changing techniques and paradigms for the evaluation of magmatic processes the study of magmatic processes is at a juncture new continuum models for the complex physics of crystallization and melting with convection and crystal melt segregation are now available and the models are amenable to exploration by numerical methods these models which were largely developed in the fields of material science and mechanical engineering will allow petrologists to consider a wide range of transport phenomena crystal settling with reaction as fully multiphase flow combined thermal and compositional buoyancy with realistic material property variations open system and eruptive behavior and the appropriate system geometry and boundary conditions the traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal melt mush subject to reintrusion or mobilization percolative flow and reaction in this mush including contaminants may be as important as near liquidus processes in driving petrological diversity thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes melting of the crust following intrusion by basalt can be considered in light of two end members the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes although more commonly cartooned the large sill type may not be nearly as efficient as repeated diking in driving crustal melting magma mixing and mingling the greatest challenge for those studying magma dynamics will be to key the models to well constrained geological examples and to raise the sophistication of the content of these models in light of well defined tests that have geological significance

time scales of hybridisation of magmatic enclaves in regular and chaotic flow fields petrologic and volcanologic implications this paper describes numerical models of advection diffusion between enclaves and host magmas applied with the aim of estimating time scales during which enclaves can be homogenised in particular advection was simulated using a numerical system consisting of regular and chaotic regions results indicate that the homogenisation time of enclaves in chaotic regions is several orders of magnitude faster than in regular regions for instance an enclave with a diameter of 100 cm may be homogenised in the chaotic region in 380 years assuming an advection velocity of 10 cm year whereas in the regular region it would require 6 5 105 years for complete homogenisation this implies that in the same magmatic system large differences in the degree of homogenisation may co exist generating magmatic masses with large spatial and temporal inhomogeneities

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

volatiles in lunar felsite clasts impact related delivery of hydrous material to an ancient dry lunar crust abstract in this detailed geochemical petrological and microstructural study of felsite clast materials contained in apollo breccia samples 12013 14321 and 15405 little evidence was found for relatively enriched reservoirs of endogenic lunar volatiles nanosims measurements have revealed very low volatile abundances 2 18 ppm hydrogen in nominally anhydrous minerals nams plagioclase potassic alkali feldspar and sio2 that make up a majority of these felsic lithologies yet these mineral assemblages and clast geochemistries on earth would normally yield relatively high volatiles contents in their nams 20 to 80 ppm hydrogen this difference is particularly notable in felsite 14321 1062 that exhibits extremely low volatile abundances 2 ppm hydrogen and a relatively low amount of microstructural evidence for shock metamorphism given that it is a clast of the most evolved 74 wt sio2 rock type returned from the moon if taken at face value wet felsic magmas 1 2 1 7 wt water are implied by the relatively high hydrogen contents of feldspar in felsite clasts in apollo samples 12013 and 15405 but these results are likely misleading these felsic clasts have microstructural features indicative of significantly higher shock stress than 14321 1062 these crustal lithologies likely obtained no more water from the lunar interior than the magma body producing 14321 1062 rather we suggest hydrogen was enriched in samples 12013 and 15405 by impact induced exchange and or partial assimilation of volatiles added to the surface of the moon by a hydrated impactor asteroid or comet or the solar wind thus the best estimate for magmatic water contents of felsic lunar magmas comes from 14321 1062 that leads to a calculated magmatic water content of 0 2 wt this dry felsic magma has a slightly greater but comparable water content to the ancient mafic magmas implied by the other lithologies that we have studied based on this and expanding evidence for a significantly dry ancient or early degassed moon it is likely that some recent estimates 100 s ppm of the water abundances in the lunar parental magma ocean have been overestimated

formation of double layered ejecta craters on mars a glacial substrate model 1 a class of martian impact craters with particularly unusual ejecta characteristics double layered ejecta dle craters are preferentially located in the midlatitudes in both hemispheres of mars unlike today decameters thick deposits of snow and ice occupied these same latitudes for significant periods during the amazonian period we assess the hypothesis that the unusual double layer morphology could be related to impact into a snow and ice glacial substrate followed by landsliding of ejecta off of the structurally uplifted rim we find that many characteristics of dle craters e g latitudinal distribution lack of secondaries landslide like textures evidence for overthrusting relation to other crater types etc are consistent with such an origin

nd and sr isotopic study of crustal and mantle inclusions from the sierra nevada and implications for batholith petrogenesis sm nd and rb sr systematics have been examined for eight upper mantle and lower crustal xenoliths the xenolith bearing trachyandesite and the granodiorite intruded by the trachyandesite in the sierra nevada batholith isotopic heterogeneity is shown by the xenoliths with 143 nd 144 nd ranging from 0 51169 to 0 51306 and 87 sr 86 sr ranging from 0 7031 to 0 7333 these ranges are similar to those of the plutonic rocks of the batholith this similarity suggests that the xenoliths believed to be representative of mantle and crustal reservoirs may approximate the diverse source regions from which the varied granitic rocks of the batholith were derived and that the sr nd isotopic correlation seen in granitic rocks may not be due to simple mixing of two end member components the subgranitic crust in this part of the sierra nevada apparently consists of metamorphosed sedimentary and igneous rocks which are highly variable isotopically the mantle beneath the sierra nevada is also isotopically heterogeneous both vertically and laterally

rapid assembly and rejuvenation of a large silicic magmatic system insights from mineral diffusive profiles in the kidnappers and rocky hill deposits new zealand abstract the timescales over which magmas in large silicic systems are reactivated assembled and stored remains a fundamental question in volcanology to address this question we study timescales from fe mg interdiffusion in orthopyroxenes and ti diffusion in quartz from the caldera forming 1200 km3 kidnappers and 200 km3 rocky hill eruptions from the mangakino volcanic centre taupo volcanic zone new zealand the two eruptions came from the same source area have indistinguishable 40ar 39ar ages 1 0 ma and zircon u pb age spectra but their respective deposits are separated by a short period of erosion compositions of pumice glass and mineral species in the collective eruption deposits define multiple melt dominant bodies but indicate that these shared a common magmatic mush zone diffusion timescales from both eruptions are used to build on chemical and textural crystal signatures and interpret both the crystal growth histories and the timing of magma accumulation fe mg interdiffusion profiles in orthopyroxenes imply that the three melt dominant bodies established through extraction of melt and crystals from the common source were generated within 600 years and with peak accumulation rates within 100 years of each eruption in addition a less evolved melt interacted with the kidnappers magma beginning 30 years prior to and peaking within 3 years of the eruption this interaction did not directly trigger the eruption but may have primed the magmatic system orthopyroxene crystals with the same zoning patterns from the kidnappers and rocky hill pumices yield consistently different diffusion timescales suggesting a time break between the eruptions of 20 years from core rim zones to 10 years outer rim zones diffusion of ti in quartz reveals similarly short timescales and magmatic residence times of 200 km 3 eruption our data support concepts of large silicic systems being stored as long lived crystal mushes with eruptible melts generated over extraordinarily short timescales prior to eruption

