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

travertine deposits of soda dam new mexico and their implications for the age and evolution of the valles caldera hydrothermal system uranium thorium disequilibrium dates and stable isotope analyses of travertine deposits at soda dam new mexico have been used to determine the age and to document the evolution of the valles caldera hydrothermal system soda dam discharges from paleozoic and precambrian rocks in san diego canyon southwest of the caldera and the canyon was filled with several hundred metres of upper bandelier tuff during formation of valles caldera 1 12 ma according to the dates the valles hydrothermal system is at least 106 yr old or nearly as old as the caldera the basic hydrology of the system has thus remained virtually unchanged during this time travertine ages indicate pulses in travertine deposition from 1 0 to 0 48 ma from 0 107 to 0 058 ma and from 0 005 ma to present the volume of various travertine deposits suggests that the hot spring system at soda dam was once larger than it is today possibly discharging as much as 5 to 10 times more fluid of na hco3 cl composition stable isotopes of the travertines indicate however that the hot spring system has probably never been more than about 10 c hotter than the present maximum temperature of 48 c stable isotopes also show that the 13c of co2 and 18o of h2o in hot spring waters has remained relatively constant for 106 yr the similar ages between caldera formation and the onset of large scale hydrothermal circulation and travertine deposition on pre bandelier tuff rocks imply that incision rates by the jemez river in bandelier tuff were relatively rapid as much as 400 m of tuff were cut by the ancestral jemez river in 105 yr or less

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

influence of grain size and shape on volcanic ash electrical conductivity abstract few studies have examined the electrical properties of volcanic ash or considered the effects of physical characteristics such as grain size and shape on its electrification this study measures the resistivity of eight volcanic ash samples three milled samples and five natural ashfall samples from alaska u s a using a current amplifier and examines the influence of particle size and particle shape on calculated grain conductivity grain conductivities are calculated by applying spherical oblate and prolate ellipsoidal shape parameters within a general effective media equation volcanic ash is an electrically insulating material with average resistivity values ranging from 1 732e7 to 1 606e12 m results show that grain size has a limited impact on resistivity calculated grain conductivities remained within the same order of magnitude between the three different shape calculations ranging between 1 367e 7 and 2 196e 12 s m using the spherical shape parameter electrical measurements of heterogeneous powders are complex and challenging this study aims to expand the community s dataset and understanding of the electrical properties of volcanic ash and the impact of different physical characteristics this has implication for understanding ash hazards to electrical infrastructure and volcanic lightning

volcanism and sedimentation along the western margin of the rio grande rift between caldera forming eruptions of the jemez mountains volcanic field north central new mexico usa abstract the cerro toledo formation ctf a series of intracaldera rhyolitic dome complexes and their associated extracaldera tephras and epiclastic sedimentary deposits records the dynamic interplay between volcanic tectonic and geomorphic processes that were occurring along the western margin of the rio grande rift between major caldera forming eruptions of the bandelier tuff 1 65 1 26 ma the alamo canyon and pueblo canyon members differ significantly despite deposition within a few kilometers of each other on the pajarito plateau these differences highlight spatial distinctions in vent sources eruptive styles and depositional environments along the eastern side of the jemez mountains volcanic field during this ca 400 000 year interval intercalated pyroclastic fall deposits and sandstones of the pueblo canyon member reflect deposition with a basin thick alamo canyon member deposits of block and ash flow tuff and pyroclastic fall deposits fill a paleovalley carved into coarse grained sedimentary units reflecting deposition along the mountain front chemistry and ages of glass from fall deposits together with clast lithologies of sedimentary units allow correlation of outcrops subsurface units and sources dates on pyroclastic fall deposits from alamo canyon record deep incision into the underlying otowi member in the southern part of the pajarito plateau within 100 k y of the toledo caldera forming eruption reconstruction of the ctf surface shows that this period of rapid incision was followed by aggradation where sediments largely filled pre existing paleocanyons complex sequences within the upper portion of the otowi member in outcrop and in the subsurface record changes in the style of eruptive activity during the waning stages of the toledo caldera forming eruption

