the lunar far uv albedo indicator of hydration and weathering 1 we present an analysis of lunar reconnaissance orbiter lro lyman alpha mapping project lamp measurements of the dayside lunar surface at far ultraviolet wavelengths we use the strong 165 nm h2o absorption edge to look for diurnal variations in hydration we find that diurnal variations in spectral slope are indeed present they are superimposed on latitudinal and spatial variations related to composition and weathering we use two different spectral regions 164 173 nm and 175 190 nm to separate out these effects highlands and mare regions have distinct reflectance spectra with mare regions being spectrally bluer than highlands regions a consequence of the greater abundance of opaque minerals in mare regions bright ray terrains and areas known to be young such as giordano bruno crater are found to be relatively spectrally flat or red in the far uv this is consistent with a lack of space weathering which tends to make the far uv spectrum bluer due to the spectral behavior of nanophase iron large scale latitudinal variations in fuv slope are distinct and are likely due to a gradient in space weathering the diurnal variation in hydration is consistent with a solar wind origin and with loss of h2o at temperatures above 320 k far uv spectroscopy is thus shown to represent a viable method for mapping aqueous alteration even on the dayside of the moon and potentially elsewhere in the solar system

high resolution mapping of tio2 abundances on the moon using the hubble space telescope 1 samples of the lunar regolith returned by apollo astronauts show large variations 0 to 10 wt in tio2 abundance indicating complex compositional zonation within the lunar mantle a long held goal of the lunar science community is the accurate determination of tio2 abundances on the lunar surface through remote sensing methods to date only limited progress has been made in this area using spacecraft spectral measurements acquired in visible through near infrared wavelengths here we show that variations in the ratios of ultraviolet uv to visible vis reflectances in images taken by the hubble space telescope indicate a strong correlation with tio2 abundances determined from returned samples at the apollo 17 landing site and little correlation with the maturity of lunar soils these new findings imply that uv vis observations may lead to an alternate and improved method of remotely mapping tio2 bearing materials probably as ilmenite across the lunar surface and enable more refined studies of lunar crustal composition surface volcanism and subsurface magma evolution processes additionally accurate identification and quantification of tio2 rich deposits serves to guide future human exploration of the moon

compositional heterogeneity of central peaks within the south pole aitken basin 1 using high spectral and spatial resolution moon mineralogy mapper data we investigate compositional variations across the central peak structures of four impact craters within the south pole aitken basin spa two distinct causes of spectral diversity are observed spectral variations across the central peaks of bhabha finsen and lyman are dominated by soil development including the effects of space weathering and mixing with local materials for these craters the central peak structure is homogeneous in composition although small compositional differences between the craters are observed this group of craters is located within the estimated transient cavity of spa and their central uplifts exhibit similar mafic abundances therefore it is plausible that they have all uplifted material associated with melts of the lower crust or upper mantle produced during the spa impact compositional differences observed between the peaks of these craters reflect heterogeneities in the spa subsurface although the origin of this heterogeneity is uncertain in contrast to these craters leeuwenhoek exhibits compositional heterogeneity across its central peak structure the peak is areally dominated by feldspathic materials interspersed with several smaller exposures exhibiting a mafic spectral signature leeuwenhoek is the largest crater included in the study and is located in a region of complex stratigraphy involving both crustal feldspathic and spa mafic melt and ejecta materials the compositional diversity observed in leeuwenhoek s central peak indicates that kilometer scale heterogeneities persist to depths of more than 10 km in this region

comparative proximal features of the main plinian deposits campanian ignimbrite and pomici di base of campi flegrei and vesuvius abstract the proximal plinian fall deposits of the campanian ignimbrite ci 38 ky fedele et al 2008 and pomici di base pdb 18 ky bertagnini et al 1998 have been investigated in order to understand the contribution of each part of the plume to the proximal sedimentation following houghton et al 2004b we consider three main transport regimes jet phase producing facies fb buoyant region of the plume producing facies fa and direct lateral ejection producing facies fc as well documented in medial locations sparks et al 1992 1997 ernst et al 1996 transport regimes can develop different facies even in proximal locations according to the dynamics of the eruptive column our proximal deposits show stratification and diffuse bedding allowing us to introduce two new facies stratified fa sfa and diffuse bedded fb dbfb these facies retain the transport regime previously proposed for fa buoyant plume and fb jet phase but their lithological features are influenced by near vent depositional conditions lithology and sedimentological data grain size componentry maximum clasts suggest that most of the sedimentation occurred mainly from the buoyant plume with simultaneous contribution from the other two different dynamic regimes coarse clasts falling from the lower margins of the plume strongly affected the sedimentation of the ci proximal fall deposit with a minor contribution from lithic clasts ballistically emplaced and partial collapses of the plume forming pyroclastic density currents in contrast the pdb proximal fall deposit was strongly affected by coarse clasts emplaced directly from the vent through parabolic trajectories with very little contribution of material emplaced from the lower part of the plume these differences can be attributed to different vent conduit processes acting during the eruptions

complexities of plinian fall deposition at vent an example from the 1912 novarupta eruption alaska abstract an extremely proximal ejecta ring with exposures to within 100 m of vent was deposited during later stage plinian fall activity during the 1912 novarupta eruption in alaska one bed in the ejecta ring bed s contains predominantly andesitic clasts which serve to delineate the striking contrast in thinning rates along dispersal axis of the ejecta ring pyle bt values of 70 m bed s alone or 190 m whole ejecta ring and the coeval dacitic plinian fall deposits pyle bt values of 4 km proximal and 37 km medial distal the locally deposited andesitic and dacitic clasts of the ejecta ring are interpreted as products of an irregular collar of low fountaining ejecta partially sheathing the core of higher velocity dacitic ejecta that fed the stable converting 23 km high column the presence of such an extremely proximal accumulation of ejecta appears to be a feature common to several other historic eruptions that generated widespread fall deposits this feature in part accounts for conflicts between measured and calculated values for thickness maxima in plinian fall deposits and suggests that modifications may be required of existing models for plinian eruption columns

s type asteroids ordinary chondrites and space weathering the evidence from galileo s fly bys of gaspra and ida new observations of the s type asteroids gaspra and ida especially by the galileo spacecraft demonstrate that a space weathering process operates which modifies the reflectance spectra of fresh material to be redder straighter and have shallower absorption bands the weathering process appears similar to but less potent than that observed on the moon it operates in the sense that it would tend to convert spectra of ordinary chondrites oc to having the spectral traits of s type asteroids these results appear to resolve the major obstacles of the long standing s type conundrum about the provenance of ordinary chondrite meteorites a wide body of recent research which is reviewed here builds on previous meteoritical evidence to support a new developing consensus that the larger s type asteroids are a diverse assortment of silicaceous assemblages which includes the ordinary chrondite parent bodies recent fairly realistic laboratory simulations of space weathering processes have changed oc spectra to resemble s type spectra j f bell s highly regarded earlier paradigm that oc parent bodies would be found only among sub 10 km main belt asteroids has been tested by an extensive observational survey by r binzel and his proteges the result is that no small main belt analog for the ocs has been found not even the somewhat oc like object boznemcova this article reviews the history of the s type conundrum which set the stage for galileo s historic flybys i review the findings about gaspra and ida including results about their geology their geophysical structure and probable origins and about ida s small moonlet dactyl density constraints on ida set by dactyl s orbit rule out for ida at least the classic view of s types as metallic stripped cores of differentiated precursor asteroids new analysis of the galileo spectral images of ida is presented that provides strong evidence that space weathering occurs on ida and that ida and the koronis family asteroids in general are plausibly oc like in composition after reviewing 1990s developments on the s type conundrum i advocate a new perspective that the ordinary chondrite parent bodies are among the s type asteroids a diverse grouping that also contains other silicaceous and silicate metal assemblages presumably including various stony irons and primitive achondrites represented in meteorite collections gaspra may be such a metal rich assemblage