microchemical and sr isotopic investigation of zoned k feldspar megacrysts insights into the petrogenesis of a granitic system and disequilibrium crystal growth k feldspar megacrysts kfm are used to investigate the magmatic evolution of the 7ma monte capanne mc monzogranite elba italy dissolution and regrowth of kfm during magma mixing or mingling events produce indented resorption surfaces associated with high ba contents diffusion calculations demonstrate that kfm chemical zoning is primary core to rim variations in ba rb sr li and p support magma mixing i e high ba and p and low rb sr at rims but more complex variations require other mechanisms in particular we show that disequilibrium growth related to variations in diffusion rates in the melt may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber initial 87 sr 86 sr ratios isr obtained by microdrilling decrease from core to rim inner core analyses define a mixing trend extending towards a high isr rb sr melt component whereas the outer cores and rims display a more restricted range of isr but a larger range of rb sr lower isr at the rim of one megacryst suggests mixing with high k calc alkaline mantle derived volcanics of similar age on capraia trace element and isotopic profiles suggest 1 early megacryst growth in magmas contaminated by crust and refreshed by high isr silicic melts as seen in the inner cores and 2 later recharge with mafic magmas as seen in the outer cores followed by 3 crystal fractionation with possible interaction with hydrothermal fluids as seen in the rim the model is compatible with the field occurrence of mafic enclaves and xenoliths

diffusional gradients at the crystal melt interface and their effect on the compositions of melt inclusions melt inclusions are widely believed to represent melts from which host crystals grew melt inclusions represent melt adjacent to growing crystals where compositional gradients exist due to preferential incorporation or exclusion of components by the crystallizing mineral the possibility arises therefore that melt in inclusions may differ significantly from melt which was more remote from growing crystals at the time the crystals grew we have tested this possibility by analyzing 45 major minor and trace components in 50 to 400 m diameter melt inclusions in phenocrysts from the rhyolitic bishop tuff california the following observations indicate that the effects of compositional gradients on chemical compositions of melt inclusions are negligible 1 melt inclusions in quartz and sanidine phenocrysts have indistinguishable compositions 2 no correlation is observed between sizes of melt inclusions and their chemical compositions 3 ten melt inclusions in four quartz phenocrysts from a single

friction and dynamics of rock avalanches travelling on glaciers rock avalanches travelling on glaciers often exhibit effective friction coefficient lower than those on a rocky terrain after briefly considering some data of rock avalanches on glaciers the physics of sliding of solid objects on icy surfaces is reviewed and a model is put forward for the mechanics of rock avalanche sliding on ice accounting for the formation of a natural lubricating layer it is suggested that at the beginning of the flow of a rock avalanche friction results from rocky blocks ploughing on ice as the erosion continues a gouge of ice particles results which clogs the interstices between blocks and may partially melt as a consequence of the production of frictional heat this conceptual model is numerically investigated for a slab travelling on ice the results show an increase in mobility as a function of slab thickness travelled length and the gravity field in agreement with case studies the results are useful to interpret the peculiar features of rock avalanches travelling on icy surfaces such as digitations out runner blocks and longitudinal furrows the lubrication theory for landslides on ice proposed here may provide a framework for understanding landslides on earth and for future modelling in addition it may help elucidate the presence of similar landslide deposits on the surface of mars

a multi system geochronology in the ad 3 borehole pannonian basin hungary with implications for dating volcanic rocks by low temperature thermochronology and for interpretation of u th he data independent geochronological and thermal modelling approaches are applied to a biostratigraphically exceptionally well controlled borehole alcs ad 3 in order to constrain the age of cenozoic geodynamic events in the western pannonian basin and to test the efficacy of the methods for dating volcanic rocks apatite fission track and zircon u pb data show two volcanic phases of middle eocene 43 4 39 0 ma and early oligocene 32 72 0 15 ma age respectively apatite u th he ages 23 8 14 8 ma and independent thermal and subsidence history models reveal a brief period of heating to 55 70 ca t 17 ma caused by an increased heat flow related to crustal thinning and mantle upwelling our results demonstrate that contrary to common perception the apatite u th he method is likely to record apparent or mixed ages resulting from subsequent thermal events rather than cooling or eruption ages directly related to distinct geological events it follows that a direct conversion of apparent or mixed u th he ages into cooling exhumation or erosion rates is incorrect

a study of strontium diffusion in k feldspar na k feldspar and anorthite using rutherford backscattering spectroscopy abstract sr chemical diffusion has been measured in orthoclase anorthoclase and anorthite under dry 1 atm conditions a strontium oxide aluminosilicate powder mixture was used as the source material with rutherford backscattering spectroscopy rbs used to measure diffusion profiles over the temperature range 725 1075 c the following arrhenius relations were obtained orthoclase normal to 001 d 5 97 10 0 0060 0 0030exp 67900 1600rtcm 2 s 1 measurements of diffusion perpendicular to 010 and 100 indicate little diffusional anisotropy in orthoclase anorthoclase normal to 010 d 4 51 10 1 358 5 6exp 89100 4800rt cm 2 s 1 normal to 001 d 2 25 10 2 1615 31exp 89300 4600rt cm 2 s 1 anorthite normal to 010 d 3 85 10 2 0 380 0 0039 exp 78800 5400rt cm 2 s 1 the correlation of sr uptake with a reduction of k and ca in orthoclase and anorthite respectively indicates that sr is exchanging with these species nuclear reaction analysis nra measurements of al in orthoclase suggests that the coupled exchange sr 2 al 3 k 1 si 4 is taking place as expected sr closure temperatures for feldspars calculated with the above parameters are relatively high compared to those of some other minerals e g biotite muscovite apatite however closure temperatures of feldspars may be significantly depressed if they are exsolved or twinned leading to small effective diffusion lengths feldspar crystals of millimeter size should retain 87 sr 86 sr ratios except when subjected to thermal events of extreme temperature or duration

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

global scale analysis of martian landslide mobility and paleoenvironmental clues the mobility of landslides on mars is studied based on a database of 3 118 events to establish the volume of the landslides for the whole dataset based on the deposit area a new volume area relationship based on a representative dataset of 222 landslides is used plotting the h l ratio between fall height h and runout l versus volume landslide mobility is analyzed and compared with existing empirical relationships for martian and terrestrial landslides analyzing the mobility in terms of normalized residuals i e the relative deviation of the h l ratio from the dataset best fit line mobility is found to depend on both the landslide location on mars and landslide typology this allows us to identify four different types of high mobility hypermobile landslides three classes of high mobility landslides are associated respectively to meteoroid impact the olympus mons aureoles and landslides with toreva block failure style and their mobility can be explained by the peculiar flow mechanics the fourth class includes landslides associated with isolated craters those in the regions wetted by the putative oceanus borealis and the ones at high latitudes we suggest that the common factor behind all the hypermobile landslides of this fourth kind is the presence of ice this is confirmed by our data showing that landslides increase in mobility with latitude the latitudinal trend mirrors the distribution of ice as detected by radar neutron probes and the presence of glacial and layered ejecta morphologies because the overall landslide distribution supports the presence of ice lubricated conditions two ice lubrication models are presented showing how ice melting within or underneath the landslides could enhance mobility by proper analysis in terms of apparent friction residuals we find that the mobility of landslides in valles marineris with the largest landslide concentration is lower than average we explain this circumstance partly from the smaller role of ice in equatorial valles marineris and partly because the collapses from high slope relief imply high speed impact with the floor valley confinement loss of momentum and decrease in mobility environmental consequences imply that the present subsurface ice distribution may have been persistent throughout the amazonian period

