comment on paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing by hua ning qiu and j r wijbrans abstract in a recent study qiu and wijbrans 2006 qiu h n and wijbrans j r 2006 paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing geochimica et cosmochimica acta 70 2354 2370 analyzed ar isotopes extracted by crushing garnets from the dabie shan bixiling eclogite and claimed to have constrained a pre triassic 450 ma episode of uhp metamorphism from primary fluid inclusions however in the absence of careful sample characterization and stepped heating analyses the reported ages are more easily explained as experimental artifacts related to ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix dabie shan garnets commonly contain mineral impurities such as k rich omphacite and or k feldspar if dabie shan uhp metamorphism is of the generally accepted triassic age 210 240 ma the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous 40ar component with mean 40ar 36ar value of 5000 this value is similar to the composition of extraneous 40ar in other eclogite facies terrane

an integrated palaeomagnetic and ams study of the tertiary flysch from the outer western carpathians summary the carpathians belong to the european alpine system which was formed during the convergence and collision of the european and african plates the study area which comprises the magura and silesian nappes of the outer western carpathians were the locations of intensive folding and nappe transport during the tertiary there are a number of models depicting the changes in shapes and positions of the original sedimentary basins as a consequence of the above processes however it is this study that provides the first palaeomagnetic constraints for reconstructing the displacement history of the silesian and magura nappes during tertiary for the palaeomagnetic and anisotropy of the low field magnetic susceptibility ams studies 554 independently oriented cores were drilled mostly from claystones at 57 geographically distributed localities of oligocene and partly of late eocene in the magura nappe age in the laboratory the magnetic mineral identified was always magnetite invariably accompanied by paramagnetic pyrite the ams measurements showed that the magnetic fabric was dominantly foliated the foliation and bedding poles were very close at most of the localities suggesting that the foliation was of sedimentary compaction origin magnetic lineations even if clustered at the locality level did not exhibit a regional pattern for the magura nappe on the other hand lineations related to folding were ne sw oriented in the western and e w oriented in the central and eastern segments of the silesian nappe following the ams measurements the samples were subjected to detailed stepwise demagnetization and analysis of the demagnetization curves that provided the input data for regional tilt tests both the tilt and inclination only tests suggested that the remanence is of post folding age for the magura nappe showing a general 50 counterclockwise ccw rotation similar palaeomagnetic directions were observed for the central and eastern segments of the silesian nappe whereas a significantly larger ccw rotation for the western segment the orientations of the ams lineation directions averaged for the central the eastern and the western segments of the silesian nappe correspond to a rotated palaeomagnetic pattern as they are practically e w oriented for the first two and ne sw oriented for the third the palaeomagnetic results of this study are interpreted in terms of a 50 en bloc rotation in the ccw sense of the magura and silesian nappes during their miocene emplacement this result has two important implications the first is that the ccw rotation of the nappes that accompanied the displacement of the flysch basins from sw to ne is not of palaeogene age as earlier suggested but younger the second is that the change in the stress field orientation during the miocene observed for the silesian nappe is not a cw far field stress rotation but the consequence of ccw block rotations the larger ccw rotation observed in both the palaeomagnetic and ams data sets for the western segment of the silesian nappe relative to the central and eastern segments is connected to local block rotations in a left lateral wrench corridor

an expression of philippine sea plate rotation the parece vela and shikoku basins abstract the philippine sea plate located between the pacific eurasian and australian plates is the world s largest marginal basin plate the motion of the philippine sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence previous studies have relied on palaeomagnetic analysis to constrain its rotation we present a comprehensive analysis of geophysical data within the parece vela and shikoku basins two oligocene to miocene back arc basins which provide independent constraints on the rotational history of the philippine sea plate by means of their seafloor spreading record we have created a detailed plate model for the opening of the parece vela and shikoku basins based on an analysis of all available magnetic gravity and bathymetric data in the region subduction along the izu bonin mariana trench led to trench roll back arc rupture and back arc rifting in the parece vela and shikoku basins at 30 ma seafloor spreading in both basins developed by chron 9o 28 ma and possibly by chron 10o 29 ma as a northward and southward propagating rift respectively the spreading orientation in the parece vela basin was e w as opposed to ene wsw in the shikoku basin the spreading ridges joined by chron 6by 23 ma and formed a r r r triple junction to accommodate the difference in spreading orientations in both basins at chron 6no 20 ma the spreading direction in the parece vela basin changed from e w to ne sw at chron 5ey 19 ma the spreading direction in the shikoku basin changed from ene wsw to ne sw this change was accompanied by a marked decrease in spreading rate cessation of back arc opening occurred at 15 ma a time of regional plate reorganisation in se asia we interpret the dramatic change in spreading rate and direction from e w to ne sw at 20 1 3 ma as an expression of philippine sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones this rotation was previously thought to have begun at 25 ma as a result of a global change in plate motions our results suggest that the philippine sea plate rotated clockwise by about 4 between 20 and 15 ma about a pole located 35 n 84 e this implies that the majority of the 34 clockwise rotation inferred to have occurred between 25 and 5 ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 ma

