chemical differentiation of the bishop tuff and other high silica magmas through crystallization processes re examination of petrochemical data for high silica ash flow tuffs suggests that crystal liquid fractionation plays a major role in the differentiation of high silica magmas for the high silica bishop tuff the enrichment and depletion of 27 major and trace elements correlates with the bulk crystal liquid partition coefficients calculated from the data of hildreth in ways that are consistent with crystal fractionation comparison of the bishop tuff with other high silica tuffs and rhyolites with different compositional variations and different phenocryst assemblages shows the correspondence of chemical gradients with mineralogy crystal zoning studies in the cordillera paine granite in southern chile suggest that the evolution of differentiated liquids similar to those in the bishop tuff results from crystal fractionation the physical mechanisms of separation of liquid and crystals for the paine granite and bishop tuff were quite different however differentiation of the high silica ash flows might result from advection of a less dense liquid from a crystallizing boundary into the top of the magma chamber

radiocarbon dates as data quantitative strategies for estimating colonization front speeds and event densities abstract archaeological analysis of large scale prehistoric population history requires us to estimate rates of spatial spread during dispersals and rates and magnitudes of temporal contraction during crashes using oxcal s mcmc sampling routine i introduce and demonstrate a simple and easily implemented method of estimating front speeds that takes due account of the uncertainty in the archaeological data in both dates and distances and argue that this method is more appropriate than those most often used in front speed estimation at present i also propose a simple and easily implemented method of estimating event densities as a demographic proxy as an alternative to summed calibrated probability distributions i argue that this alternative is a significantly better technique and show that its use also enables us to identify individual archaeological dates that are exerting particularly strong influence on the results so that we can efficiently allocate our attention when assessing the possible effects of exogenous sampling uncertainty to illustrate these methods i re analyse two published datasets relating to the early paleoindian colonization of north america my results with the new technique indicate that even with a very noisy dataset there was clear evidence in the framework of the intcal04 calibration curve for a drastic reduction in archaeological event densities following the younger dryas onset followed by a prolonged period of reduced human activity and a possible renewed phase of rapid growth after the younger dryas termination and onset of the holocene however the revised estimate of the younger dryas marine reservoir offset in the intcal09 calibration curve for 12 550 12 900 cal bp changes the picture significantly by flattening the peak in clovis age events and pushing it forward in time into the early younger dryas itself

experimental zircon melt and zircon garnet trace element partitioning and implications for the geochronology of crustal rocks garnet is the most commonly used mineral in thermobarometry whereas zircon is the most robust chronometer to date high grade metamorphic rocks to provide a basis for correlation of zircon and garnet growth we determined experimentally the trace element partitioningbetweenzircon ahydrousgraniticmeltandgarnetat20kbarand800 1000 cforp y rareearthelements ree zr hf th andu inrespecttomelt zirconpreferentiallyincorporatesallinvestigatedelementsapartfromreewithatomicnumberlowerthansm at this pressure and in the chosen composition the distribution coefficient between zircon and melt d zrn melt for ree increases with increasingatomicnumberofthereeandwithdecreasingtemperature dyb is 20at1000 c butmorethananorderofmagnitude higher at800 c the solubilityofzrinhydrousgraniticmelts buffered by zirconisabouta factoroftwo lower at20kbar than what has been previously established for mid crustal pressures large garnet produced in the experiments allowed determination of garnet melt trace element partitioning d grt melt at temperatures of 800 1000 c conditions relevant for partial melting of crustal rocks there is a systematic increase indree with decreasing temperature zircon contains significantly more heavy ree than garnet at temperatures of 800 850 c zircon garnetpartitioncoefficientsofheavy reedecreasewithincreasingtemperaturefromdlu of12at800 cto 1 4at1000 c middle reepartitioningisclosetounityforthewholeinvestigatedtemperaturerange differentempiricallydetermined zircon garnet partition coefficients from granulites and ultra high temperature granulites can potentially be explained by the experimentallydeterminedchangeofpartitioningasafunctionoftemperature thesedatacanassistinestablishingequilibriumbetween garnetandzirconzonesinnaturalrocks andintheconstructionofdetailedpressure temperature timepathsinhigh grademetamorphic rocks

reports venus identification of banded terrain in the mountains of ishtar terra high resolution images obtained with the arecibo radar system at a wavelength of 12 6 centimeters reveal numerous parallel 10 to 20 kilometer wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the ishtar terra region of venus geometric and morphological characteristics suggest that the bands represent earth like tectonic deformational features such as folds and faults

viscoelastic relaxation of topographic highs on venus to produce coronae coronae on venus are believed to result from the gravitationally driven relaxation of topography that was originally raised by mantle diapirs we examine this relaxation using a viscoelastic finite element code and show that an initially plateau shaped load will evolve to the characteristic corona topography of central raised bowl annular rim and surrounding moat stresses induced by the relaxation are consistent with the development of concentric extensional fracturing common on the outer margins of corona moats however relaxation is not expected to produce the concentric faulting often observed on the annular rim the relaxation timescale is shorter than the diapir cooling timescale so loss of thermal support controls the rate at which topography is reduced the final corona shape is supported by buoyancy and flexural stresses and will persist through geologic time development of lower flatter central bowls and narrower and more pronounced annular rims and moats enhanced by thicker crusts higher thermal gradients and crustal thinning over the diapir

corona associations and their implications for venus abstract geologic mapping principles were applied to determine genetic relations between coronae and surrounding geomorphologic features within two study areas in order to better understand venusian coronae the study areas contain coronae in a cluster versus a contrasting chain and are 1 directly west of phoebe regio quadrangle v 40 centered at latitude 15 s longitude 250 and 2 west of asteria and beta regiones between latitude 23 n longitude 239 and latitude 43 n longitude 275 results of this research indicate two groups of coronae on venus 1 those that are older and nearly coeval with regional plains and occur globally and 2 those that are younger and occur between beta atla and themis regiones or along extensional rifts elsewhere sometimes showing systematic age progressions mapping relations and earth analogs suggest that older plains coronae may be related to a near global resurfacing event perhaps initiated by a mantle superplume or plumes younger coronae of this study that show age progression may be related to 1 a tectonic junction of connecting rifts resulting from local mantle upwelling and spread of a quasi stationary hotspot plume and 2 localized spread of post plains volcanism we postulate that on venus most of the young post resurfacing coronal plumes may be concentrated within an area defined by the bounds of beta atla and themis regiones

insights into corona formation through statistical analyses 1 statistical analysis of an expanded database of coronae on venus indicates that the populations of type 1 with fracture annuli and 2 without fracture annuli corona diameters are statistically indistinguishable and therefore we have no basis for assuming different formation mechanisms analysis of the topography and diameters of coronae shows that coronae that are depressions rimmed depressions and domes tend to be significantly smaller than those that are plateaus rimmed plateaus or domes with surrounding rims this is consistent with the model of smrekar and stofan 1997 and inconsistent with predictions of the spreading drop model of koch and manga 1996 the diameter range for domes the initial stage of corona formation provides a broad constraint on the buoyancy of corona forming plumes coronae are only slightly more likely to be topographically raised than depressions with type 1 coronae most frequently occurring as rimmed depressions and type 2 coronae most frequently occurring with flat interiors and raised rims most type 1 coronae are located along chasmata systems or fracture belts while type 2 coronae are found predominantly as isolated features in the plains coronae at hot spot rises tend to be significantly larger than coronae in other settings consistent with a hotter upper mantle at hot spot rises and their active state

coronae on venus morphology classification and distribution abstract two major types of circular features with no direct morphologic analog on the terrestrial planets were identified in venera 15 16 radar images of venus coronae and corona like features coronae are irregular to circular structures characterized by a complex interior zone and maximum widths of 170 690 km corona like features resemble coronae but lack an annulus of ridges twenty one structures that fit the definition of coronae have been identified in venera 15 16 coverage along with eleven corona like features each group has been subdivided into three classes based on general morphology symmetrical coronae are characterized by circular to oval form with interiors characterized by flows domes and tectonic ridges asymmetrical coronae have similar morphology but have only a partial annulus or are asymmetrical about a central axis subdued coronae are dominated by smooth plains with flooded and embayed annuli double corona like structures are composed of two linked rings and are found along ridge belts in the region east of atalanta planitia irregular corona like features are surrounded by a rim rather than an annulus as are the double structures and are very subdued in appearance heart shaped corona like features are rounded at one end and form a wedge at the other end we interpret the classes of coronae and corona like features to have a similar mode of origin with differences in morphology due to the stage of evolution and or varying amounts and significance of geologic processes i e tectonism volcanism gravitational relaxation that occur during their evolution corona evolution is characterized by early uplift and volcanism followed by annulus formation formation of a peripheral trough may occur in the middle to late stages of corona evolution followed by extensive volcanism and lowering of topographic relief both coronae and corona like features are interpreted to have formed as a result of uplift and volcanism caused by a thermal anomaly at depth followed by gravitational relaxation