reassessing rock mass properties and slope instability triggering conditions in valles marineris mars the rock walls of the valles marineris valleys vm in the equatorial area of mars exhibit several gravitational failures which resulted in a series of large landslides up to several hundred cubic kilometers in volume questions arise as to forces at play and rock strength in the stability of the walls of vm in this work we address the stability analysis of the walls of vm by considering the strength of the materials of the walls and the causes of landslides using finite element calculations and the limit analysis upper bound method we explore the range of cohesion and friction angle values associated with realistic failure geometries and compare predictions with the more classical culmann s translational failure model our analysis is based both on synthetic simplified slope profiles and on the real shape of the walls of vm taken from the mola topographic data validation of the calibrated cohesion and friction angle values is performed by comparing the computed unstable cross sectional areas with the observed pre and post failure profiles the estimated failure surface geometry and ridge crest retreat this offers a link between rock mass properties slope geometry and volume of the observed failure represented in dimensionless charts the role of groundwater flow and seismic action on the decrease of slope stability is also estimated pseudo static seismic analyses provide another set of dimensionless charts and show that low seismicity events induced by meteoroid impacts consistent with the size of craters could be a cause for some of the observed landslides if poor rock properties for vm are assumed analyses suggest that rock mass properties are more similar to their earth equivalents with respect to what has been previously supposed

the stability of ground ice in the equatorial region of mars consideration of the partial pressure of h2o in the martian atmosphere and the range of mean annual temperatures at the martian surface suggests that the occurrence of ground ice in equilibrium with the atmosphere is restricted to latitudes poleward of 40 however there is a growing body of morphologic evidence that indicates that substantial quantities of ground ice may have been present in the equatorial regolith throughout martian geologic time the accepted explanation for this apparent contradiction has been that the h2o found near the equator is a relic emplaced very early in martian geologic history 3 5 b y ago and under substantially different climatic conditions it is generally believed that this fossil ground ice layer has been preserved to the present day by the diffusion limiting properties of a relatively shallow layer 10 m of fine grained regolith to evaluate this hypothesis the lifetime of a 200 m layer of ground ice buried below 100 m of ice free regolith has been examined for the latitudes between 30 on mars twelve model pore size distributions representative of silt and clay type soils found on earth were selected to simulate the pore structure of the martian regolith the parallel pore model of gaseous diffusion was then used to calculate the flux of escaping h2o molecules from the buried ground ice layer the potential effects of such factors as depth of burial the martian geothermal gradient regolith porosity adsorption surface diffusion and climatic change were also considered based on our analysis the most important factors affecting the stability of equatorial ground ice appear to be 1 soil structure 2 the magnitude of the geothermal gradient and 3 the climatic desorption of co2 from the regolith we conclude that while the conditions necessary for the long term survival of ground ice may exist in isolated regions within the equatorial regolith on the global scale it appears unlikely that a fossil ground ice layer has been preserved throughout martian geologic history if massive ground ice does presently exist in the equatorial regolith then its presence may imply the existence of some continuous or periodic process of replenishment

links between magma tectonic rate balances plutonism and volcanism with smith s 1979 model of ash flow magmatism as the point of departure quantitative regimes of crustal magmatic evolution are outlined the average extrusive regime of ash flow magmatism operates at a rate of about 0 001 km3 yr progressively larger episodes of increasing repose times characterize the evolution of caldera forming eruptions the intrusive regime operates at an inferred average rate of about 0 01 km3 yr expressed in terms of the volumetric influx of mafic magma from the mantle based on the record of silicic rates of extrusion and a 10 1 intrusive to extrusive ratio derived from evidence of zoned silicic systems this rate is almost identical with the mean cumulative rate for the hawaiian emperor system of basaltic volcanism which is determined to be 0 015 km3 yr fluctuations about a mean rate of 10 2 km3 yr lead to a threefold classification of magmatic regimes in terms of source power depending on whether they are persistently greater than hher regime about equal to mher regime or lower than lher regime the mean hawaiian emperor rate ash flow magmatism correlates with the mher regime averaged over a sufficiently long time thus the conditions of hawaiian emperor magmatism are identified with a characteristic power of magma transfer rates that optimize mechanisms for creation of high level chambers of derivative and fractionated silicic magma terminology of smith 1979 within sufficiently localized regions of high residence times these conditions represent a balance point and a bifurcation in dynamic paths of magmatic evolution leading to widely divergent types of plutonic volcanic associations categorical examples of the principal types of igneous path are discussed relative to continental basalt silicic ash flow batholithic granite and intrusive porphyry systems the henry mountains utah are thought to represent an example of an intrusive porphyry generated in the lher regime of a mafic source system lacking sufficient power for volcanic expression or significant mineralization long valley california and possibly other systems along the eastern front of the sierra nevada is an example operating at the mean rate with transient excursions to higher and lower rates in regions of rapid extension and or volcanic propagation whether linear or areal these regimes give rise to basaltic eruptions ranging from isolated cinder cones to flood basalts

new insights on the runout of large landslides in the valles marineris canyons mars 1 analogy with lab scale dry granular flow experiments demonstrates that runouts and deposits heights of valles marineris vm landslides can be scaled on a curve varying primarily with the initial aspect ratio of the mobilized rock mass before slope failure this results suggests both that any interstitial fluid played a negligible part in the vm landslides dynamics and that mobility is not an appropriate tool to characterize their dynamics

raising the colorado plateau shallow marine rocks exposed on the 2 km high 45 km thick colorado plateau in the western united states indicate that it was near sea level during much of the phanerozoic isostatic calculations however illuminate the difficulty in maintaining a 45 km thick crust at or near sea level we propose that an isostatically balanced 30 km thick proto colorado plateau crust was thickened during the late cretaceous to early tertiary by intracrustal flow out of an overthickened sevier orogenic hinterland this plateau would have been supported by a thick 70 km crustal root which is proposed to have been the source region for hot and weak mid crustal material that flowed eastward from the plateau toward the low elevation proto colorado plateau

sr isotope systematics of k feldspars in plutonic rocks revealed by the rb sr microdrilling technique sr 86 sr isotope variation was studied in subareas of k feldspar megacrysts from late palaeozoic granodiorite diorite and syenite of the bavarian forest most samples were collected close to the bavarian pfahl shear zone and feldspars from these sites reveal remarkably constant initial 87 sr 86 sr isotope ratios these ratios are similar to those of the bulk groundmass or feldspar crystals from the groundmass individual feldspar grains however define isochron ages that are considerably younger by 15 to 30 ma than the crystallisation age of the host rocks as defined by u pb and pb pb zircon geochronology the rb sr feldspar data probably reflect diffusion controlled isotopic homogenisation under hydrous conditions between 290 310 ma a pink feldspar crystal from a hydrothermally altered syenite reveals comparatively large degree of isotopic variation indicating local disturbance of the rb sr system caused by hydrothermal alteration during permian triassic pfahl quartz precipitation a diorite from the furstenstein pluton displays some isotopic scatter and slight differences in the initial 87 sr 86 sr ratios between the more radiogenic feldspar megacryst and the less radiogenic bulk groundmass this rock type was subsequently intruded by the saldenburg granite and the more radiogenic 87 sr 86 sr isotope signature of the feldspar relative to the groundmass is interpreted as being consistent with the feldspar having exchanged with the fluids derived from the granite this study shows that k feldspar crystals from slowly cooled plutonic rocks do not necessarily preserve their original sr isotope composition however sr isotope traverses through single crystals can give robust records of the mm scale isotope system behaviour also in older plutonic rocks relevant for solving petrological or geochronological problems d 2005 elsevier b v all rights reserved

mobility and topographic effects for large valles marineris landslides on mars 1 recent experiments on dry granular flows over horizontal plane bare some similarities with large martian landslides observed in valles marineris vm however martian normalized runout are twice as large as those that observed in dry granular flow experiments numerical simulations on theoretical 2d and real 3d topographies reconstructed from remote sensing data show that slope effects significantly reduce the shift between experimental results and martian observation however topography effects are not strong enough to explain the high mobility of martian landslides as a result other physical and or geological processes should play a key role into the dynamics of martian landslides a new mobility is defined that makes it possible to characterize the dynamics of the flow regardless of the geometry of the released mass and of the underlying topography

