origin timing and temperature of secondary calcite silica mineral formation at yucca mountain nevada the origin of secondary calcite silica minerals in primary and secondary porosity of the host miocene tuffs at yucca mountain has been hotly debated during the last decade proponents of a high level nuclear waste repository beneath yucca mountain have interpreted the secondary minerals to have formed from cool descending meteoric fluids in the vadose zone critics citing the presence of two phase fluid inclusions argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids understanding the origin temperature and timing of these minerals is critical in characterizing geologically recent fluid flux at the site and has significant implications to whether waste should be stored at yucca mountain petrographic and paragenetic studies of 155 samples collected from the exploratory studies facility esf and repository block cross drift ecrb tunnels indicate that heterogeneously distributed calcite with lesser chalcedony quartz opal and fluorite comprise the oldest secondary minerals these are typically overgrown by intermediate aged calcite often exhibiting distinctive bladed habits the youngest event recorded across the site is the deposition of mg enriched up to 1 wt and depleted growth zoned calcite intergrown with u enriched opal the cyclical variation in mg enrichment and depletion is probably related to climate changes that have occurred during the last few million years the distribution of secondary minerals is consistent with precipitation in the vadose zone fluid inclusion petrography of sections from the 155 samples determined that 96 of the fluid inclusion assemblages fias contained liquid only inclusions that formed at ambient temperatures 35 c however 50 of the samples n 78 contained relatively rare fia that contain both liquid only and liquid plus vapor inclusions herein termed two phase fias that formed at temperatures above 35 c virtually all of these two phase fias occur in paragenetically old calcite rare two phase inclusion assemblages were also observed in early fluorite and quartz and early intermediate calcite homogenization temperatures trapping temperatures across yucca mountain are generally 45 to 60 c but higher temperatures reaching 83 c were recorded in calcite from the north portal and ramp of the esf cooler temperatures of 35 to 45 c were recorded in the intensely fractured zone multiple populations of two phase fias from lithophysal cavities in the esf and ecrb cross drift indicate early fluid cooling with time from temperatures 45 c in early calcite to 35 to 45 c in paragenetically younger calcite freezing point depressions range from 0 2 to 1 6 c indicating trapping of a low salinity fluid the majority of intermediate calcite and all outermost mg enriched calcite contains rare all liquid inclusions and formed from ambient temperature 35 c fluids carbon and oxygen isotope data reveal a consistent trend of decreasing 13 c from 9 5 to 8 5 and increasing 18 o from 5 2 to 22 1 values from paragenetically early calcite to mg enriched growth zoned calcite depleted d values 131 to 90 of inclusion fluids from intermediate and the youngest mg enriched calcite indicate derivation from surface meteoric fluids recalculation of 18 oh2o values of 12 to 10 is consistent with derivation from paleometeoric fluids results of integrated u pb dating opal and chalcedony and fluid inclusion microthermometry indicate that two phase fias that trapped fluids of 50 c are older than 6 29 0 30 ma two phase fias in parage netically later calcite which formed from fluids of 35 to 45 c are older than 5 32 0 02 ma there is no evidence for trapping of fluids with elevated temperatures during the past 5 32 my the youngest mg enriched calcite intergrown with opal began to precipitate between about 1 9 to 2 9 ma and has continued to precipitate within the past half million years the presence of liquid only inclusions and the consistent occurrence of mg enriched calcite and opal as the youngest event indicate a minor but chemically distinct ambient temperature 35 c fluid flux during the past 2 to 3 my copyright 2003 elsevier science ltd

magmatic and hydromagmatic conduit development during the 1975 tolbachik eruption kamchatka with implications for hazards assessment at yucca mountain nv abstract xenoliths in pyroclastic fall deposits from the 1975 tolbachik eruption constrain the timing and development of subsurface conduits associated with basaltic cinder cone eruptions the two largest tolbachik vents contain xenoliths derived from magmatic and hydromagmatic processes which can be correlated with observed styles of eruption activity although many basaltic eruptions progress from early hydromagmatic activity to late magmatic activity transient hydromagmatic events occurred relatively late in the 1975 eruption sequence magmatic fall deposits contain 0 01 0 3 vol xenoliths from 500 m depth into the dry out zone around the conduit disrupting and ejecting 10 5 10 6 m 3 of wall rock through hydromagmatic processes with conduits widening to 8 48 m hydromagmatic falls contain 60 75 vol of highly fragmented xenoliths with juvenile clasts displaying obvious magma water interaction features during the largest hydromagmatic event unusual breccia bombs formed containing a wide range of fresh and pyrometamorphic xenoliths suspended in a quenched basaltic matrix hydromagmatic activity during the 1975 tolbachik eruption occurred below likely fragmentation depths for a basalt containing 2 2 wt magmatic water this activity is more likely related to conduit wall collapse rather than variations in conduit flow pressure in contrast larger volume silicic eruptions may have transient hydromagmatic events in response to conduit flow dynamics above the magma fragmentation depth the 1975 tolbachik volcanoes are reasonably analogous to quaternary basaltic volcanoes in the yucca mountain region and can guide interpretations of their poorly preserved deposits the youngest basaltic volcanoes near yucca mountain have cone deposits characterized by elevated xenolith abundances and distinctive xenolith breccia bombs remarkably similar to 1975 tolbachik deposits extrapolation of 1975 tolbachik data suggests conduits for some yucca mountain basaltic volcanoes may have widened locally on the order of 50 m in response to late stage hydromagmatic events

single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

polygenetic quaternary volcanism at crater flat nevada abstract alkali basalts erupted during the quaternary at crater flat nevada record a complex history of polycyclic and polygenetic volcanism magmas from the two main centers black cone and red cone are petrographically and geochemically similar although field evidence suggests a number of separate eruptive events high incompatible element concentrations low nb la and high zr y indicate that the magmas were derived by small degrees of partial melting from the lithospheric mantle at red cone a significant range of sr la ce ba and th concentrations is observed with time e g sr range 1308 1848 ppm the youngest samples having the more elevated values however there is only limited variation in the compatible trace elements e g sc and ni the array of compositions at red cone could not have been produced by changes in the degree of partial melting or by fractional crystallization rather a model of magma mixing is proposed between relatively enriched and depleted end members the cluster of black cone data falls consistently at the least enriched end of the red cone sample arrays suggesting that the black cone magma represents one of the mixing end members the modeling indicates that the magmatic plumbing systems of the two centers were linked at least during the early stages of volcanism moreover volcanic activity may have occurred at a number of sites along the length of the magmatic feeder zone during a single eruptive phase this could have significant implications for volcanic hazard assessment in the region around yucca mountain and the proposed nuclear waste repository