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

dating of young 1 ma tephras using u pb zircon and u th sm he zircon apatite magnetite chronometers to unravel the eruption age of a tephra in the woodlark rift of papua new guinea

three dimensional kirchhoff migration imaging of the jemez volcanic field using teleseismic data 1 we computed a reflectivity image for the earth beneath the jemez volcanic field new mexico using a novel adaptation of petroleum exploration seismic imaging this image was obtained by applying the kirchhoff wave field imaging method to digitally recorded teleseismic data the volume imaged has a lateral extent of 30 km extends to 45 km depth and lies beneath the valles caldera to the west of los alamos new mexico the derived picture of the earth is a three dimensional 3 d map of the locations of impedance changes within the crust and upper mantle below the jemez volcanic field with a spatial resolution on the order of a kilometer significant features seen in the image include the base of the caldera fill several reflectors in the crust we interpret to be associated with intrusions coming from the mantle and or other crystallized chambers such as a low velocity zone seen in the tomographic image two strong reflectors coincident with the crust mantle interface and a zone of layered reflections at the base of the crust which may be basaltic underplating this method proved to be a powerful tool for imaging the earth s crust and mantle when combined with other methods such as tomography

is the valles caldera entering a new cycle of activity the valles caldera formed during two major rhyolitic ignimbrite eruptive episodes the bandelier tuff at 1 61 and 1 22 ma after some 12 m y of activity in the jemez mountains volcanic field new mexico several subsequent eruptions between 1 22 and 0 52 ma produced dominantly high silica rhyolite lava domes and tephras within the caldera these were followed by a dormancy of 0 46 m y prior to the most recent intracaldera activity the longest hiatus since the inception of the bandelier magma system at 1 8 ma the youngest volcanic activity at 60 ka produced the sw moat rhyolites a series of lavas and tuffs that display abundant petrologic evidence of being newly generated melts petrographic textures conform closely to published predictions for silicic magmas generated by intrusion of basaltic magma into continental crust the valles caldera may currently be the site of renewed silicic magma generation induced by intrusion of mafic magma at depth recent seismic investigations revealed the presence of a large low velocity anomaly in the lower crust beneath the caldera the generally aseismic character of the caldera despite abundant regional seismicity may be attributed to a heated crustal column the local effect of 13 m y of magmatism and emplacement of mid crustal plutons seismic signals of magma movement in the deep to mid crust may therefore be masked and clear seismic indications of intrusion may only be generated within a few kilometres of the surface we therefore encourage the establishment of a local dedicated volcanic monitoring system

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

new evidence for the age of the youngest eruptions in the valles caldera new mexico new geochronologic data provide evidence for an age of about 50 to 60 ka for the youngest volcanic eruptions within the valles caldera new mexico an age that is significantly younger than most previous age determinations thermoluminescence age estimates for buried soils beneath the el cajete pumice a key stratigraphic marker in the region range from 48 to 61 ka and sup 14 c analyses of burnt logs within volcanic surge beds near the el cajete vent yield similar ages of 50 to 58 ka these data conflict with fissontrack sup 40 ar sup 39 ar and k ar ages of 130 to 200 ka but are supported by recent analyses by electron spin resonance the results of this study reinforce the need to apply a variety of dating methods when evaluating the age of young volcanic events and support the hypothesis that the el cajete eruptions were part of a new cycle of volcanic activity in the valles caldera after an exceptionally long period of quiescence of nearly 460 ka the new age constraints also suggest a previously unrecognized link between cycles of volcanism and pulses of hydrothermal activity in the caldera such that hydrothermal outflow appears to decrease significantly following completion ofmore eruptive cycles 24 refs 3 figs 2 tabs less

reconstruction of the most recent volcanic eruptions from the valles caldera new mexico abstract products of the latest eruptions from the valles caldera new mexico consist of the el cajete pyroclastic beds and battleship rock ignimbrite a sequence of pyroclastic fall and density current deposits erupted at 55 ka capped by the later banco bonito flow erupted at 40 ka and collectively named the east fork member of the valles rhyolite the stratigraphy of the east fork member has been the subject of conflicting interpretations in the past a long running investigation of short lived exposures over a period of many years enables us to present a more complete event stratigraphy for these eruptions than has hitherto been possible the volume of rhyolitic magma erupted during the 55 ka event may have been more than 10 km3 and for the 40 ka event can be estimated with rather more confidence at 4 km3 during the earlier event plinian eruptions dispersed fallout pumice over much of the valles caldera the southern jemez mountains and the rio grande rift we infer a fallout thickness of several decimeters at the site of the city of santa fe and significant ash fall in eastern new mexico in contrast pyroclastic density currents were channeled within the caldera moat and southwestward into the head of canon de san diego the principal drainage from the caldera simultaneous or rapidly alternating pyroclastic fallout and density current activity characterized the 55 ka event with density currents becoming more frequent as the eruption progressed through two distinct stages separated by a brief hiatus one early pyroclastic surge razed a forest in the southern caldera moat in a similar manner to the initial blast of the may 18 1980 eruption of mt st helens ignimbrite outflow from the caldera through the drainage notch may have been restricted in runout distance due to steep rugged topography in this vicinity promoting mixing between flows and air and the formation of phoenix clouds lavas erupted during both the 55 and 40 ka events were largely confined to the caldera moat any future rhyolitic eruptions of similar magnitude in the southern or western parts of the valles caldera will likely affect similar areas

