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

dextral shear along the eastern margin of the colorado plateau a kinematic link between laramide contraction and rio grande rifting ca 75 13 ma abstract kinematic data associated with both laramide age and style and rio grande rift related structures show that the latest cretaceous to neogene interaction between the colorado plateau and the north american craton was dominantly coupled with a component of dextral shear consistent with earlier studies minor fault data in this study yielded results of varied kinematics inverted to a common northeast oriented hemisphere the mean trend of kinematic shortening associated with laramide age structures is documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

ar ar dating of k feldspar in low grade metamorphic rocks example of an exhumed mesozoic accretionary wedge and forearc south island new zealand 1 new 40ar 39ar ages from detrital k feldspars and metamorphic white micas from the eastern province terranes of new zealand have been used to investigate the thermo tectonic history of different parts of an exhumed mesozoic forearc basin and accretionary wedge k feldspars from barely metamorphosed sedimentary host rocks mainly record detrital source area ages whereas those from zeolite and prehnite pumpellyite facies host rocks have textures and argon age spectra that indicate recrystallization during regional low temperature metamorphism the results contribute to a model that genetically links thermo tectonic events across the accretionary wedge and forearc basin elements of the convergent margin and into the median batholith arc probably by the early cretaceous and possibly by the middle jurassic thus even though multidiffusion domain mdd models cannot be used to make inference on cooling histories in such situations the k feldspar argon thermochronometer can provide useful information on the timing of geological events in sub greenschist facies rocks

sedimentary features of tsunami backwash deposits in a shallow marine miocene setting mejillones peninsula northern chile abstract miocene shoreface sandstones in the caleta herradura half graben northern chile contain an exceptionally coarse deposit that based on sedimentologic and stratigraphic features is regarded as having been laid down during a tsunami event by non cohesive and sediment laden subaqueous density flows interpretations of the principal sediment depositing mechanisms effective in the tsunami surges rely largely on field observations of deposit geometry and internal sedimentary characteristics this example comprises two erosively based sedimentation units that were probably deposited by successive waves in the tsunami wave train the lower unit consists of a clast supported polymodal boulder bearing breccia composed mostly of angular clasts and fewer well rounded clasts framework components are mostly chaotic but may also exhibit either inverse to normal grading or crude normal grading laterally changes in characters of depositional facies are common and abrupt the sand sized bioclastic rich matrix is poorly sorted and very similar to the underlying lower shoreface bioclastic sandstone implying that soft sediments eroded at the lower erosional surface contributed to the tsunami deposit the bulk of the upper unit is a poorly sorted breccia bearing sandstone pebbles and cobbles are scattered massive or normally graded sporadic outsized boulders emplaced as debris fall deposits may occur along the erosional base an array of signatures such as unusually coarse grain size in comparison to the surrounding deposits erosional bases the mixed sources of sediments multiple erosional and depositional events normal size grading or massive texture are all considered distinctive features of tsunamigenic deposits backwash deposition is indicated by the incorporation within the tsunami deposits of sediments derived from mixed sources such as angular clasts from nearby subaerial settings rounded clasts reworked from beach gravels and bioclastic sand eroded from older and unconsolidated shoreface deposits notwithstanding the absence of soft sediment deformation features the marked facies change from a lower shoreface into an upper shoreface environment through the tsunamiites provides directional information about the origin of the tsunami wave it was most likely produced by a sea floor fault displacement associated with an episode of sudden probably coseismic coastal uplift

antidunes and traction carpet deposits in deep water channel sandstones cretaceous british columbia canada abstract sandstone beds intercalated with thick shale of the upper member of the haida formation albian turonian in the queen charlotte islands british columbia canada show thinly laminated graded bedding the sandstones generally are present as successions of more than a few tens of inversely to normally graded thin beds the beds at the base of each succession contain much coarser detritus than the beds at the top each succession of graded beds shows a lens shaped channel geometry wavy bed forms similar in shape to the hummock and swale topography that is thought to form hummocky cross stratification hcs characterize the uppermost part of the succession however grain fabric and spacing of the bed forms indicate that they are of antidune origin it is concluded that the successions of multiple laminated sandstone beds were produced by high density turbidity currents that were probably associated with a gravity transformation in which a successive freezing of traction carpets took place lack of shale in the succession together with the common presence of antidunes suggest that each succession was the product of one sedimentation event without decrease in stream power

k ca and rb sr dating of two lunar granites relative chronometer resetting k ca and rb sr age determinations were made for lunar granite 12033 576 this small fragment is a subsample of soil particle 12033 507 which was collected subsurface at the north rim of head crater the k ca mineral isochron is well defined and yields an age of 3 62 0 11 2 ga for 40k 0 5543ga 1 and an initial 40ca44ca of 47 160 0 006 2 normalized to 42ca44ca 0 31221 the rb sr mineral isochron is quite disturbed and yields an imprecise young age of 2 20 0 65 ga for 87rb 0 0139 ga 1 and a high initial 87sr86sr of 0 775 0 022 assuming that the granite crystallized 4 0 ga ago a modeled formation age and was shock heated 0 8 ga ago 39ar 40ar age the intermediate rb sr and k ca ages are interpreted as due to partial resetting by thermal diffusion of strontium and calcium diffusion data determined for argon in 12033 507 give a value of da2 1 10 6 s 1 for a heating temperature of 700 c at this temperature it would require a time on the order of days to totally degas argon the required diffusion coefficients d for calcium 7 9 10 13cm2s 1 and sr 8 10 12cm2s 1 that are predicted from the observed age resetting are in good agreement with those determined experimentally in granitic melts the k ca system was about a factor of ten more resistant to resetting than the rb sr system which in turn was 10 times more resistant to resetting than the k ar system diffusion data indicate that isotopic exchange of calcium and strontium was responsible for the partial resetting of the k ca and rb sr chronometers rather than elemental exchange of potassium and rubidium among mineral phases rb sr isotopic data also were obtained for granite 14303 206 a subsample of breccia clast 14303 204 which was collected at the fra mauro region about 180 km from the apollo 12 site the mineral isochron is quite disturbed and yields an imprecise age of 3 95 0 38 ga for 87rb 0 0139 ga 1 and initial 87sr86sr 0 7046 0 0051 the rb sr age is comparable to the previously determined k ca age of 4 04 0 64 ga the rb sr and k ca ages of this clast are imprecise and are statistically indistinguishable from the well defined 39ar 40ar degassing age of 3 83 0 03 ga or the rb sr and k ca model ages of 4 30 0 06 ga and 4 25 0 12 ga respectively because these model ages are within error of a reported zircon u pb age of 4 31 ga we assume this granitic clast crystallized 4 3 ga ago and was shock heated 3 83 ga ago using the apparent resetting of the k ar rb sr and k ca isotopic systems and literature data for relative strontium and calcium diffusivity we estimate a shock reheating temperature of 860 c would be necessary to partially reset the rb sr and k ca ages to their nominal values this reheating temperature is uncertain because of the large errors on the ages but would be consistent with complete resetting of the k ar age and partial resetting of the rb sr and k ca ages if the hot clast was buried under 1 m of the ejecta blanket for 1 day