geology mineralogy and stable isotope geochemistry of the kabwe carbonate hosted pb zn deposit central zambia abstract the carbonate hosted kabwe pb zn deposit central zambia has produced at least 2 6 mt of zn and pb metal as well as minor amounts of v cd ag and cu the deposit consists of four main epigenetic pipe like orebodies structurally controlled along ne sw faults sphalerite galena pyrite minor chalcopyrite and accessory ge sulphides of briartite and renierite constitute the primary ore mineral assemblage cores of massive sulphide orebodies are surrounded by oxide zones of silicate ore willemite and mineralized jasperoid that consists largely of quartz willemite cerussite smithsonite goethite and hematite as well as numerous other secondary minerals including vanadates phosphates and carbonates of zn pb v and cu galena sphalerite and pyrite from the pb zn rich massive orebodies have homogeneous negative sulphur isotope ratios with mean 34 s cdt permil values of 17 75 0 28 1 16 54 0 0 27 and 15 82 0 25 respectively the zn rich and pb poor no 2 orebody shows slightly heavier ratios of 11 70 0 5 34 s for sphalerite and of 11 91 0 71 34 s for pyrite the negative sulphur isotope ratios are considered to be typical of sedimentary sulphides produced through bacterial reduction of seawater sulphate and suggest a sedimentary source for the sulphur carbon and oxygen isotope ratios of the host dolomite have mean 13 c pdb and 18 o smow values of 2 89 and 27 68 respectively which are typical of marine carbonates the oxygen isotope ratios of dolomite correlate negatively to the sio 2 content introduced during silicification of the host dolomite the depletion in 18 o in dolomite indicates high temperature fluid rock interaction involving a silica and 18 o rich hydrothermal solution two types of secondary fluid inclusions in dolomite both of which are thought to be related to ore deposition indicate temperatures of ore deposition in the range of 257 to 385 and 98 to 178 c respectively the high temperature fluid inclusions contain liquid vapour solid phases and have salinities of 15 to 31 eq wt nacl whereas the low temperature inclusions consist of liquid vapour with a salinity of 11 5 eq wt nacl fluid transport may have been caused by tectonic movements associated with the early stages of the pan african lufilian orogeny whereas ore deposition within favourable structures occurred due to changes in pressure temperature and ph in the ore solution during metasomatic replacement of the host dolomite the termination of the kabwe orebodies at the mine club fault zone and observed deformation textures of the ore sulphides as well as analysis of joint structures in the host dolomite indicate that ore emplacement occurred prior to the latest deformation phase of the neoproterozoic lufilian orogeny

reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits 1 methods two inversion methods are developed to reconstruct eruption column dynamics from granulometric data of plinian deposits they are based on the steady tephra dispersal model from umbrella clouds which spread as gravity currents the methods provide 1 expansion rate of an umbrella cloud 2 grain size distribution at the top of the eruption column and 3 total amount of ejecta including very fine particles which are extensively dispersed in the atmosphere the first quantity is correlated to the magma discharge rate the second quantity provides information about sorting processes during column rise and hence about deceleration in the gas thrust region of the eruption column and the timing of column collapse the third quantity together with the estimated magma discharge rate provides the effective duration of the eruption the two methods are based on the same dispersal model but use different averaging combining the two methods can provide additional information about eruption dynamics such as minor flow structures of the umbrella cloud and fluctuations of eruption rate the methods are applicable to large scale eruptions where the radial expansion velocity of the umbrella cloud exceeds wind velocity the applicability of the methods is tested from the viewpoint of comparison with real time observations in the companion paper paper 2

the geotectonic environment of zambian copper cobalt mineralization suggestions that the ore shale of the lower roan group is part of a series of sedimentary fades produced by the north easterly transgression of the sea over dominantly arenaceous strata of littoral deltaic lacustrine and continental origin have been viewed with scepticism by the author the lack of proven transition facies from silty carbonaceous argillites of the type found close to the south western limits of the ore shale in the zambian copperbelt to dolomites found further to the sw has tended to accentuate this feeling the intensity of exploration drilling in these areas must imply that whatever the cause the cut out of the ore shale is rapid at the same time a 9normal9 lower roan succession of the chambishi type suddenly becomes attenuated and dolomites and dolomitic chloritic schists are found immediately overlying argillaceous gritty sandstones conglomerates and arkosic quartzites characteristic of footwall formations locally this contact may be reddened porous and brecciated amphibolite bodies of the type normally found at a much higher stratigraphic level in the upper roan group are present a short distance above this contact it is thus proposed that this feature represents a hiatus in the succession and that all rocks from the ore shale to the top of the lower roan and possibly into the base of the upper roan are missing due to non deposition and or erosion this interruption of the succession coincides with the western cut out of the ore shale from the konkola dome in the nw down to the western end of the roan basin in the s in view of various authors9 contention that the zambian copperbelt is the site of an ancient aulacogen the above observations take on an important genetic significance the present paper thus reviews the location of copper cobalt mineralization in relation to sedimentary facies changes and cut outs structural features and amphibolitic bodies and proposes a geotectonic model for the copperbelt which it is hoped provides a more satisfactory and comprehensive explanation for the facts this model also provides a source for the metals which were converted to sulphides by diagenetic processes it is believed that the complex mineral chemistry of the ore shale associated ores and in particular the presence of localized concentrations of mo co ag as bi and te bearing phases is best explained by high level seepage of magmatically enriched hydrothermal solutions along precambrian rift systems and associated fractures these metals were either dispersed by wave and current action in the sedimentary environment or were concentrated by porosity and physico chemical and bacteriogenic controls in the diagenetic environment

mapping iron in the lunar mare an improved approach 1 previous methods for mapping iron lucey et al 1995 1998 2000a give inconsistent results in terms of suppressing the effects of maturity making it difficult to distinguish true units of contrasting iron from regions that simply show the residual effects of incomplete separation of maturity these methods rely on an assumption that materials of similar iron content but varying maturity form radial trends on a plot of vis reflectance versus nir vis ratio recently the assumption of radial behavior of maturity trends in mare regions has been called into question by staid and pieters 2000 they observed trends in the mare that appeared more parallel in nature in this work we study nearly 10 000 craters in six mare regions and model the effects of maturity with radiative transfer theory to quantify and better understand the spectral behavior of maturity variations in the mare we confirm that the maturity trends in mare regions are more parallel than radial and we exploit this fact to develop a new algorithm for determination of iron content in mare regions this new mare iron algorithm better compensates for maturity than previous methods and uncertainties due to maturity variations are less than 0 5 wt feo absolute uncertainties are similar to previous algorithms 1 5 wt feo results from this work provide the ability to detect iron anomalies in the ejecta of craters and as a result more confidently map vertical stratigraphy in the lunar mare in terms of iron composition

space weathering on airless bodies resolving a mystery with lunar samples using new techniques to examine the products of space weathering of lunar soils we demonstrate that nanophase reduced iron npfe0 is produced on the surface of grains by a combination of vapor deposition and irradiation effects the optical properties of soils both measured and modeled are shown to be highly dependent on the cumulative amount of npfe0 which varies with different starting materials and the energetics of different parts of the solar system the measured properties of intermediate albedo asteroids the abundant s type asteroids in particular are shown to directly mimic the effects predicted for small amounts of npfe0 on grains of an ordinary chondrite regolith this measurement and characterization of space weathering products seems to remove a final obstacle hindering a link between the abundant ordinary chondrite meteorites and common asteroids