cosmogenic 3he in himalayan garnets indicating an altitude dependence of the 3he 10be production ratio to investigate the potential of using cosmogenic helium in garnet as a dating tool 3hec and 10bec have been measured within garnets and coexisting quartz respectively sampled in the himalayan range at various elevations comparison and correlation of 3hec and 10bec concentrations demonstrate that cosmogenic 3he is well retained in himalayan garnets and can thus be used in this mineral to quantify precisely earth s surface processes this in the case of recently cooled orogenic samples which contain limited amounts of non cosmogenic helium for these high elevation samples the exposure ages derived from 3hec using classical scaling factors lead however to an overestimation of 3hec ages compared to 10bec ages 0 1 to 28 ky by up to a factor of 2 additionally over the samples altitude range 3000 4622 m the theoretically invariant polar 3hec 10bec production ratio 22 5 increases with increasing elevation challenging classical models of production rate evolution to explain this observation we propose a new 3hec production mechanism based on the altitudinal dependence of the energy spectrum of cosmic rays and according to which neutrons or protons resulting from a first spallation reaction within the rock tertiary particles may have sufficient energy to induce an additional in situ spallation reaction producing a second 3h or 3he but insufficient energy to produce 10be the probability of such a mechanism increasing with altitude according to these observations we propose for future and ongoing studies to use the empirically determined attenuation length of 121 g cm2 for helium production at high altitude

a noble gas study of a granulite sample from the nilgiri hills southern india implications for granulite formation the he and ar isotopic composition of several mineral separates from a 2 5 ga old enderbite sample from the nilgiri hills southern india were analyzed the sample contains abundant high density carbonic fluid inclusions which have been well characterized in previous studies 1 2 on this particular sample garnet contains synmetamorphic primary fluid inclusions while quartz and plagioclase contain re equilibrated secondary inclusions the noble gases were extracted by crushing or heating of the minerals all measured 3he 4he ratios are above the typical value of old crustal rocks which is about 0 01 0 02 times the atmospheric ratio ra fluid inclusions from the crushed garnet sample contain helium with the highest 3he 4he ratio of 2 34 0 36 ra the he excess is mantle derived and cannot be explained by a cosmogenic or nucleogenic 3he rich component the elemental and isotopic ratios of he ne and ar in all minerals can be explained by a combination of elemental fractionation during diffusional loss of noble gases production of radiogenic nucleogenic4he 21ne 22ne and40ar and atmospheric contamination depending on the assumptions made for the evolution of the he trapped in the garnet fluid inclusions we obtain mixing ratios between mantle derived he and crustal he ranging from 1 5 5 to essentially pure mantle he however probably pure mantle he was trapped in the minerals 2 5 ga ago these data suggest that the enderbite sample represents a former mantle derived tonalitic magma which exsolved its volatiles upon crystallization in the lower crust liberation of a huge amount of co2 by similar intrusions together with the heat provided by them might be responsible for the dehydration of the former amphibolite facies rocks to the north bordering the nilgiri hills

miocene pliocene tectonic evolution of the slovenian periadriatic fault implications for alpine carpathian extrusion models the periadriatic line pal is a remarkable several hundred kilometer long fault system of the alpine orogen its dextral character was documented by several authors using diverse criteria but detailed kinematics and timing of movements had not been investigated along its whole length structural and paleomagnetic measurements mapping and stratigraphic and sedimentological studies have helped to unravel the miocene pliocene evolution of the slovenian segment of the pal brittle deformation was characterized by nw se to n s compression and perpendicular tension deformation has resulted in dextral strike slip faulting folding and tilting of beds the first transpressional event corresponds to the first phase of lateral extrusion of the east alpine western carpathian northern pannonian block in the early miocene 24 17 5 ma after a short period of transtension during the karpatian 17 5 16 5 ma dextral transpression reoccurred during the middle miocene to pliocene and lasted up to the quaternary middle miocene dextral slip can be connected to the second phase of extrusion the highly deformed rocks within the dextral shear zones show variable clockwise sometimes counterclockwise rotations the mechanism of rotation seems to be complex ranging from regional rotation to local folding due to pure or simple shear domino type rotation