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

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

40ar 39ar age constraints for the jaramillo normal subchron and the matuyama brunhes geomagnetic boundary our mid pleistocene 4oar 39ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope climate record calibration of the astronomical timescale proposed by johnson 1982 and shackleton et al 1990 4oar 39ar ages of a normally magnetized rhyolite dome in the valles caldera northern mexico yielded a weighted mean age of 1 004 0 019 ma a k ar age of 0 909 0 019 ma for this rock by doell and dalrymple 1966 was the linchpin for the recognition and calibration of the jaramillo normal subchron jns other 4oar 39ar ages from the vailes caldera and 4oar 39ar ages of ivory coast tektites indicate that the jns began at about 1 11 ma and ended before 0 92 ma probably near 0 97 ma the matuyama brunhes boundary occurred between 0 79 ma and 0 76 ma on the basis of 4oar 39ar sanidine ages from 1 three reversely magnetized rhyolite domes of the valles caldera 0 793 0 018 ma 0 794 0 007 ma and 0 812 0 023 ma and pumice 0 789 0 006 ma from the reversely magnetized oldest toba tuff of sumatra and 2 pumice 0 764 0 005 ma and 0 757 0 009 ma from the lower and upper units of the normally magnetized bishop tuff the age of the boundary may be close to 0 77 ma as deduced from rates of sedimentation in ancient lake bonneville utah increasingly aware that the polarity of earth s magnetic field has changed repeatedly through geologic time and isotopic dating of these polarity reversals resulted in a geomagnetic polarity time scale gpts for the late cenozoic this combined paleomagnetic and isotopic research played a pivotal role in confirming plate tectonic theory glen 19821 which revolutionized the earth sciences continual refinement of isotopic ages which define the gpts has increased the usefulness of geomagnetic polarity reversals as marker hori zons in many types of geologic studies we present 4oar 39ar data and ages for some key volcanic units that support changes in the isotopic age chronology of the gpts using volcanic rocks erupted near the time of the geologically youngest two geomagnetic polarity reversals figure 1 the older of the two reversals originally called by doell and dalrymple 1966 the jaramillo event is now named the jaramillo normal subchron jns it is recorded in middle pleistocene rocks as a brief episode of normal polar ity within the late matuyama chron the younger of the two called the matuyama brunhes boundary mbb is regis tered in middle pleistocene rocks worldwide and consists of

linking tephrochronology and soil characteristics in the sila and nebrodi mountains italy recent studies have demonstrated that soils formed on pyroclastic ash deposits are much more common in the mediterranean area than previously assumed these soils are an important key to understanding past volcanic events and landscape evolution chronological information in soils of quaternary volcanic events however remains still poorly understood in southern italy using a multi method forensic approach we explore the origin and age of volcanic deposits soils in sicily and calabria the geochemical signature of the soil was compared to the chemical fingerprint of the magmas of potential source areas of southern italian volcanoes the results indicate that the investigated soils on the nebrodi sicily and sila calabria mountains were both impacted by materials having a high k calc alkaline series volcanism the aeolian islands in particular lipari and vulcano are the most likely source of origin but contributions also from the etna particularly the biancavilla ignimbrites and plinian eruptions occurred weathering and leaching processes along with a potential contribution from the underlying non volcanic bedrock has altered the main chemical composition of soils often precluding direct relation to potential source areas immobile elements and their ratios e g the nb y vs zr ti plot or trace elements co th and rare earth elements laser ablation icp ms analyses of glass particles volcanic clasts and pumice like materials gave precious hints of the origin of the volcanic deposits radiocarbon dating of the h2o2 resistant soil organic fraction indicates a minimum age of 8 10 ka of the soils the weathering index wip weathering index according to parker and the chemical composition of volcanic glasses and clasts were tested as proxies for the age of the volcanic deposits and time for soil formation the soils and landscape are characterised by multiple volcanic depositional phases for the last about 50 ka in the sila mountains and about 70 ka or more in the nebrodi mountains chemical mineralogical analyses enabled the detection of deposition phases during the pleistocene and also holocene the multi method approach enabled the identification of potential source areas provided a tentative age estimate of the start and in part duration of ash deposits and therefore improved our understanding of volcanic landscape evolution

