic compounds on the surfaces of detrital minerals and other detrital nonorganic grains mayer 1994 results of our analysis indicate that it is not the surface roughness of the terrigenous detrital framework grains that controls the specific surface area of most continental margin sediments even in the sand and silt fractions but rather the presence of nonspherical high surface area to volume particles primarily clays but also oxy hydroxides and ultra structured nonorganic bioclasts such as diatom frustules analysis of the mineralogy organic carbon content surface area and in situ microfabric of continental margin sediments off california in the mendocino and san luis obispo areas and off the state of washington indicate a strong correlation between the amount of matrix linked organic matter present and the suite of clay minerals data indicate that organic carbon appears to be preferentially sequestered in smectite rich sediments compared to those whose clay fractions are dominated by chlorite we suggest that this association is a function of differences in the site density and chemistry of the clays and differences in their flocculation behavior our data also indicate that organic carbon preservation on the three california transects all of which have a suboxic oxygen minimum zone that impinges on the sea floor is not significantly influenced by differences in bottom water oxygen concentration and carbon stable isotope data suggest no preferential preservation of continental organic matter over that of marine origin

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

micropore surface area variation with grain size in unweathered alkali feldspars implications for surface roughness and dissolution studies abstract the bet and micropore surface areas of different size fractions of four different unweathered alkali feldspars were calculated using gas adsorption data bet surface area decreased with increasing grain size micropore surface area accounted for 12 84 of bet surface area for two of the feldspars eifel sanidine and kb14 micropore surface area increased with decreasing grain size with regard to the other two perth perthite and keystone microcline surface area varied randomly with grain size micropores have diameters 2 nm which is on the same scale as alkali feldspar unit cell dimensions as there are no channels in the alkali feldspar crystal structure it was concluded that the measured micropore surface area was due to cracks generated during the grinding involved in sample preparation values for the surface roughness of the grains were calculated assuming that micropore surface area equaled internal surface area values varied between 1 and 7 for the different grain size fractions of perth perthite keystone microcline and eifel sanidine for kb14 surface roughness varied between 7 and 24 it is possible that the dissolution of feldspars proceeds at different rates down cracks and at the outer surface of grains if this is the case then the present data suggest that care must be taken when comparing surface area normalised dissolution rates obtained from chemically similar samples which have been ground for different periods of time with different degrees of force

baltica a synopsis of vendian permian palaeomagnetic data and their palaeotectonic implications torsvlk t h smethurst m a van der voo r trench a abrahamsen n and halvorsen e 1992 baltlca a synopsis of vendlan permmn palaeomagnetic data and their palaeotectonlc implications earth sci rev 33 133 152 in light of recent additions to the palaeozoic palaeo magnetlc data base particularly for the ordovlclan era a revised apparent polar wander apw path for baltica has been constructed following a rigorous synthesis of all late precambrlan perm lan data the apw path is characterized by two prominent loops firstly a late precambrlan camb rlan loop probably relating to a rifting event and secondly a younger loop relating to a mid silurian scandlan colhsion event these features imply major change in plate tectonic reconfiguratlon baltlca probably represented an individual continental unit m early palaeozoic times and was positioned m high southerly latitudes in an inverted geographic orientation in such a reconstruction baltlca was separated from the northern margin of gondwana by the tornqulst sea and from laurentla by the iapetus ocean the tornqulst zone is thus interpreted as a passive or dextral transform margin during the early palaeozoic while undergoing counter clockwise rotations up to 1 6 ma baltica drifted northward through most ol the palaeozoic except for a short period of southerly movement in late silurian early devonian times after colhslon with laurentla rapid movements in latitude up to 9 cm yr are noted m late precambrlan earl y palaeozoic times and slgmficant decrease in velocities throughout palaeozoic time probably reflect the progressive amalgamation of a larger continent by early devonian euramerlca and permian pangea times the tornqulst sea had a principal component of palaeo east west orientation hence it is difficult to be precise in the timing of when micro continents such as eastern avalonla and the european masslfs ultimately collided along the southwestern margin of baltica these micro continents are considered to have been peripheral to gondwana m high southerly latitudes during the early ordovlclan eastern avalonla clearly had rifted off gondwana by llanvlrn llanded o times and may have collided with baltlca during late ordovlclan times although the present available sdunan palaeomagnetlc data from eastern avalonia may suggest colhslon in late silurian times across the lapetus facing margin of baltica laurentla was s tuated in equatorml to southerly latitudes during most of the lower palaeozoic these continents collided in mid silurian times l e a first collision between southwestern norway and greenland scotla nd which gave rise to the early scandlan orogeny 425 ma in southwestern norway possible followed by a later but less dramatic scandlan event in northern norway at around 410 ma since baltlca was geographically reverted m early palaeozoic times the colhsional margin could not have been a margin that once rifted off laurentm as assumed in a number of plate tectonic models