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 3 application to the peach spring tuff arizona california nevada usa establishing the depths of magma accumulation is critical to understanding how magmas evolve and erupt but developing methods to constrain these pressures is challenging we apply the new rhyolite melts phase equilibria geobarometer based on the equilibrium between melt quartz and two feldspars to matrix glass compositions from peach spring tuff arizona california nevada usa high silica rhyolite we compare the results to those from amphibole geothermobarometry projection of glass compositions onto the haplogranitic ternary and glass sio2 geobarometry quartz 2 feldspar rhyolite melts pressures span a relatively small range 185 230 mpa consistent with nearly homogeneous crystal compositions and are similar to estimates based on projection onto the haplogranitic ternary 250 50 mpa and on glass sio2 255 275 mpa amphibole geothermobarometry gives much wider pressure ranges temperature independent 65 300 mpa temperature dependent 75 295 mpa amphibole only 80 950 mpa average anderson and smith am mineral 80 549 559 1995 blundy and holland contrib miner petrol 104 208 224 1990 or holland and blundy contrib miner petrol 116 433 447 1994 thermometer a b pressures are most similar to phase equilibria results 220 210 190 mpa respectively crystallization temperatures determined previously with rhyolite melts 742 c zr in sphene 769 20 c and zircon saturation 770 780 c geothermometry are similar but temperatures from amphibole geothermometry 450 955 c are notably different the average anderson and smith holland and blundy 1994 thermometer b 710 c temperature is most consistent with previous estimates the rhyolite melts geobarometer effectively culls glass compositions affected by alteration or analytical issues peach spring glass compositions that yield pressure estimates reveal a tight range of plausible na2o and k2o contents suggesting that low na2o and high k2o contents of many peach spring samples are due to alteration use of altered whole pumice compositions in rhyolite melts simulations is likely the cause of the incorrect crystallization sequence reported previously for peach spring compositions using the rhyolite melts geobarometer we estimate a more realistic composition for peach spring tuff high silica rhyolite and the calculated composition finds close matches with some analyzed rocks and results in the expected sequence of crystallization

the younger dryas impact hypothesis a cosmic catastrophe in this paper we review the evidence for the younger dryas impact hypothesis ydih which proposes that at 12 9k cal a bp north america south america europe and the middle east were subjected to some sort of extraterrestrial event this purported event is proposed as a catastrophic process responsible for terminal pleistocene environmental changes onset of yd cooling continent scale wildfires extinction of late pleistocene mammals and demise of the clovis culture in north america the earliest well documented continent scale settlement of the region the basic physics in the ydih is not in accord with the physics of impacts nor the basic laws of physics no yd boundary ydb crater craters or other direct indicators of an impact are known age control is weak to non existent at 26 of the 29 localities claimed to have evidence for the ydih attempts to reproduce the results of physical and geochemical analyses used to support the ydih have failed or show that many indicators are not unique to an impact nor to 12 9k cal a bp the depositional environments of purported indicators at most sites tend to concentrate particulate matter and probably created many ydb zones geomorphic stratigraphic and fire records show no evidence of any sort of catastrophic changes in the environment at or immediately following the ydb late pleistocene extinctions varied in time and across space archeological data provide no indication of population decline demographic collapse or major adaptive shifts at or just after 12 9 ka the data and the hypotheses generated by ydih proponents are contradictory inconsistent and incoherent copyright 2014 john wiley sons ltd

an experimental study of trace element partitioning between zircon and melt as a function of oxygen fugacity abstract the partitioning of p sc ti sr y nb the rare earth elements ree hf ta th and u between zircon and a synthetic andesitic melt was determined as a function of oxygen fugacity f o 2 over a range of fourteen log units from qfm 8 to qfm 6 where qfm is the quartz fayalite magnetite oxygen buffer at 1300 c and 1 atm using sims and la icp ms the partition coefficients of ce and eu were found to vary systematically with f o 2 relative to those of the other ree producing ce and eu anomalies similar to those that are characteristic of natural igneous zircon these anomalies coexist at terrestrial f o 2 s and were used to quantify eu 3 eu 2 eu 3 and ce 4 ce 3 ce 4 in the melt the partition coefficients of the redox invariant trivalent cations are in excellent agreement with lattice strain theory the values of d ree are related to those determined for natural samples by the expected dependence on temperature the incorporation of ree 3 in zircon was independent of the presence of p d u d th was found to vary systematically with f o 2 indicating the presence of u 4 u 5 and u 6 in the melt at terrestrial conditions d ti was independent of f o 2 despite an expectation of significant ti 3 in the most reduced experiments

corona structures on venus models of origin coronae on venus are circular to elongate structures with maximum widths of 150 1000 km characterized by annuli of concentric ridges surrounding complex interiors the features have raised topography relative to the surroundings they are associated with volcanic activity and most are partially surrounded by a peripheral trough variations in morphology between individual coronae are due to differences in their stage of evolution and or differences in the relative significance of the geologic processes that occur in each stage we examine models for three processes that may be involved in corona origin and evolution 1 a hotspot or rising mantle diapir model 2 a sinking mantle diapir model and 3 gravitational relaxation of topography rising mantle diapirs are caused by heating at depth e g hotspot while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere the hotspot model is most consistent with the major characteristics of coronae with gravitational relaxation occurring as a modificational process the sinking mantle diapir would produce dominant central compression that has not been observed at coronae however higher resolution image and altimetry data from magellan can be used to distinguish more fully between the two models coronae in various states of formation and degradation can be identified in the venera 15 16 data suggesting that the process may be continuing today

coronae of mnemosyne regio morphology and origin abstract coronae are circular to elliptical structures 150 600 km across characterized in venera 15 16 and arecibo radar images by an annulus of concentric ridges the interiors of the structures are geologically complex displaying volcanic flows domes and radial concentric and or obliquely trending ridges and are generally distinct from the exterior the mnemosyne regio area to the west of ishtar terra contains seven coronae the largest cluster of coronae identified on venus on the basis of the morphologic and topographic characteristics of the coronae in the mnemosyne region we assess several possible models of corona origin using cross cutting and stratigraphic relationships we have been able to identify a sequence of features and events for this group of coronae and have determined that a variety of processes contribute to the origin and evolution of coronae we find that several processes associated with diapirism or hotspots and gravitational relaxation of high topography are most consistent with observed corona topography and morphology and are thus likely to have been significant in the formation and evolution of coronae on the basis of these observations and stratigraphic relationships the following sequence of events is outlined for the formation and evolution of coronae in this region 1 regional volcanic and tectonic activity 2 localized uplift and volcanism caused by diapirism or hotspots 3 annulus and trough formation related to gravitational relaxation accompanied by further volcanic activity 4 continued local volcanic flooding of the corona annulus and trough differences in morphology between coronae reflect variations in amount and significance of these geologic processes coronae in other regions exhibit the same overall characteristics as the mnemosyne coronae and support a similar origin for the majority of coronae

origin and provenance of spherules and magnetic grains at the younger dryas boundary abstract one or more bolide impacts are hypothesized to have triggered the younger dryas cooling at 12 9 ka in support of this hypothesis varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the younger dryas boundary ydb we show that bulk sediment and or magnetic grains microspherules collected from the ydb sites in arizona michigan new mexico new jersey and ohio have 187os 188os ratios 1 0 similar to average upper continental crust 1 3 indicating a terrestrial origin of osmium os in these samples in contrast bulk sediments from ydb sites in belgium and pennsylvania exhibit 187os 188os ratios 1 0 and at face value suggest mixing with extraterrestrial os with 187os 188os of 0 13 however the os concentration in bulk sample and magnetic grains from belgium is 2 8 pg g and 15 pg g respectively much lower than that in average upper continental crust 31 pg g indicating no meteoritic contribution the ydb site in pennsylvania is remarkable in yielding 2 to 5 mm diameter spherules containing minerals such as suessite fe ni silicide that form at temperatures in excess of 2000 c gross texture mineralogy and age of the spherules appear consistent with their formation as ejecta from an impact 12 9 ka ago the 187os 188os ratios of the spherules and their leachates are often low but os in these objects is likely terrestrially derived the rare earth element patterns and sr and nd isotopes of the spherules indicate that their source lies in 1 5 ga quebecia terrain in the grenville province of northeastern north america

