age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

the cosmogenic 21ne production rate in quartz evaluated on a large set of existing 21ne 10be data abstract based on a compilation of published combined 10 be and 21 ne cosmogenic nuclide data sets from quartz samples obtained at eth zurich we assess the 21 ne 10 be p21 p10 production rate ratio with the goal to determine the 21 ne production rate p21 in quartz a variety of sliding erosion islands in a 21 ne 10 be versus 10 be diagram were evaluated to find the one that fits the data best which in turn yields the most probable p21 if the 10 be production rate is known the approach minimizes the influence of samples with a complex exposure history a best fit value for p21 p10 sp sp the 10 be fraction being produced by spallation as opposed to production by muons of 4 23 0 17 is obtained for a 10 be half life of 1 39 ma adopting a p10 sp value in quartz of 4 41 0 52 at g 1 yr 1 this yields a p21 of 18 7 2 3 at g 1 yr 1 it is possible that 2 of the 21 ne is produced by fast muons

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

complex multiple cosmogenic nuclide concentration and histories in the arid rio lluta catchment northern chile terrestrial cosmogenic nuclide tcn concentrations measured in river sediments can be used to estimate catchment wide denudation rates by investigating multiple tcn the steadiness of sediment generation transport and depositional processes can be tested measurements of 10be 21ne and 26al from the hyper to semi arid rio lluta catchment northern chile yield average single denudation rates ranging from 12 to 75 m myr 1 throughout the catchment paired nuclide analysis reveals complex exposure histories for most of the samples and thus the single nuclide estimates do not exclusively represent catchment wide denudation rates the lower range of single nuclide denudation rates 12 17 m myr 1 established with the noble gas 21ne is in accordance with palaeodenudation rates derived from 21ne 10be and 26al 10be ratio analysis since this denudation rate range is measured throughout the system it is suggested that a headwater signal is transported downstream but modulated by a complex admixture of sediment that has been stored and buried at proximal hillslope or terrace deposits which are released during high discharge events that is best evidenced by the stable nuclide 21ne which preserves the nuclide concentration even during storage intervals the catchment wide single 21ne denudation rates and the palaeodenuation rates contrast with previous tcn derived erosion rates from bedrock exposures at hillslope interfluves by being at least one order of magnitude higher especially in the lower river course these results support earlier studies that identified a coupling of erosional processes in the western cordillera contrasting with decoupled processes in the western escarpment and in the coastal cordillera copyright 2008 john wiley sons ltd

denudation rates and a topography driven rainfall threshold in northern chile multiple cosmogenic nuclide data and sediment yield budgets abstract the quantification of geomorphic process rates on the outcrop and the orogen scale is important to describe accurately the interaction between the relative effects of erosion tectonics and climate on landscape evolution we report single and paired cosmogenic nuclide 10 be 26 al and 21 ne derived erosion rates and exposure ages on hillslope interfluves from the tectonically active western central andes that show a distinct spatial variation a positive correlation of erosion rates with elevation and present day rainfall rates is observed erosion rates at lower altitudes the hyperarid coastal cordillera and the western escarpment with the northern part of the atacama desert are extremely low and of the order of 10 100 cm my nominal exposure ages 1 6 my in contrast erosion rates at higher altitudes the semiarid western cordillera range up to 4600 cm my nominal exposure ages 0 02 0 1 my this latter average long term bedrock erosion rate record suggested to be coupled to an orographically controlled pattern of rainfall is also reflected in the pattern of denudation rates derived from a short term decadal record of limited sediment yield data specifically denudation rates calculated from sediment flux data are of a similar order of magnitude as erosion rates deduced from long lived cosmogenic nuclides from bedrock hillslope interfluves of the western cordillera nevertheless the production and the supply of sediment from the western andean slope are very limited analysis of multiple cosmogenic nuclides allows simultaneous determination of erosion rates and exposure ages but also reveals complex exposure histories of non bedrock samples such as boulders or amalgamated clast samples notably this study shows that saturation of nuclides usually assumed in studies where only a single nuclide is analyzed is rather the exception than the rule as revealed by erosion island plots constant erosion that started much later than the formation age of the rocks or episodic erosion by spalling can partially explain non steady state concentrations and more complicated exposure scenarios furthermore the use of multiple nuclides with different half lives allowed us to infer that no significant variations in long term erosion rates have occurred and that at the western escarpment erosion rates have been low and constant for most of the late neogene nevertheless the time intervals necessary to reach steady state concentrations for cosmogenic nuclides can be quite different from those needed for landscapes to reach steady state

climatic versus tectonic control on river incision at the margin of ne tibet 10be exposure dating of river terraces at the mountain front of the qilian shan 1 we document late pleistocene holocene aggradation and incision processes at the mountain front of the qilian shan an active intracontinental fold and thrust belt accommodating a significant portion of the india asia convergence the shiyou river cuts through a nne vergent fault propagation fold with miocene red beds in the core and pliocene quaternary growth strata on the northern forelimb south of the anticline miocene strata dip 20 ssw suggesting a similar orientation for the basal decollement after aggradation of an 150 m thick late pleistocene valley fill the shiyou river formed three terraces the highest terrace located 170 m above the river constitutes the top of the fill the other terraces are fill cut terraces their treads are located 130 105 m and 37 m above the river respectively the 10be exposure dating of the terraces suggests that river incision accelerated from 0 8 0 2 mm yr 1 to 10 mm yr 1 at 10 15 kyr we interpret fast holocene river incision as largely unrelated to tectonic forcing the late pleistocene incision rate of 0 8 0 2 mm yr 1 places an upper limit of 2 2 0 5 mm yr 1 on the horizontal shortening rate assuming that incision is solely caused by rock uplift above a decollement dipping 20 however the actual shortening rate may lie between 2 2 mm yr 1 and zero because deformation of the terraces and the valley fill cannot be unequivocally demonstrated our estimate is consistent with the bulk shortening rate of 5 10 mm yr 1 across several faults in ne tibet derived from neotectonic and gps data although in case of the shiyou river holocene deformation is barely discernible owing to intense climate induced river incision

global cooling initiated stony deserts in central australia 2 4 ma dated by cosmogenic 21ne 10be stony deserts are durable indicators of aridity but until now have not been directly dated using 21ne and 10be produced in surface rocks by cosmic rays we show that australian stony deserts formed 2 4 ma at the time when global cooling initiated the quaternary ice ages and intensified aridity induced major landscape changes in central australia this is the first direct determination of stony desert ages using a new method for determining cosmogenic 21ne in the presence of various neon components from other sources

the 21ne production rate in quartz revisited abstract i have revised our earlier production rate determination of 21ne in terrestrial quartz s niedermann th graf j s kim c p kohl k marti and k nishiizumi 1994 earth planet sci lett 125 341 355 taking account of improved knowledge about the exposure age of samples used in that work and applying distinct procedures for scaling to sea level and high latitudes the corrected value of 19 0 3 7 atoms g 1 a 1 is in excellent agreement with expectations based on nuclear properties j masarik and r c reedy 1995 earth planet sci lett 136 381 395 and closely corresponds to the present day production rate in the case of substantial variations of the geomagnetic field in the past as proposed by several studies the revised value can consistently be used with suitable correction methods

calibration of cosmogenic 3he production rates from holocene lava flows in oregon usa and effects of the earth s magnetic field abstract we have measured cosmogenic 3he production rates in olivine phenocrysts from four radiocarbon dated holocene lava flows in oregon the flows span the period between 2 and 7 ka when there were significant fluctuations in the intensity of the earth s dipole moment our individual 3he production rate determinations are consistent with previous estimates and reinforce the feasibility of dating very young late holocene surfaces with the cosmogenic 3he method integrated cosmogenic 3he production rates exhibit small temporal variations during the holocene supporting predictions that production rates at mid latitudes are weakly affected by geomagnetic modulation of cosmic ray flux however the time varying difference between geographic and geomagnetic latitude caused by secular variation of dipole axis position may represent an important source of error as much as 5 in holocene surface exposure ages and production rate calibrations the best value for the integrated holocene production rate of cosmogenic 3he from calibration sites in this study is 116 3 atoms g 1 yr 1

multiple cosmogenic nuclides document complex pleistocene exposure history of glacial drifts in terra nova bay northern victoria land antarctica geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses 10be 26al and 21ne allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the east antarctic ice sheet eais and valley glaciers in the terra nova bay region glacially scoured coastal piedmonts with round topped mountains occur below the highest local erosional trimline they represent relict landscape features eroded by extensive ice overriding the whole coastal area before at least 6 ma pre dating the build up of the mt melbourne volcanic field since then summit surfaces were continuously exposed and well preserved under polar condition with negligible erosion rates on the order of 17 cm ma complex older drifts rest on deglaciated areas above the younger late pleistocene glacial drift and below the previously overridden summits the combination of stable and radionuclide isotopes documents complex exposure histories with substantial periods of burial combined with minimal erosion the areas below rounded summits were repeatedly exposed and buried by ice from local and outlet glaciers the exposure ages of the older drift s indicate multiple pleistocene glacial cycles which did not significantly modify the pre existing landscape

