cation ratios of fluid inclusions in a gold quartz vein at brusson val d ayas northwestern italian alps comparison of bulk crush leach results with sims analyses of individual inclusions abstract this study tests the widely made assumption that with favourable fluid inclusion samples bulk crush leach analyses accurately reflect the electrolyte compositions of individual fluid inclusions secondary ion mass spectrometric sims analyses of individual inclusions calibrated by synthetic fluid inclusion standards are compared to crush leach analyses of the same natural quartz sample quartz from the bechaz mesothermal au vein at brusson northwestern italian alps is dominated by fluid inclusions of uniform low salinity low x co 2 composition these inclusions represent the liquid phase of an originally immiscible hydrothermal system whereas much rarer high x co 2 inclusions of even lower salinity represent the conjugate vapour phase sims analyses of the low x co 2 inclusions yield mean molar k n 0 050 and ca na 0 016 these results are identical well within experimental uncertainty to replicate crush leach analyses made in two different laboratories the excellent agreement between the sims and crush leach results confirms the accuracy of both analytical methods despite the low salinity of the bechaz fluid inclusions and the rare muscovite and dolomite impurities in the host quartz the crush leach analyses are both highly precise and accurate furthermore the agreement between methods demonstrates that sims analyses of natural multi component fluid inclusions may be reliably calibrated by synthetic inclusions with simple compositions a sims analysis of a high x co 2 vapour inclusion yields k na and ca na ratios that are not significantly different from those of the coexisting low x co 2 inclusions this suggests that under the p t conditions of immiscibility in the bechaz vein 230 c and 600 bar liquid vapour fractionation of k and ca relative to na was only very slight

the iron content of some red mediterranean soils from northeast spain and its pedogenic significance abstract alfisols cover 18 of the surface of the ne edge of the garraf massif se of barcelona catalonia most of these are mediterranean red soils some representative pedons especially over mesozoic limestones are discussed in this paper climate of the zone is littoral mediterranean mean temperature 16 c precipitation 650 mm pedoclimatic regimes are xeric and thermic vegetation is quercetum ilicis galloprovinciale horizon b is 5yr and 2 5yr in munsell hue with a phw of 7 2 to 8 6 all of them are base saturated and the majority recarbonated organic matter content is 2 5 some have an isohumic character the illuviation clay index ranges from 0 37 to 0 87 and the mean value of the illuviation fe index is 0 76 the nearly constant fed clay ratio indicates co illuviation values for fed are between 0 40 and 2 81 the mean feo is 0 057 and it ranges between 0 to 0 14 generally redness rating rr increases with fed values of fed fe1 vary from 0 22 to 0 82 and the mean is 0 54 rubefaction according to redness rating is moderate taxonomically calcic haploxeralfs predominate and are located at 50 70 m above sea level calcic palexeralfs at 70 80 m and again at 150 m and petrocalcic palexeralfs at 80 130 m are also found typic haploxeralfs are frequent over limestones at higher altitudes 260 m or overlying silicic rocks found at 35 65 m according to the 1967 french classification system they are red fersialitic and brown fersialitic soils and some are integrades with cinnamotnic soils marron encroute

recrystallization or cooling ages in situ uv laser 40ar 39ar geochronology of muscovite in mylonitic rocks the intra grain 40 ar 39 ar age distributions of muscovite from greenschist facies mylonite zones are shown to be sensors of metamorphic history textures compositions and in situ uv laser ablation 40 ar 39 ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo or recrystallization ages deformed muscovite porphyroclasts and recrystallized shear band muscovite from the pogallo shear zone ivrea zone southern alps reveal variable intra grain 40 ar 39 ar ages with internal age variations of more than 65 ma the wide range of 40 ar 39 ar ages within compositionally homogeneous grains is consistent with diffusion dominated argon loss controlled by observable intra grain microstructures in contrast intra grain 40 ar 39 ar ages of muscovite fish from proterozoic mylonite of the porsgrunn kristiansand shear zone southern norway display only minor age dispersion and age variations are correlated with changes in muscovite composition the sympathetic age composition correlations in different muscovite microstructures directly relate to sequential neo and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the porsgrunn kristiansand shear zone

basal slip and mechanical anisotropy of biotite the basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on 001 samples loaded at 45 to 001 exhibit gentle external rotations associated with dislocation glide high angle kink bands in these samples unlike those developed in micas loaded parallel to 001 are limited to sample corners samples shortened perpendicular to 001 show no evidence of nonbasal slip and fail by fracture over all conditions tested the mechanical response of biotite shortened at 45 to 001 is nearly perfectly elastic plastic stress strain curves are characterized by a steep elastic slope a sharply defined yield point and continued deformation at low mostly 1 stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 mpa and exhibit weak dependencies upon strain rate and temperature assuming an exponential relationship between differential stress d and strain rate e of the form e cexp d exp q rt the data collected over strain rates and temperatures of 10 7 to 10 4 s 1 and 20 to 400 c respectively are best fit by an exponential constant of 0 41 0 08 mpa 1 and an activation energy q of 82 13 kj mol a power law fits the data equally well with n 18 4 and q 51 9 kj mol samples oriented favorably for slip in directions 100 and 110 are measurably weaker than those shortened at 45 to 010 and 310 consistent with the reported burgers vectors 100 1 2 110 and 1 2 110 the anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to 001 previous studies have shown that biotite loaded in the 001 plane is strong prior to the nucleation of kink bands the strength of biotite shortened perpendicular to 001 exceeds that measured parallel to 001 and is pressure dependent application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions its presence within foliated rocks and shear zones may limit locally the stresses that can be supported

dynamic pedogenesis new views on some key soil concepts and a model for interpreting quaternary soils abstract inasmuch as soils are open systems and rarely if ever achieve equilibrium with their environments it is philosophically sound to view all soils as expressing varying levels of polygenesis as that term has been redefined soil genesis and resultant morphology may then be viewed in a comprehensive framework of soil evolution that consists of two linked pathways one developmental and the other regressive that reflect interactions of both exogenous and endogenous vectors vectors are factors processes and conditions of pedogenesis following this philosophy a model of pedogenesis is framed in an evolutional paradigm that emphasizes the integrated effects of dynamic and passive pedogenic vectors in directing pathways and in controlling rates of soil genesis through time the dynamic vectors include energy and mass fluxes frequencies of soil wetting and drying water table dynamics organisms feedback processes and pedoturbation the passive vectors include parent materials soil chemical environment stability of geomorphic surfaces and various evolved soil properties and conditions accessions both sets of vectors vary spatially and the dynamic vectors more so than passive vectors fluctuate through time the model is expressed as s f d p dd dt dd dt where s is the state of the soil or degree of profile evolution d is the set of dynamic vectors p is the set of passive vectors and dd dt and dp dt denote change through time t the model helps explain the apparent minimal development and regressed character of some old soils and the rapid and strong development of some young ones