sedimentological processes in a scarp controlled rocky shoreline to upper continental slope environment as revealed by unusual sedimentary features in the neogene coquimbo formation north central chile abstract exceptionally good outcrops of miocene to pliocene deposits in the vicinity of submarine paleozoic basement scarps at carrizalillo north of la serena reveal a wealth of sedimentary features not commonly observed the most proximal facies consist of rock fall and coarse grained debris flow deposits directly abutting the basement wall from which they originated angular basement clasts are mixed with well rounded cobbles which probably formed as a basal gravel on a wave cut platform at the beginning of marine flooding subsequently accumulated at the scarp edge and were incorporated into the debris when the latter collapsed the poor sorting inverse grading and protruding cobbles and boulders are classical debris flow features with good clast imbrication indicating a laminar shearing action a medial facies is represented by secondary channels running parallel to the major scarp about 1 km downslope of the first locality in the largest channel megaflutes at the base indicate the passage of highly turbulent nondepositing flows eroding the soft silty substrate in the deepest central part of the channel a pebbly coquina shows horizontal and trough cross stratification with most of the bivalves oriented convex side up meter scale rip up clasts of the underlying siltstone are also present indicating turbulent flow with a density sufficiently high to retard settling the coquina is interpreted as a detachment deposit resulting from a hydroplaning debris flow along the central part of the channel where the velocity and rate of pore pressure decay were highest this deposit is overlain by fining upward massive to horizontally stratified sandstone very similar in texture and composition to the matrix of the debris flow suggesting its formation by surface transformation and elutriation of the latter along the channel margin a basal centimeter scale sandstone layer is virtually unaffected by the megaflute topography and clearly represents a subsequent event it is interpreted as a basal shear carpet driven by the overlying debris flow within the shear carpet a basal friction zone and an overlying collision zone containing a higher concentration of shell hash can be distinguished the overlying debris flow deposit is represented by massive coquina with scattered angular to rounded basement clasts it contains disarticulated bivalves oriented with their concave side up indicating large scale upward fluid escape during deposition a smaller secondary channel shows large rip up rafts of the underlying substrate some rafts appear to have been plucked from the substrate by a process of sand injection from an overriding high density sandy debris flow which probably originated during a tsunami such clasts can climb upward into a laminar flow by down current tilting and tumbling the most distal facies occurs below a second scarp oriented more or less parallel to the present coastline where finer grained turbidites onlap and backlap onto the stoss and lee sides of an obstacle formed by eroded boulder conglomerates the onlap deposits resemble inclined sandy macroforms recently described in submarine canyon settings they are interbedded with diatom containing volcanic ash beds with cross stratification dipping eastwards and containing deepwater microflora typical of continental upwelling zones

bed forms in bimodal sand gravel sediments laboratory and field analysis bed forms were studied in goodwin creek and a laboratory flume channel the bed sediment of the field site and flume had median diameters of 8ae3 modes of 0ae4 22ae6 mm and 1ae82 mm modes of 0ae5 5ae6 mm respectively the laboratory andfieldchannelshadsimilarvaluesofbimodalparameters ratiosofflowdepth to median bed material diameter and ratios of shear stress to critical shear stress and were judged to be comparable in the transport of bed load sediment and the resulting bed forms three groupings of bed forms from the laboratory flume experiments ripple like bed forms bed load sheets low relief bed waves were identified using the height and period of the bed forms for the range of flow depths and discharges investigated in the flume bed forms became higher and longer with increasing bed shear stress bed forms from goodwin creek were similar to those from theflume with comparable ratios between bed form length height and flow depth the bed forms in the flume provide a positive link between rate and size fluctuations measured in the field and the bed forms the smaller bed forms identified were sediment starved and are not considered to be dunes while the largest bed forms in which all of the bed material sizes were mobilized in the field and laboratory were judged to be dunes

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

neon diffusion kinetics in olivine pyroxene and feldspar retentivity of cosmogenic and nucleogenic neon abstract we performed stepwise degassing experiments by heating single crystals of neutron or proton irradiated olivine pyroxene and feldspar to study diffusion kinetics of neon this is important in evaluating the utility of these minerals for cosmogenic 21 ne measurements and potentially for ne thermochronometry degassing patterns are only partially explained by simple arrhenius relationships most samples do not exhibit a precisely determined activation energy in an individual diffusion domain regardless we find clear differences in diffusion kinetics among these minerals based on sub selected data our estimates for neon diffusion kinetics activation energy e a and pre exponential factor d o assuming the analyzed fragments approximate the diffusion domain in each mineral are as follows for the feldspars e a ranges from 65 to 115 kj mol and d o from 3 9 10 3 to 7 1 10 2 cm 2 s 1 for the pyroxenes e a ranges from 292 to 480 kj mol and d o from 1 6 10 2 to 2 9 10 11 cm 2 s 1 for the olivines e a ranges from 360 to 370 kj mol and d o from 1 5 10 6 to 5 0 10 6 cm 2 s 1 differences in these parameters are broadly consistent with the expected effect of structural differences between feldspar and olivine and pyroxene these results indicate that cosmogenic 21 ne will be quantitatively retained within olivine and pyroxene at earth surface temperatures over geological timescales the diffusion kinetics for feldspars on the other hand predicts that 21 ne retention at earth surface temperatures will vary significantly with domain size crystal microtexture surface temperature and exposure duration quantitative retention is expected only in favorable conditions this conclusion is reinforced by additional measurements of cosmogenic 21 ne in coexisting quartz and feldspar from naturally irradiated surface samples sanidine from a variety of rhyolitic ignimbrites exhibits quantitative retention whereas alkali feldspar from several granites does not

small scale hummocky cross stratification in turbidites a form of antidune stratification small scale hummocky cross stratification occurs in upper cretaceous calciclastic turbidites exposed in the western basque pyrenees facies associations and microfossil assemblages indicate slope to base of slope bathyal depositional environments it is developed in the fine grained portion of beds and displays spacings mostly between 0 2 and 0 7 m the beds fine upward with no sharp grain size breaks or mud partings suggesting that deposition occurred during a single flow event hummocky intervals are 0 1 0 8 m thick and consistently grade laterally and vertically into flat planar laminations of bouma b divisions suggesting that deposition occurred under upper flow regime conditions they have wave like geometries with laminae continuous across crests and troughs and display a ratio of wavelength to estimated underflow thickness of 11 3 12 8 combining the above observations and inferences these examples of small scale hummocky cross stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner denser underflow main body tail of the turbidity current and an overlying thicker low density layer this occurrence is further evidence that small scale hummocky cross stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment

implications of jurassic cretaceous and proterozoic piercing lines for laramide oblique slip faulting in new mexico and rotation of the colorado plateau discussion and reply discussion cather 1999 1 provided a useful compilation of stratigraphic data relevant to tests of models for possible dextral offset along the east side of the colorado plateau during the laramide orogeny his paper allowed the reader to judge whether this independent data set is consistent with