experimental measurements of zircon melt trace element partition coefficients abstract zircon was grown from trace element doped hydrous peralkaline rhyolite melts with buffered oxygen fugacities in cold seal experiments at 0 1 and 0 2 gpa and 800 c and piston cylinder experiments at 1 5 gpa and 900 1300 c zircon and glass were present in all run products and small monazite crystals were present in eight of the 12 experiments average diameters of zircon crystals ranged from 5 to 20 m at 800 c to 30 50 m at 1300 c zircon crystals have thin rims and adjacent glass has a narrow 1 m thick compositional boundary layer concentrations obtained through in situ analysis of cores of run product zircon crystals and melt pools were used to calculate trace element partition coefficients dzircon melt for p sc ti v y la ce pr nd eu gd ho yb lu hf th and u in most cases lu was the most d 12 105 and la the least 0 06 0 95 compatible elements d values from this study fall within the range of previously measured values for rare earth elements ree however d values measured experimentally show less fractionation than those recently measured using natural phenocryst matrix pairs for example dlu dla measured experimentally in this study range between 27 and 206 compared to a value of 706 522 for a natural zircon dacite pair sano y terada k and fukuoka t 2002 high mass resolution ion microprobe analysis of rare earth elements in silicate glass apatite and zircon lack of matrix dependency chem geol 184 217 230 although d values from this study show good agreement with the lattice strain model d values from natural phenocryst matrix pairs combined with measured zircon compositions better reproduce host rock magma compositions of igneous rocks they also yield more reasonable estimates of magma compositions when combined with compositions of out of context zircons for example compositions of the hadean detrital zircons from jack hills australia yield lree enriched magmas when combined with d values from phenocryst matrix pairs yields but yield lree depleted magmas when experimentally determined d values are used we infer that experimentally measured dzircon melt values represent disequilibrium partitioning resulting from rapid zircon growth during short laboratory timescales rapid growth causes development of observed diffusive boundary layers in the melt adjacent to zircon crystals d values from phenocryst matrix pairs are therefore recommended for petrogenetic modeling

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

petrology of the anorogenic oxidised jamon and musa granites amazonian craton implications for the genesis of proterozoic a type granites the 1 88 ga jamon and musa granites are magnetite bearing anorogenic a type granites of paleoproterozoic age they intrude the archaean rocks of the rio maria granite greenstone terrain in the eastern part of the amazonian craton in northern brazil a suite of biotite amphibole monzogranite to syenogranite with associated dacite porphyry dp and granite porphyry gp dykes dominates in these subalkaline granites that vary from metaluminous to peraluminous and show high feo feo mgo and k2o na2o in spite of their broad geochemical similarities the jamon and musa granites show some significant differences in their ree patterns and in the behaviour of y the jamon granites are related by fractional crystallisation of plagioclase potassium feldspar quartz biotite magnetite amphibole apatite ilmenite geochemical modelling and nd isotopic data indicate that the archaean granodiorites trondhjemites and tonalites of the rio maria region are not the source of the jamon granite and associated dyke magmas archaean quartz diorites differentiated from the mantle at least 1000 m y before the emplacement of the granites have a composition adequate to generate dp and the hornblende biotite monzogranite magmas by different degrees of partial melting a larger extent of amphibole fractionation during the evolution of the musa pluton can explain some of the observed differences between it and the jamon pluton the studied granites crystallised at relatively high fo2 and are anorogenic magnetite series granites in this aspect as well as concerning geochemical characteristics they display many affinities with the proterozoic a type granites of south western united states the jamon and musa granites differ from the anorthosite mangerite charnockite rapakivi granite suites of north eastern canada and from the reduced rapakivi granites of the fennoscandian shield in several aspects probably because of different magmatic sources

impact craters of venus analysis of venera 15 and 16 data from description of impact craters on venus and crater size and depth distributions models are developed that analyze the size distribution curve and radar bright haloes around fresh craters the dense venusian atmosphere has a strong influence on cratering the estimated age of the surface of the area under study is from 0 5 to 1 0 109 years the estimated rate of resurfacing during this time interval is about 0 04 to 0 8 cm 106 years the lithosphere of venus seems to be sufficiently rigid to sustain craters of 100 140 km diameter and 0 8 2 5 km depth for about 108 years

the geomorphology and evolution of hecate chasma venus abstract hecate chasma is a discontinuous trough and fracture system extending from atla regio through asteria regio a detailed examination of hecate chasma using magellan image and altimetry data reveals morphologic features such as normal faults including graben compressional ridges a variety of volcanic edifices and coronae there is no evidence for a systematic age progression among coronae in chains thus a single stationary thermal anomaly underlying a moving lithosphere is an unlikely origin for the chains a comparison of the observed features with two possible models of evolution subduction delamination or lithospheric extension with limited rifting shows some similarities between the predicted morphologies of each model and hecate chasma the dominance of features related to upwelling and extensional tectonism strongly favor an extensional origin for this zone the corona chain may have been formed by upwellings related to rayleigh taylor instabilities similar to those observed at terrestrial mid ocean ridges but in an environment of limited spreading

crystal melt elemental partitioning in silicic magmatic systems an example from the peach spring tuff high silica rhyolite southwest usa partition coefficients kd are critical for the quantitative modeling of the evolution of magmatic systems high silica rhyolites hsr are characterized by saturation in numerous phases such that a comprehensive kd database is necessary while there exists a large body of published kd values various methods have been used by different authors over time focusing on different compositions and conditions and most investigations are limited to small suites of elements e g rare earth elements ree typically in few 2 3 mineral phases in this study we present kd values for a suite of 45 elements in 8 mineral phases from a hsr pumice clast from the peach spring tuff pst sanidine plagioclase biotite amphibole titanite apatite zircon and chevkinite we determine kd by measuring crystal rim compositions and unaltered glass from a single volcanic sample which reflect equilibration between crystals and melt in the system at or near the time of eruption we measure compositions using a combination of sem eds and la icpms analysis we show that titanite strongly partitions ree from the melt with a significant preference for middle mree over light lree and heavy ree hree apatite and amphibole share a similar mree enriched pattern though with absolute kd values more than an order of magnitude lower than titanite while apatite displays a slight preference for lree over hree amphibole shows a higher affinity for hree than lree zircon strongly partitions hree hf and u with little affinity for lree chevkinite concentrates lree and th significantly more than any other phase we analyzed biotite kd are generally low 10 with the highest values for transition metals and ba sanidine and plagioclase display a strong preference for sr ba and eu our dataset is largely consistent with published studies that use similar in situ techniques in samples of similar composition hsr we observe a large variation in other studies that we attribute in part to contamination from inclusions in crystals particularly when measuring compositions by whole crystal methods our dataset suggests that accessory minerals play a dominant role in the partitioning of trace elements they collectively control the distribution of ree and high field strength elements hfse in the system on the other hand large ion lithophile elements lile are most dominantly influenced by feldspars but if abundant phases such as biotite and apatite may also influence the partitioning of lile from the melt we analyze our kd values in light of the theoretical crystal lattice strain model lsm by fitting our data to curves that have the expected form in ln kd versus ionic radius space onuma diagrams the fact that we are successful in finding good fits for our kd values especially the ree reinforces the idea that mineral melt equilibrium was attained and analytical data are of appropriate quality additionally we use the fitted curves to estimate ce3 ce and eu2 eu ratios in the melt which are controlled by the oxidation state of the magma we estimate that 97 of all ce in the pst melt existed as ce3 the proportion of eu2 in the melt is not as well constrained we estimate eu2 ranges mainly from 10 to 50 of the total eu