a soil chronosequence in the semi arid environment of patagonia argentina abstract soil development with time was investigated on beach ridges with ages ranging from about 1380 to 6240 14c years bp at the eastern coast of central patagonia the main pedogenic processes are accumulation of organic matter and carbonate leaching and accumulation within the upper part of the soils soil formation is strongly influenced by incorporation of eolian sediments into the interstitial spaces between the gravel of which the beach ridges are composed different amounts of eolian material in the soils lead to differentiation into leptosols containing 10 fine earth in the upper 75 cm and regosols containing 10 fine earth soil depth functions and chronofunctions of organic carbon calcium carbonate ph ca zr mg zr k zr na zr fe zr mn zr and si al obtained from x ray fluorescence analysis were evaluated to establish soil chronofunctions mean values of the horizon data of 0 10 cm below the desert pavement were used which were weighted according to the horizon thicknesses the depth function of ph shows a decrease towards the surface indicating leaching of bases from the upper centimeters chronofunctions of ph show that within 6000 radiocarbon years of soil development ph drops from 7 0 to 6 6 in the leptosols and from 8 1 to 7 5 in the regosols the higher ph of the regosols is due to input of additional bases from the eolian sediments chronofunctions of ca zr and k zr indicate progressive leaching of ca and k in the regosols showing close relationships to time r2 0 972 and 0 995 na leaching as indicated by decreasing na zr ratios shows a strong correlation to time only in the leptosols r2 0 999 both leptosols and regosols show close relationships to time for fe zr r2 0 817 and 0 824 mn zr r2 0 940 and 0 803 and si al 0 971 and 0 977 indicating enrichment of fe and mn and leaching of si leaching of mobile elements takes place on a higher level in the regosols than in the leptosols from the beginning of soil formation hence a significant part of the eolian sediments must have been incorporated into the beach ridges very soon after their formation

uranium series age of the eel point terrace san clemente island california uranium series analysis of the coral allopora californica verrill from the 2nd 32 m eel point terrace on san clemente island california has yielded an age of 127 000 7 000 yr the eel point terrace is thus correlative with numerous terrace localities on the southern california mainland with coral reefs on barbados and new guinea dated about 120 000 yr and with substage 5e of the marine oxygen isotope record a tectonic uplift rate of about 0 20 m 1 000 yr has been calculated assuming a sea level slightly higher than the present one at the time of terrace formation extrapolation of this uplift rate allows age estimates to be made for other terraces on the island

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

40ar loss in experimentally deformed muscovite and biotite with implications for 40ar 39ar geochronology of naturally deformed rocks abstract the effects of deformation on radiogenic argon 40ar retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite cylindrical cores 15 mm in length and 6 25 mm in diameter were drilled from granite collected from the south armorican massif in northwestern france loaded into gold capsules and weld sealed in the presence of excess water the samples were deformed at a pressure of 10 kb and a temperature of 600 c over a period 29 of hours within a solid medium assembly in a griggs type triaxial hydraulic deformation apparatus overall shortening in the experiments was approximately 10 transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking cracking and grain segmentation of the micas infrared ir laser co2 heating of individual 1 5 2 5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well defined 40ar 39ar plateau ages of 311 2 ma 2 identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40ar loss of 0 35 in muscovite and 2 3 40ar loss in biotite intragrain in situ ultraviolet uv laser ablation 40ar 39ar ages 4 10 1 of deformed muscovites range from 309 13 to 264 7 ma consistent with 0 16 40ar loss relative to the undeformed muscovite the in situ uv laser ablation 40ar 39ar ages of deformed biotite vary from 301 to 217 ma consistent with up to 32 40ar loss no spatial correlation is observed between in situ 40ar 39ar age and position within individual grains using available argon diffusion data for muscovite the observed 40ar loss in the experimentally treated muscovite can be utilized to predict average 40ar diffusion dimensions maximum 40ar 39ar ages obtained by uv laser ablation overlap those of the undeformed muscovite indicating argon loss of

kinks in mica role of dislocations and 001 cleavage abstract kinking in natural and experimentally deformed biotite and muscovite has been investigated by transmission electron microscopy tem it is found that complex microstructures exist in the vicinity of kink band boundaries kbbs gentle bending of 001 planes plays an important role in the kinking process where 001 planes are bent several degrees edge dislocations are observed in complex walls between active slip planes together with broad bands of dislocations parallel to 001 microfracturing and dilation occur parallel to 001 planes and the kbb a model is presented to explain the origin of observed kbbs which may be applicable to other crystalline materials with only one active slip plane

on the loss of 40ar from muscovite during polymetamorphism abstract recent laser studies have documented 40ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years a fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or alternatively loss during chemical reactions and deformation new data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40ar via lattice diffusion during overprinting events than has been inferred previously kyanite muscovite assemblages in proterozoic schist along the eastern flank of the green mountain massif central vermont appalachians formed during proterozoic metamorphism at ca 1 1 ga and were remetamorphosed in the greenschist facies during both taconian ca 465 ma and acadian ca 390 ma events the overprinting events resulted in deformation of earlier kyanite and muscovite growth of chemically and isotopically distinct rims on earlier muscovite and growth of neoblastic muscovite and chloritoid with each event at inferred minimum temperatures of ca 425 c laser spot analyses of the grenville muscovite range from 988 11 ma in the centers of porphyroclasts to 397 7 ma along rims ages younger than 750 ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals parts of early formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40ar laser spot analyses along the outer 100 m of an undeformed growth face yield ages close to the timing of proterozoic metamorphism and cooling the present study suggests episodic heating events are ineffective for producing measurable diffusive argon loss along the edges of large muscovite crystals ca 1 mm in diameter particularly in comparison with deformation and reaction mechanisms and are unlikely to result in complete 40 ar 39 ar age resetting of such crystals the results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work