thermotectonic evolution of the western fold and thrust belt southern uralides russia as revealed by apatite fission track data abstract the uralides a linear n s trending palaeozoic fold belt reveals an intact well preserved orogen with a deep crustal root within a stable continental interior in the western fold and thrust belt of the southern uralides devonian to carboniferous siliciclastic and carbonate rocks overlay mesoproterozoic to neoproterozoic sedimentary rocks deformation in the devonian carboniferous and permian caused thick skinned tectonic features in the western and central parts of the western fold and thrust belt a stack of several nappes characterizes the deformation in the eastern part along the e w transect ac ts 96 that crosses the western fold and thrust belt apatite fission track data record various stages of the geodynamic evolution of the uralide orogeny such as basin evolution during the palaeozoic synorogenic movements along major thrusts synorogenic to postorogenic exhumation and a change in the regional stress field during the upper jurassic and lower cretaceous the palaeozoic sedimentary cover and the neoproterozoic basement of the ala tau anticlinorium never exceed the upper limit of the paz since the devonian a temperature gradient similar to the recent one 20 c km would account for the ft data reactivation of the neoproterozoic zilmerdak thrust was time equivalent to the onset of the devonian and carboniferous collision related deformation in the east west directed movement along the tashli thrust occurred in the lower permian the devonian and carboniferous exhumation path of the neoproterozoic siliciclastic units of the tirlyan synclinorium mirrors the onset of the uralian orogeny the emplacement of the tirlyan nappe and the continuous west directed compression the five main tectonic segments inzer synclinorium beloretzk terrane ala tau anticlinorium yamantau anticlinorium and zilair synclinorium were exhumed one after another to a stable position in the crust between 290 and 230 ma each segment has its own t t path but the exhumation rate was nearly the same final denudation of the western fold and thrust belt and exhumation to the present surface probably began in late tertiary in jurassic and cretaceous south directed movements along w e trending normal faults indicate a change in the tectonic regime in the southern uralides

microtextural controls of weathering of perthitic alkali feldspars the relationship between the microtexture and dissolution behaviour of fresh hf acid etched and naturally weathered alkali feldspar phenocrysts from the lower devonian shap granite has been investigated by sem and tem a novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface further electron microscope work suggests that shap phenocrysts are representative of the alkali feldspar in the protolith of many soils fresh and unweathered shap feldspars have a complex microtexture comprising areas of pristine cryptoperthite and lamellar microperthite cross cut by volumes of microporous altered feldspar or patch perthites cryptoperthites are made up of 75 nm wide albite films the platelets are coherent but albite films have numerous edge dislocations along their interface with orthoclase 001 and 010 cleavage surfaces intersect 2 3 edge dislocations m2 in three dimensions these edge dislocations form an orthogonal net in the murchison plane of easy fracture close to 601 patch perthites are irregular semicoherent to incoherent intergrowths of albite and irregular microcline subgrains with 0 65 0 70 sub m to m sized pores m2 microporous patch perthites form by dissolution reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime dissolution of shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture especially by dislocations and exsolution lamellae the core and strain energy associated with dislocation outcrops on 001 and 010 cleavage surfaces promotes rapid dissolution at those sites and formation of nn sized etch pits after 30 s of laboratory etching with hf acid vapour with progressive hf etching crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite naturally weathered feldspars were collected from the glacial erratic boulders fine gravels surrounding exposed granite surfaces and from peat soil overlying the granite during natural weathering etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate 15 m below the cleavage surface forming an interconnecting ladder like grid of submicrometer wide channels in the murchison plane coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly but faster than orthoclase this is probably because the albite lamellae have significant elastic coherency strain but this is much less per unit volume of albite than the core and strain energy associated with edge dislocations patch perthites etch in hf vapour and weather rapidly in nature to produce a honeycomb like texture of interconnecting nanometer to micrometer sized pits which nucleate at preexisting micropores or incoherent subgrain boundaries the size and density of etch pits on microperthite surfaces which is determined by rates of growth and coalescence may be a useful progress variable for natural and experimental dissolution all alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale these microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains the controls on absolute dissolution rates and why they commonly change over time the nonlinear variation of dissolution rate with grain size the ratio of alkali ions released into solution and disparities between laboratory dissolution rates and those observed in the field