application of proton microprobe data to trace element partitioning in volcanic rocks abstract in situ proton microprobe analyses are used to calculate partition coefficients d for mn fe cu zr ga rb sr y zr ba and pb and more restricted data for ni ge nb and as in pyroxenes olivines biotites hornblendes feldspars and fe ti oxides from 32 volcanic rocks all with glassy matrices these range from leucitite and picritic basalt to high silica and peralkaline rhyolites data for pigeonite aenigmatite allanite and leucite are included where these occur as additional phases coefficients have also been rayleigh fractionation corrected this correction being significant for those elements with high coefficients in specific mineral phases and where pronounced crystallisation has occurred partition coefficients are presented as plots vs nbo t ratios of coexisting glasses melts negative correlations are observed based on log transformed data for mn zn fe and y pyroxenes mn zn and fe olivines mn and zn fe ti oxides nb y sr and zr hornblendes sr plagioclase and ba cu zn and fe feldspars these element partition coefficients are not correlated with bulk mineral compositions zr and ga are not correlated with either glass or mineral compositions except zr in hornblende the correlations support the role of melt polymerisation in controlling element partitioning at least for those elements listed although possible effects of temperature cannot be uniquely isolated in the present data set the coefficients presented here are generally consistent with published values for equivalent rock compositions one difference being that some of the higher published coefficients e g sr in plagioclase are not matched in the present data set from a practical viewpoint the relative regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic modelling procedures

on venus impact basins viscous relaxation of topographic relief the question of whether impact basins dating from the time of heavy bombardment of the inner solar system are preserved on the surface of venus is of paramount importance for an understanding of the geological processes that have shaped that planet two types of features have been suggested as candidates for impact basins on venus 1 large quasi circular topographic depressions and 2 large quasi circular regions of low radar backscatter surrounded by terrain of greater radar backscatter and generally displaying little topographic relief as a guide to the expected character of an impact basin on venus we have calculated the viscous relaxation with time of the topographic relief of such a basin under the assumption that the surface temperature of the planet has been at or near its presently high value since the time of basin formation the viscous relaxation model includes the effects of a decrease in effective viscosity with depth and the partial to complete isostatic compensation of initial topography we conclude that an impact basin several hundred kilometers or greater in diameter and 3 b y old or greater in age should have negligible topographic relief at present this conclusion does not exclude the hypothesis that quasi circular features of low radar backscatter but little topographic relief may be relaxed remnants of ancient impact basins large quasi circular depressions on venus such as atalanta planitia however cannot exceed a few hundred million years in age otherwise their initial topographic relief would be excessive such large depressions are thus likely to be geologically young and to have resulted from processes other than impact an additional process must therefore have operated on venus to create large roughly circular areas of lowland terrain one possibility is that these topographic features are analogs on venus to terrestrial platform basins produced by lithospheric extension and thermal subsidence

variations in trace element partition coefficients in sanidine in the cerro toledo rhyolite jemez mountains new mexico effects of composition temperature and volatiles trace element partition coefficients have been measured for one plagioclase and five sanidine mineral separates from the cerro toledo rhyolite new mexico sanidine partition coefficients vary substantially and systematically within the cerro toledo rhyolite partition coefficients for ca sr zn la and eu are lowest in the most evolved rhyolites whereas sm and hree partition coefficients are highest rubidium partition coefficients remain constant while those for ba ce and th are variable variations of the sr zn la and eu partition coefficients are correlated with the ca contents and partition coefficients of the sanidines calcium may have controlled the distribution of these elements in the sanidine by modifying the feldspar structure the low ca partition coefficients in sanidines for the most evolved rhyolites may be the consequence of modification of the melt structure possibly due to increased volatile contents at the top of the magma chamber s during evolution of the cerro toledo rhyolite the zn and la partition coefficients between sanidine and melt also may have been controlled by this change in melt structure modelling using major elements and the constant rb partition coefficient for sanidine indicates 70 crystallization of magma during cerro toledo rhyolite time by removal of 68 sanidine and 32 quartz estimates of the volume 1 of initial parental magma and 2 of the magma that crystallized during this period are 11 670 km3 and 8170 km3 respectively the average intrusion rate of silicic magma during cerro toledo rhyolite activity was 35 10 3 km3 a

accessory mineral behavior during differentiation of a granite suite monazite xenotime and zircon in the sweetwater wash pluton southeastern california u s a abstract compositional and textural characteristics of the accessory minerals monazite xenotime and zircon in the sweetwater wash granites and related aplites indicate that these phases not only controlled much of the trace element geochemistry of the suite but that they also record the melt compositional changes that occurred during magmatic differentiation fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare earth element ree depletion this was accompanied by increasing heavy ree concentrations until xenotime joined the crystallizing assemblage subsequently combined monazite xenotime fractionation resulted in lowering of the entire ree budget zircon which contains inherited cores that were apparently resorbed and rounded during initial anatexis was saturated throughout the differentiation history of the sweetwater wash suite accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales the differences are best displayed by monazite on average crystals in more differentiated rocks aplites are relatively depleted in light ree s and contain higher concentrations of the substituting elements u and th zircon displays complimentary increases in hf and y while xenotime shows a slight increase in th and in gd ho ratios both phases also exhibit higher u concentrations in aplites than in granites these differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites but also within individual thin sections due to in situ fractionation on an even smaller scale strong compositional variations are present within single crystals possibly due to diffusion controlled melt compositional gradients in the regions a adjacent to growing major phases and b adjacent to the growing accessory crystal itself our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes but that the scale of observed variations must be carefully considered

record of magma chamber processes preserved in accessory mineral assemblages aztec wash pluton nevada field relations and geochemistry indicate that aztec wash pluton had a complex open system history the tilted pluton represents a 2 5 km thick chamber that was recharged with both felsic and mafic magma the lower portion is highly heterogeneous with mafic sheets cumulates hybrid rocks mafic felsic and composite dikes and sheets and pods of granite heterogeneous h zone the upper part is granite that is generally homogeneous in texture and geochemistry granite g zone at the base of the g zone a discontinuous zone buffer b zone records interaction between the g and h zones complexity of the h zone makes detailed reconstruction of magma chamber history difficult and the relatively homogeneous g zone appears to offer few clues about the evolution of the pluton or the interaction between the felsic and underlying more mafic magmas accessory mineral textures zoning and assemblages in the g zone however are far from homogeneous and provide clear evidence for fluctuating conditions that elucidates magma chamber history mafic rocks of the h zone contain the accessory mineral assemblage ilmenite magnetite quench apatite or late sphene and zircon g zone rocks have magnetite apatite sphene zircon or allanite ilmenite and chevkinite the magnetite allanite early sphene apatite and zircon association that characterizes much of the g zone indicates a lower temperature and possibly higher f o2 than the h zone assemblage mineral textures and zoning however document fluctuations in the stable g zone assemblage 1 as many as five rounded surfaces truncate internal zones in zircon each indicating a dissolution event 2 in addition to euhedral concentric zoning sphene contains regions of highly irregular zoning that are rich in inclusions especially anhedral ilmenite 3 ilmenite and allanite are mutually exclusive but allanite is present in the matrix of rocks that contain sphene with ilmenite inclusions and sphene grains in some samples have alternating regions with allanite and ilmenite inclusions we attribute fluctuations in the stable g zone accessory assemblage to fluctuations in temperature and possibly f o2 with appearance of the high t reduced assemblage indicating interaction with hot mafic h zone magma these interactions certainly involve heat transfer and may involve limited chemical contamination we infer that they must have taken place near the h zone g zone boundary the most frequent and intense fluctuations marked by zircon with the highest number of truncation surfaces and by sphene with irregular zoning and abundant ilmenite inclusions affected rocks that are near the boundary but ilmenite inclusions in sphene and truncation surfaces in zircon are present to the top of the pluton we conclude that granitic magma was subjected to multiple cycles of thermally induced vertical transfer convection that at least initially affected the entire upper part of the chamber