compositional variations of the lunar crust results from radiative transfer modeling of central peak spectra 1 we present model mineralogy of impact crater central peaks combined with crustal thickness and crater central peak depth of origin models to report multiple perspectives of lunar crustal composition with depth here we report the analyses of 55 impact crater central peaks and how their compositions directly relate to the lunar highlands sample suite a radiative transfer model is used to analyze clementine visible plus near infrared spectra to place compositional constraints on these central peak materials central peaks analyzed are dominantly magnesian and plagioclase poor strong compositional similarities to lunar mg suite materials are evident relative to crustal thickness estimates central peak mineralogy becomes more plagioclase rich as the crust thickens relative to the crust mantle boundary the origin of peaks with dominantly mafic mineralogy are confined to the lower crust and primarily within the south pole aitken and procellarum kreep terranes pkt additionally central peaks with anorthositic mineralogy 60 vol plagioclase are transported to the surface from all depths in the crustal column and confined to the feldspathic highlands terrane fht the discovery of mafic and magnesian materials consistent with mg suite rocks of the sample collection in all lunar terranes suggests that the process and sources that give rise to these types of rocks is not unique to the pkt and not necessarily dependent on incompatible elements for formation the identification of ferroan and magnesian anorthositic material near the crust mantle boundary of the fht is also inconsistent with an increasing mafic feldspar ratio and mg with depth in the crust

remote sensing studies of the lomonosov fleming region of the moon 1 lomonosov fleming is a 620 km pre nectarian impact basin centered at 19 n 105 e clementine multispectral images and a variety of spacecraft photographs were used to investigate the composition and origin of geologic units in the region mare basalt deposits in the lomonosov fleming region are small and exhibit varying degrees of highlands contamination four previously unrecognized dark mantle deposits of probable pyroclastic origin were identified and mapped a major expanse of cryptomare was mapped in and around the lomonosov fleming basin spectral and chemical data obtained for dark haloed craters dhc established that these impact craters excavated mare basalt from beneath higher albedo highlands rich surface units the buried basalts exposed by dhcs in the region exhibit a variety of compositions which range from very low titanium basalt to intermediate tio2 mare basalt nectarian aged units with subjacent cryptomare are much more extensive than imbrian aged deposits with underlying basalt at least some of the buried mare basalts in the region may have been emplaced during pre nectarian time while both impact basins and craters played a role in cryptomare formation by transporting highlands material to mare surfaces the evidence indicates that most of the cryptomare inside the basin are of the distal basin ejecta type the enhanced feo and tio2 abundances exhibited by these cryptomare surfaces usually light plains units demonstrate that ballistic erosion and sedimentation played an important role in their formation

stratigraphy and physical parameters of the plinian phase of the campanian ignimbrite eruption the campanian ignimbrite eruption 39 ka was the most powerful eruptive event of the campi flegrei caldera southern italy this event coincided with the onset of a cold climatic phase and the paleolithic transition from neanderthals to modern humans the eruption started with a sustained column that emplaced a widespread pyroclastic fall deposit covering an area 4000 km2 within the 15 cm isopach for the first time we present the complete longitudinal variation from the coarse and 10 m thick proximal down to 15 km sequence through the well stratified pumice lapilli deposit in medial areas 30 80 km to the distal tephra hundreds to thousands of kilometers distant the plinian pumice fall deposit shows a strong lateral heterogeneity due to variations in stratification grading and abundance of components all campanian ignimbrite fall layers display plinian dispersal variation of grain size with stratigraphic height suggests that the convective plume was far from steady state initially the plume dispersed 1 3 km3 of tephra toward the ene n75 e from a height of 29 km layer a a gradual increase in intensity resulted in inverse grading of layer b column height increased from 26 to 37 km at a vertical velocity of 3 6 m s it had a main dispersal axis to the east n97 e and a secondary lobe to the southeast n137 e during this phase a maximum volume of 1 73 km3 of tephra was emplaced accessory lithics concentrated in layer c are possibly due to vent clearing after partial blockage of the vent the 33 km high column dispersed ejecta to the east n95 e with only 0 2 km3 of tephra erupted during this phase during the successive gradual decline in eruption intensity column height decreased from 38 to 32 km at a velocity of 4 2 m s a normally graded layer d with a volume of 1 km3 accumulated to the east n95 e the sustained column phase ended with a pulsating and partially collapsing column that reached 23 km in height and dispersed 1 1 km3 of a stratified and lithic rich succession layer e to the southeast n112 e if we include the distal co plinian deposit a total volume of 7 8 km3 dense rock equivalent dre of magma was released containing 0 09 km3 of accessory lithics tephra mass for the single layers is on the order of 1011 kg and the total mass is 2 1012 kg 2 1013 kg including the co plinian ash mass discharge rates ranged from 0 9 to 6 7 108 kg s the calculated magnitude of the sustained column phase is 6 3 the duration of the plinian phase of this eruption based on the ratio of two parameters erupted mass divided by discharge rate is estimated to have been 20 h including co plinian ash this study shows that plinian deposits are not always homogeneous and as for pyroclastic current deposits can show an articulate architecture only the complete reconstruction of vertical and lateral variations in components stratification and grading might provide insights into the temporal and spatial evolution of the sustained plume

the effect of intensity of turbulence in umbrella cloud on tephra dispersion during explosive volcanic eruptions experimental and numerical approaches abstract during an explosive volcanic eruption tephra fall out from the umbrella region of the eruption cloud to the ground surface we investigated the effect of the intensity of turbulence in the umbrella cloud on dispersion and sedimentation of tephra by performing a series of laboratory experiments and three dimensional 3 d numerical simulations in the laboratory experiments spherical glass bead particles are mixed in stirred water with various intensities of turbulence and the spatial distribution and the temporal evolution of the particle concentration are measured the experimental results show that when the root mean square of velocity fluctuation in the fluid w rms is much greater than the particle terminal velocity v t the particles are homogeneously distributed in the fluid and settle at their terminal velocities at the base of the fluid where turbulence diminishes on the other hand when w rms is as small as or smaller than v t the particle concentration increases toward the base of the fluid during settling which substantially increases the rate of particle settling the results of the 3 d simulations of eruption cloud indicate that w rms is up to 40 m s in most of the umbrella cloud even during a large scale plinian eruption with a magma discharge rate of 10 9 kg s these results suggest that relatively coarse pyroclasts more than a few mm in diameter tend to concentrate around the base of the umbrella cloud whereas fine pyroclasts less than 1 8 mm in diameter may be distributed homogeneously throughout the umbrella cloud during tephra dispersion the effect of the gradient of particle concentration in the umbrella cloud explains the granulometric data of the pinatubo 1991 plinian deposits

reappraisal of the 1982 eruptions of el chichon volcano chiapas mexico new data from proximal deposits additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases iii and iv of the 1982 eruption of el chichon volcano phase iii began on 4 april at 0135 gmt with a powerful hydromagmatic explosion that generated radially fast moving 100 ms 1 pyroclastic clouds that produced a surge deposit s1 due to the sudden reduction in the confining pressure the process continued by tapping of magma from a deeper source causing a new explosion the ejected juvenile material mixed with large amounts of fragmented dome and wall rock which were dispersed laterally in several pulses as lithic rich block and ash flow f1 partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate a series of hydromagmatic explosions that dispersed moisture rich surge clouds and small volume block and ash flows iu up to distances of 3 km from the crater the eruption continued by further decompression of the magmatic system with the ensuing emission of smaller amounts of gas rich magma which with the strong erosion of the volcanic conduit formed a lithic rich plinian column that deposited fallout layer b associated with the widening of the vent an increase in the effective density of the uprising column took place causing its collapse block and ash flows arising from the column collapse traveled along valleys as a dense laminar flow f2 in some places flow regime changes due to topographic obstacles promoted transformation into a turbulent surge s2 which attained minimum velocities of approximately 77 ms 1 near the volcano the process continued with the formation of a new column on 4 april at 1135 gmt phase iv that emplaced fall deposit c and was followed by hydromagmatic explosions which produced pyroclastic surges s3