comparison of element and isotope diffusion of k and ca in multicomponent silicate melts recent experimental work has shown that the homogenization of elemental concentrations can be much slower than that of isotopic ratios when there are strong concentration gradients in sio 2 and al 2o 3 the ramifications of this result for magma homogenization and other petrological problems related to diffusion are significant we report here a comparison of experimental profiles of elemental concentrations and isotopic fractions of k and ca in rhyolite andesite large concentration gradients and rhyolite rhyolite small concentration gradients melt couples when the concentration profile and the isotopic profile of the same element in a single couple are compared the former is much shorter than the latter in the rhyolite andesite couple consistent with other recent studies however the lengths of both concentration and isotopic profiles are similar in the rhyolite rhyolite couple therefore diffusion of an element or oxide may be decoupled from or coupled with isotopic diffusion depending on whether large concentration gradients of major components are present when the two couples are compared the intrinsic effective binary diffusivities obtained from isotopic profiles are similar for each element in the two couples whereas the effective binary diffusivity of k obtained from the concentration profile in the rhyolite rhyolite couple is 37 times that in the rhyolite andesite couple therefore isotopic homogenization is roughly independent of elemental homogenization and the presence of sio 2 al 2o 3 and other concentration gradients whereas elemental homogenization is strongly affected by concentration gradients our experimental data isotopic and concentration profiles including uphill diffusion profiles can be modeled quantitatively to a good approximation using a modified effective binary diffusion model in which the flux of a component is assumed to be proportional to its activity gradient instead of its concentration gradient therefore the multicomponent diffusion effect in the silicate systems of our experiments seems to be largely due to contributions of non ideal mixing to the cross terms of the diffusivity matrix

comment on evidence for long residence times of rhyolitic magma in the long valley magmatic system the isotopic record in precaldera lavas of glass mountain by a n halliday g a mahood p holden j m metz t j dempster and j p davidson comment and replies on the paper of a n halliday g a mahood p holden j m metz t j dempster and j p davidson 1989 earth planet sci lett 94 274 290

a pliocene ignimbrite flare up along the tepic zacoalco rift evidence for the initial stages of rifting between the jalisco block mexico and north america the tepic zacoalco rift a nw trending corridor 50 250 km is one arm of a triple rift system in western mexico together with the colima rift and the middle america trench it bounds the jalisco block a portion of western mexico that may be moving independently of north america the predominant basement rock types in the tepiczacoalco rift are rhyolitic ash fl ow tuffs and lavas which were previously assumed to be oligocene miocene in age related to the sierra madre occidental volcanic province or older new 40 ar 39 ar dates on 41 volcanic samples reveal a previously unrecognized voluminous fl are up of rhyolitic ignimbrites between 5 and 3 ma throughout the entire corridor of the tepic zacoalco rift they are often associated with pliocene high ti basalts the eruption rate during this pliocene time period was an order of magnitude higher hundreds of m m y than that documented in the tepic zacoalco rift over the last 1 m y the pliocene ash fl ow tuffs have been faulted along nw trending lineaments producing vertical offsets up to at least 500 m the voluminous ignimbrite fl are up in the tepiczacoalco rift at 5 3 ma may refl ect the initial stages of rifting of the jalisco block away from north america analogous to what occurred in the proto gulf region at 12 6 ma prior to the transfer of baja california from north america to the pacifi c plate additionally new 40 ar 39 ar dates show that the sierra madre occidental volcanic province extends across the entire width of the tepic zacoalco rift and terminates abruptly at the northern boundary of the jalisco block near the rio ameca in contrast paleoceneeocene basement from the jalisco block extends northward into the tepic zacoalco rift where it is locally overlain by sierra madre occidental rhyolites

large rock slides in impact craters on the moon and mercury abstract impact craters are the most common surface features on the moon and mercury on these two bodies we recognized and mapped large landslides on the walls of impact craters through visual inspection of high resolution imagery we compiled an inventory of 60 landslides on the moon and a second inventory of 58 landslides on mercury adopting categories used to catalog terrestrial mass movements we classified the landslides on the moon and mercury as rock slides we determined the probability density distribution of their planimetric area and we compared the distributions with similar distributions for terrestrial and martian landslides using data from the literature we found that rock slides mapped in impact craters on the moon are on average larger than analogous rock slides on mercury the relationship between the area of the individual rock slides and the area of the hosting crater suggests that rock slides on mercury initiate in smaller craters we hypothesize that the above findings are an effect of the weaker surface gravity of the moon compared to that of mercury and or an effect of the rock material properties

field and geochemical constraints on mafic felsic interactions and processes in high level arc magma chambers an example from the halfmoon pluton new zealand the 140 ma halfmoon pluton of stewart island new zealand provides direct evidence for a number of physico chemical processes that operate at depth within active arc settings it is characterized by a sequence of mingled mafic sheets and enclaves that are preserved within intermediate felsic host rocks way up structures and textures are consistent with a younging direction to the south and allow a south dipping magma chamber stratigraphy to be constructed the similarity in modal mineralogy geochemistry and isotopic signature between the various components and their proximity in space and time indicates that the system can be considered as a single composite pluton with discrete portions active at different times interpretation of mingling structures and textures in the field combined with detailed geochemical and geochronological analyses has allowed the identification of several physical and chemical processes operating within the chamber including 1 multiple mafic magma pulses that were extensively mingled and sometimes physically mixed at the exposed level and at depth 2 the development of mineral fabrics e g aligned plagioclase and hornblende crystals and alignment of mafic enclaves caused by processes of crystal accumulation magmatic flow and compaction 3 shortening of the chamber during crystallization as a result of magmatic loading 4 fractional crystallization of the intermediate felsic host magma that was interrupted periodically by mafic magma intrusion 5 incremental assembly of the pluton from the base up 6 limited chemical transfer between the mingled mafic sheets and enclaves and the surrounding intermediate felsic host rocks variations in morphology chemistry and texture of the mingled mafic rocks relative to their position within the magmatic stratigraphy indicate that processes operating within the chamber varied in both space and time mineral assemblages chemical characteristics primitive isotopic signatures and a lack of zircon inheritance indicate that the amphibole rich calc alkaline halfmoon pluton was emplaced into a juvenile arc setting and experienced no contamination with ancient crustal materials a model of pluton construction and evolution is presented that shows that the halfmoon pluton consisted of at least two adjacent magma pods that grew incrementally by episodic replenishments of mafic magma into a magma chamber of evolving intermediate felsic composition data are consistent with a model whereby hydrous amphibole rich basaltic magmas ponded at the crust mantle interface and episodically rose injected and mingled

raising the colorado plateau comment and reply comment mcquarrie and chase 2000 1 proposed three tests for their novel model first that it is possible to account for the elevation of the colorado plateau in their model by reasonable variation of crustal thickness second that mechanical parameters required of their model assume reasonable values