accessory mineral chemistry as a monitor of petrogenetic and metallogenetic processes a comparative study of zircon and apatite from wushan cu and zhuxiling w mo mineralization related granitoids abstract in this study we explore the chemistry of accessory apatite and zircon as tools to monitor the petrogenetic and metallogenetic processes involved in the formation of the wushan cu and zhuxiling w mo mineralization related granodiorite porphyry the wushan granitoids are metaluminous with a cnk of 0 88 1 01 and the zhuxiling intrusions are weakly peraluminous with higher a cnk values 1 02 1 12 the wushan granitoids have end t values of 4 6 to 4 7 which combined with the relatively elevated mg values 57 62 and adakitic features high sr y ratios of 51 60 indicate that they were derived from melting of a thickened lower crust of the yangtze block with addition of small amounts of mantle components compared to the wushan granitoids the zhuxiling granitoids have relatively high 87sr 86sr i 0 70945 0 70951 and low end t values 5 9 to 6 3 and zircon ehf t values 3 8 to 9 0 suggesting their derivation dominantly from partial melting of the neoproterozoic crust the apatites from the zhuxiling samples are characterized by relatively high mno and feo variable y and low ce which is consistent with the weakly peraluminous feature of whole rock chemistry these apatites show notable depletion of th and lree with a lower la sm n ratio 0 74 1 44 and la yb n ratio 6 2 17 9 relative to those of the host rock 3 04 3 61 and 19 1 23 4 respectively suggesting early crystallization of some lree and th enriched minerals such as allanite monazite before apatite crystallization the steep trend of ce sm versus yb gd ratios shown by zircons from the zhuxiling granitoids suggests that the crystallization of zircon is affected by the crystallization of titanite and apatite thus the crystallization sequence of accessory minerals in the zhuxiling intrusion is allanite apatite titanite and zircon the apatites from the wushan granitoids however show strongly lree enriched ree patterns with weak negative eu anomalies which indicate minor feldspar fractionation and mildly magmatic differentiation the flat trend of ce sm versus yb gd ratios shown by zircon from the wushan pluton indicates that its ree composition is affected by hornblende saturation zircon grains from the metaluminous wushan granitoids exhibit remarkably higher ce4 ce3 ratios and weaker eu eu anomalies than those of the peraluminous zhuxiling samples and the apatites from the wushan samples show higher eu eu 0 55 0 62 and lower ce ce 0 91 1 04 and mno 0 05 0 09 compared to those of the zhuxiling pluton with eu eu 0 41 0 50 ce ce 1 06 1 11 and mno 0 15 0 18 respectively the apatite and zircon chemistry suggest that the cu bearing granitoid formed at an oxygen fugacity higher than the w bearing granitoid which agrees with the fact that the wushan granitoids show whole rock fe3 fe2 ratios higher than the zhuxiling granitoids in addition apatites from the wushan intrusion show enrichment of cl and higher f cl ratios compared to those of the zhuxiling samples suggesting that a water rich parental magma with relatively high cl content is favourable for copper mineralization this case study shows that apatite and zircon chemistry could reflect the geochemical characteristics of their host rock and magma evolution moreover they provide insights into the oxidation state halogen fugacity and magma differentiation of the magmatic system which are key factors for mineralization our data suggest that magma associated with cu mineralization is relatively oxidizing chlorine enriched and less fractionated compared to w bearing magma

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

lithium and nd isotopic constraints on the origin of li poor pegmatite with implications for li mineralization abstract although volumetrically minor in the upper continental crust pegmatites host lots of rare metal mineralization e g li be pegmatites and associated mineralization were generally thought to originate from extreme differentiation of a parent granite system we propose an alternative model to interpret the genesis of pegmatite and factors controlling li mineralization based on combined nd and li isotopic studies on the li poor pegmatites barren from the qinghe pegmatite field of the altai orogen nw china the qinghe pegmatites show significantly varied nd isotopic compositions end t 4 3 1 7 which overlap the surrounding schists 6 5 1 5 but lower than the granites 2 2 9 of the orogen these data together with the low ree 3 3 41 ppm and absence of biotite in the pegmatites suggest they were not derived from extreme differentiation of a parent granite as traditionally thought but from low degrees of partial melting of the schists involving muscovite dehydration melting under amphibolite facies conditions the pegmatites are characterized by heavy li 7li 4 1 14 5 and low li abundance 3 6 50 ppm which contrast to that of local schist 7li 0 9 3 0 li 24 123 ppm and granite 7li 0 9 3 0 li 24 70 ppm and also to that of global li rich pegmatites 7li 1 0 10 li 500 ppm modeling study indicates that li poor pegmatites always have heavy li signature compared with li rich pegmatites due to the abundance of biotite in the source the formation of li rich pegmatite with li mineralization is attributable to the distinct source characteristics with abundant fluxing components like li na co32 and hco3 and sufficient alumina rich clay phases and the fluxing components probably originated from evaporate interlayers in the source

trace element incorporation in titanite constraints from experimentally determined solid liquid partition coefficients titanite liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite saturated liquids equilibrated at 1 5 gpa and 850 c starting from lamproitic compositions the high compatibility observed for ree hfse sr v and sc and the strong incompatibility observed for actinides large ion lithophile and light elements are discussed in terms of available crystal chemical mechanisms for incorporation and crystal structure control the exchange vectors na1ca 1 and al1ti 1 allow local charge balance to be achieved after incorporation of ree3 and r5 at the ca and ti site respectively the significant amounts of h measured are also relevant in this regard the exchange mechanism being oh 1o 1 at the o1 site the incorporation of u4 and th4 at the ca site is scarce and is likely balanced by that of mg2 at the ti site both these substitutions are responsible for strong structural strain titanite can thus be considered an important repository for ree and hfse in metamorphic and igneous rocks and its role must be accounted for when modelling trace element residence during metamorphic reactions and late magmatic crystallisation history due to the measured differences in compatibility titanite crystallisation increases the values of nb ta and lree hree ratios in residual liquids the similar compatibility of u and pb makes titanite suitable for u pb geochronology of igneous rocks only after common pb correction finally this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal chemical constraints and that titanite based waste forms should contain high amounts of na and mg2

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

apatite liquid partition coefficients for the rare earth elements and strontium abstract sixteen sets of apatite liquid partition coefficients dap liq for the rare earth elements ree la sm dy lu and six values for sr were experimentally determined in natural systems ranging from basanite to granite the apatite melt glass assemblages were obtained from starting glasses artificially enriched in ree sr and fluorapatite components these were run under dry and hydrous conditions of 7 5 20 kbar and 950 1120 c in a solid media piston cylinder apparatus an sem equipped electron microprobe was used for subsequent measurement of ree and sr concentrations in coexisting apatites and quenched glasses the resulting partition coefficient patterns resemble previously determined apatite phenocryst groundmass concentration ratios in the following respects 1 the rare earth patterns are uniformly concave downward i e the middle ree are more compatible in apatite than the light and heavy ree 2 dreeap liq is much higher for silicic melts than for basic ones and 3 strontium and therefore eu2 is less concentrated by apatite than are the trivalent ree the effects of both temperature and melt composition on dreeap liq are systematic and pronounced at 950 c for example a change in melt sio2 content from 50 to 68 wt causes the average ree partition coefficient to increase from 7 to 30 a 130 c increase in temperature on the other hand results in a two fold decrease in dreeap liq partitioning of sr is insenstitive to changes in melt composition and temperature and neither the sr nor the ree partition coefficients appear to be affected by variations in pressure or h2o content of the melt the experimentally determined partition coefficients can be used not only in trace element modelling but also to distinguish apatite phenocrysts from xenocrysts in rocks reported apatite megacryst host basalt ree concentration ratios 12 for example are considerably higher than the equilibrium partition coefficients which suggest that in this particular case the apatite is actually xenocrystic a reversal experiment incorporated in our study yielded diffusion profiles of ree in apatite from which we extracted a ree ca interdiffusion coefficient of 2 4 10 14 cm2 s at 1120 c extrapolated downward to crustal temperatures this low value suggests that complete ree equilibrium between felsic partial melts and residual apatite is rarely established

styles of tectonic deformations on venus analysis of venera 15 and 16 data analysis of the radar images obtained by veneras 15 and 16 leads to the conclusion that the ridge and groove structures on the surface of venus are the result of tectonic deformation although the mechanism of such deformation cannot yet be unequivocally deduced several styles of deformation can be described areal deformation occurs where horizontal stresses have operated over large areas shear deformation appears in bands showing differential longitudinal deformation transversal stresses operating over long and relatively narrow areas have produced belt deformation circular deformation is related to a specific locus of the stresses the absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment the origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity induced spreading of surface material over upwellings a general conclusion is that in the surveyed area of venus neither terrestrial plate tectonics nor lunar highland type terrain exists