planar defects as ar traps in trioctahedral micas a mechanism for increased ar retentivity in phlogopite abstract the effects of planar defects and composition on ar mobility in trioctahedral micas have been investigated in samples from a small marble outcrop 500 m2 in the frontenac terrane grenville province ontario these micas crystallized during amphibolite facies metamorphism at 1170 ma and experienced a thermal pulse 100 ma later at shallow crustal levels associated with the emplacement of plutons 87rb 86sr ages of the phlogopites range from 950 to 1050 ma consistent with resetting during the later thermal event the same phlogopites however give 40ar 39ar ages between 950 and 1160 ma spanning the age range of the two thermal events this result is intriguing because these micas have undergone the same thermal history and were not deformed after peak metamorphic conditions in order to understand this phenomenon the chemical crystallographical and microstructural nature of four mica samples has been characterized in detail using a wide range of analytical techniques the scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectrometry la icp ms data show that the micas are chemically homogeneous with the exception of ba and similar in composition the fourier transform infrared spectroscopy and mossbauer results show that the m sites for three of the micas are dominated by divalent cations and the fe3 fe2 fe3 ratio for all four phlogopites ranges from 0 10 to 0 25 the stable isotopic data for calcite indicate that this outcrop was not affected by hydrothermal fluids after peak metamorphism no correlation between chemical composition and 87rb 86sr and 40ar 39ar age or between crystal size and 40ar 39ar age is observed the only major difference among all of the micas was revealed through transmitted electron microscope tem which shows that the older 1m micas contain significantly more layer stacking defects associated with crystallization than the younger micas we propose that these defect structures which are enclosed entirely within the mineral grain may serve as ar traps and effectively increase the ar retentivity of the mineral as this phenomenon has not been previously documented in micas this may have significant implications for the interpretation of 40ar 39ar ages of minerals which have similar defect structures

geochemical evidence of saharan dust parent material for soils developed on quaternary limestones of caribbean and western atlantic islands abstract most previous workers have regarded the insoluble residues of high purity quaternary limestones coral reefs and oolites as the most important parent material for well developed clay rich soils on caribbean and western atlantic islands but this genetic mechanism requires unreasonable amounts of limestone solution in quaternary time other possible parent materials from external sources are volcanic ash from the lesser antilles island arc and saharan dust carried across the atlantic ocean on the northeast trade winds soils on quaternary coral terraces and carbonate eolianites on barbados jamaica the florida keys united states and new providence island bahamas were studied to determine which if either external source was important caribbean volcanic ashes and saharan dust can be clearly distinguished using ratios of relatively immobile elements al 2 o 3 tio 2 ti y ti zr and ti th comparison of these ratios in 25 soils where estimated ages range from 125 000 to about 870 000 yr shows that saharan dust is the most important parent material for soils on all islands these results indicate that the northeast trade winds have been an important component of the regional climatology for much of the quaterary saharan dust may also be an important parent material for caribbean island bauxites of much greater age

in situ and step heating 40ar 39ar dating of white mica in low temperature shear zones tenda massif alpine corsica france in order to clarify the link between 40ar 39ar record in white mica and deformation we performed in situ and bulkwise 40ar 39ar dating over the east tenda shear zone tenda massif alpine corsica white micas from 11 samples were selected and extensively analyzed using in situ techniques across nested scales of strain intensity gradients developed at the expense of a late variscan protolith 40ar 39ar systematics are unaffected by inherited ar and directly linked to deformation with little or no ar lattice volume diffusion extensive sampling allows constraining the end of deformation related to burial and exhumation respectively at 34 and 22 ma bracketing the duration of regional extensional shear to 12 myr results also highlight a regional strain localization toward the upper contact of the unit with smaller scale localization in specific lithologies notably meta aplites second order complications exist such as local ill defined correlations between ages and finite strain microstructures thus the use of a strain gradient as a proxy for strain localization in time is regionally valid but sometimes locally too complex to track or resolve strain partitioning localization trends at the meter outcrop scale and below age mixing and incomplete isotopic homogenization by dissolution precipitation are identified as the main causes of local discrepancies that complicate the link between age and microstructure and the derivation of strain localization rates tracking temporal trends in shear distribution across regional scale deformation gradients in such settings is possible but requires a multi scale approach as implemented here to reveal younging patterns associated to strain localization

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

the effect of dislocations on bulk diffusion in feldspars during metamorphism there is no significant difference in the diffusion profiles across albite adularia bicrystals that were simultaneously deformed at a strain rate of 10 6s 1 and those from hydrostatic experiments at the same conditions 1500 mpa and 1000 c for 156 h this indicates that the bulk alkali diffusion rate which is the sum of lattice diffusion d 1 and dislocation pipe diffusion dp is not significantly enhanced by dislocations at these conditions and that the maximum value for the ratio of dp d1 is about 105 this is equal to the value previously reported for oxygen diffusion in albite if this ratio is independent of temperature the contribution of either static pre deformed or moving syn deformed dislocations to the bulk diffusion rate of alkalis is probably minor at all metamorphic conditions for al and si diffusion the ratio of dp d1 may be larger if d1 is lower thus a significant contribution from dislocations to bulk diffusion cannot be ruled out especially during simultaneous deformation

rb sr isotopic and compositional retentivity of muscovite during deformation abstract permian metapegmatite muscovite from the upper austroalpine matsch unit in southern tyrol italy was investigated regarding its rb sr and compositional retentivity during cretaceous upper greenschist facies deformation the data imply that microstructurally relic permian magmatic muscovite largely maintained its major and trace element compositions during deformation whereas the rb sr geochronometer is strongly affected by a net loss of sr lower sr concentrations of muscovite correlate with higher 87rb 86sr and 87sr 86sr ratios in most samples the muscovite grain size and magnetic fractions with the lowest 87rb 86sr and 87sr 86sr ratios preserve a permo triassic muscovite whole rock rb sr apparent age interpreted as to reflect formation during or cooling after pegmatite emplacement contrastingly muscovite fractions with higher 87rb 86sr and 87sr 86sr ratios are arranged along a roughly linear array with a positive correlation of the 87rb 86sr and 87sr 86sr ratios in the 87rb 86sr vs 87sr 86sr space they yield successively lower muscovite whole rock rb sr apparent ages we explain the variations in the rb sr isotopic character of microstructurally relic muscovite by a presumably deformation related loss of sr during the cretaceous event contemporaneously only very limited amounts of isotopically different sr from the matrix reservoir might possibly have entered the muscovite consequently the rb sr of the relic muscovite is affected by a net loss of sr the results imply that at temperatures of