plinian pumice fall deposit of the campanian ignimbrite eruption phlegraean fields italy abstract a plinian pumice fall deposit associated with the campanian ignimbrite eruption 36 ka phlegraean fields caldera italy occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km2 between benevento and the sorrentina peninsula in the thickest stratigraphic section at voscone 130 cm 45 km east of the phlegraean caldera centre pozzuoli the deposit consists of two units the lower fall unit lfu is well sorted exhibits reverse size grading and is composed of equidimensional light grey pumice clasts with very subordinate accidental lithics the upper fall unit ufu is from well to poorly sorted crudely stratified richer in lithics and composed of both equidimensional and prolate pumice clasts the two fall units show slightly different dispersal axis n90 for the lfu and n95 for the ufu volumes calculated with the method of pyle 1989 are about 8 km3 for the lfu and 7 km3 for the ufu the maximum height of the eruptive columns are estimated using the model of the maximum lithic clasts dispersal at 44 km for the lfu and 40 km for the ufu classifying both fall units as ultraplinian in character reverse size grading within the lfu suggests an increase of the height of the column and magma discharge rate with time moderate sorting and crude stratification of the ufu are consistent with short period oscillation of the column possibly associated with repeated partial column collapses sharp increases in lithic content at the transition to ufu and within the ufu suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit vent erosion the phase of column instability preceded the emplacement of widely dispersed pyroclastic flow the ultraplinian nature of the fall fits well with the wide dispersal of the campanian ignimbrite with an estimated aspect ratio of 3 4 10 4 lari

the plinian eruptions of 1912 at novarupta katmai national park alaska the three day eruption at novarupta in 1912 consisted of three discrete episodes episode i began with plinian dispersal of rhyolitic fallout layer a and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers the plinian column was sustained throughout most of the interval of ash flow generation in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite accordingly plinian layer b which fell in unbroken continuity with purely rhyolitic layer a is zoned from 99 to 15 rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in mageik creek and the valley of ten thousand smokes vtts only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent indicating that ignimbrite emplacement barely outlasted the plinian phase on near vent ridges the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley filling equivalents but exhibit evidence for turbulent deposition and recurrent scour episode ii began after a break of a few hours and was dominated by plinian dispersal of dacitic layers c and d punctuated by minor proximal intraplinian flows and surges after another break dacitic layers f and g resulted from a third plinian episode iii intercalated with these proximally are thin intraplinian ignimbrites and several andesite rich fall flow layers both cd and fg were ejected from an inner vent 400 m wide nested within that of episode i into which the rhyolitic lava dome novarupta was still later extruded two finer grained ash layers settled from composite regional dust clouds layer e which accumulated during the d f hiatus includes a contribution from small contemporaneous ash flows and layer h settled after the main eruption was over both are distinct layers in and near the vtts but distally they merge with cd and fg respectively they are largely dacitic but include rhyolitic shards that erupted during episode i and were kept aloft by atmospheric turbulence published models yield column heights of 23 26 km for a 22 25 km for cd and 17 23 km for fg and peak mass eruption rates of 0 7 1x108 0 6 2x108 and 0 2 0 4x108 kg s 1 respectively fallout volumes adjusted to reflect calculated redistribution of rhyolitic glass shards are 8 8 km3 4 8 km3 and 3 4 km3 for episodes i ii and iii microprobe analyses of glass show that as much as 0 4 km3 of rhyolitic glass shards from eruptive episode i fell with cde and 1 1 km3 with fgh most of the rhyolitic ash in the dacitic fallout layers fell far downwind se of the vent near the rhyolite dominated ignimbrite however nearly all of layers e and h are dacitic showing that the downwind rhyolitic ash is of co plinian rather than co ignimbrite origin

geochemistry and geotectonic setting of late proterozoic katangan basic rocks from kibambale in central shaba zaire the late proterozoic katangan basic rocks dolerites and basalts outcropping at kibambale in central shaba zaire are classified as extension related tholeiites the concentrations of immobile major and trace elements resemble those encountered in phanerozoic enriched oceanic ridge basalts e morb emplaced in incipient oceanic rifts such as red sea and tadjura gulf troughs this conclusion is also sustained by elemental ratios which are morb like multielement chondrite and morb normalized patterns sedimentological data especially the shallow water conditions which prevailed during the deposition of lower kundelungu tillite in which these basic rocks are interstratified and the geological and structural framework notably the emplacement of these igneous rocks in a basin not distant from the kibaran continental land mass

grain size variation of tephra derived from volcanic umbrella clouds the relationships among the thickness and grain size of tephra fall deposits and the volumetric flow rate of their source umbrella clouds are analytically obtained the logarithm of the ratio of the probability distribution function based on grain size inr f in fall deposits at two localities from the vent r 1 and r 2 respectively has a linear relationship with the particle settling velocity v as in 1n r f frac pi r 2 2 r 1 2 nu q 1n a where q is the volumetric flow rate of the umbrella cloud and a is a constant for a given pair of localities the volumetric flow rate of the umbrella cloud can be estimated from granulometric data using this formula generally the thickness distance relationship of tephra fall deposits depends on the initial grain size distribution and the volumetric flow rate of the umbrella cloud the empirical relationship of the exponential thinning behaviour can be extrapolated towards infinite distance only for a specific initial grain size which is similar to a log normal distribution with 2 5 otherwise it holds only in a limited range of distances in applying these results to the 1991 eruption of mt pinatubo it is shown that the volumetric flow rate of the umbrella cloud during the climactic phase of 15 june was approximately 5x1010 m3 s which is fairly consistent with the expansion rate of the umbrella cloud observed in the satellite images

volcanic jets plumes and collapsing fountains evidence from large scale experiments with particular emphasis on the entrainment rate the source conditions of volcanic plumes and collapsing fountains are investigated by means of large scale experiments in the experiments gas particle jets issuing from a cylindrical conduit are forced into the atmosphere at different mass flow rates dense jets high particle volumetric concentration e g c 0 0 01 generate collapsing fountains whose height scales with the squared exit velocity this is consistent with bernoulli s equation which is a good approximation if air entrainment is negligible in this case kinetic energy is transformed into potential energy without any significant loss by friction with the atmosphere the dense collapsing fountain on hitting the ground generates an intense shear flow similar to a pyroclastic density current dilute hot jets low particle volumetric concentration e g c 0 3

origin of the nchanga copper cobalt deposits of the zambian copperbelt the zambian copperbelt forms the southeastern part of the 900 km long neoproterozoic lufilian arc and contains one of the world s largest accumulations of sediment hosted stratiform copper mineralization the nchanga deposit is one of the most significant ore systems in the zambian copperbelt and contains two major economic concentrations of copper and cobalt hosted within the lower roan group of the katangan supergroup a lower orebody copper only and upper orebody copper and cobalt occur towards the top of arkosic units and within the base of overlying shales the sulfide mineralogy includes pyrite bornite chalcopyrite and chalcocite although in the lower orebody sulfide phases are partially or completely replaced by malachite and copper oxides carrollite is the major cobalt bearing phase and is restricted to fault propagation fold zones within a feldspathic arenite hydrothermal alteration minerals include dolomite phlogophite sericite rutile quartz tourmaline and chlorite quartz veins from the mine sequence show halite saturated fluid inclusions ranging from 31 to 38 wt equivalent nacl with homogenisation temperatures thtot ranging between 140 and 180 c diagenetic pyrites in the lower orebody show distinct relatively low 34s ranging from 1 to 17 whereas arenite and shale hosted copper and cobalt sulfides reveal distinctly different 34s from 1 to 12 for the lower orebody and 5 to 18 for the upper orebody there is also a clear distinction between the 34s mean of 12 1 3 3 n 65 for the upper orebody compared with 5 2 3 6 n 23 for the lower orebody the 13c of dolomites from units above the upper orebody give 13c values of 1 4 to 2 5 consistent with marine carbon however dolomite from the shear zones and the alteration assemblages within the upper orebody show more negative 13c values 2 9 to 4 0 and 5 6 to 8 3 respectively similarly shear zone and upper orebody dolomites give a 18o of 11 7 to 16 9 compared to lower roan dolomites which show 18o of 22 4 to 23 0 two distinct structural regimes are recognized in the nchanga area a weakly deformed zone consisting of basement and overlying footwall siliciclastics and a moderate to tightly folded zone of meta sediments of the katangan succession the fold geometry of the lower roan package is controlled by internal thrust fault propagation folds which detach at the top of the lowermost arkose or within the base of the overlying stratigraphy and show vergence towards the ne faulting and folding are considered to be synchronous as folding predominantly occurred at the tips of propagating thrust faults with local thrust breakthrough the data from nchanga suggests a strong link between ore formation and the development of structures during basin inversion as part of the lufilian orogeny sulfides tend to be concentrated within arenites or coarser grained layers within shale units suggesting that host rock porosity and possibly permeability played a role in ore formation however sulfides are also commonly orientated along but not deformed by a tectonic fabric or hosted within small fractures that suggest a significant role for deformation in the development of the mineralization the ore mineralogy hydrothermal alteration and stable isotope data lend support to models consistent with the thermochemical reduction of a sulfate and metal enriched hydrothermal fluid at the site of mineralization there is no evidence at nchanga for a contribution of bacteriogenic sulfide produced during sedimentation or early diagenesis to the ores