mechanisms for transition in eruptive style at a monogenetic scoria cone revealed by microtextural analyses lathrop wells volcano nevada u s a explosive activity at lathrop wells volcano nevada u s a originated with weak strombolian ws eruptions along a short fissure and transitioned to violent strombolian vs activity from a central vent with lava effusion during both stages the cause for this transition is unknown it does not reflect a compositional change as evidenced by the consistent bulk geochemistry of all the eruptive products however comparison of agglutinate samples from the early ws events with samples of scoria from the later vs events reveal differences in the abundance and morphology of groundmass phases and variable textures in the rims of olivine phenocrysts scanning electron microscope sem examination of thin sections from the ws samples show euhedral magnetite microlites in the groundmass glass and olivine phenocrysts show symplectite lamellae in their rims secondary ion mass spectrometry sims depth profiles of these symplectites indicate they are diffusion controlled the calculated dfe mg allows an estimation of the oxygen fugacity fo2 and indicates an increased fo2 during eruption of the ws products conversely the vs samples show virtually no magnetite microlites in the groundmass glass a lack of symplectites in the olivines and a lower calculated fo2 these microtextural features suggest that the lathrop wells trachybasalt experienced increased oxidation during ws activity as magma ascended through the original fissure exsolved bubbles were concentrated in the wider part s the protoconduit and this bubble flux drove convective circulation that oxidized the magma through exposure to atmosphere and recirculation this oxidation resulted in groundmass crystallization of magnetite within the melt and formation of symplectites within the olivine phenocrysts bubble driven convection mixed magma vertically within the protoconduit keeping it fluid and driving strombolian bursts while microlite crystallization in narrower parts of the fissure helped to focus flow development of a central conduit increased the magma ascent velocity due to a greater product volume in the later eruptive stages and caused the shift in eruption intensity consequently variations in microtextures of the lathrop wells products reveal how a combination of fluid dynamic and crystallization processes in the ascending magma resulted in different styles of activity while the products maintained a consistent bulk composition

dispersion of channel sediment contaminants in distributary fluvial systems application to fluvial tephra and radionuclide redistribution following a potential volcanic eruption at yucca mountain abstract predicting the fluvial transport and mixing of channel sediment contaminants is necessary for assessing and mitigating heavy metal and nuclear waste contamination in rivers the dilution mixing model is widely used for this purpose in tributary channel systems that transport contaminants as bed material load without significant overbank sedimentation here a more general three dimensional 3d contaminant transport numerical model is developed and tested based on bed scour turbulent mixing of contaminant material with uncontaminated channel bed sediments and re deposition of the mixture by the cumulative effect of many flood events first the model is applied to a synthetic alluvial fan environment downstream from a localized contaminant source in order to illustrate the model behavior second the model is validated against measured tephra concentrations in channels downstream from the lathrop wells scoria cone volcano a localized source of basaltic tephra to downstream channels otherwise comprised of non basaltic sediments third the model is applied to the problem of predicting the concentration of radionuclide bound tephra in channels downstream from the proposed nuclear waste repository at yucca mountain nevada in the event of a volcanic eruption through the repository contaminated tephra is mobilized from the landscape in this model using threshold criteria for hillslope gradient and channel stream power mobilized contaminated tephra is mixed with uncontaminated channel bed sediments using the contaminant transport model and deposited in channels of the fortymile wash alluvial fan where the residents nearest to the proposed repository live the results of twenty monte carlo simulations of eruption fallout and post eruption redistribution corresponding to a range of wind conditions and eruption magnitudes provide information on the mean and variability of contaminated tephra concentrations to be expected in channels of the fortymile wash alluvial fan in the event of an eruption mean tephra concentrations are approximately 1 but vary from nearly zero to as high as 26 reflecting the combined effects of wind direction eruption magnitude and dilution of tephra with uncontaminated channel bed sediments during transport

eruptive history of the late quaternary ciomadul csomad volcano east carpathians part i timing of lava dome activity located at the southern tip of the intra carpathian volcanic range in romania and composed of a dozen dacitic lava domes the ciomadul csomad volcanic complex is the youngest eruptive centre of the carpatho pannonian region whereas in the last decade the explosive history of ciomadul since 50 ka has been well constrained by numerous studies the chronology of the dome sequence still lacks robust chronological constraints and an extended analysis of all available data here we apply a detailed k ar dating approach to refine the chronology of the lava dome eruptions using the unspiked k ar cassignol gillot technique our dating focused on the most voluminous central part of the lava dome complex new eruption ages were determined following a strict separation of 10 g of groundmass from about 3 kg of unaltered sample rocks thereby isolating material whose cooling was contemporaneous with the eruption the newly applied methodology mainly consisting of a double full preparation first at larger grain size 0 4 mm and then at 100 m provides an appropriate procedure to separate suitable material to obtain the k ar age of the eruption i e the sample s groundmass in which there is no risk of the presence of older inherited crystals our new geochronological data set gives an improved insight into the temporal construction of the ciomadul volcanic complex where due to the method applied here all ages are younger than those from previous studies that used whole rock k ar ages our new results show that ciomadul s volcanic activity began with the construction of the south eastern peripheral domes from ca 850 ka to 440 ka after a ca 250 ky long repose period the activity resumed in the northern part at around 200 ka with subsequent domes emplaced between 200 and 130 ka aligned roughly north south in the western central part of the complex following a 30 ky long quiescence period the eastern central domes formed between 100 and 60 ka in addition to the chronological history of lava dome volcanism we also investigated the sequence of crystallization of mineral phases present in the lavas with respect to the modification of eruption ages ages obtained on pure minerals plagioclase amphibole and biotite are systematically older than those obtained on groundmass showing that most of them formed up to 1 85 myr before eruption in a long lived pre ciomadul magmatic system crystal size distribution csd data support the age contrasts between juvenile groundmass and older inherited minerals after injection of new magma and convective mixing with crystal clots ascent of the resulting mixture led to eruptions of material representing contrasting ages

aspects of potential magmatic disruption of a high level radioactive waste repository in southern nevada volcanic hazard studies combining standard techniques of hazard appraisal and risk assessment probability x s consequences are being undertaken with respect to storage of high level radioactive waste in southern nevada consequence studies the emphasis of this work are evaluated by tracing the steps of ascent of basaltic magma including intersection and disruption of a repository followed by surface eruption outhern nevada is cut obliquely by a n ne trending belt of basaltic volcanism that contains a number of major volcanic fields separated by areas with scattered basalt centers the basalts of the belt range in age from 8 m y to recent they are primarily of hawaiite composition and a number of the fields exhibit the straddle type associations of the alkalic subsuite the basalts are characterized by high 87 sr 86 sr isotopic ratios 0 707 and high sr contents and are evolved from parental compositions by crystal fractionation theoretical considerations suggest basalt magma ascends

the age of lathrop wells volcanic center an 40ar 39ar dating investigation the sup 40 ar sup 39 ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the lathrop wells volcanic center nevada yield eruption ages of 77 3 plus minus 6 0 and 76 6 plus minus 4 9 ka 2 sigma for the ql sub 1 and ql sub 2 flows respectively basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the ql sub 2 the sanidines from the ql sub 2 xenoliths yielded apparent plateau ages n 10 ranging from 69 plus minus 17 to 309 plus minus 11 ka with a strong cluster n 6 at approximately 77 ka because the diffusion dimension of sanidine is approximated by the physical crystal size cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and hf acid etching lead to anomalously old apparent ages these new ages for the ql sub 1 and ql sub 2 flows are analytically indistinguishable and agree well with published eruption ages determined by sup 3 he and sup 36 cl cosmogenic surface exposure dating methods we conclude that the ql submore 1 and ql sub 2 flows are temporally equivalent additionally the agreement between our eruption ages for these units and the published 76 plus minus 20 ka ital zreda ital et al 1993 sup 36 cl exposure age for the lathrop wells scoria cone qs sub 3 suggests that the cone may be temporally equivalent to the ql sub 1 and ql sub 2 flows we believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic copyright 1999 american geophysical union less