sorption and diffusion of helium in garnet implications for volatile tracing and dating abstract we present data on he sorption in garnet obtained by loading garnet samples with he at 250 10 bar in the temperature range 800 990 c for this temperature range we find a three fold decrease in he sorption from 2 1 10 5 800 c to 0 72 10 5 cm 3 stpg 1 bar 1 990 c we use the sorption data of partially saturated samples to obtain the activation energy e a 660 120 190 kj mol and frequency factor d 0 10 15 6 8 5 2 cm 2 s for he diffusion in garnet the results of our study suggest that garnet is probably one of the most retentive silicate minerals for he which is in agreement with its very low ion porosity the high retentivity results in a high closure temperature t c for the u th he system for cooling rates as low as 0 1 1 c ma t c lies between 590 and 630 c accordingly u th he dating of garnet may provide information about the medium to high temperature cooling path of a rock at the lower end of the temperature range of currently used isotopic systems in garnet in addition the high retentivity for he makes garnet a potentially useful mineral in the field of exposure age dating currently it is the only known common silicate mineral occurring in upper crustal rocks which probably retains cosmogenic noble gases under all climatic conditions for geochemical studies using noble gases as tracers for geofluids garnet is a perfect container if temperatures never exceed 600 c that is all originally trapped noble gases are quantitatively retained

diffusion of cosmogenic3he in olivine and quartz implications for surface exposure dating abstract thein situ production of3he in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool to evaluate helium loss problems in this technique cosmogenic3he diffusivities were measured in quartz and olivine by incremental heating at 150 600 c arrhenius temperature dependences were observed in both minerals with similar activation energies ea 25 4 kcal mole logd0 3 7 0 9 in olivine andea 25 2 0 9 logd0 0 2 0 4 in quartz and imply very low diffusivities when extrapolated to environmental temperatures less than 10 18 cm2 s in quartz and 10 22 cm2 s in olivine at 20 c these low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 106 years in quartz and 109 years in olivine provided large 2 mm sample grains are used exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose cosmogenic3he diffuses more than 100 times faster than trapped magmatic3he in olivine or radiogenic4he in quartz the mechanisms responsible for these differences have yet to be determined but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage therefore it is possible that helium diffusivities may depend on a sample s cosmic ray exposure or radiogenic production history whatever the precise mechanism the higher mobility of3he suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies comparison of the diffusivity results with total cosmogenic3he contents of different quartz size fractions for an antarctic quartzite suggests that he loss may have occurred more rapidly than predicted by the laboratory measurements nonetheless retention of cosmogenic3he is quite high equivalent to more than a million years of exposure and cosmogenic3he geochronology is feasible in this and other quartz containing rocks

temperature induced 4he degassing of specularite and botryoidal hematite a 4he retentivity study degassing of radiogenic 4he induced by stepwise heating in a vacuum was carried out on various grain size fractions of specular and botryoidal hematite in order to place constraints on their 4he retentivities samples from the vosges odenwald saarbergland and cumbria were used below 900 c specularite yields straight lines in arrhenius plots this is consistent with volume diffusion the diffusion distances correspond with the macroscopic mean grain size by contrast botryoidal hematite yields curved lines which are straight only below 250 c the actual diffusion distances appear to be commensurate with the microscopic size of the constituent crystals 0 1 to 10 m the diffusion coefficients of specular d20 and botryoidal hematite d20 r2 are 10 28 cm s 1 and 10 22 s 1 and the activation energies range from 110 to 130 and 130 to 210 kj mol respectively helium 4 closure temperatures tc of specularite grains with diameters of about 5 mm are above 200 c and for all grain fractions in botryoidal hematite 10 m the tc is above 90 160 c on the basis of tc both hematite varieties should be useful thermochronometers low temperature 4he losses of the order of a few percent during a few 100 m y are conceivable for botryoidal hematite