the lunar rock and mineral characterization consortium deconstruction and integrated mineralogical petrologic and spectroscopic analyses of mare basalts the lunar rock and mineral characterization consortium lrmcc has conducted coordinated mineralogy petrography spectroscopy analyses of a suite of pristine lunar basalts four basalt slabs two low ti two high ti and paired thin sections were analyzed thin sections were analyzed for mineralogy petrography while the slabs were used to prepare particulate separates of major mineral phases and bulk samples mineral separates and particulate bulk samples were crushed to controlled grain sizes and their reflectance spectra measured in the nasa relab at brown university the resulting data set provides an essential foundation for spectral mixing models offers valuable endmember constraints for space weathering analyses and represents critical new ground truth results for lunar science and exploration efforts

space weathering from mercury to the asteroid belt the variety of evidence bearing on the nature of space weathering is reviewed the effects of space weathering include spectral darkening reddening and subdued absorption bands and the distinctive magnetic electron spin resonance caused by single domain metallic iron particles ever since the apollo missions two paradigms have dominated the thinking of the planetary science community concerning space weathering 1 the optical effects are caused by impact vitrified glass in agglutinates and 2 the submicroscopic metallic iron results from the reduction of ferrous iron by the impact melting of minerals whose surfaces have been saturated with hydrogen from the solar wind however studies carried out since the apollo program showed that both of these paradigms are invalid a hypothesis first suggested by the author and his colleagues 26 years ago but not generally accepted at that time now appears to be essentially correct both the optical and magnetic effects are caused by metallic iron particles smaller than the wavelength in ubiquitous vapor deposited coatings on soil particle surfaces and inside agglutinates the vapor is generated by both solar wind sputtering and micrometeorite impact vaporization and injected preferentially downward into the porous regolith the iron is reduced by a physical process the selective loss of oxygen that occurs during deposition of the vapor and does not require heating melting or a reducing environment a mathematical theory that describes the optical effects of the submicroscopic iron quantitatively is derived and applied to the regoliths of the moon mercury and an s asteroid

mapping the feo and tio2 content of the lunar surface with multispectral imagery the derivation of quantitative elemental concentrations from multispectral imaging of the moon has long been a goal of lunar remote sensing concentration maps at the spatial resolutions available from the recent clementine mission would provide a revolutionary new tool for understanding the origin and evolution of the lunar crust lucey et al 1995 presented a method for extracting the concentration of fe from multispectral imaging of the moon this paper examines and quantifies important aspects ofthat technique left unexamined by lucey et al which had the potential to severely limit its utility these aspects include the effects of maturity grain size mineralogy shading due to topography and the presence of glass we also present a new algorithm for derivation of tio2 from multispectral imaging of both mare and highland units we find that both techniques are only weakly sensitive to maturity and that they have about 1 wt accuracy based on examination of the spectral properties and compositions of resolved lunar sampling stations presented by blewett et al 1997 we also discuss these findings in the context of two contrasting views of the effect of composition on lunar spectral properties presented by pieters and coworkers and hapke and coworkers we find the view of hapke and coworkers to be more consistent with our observations using a global mosaic of clementine multispectral data and these element derivation algorithms we find that the global modal abundance of feo is 4 5 wt 1 wt and the global modal abundance of tio2 is 0 45 wt 1 wt

sedimentation from turbulent jets and plumes theoretical models are developed for the sedimentation from the margins of a particle laden axisymmetric turbulent buoyant plume in a still environment and for an axisymmetric turbulent momentum jet the models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time for relatively coarse particles the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r1 3 for the buoyant plume the exponential decay constant j is proportional to the terminal fall velocity vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux m0 for the jet fallout jj vtmo 1 2 and to the third power of the initial buoyancy flux fo for the plume fallout jp vtfo 1 3 smaller particles are affected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights this is taken into account by introducing a reentrainment coefficient into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength in new experiments fallout occurs from the margins of particle laden fresh water buoyant jets and plumes in a tank of salty water and sedimentation is measured on the tank floor two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory for smaller particles and increasingly buoyant plumes and strong jets particle reentrainment is important the experimental data are fitted by the new reentrainment theory confirming that values of the reentrainment coefficient are approximately constant for a given flow a settling number is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity for 1 reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0 1 for jets and 0 4 for plumes irrespective of flow strength or particle size the plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for slightly less than 1 the general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor for volcanic eruptions the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small lapilli up to at least 1 m diameter blocks are efficiently reentrained into eruption columns the size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 m in weakly buoyant plumes up to over 1000 m in megaplumes

sedimentation from strong volcanic plumes 1 we develop descriptions of the key processes influencing tephra dispersal from strong volcanic plumes these are characterized by the development of a subvertical eruption column in the atmosphere that forms a spreading current at a level of neutral buoyancy we describe the propagation of the spreading current due to both gravity and wind advection using scaling arguments and a simplified geometry to model particle sedimentation in a windy as well as a wind free environment new parameterizations are used to describe the wind field below the spreading current particle aggregation and particle density variations we conducted a broad study to investigate the effects of these processes and made comparisons with field observations the greatest variations resulted from wind advection below the spreading current which shifts the plume corner mass accumulation and the position of transitions in fallout regimes downwind particle aggregation strongly depends on the initial grain size distribution and significantly affects deposit thinning depending on the aggregate size and density and on the relative amount of aggregating particles small aggregates can reproduce secondary maxima of mass accumulation when sedimenting in a wind field variation of particle density also affects the resulting thinning trend the model provides acceptable reproduction of observations of the propagation of the spreading current tested with the plinian phase of the 18 may 1980 eruption of mount st helens and of observed thinning of tephra fallout deposits tested with deposits from mount st helens 1980 askja d 1875 quizapu 1932 minoan eruption 3 6 ka b p novarupta a 1912 and hudson 1991

mobility of a large volume pyroclastic flow emplacement of the campanian ignimbrite italy abstract the trachytic campanian ignimbrite originally exposed over a 30 000 km 2 area around naples italy is the product of a highly energetic gas rich eruption the deposit lies in valleys and isolated watersheds and in its medial and distal extent to the south north and east of naples the campanian pyroclastic current encountered mountains exceeding 1000 meters anisotropy of magnetic susceptibility ams measurements indicate that the campanian pyroclastic current the transport system traveled radially outward from the phlegrean fields area but the ignimbrite forming flow the deposition system that developed from the base of the transport system moved downslope from mountainsides to valleys including slopes facing the eruption source and flowed down drainage systems from intermontane basins the campanian pyroclastic current flowed 35 km over the water of the bay of naples to deposit 43 m of ignimbrite on the south shore and also overtopped a 685 1000 m high ridge of the sorrento peninsula to deposit more on the other side the distribution of the ignimbrite and the measured flow directions suggest that the campanian pyroclastic current moved across the landscape as an expanded and therefore turbulent decompressing flow rather than as a high density nonturbulent sheet like current moving over mountains by momentum acquired by eruption column collapse strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80 by volume of the original magma