the ree ti mineral chevkinite in comenditic magmas from gran canaria spain a syxrf probe study the ree ti silicate chevkinite has been recognised previously in miocene ignimbrites from gran canaria and in correlative offshore syn ignimbrite turbidites we have estimated the partition coefficients of ree y zr and nb for chevkinite and co existing peralkaline rhyolitic comendite glass using synchrotron xrf probe analyses syxrf in order to evaluate the role of this mineral in the ree budget of felsic peralkaline magmas the zr nb ratio of the chevkinite is 1 55 1 7 strongly contrasting with zr nb of 6 5 in the associated glass zr shows a three fold enrichment in chevkinite relative to the residual melt whereas nb is enriched by a factor 10 the enrichment of ce and la in chevkinite is even more significant namely 19 wt ce and 12 wt la compared to 236 ppm ce and 119 ppm la in the glass chevkinite glass ratios are 988 30 for la 806 30 for ce 626 30 for pr 615 40 for nd 392 50 for sm 225 30 for eu 142 25 for gd 72 20 for dy for trace elements we derived kdte of 74 25 for y 8 for hf 50 for th 15 5 for nb and 3 55 0 4 for zr mineral glass ratios for co existing titanite are 28 10 for la 86 20 for ce 98 30 for pr 134 35 for nd 240 50 for sm 50 20 for eu 96 25 for gd 82 25 for dy 99 30 for y 45 10 for nb and 3 0 5 for zr based on these data the removal of only 0 05 wt of chevkinite from a magma with initially 300 ppm ce would deplete the melt by 93 ppm to yield 207 ppm ce in the residual liquid chevkinite thus appears when present to be the controlling mineral within the lree budget of evolved peralkaline magmas

characterization of venera 15 16 geologic units from pioneer venus reflectivity and roughness data abstract geologic units have been defined for the surface of venus from venera 15 16 image data a characterization of these geologic units is carried out using information on surface properties derived from pioneer venus pv reflectivity and rms slope data the geologic context provided by venera 15 16 units allows additional more specific interpretations of surface radar properties to be made characterization of venera units results in the definition of four groups of venera units i smooth rocky units ii rough rocky units iii rough high dielectric units and iv diffusely scattering units on the basis of correlations of surface morphology to spatial and statistical distributions in rms slope and reflectivity data we test models for the origin of the surface properties of some units we conclude that plains and tectonic units can be contrasted in terms of the average roughness of the surface and that tectonic deformation appears to roughen the surface at 0 5 to 10 m and 5 to 50 cm scales this tectonic weathering process appears to dominate the erosional regime of venus unlike earth or mars production and transport of soils dominates only a small portion 5 of the surface some of the venera units display distinctive spatial and statistical distributions of pv radar data in particular apparent low reflectivity in the tesserae appears to be caused by small 5 50 cm rock fragments on the surface which cause diffuse scattering at pioneer venus wavelenghts analysis of models for the formation of these fragments suggests that they are due to the pervasive deformation undergone by the tesserae finally aspects of this study have been used to extend results of venera image data analysis southward of 30 n lat resulting in a prediction of the distribution of tessera such results can aid in magellan investigations

hot spot heat transfer its application to venus and implications to venus and earth gross similarities between earth and venus suggest that both planets might be expected to lose heat by the same mechanism available data do not resolve plate tectonics on venus however and other heat loss mechanisms such as hot spot heat loss have been suggested using a model which gives a relationship among surface elevation lithospheric thickness and heat flux we test the hot spot heat loss mechanism for venus and determine that it can easily explain the predicted heat loss of the planet with a modest number of hot spots of the order of 35 approximately 93 of the mapped topography of venus can be explained solely on the basis of lithospheric thickness variations additional compensation is required for topography above a radius of 6053 km and this can be effected by incorporating a variable thickness crust into the model if crust is assumed to be generated on the crests of the hot spots probably by processes associated with volcanism the model is consistent with almost 99 of the mapped venus topography the model is also basically consistent with available gravity data and interpretations which indicate compensated topography and great depths of compensation 100 1000 km for the mid latitudes of the planet the remaining approximately 1 of the topography not explained by hot spot crustal generation is thought to be compensated at a shallower depth primarily by variations in crustal thickness which are not directly related to hot spot volcanism the models relating surface elevation to lithospheric thickness and heat flow are grossly consistent with either a plate tectonic or hot spot heat loss mechanism for venus the range of lithospheric thicknesses implied by the range of elevations on venus implies that if plate tectonics is the dominant heat loss mechanism on venus the mean plate velocities are slower than on earth slow spreading ridges should be resolvable by the available venus topographic data but are not apparent comparison and analysis of the hypsometric curves for earth and venus suggest basic differences between the curves the skewed terrestrial ocean floor curve being consistent with plate tectonics but the relatively symmetrical venusian curve indicates that either plate tectonics differs on venus from the terrestrial analogue or that plate tectonics does not dominate the topographic expression of venus tectonism the venusian curve is consistent with a hot spot heat loss mechanism a hot spot crustal genesis mechanism is proposed which would probably result in crust very similar to the granite greenstone terranes of many terrestrial archaean cratons we speculate that hot spot heat loss may have preceded plate tectonics as the dominant terrestrial heat loss mechanism

chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12 800 years ago according to the younger dryas impact hypothesis ydih 12 800 calendar years before present north america experienced an extraterrestrial impact that triggered the younger dryas and devastated human populations and biotic communities on this continent and elsewhere this supposed event is reportedly marked by multiple impact indicators but critics have challenged this evidence and considerable controversy now surrounds the ydih proponents of the ydih state that a key test of the hypothesis is whether those indicators are isochronous and securely dated to the younger dryas onset they are not we have examined the age basis of the supposed younger dryas boundary layer at the 29 sites and regions in north and south america europe and the middle east in which proponents report its occurrence several of the sites lack any age control others have radiometric ages that are chronologically irrelevant nearly a dozen have ages inferred by statistically and chronologically flawed age depth interpolations and in several the ages directly on the supposed impact layer are older or younger than 12 800 calendar years ago only 3 of the 29 sites fall within the temporal window of the yd onset as defined by ydih proponents the ydih fails the critical chronological test of an isochronous event at the yd onset which coupled with the many published concerns about the extraterrestrial origin of the purported impact markers renders the ydih unsupported there is no reason or compelling evidence to accept the claim that a cosmic impact occurred 12 800 y ago and caused the younger dryas

protracted near solidus storage and pre eruptive rejuvenation of large magma reservoirs building super eruptive magma reservoirs in the cold upper parts of earth s crust requires a significant influx of magma over an extended period sufficient to allow the magma to accumulate differentiate and periodically erupt some models favour magma storage in a cold non eruptible state requiring extensive reactivation of the reservoirs before eruption whereas others suggest storage at higher temperature and lower crystallinity implying that magma in such reservoirs is readily eruptible consequently constraining volcanic hazards requires observations directly linking magma residence timescales to the thermal state and crystallinity of storage here we simultaneously determine crystallization temperatures and ages of magmatic crystals of zircon and titanite in the 900 km3 kneeling nun tuff new mexico usa which allows us to place tight constraints on the long term thermal evolution of the magma reservoir we show that zircon and titanite crystals record more than 600 000 years of magma assembly and constrain the dominant storage conditions to low temperatures set between the granitic solidus 680 to 700 c and the temperature of the onset of titanite crystallization about 720 to 730 c we apply the zircon titanite systematics to a suite of other super eruptions and suggest that protracted low temperature storage culminating in late stage reheating is a widespread feature of large crystal rich eruptions super eruptions are fed by large magma reservoirs geochemical analyses of volcanic rocks erupted in new mexico suggest the magma was stored under cool conditions in the crust for 600 000 years before late stage heating triggered an eruption

experimental determination of ree partition coefficients between zircon garnet and melt a key to understanding high t crustal processes the partitioning of rare earth elements ree between zircon garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis the experiments performed using internally heated gas pressure vessels at 7 kbar and 900 1000 c represent equilibrium partitioning of the middle to heavy ree between zircon and garnet during high grade metamorphism in the mid to lower crust the dree zircon garnet values show a clear partitioning signature close to unity from gd to lu because the light ree have low concentrations in both minerals values are calculated from strain modelling of the middle to heavy ree experimental data these results show that zircon is favoured over garnet by up to two orders of magnitude the resulting general concave up shape to the partitioning pattern across the ree reflects the preferential incorporation of middle ree into garnet with dgd zircon garnet ranging from 0 7 to 1 1 dho zircon garnet from 0 4 to 0 7 and dlu zircon garnet from 0 6 to 1 3 there is no significant temperature dependence in the zircon garnet ree partitioning at 7 kbar and 900 1000 c suggesting that these values can be applied to the interpretation of zircon garnet equilibrium and timing relationships in the ultrahigh t metamorphism of low ca pelitic and aluminous granulites