rhyolite lava emplacement dynamics inferred from surface morphology abstract the morphology of a silicic lava is created by the fluid dynamic conditions that operated during flow careful analysis of these features can thus be used to reconstruct emplacement processes we used unoccupied aerial system and structure from motion photogrammetry techniques to collect centimeter scale spatial resolution imagery of the surfaces of south coulee and obsidian dome rhyolite lavas mono inyo craters usa we supplement the newly acquired orthomosaics and digital elevation models with existing imagery of other holocene rhyolite lavas from across the western usa including rock mesa dome south sister or newberry flow south sister or obsidian flow newberry or interlake flow newberry or and banco bonito flow valles caldera nm although many morphologic features exist we restrict our quantitative analyses to the recurrent ridges and blocks of loose rubble that are common to the flows ridge spacing and ridge vergence indicate the ridges are likely folds produced by compression during emplacement pressure ridge spacings range from 27 to 45 m at south coulee and 18 to 34 m on obsidian dome spacing generally decreases with distance from the vent across both lavas ridge amplitudes range from 4 to 17 m at both lavas and show little correlation with distance from the vent the vergence of the crest of the ridges in the flow interiors points back to the source vent which provides strong evidence for endogenous flow and the backward rotation of folds produced by undertow of the underlying lava ridges near flow margins verge towards the flow front demonstrating that exogenous flow becomes increasingly important at the point of advance in both south coulee and obsidian dome block sizes are largest near the vent and gradually decrease towards the flow front the gradual decrease in block size with increasing distance from the vent likely reflects decreasing effusion rates from the conduit we interpret our high resolution field measurements under the lens of analytical solutions to fluid dynamic models to estimate emplacement timescales and associated rates we calculate relatively abbreviated eruptive timescales ranging from 1 month to

3d seismic attribute assisted stratigraphic framework and depositional setting characterization of frontier miocene to pliocene aged agbada formation reservoirs deep offshore niger delta basin abstract in the freeman field located about 120 km 75 miles offshore southwestern niger delta at about 1300 m 4260 ft water depth 3d seismic attributes based analyses and stratigraphic based geometric models enhance and constrain the characterization of the stratigraphic framework and depositional setting of miocene pliocene agbada formation reservoirs in the deep offshore niger delta basin for improved oil and gas exploration and exploitation an integrated approach was employed involving 3d seismic interpretation prospectivity mapping subsurface modelling as well as spectral decomposition and amplitude attributes analyses attribute assisted seismic interpretation and mapping results reveal a massive northwest southeast trending shale cored detachment anticline containing numerous associated normal faults strata in the freeman field are interpreted to have been deformed by extension in the crestal region of a compressional fault related fold and secondarily by inflation of the anticline by flow of the ductile undercompacted and overpressured akata shale the mid miocene to pliocene agbada formation reservoirs occur as part of a channelized fan system mostly deposited as turbidites in an unconfined distributary environment on a massive northwest southeast trending shale cored detachment anticline containing numerous associated normal faults except a reservoir sand that occurs as channel fill in a confined depositional environment within a submarine canyon that came across and eroded a previously deposited distributary fan complex suggesting likely presence of prospective areas for hydrocarbon exploration southwest of the freeman field

migration of volcanism in the san francisco volcanic field arizona the remanent magnetization of volcanic rocks has been determined at 650 sites in the san francisco volcanic field in the southern part of the colorado plateau the polarity of remanent magnetization combined with k ar age determinations spatial and petrographic associations stratigraphic relations and state of preservation of the cinder cones provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and 100 intermediate to silicic flows flow sequences and vents the age assignments for basaltic rocks include 243 brunhes 0 73 ma vents 220 matuyama 0 73 to 2 48 ma vents and 147 pre matuyama 2 48 to about 5 0 ma vents basaltic volcanism migrated northeastward before matuyama time at a rate of 1 2 cm yr and eastward s87 5 e over the past 2 5 m y at a rate of 2 9 0 3 cm yr concomitant acceleration in total magma production from 75 to 1 400 10 6 km3 yr and frequency of basaltic eruptions from 1 per 17 000 yr to 1 per 3 000 yr occurred between 5 and 0 25 ma for the past 0 25 m y magma production 180 10 6 km3 yr and perhaps eruption frequency have decreased this evolutionary sequence coupled with the lead and strontium isotopic composition of the rocks can be explained by magmatism caused by shear heating at the base of the lithosphere we propose that this eastward drift of volcanic activity represents absolute westward motion of the north american plate our model is in agreement with a model in which the african plate is fixed to the deep mantle