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

ca isotope fingerprints of early crust mantle evolution abstract the utility of 40 ca 44 ca as a tracer of pre existing crustal contributions in early archaean cratons has been explored to identify traces of hadean crust and to assess the style of continental growth the relatively short half life of 40 k 1 3 gy means that its decay to 40 ca occurs dominantly during early earth history if archaean crust had a significant component derived from a more ancient protolith as anticipated by steady state crustal evolution models this should be clearly reflected in radiogenic 40 ca 44 ca ratios or positive initial e ca in different archaean cratons a high precision thermal ionisation technique has been used to analyse the 40 ca 44 ca ratios of plagioclase separates and associated whole rocks in 3 6 ga early archaean samples from zimbabwe and west greenland three out of four tonalite trondhjemite granodiorite ttg suite samples from zimbabwe display initial 40 ca 44 ca ratios indistinguishable from our measured modern morb value i e e ca 3 6 0 greenland samples however are very diverse ranging from e ca 3 7 0 1 in mafic pillow lavas and felsic sheets from the isua supracrustal belt up to very radiogenic signatures e ca 3 7 2 9 in both mafic rocks of the akilia association and felsic ttg from the coastal amitsoq gneisses at face value these results imply the zimbabwe crust is juvenile whereas most greenland samples include an earlier crustal component yet the west greenland craton as with many archaean localities has experienced a complex geological history and the interpretation of age corrected initial isotope values requires great care both felsic and mafic samples from greenland display e ca 3 7 so radiogenic that they are not readily explained by crustal growth scenarios the presence of such radiogenic 40 ca 44 ca found in low k ca plagioclases requires ca isotope exchange between plagioclase and whole rock during later metamorphic event s in addition the unexpectedly radiogenic ca isotope ratios in some mafic samples reflect anomalous k ca ratios as a result of intense k metasomatism 3 6 ga thus ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono metamorphic processes coupled with other isotope data however the degree of overprint can be estimated and the 40 ca 44 ca ratio of a little disturbed sample hints at a small contribution of hadean protocrust in the coastal part of the godthabsfjord area southwest greenland in the majority of zimbabwe ttg samples unradiogenic initial ca isotope ratios point to very little prior crustal history and minor subsequent disturbance we thus infer that the modest initial e nd 0 8 of the zimbabwean samples is representative of the depleted mantle at 3 6 ga furthermore ca isotope systematics provide little support for a steady state model of crustal growth

systematic variations of argon diffusion in feldspars and implications for thermochronometry abstract coupled information about the time dependent production and temperature dependent diffusion of radiogenic argon in feldspars can be used to constrain the thermal evolution attending a host of earth and planetary processes to better assess the accuracy of thermal models an understanding of the mechanisms and pathways by which argon diffuses in feldspars is desirable here we present step heating ar diffusion experiments conducted on feldspars with diverse compositions structural states and microstructural characteristics the experiments reveal systematic variations in diffusive behavior that appear closely related to these variables with apparent closure temperatures for 0 1 1 mm grains of 200 400 c assuming a 10 c ma cooling rate given such variability there is no broadly applicable set of diffusion parameters that can be utilized in feldspar thermal modeling sample specific data are required diffusion experiments conducted on oriented cleavage flakes do not reveal directionally dependent diffusive anisotropy to within the resolution limits of our approach approximately a factor of 2 additional experiments aimed at constraining the physical significance of the diffusion domain are presented and indicate that unaltered feldspar crystals with or without coherent exsolution lamellae diffuse at the grain scale whereas feldspars containing hydrothermal alteration and or incoherent sub grain intergrowths do not arrhenius plots for argon diffusion in plagioclase and alkali feldspars appear to reflect a confluence of intrinsic diffusion kinetics and structural transitions that occur during incremental heating experiments these structural transitions along with sub grain domain size variations cause deviations from linearity i e upward and downward curvature on arrhenius plots an atomistic model for arrhenius behavior is proposed that incorporates the variable lattice deformations of different feldspars in response to heating and compression the resulting implications for accurately extrapolating laboratory derived diffusion parameters to natural settings and over geologic time are discussed we find that considerable inaccuracies may exist in published thermal histories obtained using multiple diffusion domain mdd models fit to arrhenius plots for exsolved alkali feldspar where the inferred ar partial retention zones may be spuriously hot

diffusion vs fluid alteration in alkali feldspar 40ar 39ar thermochronology does cross correlation of log r r0 and age spectra validate thermal histories abstract for six decades geoscientists have been trying to quantitatively understand the nature of radiogenic ar loss from alkali feldspar some researchers suggest that volume diffusion is the dominant mechanism and they use conventional step heating 40ar 39ar data from alkali feldspar to recover the thermal histories of rocks they argue that a high degree of correlation between log r r0 and 40ar 39ar age spectra which is observed in a number of natural examples justifies this hypothesis in contrast other investigators suggest that fluid mediated recrystallisation and alteration control the radiogenic ar redistribution hence rendering alkali feldspar useless as a thermochronometer by means of numerical modelling we found that the latter mechanism as well is able to produce samples with highly correlated log r r0 and 40ar 39ar age spectra in addition we show that apparent thermal histories recovered for altered alkali feldspar crystals by interpreting step heating 40ar 39ar data may be grossly inaccurate and yet seemingly fit the prevailing understanding of regional geology such inaccurate apparent thermal histories can be obtained even from alkali feldspar crystals that underwent volumetrically low degrees of alteration therefore we conclude that conventional step heating 40ar 39ar data are insufficient to support the assumption that radiogenic ar loss from alkali feldspar occurred solely by volume diffusion and validate the constrained thermal histories even if upheld by a priori knowledge of regional tectonics we further suggest that all thermochronological constraints obtained using such data should be supported by detailed petrological characterisation of alkali feldspar