cosmogenic 3he and 21ne production rates calibrated against 10be in minerals from the coso volcanic field this study calibrates the production rate of cosmogenic 3 he in pyroxene olivine garnet zircon and apatite as well as 21 ne in quartz and pyroxene against the known production rate of 10 be in quartz the devil s kitchen rhyolite from the coso volcanic field in southeastern california elev 1300 m was chosen for this study due to its young age 610 ka and diverse mineral assemblage based on 10 be our two rhyolite samples have apparent exposure ages of 49 and 93 ka indicating substantial erosion after eruption combining data from the two samples we estimate sea level high latitude 3 he spallation production rates of 145 11 141 16 and 144 30 at g 1 a 1 2 for pyroxene olivine and spessartine garnet respectively for zircon and apatite we estimate apparent 3he spallation production rates of 114 8 and 149 28 at g 1 a 1 2 respectively the rates for zircon and apatite are reported as apparent production rates because we do not explicitly address the redistribution of spallation produced 3 he from adjacent minerals these estimates quantitatively account for production of 3 he from both cosmogenic and radiogenic neutron reactions on 6 li within the analyzed phases and also implanted from nuclear reactions in neighboring minerals the high u th and li content of this rhyolite provides a particularly rigorous test of this correction we estimate 21 ne production rates of 17 7 1 6 and 34 1 3 2 at g 1 a 1 2 in quartz and pyroxene fe mg 0 7 by mass respectively although high u and th contents create the potential for significant production of nucleogenic 21 ne this component is small due to the young eruption age of the rhyolite

the cosmic ray produced he 21ne ratio in ultramafic rocks although production of noble gases in meteorites are well studied since many years e g bogard and cressy 1973 the first analysis of cosmic ray produced helium in terrestrial rocks were performed only in 1986 by kurz and by craig and poreda later marti and craig 1987 reported cosmic ray produced neon in summit lavas from maui more data on cosmogenic helium and neon were reported by staudacher and allegre 1991 on ultramafic nodules from asia the absolute production rate of cosmic ray produced helium has been determined by kurz 1986 1987 in olivine rich basalts from hawaii with known k ar or 14c ages the resulting hec production rate is 3 72 10 18cm stp g a this allows cosmogenic he to be used to determine cosmic ray exposure ages of surface samples for neon no such absolute production rate has yet been determined in order to develop a complementary dating method using cosmogenic 21ne staudacher and allegre 1991 measured the cosmogenic he 21ne ratio in ultramafic nodules their value was 1 4 which is significantly smaller than the value of 3 5 obtained in this work here we discuss the cosmogenic he 21ne ratio in view of new noble gas data on olivine separates of oceanites from reunion island and on ultramafic nodules from mongolia

solubility and partitioning of ne ar kr and xe in minerals and synthetic basaltic melts we have measured the solubilities of ne ar kr and xe in natural samples of anorthite diopside forsterite spinel and in synthetic basaltic melts the minerals and melts represent equilibrium pairs in the fo an di sio2 system samples were suspended in individual crucibles in a one bar flowing mixed noble gas atmosphere at 1300 or 1332 c for times ranging from 7 18 days the solubilities in the minerals increase with increasing noble gas atomic number and typical solubility values are surprisingly high samples of a particular mineral i e anorthite that came from different localities yield distinctly different and reproducible solubility values this indicates that properties intrinsic to individual samples can influence solubility noble gases are likely to be sited in lattice vacancy defects in contrast the solubilities in the melts decrease with increasing atomic number our data overlap the low end of the range defined previously for natural basalts however the dynamic range of the values from ne to xe is not as great as for the natural melts solubilities correlate well with melt molar volume but poorly with density the partition coefficients increase with increasing noble gas atomic number for all mineral melt pairs such trends imply that the terrestrial planet atmospheres were not derived from partial melting of chondritic source material significant fractionation of kr relative to xe is not observed ruling out an origin for earth s missing xe via magmatic fractionation partition coefficient absolute values are frequently near or greater than unity for example the ranges for five diopside measurements are the following dnel 0 013 0 37 darl 0 15 0 84 dkrl 0 31 2 4 and dxel 3 2 47 this indicates that magmatic transport is not an efficient mechanism for degassing the terrestrial planets our results favor a catastrophic origin for the atmospheres