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

surface characteristics of venus derived from pioneer venus altimetry roughness and reflectivity measurements the three primary data sets for the pioneer venus orbiter radar experiment topography roughness and reflectivity contain important information about the geological and textural characteristics of the surface of venus we have subdivided the range of roughness and reflectivity values into three categories as follows roughness in degrees rms slope relatively smooth 1 2 5 transitional from smooth to rough 2 5 5 and relatively rough 5 and fresnel reflectivity surfaces dominated by soil or porous material 0 2 we have analyzed each of these data sets and their relationships to each other in order to define areas of the surface that are characterized by distinctive properties e g rough rocky surfaces smooth soil surfaces we then describe the abundance and areal distribution of such areas and locally calibrate the geological significance of some of the surface types by examining high resolution images from spacecraft and earth based observatories we find that the majority of venus is covered by regionally contiguous rock and bedrock surfaces many of the smooth surfaces we interpret to be of volcanic origin most likely lava flows while rougher surfaces are locally characterized by tectonic deformation of several types soil surfaces cover less than about 27 of the planet and are generally patchy in their distribution on the basis of the distribution of these surfaces we see no evidence for the extensive preservation of an ancient global regolith or for widespread topographically controlled erosion lateral transport and sedimentation the small percentage of the surface of venus characterized by high dielectric material appears to originate from several processes including primary lava flows probably containing enrichments of high dielectric materials such as metal or metal oxides e g theia mons in beta regio and exposure of high dielectric materials by tectonic deformation e g maxwell montes in ishtar terra these global data set correlations provide a fundamental frame work for understanding the nature of the surface of venus and will permit extrapolation of local and regional findings from future geochemical and imaging experiments to a global context

younger dryas black mats and the rancholabrean termination in north america of the 97 geoarchaeological sites of this study that bridge the pleistocene holocene transition last deglaciation approximately two thirds have a black organic rich layer or black mat in the form of mollic paleosols aquolls diatomites or algal mats with radiocarbon ages suggesting they are stratigraphic manifestations of the younger dryas cooling episode 10 900 b p to 9 800 b p radiocarbon years this layer or mat covers the clovis age landscape or surface on which the last remnants of the terminal pleistocene megafauna are recorded stratigraphically and chronologically the extinction appears to have been catastrophic seemingly too sudden and extensive for either human predation or climate change to have been the primary cause this sudden rancholabrean termination at 10 900 50 b p appears to have coincided with the sudden climatic switch from allerod warming to younger dryas cooling recent evidence for extraterrestrial impact although not yet compelling needs further testing because a remarkable major perturbation occurred at 10 900 b p that needs to be explained

orogenic belts on venus recent radar observations of venus show evidence for three types of deformational features in the linear mountain belts of ishtar terra linear ridges and troughs oriented parallel to the strike of the mountains and interpreted to be anticlines and synclines broad low arches similar to mare ridge type features seen on the moon and interpreted to represent low angle thrusting and buckling and linear discontinuities that cut across the strike of the ridges and troughs and are interpreted to represent strike slip movement on the basis of the localized concentration of these features their orientations and patterns of distribution and their association with linear mountainous topography we conclude that they mark the location of concentrated horizontal compressional deformation and that akna and freyja montes represent erogenic belts on venus

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

radiocarbon calibration curves summed probability distributions and early paleoindian population trends in north america archaeologists increasingly examine summed probability distributions of radiocarbon dates to search for temporal trends in ancient human populations including early north american population trends across the onset of the younger dryas climatic period 10 900 bc we use both intcal04 and intcal09 to simulate and calibrate sequences of radiocarbon dates that represent evenly spaced calendar ages from 12 000 to 9000 bc and use intcal09 to simulate two different demographic processes across this interval the shapes of the calibrated summed probability distribution from our first set of simulations mirror many trends that have been interpreted in demographic terms and this shape shows clear links to the shape of the radiocarbon calibration curve this is true for both intcal04 and intcal09 although these different curves produce different probability distributions the shapes of the calibrated summed probability distribution from our second set of simulations differ somewhat but show virtually identical trends at points where the actual frequencies of calendar dates are very different we conclude that changing frequencies of radiocarbon dates over time probably do contain demographic information but that extracting this information is more difficult than archaeologists have acknowledged

redox evolution of silicic magmas insights from xanes measurements of ce valence in bishop tuff zircons the potential for zircon to record continuous and evolving magmatic redox conditions is investigated by quantifying ce valence in natural and synthetic crystals by x ray absorption near edge structure xanes valence was determined at high spatial resolution 2 4 m by analysis of the ce l3 edge for synthetic zircons and crystals from the bishop tuff ig2e sequence analyses included both core to rim and cross sector measurements core to rim zonation among natural grains reveals a systematic increase in ce4 ce with core regions that range from 0 4 to 0 6 ce4 ce i e 40 60 ce4 while zircon rims range from 0 7 to 1 0 ce4 ce i e 70 100 ce4 repeat analysis on an individual point suggests on average a ce4 ce reproducibility at the 5 level or less changes in spectral features with grain orientation were also investigated by rotating and analyzing synthetic zircons every 45o this resulted in changes to the calculated ce valence of 5 which is much smaller than the range observed in natural samples the core to rim increase in ce4 ce of bishop tuff samples may indicate a continuous crystal melt evolution to more oxidizing conditions prior to eruption but this cannot be uniquely decoupled from other effects that may influence ce valence in zircon which potentially include temperature changes or kinetic processes related to the mineral growth surface cathodoluminescence imaging couples with xanes spectra for bishop tuff samples reveal that different sectors yield notably different ce4 ce implying anisotropic decoupling of ce3 and ce4 in the zircon near surface during crystallization broadly correlative albeit with lower spatial resolution ti thermometry and light rare earth element la icp ms data are also reported for zircon grains there is no correlation between measured ce anomalies and ce4 ce cerium valence measurements of zircon may be able to constrain magma redox evolution with time without relying on the nearly ubiquitous low concentrations of la and pr which are classically used to calculate ce anomalies

rare earth element partitioning between titanite and silicate melts henry s law revisited we present detailed experimental results on the partitioning of rare earth elements ree between titanite and a range of different silicate melts our results show that henry s law of trace element partitioning depends on bulk composition the available partners for heterovalent substitution crystal composition and melt composition we illustrate that the partition coefficients for sm depend very strongly on the bulk concentration of sm in the system the substitution mechanism by which rare earth elements are incorporated into the crystal structure plays an important role for trace element partitioning and also for the onset of henry s law our data show that there are clear differences between substitution mechanisms of major elements compared to elements which are present only as traces our experiments also clearly show that the onset of henry s law depends on the concentrations of the sum of all trace elements which are incorporated into the crystal by the same substitution mechanism for geochemical modelling of magmatic processes involving titanite and indeed other accessory phases it is of crucial importance to first evaluate whether the ree and other trace elements are present as traces or as major elements only then appropriate d values may be chosen

partition coefficients for rare earth elements in mafic minerals of high silica rhyolites the importance of accessory mineral inclusions abstract ree concentrations of mafic mineral separates from high silica rhyolites measured by inaa are high and variable compared to electron microprobe analyses of the minerals themselves the mafic phases commonly contain inclusions or have adhering grains of accessory rare earth element ree rich minerals optical and electron microscopic observation revealed discrete grains of chevkinite rare earth titano silicate included within clinopyroxenes from the sierra la primavera mexico rhyolite and monazite grains adhering to ortho and clinopyroxenes from the bishop tuff california during hand picking of mineral separates inclusions are only partly removed as a result the magnitude and variability of true mineral melt partition coefficients for light ree have been overestimated the true ree partition coefficients of la primavera and bishop tuff pyroxenes obtained by microprobe are only slightly higher than they are in lower silica rhyolites and are not as variable as previously thought the partitioning slope is positive as in less silicic systems the relative partitioning behavior of ree in high silica rhyolites is dominated by crystal chemical controls and not by liquid structural effects when a partition coefficient is used for crystallization calculations adhering phases and inclusions should be retained or the minor phases must be accounted for separately alternatively a bulk distribution coefficient can be calculated using whole rock and glass compositions and the glass mode