the ascent of felsic magmas in dykes abstract the results of a thermal and fluid dynamical analysis are presented that allow geologists to estimate the critical dyke or fracture width w c necessary for the transport of felsic magma through the crust without control by solidification the principal geological variables required to estimate w c are the magmatic viscosity the density and temperature contrasts and t between magma and country rock and the height of ascent h the results show that for typical values of and and for transport distances of 20 to 30 km the critical dyke width is constrained to lie between about 2 and 20 m

cosmogenic beryllium 10 and neon 21 dating of late pleistocene glaciations in nyalam monsoonal himalayas we present cosmogenic 10be and 21ne chronologies from 21 erratic boulders on three moraine sequences in nyalam county monsoonal himalayas southern margin of the tibetan plateau the surface exposure ages provide evidence for at least two distinct glacial advances during the late stage of the last glacial cycle and for one or more significantly older glaciations the distribution of cosmogenic ages from the three ridges of the old moraine sequence is inconsistent with their stratigraphic order because exposure periods of the erratics prior to deposition on the moraine surface is shown to be small the chronology stratigraphy mismatch suggests that the cosmogenic ages do not date moraine deposition but most likely significant moraine ridge denudation and related boulder exhumation after initial deposition of the moraines during the penultimate glacial cycle or earlier the surface exposure ages based on various currently accepted production rate scaling protocols yield age differences of up to 35 reflecting the poor knowledge of terrestrial cosmogenic production rates at low latitude high altitude sites even within this conservative uncertainty range our results do not yield evidence for late pleistocene glaciations in monsoonal tibet to be asynchronous to those in mid latitudes on both hemispheres there is an urgent need to improve the knowledge of terrestrial cosmogenic nuclide production rates and their scaling to low latitude regions to fully exploit the climate information archived in tropical moraine sequences

climatic influence in nrm and 10be derived geomagnetic paleointensity data abstract one can determine geomagnetic paleointensities from natural remanent magnetizations nrm and by inverting production rates of cosmogenic isotopes such as 10 be and 14 c recently two independently derived 200 kyr stacks y guyodo j p valet relative variations in geomagnetic intensity from sedimentary records the past 200 000 years earth planet sci lett 143 1996 23 36 m frank b schwarz s baumann p w kubik m suter a mangini a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10 be in globally stacked deep sea sediments earth planet sci lett 149 1997 121 129 were compared and the good agreement was suggested to validate the use of sedimentary cores for studies both compilations use mainly the astronomically forced and climatically controlled oxygen isotope stratigraphy to date and synchronize the sedimentary records while this very curve has several coherent features with the supposedly pure geomagnetic records an nrm relative paleointensity record which was included in the conventional paleointensity stack shows correspondence with climatic features which is explained by an inadequacy in the normalization technique therefore it is possible that the extraction of the pure paleointensity signal from marine sediments has not always been accomplished

cosmic ray produced 21ne in terrestrial quartz the neon inventory of sierra nevada quartz separates the study of cosmic ray produced radioactive and stable nuclides on the surface of the earth can provide relevant geomorphological and glaciological information at present the cosmic ray production rates of stable 21ne are not well known this study attempts to remedy the situation by determining the production rate ratio of 21ne and 26al p21p26 in quartz 26al concentrations and p26 rates have previously been investigated for quartz separates of sierra nevada rocks which were brought to the surface by glacial scouring during the tioga period at the end of the last ice age 1 we used splits of the same samples for our studies and found that ne in these rocks represents a mixture of several components trapped ne nucleogenic 21ne and 22ne produced by n reactions in oxygen and fluorine respectively as well as cosmic ray produced ne which is the component of interest in this study the trapped component was substantially lost in one sample w86 12 by crushing and by a density separation of the grain sizes 38 90 m and 90 125 m permitting the resolution of the in situ produced 21ne into cosmic ray spallation and n produced components and the determination of a lower limit to p21p26 in a second sample w86 8 one split contained small enough amounts of nucleogenic 21ne to permit the determination of a reasonable upper limit to p21p26 the two ratio determinations are consistent within error limits and the value adopted 0 65 0 11 2 agrees with ratios observed in extraterrestrial matter apparently p21p26 is thus not very sensitive to the neutron spectrum however the observed production rate ratio is substantially larger than theoretical estimates for si targets reflecting poorly known neutron excitation functions the above p21p26 value coupled to the observed 26al production rate 1 corresponds to a 21ne production rate of p21 21 atoms g 1 a 1 in quartz or to p21 45 atoms g si 1 a 1 at sea level and high latitudes this rate is based on an adopted exposure age of 11 000 yr for our quartz samples

a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10be in globally stacked deep sea sediments the reconstruction of geomagnetic field intensity variations during the last 200 kyr from paleomagnetic data is at present the subject of numerous studies and major debate there is currently no generally accepted record here we present a global stacked record of 230thex normalized 10be deposition in marine sediments representing relative variations in 10be production rate which are translated into field intensity variations the record shows major periods during which the field intensity was between 10 and 40 of the present day value namely 30 42 60 75 85 110 and 180 192 kyr b p our results are compared to independently derived paleomagnetic studies and th u calibrations of 14c dates on corals during most of the observed period the geomagnetic field intensity was weaker than today resulting in an overall 30 reduced value for the last 200 kyr

progressive crystallization of a granitoid diapir and its relationship to stages of emplacement the cannibal creek granite in northeastern australia consists of two adamellite plutons and several dykes each group representing two very similar batches of peraluminous magma the magma ascended through the crust and fractionated plagioclase biotite and zircon producing a zonation within the larger of the two plutons in these minerals and their constituent elements fe mg ca zr sr v ti and ba minor amounts of quartz and k feldspar were probably also fractionated during this period but not sufficiently to result in a detectable zonation across the main pluton at this stage the magma probably contained less than 50 crystals and hence was theologically weak which enabled it to ascend via fractures and by kinking and stoping the country rock slates the later stages of crystallization proceeded when the crystals formed a continuous framework they could not segregate from the melt and hence no zonation was developed in alkali feldspars or quartz or in si al na or k with falling temperat

improving in situ cosmogenic chronometers new radiocarbon ages for sierra nevada deglaciation the first 10 be measurements from the laurentide terminal moraine and calculations based on paleomagnetic field strength have the potential to substantially improve the accuracy of cosmogenic age estimates specifically three new constraints apply to the interpretation of measured abundances of in situ produced cosmogenic 10be and 26al 1 a suite of minimum limiting radiocarbon dates indicates that the sierra nevada was deglaciated at least several thousand years earlier than assumed when nishiizumi et al 1989 first calibrated 10be and 26 al production rates based on polished bedrock surfaces in the range with retreat beginning by 18 000 cal yr b p and completed by 13 000 cal yr b p 2 concentrations of 10be in moraine boulders and glacier polished bedrock in new jersey show little variance 10 1 and can be used to calculate a preliminary 10be production rate integrated over the past 21 000 22 000 cal yr b p at 41 200 300 m altitude that is about 20 lower than currently accepted 3 calculations of the effect of past geomagnetic field strength variations on production rates suggest that the use of temporally averaged production rates may generate age errors of 20 however cosmogenic exposure ages can be corrected for this effect although the corrections currently are imprecise many previously reported late pleistocene 10be and 26al exposure ages are probably too young and are less accurate and less precise than implied by reported uncertainties the discrepancy between accepted production rates and those calculated from laurentide exposures when considered together with the sierran deglacial chronology and the model results suggest that correlations between cosmogenic and other numerical ages especially for brief events like the younger dryas and heinrich events will not be robust until temporal variations and the altitude latitude scaling of production rates are fully understood and quantified at levels comparable to current analytic uncertainties 3

plio pleistocene history of ferrar glacier antarctica implications for climate and ice sheet stability abstract the areal distribution and elevation of glacial drifts in vernier valley southern victoria land are used to reconstruct the plio pleistocene history of upper ferrar glacier 21 ne cosmogenic nuclide analyses of surface cobbles on four moraines ferrar 1 2 3 and 4 provide age control a minimum age estimate for ferrar drifts calculated by assuming zero surface erosion indicates that the oldest moraine ferrar 4 was deposited at least 3400 ka our preferred age model which applies a very conservative erosion rate of 5 cm ma 1 in age calculations suggests that ferrar 4 is 4000 ka ferrar 3 is 1200 ka and ferrar 2 is 700 ka based on glacial geologic data ferrar 1 is modern cosmogenic ages for cobbles on this moraine suggest a value for nuclide inheritance of 50 ka the ferrar drifts are most easily interpreted in terms of a progressive reduction in the ice surface elevation of upper ferrar glacier during plio pleistocene time relative to today the surface of upper ferrar glacier was 100 to 125 m higher during the pliocene climatic optimum and 50 m higher during early to mid quaternary time conversely during mis 2 the ice surface elevation of upper ferrar glacier was likely no larger than today and may have stood below modern levels the texture and sedimentology of all ferrar drifts indicate that during ice recession from vernier valley the upper ferrar glacier lacked surface melting ablation zones even during the pliocene climatic optimum results from a simple 2 d glaciological flow band model demonstrate that upper ferrar glacier also lacked basal melting zones during ice recession we show that the development of weathering pits and desert varnish on cobbles exposed at the surface of ferrar drifts varies in accord with cosmogenic age the mean width and depth of the largest surface pits on boulders from ferrar drifts increases by 10 mm ma 1 and 6 7 mm ma 1 respectively the maximum thickness of desert varnish on surface boulders increases by 1 5 mm ma 1 these rates may be used to help calculate ages for dolerite rich drifts elsewhere in the western dry valleys region the general stability of the ice surface elevation of upper ferrar glacier and of the landscape in vernier valley suggests minimal climatic amelioration in the upland region of the dry valleys during the last 4 ma