artificial weathering of spanish granites subjected to salt crystallization tests surface roughness quantification for hundreds of years two types of granite zarzalejo and alpedrete from the madrid region spain have been extensively used as building stones fresh specimens of both stone types have been sampled from their respective quarries and subjected to sodium sulphate salt crystallization test sct the resulting physical and chemical weathering patterns have been characterized by polarized light optical and environmental scanning electron microscopy water absorption under vacuum conditions and mercury intrusion porosimetry techniques were used to determine the pre and post sct porosity and pore size distribution the following non destructive techniques were performed to assess stone durability and decay ultrasound velocity us and surface roughness determination sr of intra and inter granular quartz feldspar and biotite minerals at the centre as well as at the corners and edges of specimen surfaces before the sct us values were lower and sr values higher in zarzalejo zar than alpedrete alp granite after sct the us values declined while sr rose in both types of granites with greater average differences in zar than alp for both parameters feldspar and biotite and their inter granular contacts were found to be the weakest and therefore the most decay prone areas of the stone the initial sr parameters were generally higher and rose more steeply after sct at the corners and around the edges of the specimens while behaviour was found to be similar in the two types of granite variations were greater in zar the less durable and more decay prone of the two surface roughness measurement of mineral grains in granite stones is a very useful in situ non destructive technique for quantifying salt crystallization mediated physical and chemical weathering the resulting quantification of decay and of related durability provides insight into the future behaviour of this type of stone commonly used in historic buildings

carbon isotope chemostratigraphy of the middle riphean type section avzyan formation southern urals russia signal recovery in a fold and thrust belt abstract the avzyan formation of the southern ural mountains russia forms part of the middle riphean 1300 1000 ma type section its age is constrained to be younger than 1348 ma and older than 1080 ma and thus may preserve all or part of the mid mesoproterozoic isotopic shift this shift represents a significant reorganization of the carbon cycle and separates pre 1300 ma carbonates with 13c near 0 from post 1200 ma mesoproterozoic successions having average 13c values greater than 3 5 because the avzyan formation is a significant reference section for the mesoproterozoic and because it has the potential to record this mid mesoproterozoic shift a chemostratigraphic profile was constructed for this strongly folded and thrust faulted succession best preserved 13c values from carbonates of the avzyan formation lie mainly between 0 and 2 but the unit contains a single excursion represented by several data points to values greater than 4 the chemostratigraphy of the avzyan formation does not provide a definitive match to a better dated succession however the character of the carbon isotopic curve is most similar to that of the dismal lakes group arctic canada suggesting that the avzyan formation was deposited during the mid mesoproterozoic isotopic shift and may have an age near 1270 ma best preserved sr isotopic values of 0 70587 are also consistent with a later mesoproterozoic age