a comparison of mineralogical techniques and potassium adsorption isotherm analysis for relative dating and correlation of late quaternary soil chronosequences abstract the utility of potassium adsorption isotherm analysis kia for relative dating and correlation of soil chronosequences was assessed by a comparison with some widely used mineralogical techniques the soils studied near detroit michigan are brown 10yr calcareous entisols developed in holocene 4 ka floodplain sediments and inceptisols and alfisols with redder 7 5yr 5yr cambic and argillic b horizons formed on late pleistocene 14 5 12 4 ka fluvial terraces fluvial paleochannels and paleoshorelines beach ridges of proglacial lakes the soils show a positive linear correlation between soil age and clay content depth of carbonate leaching and fe oxide accumulation in contrast the duration of soil formation is too short for variations in heavy mineral frequencies to discriminate soil age reliably scanning electron microscopy shows that chemical weathering stage is indicated by progressive etching and pitting of calcite apatite and amphibole with increasing soil age k isotherms are linear or slightly curvilinear indicating that ion exchange is dominant over specific adsorption a statistically significant linear correlation between k distribution constant and soil age attributed to progressive weathering of mica suggests that kia is useful for relative dating of genetically related soils once a well dated chronosequence is established kia produces results comparable to or better than mineralogical techniques however soil morphology and a clay redness index are more effective than mineralogical and chemical techniques for interchronosequence correlation

geological characteristics and tectonic settings of paleozoic epithermal gold deposits in northern xinjiang the basic characteristics of the epithermal gold deposits in northern xinjiang are described briefly in this paper the epithermal gold deposits in northern xinjiang can be divided into high sulfidation and low sulfidation types with the latter being the main type the distribution and formation of epithermal gold deposits in northern xinjiang are structurally controlled by calderas craters diatremes and related faults volcanic rocks related to the gold deposits are mainly a suite of calc alkaline continental volcanic rocks which are composed mainly of basalt andesite dacite rhyolite and relevent pyroclastic rocks together with minor marine and continental volcanic rocks gold orebodies are mainly hosted in volcanic rocks and subvolcanic intrusions with a few occurring in the porphyry the alteration assemblages are mainly adularia chalcedony sericite in low sulfidation gold deposits and alunite kaolinite quartz chalcedony in high sulfidation gold deposits the ore forming ages of the epithermal gold deposits in northern xinjiang are in the range of 340 244 ma corresponding to the large scale volcanic subvolcanic activities at 350 10 ma 300 10 ma and 250 10 ma it is held that all the epithermal gold deposits in northern xinjiang are products of magmatic heat events in the extensional tectonic setting or the local extensional environment under the large scale compressional background

laser 40ar 39ar evaluation of slow cooling and episodic loss of 40ar from a sample of polymetamorphic muscovite volume diffusion models predict that crystals with large diffusion dimensions can record a wide range of thermal conditions in the earth s crust direct measurements of the zoning of radiogenic argon 40 in single muscovite porphyroblasts from a complex terrain in the vermont appalachians record multiple crustal events that span 150 million years the crystal radius was the effective dimension for argon diffusion approximately 1000 micrometers late deformation features inside the crystals locally decreased the diffusion dimension and promoted loss of argon 40 zoning patterns of radiogenic isotopes as observed in this study are an increasingly important diagnostic tool for studying the thermal record of tectonic processes

diversity of diagnostic horizons in soils of the contiguous usa a case study abstract many previous studies of pedodiversity have emphasized the lateral variability of soils in this study the vertical distribution of diagnostic horizons is examined at the soil series level unlike genetic horizons diagnostic soil horizons are based on quantitatively defined morphological and analytical properties and they form the basis of modern soil classification systems an examination of 1600 soil series in 63 representative out of 226 major land resource areas mlras in contiguous usa revealed that they contain from one to seven diagnostic soil horizons with an average of 2 6 1 2 about 20 of the soil series contain 4 or more diagnostic horizons 80 of these soil series are in seven great groups the glossudalfs haplorthods fragiorthods endoaquods epiaquods fraglossudalfs and glossaqualfs the laurentide great lakes region contains a discordant number 57 of soils with 4 or more diagnostic horizons the most common being ochric 100 albic 91 argillic 85 glossic 69 spodic 51 and a fragipan 19 several factors appear to contribute to the diversity of diagnostic horizons including proximity to the great lakes where abundant snowfall precludes soil freezing and enables water to percolate rapidly through the soil in a short interval during the spring the prevalence of stratified glacial deposits often containing a lithologic discontinuity and a fluctuating water table all of which enable specific weathering products to accumulate at different depths

deformation of biotite and muscovite tem microstructure and deformation model abstract growth and deformation defects have been observed in 1m and 2m 1 biotites and 2m 1 muscovites using transmission electron microscope tem techniques these have been related to optical observations in deformed biotite the dislocation density increases as the strain increases the density of stacking faults also increases with strain the spatial ordering of stacking faults decreases as the strain increases in muscovites selected from the same areas as the biotites stacking fault and dislocation densities show little or no variation with deformation the different responses of biotite and muscovite to deformation are explained in part by the physical and chemical differences between 1m biotite and 2m 1 muscovite there is strong evidence in both the biotites and muscovites that stacking faults may be result of deformation as well as a crystal growth process the grain size of the deformed mica aggregates is reduced by the nucleation of new micas there is also evidence for a grain size reduction mechanism segmentation in biotites this mechanism appears to achieve the same result as recovery recrystallization mechanisms but not by the same method segmentation may be a mechanism which is favoured in strongly anisotropic minerals with a limited number of slip systems a model is proposed that attempts to correlate response of the biotite to deformation the deformation is synchronous with and post dates the nucleation of the new metamorphic micas