sedimentology and tectonic implications of cretaceous fan delta conglomerates queen charlotte islands canada the honna formation of coniacian age consists of several hundred metres of polymictic clast supported conglomerate associated with sandstone and mudstone five conglomerate facies are recognized ungraded beds inverse graded beds normal graded beds inverse to normal graded beds and parallel stratified beds these facies are interpreted as the deposits of subaqueous cohesionless debris flows and or high density turbidity currents the depositional environment was a deep water gravelly fan that draped a fault controlled basin margin slope the fan is inferred to have passed upslope directly into an alluvial fan unpreserved hence the name fan delta can be applied to the overall depositional system this type of fan delta of which the brae oilfield in the north sea is an example is defined here as a deep water fan delta the lack of a shelf is in marked contrast to other types of fan delta three facies associations are recognized in the honna formation subaqueous proximal fan conglomerates distal fan turbiditic sandstones and pro fan interfan mudstones with thin sandy turbidites the proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km such a long subaqueous gravel belt lacks a known modern analogue the distal fan was an unchannelled sandy extension of the proximal gravel belt it is postulated that the honna formation accumulated in a foreland basin which migrated westwards from the coast mountains where the wrangellia alexander terrane was colliding with north america in this model the honna fan delta was sourced by a west verging thrust sheet whose sole thrust was the sandspit fault immediately to the east deep water fan deltas appear to develop preferentially when eustatic sea level is relatively high so that the feeder alluvial fan is small and gravelly throughout in petroleum exploration and field development care should be taken to distinguish deep water fan deltas from base of slope canyon fed submarine fans because the two systems differ significantly in terms of coarse sediment distribution

in situ 40k 40ca double plus sims dating resolves klokken feldspar 40k 40ar paradox abstract the 40k 40ca decay system has not been widely utilized as a geochronometer because quantification of radiogenic daughter is difficult except in old extremely high k ca domains even these environments have not heretofore been exploited by ion microprobe analysis due to the very high mass resolving power mrp of 25 000 required to separate 40k from 40ca we introduce a method that utilizes doubly charged k and ca species which permits isotopic measurements to be made at relatively low mrp 5000 we used this k ca double plus approach to address an enduring controversy in 40ar 39ar thermochronology revolving around exsolved alkali feldspars from the 1166 ma klokken syenite southern greenland ion microprobe 40k 40ca analysis of klokken samples reveal both isochron and pseudoisochron behaviors that reflect episodic isotopic and chemical exchange of coarsely exsolved perthites and a near end member k feldspar until 719 ma and perhaps as late at 400 ma feldspar microtextures in the klokken syenite evolved over a protracted interval by non thermal processes fluid assisted recrystallization and thus this sample makes a poor model from which to address the general validity of 40ar 39ar thermochronological methodologies

40ar39ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the klokken syenite south greenland laser probe andin vacuo crushing have been used to apply the40ar39ar technique to microtexturally well characterised pristine and turbid alkali feldspar from the 1166 ma klokken syenite individual crystals are several millimetres in size but sem and tem studies reveal complex microstructures which define a range of sub grain sizes over three orders of magnitude from 100 m in pristine areas with fine scale cryptoperthite down to 200 nm in turbid areas with coarsened perthites and numerous micropores laser probe40ar39ar measurements of turbid feldspar gave low apparent ages indicating significant40ar loss 40 average which can be accounted for by sustained heating of the small sub grains 1 m for 1166 ma at low temperature 150 c pristine feldspar gives high apparent ages indicating the presence of excess40ar this component was readily lost from the feldspar by laboratory heating at low temperature 600 c and was also released by crushing which indicates that it was released from fluid inclusions during crushing release of excess40ar is correlated with cl release and the40ar cl ratio of the fluid is similar to that of mantle fluids turbid feldspar also released a fluid during crushing but the40ar cl ratio was lower probably as a result of evolution of the original fluid during boiling anomalously high apparent ages are often a feature of low temperature ar release in stepped heating age spectra of feldspars the presence of excess40ar in fluid inclusions may substantially affect the lower temperature release in feldspar stepped heating spectra the character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite

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

hummocky cross stratification like structures and combined flow ripples in the punta negra formation lower middle devonian argentine precordillera a turbiditic deep water or storm dominated prodelta inner shelf system abstract turbidity current and storm induced deposits may exhibit similarities in particularly when the latter is laid down by a combination of oscillatory and unidirectional flows recent progress in facies analysis helps to discriminate the sedimentary effects of oscillatory from unidirectional components of the flow on the basis of detailed analysis of sedimentary facies strata geometry and palaeocurrent data the present study reinterprets the punta negra formation pnf lower middle devonian argentine precordillera previously considered as a depositional system of deep water as a storm dominated prodeltaic shelf depositional system in the sandstone beds of the pnf planar low angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms combined flow ripples accretionary hummocky cross stratification like hcs like and anisotropic hcs like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits different hypotheses on the origin of the oscillatory currents have been examined the most convincing interpretation is that the oscillatory component of the velocity is attributed to storm induced waves the palaeocurrent data indicate offshore current directions suggesting that the unidirectional flow was a gravity induced bottom current inverse grading at the base and overlying normally graded divisions of the sandstone beds testify to waxing waning behaviour of the depositional flows interbedding of sedimentary structures undulating laminations low angle and parallel laminations and combined flow ripples in the lower and intermediate divisions of the beds indicate fluctuations of flow velocity this organisation of the sedimentary structures permits association of the unidirectional component with hyperpycnal bottom currents the terrestrial origin of the hyperpycnal flows is suggested by the abundance of terrestrial plant remains the mineralogical and textural immaturity of the sandstone composition and the relative scarcity of bioturbation which was likely controlled by fresh water input and a high rate of sedimentation storm influenced hyperpycnal flows generated subaqueous channelised forms at the mouth of the river deltas which later filled with sand at the distal end of the channelised forms lobe shaped sandstone beds were deposited evolving distally into thin sandstone beds alternating with sandy mudstone

erosion and deposition by supercritical density flows during channel avulsion and backfilling field examples from coarse grained deepwater channel levee complexes sandino forearc basin southern central america abstract erosion and deposition by supercritical density flows can strongly impact the facies distribution and architecture of submarine fans field examples from coarse grained channel levee complexes from the sandino forearc basin southern central america show that cyclic step and antidune deposits represent common sedimentary facies of these depositional systems and relate to the different stages of avulsion bypass levee construction and channel backfilling during channel avulsion large scale scour fill complexes 18 to 29 m deep 18 to 25 m wide 60 to 120 m long were incised by supercritical density flows the multi storey infill of the large scale scour fill complexes comprises amalgamated massive normally coarse tail graded or widely spaced subhorizontally stratified conglomerates and pebbly sandstones interpreted as deposits of the hydraulic jump zone of cyclic steps the large scale scour fill complexes can be distinguished from small scale channel fills based on the preservation of a steep upper margin and a coarse grained infill comprising mainly amalgamated hydraulic jump zone deposits channel fills include repeated successions deposited by cyclic steps with superimposed antidunes the deposits of the hydraulic jump zone of cyclic steps comprise regularly spaced scours 0 2 to 2 6 m deep 0 8 to 23 m long infilled by intraclast rich conglomerates or pebbly sandstones displaying normal coarse tail grading or backsets these deposits are laterally and vertically associated with subhorizontally stratified low angle cross stratified or sinusoidally stratified sandstones and pebbly sandstones which were deposited by antidunes on the stoss side of the cyclic steps during flow re acceleration the field examples indicate that so called spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate grain size distribution and amalgamation the deposits of small scale cyclic steps with superimposed antidunes form fining upwards successions with decreasing antidune wavelengths indicating waning flows such cyclic step antidune successions form the characteristic basal infill of mid fan channels and are inferred to be related to successive supercritical high density turbidity flows triggered by retrogressive slope failures