4he 3he thermochronometry using classical diffusion theory we present a mathematical technique for the determination of 4he concentration profiles in minerals this approach should prove useful for constraining the low temperature cooling histories of individual samples and for correcting u th he ages for partial diffusive loss the calculation assumes that the mineral of interest contains an artificially produced and uniform distribution of 3he obtained by proton irradiation shuster et al earth planet sci lett 217 2004 19 32 in minerals devoid of natural helium this isotope allows measurement of he diffusion coefficients in minerals with measurable radiogenic he it permits determination of 4he profiles arising during ingrowth and diffusion in nature the 4he profile can be extracted from stepwise degassing experiments in which the 4he 3he ratio is measured the evolution of the 4he 3he ratio as a function of cumulative 3he released can be compared with forward models to constrain the shape of the profile alternatively we present a linear inversion that can be used to directly solve for the unknown 4he distribution the inversion incorporates a standard regularization technique to filter the influence of random measurement errors on the solution using either approach we show that stepwise degassing data can yield robust and high resolution information on the 4he profile profiles of radiogenic he are a sensitive function of the time temperate t t path that a cooling sample experienced thus by step heating a proton irradiated sample it is possible to restrict the sample s acceptable t t paths the sensitivity of this approach was explored by forward modeling 4he profiles resulting from a range of realistic t t paths using apatite as an example results indicate that 4he profiles provide rich information on t t paths especially when the profiles are coupled with u th he cooling ages on the same sample samples that experienced only moderate diffusive loss have 4he concentration profiles that are rounded at the edge but uniform in the core of the diffusion domain such profiles can be identified by nearly invariant 4he 3he ratios after the first few to few tens of percent of 3he have been extracted by step heating we show how such data can be used to correct u th he ages for partial diffusive loss

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

texturally early fluid inclusions in garnets evidence of the prograde metamorphic path well formed texturally early fluid inclusions in garnets from the archean pikwitonei granulite domain manitoba canada have been analyzed using microthermometric methods the mean co2 homogenization temperature to liquid for inclusions in 12 of 13 samples from the cauchon lake nelson river area is 15 2 c n 125 2 8 2 c corresponding to a co2 density of 0 82 g cm3 inclusions in the remaining sample have somewhat lower co2 homogenization temperatures mean 5 4 c n 24 the studied inclusions contain an estimated 10 to 20 vol percent h2o with minor amounts of other fluid species such as ch4 n2 and or h2s the fluid inclusions were probably trapped during early garnet growth at relatively low pressures 5 kbar if at 750 c and appear to have undergone only limited or possibly no subsequent re equilibration this interpretation is consistent with the anti clock wise p t t path heating before loading determined for the pikwitonei region by other workers for such a prograde path inclusions entrapped early at high temperatures but at relatively low pressures would experience internal underpressures during most of the subsequent prograde and retrograde phases of metamorphism the texturally early fluid inclusions in garnets from the pikwitonei region therefore cannot be used to provide direct information about the highest metamorphic temperature and pressure conditions 750 c and 7 kbar however the results obtained in this study suggest that texturally early fluid inclusions in garnets may in some cases retain evidence of the prograde metamorphic path

the magnetic fabric of the ždanice thrust sheet of the flysch belt of the west carpathians sedimentological and tectonic implications abstract the magnetic fabric of sandstone of the danice hustopece formation flysch belt of the west carpathians is essentially sedimentary in origin although affected very weakly by ductile deformation the magnetic lineation is mostly parallel to the flow direction the sandstone was probably deposited from thin suspension and the rotation of grains in flowing water was the dominating orientation mechanism two systems of currents are indicated one supplying the basin with clastic material from the source the second transporting it along the trough the weak deformation was lengthening parallel to bedding and was due to the strain associated with the horizontal movement of the danice thrust sheet

tertiary paleomagnetic results and structural analysis from the transdanubian range hungary rotational disintegration of the alcapa unit abstract we carried out an integrated paleomagnetic and structural study in the transdanubian range western and central hungary as a result the tertiary tectonic history of this area can be characterized by three events of counterclockwise ccw rotation and four or five phases of brittle deformation the change of the orientation of stress axes between phases is mainly apparent and reflects the rotation of the faults predating a particular rotation event the first two rotation events r1 and r2 were probably governed by the rollback mechanism of the subducting european plate we suggest that these rotations were taking place from 18 17 and 16 14 5 ma respectively i e simultaneously with the rotations of the north hungarian paleogene basin and the main part of the western carpathians however the angle of both rotations was less in the transdanubian range due to increasing distance from the subduction front the differential rotation was accommodated by extensional faulting by formation of a graben system on the other hand the youngest rotation event r3 seems to be connected to the renewed rotation of the adriatic plate around 5 ma our combined data set strongly supports earlier conclusions namely that the different subunits of the eastern alpine western carpathian northern pannonian unit alcapa did not form a rigid unit although they moved in similar manner