the 40ar 39ar age constraints on the duration of resurgence at the valles caldera new mexico 1 the 40ar 39ar ages for rocks from the valles caldera new mexico imply that resurgent uplift of the caldera floor occurred within 27 27 ka of caldera collapse the structural resurgent dome of the valles caldera was uplifted approximately 1000 m above the surrounding caldera floor during resurgence the upper bandelier tuff which yields an age of 1 256 0 010 ma 2 ages relative to fish canyon tuff sanidine at 28 02 ma is the lower constraint on the timing of resurgence the oldest postcollapse ring fracture dome cerro del medio is the upper time constraint and yields an age of 1 229 0 017 ma for one of its oldest flow lobes therefore resurgent uplift probably occurred within 54 ka at a minimum rate of 1 9 cm yr sanidine phenocrysts from rhyolites of the postcollapse intracaldera deer canyon and redondo creek members yield 40ar 39ar single crystal laser fusion ages ranging from 1 229 0 013 ma to 1 283 0 017 ma deer canyon member n 8 and 1 208 0 017 ma to 1 239 0 017 ma redondo creek member n 4 with one exception from each unit these ages are statistically indistinguishable from the upper bandelier tuff indicating that eruption of these postcollapse rhyolites probably commenced shortly after caldera formation melt inclusion hosted excess argon was not found to have a measurable effect on the age of sanidine from the upper bandelier tuff or the deer canyon member

the enigmatic origin and emplacement of the samosir island lava domes toba caldera sumatra indonesia the 60 20 km samosir resurgent dome within the 74 ka toba caldera features several clusters of rhyolitic lava domes because the previously known tuk tuk and samosir fault lava domes occur near the base of a major resurgent dome fault they appear associated with resurgent uplift and dating them could provide constraints on the timing of resurgence towards that goal we mapped and sampled the tuk tuk and samosir fault lava domes discovered two additional lava dome clusters in the uplifted interior of samosir island then determined modal mineralogy major and trace element geochemistry and 40ar 39ar sanidine ages on several samples from each cluster these quartz bearing rhyolite lava domes have sio2 contents ranging from 70 76 wt crystallinities of 28 54 and 40ar 39ar ages 74 76 ka that overlap with those of the youngest toba tuff ytt 74 75 ka our field observations and the remarkable similarity in mineralogy geochemistry and ages among the dome clusters and ytt support the interpretation that these domes represent remnant ytt magma that erupted shortly after the climactic ytt eruption later during resurgence these pre resurgent lava domes were uplifted to their present locations some near current lake level others up to 440 m above lake level

zircon reveals protracted magma storage and recycling beneath mount st helens current data and models for mount st helens volcano washington united states suggest relatively rapid transport from magma genesis to eruption with no evidence for protracted storage or recycling of magmas however we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust cool and crystallize beneath the volcano and are then rejuvenated and incorporated by hotter young magmas on their way to the surface estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century zircon elemental compositions refl ect the increasing infl uence of mafi c input into the system through time recording growth from hotter less evolved magmas tens of thousands of years prior to the appearance of mafi c magmas at the surface or changes in whole rock geochemistry and petrology and providing a new time correlated record of this evolution independent of the eruption history zircon data thus reveal the history of the hidden long lived intrusive portion of the mount st helens system where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh infl uxes of magmas that traverse the intrusive reservoir before erupting