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

the cadomian granites of mancellia northeast armorican massif of france relationship to the st malo migmatite belt petrogenesis and tectonic setting abstract the north armorican shear zone nasz divides the cadomian belt of the armorican massif of france into two major terranes the north armorican composite terrane nact and the central armorican terrane cat the nact is a pastiche of displaced blocks and terranes st brieuc terrane st malo terrane and mancellian terrane which results from the amalgamation of cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone in the nact the neoproterozoic brioverian succession was deformed and metamorphosed during the cadomian orogeny and intruded by syn to post tectonic plutonic complexes the st malo migmatite belt is a syn tectonic suite of anatectic migmatites derived through partial melting of the brioverian succession using the t test on whole rock geochemical data there is no evidence to suggest that a sample of st malo diatexites comes from a population with a different mean than some samples of the brioverian succession sandstones resulting homogeneous diatexites anatectic granites of the core were emplaced syn kinematically into cadomian strike slip shear zones thus the age of anatexis of 540 ma is also the date of strike slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the nact the mancellian granites to the southeast of the st malo migmatites were emplaced at 540 ma and are overlain unconformably by cambrian sediments overall geochemistry and a variety of specific geochemical parameters show that the mancellian granites are similar to the diatexites and anatectic granites of the st malo migmatite belt using the t test on whole rock geochemical data there is no evidence to suggest that the sample of mancellian granites comes from a population with a different mean than the sample of st malo diatexites additionally both suites of rocks are peraluminous they have similar distributions of rb sr and ba they have k rb ratios around 225 and they plot largely within the vag field using trace element discrimination diagrams for the interpretation of tectonic setting samples of the brioverian succession sandstones the st malo migmatites and the mancellian granites all exhibit similar primordial mantle normalized element patterns preliminary results of nd and sr isotope studies reveal a similar restricted range of values for nd between 4 0 and 7 3 and sr between 3 and 42 and depleted model ages in the range 1 7 to 1 5 ga tdmur for both mancellian granites and st malo migmatites isotopic features which set these rock types apart from other cadomian granites within the nact the st malo migmatite belt and the mancellian granites are coeval at 540 ma and both developed by anatexis at intermediate crustal depths of the brioverian succession within an inverted but only moderately overthickened behindarc basin in comparison with the st malo migmatites the mancellian granites represent farther travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source

measurement of ancient cosmogenic 21ne in quartz from the 28 ma fish canyon tuff colorado abstract measurement of 21 ne in quartz from the 28 ma buried fish canyon tuff of colorado yielded a maximum of 2 75 0 57 10 6 21 ne atoms g sio 2 1 above nucleogenic and modern concentrations this quantity represents cosmogenic 21 ne produced on the fish canyon tuff surface prior to the deposition of the overlying carpenter ridge tuff ancient 21 ne was isolated through the analysis and identification of 1 background 21 ne produced from particle interactions or inherited and 2 cosmogenic 21 ne produced from modern radiation multiple samples of fish canyon tuff shielded during both exposure periods 28 ma ago and in modern time yielded the background that was then subtracted from all samples likewise samples of fish canyon exposed today but well below the fish canyon carpenter ridge contact and therefore shielded 28 ma ago yielded a modern component that was also removed from all sample concentrations the presence of an ancient cosmogenic isotope in a buried surface has several potential applications including the quantification of paleoelevation paleolatitude and pre quaternary erosion rates

long term cosmogenic 3he production rates 152 ka 1 35 ma from 40ar 39ar dated basalt flows at 29 n latitude a set of time integrated cosmogenic 3he production rates in olivines for the last 1 35 ma are presented we investigated three basaltic lava flow tops from lanzarote canary islands spain the 40ar 39ar ages determined for those basalt flows by incremental laser heating of leached groundmass samples are 152 26 ka 281 19 ka and 1 35 0 01 ma 2 three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic 3he abundances the resulting 3he production rates in olivine at sea level at 29 latitude are 82 14 and 82 8 atoms g 1 a 1 as obtained from the 152 ka and 281 ka old flows respectively considering effects of erosion on the 1 35 ma old flow we find that the production rate of 82 8 atoms g 1 a 1 is consistent with the cosmogenic 3he production during the last 1 35 ma there appears to be a 14 discrepancy between previously published production rates derived at higher latitudes and altitudes if the scaling factors of lal are used to compare results this discrepancy is greatly reduced however if the revised scaling factors of dunai this issue are applied using the new scaling factors we derive a production rate for cosmogenic 3he in olivine at sea level and high latitudes 60 of 118 11 atoms g 1 a 1 2 the correspondingly revised value of cerling and craig and ackert et al is 123 6 atoms g 1 a 1 2 the mean value of these two calibrations is 121 6 atoms g 1 a 1 2 we suggest that the production rate of 121 6 atoms g 1 a 1 at sea level and high latitudes may be applied to the complete time range where paleomagnetic data indicate that there was no long term averaged intensity variation in the earth s magnetic field i e over the last 10 ma

a comprehensive study of noble gases and nitrogen in hypatia a diamond rich pebble from sw egypt this is a follow up study of a work by kramers et al 2013 on a very unusual diamond rich rock fragment found in the area of south west egypt in the south western side of the libyan desert glass strewn field this pebble called hypatia is composed of almost pure carbon transmission electron microscopy tem and x ray diffraction xrd results reveal that hypatia is mainly made of defect rich diamond containing lonsdaleite and multiple deformation bands these characteristics are compatible with an impact origin on earth and or in space we also analyzed concentrations and isotopic compositions of all five noble gases and nitrogen in several mg sized hypatia samples these data confirm the conclusion by kramers et al 2013 that hypatia is extra terrestrial the sample is relatively rich in trapped noble gases with an isotopic composition being close to the q component found in many types of meteorites 40ar 36ar ratios in individual steps are as low as 0 4 0 3 cosmicray produced cosmogenic 21ne is present in concentrations corresponding to a nominal cosmic ray exposure cre age of roughly 0 1 myr if produced in a typical meter sized meteoroid such an atypically low nominal cre age suggests high shielding in a considerably larger body in addition to the xe q composition an excess of radiogenic 129xe from the decay of short lived radioactive 129i is observed 129xe 132xe 1 18 0 03 two isotopically distinct n components are present an isotopically heavy component 15n 20 released at low temperatures and a major isotopically light component 15n 10 at higher temperatures this disequilibrium in n suggests that the diamonds in hypatia were formed in space rather than upon impact on earth 15natm 0 all our data are broadly consistent with concentrations and isotopic compositions of noble gases in at least three different types of carbon rich meteoritic materials carbon rich veins in ureilites graphite in acapulcoites lodranites and graphite nodules in iron meteorites however hypatia does not seem to be directly related to any of these materials but may have sampled a similar cosmochemical reservoir our study does not confirm the presence of exotic noble gases e g g component that led kramers et al 2013 to propose that hypatia is a remnant of a comet nucleus that impacted the earth

single grain cosmogenic 21ne concentrations in fluvial sediments reveal spatially variable erosion rates we evaluated the hypothesis that the spatial variation in erosion in a catchment is reflected in the distribution of the cosmogenic nuclide concentrations in sediments leaving the catchment using published data and four new 10be measurements in fluvial sediment collected from the outlets of small river catchments we constrained the spatial variability of erosion rates in the gaub river catchment in namibia we combined these catchment averaged erosion rates and the mean slope values with which they are associated in a digital elevation model dem based analysis to predict distributions of cosmogenic 21ne concentrations in the sediment leaving the gaub catchment we compared these synthetic distributions with the distribution of concentrations of cosmogenic 21ne 21nec in 32 quartz fluvial pebbles 16 21 mm collected from the catchment outlet the 21nec concentrations span nearly two orders of magnitude 2 6 160 106 atoms g and are highly skewed toward low values the dem based analysis confirms this skew the measured 21nec distribution plots within the envelope of distributions predicted for the catchment this match between measured and synthetic 21ne distributions implies that the measured distribution is a signature of the spatial variation in erosion rates