a comparison of mercurian reflectance and spectral quantities with those of the moon we have examined previously published earth based telescopic spectra of mercury with the goal of obtaining clues to the planet s surface composition the study has three parts first because of suggestions that mercury s crust may be anorthositic we compare near infrared spectra of mercury with spectra of lunar anorthosites the spectral slopes and lack of strong mineralogic absorption features make the mercurian spectra similar to spectra of lunar anorthosites second we compare albedo quantities for mercury and the moon computed from telescopically derived photometric parameters with the laboratory albedos of returned lunar samples the albedos calculated from the photometric parameters are much lower than those of lunar samples measured in the laboratory third the mercurian spectra are interpreted in terms of the current model for the reflectance of an iron bearing silicate regolith on an airless body recent steps toward quantifying this model permit estimates of the feo content and opaque mineral abundance of the surface to be made we find that the mercurian spectra least affected by telluric absorptions imply feo tio2 4

remote sensing studies of the dionysius region of the moon 1 the dionysius region is located near the western edge of mare tranquillitatis and is centered on dionysius crater which exhibits a well developed dark ray system proposed origins for these dark rays included impact melt deposits and dark primary ejecta the region also contains extensive deposits of cayley type light plains clementine multispectral images and a variety of spacecraft photography were utilized to investigate the composition and origin of geologic units in the dionysius region the portions of the dark rays for which spectral and chemical data were obtained are composed of mare debris contaminated with minor amounts of highland material both five point spectra and values of the optical maturity omat parameter indicate that the dark rays are dominated by mare basalts not glassy impact melts the high albedo rays associated with dionysius exhibit feo and tio2 values that are lower than those of the adjacent dark ray surfaces and omat values that indicate that bright ray surfaces are not fully mature the high albedo rays are bright largely because of the contrast in albedo between ray material containing highlands rich ejecta and the adjacent mare rich surfaces the mafic debris ejected by dionysius was derived from a dark iron rich unit exposed high on the inner wall of the crater this layer probably represents a mare deposit that was present at the surface of the preimpact target site with one possible exception there is no evidence for buried mare basalts associated with cayley plains in the region

alteration and ore distribution in the proterozoic mines series tenke fungurume cu co district democratic republic of congo two sediment hosted stratiform cu co deposits in the tenke fungurume district of the central african copperbelt were examined to evaluate the alteration history of the ore hosting mines series and its implications for ore distribution and processing core logging and petrography focused on lithology and timing relationships outlined a complex alteration sequence whose earliest features include formation of anhydrite nodules and laths followed by precipitation of dolomite later alteration episodes include at least two silica introductions accompanied by or alternating with two dolomite introductions into the existing gangue assemblages one introduction of cu co sulfides accompanied the last episode of dolomite alteration overprinting an earlier generation of ore whose gangue association was unidentifiable sulfides and some carbonates were subsequently modified by supergene oxidation transport and reprecipitation to 100 200 m depth present day ore distribution resulted from these successive processes ore is concentrated in two shale dominated units on either side of a cavernous silicified dolomite which is interpreted as the main conduit for the mineralizing fluids sulfide ores precipitated at the redox or sulfidation contacts between this dolomite and the shales later supergene fluids dissolved and moved some of the metals redepositing them as oxides and carbonates solubility differences between cu and co in supergene conditions caused them to precipitate separately thus modern ore distribution at tenke fungurume results both from original hypogene lithology and contact related precipitation and from supergene oxidation transport and cu co decoupling the supergene fluid flow also redistributed gangue minerals such as dolomite which has an economically important influence on the processing costs of supergene ores

the nature and origin of rims on lunar soil grains abstract space weathering processes that operate in the lunar regolith modify the surfaces of lunar soil grains transmission electron microscope analysis of the lunar soil grains from the fine size fraction of several lunar soils show that most grains are surrounded by thin 60 200 nm thick rims the microstructure and chemical compositions of the rims can be used to classify rims into four broad categories amorphous inclusion rich multiple and vesicular amorphous rims are noncrystalline generally lack crystalline inclusions show evidence for preferential sputtering of cations and are produced largely by solar wind irradiation damage inclusion rich rims contain abundant nanometer sized grains of fe metal as randomly dispersed inclusions or as distinct layers embedded in an amorphous silica rich matrix inclusion rich rims are compositionally distinct from their host grains and typically contain accumulations of elements that are not indigenous to the host inclusion rich rims are formed largely by the deposition of impact generated vapors with a contribution from the deposition of sputtered ions a continuum in the chemical and microstructural properties exists between typical amorphous rims and typical inclusion rich rims multiple rims consist of a distinct radiation damaged layer up to 50 nm thick that is overlain by vapor deposited material of comparable thickness vesicular rims are compositionally similar to their hosts and are characterized by an abundance of small the formation of rims on lunar soils is complex and involves several processes whose effects may be superimposed from this study it is shown that one process does not dominate and that the relative importance of vapor deposition is comparable to radiation damage in the formation of rims on lunar silicate grains the presence of rims on lunar soil grains particularly those with nanometer sized fe metal inclusions may have a major influence on the optical and magnetic properties of lunar soils

neoproterozoic allochthonous salt tectonics during the lufilian orogeny in the katangan copperbelt central africa the neoproterozoic katangan rocks in the copperbelt of shaba democratic republic of the congo and zambia contain the world s largest concentration of stratiform copper cobalt ores we propose that during the lufilian orogeny the katangan basin was radically transformed first by extrusion of allochthonous evaporites then by orogenic shortening salt tectonics plausibly explains spectacular breccias underlying 25 000 km2 and containing gigaclasts up to 10 km long the brecciated shaban roan supergroup the lowermost katangan unit forms regional detachments and diapirs the former existence of roan evaporites is indicated by sabkha facies crystals and pseudomorphs of gypsum and anhydrite stratigraphic gaps underlain by collapse breccias chloride inclusions in ores hydrothermally propylitized ore hosts and saline springs salt tectonics began during deposition of the roan supergroup between 1050 and 950 ma in mid roan time small walls and extrusions of evaporite gigabreccia began to be emplaced in early kundelungu time 940 850 ma evaporitic diapirs enlarged lufilian deformation began between 850 and 650 ma by laterally squeezing diapirs to form salt welds then a large sheet of commingled roan evaporite and carbonate dominated sediments and ores was extruded northward by 65 km extrusion was fast enough to blanket a uniform preorogenic footwall unit without overriding any synorogenic deposits continued shortening then emplaced large thrust sheets lubricated by the preexisting salt sediment extrusive sheet restoration suggests that the lufilian foreland in this area shortened from 193 km to 85 km the depositional northern edge of the evaporite basin controlled the shape of the outer lufilian arc we infer three original salt provinces the large scale salt tectonics inferred here has implications for ore genesis and may yet be recognized in other precambrian basins

quantifying volcanic ash dispersal and impact of the campanian ignimbrite super eruption 1 we apply a novel computational approach to assess for the first time volcanic ash dispersal during the campanian ignimbrite italy super eruption providing insights into eruption dynamics and the impact of this gigantic event the method uses a 3d time dependent computational ash dispersion model a set of wind fields and more than 100 thickness measurements of the ci tephra deposit results reveal that the ci eruption dispersed 250 300 km 3 of ash over 3 7 million km 2 the injection of such a large quantity of ash and volatiles into the atmosphere would have caused a volcanic winter during the heinrich event 4 the coldest and driest climatic episode of the last glacial period fluorine bearing leachate from the volcanic ash and acid rain would have further affected food sources and severely impacted late middle early upper paleolithic groups in southern and eastern europe citation costa a a folch g macedonio b giaccio r isaia and v c smith 2012 quantifying volcanic ash dispersal and impact of the campanian ignimbrite super eruption geophys res lett 39 l10310 doi 10 1029 2012gl051605