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

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

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

hummocky cross stratification like structures in deep sea turbidites upper cretaceous basque basins western pyrenees france by mulder et al 2009 sedimentology 56 997 1015 discussion

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

detailed anatomy of a deep water carbonate breccia lobe upper jurassic french subalpine basin abstract detailed correlations across tithonian carbonate breccia deposits in the drome river area northern part of the so called vocontian through suggest the depositional system was that of an elongated deep water lobe up to 70 km long and 20 to 30 km wide for a thickness reaching 200 m the drome lobe as it is now called is mainly made of slope to basinal mudstones breccias with minor platform components interpreted as debris flow and mud flow deposits associated with slump deposits it is basically a base of slope system whose elongated depositional area implies it was a point sourced gravity system thus perhaps connected to a small canyon cut onto the western slope of the basin but the mostly mudstone material of the breccias also suggests that the walls of this inferred canyon were the main supplier of the lobe not the carbonate platform proper the updip part of the lobe has a complex internal geometry as the deposition of breccia bed packages is interrupted by scourings locally 50 m deep indicating maybe a canyon mouth environment the middle part of the lobe is dominated by pure vertical aggradation of breccia beds with minor intervening erosion in the downdip part of the system a morphological compensation mechanism occurs as breccia beds tend to spread laterally a huge slump carrying large mudstone olistoliths ends the breccia deposition at the beginning of the berriasian this megaslump deposit was mostly emplaced on the right side of the breccia lobe supporting the idea of a depositional relief our observations thus show that previous interpretations as a submarine canyon infilling or as shallow water breccias formed in situ by cyclic loading under attenuating hurricane waves approaching the platform are not consistent with our observations the internal geometry of the system studied brings new data about a poorly studied kind of turbidite systems that of deep water carbonate breccias

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

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

mesozoic stratigraphic constraints on laramide right slip on the east side of the colorado plateau regional eastward trending piercing lines defined by stratigraphic truncations and depositional pinchouts in jurassic and cretaceous strata in north central new mexico allow 5 to 20 km of right slip between the eastern margin of the colorado plateau and the craton to the east in laramide late cretaceous paleogene and younger time estimates of 60 170 km of laramide right slip by previous workers were based largely on 1 the amount of presumed crustal shortening across thrust and reverse faults in the wyoming province north of the plateau or 2 offset of piercing lines defined by a proterozoic regionally subhorizontal isobaric metamorphic surface that intersects preexisting structures and steep stratigraphic contacts major right slip inferred on the basis of these metamorphically defined piercing lines in proterozoic rocks must therefore predate the laramide orogeny and cannot be used to support a model postulating large scale laramide right slip restoration of 60 170 km of right slip creates lateral offset of the north south extent of the jurassic todilto formation producing an untenable paleodepositional basin geometry

ca diffusion in k feldspar abstract diffusion of ca has been characterized in natural k feldspar under dry 1 atm conditions using a 44ca tracer polished sections of feldspar were surrounded by source powders in pt capsules and annealed in air or with solid buffers to buffer at nno or iw one hydrous experiment was conducted in a piston cylinder device at 800 c and 1 gpa in all cases the source of diffusant was pre synthesized anorthitic powder doped with 44ca prepared sample capsules were held at run conditions for times ranging from a few hours to a few months at temperatures from 750 to 1050 c the 44ca distributions in the feldspars were profiled by rutherford backscattering spectrometry rbs the following arrhenius relation is obtained for ca diffusion in k feldspar for diffusion normal to 001 d 3 91 10 9 exp 276 41 kj mol rt m2 sec 1 diffusivities normal to 010 appear similar suggesting little anisotropy for ca diffusion diffusion coefficients for experiments buffered at nno and iw agree within experimental uncertainty with those for experiments run in air indicating little dependence of ca diffusion on oxygen fugacity under the investigated range of conditions on the basis of the single high pressure experiment neither pressure nor the presence of h2o appear to affect ca diffusion ca diffusion in k feldspar is slower than the diffusivities of the larger divalent cations sr and pb and also slower than ca diffusion in plagioclase which may be attributable to the large size mismatch of ca in the k feldspar structure ca diffusion is 7 8 orders of magnitude slower than ar diffusion in k feldspar indicating that k feldspars will be much more retentive of ca chemical signatures than of ar as an example 1 mm diameter feldspar grains will experience 50 ar loss in 1500 years at 500 c while ca loss of only 1 would require 100 million years at this temperature

aseismic controls on in situ soft sediment deformation processes and products in submarine slope deposits of the karoo basin south africa intervals of soft sediment deformation features including vertical fluid escape and load structures are common and well exposed in permian lower slope deposits of the tanqua depocentre karoo basin the structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones observed over a range of scales the presence of an upper structureless siltstone layer linked to the flames interpreted as a product of the debouching of fine grained material transported through the flame onto the palaeo seabed together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment rich fluids flames are oriented parallel to the deep water palaeoslope in lateral splay deposits between two major slope channel complexes statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism importantly deformation occurred in an instantaneous and synchronous manner liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north west facing slope seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep water succession the driving mechanisms that resulted in the final configuration of the soft sediment deformation structures involved a combination of vertical shear stress caused by fluidization development of an inverse density gradient and a downslope component of force associated with the local slope ground penetrating radar profiles confirm the overall north east orientation of the flame structures and provide a basis for recognition of potential larger scale examples of flames in seismic reflection data sets