composition of the uralide crust from seismic velocity v p v s heat flow gravity and magnetic data abstract p wave velocity vp s wave velocity vs poisson s ratio heat flow potential field and surface geological data are integrated to constrain a model for the composition of the uralide crust along the urseis transect the model is constructed using published laboratory measurements of vp vs and density for a variety of crustal rock types these laboratory data have been corrected for depth pressure and the uralides temperature depth function the model shows clear differences between the composition of the old continental crustal nucleus of the east european craton and the newly added crust of the accreted arc terranes to the east the crust of the east european craton is more felsic than that of the magnitogorsk and east uralian zones and the latter two have a lowermost crust whose characteristics indicate a high garnet content mafic garnet granulite and or the presence of hornblendite the overall composition of the arc terranes is basaltic physical properties suggest that eclogite is not present in the arc terranes or if present it exists in small amounts that are below the resolution of the data set the lack of eclogite in the lower crust favours an intracrustal differentiation model for the evolution of the bulk composition of the continental crust nevertheless the absence of surface uplift the lack of metamorphism and late orogenic mantle melts and the current crustal thickness indicate that crustal thinning did not affect the bulk composition of the uralide crust

proterozoic magmatic and tectonometamorphic evolution of the taratash complex central urals russia the taratash complex tc in the northernmost bashkirian anticlinorium middle urals is unique among the pre uralian polymetamorphic complexes along the eastern margin of the east european craton because it experienced granulite facies peak metamorphic conditions 850 900 c 10 kbar herein we constrain the post granulite facies polystage evolution of the complex which records various increments of the geodynamic history of the east european continental margin formation of granite and migmatite associated with amphibolite facies events are dated at 2 344 29 and 2 044 8 ma u pb zircon in different structural units at 1 810 41 ma the tc was affected by a greenschist facies retrogressive metamorphism which was probably related to a stage of granite formation in the eastern part of the east european craton this is confirmed by a u pb zircon age of 1 848 8 ma obtained from a sheared granite in the adjacent alexandrovskiy complex ac greenschist facies shear zones which separate different structural units of the tc formed before 1 350 ma partial re equilibration of rb sr and k ar isotope systems between 1 350 ma and 1 200 ma is attributed to fluid flow probably induced by anorogenic magmatism in the bashkirian anticlinorium meso to neoproterozoic basaltic dykes indicate that the tc had been exhumed to upper crustal levels at that time evidence for a grenvillian event or for the timanian orogeny which affected other pre uralian complexes in the urals is lacking uralian orogenic shortening and thrusting on devonian limestones is recorded by shear zones in the ac to the east of the tc and has been dated at 300 ma rb sr 40ar 39ar

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

cambrian ordovician paleogeography of baltica recent paleomagnetic data and existing paleontological evidence show that baltica occupied temperate southern latitudes during cambrian and early ordovician time but was inverted with reference to its present orientation hence the currently opposed margins of the baltic and laurentian shield were not conterminous in the early paleozoic laurentian and baltic late precambrian rifting episodes may therefore have occurred along unrelated continental margins closure of the tornquist sea was accompanied by the counterclockwise rotation of baltica relative to avalonia in early to middle ordovician time this rotation initiated in late cambrian early ordovician time was coeval with the finnmarkian orogeny in the northern scandinavian caledonides

from cadomian subduction to early palaeozoic rifting the evolution of saxo thuringia at the margin of gondwana in the light of single zircon geochronology and basin development central european variscides germany