the effect of sub seismic fault slip processes on the isotopic signature of clay minerals implications for k ar dating of fault zones abstract clay minerals are abundant in fault gouges and isotope dating of the radiogenic noble gases that they contain may be used to constrain timing of fault initiation reactivation and to estimate recurrence intervals of earthquakes the potential influence of fault slip and ambient temperature on the isotope signature of authigenic clay minerals however remains unknown this study reports measurements of the k ar isotope signature of samples that have been sheared experimentally at sub seismic slip rates within a range of ambient temperatures as well as of samples undergoing standardised clay heating degas experiments in the investigation and comparison of the influence of sub seismic frictional slip and external heating on clay gouge the age data confirm that the sub seismic frictional slip of clay gouge affects the clay microstructures and releases radiogenic 40ar the experimental data indicate that radiogenic 40ar release due to external heating can be disregarded until 350 c which is supported by ar diffusion modelling frictional slip at sub seismic slip rates and at room temperature results in a significant radiogenic 40ar loss of 56 and the remaining clay will inherit 44 of radiogenic 40ar sub seismic frictional slip experiments at imposed elevated temperatures from 150 to 450 c increase the amount of radiogenic 40ar loss and decrease the amount of inherited radiogenic 40ar at a relatively high temperature of 450 c about 16 of radiogenic 40ar will be present in the remaining sample portion reinforcing the importance of rigorous clay grain size separation processes required prior to fault gouge dating

a groundwater hypothesis for the origin of fire areas on the northern channel islands california abstract pleistocene areas of red sediments and carbonized vegetation on the northern channel islands california have in the past been interpreted as caused by fires of either natural or human origin some are associated with darkened mammoth and bird fossils and these fossils have been considered as having been burned by early man reevaluation of these so called fire areas indicates that the above phenomena are the result of low temperature 100 c nonheating processes occurring in groundwater evidence for this conclusion is derived from field observations on fossil carbonized vegetation and the geology of the areas additional evidence derives from experiments on the red sediments and fossil wood x ray diffraction analyses magnetic analyses studies on the clay minerals smectite and illite and the demonstration that fossil mammoth bone contains sufficient fe and mn to account for their discoloration much of the carbonization of vegetation probably occurred in water rather than in fire radiocarbon dates from the islands will probably need to be reevaluated these data provide evidence contrary to the concept of the occurrence of significant fires either natural or set by early man on the northern channel islands

a soil chronosequence on quaternary marine terraces san clemente island california a soil chronosequence on marine terraces was examined on san clemente island california in an arid mediterranean climate in order to see which soil properties differ most systematically with time ages of the geomorphic surfaces were determined by radiocarbon uranium series and amino acid racemization techniques and range from 1 000 000 yrs soils on surfaces 200 000 yrs old are vertisols or alfisols with vertic properties such as slickensides and cracks and they exhibit both prismatic and angular blocky sphenoidal or only angular blocky structure solum thickness increases linearly with age of geomorphic surface clay soluble salts ald smectite mica ratios in the clay fraction and quartz plagioclase ratios in the silt fraction when expressed as profile summations show increases with age of geomorphic surface the best fit regression equations are linear with the exception of ald which is best expressed as a power function important soilforming processes appear to be alteration of mica to smectite accumulation at depth of soluble salts derived from marine aerosols possibly additions of eolian materials clay illuviation in the younger soils and pedoturbation in the older soils the observations reported here allow the following conclusions to be drawn about soils in this environment 1 certain soil properties which show distinct trends with time can be used for relative age dating in geomorphic and stratigraphic studies 2 soils have not reached a steady state condition with respect to many of their properties and 3 alfisols and mollisols with natric b horizons may be precursors to vertisols

the effects of deformation induced microstructures on intragrain 40ar 39ar ages in potassium feldspar a combination of forescattered scanning electron microscopy and both ultraviolet uv laser and cycle stepped heating 40 ar 39 ar dating techniques have been applied to a single deformed grain of potassium feldspar to investigate the relationship between microstructure and 40 ar 39 ar isotope systematics the form and distribution of deformation induced microstructures have been mapped by using intragrain variations in lattice orientation these variations are related to thin section and regional structural data to provide a well constrained deformation history for the feldspar grain thirty uv laser 40 ar 39 ar analyses show age variations that correlate with the internal microstructure stepped heating data provide further evidence of this relationship our approach allows the recognition of different ar reservoirs within the deformed feldspar and identifies microstructural controls on ar migration such a relationship has important implications for the understanding and interpretation of 40 ar 39 ar geochronological data and the subsequent use of such data to constrain the temperature vs time evolution of deformed rocks

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

meteoric 10be dating of highly weathered soils from fluvial terraces in taiwan abstract stream terraces are geomorphic markers that can be used to gauge surface deformation in response to tectonic uplift the surface deposits of the terraces form a chronosequence that indicates the rate and process of soil development placing a time or age constraint upon soil development has become one of the most important factors in the disciplines of soil science and fluvial geomorphology the radiocarbon 14c method is unable to date old terraces or soils but the longer half life of cosmogenic beryllium 10 10be permits this isotope to be useful to determine soil ages and rates of formation red soils redder than 7 5yr are commonly distributed on old high altitude stream terraces throughout taiwan in this study three pedons of alluvium related red soils were sampled from the taoyang ty ym pakua pk 1 and chiayi terraces cy 1 respectively located in northern central and southern taiwan according to field morphology and chemical analyses these red soils are characterized in soil taxonomy as being highly weathered and they are classified as typic hapludox ty ym and pk 1 and typic paleudult cy 1 meteoric 10be dating shows ages of 261 ka for ty ym 124 ka for pk 1 and 386 ka for cy 1 however the trend of these ages does not agree with the degree of soil development the former two ages are likely underestimated through loss of 10be due to strong leaching and considerable erosion the age obtained from cy 1 may represent the required time for soils develop into oxisols in taiwan

variation in soil surface area in a chronosequence of soils from glen feshie scotland and its implications for mineral weathering rate calculations abstract the inorganic component of soil horizons from a chronosequence of six soils developed in glen feshie scotland shows an increase in surface area with soil age and a decrease in surface area with increasing depth in the soil profiles surface area increases due to the development of discontinuous porous coatings on mineral grains the preferential dissolution of specific areas of individual minerals resulting in complex highly irregular grain shapes and a general reduction in grain size due to dissolution the discontinuous coatings consist predominantly of oxalate extractable al si oxyhydroxides and account for between 61 and 94 of the total inorganic surface area failure to take into account the increase of mineral surface area with time can lead to predictions of decreasing mineral dissolution rate with soil age in contrast to laboratory studies which assume that mineral dissolution can reach a steady state the difference between total and reactive mineral surface area may help to explain the frequently reported result that laboratory measured mineral dissolution rates are up to several orders of magnitude greater than those measured in the field