holocene secular variation in western north america paleomagnetic record from fish lake harney county oregon an 8 9 m composite section of lake sediment from fish lake steens mountain southeastern oregon has provided a high resolution record of holocene geomagnetic secular variation in western north america the section is composed of overlapping core segments with stratigraphic control based on six tephra layers and other thin distinct bands and with age control from 37 radiocarbon dates the 455 paleomagnetic samples possess a strong and stable magnetic remanence which is carried by pseudo single domain magnetite paleomagnetic records for individual core segments show low scatter and high serial correlation and where the segments overlap there is excellent agreement between records the azimuthal orientation of the composite section was recovered from the known direction of the geomagnetic field at the time of the climactic eruption of mount mazama tephra which is present in fish lake both internal and external evidence including the correspondence with an archaeomagnetic record from the southwestern united states support the conclusion that the fish lake record represents the history of geomagnetic field behavior during at least the last 10 000 years b p analysis of that record reveals that the average geomagnetic direction can not be distinguished from that of a geocentric axial dipole field and that the overall circularity of the field was clockwise but the detailed behavior was quite complex angular standard deviations of both field directions and virtual geomagnetic poles differ significantly from the values predicted from theoretical models of secular variation but are consistent with those of other holocene records when the record from fish lake is compared to the proposed type curve for east central north america there is excellent agreement for the past 6000 years provided the curves are offset by about 400 years this offset corresponds to a rate of westward drift of about 0 062 yr beyond about 6000 radiocarbon years ago good agreement between the two curves occurs when there is no offset suggesting that the field was standing rather than drifting during this time

the oligocene xanthi pluton northern greece a granodiorite emplaced during regional extension the xanthi pluton is one of a series of oligocene subduction related granodiorites in northern greece its emplacement was controlled by major faults it is located on the ene trending kavala komotini fault zone which probably originated as a strike slip fault geophysical data show that the pluton is laccolith shaped extending many kilometres south of the fault andesitic dykes several million years older than the pluton indicate a ne sw directed extensional stress field mineral lineations plunging to the sw reflect continued extension during cooling of the pluton subsequent jointing and dykes of aplite and lamprophyre reflect continued extension although this could result from extensional pull apart at a bend during dextral strike slip motion on the kavala komotini fault there is no evidence for subsequent strike slip deformation furthermore two similar laccolithic plutons are unrelated to the kavala komotini fault and the regional extent of thick oligocene sediments suggests an extensional environment during the extension that created a ramp space along the listric fault bounding the xanthi basin into which the granite was intruded the kavala komotini fault acted as a transfer fault reactivation of early joints occurred during late oligocene early miocene compression faults within the pluton parallel to the kavala komotini fault accompanied extension during neogene basin formation emplacement of subduction related magma into ramp space developed along listric faults to produce laccolith like plutons may be a common feature of back arc extension

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

an improved experimental determination of cosmogenic 10be 21ne and 26al 21ne production ratios in quartz abstract the confidence in surface exposure dating and related research such as erosion rate studies or burial dating strongly depends on the accuracy and precision of the currently used production rates of in situ produced cosmogenic nuclides reducing the uncertainties of nuclide production rates by more accurate calibrations with independently dated natural rock surfaces is crucial for further improving the quantification of earth surface processes here we use surface samples from the 760 2 ka old bishop tuff in eastern california to quantify the 10 be 21 ne and 26 al 21 ne production rate ratios in quartz our determination is based on 1 measured nuclide concentrations of cosmogenic 10 be 21 ne and 26 al 2 a conservative estimate for the erosion of the tuff in the volcanic tableland area which we base on our previously published 21 ne concentrations goethals m m niedermann s hetzel r fenton c r 2009 determining the impact of faulting on the rate of erosion in a low relief landscape a case study using in situ produced 21 ne on active normal faults in the bishop tuff california geomorphology 103 401 413 and a conservative estimate for the uncertainty of the 21 ne production rate and 3 the assumption of steady state erosion other assumptions such as the applied scaling procedure the muon contribution to nuclide production or the attenuation lengths of neutrons and muons in rock do not substantially affect the results based on 13 samples the following average production rate ratios and conservative uncertainty estimates are obtained for sea level high latitude open sky and rock surface 0 249 0 009 or 0 232 0 009 for 10 be 21 ne using 10 be half lives of 1 51 and 1 39 ma respectively and 1 80 0 09 for 26 al 21 ne for an 26 al half life of 0 72 ma the 10 be 21 ne and the 26 al 21 ne production ratios are consistent with currently used production rates but the ratios are much more precise than previous determinations the resulting 26 al 10 be production ratio of 7 23 0 45 for a 10 be half life of 1 51 ma or 7 76 0 49 for a 10 be half life of 1 39 ma is high when compared to previously published values we discuss reasons for this difference amongst them the possibility that 26 al analyses in general might be compromised by artefacts affecting the stable al concentration measurements when combined with a 10 be production rate of 5 01 at g 1 a 1 for the 1 51 ma half life or 4 61 at g 1 a 1 for the 1 39 ma half life our production ratios convert to 21 ne and 26 al production rates of 20 1 and 36 2 at g 1 a 1 or 19 9 and 35 7 at g 1 a 1 respectively

no evidence for strong fields during the r3 n3 icelandic geomagnetic reversal a previous study of the r3 n3 icelandic reversal which corresponds probably to the matuyama reunion reversal provided relatively large virtual dipole moments when the pole is close to equator to check these former results obtained using the shaw paleointensity method we resampled the very same sections and flows and reanalysed the samples using the thellier method 38 lava units and one baked horizon from 4 sections provide intermediate r3 n3 paleofield directions most of them possibly related to a single eruption display similar directions corresponding to the near equator vgp cluster unfortunately most of the intermediate lava flows are thermally unstable as attested by the irreversibility of the susceptibility versus temperature k t curves which precludes their use for paleointensity purpose the rest of the units provide fairly reversible k t curves and a near magnetite tc due to fe rich titanomagnetite resulting from oxyexsolution at high temperature 25 baked units or lava flows of this kind yield reliable thellier paleointensity data average paleostrengths of 44 7 14 9 t and 32 5 22 9 t were found for the reversed r3 and the normal n3 polarity zones respectively combining these non transitional paleostrengths with other thellier data from over the world indicates that the pliocene dipole moment was close to 8 1022 a m2 the average paleostrength is reduced to 11 3 3 0 t during the r3 n3 transition we find no indication for the strong fields 20 4 to 38 6 t previously obtained by the shaw method thus our results do not confirm the existence of large virtual dipole moments when the pole approaches the equator

cosmogenic 3he and 21ne measured in quartz targets after one year of exposure in the swiss alps abstract all currently used scaling models for terrestrial cosmogenic nuclide tcn production rates are based on neutron monitor surveys therefore an assumption underlying all tcn studies is that production rates are directly proportional to secondary cosmic ray intensities for all cosmogenic nuclides to test this crucial assumption we measured cosmogenic 3 he and 21 ne in artificial quartz targets after one year of exposure at mountain altitudes in the swiss alps the targets were inconel steel tubes containing 1 kg of artificial quartz sand 250 500 m degassed for one week at 700 c in vacuum prior to exposure from august 2006 until august 2007 ten of these targets were exposed at five locations in switzerland and italy zurich 556 m davos 1560 m santis 2502 m jungfraujoch 3571 m and monte rosa 4554 m additionally a sixth set of two blank targets was kept in storage and effectively shielded from cosmic ray exposure cosmogenic noble gases were measured at room temperature and at 700 c up to 9 of the cosmogenic 3 he was measured in the cold step indicating that 3 he diffuses out of quartz at room temperature on short time scales the remaining 3 he and all 21 ne were released at 700 c as shown by a repeat measurement at 800 c for the monte rosa target which yielded no additional cosmogenic helium and neon as expected the monte rosa target contained the highest cosmogenic nuclide content with 1 56 0 07 10 6 atoms of excess 3 he and 4 5 1 2 10 5 atoms of excess 21 ne all errors are 2 the raw measurements were corrected for non atmospheric blanks shielding roof container wall tritiogenic helium and solar modulation normalised to the average neutron flux over the past five solar cycles the 3 he 21 ne production rate ratio of 6 8 0 9 indicates that cosmogenic 3 he production by the container walls is negligible the main goal of the artificial target experiment was to determine the production rate attenuation length because all our targets had an identical design and were exposed under identical conditions all systematic errors cancel out in the calculation of an attenuation length our best estimates for the 3 he and 21 ne attenuation lengths are 134 8 5 9 g cm 2 and 135 25 g cm 2 respectively agreeing very well with currently used scaling models we conclude that tcn production rates are indeed proportional to neutron monitor count rates and that 3 he and 21 ne production rates follow the same altitudinal scaling relationships as the cosmogenic radionuclides finally the measurements were scaled to sea level and high latitude using the empirical attenuation length yielding weighted mean production rates of 107 6 6 6 at g yr for 3 he and 15 4 2 1 at g yr for 21 ne despite the significant uncertainties associated with the corrections for shielding solar modulation and especially the 3 he 3 h branching ratio these estimates are in good agreement with production rates derived from long term exposure experiments at natural calibration sites and physics based simulations

production rate of cosmogenic 21ne in quartz estimated from 10be 26al and 21ne concentrations in slowly eroding antarctic bedrock surfaces abstract we estimated the production rate of 21ne in quartz using a set of samples from slowly eroding sandstone surfaces in the antarctic dry valleys geologic evidence as well as cosmogenic 10be and 26al concentrations indicate that i these sites have experienced millions of years of surface exposure at low erosion rates and ii steady erosion has been sustained long enough that surface 10be and 26al concentrations have reached equilibrium with the erosion rate under these conditions surface cosmogenic 21ne concentrations should be a function only of the erosion rate and the 21ne production rate as the erosion rate can be determined from 10be and 26al concentrations this allows an estimate of the 21ne production rate estimating the reference 21ne production rate on this basis with the assumption that all 21ne production is by neutron spallation yields a poor fit to measured 21ne concentrations and a systematic residual that is correlated with the erosion rate of the sample site the same steady erosion assumption with a production model that includes production by deeply penetrating muons yields a good fit both to our measurements and to similar independent measurements from an antarctic bedrock core both data sets together yield a total reference 21ne production rate of 18 3 0 4 atoms g 1 a 1 of which 0 66 0 10 atoms g 1 a 1 is due to muon interactions

cosmogenic 21ne analysis of individual detrital grains opportunities and limitations we use a numerical model describing cosmogenic nuclide acquisition in sediment moving through the upper gaub river catchment to evaluate the extent to which aspects of source area geomorphology and geomorphological processes can be inferred from frequency distributions of cosmogenic 21ne 21nec concentrations in individual detrital grains the numerical model predicts the pathways of sediment grains from their source to the outlet of the catchment and calculates the total 21nec concentration that each grain acquires along its pathway the model fully accounts for variations in nuclide production due to changes in latitude altitude and topographic shielding and allows for spatially variable erosion and sediment transport rates model results show that the form of the frequency distribution of 21nec concentrations in exported sediment is sensitive to the range and spatial distribution of processes operating in the sediment s source areas and that this distribution can be used to infer the range and spatial distribution of erosion rates that characterise the catchment the results also show that lithology can affect the form of the 21nec concentration distribution indirectly by exerting control on the spatial pattern of denudation in a catchment model results further indicate that the form of the distribution of 21nec concentrations in the exported sediment can also be affected by the acquisition of 21nec after detachment from bedrock in the diffusive hillslope and or advective fluvial domains however for such post detachment nuclide acquisition to be important this effect needs to at least equal the nuclide acquisition prior to detachment from bedrock copyright 2009 john wiley and sons ltd

first results of cosmogenic dated pre last glaciation erratics from the montoz area jura mountains switzerland the jura mountains are a perialpine chain bending from the sw to the nw of switzerland alpine ice reached only marginal areas of this chain during the last glacial maximum lgm but remaining erratic boulders of alpine origin indicate older glaciations of unknown age tentatively correlated either with the classical rissian or with the most extensive glaciation presented here are the first results of dating these pre lgm boulders by using cosmogenic radionuclides 10be and noble gases 21ne both data sets are in good agreement within 1 error with radionuclide apparent exposure ages ranging from 60 to 107 ka and noble gas apparent ages from 73 to 123 ka considering the importance of erosion on older boulders the age of the erratics corresponds most probably to marine isotope stage 6 these results are encouraging for further studies on pre lgm erratic boulders in the mid latitudes

absolute paleointensity from hawaiian lavas younger than 35 ka abstract paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3 3 and 28 2 ka from the mauna loa volcano in the big island of hawaii determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in hawaii although the overall average field intensity appears to be lower since the present results were determined at higher temperatures than in the previous studies this discrepancy raises questions regarding the selection of low versus high temperature segments that are usually made for absolute paleointensity the virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows when averaged within finite time intervals the worldwide values match nicely the variations of the sint 200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period the convergence between the existing records at hawaii and the rest of the world does not favour the presence of persistent strong non dipole components beneath hawaii for this period

structural reconstruction and zonation of a tilted mid crustal magma chamber the felsic chemehuevi mountains plutonic suite structural relief resulting from middle tertiary extensional deformation in the chemehuevi mountains of california exposes a unique cross section through an extensive 280 km 2 calc alkalic compositionally zoned sill like granitic intrusion of late cretaceous age minimum estimates for emplacement pressure 4 to 6 kbar imply that the chemehuevi mountains plutonic suite was initially intruded at mid crustal depths and has undergone 10 to 15 of postemplacement tilting tectonic denudation and erosion reconstruction of the pre tertiary pre tilt configuration suggests that this metaluminous to peraluminous granitic suite exhibits crude normal vertical and temporal zonation from granodiorite to granite the zonation involves a decrease in age and an increase in silica away from the walls and roof the youngest and most evolved members being concentrated toward the center and floor of the intrusion the lower part of the intrusion had a flat floor which was penetrated by at least three feeder dikes providing magma to the chamber the chamber grew laterally in stages each pulse being more evolved than the preceding one structural reconstruction indicates that the roof is less than 1 km above the exposed top of the intrusion suggesting a tabular geometry with an aspect ratio of about 5 1 the magma apparently ponded along the contact between undeformed proterozoic basement above and subhorizontally foliated mylonitic gneisses below and was therefore structurally controlled this reconstruction provides information on the geometry of mesozonal intermediate to silicic magma chambers in addition the restoration provides a rare opportunity to observe crosscutting relations between different types of mid crustal structures thick mylonitic shear zones granitic intrusions and temporally unrelated detachment faults the geometry of which emphasizes the need for careful evaluation of seismic reflection profiles across complexly deformed and intruded continental crust

crustal scale en echelon p shear tensional bridges a possible solution to the batholithic room problem the apparent paradox of voluminous granitoid emplacement in an overall compressional magmatic are has been somewhat alleviated by the idea of pluton emplacement in tension cracks and dilational jogs within arc parallel strike slip fault systems as an alternative hypothesis we propose that an en echelon p shear array provides a continuous batholith scale zone of dilation along the trend of active magmatism and that it is favored by transpressional tectonics in the sierra nevada california the late cretaceous granitoids of the cathedral range intrusive epoch are consistent with emplacement into bridges between p shears because 1 they are elongated oblique to the magmatic trend consistent with p shear orientation within a dextral system 2 they were all emplaced within 5 10 m y along a 300 km trend resulting in a displacement rate of 1 cm yr across the arc and 3 syn to late magmatic dextral shear zones bound and crosscut the plutons as in a p shear tensional bridge

in situ cosmogenic 10be and 21ne in sanidine and in situ cosmogenic 3he in fe ti oxide minerals abstract we report concentrations of in situ cosmogenic 10 be and 21 ne from coexisting quartz and sanidine separates and of cosmogenic 3 he in coexisting fe ti oxide minerals from ignimbritic successions of northern chile oxaya and lauca ignimbrites new mineral isotope pairs such as sanidine and fe ti oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz pyroxene or olivine production rates in sanidine and fe ti oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz the experimentally determined production rates are compared to model production rates calculated with new cross sections for 3 he 21 ne and 10 be production from the individual target elements the mean experimental 21 ne production rate for five sanidine samples is 30 4 3 7 atoms g 1 yr 1 30 4 5 4 atoms g 1 yr 1 including the uncertainty of the 21 ne production rate in quartz p 21 ne qtz this is in excellent agreement with the modelled value of 28 3 atoms g 1 yr 1 which has an estimated uncertainty of 20 the 21 ne production rate in sanidine is thus about 50 higher than that in quartz the cosmogenic neon in sanidine is entirely released in low temperature steps 400 600 c and no signs of an interfering nucleogenic neon component were observed this is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies 3 he diffuses out of the sanidine structure preliminary results also indicate that sanidine is well suited for 10 be studies the mean experimental 10 be production rate from two sanidine samples is 4 45 0 38 atoms g 1 yr 1 4 45 0 42 atoms g 1 yr 1 including uncertainty of p 10 be qtz very close to the modelled value of 4 55 atoms g 1 yr 1 which has an estimated uncertainty of 20 we emphasize that 10 be and 21 ne production in sanidine is composition dependent therefore major element analyses should be carried out and production rates calculated on a sample by sample basis fe ti oxide minerals retain 3 he quantitatively experimentally derived production rates are in excellent agreement with new values derived from physical modelling the mean experimental 3 he production rate for five fe ti oxide minerals samples is 120 11 atoms g 1 yr 1 120 12 atoms g 1 yr 1 including the uncertainties of p 21 ne qtz compared to a mean modelled value of 124 atoms g 1 yr 1 which has an estimated uncertainty of 20 the variable chemical and structural composition of the solid solution lines of fe ti oxide minerals has little effect on the total 3 he production rate cosmogenic 21 ne is not produced in significant quantities in fe ti oxide minerals due to the absence of suitable target elements

emplacement kinematics of a granite diapir the chindamora batholith zimbabwe the geometric features of a large 50 40 km granitic batholith and its surrounding greenstone belt envelope are described the batholith is made up of several intrusive components and an intrusion sequence based on contact relations and xenoliths has been established the igneous rocks show a local foliation s fabric and preferred orientation of minerals the intensity of this fabric accords with the finite strain determined from xenolith shape analysis when the strain variations are analysed in detail it appears that the batholith was built up by a succession of magmatic pulses each of which led to a stretching of the previously consolidated igneous material so called balloon tectonics evidence is presented that xenolith deformation commences only when the surrounding igneous material has crystallized and a method is suggested whereby the position of the surface separating crystal supported phase plastic behaviour from melt supported phase liquid behaviour at different stages in the plutonic history can be determined ductile shear zones are present and their geometry accords well with the shape changes necessary to allow the outer parts of the pluton to stretch

multiple cosmogenic nuclides document the stability of the east antarctic ice sheet in northern victoria land since the late miocene 5 7 ma the timing and amplitude of changes in the antarctic ice level are relevant to understanding past climate fluctuations and ongoing changes in the global climate and sea levels in this study we present surface exposure ages based on in situ produced cosmogenic 10be and 21ne in the bedrock samples of glacially eroded relict surfaces from the deep freeze range northern victoria land the proximity of this region to the east antarctic ice sheet indicates that the area is sensitive to variations in inland ice volume permitting the investigation of the behavioural relationship between the east antarctic ice sheet and the alpine glacial system in northern victoria land dating erosional surfaces provides a precise chronology of northern victoria land paleoclimate evolution and allows us to correlate the east antarctic ice sheet response to global climate events and local ice level variations the 10be and 21ne concentrations from the highest peaks of the deep freeze range strongly indicate that the relict landscape features were continuously exposed for 5 7 ma denudation rates inferred from our data show that the erosion rate of the summits has been extremely low 5 cm ma for at least 5 7 ma along with evidence of persistent climate stability cold and arid conditions from other sectors of the transantarctic mountains our results indicate that the transition from the wet based to the cold based glacial regime in northern victoria land occurred after the creation of the polar east antarctic ice sheet in the middle miocene

low slip rates and long term preservation of geomorphic features in central asia in order to understand the dynamics of the india asia collision zone it is important to know the strain distribution in central asia whose determination relies on the slip rates for active faults1 2 3 4 5 many previous slip rate estimates of faults in central asia were based on the assumption that offset landforms are younger than the last glacial maximum 20 kyr ago 6 7 8 9 10 11 in contrast here we present surface exposure ages of 40 to 170 kyr obtained using cosmogenic nuclide dating for a series of terraces near a thrust at the northern margin of the tibetan plateau combined with the tectonic offset the ages imply a long term slip rate of only about 0 35 mm yr 1 for the active thrust an order of magnitude lower than rates obtained from the assumption that the terraces formed after the last glacial maximum our data demonstrate that the preservation potential of geomorphic features in central asia is higher than commonly assumed

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

dating pliocene lacustrine sediments in the central jordan valley israel implications for cosmogenic burial dating abstract cosmogenic burial dating of sediments is usually used at sites with relatively simple or known exposure burial histories such as in caves in an attempt to extend the applicability of the method to other common geological settings i e the dating of late neogene sedimentary formations where much less is known about the exposure burial history we apply the cosmogenic burial method on pliocene early pleistocene 1 5 4 5 ma lacustrine sediments in the central jordan valley israel 26al 10be and 21ne concentrations in quartz were obtained from a 170 m tectonically tilted section assuming fast burial and no post burial production we obtained burial ages which range between 3 5 and 5 3 ma integrating simple geological reasoning and the cosmogenic nuclide data post burial production is found to be insignificant we also found that the samples contain two distinct populations of grains chert and quartz from two different sources which experienced different pre burial exposure histories the cosmogenic nuclide concentrations in the samples are in accordance with those expected for the mixing of two sources and the burial ages computed for both end members agree theoretical calculations of two source mixing show that initial 26al 10be ratios are depressed relative to the expected surface ratios and may result in burial ages overestimated by as much as 500 ka using ages derived from cosmogenic nuclides independent age constraints and magnetostratigraphy we correlate the bottom of the section to the cochiti normal magnetic subchron 4 19 4 30 ma within the reverse gilbert chron and the top of the section to the reverse subchron at the top of the gilbert chron 3 60 4 19 ma

neotectonics and catastrophic failure of mountain fronts in the southern intra andean puna plateau argentina new cosmogenic nuclide exposure ages of rock avalanche deposits in front of sierra laguna blanca in the arid argentine puna plateau demonstrate the effects of westward propagation of reverse faulting and the influence of active faulting on slope oversteepening and subsequent gravitational collapse the 21 ne exposure ages of 8 superposed landslides with volumes 0 3 km 3 are between ca 150 and 430 ka and indicate an average recurrence interval of large gravitational mountain front failures of 27 5 k y for the ultimate 7 landslides landsliding did not occur after 150 ka along this sector of the sierra laguna blanca the termination of landsliding activity correlates with reverse fault propagation into the piedmont area of the sierra laguna blanca the exposure age of ca 130 ka of a tectonically uplifted terrace postdates initiation of fault activity in the piedmont region and the fault scarp height of 120 m indicates important continued uplift in the piedmont no further offsets occurred at the mountain front due to this lack of tectonic activity at the mountain front no further collapse has occurred indicating that in this environment catastrophic mountain front failure is mainly controlled by tectonically driven slope oversteepening

cryptic sr and nd isotopic variation across the leinster granite southeast ireland rb sr isotope whole rock data from the end caledonian leinster batholith define an errorchron age of 464 26 ma appreciably older than the accepted emplacement age of c 405 ma this anomalously old age is the consequence of a highly variable initial sr isotopic composition initial nd ratios and t dm model ages also show a wide range of values however these isotopic variations are neither randomly distributed nor related to petrographic changes but instead reflect in a simple way geographic position about the axis of the batholith the isotopic pattern revealed in the leinster batholith means that isochron ages obtained from s type granites may be largely dependent on the sampling scheme adopted the variable isotopic composition of the granite most likely resulted from either a heterogeneous metasedimentary source or hybridization of coeval dominantly crustal melts

the eruption of komatiites and picrites in preference to primitive basalts abstract many archaean greenstone and post archaean flood basalt provinces exhibit flows of komatiites or picrites in close spatial and temporal association with fractionated basalts it follows that picrites and komatiites can erupt through the same crustal sections in which basaltic magmas apparently pond and fractionate this is surprising since considerations of melt density alone would suggest that komatiites and picrites should be trapped and fractionated more easily than basalts some of the factors that control the ascent of magmas through the crust are examined from a theoretical viewpoint with particular reference to the eruption of komatiites and archaean basalts ascent in dykes is characterized by a local balance between the viscous pressure drop due to flow and the buoyancy forces due to the local difference in density between the melt and the host rock the level of neutral buoyancy of the magma and solidification in the dyke tip are identified as the key controls for the trapping of mafic melts in solid crust and the formation of crustal magma chambers it is suggested that komatiites and picrites can erupt in preference to basalts because they are less prone to freezing at the dyke tip during dyke propagation

assessing the age weathering correspondence of cosmogenic 21ne dated pleistocene surfaces by the schmidt hammer the schmidt hammer sh method is used to quantify the rock weathering degree and has been proposed as a relative dating tool terrestrial cosmogenic nuclide tcn methods provide absolute exposure ages for erosive surfaces few works combine both methods for surfaces older than the holocene we compare data obtained by both methods for c 150 ka bp glacial and fluvial erosive granite surfaces from northwest spain rebound values r have been firstly compared with the rock density to assess the correlation with the rock strength independently from influence of factors such as wetness and roughness in the r values for erosive glacial surfaces older than 100 ka r values are confined in a narrow range with no differences within errors stepped fluvial surfaces of 700 m to 70 m above present sea level show an inverse correspondence between tcn ages and r values although no age predictions can be done on the basis of the r values thus age inferences exclusively based on r values may not be realistic but sh studies could be a useful tool for selecting surfaces for tcn dating copyright 2009 john wiley sons ltd

the intensity of the time averaged geomagnetic field the last 5 myr the existing database for paleointensity estimates of the ancient geomagnetic field contains more than 1500 data points collected through decades of effort despite the huge amount of work put into obtaining these data there remains a strong bias in the age and global distribution of the data toward very young results from a few locations also few of the data meet strict criteria for reliability and most are of unknown quality in order to improve the age and spatial distribution of the paleointensity database we have carried out paleointensity experiments on submarine basaltic glasses from a number of dsdp sites of particular interest are the sites that provide paleointensity data spanning the time period 0 3 5 ma a time of relatively few high quality published data points our new data are concordant with contemporaneous data from the published literature that meet minimum acceptance criteria and the combined data set yields an average dipole moment of 5 49 2 36u10 22 am 2 this average value is comparable to the average paleofield for the period 5 160 ma 4 2 2 3u10 22 am 2 t juarez l tauxe j s gee and t pick 1998 nature 394 878 881 and is substantially less than the value of approximately 8u10 22 am 2 often quoted for the last 5 myr e g mcfadden

geomagnetic poles over the past 10 000 years locations of the geomagnetic pole over the past 10 000 years have been calculated by averaging the vgp positions obtained from paleomagnetic data the distribution of the geomagnetic pole was elongated to the direction parallel to the meridian of 45 and 225 longitude and westward movement of the pole was predominant throughout this period the time sequence of the polar motion can be divided into three intervals the intervals between ca 10 000 b p and ca 7000 b p between ca 7000 b p and ca 3700 b p and between ca 3700 b p and the present during the period between ca 7000 and ca 3700 b p the range of the movement of the geomagnetic pole was limited within 5 degrees around the geographical pole before and after this period the movement was very active fluctuating over 10 degrees the results of the last 2000 years show good consistency with the geomagnetic pole calculated from archaeomagnetic data by merrill and mcelhinny 1983

the geological significance of cosmogenic nuclides in large lowland river basins abstract measured from a bag of sand taken in large rivers the concentration of cosmogenic nuclides such as in situ produced 10be 26al and 14c can be used to constrain the mean sediment flux of the headwaters and assess the duration of sediment storage from source to sink we revisit these principles with examples from the amazon and ganga basins we identify two end member cases controlling the concentration of cosmogenic nuclides in lowland river sediment 1 if the time scale of floodplain sediment storage is short compared to the half life of the nuclide in situ cosmogenic nuclide concentrations are not significantly altered in lowland basins in this case the concentration of e g in situ produced 10be in the sediment taken in the lowland basin equals in most cases that exported from the sediment source area but the variability in nuclide concentrations between headwater streams is significantly averaged out thus to convert the measured river sediments in situ cosmogenic nuclide concentration into a catchment wide denudation rate production rates are scaled to include those of the sediment producing mountainous areas only rather than the entire catchment area nuclide production in the lowlands where no sediment is being produced is hence excluded this correction is termed floodplain correction 2 if sediment buried for periods of the order of the nuclides half life is episodically re entrained into the active river paired nuclides for example the ratio of 26al 10be through the differential decay of these isotopes constrain the the storage duration and re mixing of floodplain sediment as in situ cosmogenic nuclides measure denudation rates over longer time scales typically integrating between 103 and 105 years their combination with modern estimates of sediment fluxes from river load gauging offers a rich potential of deciphering the controls of earth surface fluxes across large basins from this combination we can assess how river systems react to external perturbations such as climate change or human interference and how such signals are transmitted through the alluvial reaches of lowland basins the most important results in both basins are that i lowland amazon and ganga nuclide concentrations indeed broadly reflect the spatial average of andean and himalayan denudation respectively ii the thus calculated sediment fluxes are within uncertainty of modern fluxes from sediment gauging indicating complete sediment delivery to the sea without net loss into the basin and iii only the amazon basin given its size contains a substantial fraction of sediment buried temporarily in the quaternary finally we provide a short introduction to the use of meteoric cosmogenic nuclide 10be and its ratio to stable 9be released by rock weathering in lowlands in the amazon basin we show that 10be concentrations in fine grained suspended loads depth integrated over the water column using al si ratios provide erosion rates whereas the 10be 9be ratio on be adsorbed to particles provides the denudation rate both are similar to denudation rates from in situ 10be we show that this system responds more sensitively to sediment storage than the in situ system and both accumulation and decay of meteoric concentrations may thus be used to determine sediment residence time

relative geomagnetic paleointensity and δ18o at odp site 983 gardar drift north atlantic since 350 ka abstract at odp site 983 relative geomagnetic paleointensity and planktic and benthic 18o records have been acquired for the last 350 kyr the mean sedimentation rate in this interval is 11 3 cm kyr magnetic properties and hysteresis ratios indicate that pseudo single domain magnetite is the remanence carrier volume susceptibility k anhysteretic arm and isothermal irm remanence values vary by a factor of 3 4 well within the criteria usually cited for paleointensity studies natural remanent magnetization nrm is normalized by arm and irm to acquire the paleointensity proxy arithmetic means of nrm arm and nrm irm calculated for five demagnetization steps in the 25 45 mt range constitute the relative paleointensity estimates some paleointensity lows particularly those at 40 120 and 188 ka are associated with directional excursions of the field especially the event at 188 ka referred to here as the iceland basin event that constitutes a short lived polarity reversal for the last 200 kyr the records can be correlated with other high resolution paleointensity records such as those from the labrador sea mediterranean somali basin and sulu sea implying that the millennial scale features are globally synchronous a labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages

the timing of glacier advances in the northern european alps based on surface exposure dating with cosmogenic 10be 26al 36cl and 21ne

21ne versus 10be and 26al exposure ages of fluvial terraces the influence of crustal ne in quartz abstract the accuracy of 21 ne surface exposure ages depends critically on the correction for a trapped ne component commonly the amount of cosmogenic ne used to calculate 21 ne exposure ages is considered to be the ne excess relative to a trapped component of atmospheric composition 21 ne 20 ne 0 00296 here we document a trapped ne component in quartz samples from a series of river terraces at the northern margin of tibet hetzel et al nature 417 2002 428 432 which has a non atmospheric 21 ne 20 ne ratio varying from 0 00299 to 0 00398 vacuum crushing of amalgamated samples each derived from 30 80 quartz clasts revealed that the non atmospheric trapped component is present in fluid inclusions it has probably been incorporated into the quartz crystals during their growth in veins at low grade metamorphic conditions only if the amount of cosmogenic 21 ne is determined relative to this trapped component are the calculated 21 ne exposure ages consistent with the relative ages of the tectonically induced terraces and in agreement with independent 10 be and 26 al exposure ages of the fluvial terraces a significant in situ production of nucleogenic 21 ne by the reaction 18 o n 21 ne in the paleozoic quartz minerals is ruled out by extremely low u contents of the order of 1 5 ppb all ages have been corrected for an inherited cosmogenic component that results from cosmic ray exposure during erosion of the host rock and transport of the clasts to the terraces the origin of the trapped component from crustal fluids is supported by high 40 ar 36 ar ratios of 2000 6000 and slightly elevated 136 xe 132 xe ratios relative to air which can be explained by radioactive decay of 40 k and spontaneous fission of u in the crust

origin structure and exposure history of a wave cut platform more than 1 ma in age at the coast of northern spain a multiple cosmogenic nuclide approach abstract along the asturian coast of northern spain an uplifted wave cut platform extends for 100 km east west the steep cliff which bounds the gently seaward dipping platform to the north increases in height from 30 m in the west to 100 m in the east and reflects the overall eastward increase in platform elevation the southern edge of the 2 4 km wide platform runs along the foothills of the cantabrian mountains as constrained by a high resolution digital elevation model the marine platform which was carved into deformed paleozoic bedrock with abundant quartzite beds is largely covered by weathered marine and continental sediments quartzite samples from flat bedrock outcrops which are currently not covered by sediment or soil yield cosmogenic nuclide concentrations 21ne 10be and 26al that demonstrate a long and complex exposure history including periods of burial with partial or complete shielding from cosmic rays the combination of multiple cosmogenic nuclides yields a minimum age of 1 2 ma for the platform taking into account i the horizontal and vertical extent of the platform ii the high resistance to erosion of the quartzitic bedrock and iii published data on the magnitude of past sea level fluctuations we suggest that the wave cut platform formed in the pliocene subvertical faults cutting the platform at high angles to the coastline offset the southern edge of the platform by 20 to 40 m and reactivate the pre existing anisotropy in the paleozoic bedrock uplift and crustal deformation of the coastal region have occurred after platform formation in the pliocene and may still be active the slow deformation of the northern edge of the iberian plate including the cantabrian mountains may result from the ongoing slow convergence at an incipient subduction zone extending along the coast of northern spain

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

a comparison of rift zone tectonics in iceland and hawaii observations of eroded volcanic rift zones indicate that dikes in iceland are typically several times thicker than those in hawaii geodetic and seismic observations of active rifts however suggest that dike heights in the two regions are similar provided the elastic properties of the rift zones are the same this implies that dikes are intruded with higher driving pressures magma pressure minus compressive stress perpendicular to the dike plane in iceland than hawaii a second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in iceland from this it can be infered that a lower percentage of dikes breach the surface in iceland than in hawaii thus although dikes in iceland are intruded with higher driving pressures they possess lower absolute magma pressures than in hawaii these differences can be interpreted in terms of the tectonic settings in the two regions in iceland a steady remote extension reduces the horizontal stress perpendicular to the rift zone allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available in hawaii a more continuous magma supply on the timescale over which the dike induced stresses are relaxed and perhaps a greater role for intrusions in driving long term rift extension ensure that the rift compressive stress is not relaxed significantly before the next dike is intruded thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur if the mechanism for relaxing the rift compressive stress were less efficient still then an even higher percentage of dikes would erupt and at times the rift zone trend could become an unfavorable orientation for dike intrusion such might be the case at mauna loa which lacks large rift zone faults and fissures and possesses numerous radial vents outside its two main rift zones

scaling factors for production rates of in situ produced cosmogenic nuclides a critical reevaluation abstract new scaling factors are presented describing the altitude and latitudinal dependence of production rates for in situ produced cosmogenic nuclides the new factors incorporate the influence of the non dipole contributions to the geomagnetic field on the cosmic ray flux the currently used scaling factors of lal earth planet sci lett 104 1991 424 439 are based on the assumption of a dipole field the overall strategy used in deriving the new scaling factors is however very similar to that of lal and relies on the same and or similar nuclear data in this reevaluation data are only used if the effective geomagnetic parameters inclination horizontal field strength can be reconstructed for the time of measurement the absorption free pathlengths for cosmic rays selected for this study are based on observational data at altitudes relevant for exposure age dating sea level to 7000 m at sea level and latitudes between 20 and 40 the new factors are up to 18 lower and at high altitudes more than 30 higher than those of lal in addition to accounting for the influence of the effective geomagnetic field and providing more applicable estimates of the new factors also allow for correction for the significant deviations from the standard pressure altitude relationship that exist in the atmosphere 10

glacial advances in tibet during the younger dryas evidence from cosmogenic 10be 26al and 21ne was the abrupt climate cold reversal called the younger dryas yd a global climate excursion this study presents the first evidence for yd glacial advances on the tibetan plateau derived from 10be 26al and 21ne exposure ages of two erratics with a weighted mean age of about 11 600 years bp from a moraine at the eastern margin of the tibetan plateau this implies that the yd climate signal commonly thought to be initiated by decreased north atlantic deep water formation and transferred by ocean circulation was even recorded at the inner continental high altitude site of tibet implying atmospheric transport and thus a global character for this abrupt climatic change

determining the impact of faulting on the rate of erosion in a low relief landscape a case study using in situ produced 21ne on active normal faults in the bishop tuff california abstract normal faults exposed in the volcanic tableland in the southeastern part of the bishop tuff provide a unique opportunity to investigate whether tectonic faulting and associated fracturing control the rate of erosion in the vicinity of actively growing faults if fracturing increases with fault displacement erosion close to the faults should be more rapid for faults with higher displacement here we use in situ produced cosmogenic 21 ne to quantify the erosion rate in the flat footwall blocks of three normal faults that have maximum vertical displacements between 12 and 160 m 21 ne concentrations in surface samples from volcanic bedrock and desert pavements along the three faults are nearly uniform and demonstrate that erosion is spatially invariant both along individual faults and between faults with different tectonic offsets hence faulting and fracturing of rocks adjacent to the active normal faults in the bishop tuff do not have a significant impact on the rate of erosion in the sub horizontal footwall blocks next to the faults this is further supported by samples from sites which are unaffected by faulting but record a similar erosion rate as samples located near faults using the density of the bishop tuff and a published 40 ar 39 ar eruption age for the tuff 760 2 ka the absolute amount of erosion since eruption has been quantified at most sample sites the time integrated amount of erosion ranges between 0 9 and 1 9 m which is equivalent to average erosion rates between 1 2 and 2 5 m ma samples of bedrock derived clasts from desert pavements have slightly higher 21 ne concentrations than adjacent bedrock outcrops and thus yield higher apparent exposure ages calculated assuming no erosion thus desert pavements are more suitable for determining the minimum eruption ages of yet undated welded tuffs than bedrock exposures desert pavements from the bishop tuff yield exposure ages that are 60 of the tuff s 40 ar 39 ar eruption age

a new mechanism of granite emplacement intrusion in active extensional shear zones one of the principal ways in which continental crust grows is by the incorporation from deeper within the earth of large volumes of granitic magma a space problem exists1 as to how these magmas are accommodated in the crust traditionally two main emplacement mechanisms have been emphasized forceful intrusion whereby buoyancy driven magmas physically push the crust aside creating granitic diapirs and balloons and passive emplacement characterized by replacive mechanisms such as cauldron subsidence and stoping although more recent work2 3 has demonstrated that space for granites may be created within bends and offsets of large transcurrent faults the simple view of either forceful or passive still fails to account for the intrusive mechanisms of many granites here we report the discovery of a new igneous intrusion mechanism in the superbly exposed proterozoic continental crust of south greenland we have observed that rapakivi granite was intruded as large scale sheets along ductile extensional shear zones that were active during emplacement in such a process the space problem seems to be simply resolved

cosmogenic and nucleogenic 3he in apatite titanite and zircon cosmogenic 3he was measured in apatite titanite and zircon and cosmogenic 21 ne in quartz at 13 depth intervals in a 2 7 m long drill core in a miocene ignimbrite from the altiplano of bolivia all three 3he depth profiles as well as the 21 ne profile attenuate exponentially with depth indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources the attenuation lengthscale for 3he production of 180 11 g cm 2 is consistent with expectations for neutron spallation and is identical to that found for the cosmogenic 21 ne in quartz by normalizing the measured 3he concentrations to 21 ne and using the independently known cosmogenic 21 ne production rate the apparent cosmogenic 3he production rates in apatite titanite and zircon were respectively found to be 112 97 and 87 atoms g yr at sea level and high latitude the formal uncertainty on these estimates is 20 2 and arises in equal parts from uncertainties in the measured 3he 21 ne ratios and the uncertainty in the 21 ne production rate however an additional factor affecting the apparent 3he production rate in these phases arises from the long stopping range of spalled 3he and tritium which decays to 3he because all three accessory phases have higher mean atomic number than major rock forming minerals they will have lower 3he production rates than their surroundings as a consequence the long stopping ranges will cause a net implantation of 3he and therefore higher apparent production rates than would apply for purely in situ production thus these apparent production rates apply only to the specific grain sizes analyzed analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size from 50 to 100 m cross section yields a 10 decrease in apparent production rate while this effect warrants further study the grain sizes analyzed here are typical of the accessory phases commonly encountered so the apparent rates provide an appropriate starting place for surface exposure dating using 3he in these minerals

the limited influence of glaciations in tibet on global climate over the past 170 000 yr extensive ice cover on the tibetan plateau would significantly influence earth s climate in general and the asian monsoon system in particular but extent and timing of quaternary glaciations in tibet remain highly controversial we dated erratics on top of moraines in the climatic key areas of central and east tibet using cosmogenic 10be 26al and 21ne consistent exposure ages obtained by various nuclides indicate a continuous period of exposure since the deposition of the samples our data imply that glacial advances were restricted to a few 10 km during the last 170 kyr in central tibet and during the peak of the last glaciation 24 13 kyr ago in eastern tibet advances of tibetan glaciers were much less prominent than elsewhere in the northern hemisphere most likely due to very arid conditions and high sublimation rates a proposed ice dome covering the entire plateau can be excluded thus albedo increase of tibet most likely did trigger neither northern hemisphere ice ages nor paleomonsoon changes during the last two glacial cycles the glacial advance during marine isotope stage 2 in east tibet and the absence of significant glacial events during the holocene suggest a relation of snowline lowering in east tibet to north atlantic cooling events rather than to periods of high precipitation by an intensified monsoon

young displacements on the atacama fault system northern chile from field observations and cosmogenic 21ne concentrations 1 we present the first numerical age constraint for young deformation of the atacama fault system afs in northern chile the young activity of the afs is expressed by several fault scarps which affects alluvial fan sediments of the eastern side of the coastal cordillera 23 30 23 42 s detailed mapping of alluvial fans reveals a complex relationship between fault motion erosion and alluvial fan development an older group of alluvial fans became inactive prior to the scarp formation younger alluvial fans arising directly from feeder channels and entrenched in the fault scarps posts date the scarp formation the youngest slip on the afs is recorded by headward eroding channels entrenched across the scarp which are in turn displaced vertically 0 3 0 5 m by the fault quartz fragments in four sites on the older inactive fan group were analyzed for cosmogenic 21ne concentrations yielding an average age of 424 151 ka the upper limit for the recent activity of the fault combined with the height of fault scarp we calculate a 0 01 mm yr minimum vertical fault slip rate thus young displacement on the afs is quaternary in age and confined to the late pleistocene