intragrain oxygen isotope zoning in titanite by sims cooling rates and fluid infiltration along the carthage colton mylonite zone adirondack mountains ny usa oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen bearing minerals and the dependence of mineral 18 o 16 o ratios on temperature and fluid composi tion in this study subtle intragrain oxygen isotope zoning in titanite is resolved at the 10 lm scale by secondary ion mass spectrometry the patterns of d 18 o zoning differ depending on microstructural context of individual grains and reflect multiple processes including diffusive oxy gen exchange partial recrystallization grain size reduction and grain growth using the chrono logical framework provided by structural relations these processes can be related to specific events during the grenville orogeny titanite was sampled from two outcrops within the car thage colton mylonite zone ccmz a long lived shear zone that ultimately accommodated exhumation of the adirondack highlands from beneath the adirondack lowlands during the ott awan phase 1090 1020 ma of the grenville orogeny titanite is hosted in the diana metasyenite complex which preserves three sequentially developed fabrics an early nw dipping protomylonitic fabric s1 is crosscut by near vertical ultramylonitic shear zones s2 which are locally reoriented by a nnw dipping mylonitic fabric s3 texturally early titanite pre s2 shows diffusion domi nated d 18 o zoning that records cooling from peak ottawan granulite facies conditions numerical diffusion models in the program fast grain boundary yield good fits to observed d 18 o profiles for cooling rates of 50 20 cm a 1 which are considerably faster than the 1 5 cm a 1 cooling rates previously inferred for the adirondack highlands from regional thermochronology high cool ing rates are consistent with an episode of rapid shearing and exhumation along the ccmz during gravitational collapse of the ottawan orogen at c 1050 ma texturally later titanite syn s2 h as higher overall d 18 o and shows a transition from diffusion dominated to recrystallization dominated d 18 o zoning indicating infiltration of elevated d 18 o fluids along s2 shear zones and continued shear ing below the blocking temperature for oxygen 500 c for grain sizes at the study site the tex turally latest titanite post s3 h as growth dominatedd 18 o zoning consistent with porphyroblastic grain growth following cessation of shearing along the harrisville segment of the ccmz

implications of jurassic cretaceous and proterozoic piercing lines for laramide oblique slip faulting in new mexico and rotation of the colorado plateau regional isopach patterns and pinch out data for nine stratigraphic units of jurassic and cretaceous age are either permissive or supportive of significant dextral slip of laramide age along the eastern colorado plateau boundary in new mexico the best constrained dextral offset estimates are for the sand hill nacimiento fault system 20 33 km and the previously published 13 km estimate for the defiance monocline which together yield a cumulative offset of 33 46 km mesozoic stratigraphic constraints for other laramide fault systems are less precise and typically provide only maximum limits for possible dextral offsets because of widely spaced control points and broad areas of tertiary erosion these less precise constraints allow as much as 40 60 km of laramide dextral slip along what is now the rio grande rift and as much as 110 km across the entire breadth of the laramide deformed zone in central and northern new mexico well documented dextral offsets of proterozoic lithologies and structures across the tusas picuris fault system 15 km and picuris pecos fault 37 km probably represent minimum laramide displacements because of the need to account for sinistral components related to other deformations these displacements when combined with dextral offsets along sand hill nacimiento and defiance structures yield a minimum dextral offset of 85 km for laramide structures in northern new mexico this minimum dextral offset is approximately equivalent to the amount of laramide crustal shortening on and northward of the colorado plateau a result that argues against nearby euler pole locations for laramide rotation of the colorado plateau relative to cratonic north america geological constraints allow 0 to 3 of laramide clockwise rotation of the colorado plateau additional clockwise plateau rotation during late cenozoic development of the rio grande rift was 1 to 1 5 geological constraints thus indicate that clockwise rotation of the colorado plateau from combined laramide and rio grande rift deformations was between about 1 and 4 5

cenozoic sedimentation and paleotectonics of north central new mexico implications for initiation and evolution of the rio grande rift the rio grande rift is one of the major late cenozoic continental rifts of the world sharing most geophysical geochemical and geological characteristics with other rifts cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western north american margin paleocene eocene laramide primarily amagmatic compression related to flat slab subduction oligocene intermediate to silicic volcanism related to collapse of the slab miocene to present rifting related to complex plate interactions overprinted on the previous history cenozoic paleogeographic and paleotectonic characteristics are consistent with a passive mantle mode of rifting north central new mexico provides a unique opportunity to constrain models for rift initiation and evolution it is one of the few locations within any rift where excellently exposed pre rift and syn rift basin fill has been studied thoroughly enough to allow detailed paleogeographic reconstruction for almost the entire cenozoic cenozoic paleogeography for the study area is summarized as follows 1 eocene 58 37 ma a single amagmatic sedimentary basin el rito galisteo trended northwest southeast with laramide basement uplifts on three sides 2 early to late oligocene 37 28 ma intermediate magmatism with volcaniclastic aprons derived from the san juan and ortiz cerrillos volcanic fields and residual laramide uplifts 3 late oligocene early miocene 28 21 ma initiation of bimodal volcanism with widely dispersed volcaniclastic aprons derived from the predominantly silicic san juan and latir volcanic fields 4 early to middle miocene 21 15 ma continued volcaniclastic dispersal from silicic volcanic centers concurrent with initiation of block faulting to form half grabens internal drainage and erosion of phanerozoic strata and precambrian basement 5 middle to late miocene 15 8 ma continued deepening of half grabens widespread exposure of precambrian terranes formation of complex depositional environments in basin centers and continued bimodal volcanism 6 late miocene to present 8 0 ma concentration of extension in central grabens linked by accommodation zones major bimodal volcanism for example taos plateau and jemez mountains regional uplift and integration of rio grande drainage dispersal patterns petrofacies analysis k ar ages and chemical analyses of volcanic clasts provide details concerning three primary volcanic centers 1 san juan mountains 27 29 m y high k andesite and rhyodacite 2 latir volcanic field 25 28 m y high k andesite and rhyolite and 3 the previously unrecognized servilleta plaza center 22 23 m y latite high k andesite and rhyodacite which may have been a southern extension of the latir field these three volcanic centers provided detritus to the following units respectively 1 esquibel mbr of los pinos fm upper abiquiu fm middle picuris fm bishops lodge mbr of tesuque fm and volcaniclastic units in the northern albuquerque basin zia and abiquiu 2 cordito mbr of los pinos fm and uppermost abiquiu fm and 3 chama el rito mbr of tesuque fm and upper picuris fm use of petrofacies 1 esquibel 2 cordito and 3 plaza simplifies the chaos of stratigraphic nomenclature and promotes regional correlations of poorly exposed units