the paleozoic metamorphic history of the central orogenic belt of china from 40ar 39ar geochronology of eclogite garnet fluid inclusions abstract the pressure temperature time evolution of the uhp eclogites of dabie sulu in the eastern sector of the central orogenic belt of china shows a complex pattern of predominantly triassic and to a lesser extent early paleozoic ages 40 ar 39 ar stepwise crushing analysis of eclogitic garnets yielded consistently paleozoic ages because our initial results from the bixiling eclogite were of limited areal extent their significance required further testing data for garnet bearing gneiss and eclogites from new locations in northern and southern dabieshan shuanghe ganghe and hongmiao confirm the paleozoic garnet results obtained from bixiling eclogite we also tested the technique on a garnet sample of known paleozoic age from altyn tagh in the western central orogenic belt these new results indicate that light colored garnets from dabieshan and altyn tagh yield concordant 40 ar 39 ar ages ranging from 445 465 ma by in vacuo crushing and confirm that a paleozoic u hp metamorphic event took place in the central orogenic belt from altyn tagh in western china to dabieshan in eastern china in contrast dark colored garnets from mafic metavolcanics do not yield 40 ar 39 ar plateaux or isochrons ages

4he diffusion in 40ar retentive minerals abstract 4he vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in k ar chronometry such as hornblende nepheline and others the study aims at 4he diffusion parameters for uppermost crustal temperatures and thus at he retentivities of the minerals the results indicate long time near quantitative 4he retention in hornblende pyroxene langbeinite and perhaps nepheline for 0 5 mm grains at least at temperatures below about 70 c sanidine and muscovite lose 4he over geologic periods at lower temperatures only the minerals of the first group are possible u th he chronometers or geothermometers provided that other dating problems such as alpha loss and u migration can be settled closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170 c tentative u he indices indicate that diffusion parameters of the same order of magnitude as determined for the he retentive minerals may be used to select minerals to be further examined for their u th he datability e g ore minerals

forward modeling and interpretation of u th he ages soon after the initial studies on he diffusion in apatite pointed out the use of the u th he system for low temperature geochronology zeitler et al 1987 wolf et al 1996 warnock et al 1997 the potential value of forward modeling for refined interpretation of u th he dating was indicated wolf et al 1998 while the first studies provided evidence for the low closure temperature t c dodson 1973 of the u th he system in apatite for monotonously cooling systems the latter study investigated the sensitivity of the u th he system for a wide range of geologically feasible heating cooling scenarios forward modeling of he accumulation in minerals expanded the use of the u th he geochromometer to geological systems that do not fit one of the main conditions for the meaningful application of the closure temperature concept i e monotonous cooling at a constant rate through t c forward modeling can help to give a meaning to apparent u th he ages from rocks with cooling histories with protracted residence time in the helium partial retention zone heprz e g 40 85 c in apatite wolf et al 1998 where the closure temperature is undefined fig 1 figure 1 apatite he age evolution curves dotted lines for several representative time temperature paths solid lines forward modelling was conducted using the crank nicolson method for spheres of 60 m radius note that only for history 2 the closure temperature concept dodson 1973 is defined and the age obtained relates to passing through a point t c on the time temperature path for histories that are similar to 3 5 which know a protracted residence time in the heprz there is no direct geological meaning of he ages forward modelling is required to give a geological meaning to the apparent ages obtained from he and u th concentrations in samples used with permission of elsevier from wolf

cosmogenic 10be and 3he accumulation in pleistocene beach terraces in death valley california u s a implications for cosmic ray exposure dating of young surfaces in hot climates to investigate the history of pleistocene lake levels in the southwestern u s a and to improve our understanding of the systematics of in situ produced cosmogenic nuclide accumulation in hot climates and young rocks we examined the 3he 10be and 26al contents of quartz separates from metamorphic quartzites and quartz rich felsic rocks incorporated in a series of stranded conglomerate beach ridges ranging from 3 to 160 m above sea level in death valley california calculated 10be ages range from 17 to 135 kyr with most ages between 20 and 60 kyr in contrast cosmogenic 3he contents indicate exposure ages of 1 kyr except for one sample at 18 4 kyr for which the 10be age is still significantly greater 44 11 kyr 26al10be ratios close to the cosmogenic production ratio rule out the presence of meteoric 10be and point to 3he loss as the cause of the age discrepancies comparison to laboratory diffusive loss measurements using a diurnal direct solar heating model implies the 3he loss occurs because the effective quartz grain sizes are 100 m far smaller than the 1 3 mm fracture spacings estimated from thin sections because direct insolation enhances 3he loss sampling from shallow depths 20 40 cm may be useful in some cases 4he and 20ne measurements demonstrate that small amounts of radiogenic and atmospheric 3he are also present at levels equivalent to a few kiloyears sea level exposure which should be taken into account when dating surfaces of similar age the lack of any clear progression with terrace altitude and the occurrence of two fold variations in apparent exposure ages within individual terraces suggest that the 10be contents do not solely represent accumulation in the present sample locations instead accumulation probably also occurred during sample exposure in the higher altitude beach rock source regions and during fluvial transport to the ancient lake shore thus the beach terrace exposure ages of 20 60 kyr are probably maximum ages for the times of these high lake level stands in death valley