geodynamic regimes in the precambrian of the urals abstract of all precambrian rocks in the urals the late proterozoic is best represented the archaean and lower proterozoic are reconstructed from separate blocks massifs which are roughly correlated with the crystalline basement of the adjacent platforms radiometric dates for the majority of early precambrian blocks exceed 2 ga features of very old 2 9 ga protocrustal material have been identified with grey gneiss complexes archaean blocks represent remnants of charnockite granulite and migmatite amphibolite complexes and the geodynamic regime of their generation is the most primitive and does not occur in later periods of geological history the karelian age blocks appear to be relics of nongeosynclinal mobile belts their petrochemical characteristics and ree patterns suggest rift related conditions of their formation late proterozoic deposits 1650 590 ma comprise thick terranes both at the western slope and in the axial zone of the urals revision of rock associations disclosed that these terranes were previously wrongly interpreted as geosynclinal formations they consist of platform sediments and include magmatic rocks which are typical of zones of diaschysis tectonomagmatic activation and rifting recurrent rifting of the elsonian grenville and baikal assynthian cycles of diastrophism was responsible for crustal extension and thinning followed by complete continental breakup and development of oceanic basins in the early palaeozoic only extension in the lower crust during rifting was responsible for dynamometamorphism under different temperature conditions and dropping lithostatic pressure blastomylonites and schists produced during this process were previously mistaken for products of orogenic metamorphism they were considered as evidence of geosynclinal cycles occurring in the late proterozoic and similar to those of the palaeozoic diaschysis and rifting were probably predominant geodynamic regimes in the late precambrian of the urals they resulted in destruction or extensive reworking of the original archaean and early proterozoic crust to variable degrees and in the accumulation of volcano sedimentary and terrigenous rifted depressional successions there is no reliable evidence for the existence of an oceanic basin in the late proterozoic of the urals

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 structural architecture of the footwall to the main uralian fault southern urals abstract the uralide orogen is a linear collisional belt formed during the upper paleozoic as a result of convergence between the east european craton and outboard terranes and accretion of intervening island arcs and obduction of oceanic crust onto the east european craton this paper presents a new structural subdivision of the southern urals to the west of the main suture zone that puts this part of the mountain belt in a context similar to that of other collisional orogenic belts this subdivision consists of a west vergent thrust stack that is made up of an autochthonous to parautochthonous foreland basin and a parautochthonous foreland thrust and fold belt flanked to the east by a metamorphic thrust stack the latter units are structurally overlain by allochthonous rocks of an accretionary complex slope sediments oceanic crust and high grade eclogite blueschist bearing gneiss where possible these units are defined on the basis of their internal stratigraphy and structure and on the nature of their bounding faults

definition of the continent ocean boundary of india and the surrounding oceanic regions from magsat data abstract magsat studies over the indian region and adjoining areas show that the continental oceanic contrasts appear more distinctly in the equivalent magnetization solution than in the anomaly maps the vertical component z is found to be more useful for the equatorial regions it is also noted that in general the continental crust has a higher magnetization than the oceanic crust further the continental crust seems to extend into the arabian sea across a part of the west coast a similar continuation is seen in the northern part of the bay of bengal the west coast result is corroborated using land and marine bouguer gravity anomalies

weathering and erosion of fractured bedrock systems we explore the contribution of fractures joints in controlling the rate of weathering advance for a low porosity rock by using methods of homogenization to create averaged weathering equations the rate of advance of the weathering front can be expressed as the same rate observed in non fractured media or in an individual block divided by the volume fraction of non fractured blocks in the fractured parent material in the model the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material the low porosity rock weathers by reaction transport processes as observed in field systems the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non fractured system because the volume fraction of blocks is 1 the increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density and to the increase in exposure of surface of low porosity rock to reaction transport for constant fracture aperture the weathering advance rate increases when the fracture spacing decreases equations describing weathering advance rate are summarized in the list of selected equations if erosion is imposed at a constant rate the weathering systems with fracture bounded bedrock blocks attain a steady state in the erosional transport limited regime bedrock blocks no longer emerge at the air regolith boundary because they weather away in the weathering limited or kinetic regime blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate for convex hillslopes the block size exposed at the surface increases downslope this model can explain observations of exhumed rocks weathering in the luquillo mountains of puerto rico published 2017 this article is a u s government work and is in the public domain in the usa