alpine deformation and 40ar 39ar geochronology of synkinematic white mica in the siviez mischabel nappe western pennine alps switzerland we explore the timing of deformation and exhumation of the siviez mischabel nappe western swiss alps which has been considered a classic example of a midcrustal crystalline nappe since the studies of argand 1916 this study presents 40ar 39ar ages obtained on both synkinematic white mica from permo triassic cover sediments and more complex white mica populations from basement gneisses of the siviez mischabel and middle pennine nappes primary foliation developed in cover units by nucleation growth and rigid rotation of mica grains during noncoaxial alpine deformation although some samples show a crenulation of this primary foliation mica growth appears to have occurred only during the development of primary foliation the main phase of greenschist facies deformation related to imbrication of the siviez mischabel nappe and other middle pennine nappes good agreement exists between independent estimates of the timing of deformation and reported 40ar 39ar white mica ages from cover units of the central and southern siviez mischabel nappe in cover units from the central and southern siviez mischabel regions of the study area 40ar 39ar ages appear to date synkinematic white mica growth results suggest that the siviez mischabel nappe was emplaced and developed foliation during a 5 m y period from 41 to 36 ma in cover units from the eastern siviez mischabel however 40ar 39ar white mica ages appear to date postkinematic thermal events these thermal events may be related to oligocene magmatic activity in the lower pennine nappes or to miocene development of the simplon fault zone variations in the relation between alpine age and grain size for cover samples from the central eastern and southern siviez mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism when considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western swiss alps these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high pressure units in the suture zone of the western alps are intimately related and synchronous on the scale of a few million years

using argon as a temporal tracer of large scale geologic processes abstract argon ar has proven to be one of the most useful elements in understanding the timing and duration of geological processes because of its chemical and physical properties ar is a particularly sensitive indicator of temperature variations associated with large scale geological events ar mobility in minerals is commonly described using solid state diffusion theory volume diffusion which has been used extensively to model diffusion profiles and closure temperatures in minerals because 40 ar is the radiogenic daughter product of radioactive 40 k and therefore occurs in many common rock forming minerals ar has been used effectively as a temporal tracer to understand a wide variety of geological phenomena ar isotopes have been applied with great success in three broad fields of earth science the origin and evolution of the earth s atmosphere the tectonothermal history of geologic terranes and the timing and duration of large scale tectonic events from ar isotopic data the earth s atmosphere is now known to be mainly secondary in nature although evolutionary models have undergone a recent paradigm shift due to geophysical evidence moving away from the earlier notion of a chemically layered upper and lower mantle to a whole earth mantle structure that may be chemically inhomogeneous although recent models are growing in sophistication by integrating additional information and measurements reflecting the behaviour of other noble gases through time the temporal evolution of the earth s atmosphere still remains ambiguous because measured compositions of ancient atmosphere are sparse and remain difficult to obtain 40 ar 39 ar thermochronology is one of the most commonly used tools in tectonothermal investigations because of the large range of ar closure temperatures recorded by various common k bearing minerals although more recent thermochronological models such as multidiffusion domain mdd modelling remain controversial the combination of 40 ar 39 ar thermochronology with both higher and lower temperature thermochronological methods utilizing other isotopic systems makes this an extremely versatile method of understanding the temperature time evolution of geologic regions recent developments involving the numerical modelling of ar diffusion coupled with the geodynamic modelling of orogens have led to new ways of estimating the timing and duration of a complete tectonic cycle of subduction and exhumation by applying these new methods to rocks from the bergen arcs in norway studies have shown that this cycle can be so rapid that the subducted and exhumed crust can remain relatively cool never achieving thermal equilibrium with its ambient environment throughout the entire process in summary the versatility of the 40 ar 39 ar dating method combined with the effective application of various techniques e g laser spot dating and step heating and a detailed understanding of the diffusional behaviour of ar has proven to be a powerful research tool in elucidating the timing and duration of many large scale earth system processes

the legacy of crystal plastic deformation in olivine high diffusivity pathways during serpentinization crystal plastic olivine deformation to produce subgrain boundaries composed of edge dislocations is an inevitable consequence of asthenospheric mantle flow although crystal plastic deformation and serpentinization are spatio temporally decoupled we identified compositional readjustments expressed on the micrometric level as a striped fe enriched bar x text fe 0 24 0 02 zones 0 12 0 02 bulk or fe depleted bar x text fe 0 10 0 01 zones 0 13 0 01 bulk zoning in partly serpentinized olivine grains from two upper mantle sections in norway focused ion beam sample preparation combined with transmission electron microscopy tem and aberration corrected scanning tem enabling atomic level resolved electron energy loss spectroscopic line profiling reveals that every zone is immediately associated with a subgrain boundary we infer that the zonings are a result of the environmental fe2 mg 1 exchange potential during antigorite serpentinization of olivine and the drive toward element exchange equilibrium this is facilitated by enhanced solid state diffusion along subgrain boundaries in a system which otherwise re equilibrates via dissolution reprecipitation fe enrichment or depletion is controlled by the silica activity imposed on the system by the local olivine orthopyroxene mass ratio temperature and the effect of magnetite stability the fe mg exchange coefficients k text d text atg text ol between both types of zoning and antigorite display coalescence toward exchange equilibrium with both types of zoning mn is enriched and ni depleted compared with the unaffected bulk composition nanometer sized heterogeneously distributed antigorite precipitates along olivine subgrain boundaries suggest that water was able to ingress along them crystallographic orientation relationships gained via electron backscatter diffraction between olivine grain domains and different serpentine vein generations support the hypothesis that serpentinization was initiated along olivine subgrain boundaries