biostratigraphic evidence supports paleoindian population disruption at 12 9 ka buchanan et al 1 interpret an 40 reduction in the cumulative probabilities of radiocarbon dates at the beginning of the younger dryas 12 9 ka as noisy data inconsistent with human population decreases predicted by the extraterrestrial impact hypothesis 2 this reduction is in fact consistent with other biostratigraphic evidence suggesting an abrupt environmental perturbation at the

mechanisms for lithospheric heat transport on venus implications for tectonic style and volcanism the tectonic and volcanic characteristics of the surface of venus are poorly known but these characteristics must be closely related to the mechanism by which venus rids itself of internal heat on the other solid planets and satellites of the solar system lithospheric heat transport is dominated by one of three mechanisms 1 plate recycling 2 lithospheric conduction and 3 hot spot volcanism we evaluate each mechanism as a candidate for the dominant mode of lithospheric heat transfer on venus and we explore the implications of each mechanism for the interpretation of venus surface features despite claims made to the contrary in the literature plate recycling on venus cannot be excluded on the basis of either theoretical arguments or present observations on topography and radar backscatter landforms resulting from plate convergence and divergence on venus would differ substantially from those on the earth because of the high surface temperature and the absence of oceans on venus the lack of free or hydrated water in subducted material the possibility that subduction would more commonly be accompanied by lithospheric delamination and the rapid spreading rates that would be required if plate recycling removes a significant fraction of the internal heat if plate recycling occurs on venus the rolling plains and lowlands provinces would be approximate analogs to terrestrial ocean basins in terms of age igneous rock type and formative process highlands on venus would be roughly analogous to terrestrial continents the hypothesis that lithospheric conduction dominates shallow heat transfer on venus leads to the prediction that the lithosphere is thin if venus has a global heat loss per mass equal to that for earth then temperatures marking the base of the thermal lithosphere on earth would be reached on venus at an average depth of about 40 km unless the mantle convective planform can maintain lithospheric regions of persistently low heat flow or unless the present atmospheric greenhouse on venus is geologically recent then such a lithospheric thickness leads to the conclusion that the topographic features contributing to the 13 km of relief on the planet must be geologically young the hypothesis that hot spot volcanism dominates lithospheric heat transfer on venus leads to the prediction that the surface must be covered with numerous active volcanic sources in particular if a typical venus hot spot has a volcanic flux equal to the average flux for the hawaiian hot spot for the last 40 m y then 104 such hot spots are necessary to remove the venus internal heat by volcanism such a number would produce enough volcanic material to resurface the entire planet to a depth of 1 km every 2 m y few areas of the planet would escape resurfacing for geologically long periods of time we find that none of the mechanisms for lithospheric heat transfer on venus can be excluded as unimportant at present it is likely that as on earth a combination of mechanisms operates on venus the strongest conclusion to emerge from this evaluation is that most of the major topographic features and probably many of the surface geological units on venus are young by comparison with the surfaces of the smaller terrestrial planets

venus banded terrain tectonic models for band formation and their relationship to lithospheric thermal structure recent radar images of the mountains of ishtar terra venus at approximately 3 km radar resolution show a series of linear bands of alternatively greater and lesser backscatter the bands are generally aligned with the topographic strike of the mountain ranges we test the hypothesis that these band features are tectonic in origin the product either of folding during lithospheric compression or of block faulting during lithospheric extension tested models for folding include uniform elastic and viscous plates overlying inviscid substrates layered plates and a viscous half space in which viscosity decreases exponentially with depth the uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on venus about 15 to 20 km if the elastic or high viscosity layer is at most a few kilometers in thickness though the required compressive stresses are several kilobars in magnitude for the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness and similar results hold layered elastic or viscous plates however can fold at the required wavelength with subkilobar stresses and therefore are favored over uniform plate models tested extensional models include graben and horst formation in a surface elastic brittle layer imbricate normal faulting and necking of a surficial layer that is everywhere in a state of extensional failure the spacing between adjacent extensional features can match the spacing between bands on venus if the surficial brittle layer is no more than a few kilometers in thickness both compressional and extensional models for band formation are therefore consistent with radar images of venus mountain ranges at the image resolution presently available additional evidence however including the linearity and continuity of individual bands the relationship between band trends and topographic contours in some regions near the ends of mountain ranges and the very different manifestations of lithospheric extension in other highland areas of venus leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of ishtar terra as a result of horizontal compression of the venus lithosphere independent arguments on the thermal structure of the venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of venus is approximately 1 10 km thick we therefore suggest that the surficial layer of elastic brittle or high viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the venus highlands at the time of band formation

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

allanite and chevkinite in a type granites and syenites of the graciosa province southern brazil abstract allanite and chevkinite perrierite are important accessory phases in the granitic and syenitic a type rocks of the graciosa province southern brazil allanite ce is the main ree phase in the granites of the aluminous association which includes metaluminous to weakly peraluminous granites while chevkinite ce is the typical ree phase in the metaluminous syenites and alkali feldspar syenites and the peralkaline alkali feldspar granites of the alkaline association allanite shows complex zoning patterns but is always rich in the ferriallanite molecule and in tio 2 a compositional trait that distinguishes allanite in a type aluminous associations from allanite in calcic alkali calcic and calc alkaline granitoids normalized ree patterns show strong fractionation of heavy over light ree with la n nd n ratios between 3 and 5 and la n y n ratios between 200 and 400 primary allanite is variably affected by post magmatic transformations due to the interaction with a fluid phase and metamictization caused by radioactive decay chevkinite ce is always primary and is only present in the alkaline association the early crystallization of chevkinite indicates that the parental magmas were probably close to ree saturation the ree y abundance in chevkinite 43 49 wt is almost twice that observed in primary allanite yet the normalized patterns are similar in terms of fractionation between light and heavy ree as indicated by similar la n nd n ratios in chevkinite and allanite la n y n ratios are on average lower in chevkinite 90 300 chevkinite compositions were altered by interaction with post magmatic fluids which led to marked increase in tio 2 and a reduction in ree and fe t post magmatic allanite is present in both the alkaline and aluminous associations and corresponds to ferriallanite ce in the alkaline association and allanite y in the aluminous association compared to primary allanite post magmatic allanite ce shows smaller ree fractionation 2 n nd n 5 times chondrite the mineralogy of the rocks indicates crystallization conditions close to the titanite magnetite quartz amphibole ilmenite tmqai buffer for the aluminous association and close to the fayalite quartz magnetite fqm buffer for the alkaline association and temperatures between 800 and 900 c liquidus and ca 750 c solidus the experimentally determined stability fields for chevkinite and allanite are in agreement with the textural evidence for early chevkinite crystallization and late allanite crystallization even though allanite and chevkinite only rarely coexist in the alkaline association wherever allanite and chevkinite coexist they reveal reaction textures such that it is possible that only allanite or chevkinite is stable at any given condition the distribution of allanite and chevkinite in the alkaline association suggests that the composition of the magmas is an important control on the stability fields of these minerals a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing and mingling of magmas in this sense the importance of composition on the stability of allanite and chevkinite may be larger than previously suggested

venus volcanism initial analysis from magellan data magellan images confirm that volcanism is widespread and has been fimdamentally important in the formation and evolution of the crust of venus high resolution imaging data reveal evidence for intrusion dike formation and cryptodomes and extrusion a wide range of lava flows also observed are thousands of small shield volcanoes larger edifices up to several hundred kilometers in diameter massive outpourings of lavas and local pyroclastic deposits although most features are consistent with basaltic compositions a number of large pancake like domes are morphologically similar to rhyolite dacite domes on earth flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas perhaps komatiites may be present volcanism is evident in various tectonic settings coronae linear extensional and compressional zones mountain belts upland rises highland plateaus and tesserae volcanic resurfacing rates appear to be low less than 2 km 3 yr but the significance of dike formation and intrusions and the mode of crustal formation and loss remain to be established