three dimensional records of surface displacement on the superstition hills fault zone associated with the earthquakes of 24 november 1987 seven quadrilaterals constructed at broadly distributed points on surface breaks within the superstition hills fault zone were repeatedly remeasured after the pair of 24 november 1987 earthquakes to monitor the growing surface displacement changes in the dimensions of the quadrilaterals are recalculated to right lateral and extensional components at millimeter resolution and vertical components of change are resolved at 0 2 mm precision the displacement component data for four of the seven quadrilaterals record the complete fault movement with respect to an october 1986 base these data fit with remarkable agreement the power law u t u f b t 1 b t c where u t is a displacement component at time t after the second main shock and uf b and c are constants this power law permits estimation of the final displacement uf from the data obtained within the period of observation data from one quadrilateral located near the epicenter of the second main shock and northeast trending conjugate faults allow that about 5 cm of right lateral slip may have been associated with the first main shock there data from the other quadrilaterals confirm that the surface faulting on most of the superstition hills fault zone did initiate at the time of the second main shock of the 1987 earthquakes the three dimensional motion vectors all describe nearly linear trajectories throughout the observation period and they indicate smooth shearing on their respective fault surfaces the inclination of the shear surfaces is generally nearly vertical except near the south end of the superstition hills fault zone where two strands dip northeastward at about 70 surface displacement on these strands is right reverse another kind of deformation superimposed on the fault displacements has been recorded at all quadrilateral sites it consists of a northwest southeast contraction or component of contraction that ranged from 0 to 0 1 per cent of the quadrilateral lengths between november 1987 and april 1988

the importance of ship log data reconstructing north atlantic european and mediterranean sea level pressure fields back to 1750 local to regional climate anomalies are to a large extent determined by the state of the atmospheric circulation the knowledge of large scale sea level pressure slp variations in former times is therefore crucial when addressing past climate changes across europe and the mediterranean however currently available slp reconstructions lack data from the ocean particularly in the pre 1850 period here we present a new statistically derived 5 5 resolved gridded seasonal slp dataset covering the eastern north atlantic europe and the mediterranean area 40 w 50 e 20 n 70 n back to 1750 using terrestrial instrumental pressure series and marine wind information from ship logbooks for the period 1750 1850 the new slp reconstruction provides a more accurate representation of the strength of the winter westerlies as well as the location and variability of the azores high than currently available multiproxy pressure field reconstructions these findings strongly support the potential of ship logbooks as an important source to determine past circulation variations especially for the pre 1850 period this new dataset can be further used for dynamical studies relating large scale atmospheric circulation to temperature and precipitation variability over the mediterranean and eurasia for the comparison with outputs from gcms as well as for detection and attribution studies

paricutin re examined a classic example of crustal assimilation in calc alkaline magma following its birth on the 20th of february 1943 the mexican volcano paricutin discharged a total of 1 38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952 until 1947 when 75 of the volume had been erupted the lavas varied little in chemical or isotopic composition all were basaltic andesites with 55 to 56 sio2 18o of 6 9 to 7 0 and 87sr 86sr ratios close to 0 7038 subsequent lavas were hypersthene andesites with silica contents reaching 60 18o values up to 7 6 and 87sr 86sr of 0 7040 to 0 7043 the later lavas were enriched in ba rb li and k2o and depleted in mgo cu zn cr ni sr and co the isotopic and other chemical changes which appeared abruptly over a few months in 1947 are interpreted as the result of tapping a sharply zoned and density stratified magma chamber xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70 18o of 5 6 to 9 9 and 87sr 86sr between 0 7043 and 0 7101 in many respects they resemble samples of basement rocks collected from nearby outcrops three analysed samples of the latter have silica contents of 65 to 67 18o of 7 7 to 8 6 and 87sr 86sr between 0 7047 and 0 7056 these new data provide strong support for the original interpretations of wilcox 1954 who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight continental crust except for a few trace elements particularly ba sr and zr the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late stage lavas some of the discrepancies may be explained by postulating a contaminant that was older and richer in ba sr and zr than those represented by the analysed xenoliths others can be attributed to chemical changes accompanying disequilibrium partial melting contact metamorphism and meteoric hydrothermal alteration of the country rock many of the xenoliths show evidence of having been affected by such processes the lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption the order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities

reappraisal of los humeros volcanic complex by new u th zircon and 40ar 39ar dating implications for greater geothermal potential

petrogenesis of volcanic rocks from saipan and rota mariana islands and implications for the evolution of nascent island arcs an 40 ar 39 ar age of 45 1 ma determined for lavas from northern saipan confirms that these high silica rhyolites erupted during the proto arc stage of volcanism in the izu bonin mariana system which is characterized elsewhere by eruption of boninitic lavas incompatible trace element concentrations and sr hf nd and pb isotope ratios for these rhyolites are transitional between those of c 48 ma boninitic lavas and post 38 ma first arc andesites and dacites from saipan and rota that have typical subduction related compositions these transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust a second stage of rayleigh fractionation in the upper crust is required by sio 2 concentrations that exceed 77 wt and near zero compatible element concentrations first arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first arc magmas themselves the mantle sources of the proto arc and first arc lavas from saipan and rota are similar to those of philippine back arc basin basalts based on nd and hf isotopic compositions the pb isotope compositions of these lavas are between those of pacific sea floor basalts and jurassic and younger cherty and clay rich sediments this contrasts with the boninitic proto arc volcanic rocks from guam and deep sea drilling project sites 458 and 459 that have pb isotope compositions similar to pacific basin basalts and volcaniclastic sediments the preferred explanation for the difference in the nature of proto arc volcanism between saipan and other fore arc locations is that the crust ceased extending 3 4 myr earlier beneath saipan this was caused by a change from mantle upwelling fore arc extension and shallow melting to an environment dominated by more normal mantle wedge convection stable crust and deeper melting

experimental constraints on magma ascent rate for the crater flat volcanic zone hawaiite phase equilibria and isothermal constant rate decompression experiments conducted on a hawaiite from the crater flat volcanic zone near yucca mountain nevada provide insight into the dynamics of magma transport for this magmatic province h 2 o contents up to 4 6 wt in melt inclusions the lack of plagioclase phenocrysts and the presence of amphibole all suggest that phenocrysts and melt in the crater flat volcanic zone were equilibrated at near water saturated conditions during ascent comparison of decompression induced crystallization of plagioclase microlites in experiments with natural tephra implies that magma ascent rates for the region are 0 04 m s if the magmas are water saturated estimates of minimum ascent rates increase if the magmas are water undersaturated a two stage process is needed to generate reaction rims found on natural amphibole suggesting that small batches of magma were briefly stored 3 5 days at depths as shallow as 800 m prior to eruption