influence of climate and eolian dust on the major element chemistry and clay mineralogy of soils in the northern bighorn basin u s a abstract soil chronosequences in the northern bighorn basin permit the study of chronologic changes in the major element chemistry and clay mineralogy of soils formed in different climates two chronosequences along rock creek in south central montana formed on granitic alluvium in humid and semiarid climates over the past two million years a chronosequence at the kane fans in north central wyoming formed on calcareous alluvium in an arid climate over the past 600 000 years detailed analyses of elemental chemistry indicate that the soils in all three areas gradually incorporated eolian dust that contained less zirconium considered to be chemically immobile during weathering than did the alluvium b and c horizons of soils in the wettest of the chronosequences developed mainly at logarithmic rates suggesting that leaching initially rapid but decelerating dominated the dust additions in contrast soils in the most arid of the chronosequences developed at linear rates that reflect progressive dust additions that were little affected by leaching both weathering and erosion may cause changes with time to appear logarithmic in a horizons of soils under the moist and semiarid climatic regimes clay minerals form with time in the basal b and c horizons and reflect climatic differences in the three areas vermiculite mixed layer illite smectite and smectite form in the soils of the moist climate chronosequence smectite forms in the semiarid climate chronosequence and smectite and palygorskite form in the arid climate chronosequence

geomorphic and pedogenic evolution in coastal sediments central california abstract studies of soil chronosequences on marine terraces facilitate the use of terraces for tectonic and paleoclimatic interpretation however many areas on the california coast have received substantial eolian deposits after pedogenesis began so do not qualify as chronosequences these areas are worthy of study because they are widespread coastal landscape features and they enable us to interpret pedogenic and geomorphic processes in sandy regolith the objective of this study was to use soil and deep regolith morphology and chemistry to interpret pedogenic and geomorphic evolution on a sequence of four marine terraces san luis obispo county california the terraces are numbered 1 2 3 and 4 from oldest to youngest the eroded terrace platforms have been tentatively dated at 560 420 or 480 320 and 120 ka respectively the sand deposits on each platform may be considerably younger the amount of land surface dissection and the depth of stream incision increase with increasing terrace age and elevation morphological features suggest that the soils xeropsamments on terrace 4 haploxerolls on terrace 3 on the two youngest terraces are well drained and their morphological development is typical of soils in eolian sand deposits morphological and chemical features of the basal regolith contrast with those of the soils clay fe oxides and opaline silica were deposited by groundwater flow above the bedrock platform above the shoreline angle where the deep regolith receives additional groundwater from higher terraces redoximorphic features have developed gray mottles are larger more common and more distinct in the basal regolith of progressively older terraces erosion has removed much of the overburden on terraces 2 and 1 and soils epiaqualfs on terrace 2 epiaqualfs on terrace 1 have developed in what was once deep regolith the landscape soils and deep regolith show an evolution of processes whereby the path and direction of water movement through the regolith is controlled first by terrace morphology and stratigraphy then by the development of pedogenic features in the soil and deep regolith and by terrace dissection

heterogeneous 40ar distributions in naturally deformed muscovite in situ uv laser ablation evidence for microstructurally controlled intragrain diffusion abstract micas are commonly used in 40 ar 39 ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained this study examines the relationship between deformation and deformation induced microstructures on radiogenic argon retention in muscovite a combination of furnace step heating and high spatial resolution in situ uv laser ablation 40 ar 39 ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the siviez mischabel nappe western swiss alps penninic domain brianconnais unit the pegmatite forms part of the variscan 350 ma alpine basement and exhibits a prominent alpine s c fabric including numerous mica fish that developed under greenschist facies metamorphic conditions during the dominant tertiary alpine tectonic phase of nappe emplacement furnace step heating of milligram quantities of separated muscovite grains yields an 40 ar 39 ar age spectrum with two distinct staircase segments but without any statistical plateau consistent with a previous study from the same area a single 3 5 mm muscovite porphyroclast fish was investigated by in situ uv laser ablation a histogram plot of 170 individual 40 ar 39 ar uv laser ablation ages exhibit a range from 115 to 387 ma with modes at approximately 340 and 260 ma a variogram statistical treatment of the 40 ar 39 ar results reveals ages correlated with two directions a highly correlated direction at 310 and a lesser correlation at 0 relative to the sense of shearing using the highly correlated direction a statistically generated kriging method age contour map of the 40 ar 39 ar data reveals a series of elongated contours subparallel to the c surfaces which where formed during tertiary nappe emplacement similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish the observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during alpine 35 ma greenschist facies metamorphism one dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts thus providing a network of fast diffusion pathways during alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon the structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion which is consistent with the heterogeneous nature of deformation

soil formation rates on silicate parent material in alpine environments different approaches different results abstract high mountain soils develop in particularly sensitive environments consequently deciphering and predicting what drives the rates of soil formation in such environments are a major challenge in terms of soil production or formation from chemical weathering the predominating perception for high mountain soils and cold environments is often that the chemical weathering portion of soil development is temperature inhibited often to the point of non occurrence several concepts exist to determine long term rates of soil formation and development we present three different approaches 1 quantification of soil formation from minimally eroded soils of known age using chronosequences known surface age and soil thickness sast 2 determination of soil residence times srt and production rates through chemical weathering using un stable isotopes e g 230 th 234 u activity ratios and 3 a steady state approach using cosmogenic isotopes e g 10 be for each method data from different climate zones and particularly from high mountains alpine environment are compared the sast and steady state approach give quite similar results for alpine environments european alps and the wind river range rocky mountains usa using the srt approach soil formation rates in mountain areas but having a temperate climate do not differ greatly from the sast and steady state approaches independent of the chosen approach the results seem moderately comparable soil formation rates in high mountain areas alpine climate range from very low to extremely high values and show a clear decreasing tendency with time very young soils have up to 3 4 orders of magnitude higher rates of development than old soils 10 5 to 10 6 yr this apparently is a result of kinetic limits on weathering in regions having young surfaces and supply limits to weathering on old surfaces due to the requirement for chemical weathering to occur soil production rates cannot be infinitely high consequently a speed limit must exist in the literature this limit has been set at about 320 to 450 t km 2 a our results from the sast approach show however that in alpine areas soil formation easily reaches rates of up to 800 2000 t km 2 a these data are consistent with previous studies in mountain regions demonstrating that particularly young soils intensively weather even under continuous seasonal snowpack and thus that the concept of temperature controlled soil development soil forming intervals in alpine regions must be reconsidered