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

visualization of the internal structure of the massive division in experimental sediment gravity flow deposits by mapping of grain fabric abstract a method for mapping of grain fabric is proposed for analysis of the cryptic internal structure of massive sedimentary units the method is applied to the analysis of an experimental debris flow deposit revealing a number of characteristic features of this type of mass sedimentation the debris flow was simulated in the laboratory using a channel inclined 30 opening onto a 10 slope and transverse thin sections were prepared from four longitudinal points in the depositional lobe back scattered electron images of the sections obtained by scanning electron microscopy were processed and analyzed by mapping of grain fabric using an automated image analysis procedure although the samples appear structureless by macroscopic observation the grain fabric map reveals a range of sedimentary features including distinctive lineations from lower upcurrent to upper downcurrent in the most proximal section representing synsedimentary thrusts a steepening upward trend of grain imbrication angle in intermediate samples with very low angle imbrication in the basal horizon indicative of high shear rate flow and complex imbrication features in the most distal samples this analysis reveals that massive debris flow deposits actually contain a range of distinctive structures which are characteristic of the mode of deposition and which are not identifiable by visible inspection or analysis of grain size or color the proposed method is therefore of great utility for the investigation and characterization of massive deposits

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

precise age determinations and petrogenetic studies using the kca method high precision mass spectrometric determination of calcium isotope ratios allows the 40k 40ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work 40ca42ca is used to monitor enrichments in 40ca and can be measured to 0 01 2 using an exponential mass discrimination correction russell et al 1978 and large ion currents the earth s mantle has such a low kca 0 01 that it has retained primordial 40ca42ca 151 016 0 011 normalized to 42ca44ca 0 31221 as determined by measurements on two meteorites pyroxene from an ultramafic nodule metabasalt and carbonatite 40ca42ca ratios can be conveniently expressed relative to this value as ca in units of 10 4 to test the method for age dating a mineral isochron has been obtained on a sample of pikes peak granite which has been shown to have concordant k ar rb sr and u pb ages plagioclase k feldspar biotite and whole rock yield an age of 1041 32 m y 2 in agreement with previous age determinations k 0 5543 b y 1 k 0 8952 40k 0 01167 the initial 40ca42ca of 151 024 0 016 ca 0 5 1 0 indicates that assimilation of high k ca crust was insufficient to affect calcium isotopes measurements on two mica granite from eastern nevada indicate that the magma sources had k ca 1 similar to intermediate composition crustal rocks these results show that the k ca system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks and may prove applicable to sedimentary rocks containing authigenic k minerals the relatively short half life of 40k the non volatile daughter and the fact that potassium and calcium are stoichiometric constituents of many minerals make the k ca system complementary to other dating methods and potentially applicable to a variety of geologic problems

sequential closure of k ca and rb sr isotopic systems in archaean micas comparative k ca and rb sr data for extremely radiogenic muscovites eca 2300 to 18 700 87sr 86sr 52 to 991 from two unmetamorphosed archaean pegmatitic rocks from the jack hills in the northwestern part of the yilgarn craton yield model ages ranging from 2580 ma to 2100 ma with the k ca dates being younger than rb sr the muscovites occur as two textural types radial clusters of bladed crystals rosettes in a massive pegmatitic granite and large euhedral books in cross cutting pegmatites there are marked differences between the dates from the two mineral forms as well as between the two isotopic systems and no individual date can be used to provide a point for p t t plots the data can be interpreted collectively in terms of delayed closure of the isotopic systems in a slowly cooled terrane the closure temperature modelling demonstrates that the diffusion coefficient of ca in muscovite is approximately an order of magnitude higher than that of sr a curved assumed exponential cooling trajectory with extremely slow cooling 0 1 c ma 1 at 2100 ma is required to reproduce the youngest k ca dates biotites ca 150 from unmetamorphosed archaean rocks from the southwest of the craton show differences between k ca and rb sr model ages rb sr data from libby and de laeter 1979 that resemble the differences in the jack hills muscovite dates for biotites which have been affected by regional reheating at or uplift until 500 ma the responses of the two isotopic systems are similar but the analysed samples are not sufficiently radiogenic ca 40 to permit identification of any closure sequence muscovites with a common cooling history but differing closure temperatures define empirical cooling curves in 87sr 87rb vs 40ca 40k plots these can be used in conjunction with the k ca rb sr concordia to provide an improved minimum age estimate for the samples when only single samples or very similar samples are available the difference between the k ca and rb sr dates gives a quality control indication of the discrepancy between the rb sr date and crystallisation age

two types of plane beds under upper flow regime in flume experiments evidence from grain fabric abstract laboratory experiments that aimed to produce plane beds under upper flow regime were conducted using a flume of 15 700 cm with quartz sands of mean size of 0 72 mm 0 47 two types of plane beds were formed 1 laminae formed and buried by the migration of low relief asymmetric ripples near or subcritical flow froude number fr 0 94 run 5 and 2 laminae formed without producing any apparent low relief bedform but under supercritical flow fr 1 98 run 12 the mean flow velocity 83 cm s and the mean depth 8 cm in run 5 are larger than those in run 12 62 cm s and 1 cm respectively upcurrent grain imbrications in run 12 however are better developed than those in run 5 such an imbrication pattern may be useful to recognize the formational processes of plane beds under upper flow regime

rocky mountain foreland uplifts products of a rotating stress field or strain partitioning stress tensor analysis of minor faults near the structural range fronts of diversely oriented laramide foreland uplifts in the central rocky mountains wyoming indicates that paleo 1 was horizontal and oriented nearly perpendicular to uplift trends trend directions for 1 vary from east northeast in the north trending laramie range to northeast in the northwest trending bighorn mountains to north at east trending casper mountain and in the east trending central owl creek mountains although these compression directions are compatible with models invoking temporally changing stress directions during the laramide orogeny the apparent similarity in timing of uplift in these ranges suggests that the computed stress tensors may not reflect regional stress conditions recent stress determinations in transpressional regimes suggest that strain partitioning may be an important factor in considering local stress patterns in the vicinity of mountain belt structures oblique to regional trends in light of these studies caution should be used in interpreting regional paleostress from structures measured close to the tectonic fronts of laramide uplifts