excess argon in melt inclusions in zero age anorthoclase feldspar from mt erebus antarctica as revealed by the method abstract historically erupted 1984 anorthoclase phenocrysts from mt erebus yield k ar and 40 ar 40 ar apparent ages as old as 700 ka indicating the presence of excess argon 40 ar 39 ar furnace step heating results from anorthoclase reveal a positive correlation between the cl k ratio and apparent age because chlorine up to 1700 ppm is present in melt inclusions but not in the anorthoclase crystal lattice this correlation suggests that the excess argon is associated with melt inclusions trapped within the anorthoclase during rapid crystal growth confirmation of the source of excess argon comes from step heating experiments on multiple anorthoclase aliquots separated from two phenocrysts and one glass aliquot prepared from the matrix of a volcanic bomb the anorthoclase phenocrysts were crushed and hf etched to yield aliquots containing 30 10 and 1 melt inclusions the step heated anorthoclase with 30 and 10 melt inclusions yielded the highest cl k ratios and apparent integrated ages of 640 30 ka and 179 16 ka respectively the anorthoclase with i melt inclusions yielded significantly lower cl k ratios and apparent integrated ages of 48 8 ka and 50 30 ka the step heated volcanic glass yielded the least variable cl k ratios and a total gas age of 101 16 ka argon released from the anorthoclase and the trapped melt inclusions can be distinguished by differences in their degassing behavior allowing geologically more reasonable ages to be obtained melt inclusions exposed on the exterior of anorthoclase grains principally degas during furnace extraction at temperatures less than 1200 c inclusions entirely within anorthoclase grains principally degas at temperatures greater than 1200 c when incongruent melting of the anorthoclase allows melt inclusion hosted excess argon to escape anorthoclase aliquots prepared with less than 1 inclusions can be fitted with a plateau for heating steps below 1200 c to yield ages as young as 8 2 ka whereas steps above 1200 c yield ages in excess of 100 ka however anorthoclase aliquots containing 10 30 melt inclusions yield ages in excess of 200 ka for heating steps below 1200 c minimizing the effects of the excess argon from melt inclusions relies on sample preparation and step heating fine crushing and treatment with hydrofluoric acid removes many of the larger melt inclusions small melt inclusions which remain within the anorthoclase degas primarily at temperatures above 1200 c temperatures below 1200 c yield the most accurate ages attempts at post analytically correcting for the chlorine correlated excess argon are hindered by the variations in 40 ar e cl within and between samples elevated 40 ar e cl ratios in bubbles within the melt inclusions as deduced from in vacuo crushing experiments are the most likely cause for some or all of a sample s total 40 ar e cl variation in addition relative solubilities of argon and chlorine within phonolitic melts may be partly responsible for variations in 40 ar e cl

revisions to the stratigraphy and volcanology of the post 0 5 ma units and the volcanic section of vc 1 core hole valles caldera new mexico reexamination of the continental scientific drilling project vc 1 core new field data and new isotopic ages necessitate revision of the previously described geology of the youngest eruptive units from the valles caldera the banco bonito rhyolite lava flow and minor attendant pyroclastic autoclastic and reworked deposits about 0 9 km3 dense rock equivalent volume dre are recognized as the product of the youngest eruption from the caldera the best estimate of the age of this event is 170 240 ka 1 a slightly earlier eruption produced the el cajete pumice fall deposit followed by the battleship rock ignimbrite and the vc 1 rhyolite totalling about 1 8 km3 dre we suggest that the vc 1 tuffs ignimbrites recognized only in the vc 1 core hole are not a separate unit as previously proposed but are in fact the battleship rock ignimbrite a complex volcaniclastic breccia dating from about the time of the south mountain rhyolite lava flow 529 ka is the lowermost volcanic unit in the vc 1 core the young post 500 ka eruption products from the sw part of the valles caldera are all composed of very similar low silica rhyolite and are thus distinct from earlier postcaldera high silica rhyolites included in the valles rhyolite formation this distinction marks a major change in the magmatic evolution of the valles caldera we propose that recognition of this change necessitates revisions to the stratigraphy of the upper part of the tewa group

crust and upper mantle p wave velocity structure beneath valles caldera new mexico results from the jemez teleseismic tomography experiment new results are presented from the teleseismic component of the jemez tomography experiment conducted across valles caldera in northern new mexico we invert 4872 relative ital p wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km the three principle features of our model for valles caldera are 1 near surface low velocities of minus 17 percent beneath the toledo embayment and the valle grande 2 midcrustal low velocities of minus 23 percent in an ellipsoidal volume underneath the northwest quadrant of the caldera and 3 a broad zone of low velocities minus 15 percent in the lower crust or upper mantle crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast near surface low velocities are interpreted as thick deposits of bandelier tuff and postcaldera volcaniclastic rocks lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast beneath the valle grande we interpret the midcrustal low velocity zone to contain a minimum melt fraction of 10 percent while we cannot rule out the possibility that this zone is the remnant 1 2 ma bandelier magma chamber the eruption history and geochemistry of the volcanic rocks erupted in valles calderamore following the bandelier tuff make it more likely that magma results from a new pulse of intrusion indicating that melt flux into the upper crust beneath valles caldera continues the low velocity zone near the crust mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle in either case this low velocity anomaly indicates that underplating by mantle derived melts has occurred copyright 1998 american geophysical union less