the eruptive and magmatic history of the youngest pulse of volcanism at the valles caldera implications for successfully dating late quaternary eruptions abstract new 40ar 39ar and u th ages provide insight into the youngest eruptions at the valles caldera and also reveal previously unknown pulses of magmatism the youngest eruptive units collectively termed the east fork member of the valles rhyolite include the el cajete pyroclastic beds and co erupted battleship rock ignimbrite and the disconformably overlying banco bonito lava flow previous attempts to date these units using a variety of techniques yielded ages ranging from 1 ma new 40ar 39ar ages were generated using the high sensitivity multi collector argus vi mass spectrometer which provides more than an order of magnitude increase in precision compared to most single detector mass spectrometers 40ar 39ar dating of single crystals yields a range of ages of which the youngest populations are interpreted to represent the eruption age sanidine ages indicate that the el cajete pyroclastic beds and battleship rock ignimbrite erupted at 74 4 1 3 ka whereas the banco bonito lava erupted at 68 3 1 5 ka populations of older crystals represent variably degassed xenocrysts explaining why previous 40ar 39ar bulk step heating analyses yielded spuriously old and irreproducible results u th dating of unpolished zircon surfaces also yield multiple age populations which range from the eruption age to 350 ka indicating protracted magmatism during the 453 ka long eruptive hiatus prior to the eruption of the east fork member 40ar 39ar and u th ages indicate that the east fork member represents a short 6 1 2 8 ka eruptive cycle from a longer lived magmatic system beneath the southern caldera equally short repose periods similar to the interval between the el cajete battleship rock and banco bonito eruptions are possible during future volcanism at the valles caldera results demonstrate that detailed geochronology using single crystal and in situ techniques is necessary for understanding the eruptive history and magmatic evolution at some young volcanic systems

effect of grain size and domain state on thermal demagnetization tails thermal demagnetization of viscous remanence vrm and partial thermoremanence ptrm of 20 and 135 m natural magnetites reveals a broad spectrum f tub of unblocking temperatures tub both and tb the blocking temperature in contrast 0 04 m single domain sd grains demagnetize sharply at tub tb high and low tub tails of f tub were wider for 135 m multidomain md grains than for 20 m psd grains 10 mt alternating field demagnetization rendered the vrm of the 20 m magnetite more sd like in subsequent thermal cleaning selectively erasing both low and high tub tails of f tub primary and secondary remanences are then cleanly separated and non linear thellier paleointensity determination becomes linear our results agree well with tb tub data for 105 year thermoviscous overprints in the milton monzonite over the range of log time common to the two studies because tub av tb for psd and md as well as sd grains the pullaiah et al paleothermometry method will work if tub av values are used instead of tub max values

episodic caldera volcanism in the miocene southwestern nevada volcanic field revised stratigraphic framework 40ar 39ar geochronology and implications for magmatism and extension the middle miocene southwestern nevada volcanic field swnvf is a classic example of a silicic multicaldera volcanic field in the great basin more than six major calderas formed between 15 and 7 5 ma the central swnvf caldera cluster consists of the overlapping silent canyon caldera complex the claim canyon caldera and the timber mountain caldera complex active from 14 to 11 5 ma and centered on topographic timber mountain locations of calderas older than the claim canyon caldera source of the tiva canyon tuff are uncertain except where verified by drilling younger peralkaline calderas black mountain and stonewall mountain formed northwest of the central swnvf caldera cluster we summarize major revisions of the swnvf stratigraphy that provide for correlation of lava flows and small volume tuffs with the widespread outflow sheets of the swnvf new laser fusion 40 ar 39 ar isotopic ages are used to refine and revise the timing of eruptive activity in the swnvf the use of high sensitivity mass spectrometry allowed analysis of submilligram sized samples with analytical uncertainties of 0 3 1 permitting resolution of age differences as small as 0 07 ma these results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the swnvf major caldera episodes belted range crater flat paintbrush timber mountain and thirsty canyon groups erupted widespread ash flow sheets within 100 300 k y time spans and pre and post caldera lavas erupted within 100 300 k y of the associated ash flows peak volcanism in the swnvf occurred during eruption of the paintbrush and timber mountain groups when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1 35 m y separated by a 750 k y magmatic gap peralkaline and metaluminous magmatism in the swnvf overlapped in time and space the peralkaline tub spring and grouse canyon tuffs erupted early and the peralkaline thirsty canyon group tuffs and stonewall flat tuff erupted late in the history of the swnvf flanking the central volumetrically dominant peak of metaluminous volcanism magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data particularly in the overlapping grouse canyon and area 20 calderas of the silent canyon caldera complex volcanism in the swnvf coincided with the miocene peak of extensional deformation in adjoining parts of the great basin although regional extension was concurrent with volcanism it was at a minimum in the central area of the swnvf where synvolcanic faulting was dominated by intra caldera deformation significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the swnvf between the paint brush and timber mountain caldera episodes larger magnitude detachment faulting in the bullfrog hills southwest of the central swnvf caldera cluster followed the climactic timber mountain caldera episode postvolcanic normal faulting was substantial to the north east and south of the central swnvf caldera cluster but the central area of peak volcanic activity remained relatively unextended in postvolcanic time volcanism and extension in the swnvf area were broadly concurrent but swnvf area were broadly concurrent but in detail they were episodic in time and not coincident in space

nonlinear slope dependent sediment transport in cinder cone evolution sediment flux on transport limited hillslopes is well known to vary nonlinearly with slope diverging as the angle of stability is approached to date however no study has validated the precise form of the nonlinear slope dependent transport model over geologic time scales in a non steady state landform in this paper we show how cinder cones can be used to validate the nonlinear transport model using lathrop wells cinder cone in nye county nevada as a type example cinder cones are well suited for this purpose because they can be radiometrically dated and their angles of stability can be constrained by measurement of subsurface contacts between primary fallout and overlying colluvial deposits reworked from upslope forward model results with a generalized nonlinear transport model characterized by diffusivity and nonlinear exponent n show that the evolution of the cone rim and base are most sensitive to while the cone midpoint is most sensitive to n analyses of the full cone shape therefore permit the two model parameters to be independently inferred if the cone age and angle of stability are independently known results for lathrop wells imply that n 2 in the generalized nonlinear transport model which is consistent with roering et al 9s 1999 widely used form of that model

formation of aeolian dunes on anholt denmark since ad 1560 a record of deforestation and increased storminess abstract sand dunes on the island of anholt denmark in the middle of kattegat form a relatively barren temperate climate aeolian system locally termed the desert the dunes have developed on top of a raised beach ridge system under the influence of dominant winds from westerly directions they are relatively coarse grained with an average mean grain size of 480 m the last phase of aeolian activity and dune formation on anholt started after ad 1560 when the local pine forest was removed historical sources report intense sand mobilization in the 17th century and new optically stimulated luminescence osl dates indicate that dune formation continued until the end of the 19th century this period of sand drift and dune formation took place during the later part of the little ice age which is characterized by increased summer storminess in large parts of nw europe dune stabilization in the beginning of the 20th century probably records a temporary decrease in storminess ground penetrating radar mapping of the internal structures in two dunes in the western part of the desert a parabolic dune and a linear dune indicates the importance of north westerly storm winds during dune formation

inherited argon in a pleistocene andesite lava 40ar 39ar incremental heating and laser fusion analyses of plagioclase by using 40 ar 39 ar incremental heating and laser fusion techniques xenocrystic plagioclase was discovered in a late pleistocene andesitic lava that erupted through the andean cordillera inherited argon in the xenocrysts is as much as 450 times older than the host lava the age of which is independently known and is an obstacle to dating the lava by using a whole rock sample the xenocrysts are impossible to identify from petrography or chemical parameters such as their k ca ratios holocrystalline groundmass carefully separated from plagioclase and other phenocrysts gives an accurate 40 ar 39 ar age for the lava the xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from paleozoic rocks buried under kilometers of mesozoic and tertiary arc rocks composing the cordillera in central chile the required magma ascent velocity on the order of kilometers day is extraordinarily high compared to the 10 4 km day minimum implied by the 226 ra excess in continental arc lavas these data permit magma migration and storage in the mantle and lower crust for as much as thousands of years followed abruptly by rapid ascent to the surface