ordovician metamorphism and plutonism in the sierra de quilmes metamorphic complex implications for the tectonic setting of the northern sierras pampeanas nw argentina towards unravelling the geodynamic setting of the northern sierras pampeanas nw argentina we describe the tectono metamorphic and geochronologic evolution of sub greenschist to granulite facies metamorphic sediments granitoid plutons and pegmatites in the ordovician sierra de quilmes metamorphic complex the protoliths of the metasediments are represented by a sequence of turbidites and minor calcsilicate rocks of the neoproterozoic to cambrian puncoviscana formation the metamorphic complex consists of four zones including the 1 chlorite 2 biotite muscovite 3 garnet cordierite sillimanite and 4 orthopyroxene zones zones 3 and 4 show an increasing degree of anatexis reaching large scale diatexis in the orthopyroxene zone at p t conditions exceeding 800 c and 600 mpa at or shortly after the metamorphic peak the granitic to tonalitic cafayate pluton intruded approximately along the boundary between anatectic and non anatectic rocks retrograde near isobaric cooling of the middle crust was accompanied by non penetrative ductile shearing at granulite to amphibolite facies p t conditions evidence for significant prograde deformation is absent in the sierra de quilmes metamorphic complex monazite and titanite u pb isotopic data constrain the metamorphic peak in migmatites and calcsilicate rocks to be at or slightly prior to 470 ma retrograde amphibolite facies mineral reactions led to continuous formation of monazite and titanite during slow cooling between 470 ma and 455 ma u pb data the composite cafayate pluton intruded over a time interval of several million years between 477 ma sm nd isochron and 460 ma monazite and titanite u pb isochron followed by pegmatites a younger group of pegmatites was emplaced in the country rocks at the end of the ordovician 440 ma rb sr mineral isochrons postdating most of the retrograde shear zones resetting of the muscovite k ar and 40ar 39ar system in weakly deformed pegmatites and crystallisation of new muscovite in low grade metamorphic sediments at 400 416 ma is attributed to minor late greenschist and sub greenschist facies metamorphism and deformation the massive early ordovician heat transfer the absence of major prograde deformation and subsequent prolonged phases of non penetrative deformation in the high grade metamorphic zones during slow near isobaric cooling contradict crustal thickening and continent collision models as have been proposed for the southern sierras pampeanas we suggest continuous or stepwise extensional tectonics in a back arc or a mobile belt tectonic environment for the ordovician sierra de quilmes and the northern sierras pampeanas in general an extensional setting of the northern sierras pampeanas in the ordovician is in agreement with the coeval formation of marine extensional sediment basins in vicinity of the sierras pampeanas in northwestern argentina and southern bolivia

interlayering and interlayer slip in biotite as seen by hrtem partial interlayers of brucite like sheets which form with the adjacent talc like layers of the mica single chlorite layers interstratified in the mica structure brucite interlayering is interpreted as a brucitization of an interlayer level of the mica where partial slip or cleavage has previously occurred example of a chemical process at the atomic scale favored by deformation microstructures modified journal abstract

deciphering the paleoproterozoic cooling history of the northeastern trans hudson orogen baffin island canada using 40 ar 39 ar step heating and uv laser thermochronology abstract the previously unstudied cooling and exhumation history of mid crustal rocks exposed on southeastern baffin island canada provides new insights into the post orogenic evolution of the paleoproterozoic trans hudson orogen tho new 40 ar 39 ar step heat analyses of biotite muscovite and phlogopite and core to rim intra grain 40 ar 39 ar analyses of muscovite have a range of apparent ages compatible with slow regional cooling following peak metamorphism twenty nine amphibolite to granulite facies rocks were dated using the 40 ar 39 ar step heating laser co 2 method 40 ar 39 ar spot analyses were performed across muscovite grains from three samples using an ultraviolet uv laser to investigate intra grain 40 ar 39 ar age variations step heating apparent ages range from ca 1788 1622 ma for biotite 1720 1630 ma for phlogopite and 1729 1657 ma for muscovite uv spot 40 ar 39 ar analyses in the three muscovite grains range from ca 1661 1640 ma 1675 1645 ma and 1680 1652 ma with core to rim apparent age gradients of 20 30 myr previous studies resolved peak metamorphism in this region to between ca 1860 and 1820 ma and identified late to post tho zircon and monazite populations at ca 1800 1750 ma numerical diffusion models for ar in muscovite were conducted to test different proterozoic cooling and exhumation scenarios comparisons with our 40 ar 39 ar ages attest to cooling rates of 1 2 c myr following peak metamorphism and 1 5 2 5 c myr after ca 1740 ma anomalously old apparent 40 ar 39 ar ages in cases equivalent to u pb zircon rim and monazite ages likely result from incorporation of excess ar the results suggest that mid crustal rocks on southeastern baffin island remained hotter than 420 450 c for 150 200 myr after peak metamorphism with subsequent slow cooling and denudation rates that are typical of proterozoic orogens the apparent absence of orogenic collapse implies that despite high temperatures and estimated maximum crustal thicknesses comparable to those of large hot orogens the tho remained gravitationally stable during its terminal phase

dislocation interactions in experimentally deformed biotite abstract biotite single crystals shortened experimentally in orientations that maximize critical resolved shear stresses on 001 in directions 110 010 and 310 have been examined by transmission electron microscopy in all samples dislocations are confined to the basal plane with burgers vectors of 1 2 110 and 100 consistent with previous determinations of mica slip systems occasional partial dislocations that bound stacking faults form extended dislocations of these same systems typically dislocations are concentrated in complex arrays in local regions of the samples isolated or widely spaced dislocations are the exception within these arrays a variety of interactions between the dislocations are suggested by the geometrical arrangements of dislocations that cross over one another on parallel 001 planes sufficiently close together for elastic interaction and dislocations on the same 001 plane in networks and parallel arrays crossing dislocations of opposite sign on separate 001 planes form stable dipole segments where they overlap attractive and repulsive interactions between overlapping dislocations of like sign are evident as well dislocation networks occur in all samples and may exhibit a variety of complexly distorted geometries the spacing of dislocations in parallel arrays do not match models for standard dislocation pile ups consistent with observations showing these arrays involve dislocation multipoles the observed dislocation arrays and interactions resemble those reported for stage i easy glide deformation of fcc and hcp metals basal shear strengths are probably largely controlled by the development of multipoles and dislocations that pile up behind them