the duration of prograde garnet crystallization in the uhp eclogites at lago di cignana italy abstract the distinct core to rim zonation of different rees in garnet in metamorphic rocks specifically sm relative to lu suggests that sm nd and lu hf isochron ages will record different times along a prograde garnet growth history therefore ree zonations in garnet must be measured in order to correctly interpret the isochron ages in terms of the garnet growth interval which could span several m y new ree profiles garnet crystal size distributions and garnet growth modeling combined with previously published sm nd and lu hf geochronology on a uhp eclogite of the zermatt saas fee zsf ophiolite lago di cignana italy demonstrate that prograde garnet growth of this sample occurred over a 30 to 40 m y interval relative to peak metamorphism at 38 to 40 ma garnet growth is estimated to have begun at 11 to 14 kbar pressure at 70 to 80 ma although such a protracted garnet growth interval is surprising this is supported by plate tectonic reconstructions which suggest that subduction of the liguro piemont ocean occurred through slow and oblique convergence these results demonstrate that ree zonations in garnet coupled to crystal size distributions provide a powerful means for understanding prograde metamorphic paths when combined with sm nd and lu hf geochronology

partition of noble gases between olivine and basalt melt abstract the distribution coefficients ki of noble gases were determined for synthesized olivine basalt melt pairs seven samples bh series were synthesized at 1370 1300 c under a one atmospheric pressure 105 pa noble gas mixture and four samples hpp and hpg series at 1360 1050 c at high pressures 0 2 1 5 gpa the two different experiments gave consistent results with each other some bh series olivine samples were apparently contaminated with gas rich inclusions or the glass phase and the effects of such interferences on the calculated distribution coefficients were evaluated the possible ranges of the distribution coefficients of noble gases thus obtained are khe 0 07 kne 0 006 0 08 kar 0 05 0 15 kkr 0 15 and kxe 0 3 these show remarkable features compared to solid element distribution coefficients the distribution coefficients of noble gases are rather insensitive to or even positively correlated with the atomic size of noble gases the striking contrast between the partition of noble gases and that of solid elements may be attributed to the electrical neutrality of noble gases and the increasing electronic polarizability from he to xe

evolution models of two flysch basins the northern carpathians and the southern apennines abstract the geological evolution of the external domains of the northern carpathians and southern apennines is compared and two models of flysch basins are proposed the first related to a tensile stage of the orogeny is applied only to the northern carpathian area during the cretaceous the latter related to a compressive stage is present in both areas during the cenozoic the different evolutions of the basins have counterparts in the present day continental margin and the thinning of the crust during the tensile stage controls the flysch sedimentation in the first model and during the compressive stage of subduction along mobile plate junctions enables the accumulation of deposits in trench like basins to take place

he diffusion in olivine helium diffusion in olivine dunite xenolith has been measured in the temperature range 1180 1460 c a linear arrhenius function was obtained with an activation energy of 120 27 32 kcal mole and a pre exponential factor d0 2 2 108 cm2 s diffusion mechanisms are not a viable means of degassing he from the mantle olivine phenocrysts can be expected to retain previously trapped he during cooling in extrusive basalts provided the flow units are thinner than 50 m xenoliths will retain mantle he signatures only if magma transport times are less than 50 years or if the he fugacity in the magma is high enough to prevent xenolith degassing the lower oceanic crust is probably substantially degassed of he trapped he will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 1014 c m y glass at several centimeters depth in a basalt flow near the spherulite zone will have cooling rates lower than this so he loss may be significant in many basalt glass samples