magma flow and interaction with waste packages in a geologic repository at yucca mountain nevada abstract the likelihood that a nuclear waste repository at yucca mountain could be intersected by igneous activity is very low but its potential consequences are nevertheless important to performance assessments an ongoing critical area of concern is the nature and magnitude of the thermal interaction of magma with tunnel walls high level nuclear waste packages and waste forms previous assessments consider a variety of dynamic scenarios but large uncertainties remain in understanding the rheological nature of the magma likely to be involved and its behavior within a repository drift here we specifically address the issue of magma rheology during degassing cooling and solidification as basaltic magma approaches earth s surface and enters a drift magma containing significant amounts 2 wt of dissolved water wet magma as is anticipated for this region at or near its liquidus temperature and saturated with water at 200 mpa is at a temperature near or below the 1 atm solidus temperature isentropic ascent from this near liquidus temperature promotes extensive solidification and or glassification exsolving water with approach to the surface promotes rapid vesiculation leading to fragmentation and tephra production with continued ascent the still water saturated magma traverses the solidification phase field and undergoes a combination of rapid crystallization and quenching becoming a glassy highly viscous 108 pa s mass of greatly reduced mobility this immobility is reflected in the high effective viscosity regulating flows from nearby cinder cones associated with wet basalt this also matches well with the experimentally determined rheology of dry basalt glass this rheology greatly restricts the mobility of basalt within repository drifts amounting to

eruption age of a pleistocene basalt from 40 ar 39 ar analysis of partially degassed xenoliths we have applied 40 ar 39 ar dating to potassium rich granitic xenoliths and host basalt from the pleistocene big pine volcanic field california these xenoliths had been partially degassed upon their inclusion in the basaltic lava argon released from the xenoliths at extraction temperatures below 900 c yielded plateau ages indistinguishable from the total k ar age of the basalt the best estimate of the age of eruption was 1 18 0 05 2 m y 40 ar extracted at higher temperatures included radiogenic argon not degassed from the late cretaceous xenoliths 1 18 m y ago causing an increase in the apparent age for the high temperature fractions the agreement of the low temperature xenolith plateau ages and the basalt k ar ages demonstrates that 40 ar 39 ar analysis of xenoliths may be used to measure the age of eruption of very young lavas this is significant because in many instances ages cannot be reliably determined by analysis of the lavas themselves

varve microfacies and varve preservation record of climate change and human impact for the last 6000 years at lake tiefer see ne germany the holocene sediment record of lake tiefer see exhibits striking alternations between well varved and non varved intervals here we present a high resolution multi proxy record for the past 6000

hillslope development in areas of active tectonics tectonic and geomorphic displacements of the earth s surface control topographic profile development therefore their analysis should be combined in the model presented here transient finite difference solutions to the continuity equation for material transport determine geomorphic displacements the material transport rate is a function of distance from the divide to the power m local slope to the power n and a rate constant values of m and n may be adjusted to simulate processes varying from rainsplash and soil creep i e diffusive m 0 n 1 to slope wash and river flow m 0 n 0 the actual geomorphic displacements may be transport or weathering limited depending on soil profile development superimposed edge dislocations in an elastic half plane are used to model tectonic displacements slip along a normal or reverse fault of any dip depth and down dip length may be incremental earthquake or continuous aseismic creep considering climate and material properties constant the ratio of the transport capacity rate constant to the fault slip rate roughly determines form this model extends existing morphologic diffusion erosion analyses to include other geomorphic conditions and processes transport or weathering limited conditions material flux boundary conditions and the development of gullies and knickpoints and more heterogeneous spatial and temporal distributions of tectonic displacement such as those due to slip along buried thrust faults we advocate calibration of these parameters and processes to provide a quantitative approach to modeling landform development determining deformation rates and inferring earthquake hazards

eruptive history geochronology and magmatic evolution of the puyehue cordon caulle volcanic complex chile forty three 40 ar 39 ar age determinations of lava flows domes ignimbrites and dikes plus 14 c dates from seven distal tephra layers combined with stratigraphy geochemistry and sr and th isotope data establish an eruptive chronology for the puyehue cordon caulle volcanic complex at 40 5 s in the andean southern volcanic zone svz the complex preserves 131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space approximately 80 of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka the modern puyehue stratovolcano was built on the southerly shield during the past 69 k y following a hiatus of 25 k y puyehue has erupted 15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern svz and evolved via at least six phases including 1 basaltic andesitic to dacitic lavas between 69 and 32 ka 2 a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of mgo rich basaltic andesite at 34 ka 3 dacitic to rhyolitic flows and domes from 19 to 12 ka 4 basaltic to basaltic andesitic flows between 15 and 12 ka 5 subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka followed by 6 a powerful series of phreatomagmatic and sub plinian eruptions at ca 1 1 ka that obliterated the preceding rhyolite domes and formed the present 2 5 km diameter 280 m deep summit crater along the cordon caulle fissure zone 5 km 3 of rhyodacitic to rhyolitic lavas domes and cones have formed during the past 16 5 k y including explosive and effusive eruptions in 1921 1922 and 1960 eruptive rates were nonuniform over time with background growth at 0 04 km 3 k y or less punctuated by spurts at up to 0 90 km 3 k y the time averaged rate 0 42 km 3 k y is nearly double that at the tatara san pedro complex 500 km to the north during the past 300 k y these findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable the last three stratocone building events on puyehue began during periods of deglaciation suggesting a relationship between unloading of ice and ease of magma ascent puyehue basalt exhibits subtle changes in 238 u 230 th 87 sr 86 sr and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge or the extent to which basalt was modified by assimilation of heterogeneous crustal melts the complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt despite the high flux of basalt during the past 300 k y no large silicic magma reservoir formed in the upper crust instead 238 u 230 th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust promoted perhaps by conduits that reflect strike slip faulting beneath the complex

an assessment of 40 ar 39 ar dating of incompletely degassed xenoliths the possibility of measuring the age of eruption of pleistocene lavas by 40 ar 39 ar analysis of entrapped ancient potassic xenoliths is demonstrated by a study of model systems upon inclusion in the hot magma such xenoliths are commonly only partially degassed of radiogenic 40 ar which has accumulated in them since their original crystallization the residual 40 ar will increase the apparent k ar age of the xenolith however if a xenolith is of cretaceous age or younger then a plateau in its 40 ar 39 ar age spectrum giving the age of eruption is expected to extend over 25 50 of the total 39ar released if degassing of the xenolith in the magma exceeded 90 and if the phases in the xenolith are characterized by sufficiently different diffusion dimensions or activation energies if diffusion was from a bimodal population of spheres then the radii must differ by a factor of 10 or more or the diffusion coefficients by a factor of 100 or more or if the spheres were equal in size and in diffusion coefficients then the activation energies must differ by a factor of at least 1 5 that such requirements may be realized in real xenoliths containing k feldspars is expected from published activation energies for microcline and from data determined on a granitic xenolith which was degassed in an early pleistocene basalt flow the experimental results appear to establish that old xenoliths may contain ar in distinctive phases which degas at sufficiently different temperatures as to permit determination of the age of degassing or eruption