transition to chaos and implications for time scales of magma hybridization during mixing processes in magma chambers abstract in this contribution numerical simulations of magma mixing are carried out with the aim of understanding the physico chemical conditions that trigger the development of chaotic mixing dynamics in detail the mixing process is simulated in a magma chamber where a less evolved magma underplates a more evolved magma the chaoticity of this system is quantitatively estimated via computed finite time lyapunov exponents it is shown that magma chamber dynamics are strongly controlled by the rayleigh number ra whereas other parameters such as buoyancy ratio and viscosity ratio play a subordinate role results indicate that magma chambers with ra of the order of 10 6 are dominated by non chaotic behaviour increasing ra shifts the system towards an increasing chaotic state and in particular above ra 10 7 chaotic dynamics are fully developed in the magma chamber estimates of possible time scales of hybridization of magmas during chaotic mixing suggest for example that a blob of mafic magma with an initial diameter of 100 m within a magma chamber with a width of 1000 m is stretched to a filament like structure with a thickness of 15 m after 320 years and after 1900 years its thickness is reduced to 4 0 mm using typical chemical diffusion coefficients for magmas of the order of 10 14 m 2 s it can be calculated that a 4 0 mm thick filament would homogenize by diffusion in 10 years such homogenization time scales are orders of magnitude lower that typical life times estimated for magma chambers even in the volcanic environment indicating that fast hybridization of magmas can be rapidly achieved when chaotic dynamics govern the evolution of the magmatic system

ages of valles marineris mars landslides and implications for canyon history the chronology of landslides of valles marineris the equatorial trough system of mars has been investigated by a crater population study valles marineris landslides have widespread debris aprons which offer a remarkable opportunity to study the crater population with high resolution images from mars orbiter camera moc and from mars odyssey thermal emission imaging system themis sixty six ages were determined within valles marineris including 56 landslide ages and 10 ages of the previous termcanyonnext term floor results reveal that landslides of valles marineris system of previous termcanyonsnext term occurred during a widespread period of time between 3 5 gy and 50 my in some locations the previous termcanyonnext term floor has an apparent age of 3 5 gy suggesting that at least locally within valles marineris no major refreshing processes have occurred for 3 5 gy the temporal repetitivity of landslides implies that the triggering mechanisms of the landslides are reproducible in time landslides have the same features whatever their age the dynamic of these landslides is probably the same either with intervention of water up to recently the last 100 my or without water since 3 5 gy

the aureole of olympus mons mars as the compound deposit of submarine landslides abstract the enigmatic deposits building up the olympus mons aureole on mars are likely among the largest landslide remnants in the solar system these deposits exhibit an extraordinary run out distance up to nearly 700 km in spite of a fall height some 100 times smaller after quantifying the mobility of the olympus mons aureole lobes it is suggested based on dynamical considerations and morphological analysis that the aureole could be the consequence of a series of gigantic subaqueous landslides in order to bring evidence in favor of this interpretation comparisons are drawn between the different landslide deposits on earth and mars emphasizing the similarity with the rock avalanches of the canary islands and the hawaii the results of experimental subaqueous debris flows are also analyzed and numerical calculations are introduced to simulate the dynamics of flow in analogy with certain subaqueous landslides on earth it is possible that the outstanding run out of the aureole lobes was a consequence of hydroplaning a natural lubrication by water during flow

isotopic constraints on the production rates crystallisation histories and residence times of pre caldera silicic magmas long valley california pre caldera high silica rhyolites of glass mountain california erupted episodically from 2 1 ma until the catastrophic eruption of the bishop tuff at 0 74 ma the lavas are extremely evolved with rb sr ratios between 128 to 3640 the latter being the highest recorded from a volcanic rock glass separates from pre l 2 ma lavas define two geographically controlled rb sr isoehrons lavas adjacent to the current caldera rim define an isochron age of 2 047 0 013 ma with an initial ratio of 0 7063 2 and lavas more distant from the caldera define an isochron of 1 894 0 013 ma with the same initial ratio the isochrons are consistent with the magmas forming within 26 ka which implies a minimum magma production rate of 0 75 10 3 km3 yr over this period new 4 ar 39ar ages on sanidine and biotite have established that lavas defining each isochron were erupted over a long time interval the isochron ages being up to 360 ka older than the youngest eruption age rb sr isotope data are reported for minerals from three lavas with eruption ages of 1 990 0 012 1 866 0 014 and 1 686 0 011 ma petrographically early apatite inclusions in biotite and biotite inclusions in feldspar and quartz have glass mineral rb sr ages that are indistinguishable from the relevant regional isochron sr diffusion in feldspar is slow at the magmatie temperatures inferred for glass mountain rhyolites 700 c such that over 0 5 ma the cores of large feldspars 1 ram will retain 99 9 of their original sr the cores of sanidine and plagioclase yield glass mineral ages that are up to 300 ka older than eruption ages feldspar rim ages for two samples are indistinguishable from eruption ages the rims of sanidines and plagioclases from the third sample are 110 and 280 ka older than the eruption age and 180 and 20 ka younger than the cores these mineral age data probably reflect the combination of extended periods of mineral growth and partial isotopic exchange with the host liquid during protracted residence in a magma reservoir however the ar and sr isotopic data for biotite phenocrysts are consistent with the presence of a significant component that is recycled from earlier magmatic pulses due to the extreme rb sr ratios of the rocks and minerals it is possible to very precisely resolve the time difference between the formation of different phases assuming that they crystallised from the same host magmas

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

second reply to comment of r s j sparks h e huppert and c j n wilson on evidence for long residence times of rhyolitic magma in the long valley magmatic system the isotopic record in the precaldera lavas of glass mountain

u pb dating of magmatic and xenocryst zircons from mangakino ignimbrites and their correlation with detrital zircons from the torlesse metasediments taupo volcanic zone new zealand abstract mangakino ignimbrites are a sequence of voluminous eruptive units in the rhyolitic mangakino volcanic centre mvc caldera complex taupo volcanic zone new zealand u pb dating of 113 zircons extracted from four mangakino ignimbrites reveals magmatic and xenocrystic zircons magmatic zircon ages from three ignimbrites are slightly older than k ar and 40 ar 39 ar ages magmatic zircon age spectra are tightly clustered and support a simple crystallisation history for these three ignimbrites it is possible that the assimilation of zircon antecrysts leads to magmatic zircon spectra for the ahuroa ignimbrite being 70 ka older than the 40 ar 39 ar age xenocrystic zircon age spectra correlate with spectra of detrital zircons from the underlying mesozoic torlesse metasediments our data are consistent with significant assimilation of metasediments and the incorporation of zircon prior to eruption systematic variations in the age spectra of xenocrystic zircons in successive mangakino ignimbrites may be related to the structural development of the north island new zealand

ancient ocean on mars supported by global distribution of deltas and valleys the climate of early mars could have supported a complex hydrological system analysis of ancient deltaic deposits and valley networks reveals the presence of a planet wide equipotential surface in the northern lowlands indicative of the existence of a vast ocean on mars 3 5 billion years ago

isotope disequilibrium during anatexis a case study of contact melting sierra nevada california the geochemical consequences of contact melting were investigated in the sierra nevada batholith where trachyandesite magmas have intruded and melted the host granitic rocks partial melting of the granite varies from 10 20 at contact regions to 50 70 in a granite block within the trachyandesite thermal modelling suggests that the temperature of the granite block exceeded its solidus within 3 months of trachyandesite intrusion reached a maximum of 1000 c after ca 1 5 yr and remained above 500 c for ca 40 yr sr nd and pb isotope data of granitic melts record marked sr isotope and to a lesser extent pb isotope disequilibrium both within different glass fractions and between glasses and their source rb sr isochron calculations on residual mineral glass pairs fail to yield the age of melting obtained by40ar 39ar dating 11 9 ma sr and pb isotope data of glasses establish that major rock forming minerals of the granite had not attained isotope equilibrium before the onset of melting consequently the isotopic composition of the successive liquid fractions originating from granite melting depended on the relative contribution of each mineral to the elemental budget of the melts the extrapolation of these results to the petrogenesis of crustal melts and more generally to studies of crustal evolution suggests that isotope disequilibrium during anatexis and high grade metamorphism may be a common characteristic of many geologic settings if future studies substantiate these results some geochemical models of crustal melting and assimilation may need revision in addition marked sr isotope disequilibrium in the residual mineral phases within the partially melted granite raises concerns about the reliability of radiometric dating in metamorphic terranes