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

ultra high precision 40ar 39ar ages for fish canyon tuff and alder creek rhyolite sanidine new dating standards required abstract the 40 ar 39 ar dating technique is a high precision 40 ar 39 ar standards include sanidine from the ca 28 ma fish canyon tuff fct and the ca 1 2 ma alder creek rhyolite acr despite common usage the ages of fct and acr sanidine have proven contentious with reported values varying by 2 well outside the 0 1 aspiration of earthtime www earth time org the current study presents ultra high precision multi collector 40 ar 39 ar results for fct and acr sanidine using a new generation argusvi mass spectrometer these analyses demonstrate the significantly higher by a factor of 10 level of precision achievable using the argusvi system compared to most previous studies significantly 40 ar 39 ar step heating analyses resolve distinct age gradients for both fct and acr sanidine possible explanations for these gradients include combinations of recoil loss of 39 ar instrumental bias isotopic fractionation during step heating thermally induced argon loss and or extraneous argon contamination minor recoil loss of 39 ar and partial retention of inherited argon during extended magma residence are deemed the most plausible causes of the age discordance given this assumption we calculate revised maximum eruption ages of 28 01 0 04 ma 2 for fct and 1 178 0 002 ma 2 for acr relative to the astronomically calibrated age of a1 tephra sanidine crete rivera et al 2011 earth planet sci lett 311 420 426 nonetheless the argusvi data demonstrate that fct and acr sanidine are non ideal as high precision 40 ar 39 ar standards with different heating protocols likely to induce inter laboratory bias the current findings account for at least some of the scatter in inter calibration ages reported previously for various 40 ar 39 ar standards in broader terms the results mandate a re evaluation of the astronomically calibrated ages for these and other sanidine samples while the new generation of multi collector mass spectrometers provides the means to further evaluate 40 ar 39 ar ages used to define new decay constants and standard ages

the el cajete series valles caldera new mexico three rhyolite eruptive units totaling 3 km3 dense rock equivalent volume are the youngest products from the valles caldera new mexico these pyroclastic and effusive units herein called the el cajete series were erupted over an appreciable time period with plinian and ignimbrite forming activity preceding an effusive phase by a lengthy but indeterminable interval new u th disequilibrium and published fission track dates support an age for the event between 170 and 130 ka the explosive phases formed the el cajete plinian deposit about 1 3 km3 minor dry pyroclastic surges and the battleship rock tuff about 1 0 km3 a valley confined welded ignimbrite the plinian eruption column is estimated to have been 28 km high during deposition of the most widely dispersed pumice fall unit slow effusion of the banco bonito obsidian lava flow 1 0 km3 onto a dissected surface cut into the el cajete and battleship rock pyroclastic deposits accompanied by minor explosive activity terminated the event twenty four bulk rock samples from all three eruptive stages exhibit little significant compositional variation and the three units are petrographically identical apart from differences arising from contrasting eruption styles very few phenocrysts appear to have been in equilibrium with their enclosing high silica rhyolitic liquid plagioclase grains are typically strongly resorbed while biotite and hornblende frequently occur in aggregate grains in which textures characteristic of plutonic rocks can be discerned these features result from partial melting of preexisting crustal igneous rock probably of dioritic to granodioritic composition and subsequent eruption of the resulting liquid plus restite crystals rapid generation and eruption of rhyolitic magma during this most recent phase of activity in the jemez mountains may imply that the valles magma system is presently in a state where small magma bodies are transient phenomena

teleseismic p wave image of crust and upper mantle structure beneath the valles caldera new mexico initial results from the 1993 jtex passive array teleseismic p wave relative arrival time data collected from a temporary array during the 1993 jemez tomography experiment jtex have been inverted to image velocity anomalies beneath the valles caldera in northern new mexico instruments were deployed in two 30 km long profiles one of 8 and one of 9 stations these profiles crossed the caldera trending at azimuths of n46 w and n60 e respectively two dimensional teleseismic relative arrival time inversion of the 1993 data set supplemented with data from an overlapping 1987 profile confirms the existence of a mid crustal low velocity region 30 beneath the valles caldera in the depth range of 8 to 13 km below sea level with about a 6 km horizontal extent this feature is interpreted to be the seismic expression of the remnant magma chamber a shallow low velocity anomaly beneath san antonio mountain coincides with the region of highest thermal gradient values in the caldera a lower crust upper mantle low velocity anomaly is imaged but is not as well constrained due to the limited length of the profile we tentatively correlate this anomaly with the thermal effects of basaltic magmas ponded at the crust mantle boundary

the duration of a yellowstone super eruption cycle and implications for the age of the olduvai subchron abstract small volume rhyolitic eruptions preceding and following a caldera forming eruption can provide insights into the tempo of eruption cycles and timing of magmatic recharge in this contribution high precision 40 ar 39 ar eruption ages were obtained on the three effusive eruptions bracketing the huckleberry ridge tuff which comprise yellowstone s first volcanic cycle these dates are supplemented with detailed paleomagnetic and rock magnetic analyses to resolve discrepancies with previous reported stratigraphy the huckleberry ridge tuff 2 08 ma was preceded by an eruption at 2 14 ma and followed by eruptions at 1 98 and 1 95 ma all of which occurred during four distinct periods of geomagnetic instability within the matuyama chron the first volcanic cycle of yellowstone has now been constrained to within a 200 kyr timespan or half of the previously proposed duration and similar to the duration of volcanic activity for caldera forming systems in the jemez volcanic field the maximum duration for magmatic recharge for the first yellowstone volcanic cycle is no greater than 100 kyr and likely closer to 40 kyr furthermore the combined 40 ar 39 ar eruption ages and paleomagnetic results provide polarity anchors for the pre olduvai excursion and olduvai subchron which are often used as tie points in studies of early pleistocene hominin evolution

on the eruption age and provenance of the old crow tephra abstract tephrochronology is used to correlate and reconstruct geographically disparate sedimentary records of changing environment climate and landscape throughout geologic time single tephra layers represent isochronous markers across broad regions thus accurate and precise radiometric constraints on the timing of eruption are critical to their utility the old crow tephra is found throughout eastern beringia and represents the largest preserved pleistocene ashfall event in the region despite its volume and significance as a stratigraphic marker the provenance of this tephra is debated and the interpreted eruption age of marine isotope stage mis 5 at 125 ka has vacillated to investigate provenance and eruption age we develop a geochemical fingerprint for the old crow tephra via titanomagnetite geochemistry and zircon crystallization cooling age via coupled u pb u th and u th he zircon geochronology our results indicate that old crow oxides are geochemically distinct from the commonly assumed source caldera system at the emmons lake volcanic center elvc zircon crystals from the old crow tephra range in age from proterozoic to pleistocene with concordant zircon u pb u th and u th he dates on the youngest population of grains suggesting crystallization in their parent magma and in turn eruption at 202 9 9 5 ka we discuss strengths and shortcomings of our radiogenic datasets in light of this result and review the far reaching implications of a change in old crow eruption age

carbonic springs as distal manifestations of geothermal systems highlighting the importance of fault pathways and hydrochemical mixing example from the jemez mountains new mexico abstract we examine a series of carbonic warm and hot springs in northern new mexico usa 1 tierra amarilla springs 2 penasco springs and 3 soda dam jemez and indian hot springs spring geochemistry is compared to waters from the valles caldera geothermal system and to groundwater in nearby sedimentary aquifers multiple hydrochemical tracers are applied to quantitatively evaluate flow paths and mixing at varying distances from the caldera we test three hypotheses for source and transport of waters to tierra amarilla and penasco springs san juan basin origin meteoric flow from the nacimiento mountains and or influence from the valles caldera geothermal system geochemically carbonic spring groups are distinct from meteoric and sedimentary aquifer waters based on isotopes of he and sr and concentrations of cl so4 li and b we interpret these carbonic springs to be distal manifestations of fluid circulation along faults with a mixture of valles caldera geothermal waters local meteoric and paleozoic aquifer waters with the potential for small contributions from the san juan basin aquifers semi confined fault conduits the jemez fault and nacimiento fault systems provide connectivity and help explain geochemical similarities and mixing trends within carbonic spring systems and between these systems and the distal valles caldera in addition penasco springs are interpreted to reflect a component of outflow from the geothermal system that crosses the nacimiento mountain basement block along ne trending faults input of deeply sourced waters can degrade water quality by contributing significant salinity and trace metals to groundwater at distances of 50 km from geothermal systems with faults acting as conduits for subsurface fluid flow