localization of volcanic activity 2 effects of pre existing structure abstract the capture of vertically rising dikes by faults is examined with a combination of an analytic solution numerical simulations and field observations one of the numerical solutions and the field observations also provide insight into the intrusion of sills into the hanging wall of faults that are reactivated when lubricated by magma the approximate analytic solution indicates that dike capture by a fault will be limited primarily to steeply dipping faults capture will become easier meaning that it will be possible for faults with shallower dips as the intersection of the dike with the fault occurs at smaller depths capture will also be easier as it is harder for crack to grow in the hanging wall this may be interpreted in terms of a higher tensile strength or in terms of shorter pre existing cracks or higher fracture toughness numerical analysis using a discrete element code confirms this conclusion but also shows that the analytic solutions overestimate the difficulty of intruding a pre existing fault the maximum deviation occurring for faults dipping at 45 this finding that dike capture by faults should be restricted to high angle faults and shallow depths is supported by field observations of exhumed miocene basaltic volcanic terrain in central nevada usa differences in style seen in other regions are attributed to both the rock properties and the regional stress field the numerical solutions also indicate that synintrusive fault slip is likely to lead to formation of sills in the hanging wall at depths of a few hundred meters or less a finding that is also consistent with the field observations in nevada

chronology chemistry and origin of trachytes from hualalai volcano hawaii 1 hualalai volcano is unique among hawaiian volcanoes in that it possesses a relatively high proportion of evolved trachytic lavas that were erupted at the beginning of the alkalic postshield phase of volcanism these evolved lavas yield insights into magma sources magma supply rates and the evolution of the subvolcanic magmatic plumbing system at this time trachyte lavas are exposed at the puu waawaa pumice dome and puu anahulu flow as blocks in maars on the south flank of the volcano and as flows in water wells drilled on the west flank of hualalai new 40ar 39ar dates show that the puu waawaa and puu anahulu complex is 114 ka a block from the waha pele maar is 103 ka and water well trachytes range from 107 to 92 ka in age indicating a range for trachyte volcanism of 20 ka nd and pb isotopic compositions overlap with younger alkalic basalts from hualalai but are distinct from hualalai tholeiitic basalts and pacific mid ocean ridge basalts linking the trachytes to alkalic parental magmas that underwent extensive crystallization to yield trachytic residual magmas both sr and o isotopic ratios are higher in the trachytes than in hualalai alkalic lavas which is best explained by reaction with or assimilation of altered hualalai shield basalts at shallow depth major trace element and isotopic variations between trachytes are consistent with their evolution by fractional crystallization from a puu anahulu parent the short time gap between the end of tholeiitic volcanism 133 ka and the onset of trachytic alkalic volcanism and the lack of deep origin xenoliths place the magma reservoir within which the trachytes evolved rapidly at shallow 7 km depth whereas mauna kea and kohala volcanoes produced small volumes of highly evolved lavas as magma supply rates dwindled through the postshield stage postshield magma intrusion rates at hualalai were lowest during trachyte formation and increased through a more recent period of alkalic basalt eruptions subtle rare earth element and radiogenic isotopic distinctions between trachytes from the three localities indicate that the roof of the shallow magma reservoir may have been irregular allowing some trachytes to evolve independently from others

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

clast size controls and longevity of pleistocene desert pavements at lathrop wells and red cone volcanoes southern nevada formation of desert pavement and accretionary soils are intimately linked in arid environments well sorted fallout scoria lapilli at lathrop wells 75 80 ka and red cone ca 1 ma volcanoes southern nevada formed an excellent parent material for pavement allowing infiltration of eolian silt and fine sand that first clogged the pore space of underlying tephra and then aggraded and developed vesicular a av horizons variations in original pyroclast sizes provide insight into minimum and maximum clast sizes that promote pavement and soil formation pavement becomes ineffective when clasts can saltate under the strongest winds while clasts larger than coarse lapilli are unable to form an interlocking pavement that promotes silt accumulation necessary for av development contrary to predictions that all pavements above altitudes of 400 m would have been reset in their development after late pleistocene vegetation advances the soils and pavements show clear differences in maturity between the two volcanoes this indicates that either the pavements and or soils develop slowly over many tens of thousands of years and then are very stable or if they are disrupted by vegetation advances subsequent pavements are re established with successively more mature characteristics

first order symmetry of weak field partial thermoremanence in multi domain md ferromagnetic grains 2 implications for thellier type palaeointensity determination two models derived in very different ways but both incorporating the symmetry constraints outlined in the previous paper are developed for use in simulating thellier type palaeointensity experiments these are then tested against experimental data which endorse them both as powerful tools they predict that non ideal behaviour due to md grain content is sensitive not only to the protocol employed and the vector difference between the partial thermoremanent magnetisations ptrms imparted in the laboratory and the natural remanent magnetisation nrm but also to the spacing of the temperature steps and the type of ptrm imparted during the remagnetisation stages consistent with experimental studies the models predict that ptrm checks may fail despite no alteration occurring and that any double stage experimental protocol will produce non ideal results to some degree provided that md grains are present a single heating perpendicular or high angle single heating protocol is predicted to remove non ideal effects provided that the grains obey the symmetry constraints perfectly however this remains to be experimentally tested and does not allow for the use of ptrm checks five straightforward recommendations for future palaeointensity studies to follow are outlined these are designed to minimise non ideal behaviour due to md grains as far as possible while retaining the capacity to detect and reject those samples which will nonetheless produce significant errors

modeling magma drift interaction at the proposed high level radioactive waste repository at yucca mountain nevada usa we examine the possible ascent of alkali basalt magma containing 2 wt percent water through a dike and into a horizontal subsurface drift as part of a risk assessment for the proposed high level radioactive waste repository beneath yucca mountain nevada usa on intersection of the dike with the low pressure horizontal drift the ascending magma will be diverted into the drift the fragmenting mixture expands down the drift to reach speeds of order 100 300 m s after this initial disruptive activity parts of the repository can be filled with magma within a matter of hours until a pathway is found to allow the magma to vent magma flow through the drift network will cause intense heating of any waste canisters located along the pathway between the dike and the surface conduit the assessments suggest a greater number of waste packages may be adversely affected than previously recognized

multiphase magmatic flows at yucca mountain nevada 1 the proposed yucca mountain radioactive waste repository is sited in southern nevada in a region that has experienced sporadic basaltic volcanism since the late miocene volcanic risk assessment for the proposed repository requires estimating the consequences of a new monogenetic volcano intersecting the underground facility during its 104 106 year performance period we report numerical studies aimed at understanding the range of processes and dynamic parameter values that could accompany intersection of an open repository drift by a volatile rich trachybasaltic magma as it ascends in a dike we focus on one end member type of magmatic behavior namely a fragmented magmatic mixture under pressure interacting with an underground cavity initial and boundary conditions are based upon field data and previous modeling studies of the interaction between vertically propagating dikes and a repository opening the calculations are two dimensional and time dependent and are conducted with the multiphase hydrodynamics code gmfix calculations indicate that gas particle mixtures as they rise from below and interact with horizontal openings form complex flow patterns involving varying degrees of recirculation and deposition of pyroclasts dynamic pressures are up to 106 pa but are more typically on the order of 103 to 104 pa the geometry and number of outlets play a key role in determining the types of flow patterns as do volatile contents and the degree of fragmentation the detailed numerical simulations provide information that will be used to confirm the adequacy of simplified probabilistic consequence models used in risk assessments

cosmogenic 36cl dating of a young basaltic eruption complex lathrop wells nevada it has been proposed that the lathrop wells volcanic center a late quaternary basaltic complex in southern nevada has erupted more than once in common with most quaternary basalts this volcanic center has proved difficult to date by k ar and other commonly employed methods we have measured the accumulation of 36 cl in 11 samples from lava flows and volcanic bombs and obtained a combined average age of 81 7 9 ka with no systematic differences between sample subsets collected from different volcanic features the 36 cl dates do not support a history of multiple eruptions but neither do they completely preclude the possibility