in situ sr isotopic analysis by laser ablation we demonstrate that the sr isotopic compositions of geological materials can be measured in situ through laser ablation using an icp multiple collector double focusing magnetic sector mass spectrometer mc icpms this provides rapid texturally sensitive high precision 0 004 sr isotopic analysis without the need for chemical preparation of samples the system used employs a q switched nd yag infrared laser the laser ablated material is carried by ar flow to an icp torch the resulting ions are extracted from the plasma and focused with d c quadrupole lenses and an electrostatic analyzer prior to entering a magnetic sector analyzer and collection in an array of faraday collectors a feldspar megacryst with independently known 87sr86sr of 0 703117 13 and a modern gastropod shell with independently known 87sr86sr of 0 709170 10 were used evaluate the method both have 2000 ppm sr laser power was adjusted to give an average 88sr of 2 10 11 amps during analysis to replicate the beam size during typical thermal ionization mass spectrometry tims within run precisions in 87sr86sr of 4 10 5 compare favorably with the precision from tims thirteen analyses of the cameroon feldspar megacryst yielded a weighted average 87sr86sr of 0 703106 22 in perfect agreement with the tims measurement similarly 22 analyses of the modern gastropod shell gave a weighted average of 0 709182 22 identical to the known value no memory effect was observed when switching from the cameroon feldspar sample run for several hours to the modern shell material the power of this technique is illustrated with a preliminary study of systematic differences in 87sr86sr of 0 0004 between two feldspar crystals of a long valley basalt one feldspar has identical 87sr86sr to the host basalt confirming the lack of any matrix effect with this method

dielectric map of the martian northern hemisphere and the nature of plain filling materials 1 a number of observations suggest that an extended ocean once covered a significant part of the martian northern hemisphere by probing the physical properties of the subsurface to unprecedented depth the marsis mars express provides new geophysical evidences for the former existence of a late hesperian ocean the vastitas borealis formation located inside a putative shoreline of the ancient ocean has a low dielectric constant compared with that of typical volcanic materials we show that the measured value is only consistent with low density sedimentary deposits massive deposits of ground ice or a combination of the two in contrast radar observations indicate a distribution of shallow ground ice in equilibrium with the atmosphere in the south polar region we conclude that the northern plains are filled with remnants of a late hesperian ocean fed by water and sediments from the outflow channels about 3 gy ago

an inventory and population scale analysis of martian glacier like forms abstract martian glacier like forms glfs indicate that water ice has undergone deformation on the planet within its recent geological past but the driving mechanisms behind the origin evolution and dynamics of these landforms remain poorly understood here we present the results of a comprehensive inventory of glfs derived from a database of 8058 context camera ctx images to describe the physical controls on glf concentration and morphometry the inventory identifies 1309 glfs 727 glfs in the northern hemisphere and 582 in the southern hemisphere clustered in the mid latitudes centred on a mean latitude of 39 3 in the north and 40 7 in the south and in areas of rough topography morphometric data show inter hemispheric similarity in glf length mean 4 66 km and width mean 1 27 km suggesting a common evolutionary history and a poleward orientation of glfs in both hemispheres indicating a sensitivity to climate and insolation on a local scale elevation is shown to be the most important control on glf location with most glfs occurring above an altitude threshold of 3000 m aod and in areas of moderate but not high relief we propose therefore that martian glfs may not exhibit accumulation or ablation areas as is the norm for terrestrial glacial systems but reflect ice motion in response to local relief within a widely distributed reservoir of ice deposited within prescribed latitudinal and elevational ranges

the role of superheating in the formation of glass mountain obsidians long valley ca inferred through crystallization of sanidine the glass mountain obsidians long valley ca are crystal poor 8 vol and highly evolved high sio2 low sr and therefore their formation required extremely efficient separation of melts from a crystal rich source a petrologic and experimental investigation of the mineral phases in glass mountain lavas identifies conditions under which phenocrysts grew and the driving mechanism for crystallization which places constraints on the possible processes that generated the obsidians the obsidian in this study gm 11 is saturated in nine phases sanidine quartz plagioclase titanomagnetite ilmenite zircon apatite allanite biotite and results of high resolution sem compositional mapping and electron microprobe analysis reveal that individual sanidine crystals are normally zoned and span a range of compositions or40 78 sanidines have a granophyric texture characterized by intergrowths of quartz and sanidine mineral phases in the natural sample are compared to h2o saturated phase equilibrium experiments conducted in cold seal pressure vessels over a range of conditions 700 850 c 75 225 mpa and all are found to be plausible phenocrysts comparison of sanidine compositions from the natural sample with those grown in phase equilibrium experiments demonstrates that sanidine in the natural sample occurs in a reduced abundance further comparison with phase equilibrium experiments suggests that sanidine compositions track progressive loss of dissolved melt water cooling suggesting that crystallization in the natural obsidian was driven predominantly by degassing resulting from decompression it is paradoxical that an effusively slowly erupted lava should contain multiple phenocryst phases including sanidine crystals that span a range of compositions with granophyric textures and yet remain so crystal poor to resolve this paradox it is necessary that the solidification mechanism degassing or cooling that produced the sanidine crystals and other mineral phases must have an associated kinetic effect s that efficiently hinders crystal nucleation and growth decompression experiments conducted in this study and from the literature collectively demonstrate that the simplest way to inhibit nucleation during degassing induced crystallization is to initiate degassing cooling from superliquidus conditions and therefore the glass mountain obsidians were superheated prior to crystallization

large scale landslide simulations global deformation velocities and basal friction the cause of the apparent small friction exhibited by long runout landslides has long been speculated upon in an attempt to provide some insight into the matter this paper describes results obtained from a discrete particle computer simulation of landslides composed of up to 1 000 000 two dimensional discs while simplified the results show many of the characteristics of field data the volumetric effect on runout preserved strata etc and with allowances made for the two dimensional nature of the simulation the runouts compare well with those of actual landslides the results challenge the current view that landslides travel as a nearly solid block riding atop a low friction basal layer instead they show that the mass is completely shearing and indicate that the apparent friction coefficient is an increasing function of shear rate the volumetric effect can then be understood with all other conditions being equal different size slides appear to travel with nearly the same average velocity however as the larger landslides are thicker they experience smaller shear rates and correspondingly smaller fractional resistance

kinematic implications of joint zones and isolated joints in the navajo sandstone at zion national park utah evidence for cordilleran relaxation 1 at zion national park znp the landscape is a consequence of differential weathering of the navajo sandstone where closely spaced vertical joints constitute joint zones that erode to form regularly spaced half kilometer slot canyons striking 351 between these joint zones is a set of isolated joints striking 339 fracture interaction and horsetail wing crack development indicate that the 339 striking joint set is younger than the 351 striking joint zones despite the lateral extent of stress reduction shadows in the vicinity of the large scale joint zones in addition to an older less pervasive 020 joint set this sequence of jointing records a counterclockwise rotation of the regional extension directed from wnw to wsw in the navajo sandstone at znp znp is located at the western margin of the colorado plateau 100 km east of the major normal faults of the northeastern central basin and range subprovince extension within the eastern central basin and range initiated during the miocene and exhibited a wsw extension direction anderson 1971 wernicke et al 1988 snow and wernicke 2000 the correlation between nearby basin and range extension and the extension direction for the 351 tending joint zones of znp is so close that the jointing at znp is interpreted as evidence for modest yet pervasive basin and range extension in the western margin of the colorado plateau