cosmogenic 3he in igneous and fossil tooth enamel fluorapatite igneous fluorapatite samples from a suite of six granitic rocks from the transantarctic mountains have high 3he concentrations to 5 10 9 atoms g 1 and high 3he 4he ratios to 9 10 7 these values are far higher than those found in several hundred igneous apatites from elsewhere around the world and are higher than can be attributed to nuclear reactions on 6li this 3he is almost certainly derived from cosmic ray reactions in rocks with high exposure ages at high latitude and elevation several samples of fossil tooth enamel fluorapatite from the turkana basin of kenya are similarly rich in 3he with up to 1 10 7 atoms 3he g 1 and 3he 4he ratios up to 4 10 6 again this 3he is most logically attributed to cosmic ray reactions provided that cosmogenic 3he like radiogenic 4he is quantitatively retained in fluorapatite under earth surface conditions routine 3he exposure dating of this common phase may be possible based on its chemical composition the 3he production rate in fluorapatite is about 100 atoms g 1 yr 1 at sea level and high latitude using this rate the apatites from the transantarctic mountains have apparent exposure ages of 0 5 6 2 myr in agreement with values elsewhere in the range the fossil tooth enamel samples have apparent exposure ages ranging from a few up to 130 kyr such high exposure ages suggest some of these fossils may be lag deposits with a very long residence time at or near the earth s surface 3he exposure ages can provide insights to the depositional and reworking history of enamel bearing fossils at present the major limitations to 3he exposure dating of fluorapatite are purification of sufficient amounts of material and measurement of small amounts of 3he in the presence of large quantities of 4he in addition further work is necessary to establish the nucleogenic 3he background in fluorapatite

pre metamorphic arar ages from biotite inclusions in garnet two populations of ar ar ages in biotite inclusions occluded by garnets from an indian polymetamorphic gneiss have been measured using uv laser ablation one population yields a reproducible age of 455 my identical to biotite ages in the adjacent groundmass and is interpreted as slow cooling following high grade metamorphism during the pan african event the second population yields ages in the range 1800 2100 ma and corresponds closely with zircon ages from the same rock these results imply that the ar ar system in a small population of biotite inclusions remained closed during high grade metamorphism 750 c and 5 kb the results of this first experiment accord with results for helium diffusion in garnet and suggest a mechanism for the long term storage of heavy noble gases in the deep crust

paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing abstract the 40 ar 39 ar stepwise crushing technique is applied for the first time to date garnet from ultra high pressure metamorphic uhpm eclogites three garnet samples from the bixiling eclogites analyzed by 40 ar 39 ar stepwise crushing yield regular predictable age spectra and a clear separation between excess 40 ar and concordant plateau and isochron ages all three age spectra begin with high apparent ages followed by step by step decreasing ages and finally age plateaux with apparent ages in the range from 427 20 to 444 10 ma the data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 34 to 459 58 ma corresponding to initial 40 ar 36 ar ratios from 292 1 4 5 to 294 5 6 7 statistically indistinguishable from the modern air the high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40 ar whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40 ar the plateau ages are interpreted to approximate the time of garnet growth during initial uhpm metamorphism phengite analyzed by laser stepwise heating yielded a complicated two saddle age spectrum with a scattered isochron corresponding to age of 463 116 ma and initial 40 ar 36 ar ratio of 1843 1740 indicative of the presence of extraneous 40 ar within phengite these concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that dabie uhpm eclogites were first formed in the early paleozoic during the same event that caused the qinling northern qaidam basin altyn tagh eclogites

palaeomagnetism of the teschenitic rocks lower cretaceous in the outer western carpathians of poland constraints for tectonic rotations in the silesian unit summary results of palaeomagnetic investigations of the lower cretaceous teschenitic rocks in the silesian unit of the outer western carpathians in poland bring evidence for pre folding magnetization of these rocks the mixed polarity component reveals inclinations between 56 and 69 which might be either of cretaceous or tertiary age apparently positive results of fold and contact tests in some localities and presence of pyrhotite in the contact aureole suggest that magnetization is primary although a neogene or earlier remagnetization cannot be totally excluded since inclination only test between localities gives syn folding results higher palaeoinclinations 66 69 correlate with a younger variety of teschenitic rocks dated for 122 120 ma while lower inclinations 56 60 with an older variety 138 133 ma this would support relatively high palaeolatitudes for the southern margin of the eurasian plate in the late part of the early cretaceous and relatively quick northward drift of the plate in this epoch together with the silesian basin at its southern margin declinations are similar to the cretaceous tertiary palaeodeclinations of stable europe in the eastern part of the studied area but rotated ca 14 70 counter clockwise in the western part this indicates together with older results from czech and slovakian sectors of the silesian unit a change in the rotation pattern from counter clockwise to clockwise at the meridian of 19 e the rotations took place before the final collision of the outer carpathians nappe stack with the european foreland