40ar 39ar age of the lathrop wells volcanic center yucca mountain nevada paleomagnetic and 40 ar 39 ar analyses from the lathrop wells volcanic center nevada indicate that two eruptive events have occurred there the ages 136 8 and 141 9 thousand years ago for these two events are analytically indistinguishable the small angular difference 4 7 between the paleomagnetic directions from these two events suggests they differ in age by only about 100 years these ages are consistent with the chronology of the surficial geological units in the yucca mountain area these results contradict earlier interpretations of the cinder cone geomorphology and soil profile data that suggest that at least five temporally discrete eruptive events occurred at lathrop wells approximately 20 000 years ago

evaluating the life expectancy of a desert pavement abstract this paper integrates prior scholarship on desert pavements with a case study of pavements on stream terraces in the sonoran desert to analyze the processes and site conditions that facilitate the survival of ancient desert pavements this synthesis identifies vital factors key factors and site specific factors promoting pavement stability hyperaridity is the vital factor in pavements surviving for 106 years or more aided by minimal bioturbation and clast size reduction three key factors aid in pavements surviving for 104 to 105 years accumulation of allochthonous dust underneath pavement cobbles a flat topography and a lack of headward retreating swales or gullies a unified explanation for pavement longevity however did not emerge from a literature review because a variety of site specific factors can also promote pavement antiquity including resistant bedrock beneath the pavement disk shaped cobbles to promote dust accumulation and microclimatological and ecological reasons for minimal bioturbation both key and site specific explanations for pavement longevity apply well to a case study of pavements on stream terraces in the sonoran desert central arizona the buildup of cosmogenic 10be and in situ 14c optically stimulated luminescence and varnish microlamination ages reveal stable pavements range in age between 30 and 332 ka with conditions for longevity including flat surface topography pavements underlain by consolidated granitic bedrock a lack of headward retreating gullies and swales 87sr 86sr analyses indicating the infiltration of allochthonous dust floating disk shaped pavement cobbles and a quartzite lithology resistant to disintegration however 10be ages also indicate evidence for the instability of desert pavements on stream terraces underlain by unconsolidated playa clays and unconsolidated fanglomerate these weaker materials allowed the growth of headward retreating swales that in turn promoted exposure of newer gravels by surface erosion

the evolution of holocene coastal dunefields jutland denmark a record of climate change over the past 5000 years coastal dunefields have developed on the west coast of jutland in denmark over the past 5000 years the dunefields are situated in a temperate climate zone with frequent high energy wind events dunefield development was characterized by repeated periods of transgressive dune formation punctuated by periods of dune stabilization and soil formation the chronology of dunefield evolution is based on accelerator mass spectrometry ams radiocarbon dating of peaty palaeosols 24 samples and optically stimulated luminescence osl dating of aeolian sand deposits 19 samples these dates indicate that the completeness of the stratigraphic record varies considerably but that the timing of aeolian activity events was identical in the dunefields examined here initiation of aeolian activity occurred around 2200 bc 800 bc ad 100 ad 1050 1200 and between ad 1550 and 1650 proxy climate data from bogs in southwestern sweden suggest that these periods of dunefield activity were initiated during wet cool summers most likely these climatic situations were associated with a more frequent passage of cyclones across denmark in the summer seasons increased storminess causing aeolian sand movement and dune formation the continued removal of vegetation in jutland between 4000 bc and ad 1850 allowed the aeolian sand when first activated to move more and more easily across the coastal plains the final phase of aeolian sand movement between ad 1550 1650 and 1900 had catastrophic implications for many coastal inhabitants in the region c 2008 elsevier b v all rights reserved less

decreasing magmatic footprints of individual volcanoes in a waning basaltic field the distribution and characteristics of individual basaltic volcanoes in the waning southwestern nevada volcanic field provide insight into the changing physical nature of magmatism and the controls on volcano location during pliocene pleistocene times the volumes of individual volcanoes have decreased by more than one order of magnitude as have fissure lengths and inferred lava effusion rates eruptions evolved from hawaiian style eruptions with extensive lavas to eruptions characterized by small pulses of lava and strombolian to violent strombolian mechanisms these trends indicate progressively decreasing partial melting and length scales or magmatic footprints of mantle source zones for individual volcanoes the location of each volcano is determined by the location of its magmatic footprint at depth and only by shallow structural and topographic features that are within that footprint the locations of future volcanoes in a waning system are less likely to be determined by large scale topography or structures than were older larger volume volcanoes

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

evolution of the central garlock fault zone california a major sinistral fault embedded in a dextral plate margin the garlock fault is an integral part of the plate boundary deformation system inboard of the san andreas fault california usa however the garlock is transversely oriented and has the opposite sense of shear the slip history of the garlock is critical for interpreting the deformation of the through going dextral shear of the walker lane belt eastern california shear zone the lava mountains summit range lmsr located along the central garlock fault is a miocene volcanic center that holds the key to unraveling the fault slip and development of the garlock the lmsr is also located at the intersection of the nnw striking dextral blackwater fault and contains several sinistral wsw striking structures that provide a framework for establishing the relationship between the sinistral garlock fault system and the dextral eastern california shear zone new field mapping and geochronology data 40 ar 39 ar and u pb show five distinct suites of volcanic sedimentary rock units in the lmsr overlain by pliocene exotic clast conglomerates this stratigraphy coupled with fifteen fault slip markers define a three stage history for the central garlock fault system of 11 7 ma 7 3 8 ma and 3 8 0 ma pliocene to recent slip occurs in a 12 km wide zone and accounts for 33 km or 51 of the total 64 km of left lateral offset on the garlock fault in the vicinity of the lmsr since 3 8 ma this history yields slip rates of 6 9 mm yr for the younger stage and slower rates for older stages the lmsr internally accommodates northwest directed dextral slip associated with the eastern california shear zone walker lane belt via multiple processes of lateral tectonic escape folding normal faulting and the creation of new faults the geologic slip rates for the garlock fault in the lmsr match with and explain along strike variations in neotectonic rates

eruption age of a 100 000 year old basalt from 40 ar 39 ar analysis of partially degassed xenoliths we have applied 40 ar 39 ar dating using stepwise thermal extraction of ar to five potassium rich granitic xenoliths and their host basalt from sawmill canyon in the sierra nevada california previous k ar analyses showed the age of the basalt to be roughly 100 000 years or less the xenoliths which had accumulated large amounts of radiogenic 40 ar since their crystallization 100 m y ago were partially degassed upon their inclusion in the basaltic magma ar released from the xenoliths in the laboratory at temperatures substantially below the melting temperature of the basalt was created since the host magma cooled isotopic compositions of ar released from the xenoliths in several extraction steps at temperatures below 900 c were colinear in 36 ar 40 ar versus 39 ar 40 ar diagrams and defined isochrons giving a mean age of degassing of 119 000 7000 2 years 40 ar extracted at higher temperatures included ancient radiogenic 40 ar that was never diffused from the xenoliths during immersion in the magma this 40 ar caused an increase in the apparent age for the high temperature extractions the high precision of the eruption age determined by this method is comparable to that obtained elsewhere by conventional k ar dating of sanidine 40 ar 39 ar analysis of granitic xenoliths to date young basaltic lava flows may prove to yield results superior to those found from analysis of the lava itself establishing the age of eruption of the basalt flow in sawmill canyon establishes age limits for two sierran glaciations which left moraines stratigraphically above and below the lava thus the younger glaciation must be wisconsin the older must be pre wisconsin in age

40ar 39ar geochronology of post valles caldera rhyolites jemez volcanic field new mexico the40ar 39ar dating of postcollapse rhyolites in the valles caldera 1140 kyr defines five intervals during which rhyolite domes flows and pyroclastic rocks were emplaced from ring fracture vents previously ages were based on limited k ar dating and stratigraphic relations the 40ar 39ar dates are concordant with the previous dates for about half of the samples others have ages significantly older 7 25 despite evidence for small amounts of xenocrystic material eruptive activity occurred during relatively brief periods at 1133 973 915 800 787 557 521 and 300 170 kyr separated by quiescent intervals of 115 300 kyr isochrons defined by single crystal sanidine fusions indicate trapped 40ar 39ar ranging from 282 to 326 sanidine analyses yield reliable and consistent results whereas biotite and glass data may be ambiguous biotite may have low radiogenic yields contain variable amounts of extraneous argon or define apparent isochrons discordant with cogenetic sanidine unaltered unhydrated glass in one case yields an isochron concordant with coexisting sanidine whereas in another is undateable due to large amounts of non radiogenic argon the revised chronology has implications for the evolution of the jemez magmatic system as well as for the geomagnetic polarity timescale the discrete periods of volcanism relations among rhyolites erupted within these intervals and the duration of the intervening quiescent intervals are consistent with the generation evolution and eruption of several distinct magmas following caldera collapse in addition the 40ar 39ar data suggest revision of the brunhes matuyama polarity reversal boundary from the currently accepted 730 kyr to 780 kyr

petrology and geochemistry of hawaiite lavas from crater flat nevada hawaiite type lavas were erupted in three cycles 3 7 1 2 and 0 3 m y at crater flat nevada the compositions of all three cycles considered together form a straddling alkalic series as defined by miyashiro in which the less evolved basalts plot near the normative olivine diopside divide and the more evolved basalts project into hypersthene or nepheline fields fractionation modeling based on the oldest cycle allows the removal of olivine clinopyroxene and amphibole to arrive at the more evolved hawaiite compositions in general fractionation of phlogo pite or feldspar is limited by the fractionation modeling and by eu ree relations in detail all hawaiites within one cycle 3 7 m y need not be derived from a single parent magma varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition basalts of the youngest two cycles are generally enriched in trace elements superimposed on this enrichment is a lack of rb variation leading to rb sr ratios far lower than required to generate the high 87sr 86sr ratio 0 707 typical of basalts in this region the very low rb sr ratios limit processes that may lead to trace element enrichment during magma evolution cyclic recharge of a fractionating magma chamber decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite bearing mantle rocks into a phlogopite absent mantle assemblage might explain the observed trace element enrichment with low rb sr

late proterozoic grenville deformation in central new mexico determined from single crystal muscovite 40ar 39ar age spectra abstract 40 ar 39 ar age spectrum analyses on deformed and undeformed single crystal muscovites from the manzano mountains in central new mexico provide insights on the argon systematics of deformed muscovite the post 1 4 ga cooling history of the region and the timing of low temperature deformation microstructures in narrow east side up shear zones in and around the 1427 ma priest pluton indicate post pluton greenschist facies 350 c deformation single crystal 40 ar 39 ar analysis of three samples containing undeformed muscovite yield plateau spectra with ages of 1390 3 1395 2 and 1401 3 ma suggests that by this time the pluton had cooled below 350 400 c single crystal muscovites from within the shear zones have markedly different age spectra and reveal well defined age gradients from 1 1 to 1 4 ga the age gradients are interpreted as resulting from deformation induced argon loss or reduction of the effective radius for argon diffusion thereby decreasing the closure temperature of the originally pristine magmatic muscovites these data and previously published results suggest a long period of crustal stability ca 1 4 to 1 1 ga in which average cooling rates were probably about 1 c ma the 1 1 ga ages in the deformed muscovite age spectra are interpreted as indicating that grenvillian tectonism to the southeast affected central new mexico it can not be unambiguously determined if the consequences of the grenville orogeny in new mexico was uplift and erosion cooling a discrete thermal pulse reheating or if the shear zones record grenville age contractual deformation

magnetic properties and paleointensity of pseudotachylytes from the sudbury structure canada petrologic control abstract since a pseudotachylyte is generated through a combination of comminution and frictional melting it may record an ambient earth s magnetic field as a thermal remanence during the cooling from friction melt to test the reliability of determining the paleointensity the coe modified thellier method was carried out on a small suite of pseudotachylytes in large displacement fault zones from the north range sudbury impact structure pseudotachylytes studied here are classified into two types in their protolith lithology gneiss origin and granitoid origin pseudotachylyte petrologic observations confirm the presence of abundant fine grained magnetite inclusions and coarse grained titanomagnetites with exsolved ilmenite lamella in gneiss origin pseudotachylyte in granitoid origin pseudotachylyte we found a scatter distribution of multidomain magnetite although the granitoid origin pseudotachylyte fails to estimate the paleointensity the gneiss origin pseudotachylytes yield weak paleointensity values which are consistent with previous paleointensity data from the norite in sudbury igneous complex these results suggest that the protolith petrology controls a magnetic mineralogy in pseudotachylytes and the reliability as recorder of ancient earth s magnetic field

climatic reconstructions for the northeast atlantic region ad 1685 1700 a new source of evidence from naval logbooks this paper draws on a newly developed source of data in the form of ships logbooks to cast a detailed light on the weather and climate of a critical period in recent climatic history the decades of the 1680s and 1690s possibly the coldest of the last 1000 years logbooks of the period provide daily data for wind force direction precipitation and general weather conditions these were abstracted and aggregated into monthly statistics these data indicate the highly episodic character of air flow patterns at this time with marked changes of phase between high and low zonality in the westerlies the data also provide evidence of the stormy character of the period particularly with respect to summer conditions comparisons are made with contemporary and independently derived instrumental and proxy series the data source is confirmed as providing a valuable addition to climatic sources and is additionally important because it is based on regular daily observations and made at sea an area very poorly r

comparison of conventional k ar and 40ar 39ar dating of young mafic volcanic rocks abstract k ar and 40ar 39ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the snake river plain idaho mount adams washington and crater lake oregon the k ar ages were calculated from ar measurements made by isotope dilution and k2o measurements by flame photometry the 40ar 39ar ages are incremental heating experiments using a low blank resistance heated furnace the results indicate that high quality ages can be measured on young mafic volcanic rocks using either the k ar or the 40ar 39ar technique the precision of an 40ar 39ar plateau age generally is better than the precision of a k ar age because the plateau age is calculated by pooling the ages of several gas increments the precision of a plateau age generally is better than the precision of an isotope correlation isochron age for the same sample for one sample the intercept of the isochron yielded an 40ar 36ar value significantly different from the atmospheric value of 295 5 recalculation of increment ages using the isochron intercept for the composition of nonradiogenic ar in the sample resulted in much better agreement of ages for this sample the results of this study also indicate that given suitable material and modern equipment precise k ar and 40ar 39ar ages can be measured on volcanic rocks as young as the latest pleistocene and perhaps even the holocene