diagenesis and incipient metamorphism in the western fold and thrust belt sw urals russia in the western fold and thrust belt of the southern urals the kubler and arkai indices determined on shales slates and phyllites record an increase from lower late diagenetic to epizonal grade from west to east the metamorphic grade varies strongly within the different tectonic segments which are separated by major thrusts the increase of diagenetic and incipient metamorphic grade from the footwall to the hanging wall of all major upper palaeozoic thrusts indicates a pre permo triassic origin west of the avzyan thrust zone the diagenetic to incipient metamorphic grade is related to the palaeozoic basin development and reached the final grades in late carboniferous to early permian times east of the first avzyan thrust in the yamantau anticlinorium the diagenetic to lower greenschist metamorphic grade is possibly of neoproterozoic origin and might be related to the development of the neoproterozoic basin at the eastern margin of the east european craton the eastern part of the yamantau anticlinorium was exhumed below 200 c in the late carboniferous or early permian the diagenetic grade of the autochthonous palaeozoic sedimentary units increases toward the stack of palaeozoic nappes and might partly be caused by the deformational process due to the emplacement of the palaeozoic nappes within the timirovo thrust sheet the decrease of metamorphic grade with stratigraphic age developed prior to the emplacement of the nappes the upper anchizonal metamorphic grade of the upper devonian slates of the zilair nappe results from the deformation process related to the lower carboniferous nappe emplacement

effects of silicate weathering on water chemistry in forested upland felsic terrane of the usa the authors use data from the us epa national surface water survey nsws the usgs bench mark station monitoring program and the national acid deposition program nadp to evaluate the role of weathering in supplying base cations to surface waters in forested upland felsic terrane of the northeastern northcentral and northwestern idaho batholith us multivariate regression reveals differential effects of discharge on individual base cations and silica but no secular trend in the ca na denudation rate over 24 yr 1965 1988 for the wild river catchment in the white mountains because the turn over time for na in the soil exchange complex is only ca 1 5 yr the long term behavior of the ratios ca na and si na in waters leaving this catchment indicates that weathering is compensating for base cation export in every subregion ca and mg concentrations in lakes are statistically linked to nonmarine na but the median ca na ratio is greater than the ratio in local plagioclase the authors attribute this inequality to nonstoichiometric weathering of calcium in juvenile formerly glaciated terrane not to leaching of exchangeable cations by so sub 4 because intraregional and cross regional statistical analysis reveals no effect of atmospherically derived sulfate ion the median base cation denudation rates meq m supmore minus 2 yr sup minus 1 for these american lake regions are maine granites 108 western adirondack felsic gneiss 85 vermilion batholith 42 idaho batholith 52 the regional rates are high enough to compensate for present wet deposition of acidifying anions except in some vulnerable lake watersheds in the western adirondacks less

temperature dependence of silicate weathering in nature how strong a negative feedback on long term accumulation of atmospheric co2 and global greenhouse warming estimation of the temperature dependence of natural feldspar weathering in two catchments at different elevations yields an apparent arrhenius activation energy of 18 4 kcal mol 77 0 kj mol much higher than most laboratory values this finding supports recent suggestions that hydrolytic weathering of silicate minerals may consume carbonic acid and thereby remove atmospheric carbon dioxide more rapidly with increasing temperature than previously thought this result provides a stronger negative feedback on long term greenhouse warming than has been assumed in most models of global carbon cycling the present estimate was determined from the ratio of feldspar weathering rates determined by geochemical mass balance in the southern blue ridge mountains of north carolina united states temperature a function of elevation is the only factor that differs between the two catchments parent rock type aspect hillslope hydrology and vegetation type and successional stage are the same in both

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

determination of mineral surface area in relation to the calculation of weathering rates the release of base cations by the weathering of soil minerals provides the primary buffer against the detrimental impact of acid deposition on the soil the rate of release of base cations will be governed by mineral composition and the exposed surface area of mineral grains in theory it is possible to calculate the surface area of a mixture of grains on the basis of particle size and geometry an alternative is the use of correlations it has been suggested sverdrup et al 1990 nordic council of ministers 98 that mineral surface area can be related to soil particle size distribution via a simple empirical equation this contribution compares the surface areas of soils as calculated by geometric equations and sverdrup s equation with actual bet measurements the equations tested do not appear to be applicable to the soils used in the current work modified versions of the equation suggested by sverdrup applied to subsets of our data based on sites and soil types yield better results for the soils used in the current work statistically significant improved fits are obtained when soil mineralogy as well as soil texture is taken into account