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

are arc type rocks the products of magma crystallisation observations from a simple oceanic arc volcano raoul island kermadec arc sw pacific raoul island is the emergent summit of a large intra oceanic strato volcano in what is globally one of the simplest of subduction settings in this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high mg numbers and high ni and cr expected of primitive mantle derived melts the lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene olivine orthopyroxene and spinel phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition indicating a decoupling of melt and crystal components in the system a consistent model for the raoul magmatic system is that primitive high mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low mg magmas by fractional crystallisation within the lower crust low mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone these processes are essentially those that characterise continental subduction related magmatic systems but differ only in the absence of an evolved crustal component

one million cubic kilometers of fossil ice in valles marineris relicts of a 3 5 gy old glacial landsystem along the martian equator abstract self consistent landform assemblages suggest that valles marineris the giant valley system that stretches along the martian equator was entirely glaciated during late noachian to early hesperian times and still contains huge volumes of fossil ice some of these glacial landform assemblages are illustrated here with representative examples selected in three regions ius chasma central candor chasma and the junction between coprates chasma and capri chasma a morphological boundary separating an upper spur and gully morphology from a smooth basal escarpment has been spectacularly preserved along valley walls throughout valles marineris the boundary winds around topographic obstacles and displays long wavelength variations in elevation it is associated with lateral benches hanging valleys and truncated spurs comparisons with terrestrial analogs indicate that it is most reasonably interpreted as a glacial trimline chasma floors are covered by various kinds of terrains including hummocky terrains platy terrains lateral banks layered benches and a draping mantle landforms in these terrains and their spatial relationship with the interpreted trimline suggest that they correspond to various disintegration stages of an ancient glacial fill currently protected by a superficial cover of ablation till altogether these landforms and terrains compose a full glacial landsystem with wet based glaciers that were able to flow and slide over their beds it was most probably fed by ice accumulating at low elevations directly from the atmosphere onto valley floors and walls with only minor contributions from tributary glaciers flowing down from higher elevations similar fossil glacial landsystems dating back from the early martian history are to be expected in many other low latitude troughs such as chasmata chaos valleys impact craters and other basins

analysis of a new geomorphological inventory of landslides in valles marineris mars abstract we completed a systematic mapping of landslides in a 10 5 km 2 area in tithonium and ius chasmata valles marineris mars where landslides are abundant using visual interpretation of medium to high resolution optical images we mapped and classified the geometry of 219 mass wasting features including rock slides complex compound failures rock avalanches debris flows and rock glacier like features for a total landslide area of a lt 4 4 10 4 km 2 44 of the study area a proportion larger than previously recognised studying the landslide inventory we showed that the probability density of landslide area p a l follows a power law with a scaling exponent 1 35 0 01 significantly different from the exponents found for terrestrial landslides 2 2 and 2 4 this indicates that the proportion of large landslides a l 10 7 m 2 is larger on mars than on earth we estimated the volume v l of a subset of 49 deep seated slides in our study area and found that the probability density of landslide volume p v l obeys a power law trend typical of terrestrial rock falls and rock slides with a slope 1 03 0 01 from the combined analysis of landslide area and volume measurements we obtained a power law dependency comparable to a similar relationship obtained for terrestrial bedrock landslides v l 1 2 0 8 a l 1 25 0 03 from the fall height h l and run out length l l of a subset of 83 slides unaffected by topographic confinement we obtained the mobility index heim s ratio h l l l a measure of the apparent friction angle of the failed materials 14 4 0 4 slope stability simulations and back analyses performed adopting a limit equilibrium method and using monte carlo approaches on failed and stable slopes suggest that the large landslides in valles marineris were seismically induced

mobility of large rock avalanches evidence from valles marineris mars measurements of h l height of drop length of runout vs volume for landslides in valles marineris on mars show a trend of decreasing h l with increasing volume this trend which is linear on a log log plot is parallel to but lies above the trend for terrestrial dry rock avalanches this result and estimates of 10 4 to 10 5 pa yield strength suggest that the landslides were not water saturated as suggested by previous workers the offset between the h l vs volume trends shows that a typical martian avalanche must be nearly two orders of magnitude more voluminous than a typical terrestrial avalanche in order to achieve the same mobility this offset might be explained by the effects of gravity on flows with high yield strengths these results should prove useful to future efforts to resolve the controversy over the mechan ics of long runout avalanches

minimum effective area for high resolution crater counting of martian terrains the acquisition of high resolution imagery for the surface of mars has enabled mapping of spatially limited order of 200 m on four type terrains using mars reconnaissance orbiter mro context camera ctx imagery that span the noachian hesperian and amazonian epochs the counts from each location include a region covering 10 000 km2 ten 1000 km2 subsets of that larger area and approximately one hundred 100 km2 samples the data demonstrate significant variation in the crater size frequency and derived model ages across a single terrain type for the 100 km2 samples the crater size frequency at this area scale varies across a single uniform geologic unit by up to a factor of 2 3 on the four different terrains at 1000 km2 the local pattern variations that are relevant at the 100 km2 scale become less important and the age variations are tighter in all four terrain cases the 10 000 km2 and 1000 km2 samples capture distinct crater populations km sized craters that formed before and after resurfacing event s however due to the relatively high mean distance between km sized craters the 100 km2 size area samples more commonly than not exclude a statistically significant sample at the kilometer size range masking important information about the pre resurfacing history of the terrain we therefore suggest that due to the effect of pattern variability in cratering over 100 km2 and the susceptibility of smaller craters to resurfacing crater counts derived from small area samples are suspect to major uncertainties

evolution of silicic magma through assimilation and subsequent recharge evidence from sr isotopes in sanidine phenocrysts taylor creek rhyolite nm

volume of martian midlatitude glaciers from radar observations and ice flow modeling numerous glacier like forms have been identified in the midlatitudes of mars and within recent years the acquisition of radar sounding data has revealed that the features are chiefly composed of water ice here we use radar observations in combination with ice flow models and inverse methods to calculate the volume of ice present at the midlatitudes of mars in order to obtain ice thicknesses we infer the yield stress of the ice deposits and we find that they are consistently lower than those of most terrestrial glaciers we estimate the present ice volume of lobate debris aprons identified by levy et al 2014 on mars to correspond to 1 55 105 km3 with an uncertainty of 25 this corresponds to a global ice cover of 1 1m thus the water ice found at midlatitudes is an important water reservoir and an important part of the global surface ice budget

chaotic advection fractals and diffusion during mixing of magmas evidence from lava flows structures of magma mixing from three different lava flows have been analyzed and the degree of mingling has been quantified by measuring the contact perimeter between magmas and the fractal dimension of structures in each lava flow the values of these parameters suggest that the magma mixing structures were produced by chaotic dynamics induced by stretching and folding processes between the interacting magmas the mingling of magmas has been simulated using a chaotic dynamical system consisting of repeated stretching and folding processes the simulation shows the same patterns of variation of contact perimeter and fractal dimension as those observed in natural structures and indicates that magma interaction processes acted with different intensities in the three lava flows in response to different magmatic interaction regimes since physical dispersion of one magma inside another through stretching and folding processes and chemical exchanges are closely related we performed coupled numerical simulations of chaotic advection and chemical diffusion the results show a good agreement between the computed and natural structures in particular the occurrence in the same system of well and poorly mixed regions it is shown that magma interaction processes are able to generate magmatic masses having wide spatial heterogenity at many length scales this occurrence can account for the presence of magmatic enclaves inside host rocks showing a variable degree of hybridization in both plutonic and volcanic environments