argon diffusion domains in k feldspar i microstructures in mh 10 samples of mh 10 a k feldspar thought to contain discrete diffusion domains for argon have been examined by light and tem microsopy to search for domain boudaries we examined both unheated k feldspar separates and samples heated in vacuum between 750 and 1150 c in order to characterize the initial microstructure as well as any changes resulting from laboratory heating the mh 10 rock shows almost no evidence of plastic deformation and the k feldspar does not possess networks of dislocations which might define a subgrain structure derived from external stresses three classes of substructure are present in the k feldspar but only the third appears to have been affected by laboratory heating 1 cross hatched extinction in light microscope is common but variably developed tem reveals almost no albite pericline twinning but tweed microstructure is ubiquitous 2 turbid zones occur with about 5 volume abundance micropores 1 to 2 m in diameter characterize these regions which often also contain bleds of albite up to 40 m indiameter tem shows these turbid zones to be very complex with intricate twin and tweed structures at the sub micron scale and numerous dislocation and strain features 3 albitic exsolution lamellae 1 m thick 20 m long and separated by 1 m are rarely seen in the light microscope but tem shows that 0 01 m thick by 0 2 1 m long lamellae separated by 0 1 to 0 5 m occur in about 20 of the sample these lamellae are disk shaped have a semi coherent or coherent relationship to the host k feldspar and seem to disappear upon heating to 750 c extensive diffusion studies have been conducted on hm 10 and the results analyzed in terms of distribution of sizes of diffusion domains the preferred result of the analyses was a distribution consisting of three distinct sizes the largest domain size identified in this way is approximately 50 to 100 m and this corresponds to blocks of k feldspar defined by the network of fractured turbid zones the smallest domain size inferred from diffusion analysis is 0 1 m which corresponds to the spacing between albite exsolution lamellae that may represent fast diffusion pathways alternatively the modulated argon distribution accompanying the lamellar variation in feldspar composition may result in an overestimation of the initial diffusion coefficients even though a promising agreement exists between domain analyses based upon diffusional and microstructural analyses at least two questions remain unanswered firstly no clear candidate has been identified for a domain of intermediate size secondly the diffusion analysis has been formulated with independent gas release from all domains whereas the observed structures are likely to be nested i e small domains exist inside bigger domains with the network of semi coherent lamellar boundaries having connectivity and permeability which can provide only a modest increase in argon diffusivity over bulk values

environmental response to a large explosive rhyolite eruption sedimentology of post 1 8 ka pumice rich taupo volcaniclastics in the hawke s bay region new zealand abstract the 1 8 ka eruption of the taupo ignimbrite from the taupo volcanic centre in the central north island of new zealand produced a 30 km3 ignimbrite covering an area ca 160 km in diameter centred on lake taupo remobilisation of its pyroclastic detritus triggered major alluviation in the years to decades after its emplacement in the hawke s bay region in the eastern north island three large rivers rise in the ignimbrite impacted mountainous headwaters in areas that were buried by up to 40 m of loose unconsolidated pumiceous material analysis of preserved volcaniclastic sediments along valleys shows that resedimentation occurred in two main stages characterised by contrasting sediment water ratios the first comprised erosion and redeposition of ignimbrite debris by lahars e g debris flows and hyperconcentrated flows the second was marked by decreasing sediment water ratios and the re establishment of braided fluvial systems in the former river valleys as recorded by such deposits as trough planar low angle and antidune cross bedded planar bedded and ripple cross laminated sands and gravels however there is no systematic chronological trend towards lower pumice contents in these fluvial facies four characteristic sets of co occurring lithofacies were produced during the sedimentary response 1 laharic remobilisation association 2 upper flow regime dominated stream flow association 3 lower flow regime dominated stream flow association and 4 lacustrine association individual lithofacies were strongly controlled by the unusual hydrodynamic properties of pumice particularly its low and variable density automated settling tube analysis of water saturated pumiceous sediment confirms that sieve analysis is misleading with respect to the hydrodynamic behaviour of such sediments recasting granulometric data in terms of effective hydraulic grain size shows better sorting of mixed component sediments but statistical sorting and mean grain size parameters do not discriminate between the deposits of tractional and overcapacity parent flows therefore particle behaviour is addressed by using both lithofacies and petrofacies in the description and classification of the remobilisation deposits

laser ablation rb sr dating by online chemical separation of rb and sr in an oxygen filled reaction cell abstract the rb sr beta decay dating system is one of the most attractive tools in geochronology as rb is sufficiently abundant in common k bearing minerals like biotite muscovite and k feldspar this allows dating of a wide variety of rocks e g volcanic magmatic metamorphic sedimentary and hydrothermal environments without the need of preconcentration as is often required for u pb dating of zircon however this advantage was to date negatively counteracted by the lack of a suitable in situ technique as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope a reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer icp ms allows exactly this the online chemical separation of two different elements coupled to a laser ablation la system in situ rb sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates sr from rb we present here a simple procedure in which rb sr ages can be obtained from a suite of individual phases in regular thin sections using the most established reaction gas oxygen o 2 it is possible to react part of the ablated sr to sro while no rbo is formed measurements of feldspars and micas with a spot size of 80 m were calibrated against glass standards nist srm 610 bcr 2g and biotite from la posta california 91 6 ma results are presented for a variety of magmatic rocks with well established thermal records a sample each from the klokken syenodiorite greenland 1161 ma the ulvo alkaligabbro sweden 1256 ma and a pegmatite from the bohus granite sweden 920 ma obtained in situ rb sr isochron ages are accurate 87 sr 86 sr ratios are accurate the methodology outlined in this study has significant implications for rb sr studies due to the high spatial resolution 1 quality of measurements can be better controlled by avoiding alterations and inclusions 2 large sample numbers can be investigated simply by using regular thin sections 3 several mineral generations can be targeted that would not be distinguishable in mineral separates and 4 isotope zonation within single crystals can be revealed this will open the rb sr dating system to new fields of study for example thermochronologic studies can use 87 sr 86 sr and 87 rb 86 sr zonation within crystals to better work out cooling and or reheating paths further applications are in the field of provenance studies and temporal evolution of shear zones

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

understanding k feldspar 40ar 39ar data reconciling models methods and microtextures several descriptions of argon diffusion in k feldspar have been developed including the quantitative multiple diffusion domain mdd and multipath models and the qualitative recrystallization model these models remain contentious and the natural range in behaviour remains poorly understood feldspars from the klokken syenite intrusion show a systematic relationship between 40 ar 39 ar age and microtexture suggesting that the loss of argon from different feldspars operates via different mechanisms such that the existing models represent only end member scenarios the mdd model can yield reliable thermal history information only for feldspars with pristine microtexture with this model best viewed as a cipher for the real but unknown domain structure in such samples

calcium isotopes in igneous rocks and the origin of granite abstract the k ca radioactive parent daughter system provides a tool for tracing the origins of igneous rocks it is complementary to other isotopic systems because as stoichiometric constituents of major minerals the concentrations of k and ca and the k ca ratio in rocks are simply related to mineralogy in this paper we report the first high precision calcium isotopic analyses of continental granitic rocks island arc rocks and mid ocean ridge basalts these data show that mid ocean ridge basalts have the low 40 ca 42 ca ratios expected for the earth s mantle but that island arc rocks have slightly higher 40 ca 42 ca ratios indicative of crustal calcium in their magma sources many granitic rocks have high initial 40 ca 42 ca ratios and in conjunction with independent evidence for the age of the crustal sources these ratios provide constraints on the k ca ratios and in turn on the silica contents and residual mineralogy of the deep crustal magma sources

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers