prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

on the genesis and evolution of barchan dunes morphodynamics barchan dunes are crescent shaped formations of sand that can dominate both desert and subaqueous landscapes when the supply of sand is scarce because of the complexity and scale of the underlying phenomena the mechanisms governing the entire process from the genesis to the long term evolution of barchan fields still remain to be better understood herein we attempt to present a description of this process in a subaqueous environment by employing a large eddy simulation approach that couples turbulent flow and sand bed morphodynamics we show that the seeds of the emergent structure in barchan fields are random turbulent flow motions near the initially flat bed we also provide high resolution insights into phenomena such as barchan migration and merging and show how transverse sand waves are formed and migrate over the barchan horns furthermore the transverse sand waves over the barchan horns are shown to be the seeds of the newly born barchans at the end points of the two horns of a barchan through the process known as calving to show this we examine the celerity wavelength and amplitude of the transverse sand waves over the barchan as they approach the end of its horn the celerity and wavelength of these transverse sand waves are shown to be the defining factors in determining the frequency of the calving process the amplitude of the newly born barchans through calving is also shown to be associated with the amplitude of the transverse waves near the end of the horn the simulation data also show that the wavelength of the newly born barchans the distance between individual dunes is closely related to that of the transverse sand waves over their maternal barchan finally we use the simulation results to discuss past conclusions derived from theory conventional models and field observations

a refined astronomically calibrated 40ar 39ar age for fish canyon sanidine abstract intercalibration between the astronomical and radio isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated multi chronometer geologic timescale here we report a high precision 40 ar 39 ar age for the fish canyon sanidine fcs neutron fluence monitor by multi collector noble gas mass spectrometry through cross calibration with a1 tephra sanidines a1ts of the direct astronomically tuned faneromeni section crete the astronomically intercalibrated 40 ar 39 ar age of fcs of 28 172 0 028 ma 2 external errors is within the uncertainty of but more precise 0 10 than the previous 40 ar 39 ar age determined by intercalibration with astronomically tuned tephras from the melilla basin morocco using this proposed age for fcs combined with measurements using the a1ts as the neutron fluence monitor a weighted mean bishop tuff 40 ar 39 ar sanidine age of 0 7674 0 0022 ma 2 external errors is indistinguishable from the id tims u pb zircon age 0 7671 0 0019 ma the consistency between the astronomically calibrated 40 ar 39 ar sanidine age and u pb zircon age for this quaternary unit suggests that agreement between these two radio isotopic dating techniques is now achievable at better than 0 3 2 in the youngest part of geologic time

timing of normal faulting along the indus suture in pakistan himalaya and a case of major 231 pa 235 u initial disequilibrium in zircon we report age data by tims u pb la icp ms and 39ar 40ar techniques for main magmatic events in the lower swat region of pakistan in order to constrain the tectonic evolution of the northwestern himalaya the pre himalayan history of the indian continent is documented by single zircon u pb results from the peraluminous choga granite gneiss which yielded a 468 5 ma lower concordia intercept interpreted to approximate the time of magmatic emplacement the presence of a well defined 870 7 ma inherited component upper intercept suggests a plutonic or volcanic protolith residing at unexposed levels of the indian crust zircon data for the swat granite gneiss from the northern part of the loe sar dome give an emplacement age of 267 6 3 ma which is at variance with earlier correlations favoring an early paleozoic origin subsequent to metamorphic overprint by the himalayan orogeny the swat granite was intruded by late kinematic alkali granite dykes single zircons from one of these dykes show reproducible 206pb 238u data giving a precise mean age of 29 26 0 12 ma whereas the 207pb 235u ages scatter between 34 and 81 ma pointing to huge and variable enrichment in 207pb the unsupported 207pb can be explained by incorporation of 231pa an intermediate long lived daughter nuclide in the 235u decay chain in excess of the secular equilibrium ratio la icp ms measurements confirm the presence of unsupported 207pb but do not show any correlation between the latter and other selected trace element concentrations in these zircons a concordant ar ar muscovite age of 28 4 1 1 ma obtained for the same dyke postdates regional mica cooling ages and indicates a lack of younger regional events capable of resetting the k ar system in muscovite because the dyke is pre to synkinematic relative to normal faulting and related north vergent folds its emplacement age provides a maximum age for this event or reflects already ongoing local extension a lower limit of 15 ma has previously been established by apatite fission track analysis the 29 15 ma age bracket for normal faulting is coeval with extension along the south tibetan detachment system stds on the north side of the high himalaya this suggests that the indus suture in pakistan has acted as a western continuation of the stds and that related faulting was roughly contemporaneous over most of the himalaya

high precision zircon u pb geochronology of astronomically dated volcanic ash beds from the mediterranean miocene several orbitally tuned miocene sedimentary sequences around the mediterranean contain abundant intercalated volcanic ash beds these sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology we present a large data set n 16n 16 n 166n 166 of zircon u pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims techniques for ash beds from an almost continuous orbitally tuned messinian to langhian 6 2 15 4 ma sedimentary sequence exposed along the adriatic coast south of ancona italy we use this unique data set to evaluate 1 the accuracy of zircon u pb dates 2 the significance of initial intermediate daughter product disequilibria for zircon u pb geochronology of young rocks 3 the effect of prolonged pre eruption zircon crystallization and zircon recycling on u pb derived ash bed deposition ages and 4 discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the geologic time scale

sub millennial eruptive recurrence in the silicic mangaone subgroup tephra sequence new zealand from bayesian modelling of zircon double dating and radiocarbon ages abstract accurate dating of young 38 4 1 4 1 7 ka bp and constrains the beginning of the msg eruption period to 42 7 3 5 3 7 ka bp unit a and the end to 30 6 1 5 0 6 ka bp unit l thus the entire msg sequence was emplaced in 12 100 years representing an eruption frequency of one event per 930 years on average our study demonstrates the efficacy of zdd to yield accurate eruption ages on pyroclastic deposits highlighting its potential for dating young

field measurements of intertidal bar evolution on a high energy beach system nearshore bars play a pivotal role in coastal behaviour helping to protect and restore beach systems particularly in post storm conditions examination of bar behaviour under various forcing conditions is important to help understand the short to medium term evolution of sandy beach systems this study carried out over a nine week period examines the behaviour of three intertidal bars along a high energy sandy beach system in northwest ireland using high frequency topographic surveys and detailed nearshore hydrodynamic modelling results show that in general there was onshore migration for all the bars during the study period despite the variability observed between bars which was driven mostly by wave dominated processes under the prevailing conditions migration rates of up to 1 83 m day 1 and as low as 0 07 m day 1 were observed during higher wave energy events the migration rates of the bars decelerated in their onshore route however under lower wave energy conditions they quickly accelerated maintaining their shoreward migration direction tidal influence appears to be subordinate in these conditions being restricted to moderating the localized wave energy at low tides and in maintaining runnel configurations providing accommodation space for advancing slip faces the study highlights the intricate behavioural patterns of intertidal bar behaviour along a high energy sandy coastline and provides new insights into the relative importance of wave and tidal forcing on bar behaviour over a relatively short time period copyright 2016 john wiley sons ltd

sediment erosion by gortler vortices the scour hole problem reference lhe article 2004 010view record in web of science record created on 2007 09 04 modified on 2016 08 08

large eddy simulation of interacting barchan dunes in a steady unidirectional flow we have performed large eddy simulations of turbulent flow 4 over barchan dunes in a channel with different interdune spacings in the downstream direction at reynolds number re 26000 based on the free 6 stream velocity and channel height simulations are validated against ex perimental data at re 55460 the largest interdune spacing 2 38 where is the length of the barchan model presents similar characteristics to the isolated dune in the experiment indicating that at this distance the sheltering effect of the upstream dune is rather weak we examine 3d realizations of the mean and instantaneous flow to explain features of the flow field relevant to sediment transport barchan dunes induce two counter rotating streamwise vortices along each of the horns which direct high momentum fluid toward the symmetry plane and low momentum fluid near the bed away from the centerline the flow near the bed upstream of the dune diverges from the centerline plane decelerates and then rises on the stoss side of the dune while accelerating the flow close to the centerline plane separates at the crest and reattaches on the bed away from the centerline plane and along the horns flow separation occurs intermittently the flow in the separation bubble is routed towards the horns and leaves the dune at their tips the separated flow at the crest reattaches on the bed except on the centerline symmetry plane of the dune where a weak saddle point of separation ap pears at the bed the distribution of the bed shear stress characteristics of the separation and reattachment regions and instantaneous wall turbulence are discussed characteristics of the internal boundary layer developing on the bed after the reattachment region are studied the interdune spacing isfound to affect significantly the turbulent flow over the stoss side of the downstream dunes at smaller interdune spacings coherent high and low speed streaks are shorter but stronger and the spanwise normal reynolds stress is larger the turbulent kinetic energy budgets show the importance of the pressure transport and mean flow advection in transporting energy from the overlying wake layer to the internal boundary layer over the stoss side of the closely spaced dunes the characteristics of the separated shear layer are altered slightly at smaller interdune spacing the separation bubble is smaller the separated shear layer is stronger and the bed shear stress is larger away from the dunes typical wall turbulence structures are observed but coher ent eddies generated in the separated shear layer due to the kelvin helmholtz instability are dominant near the dune coherent structures are generated more frequently at smaller interdune spacing they move farther away from the bed towards the free surface and remain in between the horns at larger interdune spacings these coherent structures are advected in the spanwise direction with the mean streamwise vortices and can be observed outside of the dunes

measurements of the aeolian sand transport saturation length the wavelength at which a dune pattern emerges from a flat sand bed is controlled by the sediment transport saturation length which is the length needed for the sand flux to adapt to a change in wind strength the influence of the wind shear velocity on this saturation length and on the subsequent dune initial wavelength has remained controversial in this paper we present direct measurements of the saturation length performed in a wind tunnel experiment complementary to this initial dune wavelengths are measured under different wind conditions in particular after storms using the linear stability analysis of dune formation it is then possible to deduce the saturation length from field data both direct and indirect measurements agree that the saturation length is almost independent of the wind strength this result supports the idea that grain inertia is the dominant dynamical mechanism limiting sediment transport saturation on dunes

barchan dune corridors field characterization and investigation of control parameters 1 the structure of the barchan field located between tarfaya and laayoune atlantic sahara morocco is quantitatively investigated and compared to that in la pampa de la joya arequipa peru on the basis of field measurements we show how the volume the velocity and the output sand flux of a dune can be computed from the value of its body and horn widths the dune size distribution is obtained from the analysis of aerial photographs it shows that these fields are in a statistically homogeneous state along the wind direction and present a corridor structure in the transverse direction in which the dunes have a rather well selected size investigating the possible external parameters controlling these corridors we demonstrate that none among topography granulometry wind and sand flux is relevant we finally discuss the dynamical processes at work in these fields collisions and wind fluctuations and investigate the way they could regulate the size of the dunes furthermore we show that the overall sand flux transported by a dune field is smaller than the maximum transport that could be reached in the absence of dunes i e in saltation over the solid ground

subaqueous barchans and plane beds from deposition of quartz silt abstractthe suspension flow of quartz silt geometric mean grain size of 4 15 m in unfavorable conditions for deposition is the motivation for the development of different bed morphologies particles deposit over a plane nonerodible surface and develop well defined barchan ripples barchanoids and plane beds in flume experiments bedload transport of quartz silt by bedform migration is several orders of magnitude smaller than the suspended transport the final bed morphology is controlled by the suspended sediment concentration and running time of the experiment the average dimensions of the bedforms after 1 day experiments are 4 93 cm wide 10 33 cm long and 0 45 cm high cohesive plane beds appear after 2 day and 3 day experiments with very high sediment concentrations 22 5 kg m3 viscous effects are deemed relevant for the formation of the beds

late pleistocene zircon ages for intracaldera domes at golcuk isparta turkey abstract pleistocene to quaternary volcanism in the isparta region sw anatolia turkey comprises potassic lavas and pyroclastic deposits which are largely centered around golcuk caldera trachytic intracaldera lava domes represent the latest eruptive event at golcuk and their eruption age is crucial for defining a minimum age for the preceding caldera forming explosive eruption here we present combined u th and u th he zircon geochronological data for two intracaldera lava domes constraining their crystallization and eruption ages respectively u th zircon crystallization ages peak between ca 15 and 25 ka in rare instances u th zircon crystallization ages date back to ca 59 and 136 ka u th zircon crystallization ages also permit u th he eruption ages from the same crystals to be individually corrected for uranium series decay chain disequilibrium which is mainly due to the deficit of the intermediate daughter 230 th in zircon average disequilibrium corrected u th he zircon ages are 14 1 0 5 and 12 9 0 4 ka 1 these ages are indistinguishable within analytical uncertainties suggesting that both lavas erupted quasi simultaneously this contradicts published k ar ages that suggest an extended hiatus from ca 52 to 24 ka between intracaldera dome eruptions evidence for protracted zircon crystallization over several thousands of years prior to eruption indicates the presence of a long lived magma reservoir underneath golcuk caldera implications of the revised eruptive geochronology presented here include younger ages for the latest effusive eruptions at golcuk and potentially also a more recent explosive eruption than previously assumed

robust classification for the joint velocity intermittency structure of turbulent flow over fixed and mobile bedforms two datasets of turbulence velocities collected over different bedform types under contrasting experimental conditions show similarity in terms of velocity intermittency characteristics and suggest a universality to the velocity intermittency structure for flow over bedforms one dataset was obtained by sampling flow over static bedforms in different locations and the other was based on a static position but mobile bedforms a flow classification based on the velocity intermittency behaviour is shown to reveal some differences from that based on an analysis of reynolds stresses boundary layer correlation and turbulent kinetic energy this may be attributed to the intermittency variable which captures the local effect of individual turbulent flow structures locations in the wake region or the outer layer of the flow are both shown to have a velocity intermittency behaviour that departs from that for idealized wakes or outer layer flow because of the superposition of localized flow structures generated by bedforms the combined effect of this yields a velocity intermittency structure unique to bedform flow the use of a time series of a single velocity component highlights the potential power of our approach for field numerical and laboratory studies the further validation of the velocity intermittency method for non idealized flows undertaken here suggests that this technique can be used for flow classification purposes in geomorphology hydraulics meteorology and environmental fluid mechanics 2014 the authors earth surface processes and landforms published by john wiley sons ltd

re anchoring the late pleistocene tephrochronology of new zealand based on concordant radiocarbon ages and combined 238u 230th disequilibrium and u th he zircon ages abstract the caldera forming rotoiti eruption from okataina volcano was one of the largest rhyolite events of the last 100 ka in the taupo volcanic zone new zealand its associated widespread tephra layer rotoehu tephra is a significant time marker in terrestrial marine paleoclimate correlation studies in the sw pacific however the accurate and precise age of this eruption and of the subsequent smaller volume earthquake flat eqf eruption has been the subject of controversy despite numerous studies we have applied combined 238u 230th disequilibrium and u th he dating of zircon from rotoiti and eqf deposits and obtained overlapping ages of 45 1 7 3 3 ka and 45 1 6 2 9 ka respectively these results are supported by new and published high precision radiocarbon data bracketing the age of the rotoiti eruption between 44 8 0 3 and 47 5 2 1 ka cal bp these age data are also in good agreement with a range of previously published estimates based on paleoclimate palynology luminescence dating of enclosing sediment and sedimentation rates in terrestrial and marine settings however these results are at variance with a commonly quoted age of 60 ka largely constrained by a single 40ar 39ar age of an overlying glassy lava flow at a distal tephra site this study demonstrates the potential for combined 238u 230th disequilibrium and u th he dating of zircon in tephra and other volcanic deposits to provide age control in the 40 100 ka time interval a period difficult to constrain using more traditional radiocarbon k ar and 40ar 39ar methods

alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of rhyolites and xenoliths from the salton trough california abstract rhyolite lavas and xenoliths from the salton sea geothermal field southern california provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust salton buttes rhyolite lavas contain xenoliths that include granophyres fine grained altered rhyolites felsite and amphibole bearing basalts zircon is present in lavas and xenoliths surprisingly even in the basaltic xenoliths where it occurs in plagioclase rich regions interpreted as pockets of crystallized partial melt zircons in the xenoliths are exclusively late pleistocene holocene in age and lack evidence for inheritance u th isochron ages are 20 5 1 2 1 2 ka granophyres 18 3 3 5 3 6 ka felsite 30 1 12 4 14 1 ka and 9 2 6 6 7 0 ka basalts all errors 1 the dominant zircon population in the rhyolite lavas yielded u th ages between 18 and 10 ka with few pre quaternary xenocrysts present 18ozircon values are lower than typical crustal basement values thus ruling out rhyolite genesis by melting of continental crust moreover 18ozircon values are 0 5 1 0 lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid ocean ridge basalt suggesting that basaltic crust and silicic plutons in the subsurface of the salton sea geothermal field isotopically exchanged with meteoric waters this is evidence for deep reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma

subaqueous barchan dunes in turbulent shear flow part 1 dune motion experiments are reported on the formation and migration of isolated dunes in a turbulent channel flow these dunes have a very robust crescentic shape with horns pointing downstream very similar to that of the barchan dunes observed in deserts at a much larger scale their main geometrical and dynamical properties are studied in detail for four types of grains the conditions for their formation their morphology the threshold shear stress for their motion their velocity erosion rate minimum size and the longitudinal stripes of grains hollowed by fluid streaks in the boundary layer in particular the law for the dune velocity is found to involve two dimensionless parameters the shields number and the sedimentation reynolds number in contrast with predictions based on classical laws for particle transport as the dune migrates its size slowly decreases because of a small leakage of particles at the horn tips and the erosion law is given a minimum size is evidenced which is shown to increase with the friction velocity and scale with a settling length

the shape of the barchan dunes of southern morocco we present detailed shape measurements of several barchan dunes in southern morocco near la youne using these data we disprove the concept of shape invariance of barchan dunes of different sizes nevertheless some parts of the barchan dune scale and we try to distinguish these from non scaling ones furthermore we point out the importance of the exact position of the brink for the overall dune shape finally we propose a simple model for dune shape using our data to determine the model parameters despite its simplicity this model is able to explain the difference between barchans where the crest and the brink coincide and those where they are separated typical side slopes can be obtained and the basic properties of the evolution of a barchan dune can be described

geology geochemistry and origin of volcanic rock hosted uranium deposits in northwestern nevada and southeastern oregon usa abstract northwestern nevada and southeastern oregon have the largest uranium u deposits in tertiary volcanic rocks in the us most deposits are in or adjacent to calderas or rhyolite lava dome fields and are hydrothermal almost all are associated with rhyolites that have high primary u concentrations 9 to 20 ppm but the rhyolites range from peralkaline to peraluminous caldera related occurrences are hosted by peralkaline rocks in the mcdermitt and virgin valley calderas at mcdermitt major deposits are along or just inboard of the southwestern kings river and northern ring fractures bretz opalite of the caldera complex the kings river deposits are strongly enriched in trace elements associated with epithermal precious metal deposits e g as au ag mo and sb much of the u occurs as hydrothermal uraniferous zircon and whole rock samples contain as much as 5 zr gangue minerals include quartz adularia fluorite and numerous sulfides 40 ar 39 ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity the bretz opalite deposits which include the large aurora resource 7 5 106 kg u3o8 are less enriched in epithermal trace elements and contain uraninite coffinite chalcedony opal and numerous sulfides at virgin valley deposits are mostly in tuffaceous sediments adjacent to ring fracture rhyolites trace element enrichment is minor u is hexavalent in all identified u minerals and associated phases include opal organic material and pyrite deposits in the lakeview u district oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements the mineral assemblage includes coffinite opal and numerous sulfides minor u occurrences at buff peak nevada the newly recognized westernmost topaz peraluminous rhyolite in the us are also trace element enriched gangue minerals include quartz adularia and sulfides on the basis of their geologic setting geochemistry age and mineralogy the mcdermitt lakeview and buff peak deposits are hydrothermal sparse published fluid inclusion data indicate temperatures between 200 c and 330 c virgin valley deposits most likely are hydrothermal minor deposits in outflow ash flow tuffs are distant from any intrusive heat source and probably formed from low temperature groundwater

evidence for initial excess 231pa in mid ocean ridge zircons abstract a limiting factor in the accuracy and precision of u pb zircon dates is accurate correction for initial disequilibrium in the 238u and 235u decay chains the longest lived and therefore most abundant intermediate daughter product in the 235u isotopic decay chain is 231pa t1 2 32 71 ka and the partitioning behavior of pa in zircon is not well constrained here we report high precision thermal ionization mass spectrometry tims u pb zircon data from two samples from ocean drilling program odp hole 735b which show evidence for incorporation of excess 231pa during zircon crystallization the most precise analyses from the two samples have consistent th corrected 206pb 238u dates with weighted means of 11 9325 0 0039 ma n 9 and 11 920 0 011 ma n 4 but distinctly older 207pb 235u dates that vary from 12 330 0 048 ma to 12 140 0 044 ma and 12 03 0 24 to 12 40 0 27 ma respectively if the excess 207pb is due to variable initial excess 231pa calculated initial 231pa 235u activity ratios for the two samples range from 5 6 1 0 to 9 6 1 1 and 3 5 5 2 to 11 4 5 8 the data from the more precisely dated sample yields estimated dpazircon duzircon from 2 2 3 8 and 5 6 9 6 assuming 231pa 235u of the melt equal to the global average of recently erupted mid ocean ridge basaltic glasses or secular equilibrium respectively high precision id tims analyses from nine additional samples from hole 735b and nearby hole 1105a suggest similar partitioning the lower range of dpazircon duzircon is consistent with ion microprobe measurements of 231pa in zircons from holocene and pleistocene rhyolitic eruptions schmitt 2007 schmitt 2011 the data suggest that 231pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions and excess initial 231pa may be more common in zircons than acknowledged the degree of initial disequilibrium in the 235u decay chain suggested by the data from this study and other recent high precision datasets leads to resolvable discordance in high precision dates of cenozoic to mesozoic zircons minor discordance in zircons of this age may therefore reflect initial excess 231pa and does not require either inheritance or pb loss

deformed barchans under alternating flows flume experiments and comparison with barchan dunes within proctor crater mars abstract it is generally considered that barchans isolated crescentic shaped dunes develop where wind is unidirectional and the available sand is insufficient to cover the entire dune field however bishop bishop m a 2001 seasonal variation of crescentic dune morphology and morphometry strzelecki simpson desert australia earth surface process and landforms 26 783 791 observed barchans that developed in areas where winds blow seasonally in opposite directions and described a peculiar deformation feature the rear slipface that is not found in ordinary barchans barchans under such bidirectional flows are poorly understood and it is necessary to study barchans that formed under many different flow conditions we conducted flume experiments to investigate the deformation of barchans under alternating water flow and observed new deformation features in addition to rear slipfaces we conclude that the deformation of barchans can be categorized into four types one of which shows morphologies similar to barchans within proctor crater mars the deformation type depends on the strength of the reverse flow relative to the forward flow and the absolute velocity of the forward flow comparison of our results with barchan dunes within proctor crater enable us to qualitatively estimate the wind strength and direction related to dune formation on mars these results are in agreement with those of fenton et al fenton l k toigo a d richardson m i 2005 aeolian processes in proctor crater on mars mesoscale modeling of dune forming winds journal of geophysical research 110 e6 e06005

ion microprobe analysis of 231pa 235u and an appraisal of protactinium partitioning in igneous zircon ion microprobe u pa measurements in zircon crystals from a holocene rhyolite salton buttes california are utilized to assess the partitioning of pa and u between zircon and melt the relative sensitivity factor rsf for ion microprobe u pa zircon analysis was calibrated on a natural high u secular equilibrium zircon from buff peak nevada zircon crystals from salton buttes rhyolite show evidence for excess 231 pa with a weighted average 231 pa 235 u activity ratio of 1 9 from this ratio model zircon melt partition coefficient ratios d pa d u 0 9 2 2 are obtained assuming plausible limits for 231 pa 235 u melt and correcting for the age of zircon crystallization 15 ka these values roughly fit lattice strain models for tetravalent cations but are more than one order of magnitude lower than predictions for pentavalent pa based on this appraisal of pa partitioning initial 231 pa disequilibrium caused by zircon melt fractionation alone can produce minor discordance or excess 207 pb in late cenozoic zircon but is insignificant for older u pb zircon ages

extreme u th disequilibrium in rift related basalts rhyolites and granophyric granite and the timescale of rhyolite generation intrusion and crystallization at alid volcanic center eritrea received september 20 2005 accepted june 14 2006 advance access publication july 14 2006rhyolite pumices and co erupted granophyric granite xenolithsyield evidence for rapid magma generation and crystallization priorto their eruption at 15 2 2 9 ka at the alid volcanic center inthe danikil depression eritrea whole rock u and th isotopicanalyses show

buried rhyolites within the active high temperature salton sea geothermal system abstract previously unrecognized pulses of rhyolite volcanism occurred in the salton trough between 420 8 ka and 479 38 ka 2 based on high spatial resolution u pb zircon geochronology presently these rhyolite lavas tuffs and shallow subvolcanic sills are buried to depths between 1 6 and 2 7 km at ambient temperatures between 200 and 300 c and are overprinted by propylitic to potassic hydrothermal alteration mineral assemblages consisting of finely intergrown quartz k feldspar chlorite epidote and minor pyrite alteration resistant geochemical indicators whole rock nd isotopes zircon oxygen isotopes reveal that these rhyolites are derived from remelting of morb type crust that was chilled and hydrothermally altered by deep circulating hydrothermal waters u pb zircon dating confirms the presence of bishop tuff in well state 2 14 at 1 7 km depth approximately 5 km ne of the geothermal wells that penetrated the buried rhyolites these results indicate accelerated subsidence towards the center of the salton trough increasing from 2 2 mm a to 3 8 mm a based on these results the present day salton sea geothermal field is identified as a focus zone of episodic rhyolitic volcanism intense heat flow and metamorphism that predates present day geothermal activity and holocene volcanism by at least 400 ka

a scaling law for aeolian dunes on mars venus earth and for subaqueous ripples abstract the linear stability analysis of the equations governing the evolution of a flat sand bed submitted to a turbulent shear flow predicts that the wavelength at which the bed destabilises to form dunes should scale with the drag length l drag s f d this scaling law is tested using existing and new measurements performed in water subaqueous ripples in air aeolian dunes and fresh snow dunes in a high pressure co2 wind tunnel reproducing conditions close to venus atmosphere and in the low pressure co2 martian atmosphere martian dunes the difficulty is to determine the diameter of saltating grains on mars a first estimate comes from photographs of aeolian ripples taken by the rovers opportunity and spirit showing grains whose diameters are smaller than on earth dunes in addition we calculate the effect of cohesion and viscosity on the dynamic and static transport thresholds it confirms that the small grains visualised by the rovers should be grains experiencing saltation finally we show that within error bars the scaling of with ldrag holds over almost five decades we conclude with a discussion on the time scales and velocities at which these bed instabilities develop and propagate on mars

obsidian hydration dating of volcanic events abstract obsidian hydration dating of volcanic events had been compared with ages of the same events determined by the 14c and k ar methods at several localities the localities ranging in age from 1200 to over 1 million yr include newberry craters oregon coso hot springs california salton sea california yellowstone national park wyoming and mineral range utah in most cases the agreement is quite good a number of factors including volcanic glass composition and exposuretemperature history must be known in order to relate hydration thickness to age the effect of composition can be determined from chemical analysis or the refractive index of the glass exposure temperature history requires a number of considerations enumerated in this paper

mean sediment residence time in barchan dunes when a barchan dune migrates the sediment trapped on its lee side is later mobilized when exposed on the stoss side then sand grains may undergo many dune turnover cycles before their ejection along the horns but the amount of time a sand grain contributes to the dune morphodynamics remains unknown to estimate such a residence time we analyze sediment particle motions in steady state barchans by tracking individual cells of a 3 d cellular automaton dune model the overall sediment flux may be decomposed into advective and dispersive fluxes to estimate the relative contribution of the underlying physical processes to the barchan shape the net lateral sediment transport from the center to the horns indicates that dispersion on the stoss slope is more efficient than the convergent sediment fluxes associated with avalanches on the lee slope the combined effect of these two antagonistic dispersive processes restricts the lateral mixing of sediment particles in the central region of barchans then for different flow strengths and dune sizes we find that the mean residence time of sediment particles in barchans is equal to the surface of the central longitudinal dune slices divided by the input sand flux we infer that this central slice contains most of the relevant information about barchan morphodynamics finally we initiate a discussion about sediment transport and memory in the presence of bed forms using the advantages of the particle tracking technique

flow effects on the morphology and dynamics of aeolian and subaqueous barchan dunes 1 barchans are aeolian sand dunes that form where unidirectional winds blow on firm ground with limited sand supply although they have been analyzed for decades the difficulty of conducting experiments in the field due to the large lengthscale and timescale involved is a considerable obstacle to understand their dynamics however crescentic patterns also appear underwater on a much smaller scale allowing easier study it is shown here that down scaled solitary barchans can be created in the laboratory using the periodic and asymmetric motion of a plate underwater to simulate the effect of a unidirectional flow those subaqueous barchans compare very well to their aeolian relatives their speeds scale as the cube of the speed of the flow times the inverse of their typical dimension finally the exploration of new topics such as the influence of the directionality of the wind or that of binary collisions illustrate that barchans can eventually adapt to any external perturbations by emitting one or several small barchans