two models for the formation of magma reservoirs by small increments abstract it is now clear that magma reservoirs develop over long time intervals out of a large number of individual intrusions evidence from some large volcanic deposits including those of the bishop tuff california and fish canyon tuff colorado indicates that such reservoirs may experience pervasive and rapid heating before catastrophic eruptions injection of hot primitive melt in the reservoir is often invoked to explain these observations but this requires the rapid emplacement of a large magma volume at least 25 km 3 in less than 100 years for the bishop tuff and efficient heat transfer through a thick cumulate pile within a magma body that grows incrementally temperatures rise progressively and may eventually lead to a permanent volume of melt temperature and crystallization follow two completely different paths depending on the thickness of individual intrusive sheets in one limit corresponding to thick sheets crystallization proceeds at equilibrium initial magma batches crystallize completely and a permanent body of melt develops progressively once ambient temperatures in the complex rise above the solidus in the other limit corresponding to thin sheets crystallization is kinetically controlled such that initial intrusions do not crystallize completely and preserve a glassy residue once temperatures within the growing magma pile reach a certain threshold value nucleation and growth of crystals get reactivated in the residual glass crystallization then proceeds catastrophically in a positive feedback loop involving latent heat release and temperature rise thus an initial phase with no evolved melt present ends with the sudden formation of a large volume of evolved melt in a crystal mush after this initial phase new intrusions get emplaced in heated surroundings and follow a thermal path similar to that of the equilibrium case with melts that become increasingly more primitive with time evidence for kinetic controls on crystallization in the natural environment is provided by the significant amounts of glass up to 40 that are found in thin isolated basaltic sills and dykes owing to sluggish crystallization kinetics more glass should be generated in andesitic and dacitic magmas emplaced in similar conditions devitrification of such quantities of glass accounts for the rapid heating of a thick sill complex the two models of magma reservoir formation involve different requirements the intrusion thickness must be smaller than a critical value in the kinetic model and the average magma input rate must exceed a threshold value in the equilibrium model for a given injection rate a permanent magma reservoir forms in a shorter time interval and over a smaller cumulative magma thickness in the kinetic model than in the equilibrium one the kinetic model is such that the amount of evolved melt generated increases with decreasing injection rate in contrast to the equilibrium model for which the opposite relationship holds

textural constraints on the kinetics of crystallization of igneous rocks natural systems provide time controlled experiments on scales unreachable in the laboratory and with complexities unapproachable by computer experiments since temperature undercooling cannot be directly scaled with time in situ crystallization experiments provide information unavailable by other means the kinetics of crystallization in simple silicate systems have been studied extensively for glass ceramic systems e g uhlmann 1982 james 1982 but predictive models for nucleation behavior in particular remain inadequate part of the problem in creating models of nucleation lies in the difficulty of experimentally isolating the nucleation process crystal growth measurements are more accessible experimentally and for this reason crystal growth is better understood e g dowty 1980 kirkpatrick 1981 james 1982 baronnet 1984 however predictive models of crystal growth e g lasaga 1982 require accurate models for the temperature and compositional dependence of both the thermodynamic and kinetic driving forces for crystallization information that is currently limited the cse parameterization of the position of ymax as a function of t tl and t suggested by dearnley 1983 provides a potentially useful model that requires further testing similarly application of the adiabatic nucleation model of meyer 1986 to simple systems suggests that the position of tmax relative to tl has an approximately constant value of 0 55 0 60 exceptions are low dsf minerals such as albite and quartz additionally while experimental crystallization rate data is a necessary link between processes in natural melts and numerical models experimental growth rate data is confined to simple systems and conditions of moderately large undercooling while growth rate estimates in complex natural systems suggest that under most conditions undercoolings remain very small quantitative textural studies of dikes suggest a pronounced dependence of growth rate on total crystallization time e g ikeda 1977 which is in turn related to effective undercooling a function of both total dike width and distance from the dike margin by analogy minimum crystallization rates may be estimated from programmed cooling experiments if a temperature interval of crystallization is assumed growth rates determined in this way show the same relationship between growth rate and total crystallization time seen in the dike studies reflecting a systematic relationship between growth rate and cooling rate effective undercooling for plagioclase in basaltic systems while the data are not available for other mineral systems there is no reason to believe that similar patterns would not exist for other silicate minerals i e the similarity in dendritic olivine growth rates estimated by fowler et al 1989 using fractal analysis to the growth rates of plagioclase under conditions of rapid cooling estimated from programmed cooling rate experiments analysis of measured crystal size frequencies provides a means of extracting effective bulk kinetic information from geologic systems that can help to constrain the dynamic feedback between physical models of magmatic systems and rates of chemical change

trace element composition of igneous zircon a thermal and compositional record of the accumulation and evolution of a large silicic batholith spirit mountain nevada hafnium u th and ree content of zircons from the spirit mountain batholith in southern nevada correlate with calculated temperatures from the ti in zircon thermometer to support field and petrologic evidence of rejuvenation of crystal mush and melt extraction events during the 2 million year accumulation of the granitoid batholith marked variation in zircon composition from sample to sample from grain to grain within individual samples and from zone to zone within individual grains documents in detail a history of fluctuating conditions with repeated episodes of replenishment reheating crystal mush rejuvenation fractional crystallization and melt segregation the zircons exhibit compositional and thermal variability indicative of variations in host melt composition due to 1 melt rejuvenation mixing and fractionation 2 coeval growth of other ree rich accessory minerals and possibly 3 fluctuation in fo2

the socompa collapse and avalanche event failure of the northwestern flank of the socompa volcano north chile about 7000 years ago created a 40 km long debris avalanche deposit the deposit is remarkably well exposed we have combined spectral reflectance data field observations and analysis of digital topographic surfaces to reconstruct the sequence of collapse events and to constrain the volumes and sources of materials comprising the avalanche collapse involved three principle components the debris avalanche deposit itself volume 26 km3 after 30 expansion large slide blocks 11 km3 and detached blocks 23 km3 which remained in the upper part of the amphitheatre the debris avalanche deposit comprised two main units and seven subunits landsat thematic mapper data enabled estimation of the proportions of spectrally distinct lithological components within these heterogeneous units about 35 of the deposit area is composed of ignimbrites and gravels derived from a 100 200 m thick slice of basement rock underlying the northwestern flank of the volcano dacite domes and lavas originally situated in the southern and western parts of the amphitheatre constitute much of the remainder of the deposit blocks composed largely of sequences of lavas and pyroclastic deposits originally located in the northern and eastern parts of the amphitheatre slid coherently 7 km northwestwards and accumulated near the mouth of the amphitheatre modelling of locations of centroids in the original cone and of displaced components shows that potential energy loss during collapse was 3 8 1017 j cf 0 7 1017 j for mount st helens 1980 significantly components of the avalanche with least potential energy travelled the furthest laterally perhaps demonstrating efficient transfer of momentum from rear to front of the avalanche retrogressive flank failures of the original cone contributed successive components to the avalanche most of these combined to form a single moving mass but some appear to have been emplaced separately