the compositional and physical properties of localized lunar pyroclastic deposits abstract lunar localized pyroclastic deposits are low albedo deposits with areas 2 these deposits were difficult to study before the turn of the millennium because of the lack of available high spatial resolution data with the launch of the lunar reconnaissance orbiter kaguya and chaandrayan 1 new sets of diverse high spatial resolution data are now available using several of these data sets we conducted a study of 34 localized pyroclastic deposits globally for each localized pyroclastic deposit we examined topography to estimate pyroclastic volume and juvenile proportions s band radar backscatter thermal infrared derived measures of surficial rock abundance and regolith density and mineral abundances our goals are to 1 quantitatively characterize the physical and mineralogical properties of each localized pyroclastic deposit 2 investigate the physical and mineralogical variations among localized pyroclastic deposits 3 compare these properties of localized 2 to regional pyroclastic deposits 2500 km 2 and 4 provide useful parameters for future volcanological modeling from this study we find that 1 localized pyroclastic deposits exhibit low relief structures 2 the surface rock abundance and circular polarization ratio of localized pyroclastic deposits display a wide range of values 0 2 0 5 and 0 3 0 6 respectively 3 the glass abundance of localized pyroclastic deposits vary between 0 and 80 wt 4 there are four types of localized pyroclastic deposits based upon the surface rock abundance and glass abundance parameters 5 pyroclastic deposits within the same floor fractured crater tend to have similar properties and 6 localized pyroclastic deposits are diverse with respect to regional pyroclastic deposits but a subset of localized pyroclastic deposits have similar physical and mineralogical properties to regional pyroclastic deposits

the influence of conduit processes on changes in style of basaltic plinian eruptions tarawera 1886 and etna 122 bc basaltic volcanism is most typically thought to produce effusion of lava with the most explosive manifestations ranging from mild strombolian activity to more energetic fire fountain eruptions however some basaltic eruptions are now recognized as extremely violent i e generating widespread phreatomagmatic subplinian and plinian fall deposits we focus here on the influence of conduit processes especially partial open system degassing in triggering abrupt changes in style and intensity that occurred during two examples of basaltic plinian volcanism we use the 1886 eruption of tarawera new zealand the youngest known basaltic plinian eruption and the only one for which there are detailed written eyewitness accounts and the well documented 122 bc eruption of mount etna italy and present new grain size and vesicularity data from the proximal deposits these data show that even during extremely powerful basaltic eruptions conduit processes play a critical role in modifying the form of the eruptions even with very high discharge and presumably ascent rates partial open system behaviour of basaltic melts becomes a critical factor that leads to development of domains of largely stagnant and outgassed melt that restricts the effective radius of the conduit the exact path taken in the waning stages of the eruptions varied in response to factors which included conduit geometry efficiency and extent of outgassing and availability of ground water but a relatively abrupt cessation to sustained high intensity discharge was an inevitable consequence of the degassing processes

the physical volcanology of the 1600 eruption of huaynaputina southern peru volcan huaynaputina is a group of four vents located at 16 36 s 70 51 w in southern peru that produced one of the largest eruptions of historical times when 11 km3 of magma was erupted during the period 19 february to 6 march 1600 the main eruptive vents are located at 4200 m within an erosion modified amphitheater of a significantly older stratovolcano the eruption proceeded in three stages stage i was an 20 h sustained plinian eruption on 19 20 february that produced an extensive dacite pumice fall deposit magma volume 2 6 km3 throughout medial distal and distal parts of the dispersal area a fine grained plinian ashfall unit overlies the pumice fall deposit this very widespread ash magma volume 6 2 km3 has been recognized in antarctic ice cores a short period of quiescence allowed local erosion of the uppermost stage i deposits and was followed by renewed but intermittent explosive activity between 22 and 26 february stage ii this activity resulted in intercalated pyroclastic flow and pumice fall deposits 1 km3 the flow deposits are valley confined whereas associated co ignimbrite ash fall is found overlying the plinian ash deposit following another period of quiescence vulcanian type explosions of stage iii commenced on 28 february and produced crudely bedded ash lapilli and bombs of dense dacite 1 km3 activity ceased on 6 march compositions erupted are predominantly high k dacites with a phenocryst assemblage of plagioclase hornblende biotite fe ti oxides apatite major elements are broadly similar in all three stages but there are a few important differences stage i pumice has less evolved glass compositions 73 sio2 lower crystal contents 17 20 lower density 1 0 1 3 g cm3 and phase equilibria suggest higher temperature and volatile contents stage ii and stage iii juvenile clasts have more evolved glass 76 sio2 compositions higher crystal contents 25 35 higher densities up to 2 2 g cm3 and lower temperature and volatile contents all juvenile clasts show mineralogical evidence for thermal disequilibrium inflections on a plot of log thickness vs area1 2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system the ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper level winds whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower altitude wind field thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24 32 km and a total column height of 34 46 km the estimated mass discharge rate for the 20 h long stage i eruption is 2 4 108 kg s and the volumetric discharge rate is 3 6 105 m3 s the pumice fall deposit has a dispersal index hildreth and drake 1992 of 4 4 and its index of fragmentation is at least 89 reflecting the dominant volume of fines produced of the 11 km3 total volume of dacite magma erupted in 1600 approximately 85 was evacuated during stage 1 the three main vents range in size from 70 to 400 m alignment of these vents and a late stage dyke parallel to the nnw sse trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre existing weaknesses during stage i this fissure evolved into a large flared vent vent 2 with a diameter of approximately 400 m this vent was active throughout stage ii at the end of which a dome was emplaced within it during stage iii this dome was eviscerated forming the youngest vent in the group vent 3 a minor extra amphitheater vent was produced during the final event of the eruptive sequence recharge may have induced magma to rise away from a deep zone of magma generation and storage subsequently vesiculation in the rising magma batch possibly enhanced by interaction with an ancient hydrothermal system triggered and fueled the sustained plinian eruption of stage i a lower volatile content in the stage ii and stage iii magma led to transitional column behavior and pyroclastic flow generation in stage ii continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage iii no caldera collapse occurred because no shallow magma chamber developed beneath this volcano

the restless resurgent campi flegrei nested caldera italy constraints on its evolution and configuration abstract we have collected new stratigraphic data in the campi flegrei caldera through surface geological mapping and study of cores drilled to various depths we integrate these data with geological geochronological and geophysical constraints available in the literature to reconstruct the volcanic and deformational history of the caldera the caldera is a resurgent nested structure resulting from two main collapses related to the campanian ignimbrite 37 ka and the neapolitan yellow tuff 12 ka eruptions while the structure is affected by a broad subsidence ongoing local resurgence and unrest occur inside the younger nested caldera structure although most of the caldera floor consists of multiple structural blocks due to long term deformations magmas were able to erupt only in one sector of the resurgent part the caldera has shown signs of unrest during the past 25 years with short term deformations that have generated a net uplift of 3 5 m the short term deformations are related to the neapolitan yellow tuff caldera resurgence and interpreted as the net result of brittle and ductile components the long term deformations likely represent the sum of the permanent brittle component of each short term deformational event

continental crustals regimes as factors in the formation of sedimentary ore deposits although the formation of metalliferous mineral deposits by processes active in sedimentary basins depends ultimately on physical and chemical conditions at the site of deposition the style scale and timing of mineralization are influenced by tectonic and magmatic events which relate to the evolution of the crustal environment as a whole during the archaean era when geothermal gradients were high and igneous activity widespread sedimentary processes played little part in mineralization the development of cratonic crustal massifs at the beginning of the proterozoic era was followed by an increase in the variety and size of deposits formed in sedimentary basins from this time onward the cratonic cover formations have provided important sites for the deposition of metals these metals entered the basins from a variety of sources some being derived from weathering and erosion of the basement and others being transferred to surface levels by magmas and volatiles originating in the mantle or lower crust it is suggested that the timing of mineralization is related to that of the major magmatic and tectonic happenings determined by the global tectonic regime

complex proximal deposition during the plinian eruptions of 1912 at novarupta alaska proximal 3 km deposits from episodes ii and iii of the 60 h long novarupta 1912 eruption exhibit a very complex stratigraphy the result of at least four transport regimes and diverse depositional mechanisms they contrast with the relatively simple stratigraphy and inferred emplacement mechanisms for the previously documented better known medial distal fall deposits and the valley of ten thousand smokes ignimbrite the proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta and numerous thin complex deposits of pyroclastic density currents pdcs with no regional analogs the locally dispersed component of the fall deposits forms sector confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100 300 m cf several kilometers for the medial distal fall deposits this locally dispersed fall material and many of the associated pdc deposits is rich in andesitic and banded pumices and richer in shallow derived wall rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition there are no marked contrasts in grain size in the near vent deposits however between locally and widely dispersed beds and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial distal samples associated pdc deposits form a spectrum of facies from fines poor avalanched beds through thin bedded landscape mantling beds to channelized lobes of pumice block rich ignimbrite the origins of the novarupta near vent deposits are considered within a spectrum of four transport regimes 1 sustained buoyant plume 2 fountaining with co current flow 3 fountaining with counter current flow and 4 direct lateral ejection the novarupta deposits suggest a model where buoyant stable regime 1 plumes characterized most of episodes ii and iii but were accompanied by transient and variable partitioning of clasts into the other three regimes only one short period of vent blockage and cessation of the plinian plume occurred separating episodes ii and iii which was followed by a single pdc interpreted as an overpressured blast involving direct lateral ejection in contrast regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume which were being strongly affected by short lived instabilities these instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent of the parameters that control explosive eruptive behavior only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products

erosional characteristics and behavior of large pyroclastic density currents factors influencing the erosive behavior of large pyroclastic density currents pdcs both mainly massive and thinly stratified are not well understood to investigate the parameters influencing the erosive behavior of pdcs produced during the flowing phase of large caldera forming plinian campanian ignimbrite and phreatoplinian neapolitan yellow tuff eruptions we use scoured fall deposits at the base of or interstratified with pdc deposits from the campanian region of italy at several localities we calculated the depth of pdc erosion by comparing the measured thickness of eroded remnants to reconstructed thickness at each site estimated by isopach mapping and recorded the 1 distance from vent 2 elevation of the locality and 3 paleoslopes furthermore we have considered how these factors can be influenced by outcrop exposure depth of erosion correlates with distance from the vent in low relief landscape while across very rugged topography the only related parameter is elevation the different erosive patterns appear to show how pyroclastic currents interact with the topography in the surrounding terrain when a pdc crosses relatively flat surfaces it decelerates away from the vent decreasing its erosive capacity but when moving through steep terrain a pdc accelerates down the valley increasing its erosive capacity

complex proximal sedimentation from plinian plumes the example of tarawera 1886 the 1886 eruption of tarawera new zealand was unusual for a plinian eruption because it involved entirely basaltic magma originated in a 17 km long fissure and produced extremely overthickened proximal deposits with a complex geometry this study focuses on an 8 km long segment cutting across mount tarawera where over 50 point source vents were active during the 5 5 h eruption a detailed characterization of the proximal deposits is developed and used to interpret the range of styles and intensities of the vents including changes with time we identify the four vents that contributed most heavily to the plinian fall and evaluate the extent to which current volcanic plume models are compatible with the depositional patterns at tarawera three proximal units are mapped that have phreatomagmatic magmatic and phreatomagmatic characteristics respectively within the magmatic proximal unit beds of like character are grouped into packages and delineated on scaled cross sections package dispersal is quantified by measuring the linear thickness half distance t1 2 in the planes of the fissure walls most packages have localized dispersals low t1 2 indicating that strombolian style activity dominated most vents the more widely dispersed packages high t1 2 reflect contributions from additional transport regimes that were more vigorous but still contributed considerable material to the proximal region we conclude that the geometry of the observed proximal deposits requires three modes of fall transport 1 fallout from the upper portions of the plinian plumes produced principally by vents in four craters 2 sedimentation from the margins of the lower portions of the plinian plumes including the jets and possibly the lower convective regions and 3 ejection by weak strombolian style explosions from vents that did not contribute significant volumes of particles to the high plume we suggest that the curvature of the velocity profile across the jet region of each plume 1 4 km height was important and that the lower velocity at the margins allowed proximal deposition of a large volume of material with a wide grain size range

a metamorphosed proterozoic carbonaceous shale hosted co ni cu deposit at kalumbila kabompo dome the copperbelt ore shale in northwestern zambia the kalumbila co ni cu prospect 12 15 s 25 20 e is located 300 km west of the zambian copperbelt on the southeastern flank of the kabompo dome in the middle unit of the lufilian arc kalumbila contains a drill indicated resource of 3 04 mt at 0 67 wt percent ni 0 2 wt percent co and 0 11 wt percent cu using a 0 08 wt percent co and 0 3 wt percent ni cutoff with an average thickness of 12 75 m ni co cu mineralization occurs as strata bound disseminated to finely laminated locally semimassive to locally discordant chalcopyrite pyrrhotite violarite and siegenite within a finely laminated highly carbonaceous and pyritic metapelite 300 500 m thick surface rock exposure in this low relief terrain of woodland and seasonal swamp is 1 percent and geophysics magnetic and electromagnetic surveys is an essential exploration tool detrital zircons in the kalumbila metasediments shrimp u pb ages of 2004 1884 ma are derived from the underlying paleoproterozoic granitic gneiss such as that exposed in the adjacent kabompo basement dome the metasedimentary sequence grades upward from siliciclastic rocks mica schists to the carbonaceous metapelite which hosts the co ni cu these units are overlain by dolomitic and mg rich schists possible meta evaporites which are in turn succeeded by calc silicate rocks and a deformed diamictite this sequence is broadly similar to the stratigraphy in the zambian copperbelt and suggests that the kalumbila carbonaceous metapelite may be the chronostratigraphic equivalent of the katangan roan ore shale a significant exploration target kalumbila occurs within a moderately 40 north northwest dipping band of wamikumbi formation carbonaceous metapelite on the flank of a recumbent south southeast verging f1 or f2 back fold during the lufilian orogeny peak metamorphic conditions at f1 were of amphibolite facies grade 650 c 13 kbars both monazites and rutiles within the kalumbila metasediments have metamorphic characteristics and late orogenic lufilian shrimp u pb ages 548 6 7 6 ma and 531 21 ma respectively sulfides in the metasediments display metamorphic textures and therefore predate the peak of lufilian tectonism and thermal metamorphism a suite of spatially associated apparently late lufilian gabbros intrude the metasedimentary rocks these gabbros are unmineralized but the thermal effects of their emplacement may have modified ore distribution a genetic model in which synsedimentary pyrite in a carbonaceous pelite was partially replaced by syndiagenetic or metamorphic co ni cu bearing fluids of unknown origin is favored

cobalt in the zambian copperbelt abstract a review of available information concerning the regional distribution of cobalt in the zambian copperbelt is presented which emphasises the limitation of this metal to a narrow possibly tectonically controlled zone on the southwestern flanks of the kafue anticline the stratigraphic location of cobalt bearing mineralisation is also examined though the majority of the known deposits are within the ore shale horizon cobalt has been found from the basement lower roan unconformity to the top of the lower roan and in one case high in upper roan dolomites the behaviour of cobalt relative to copper can best be summarised by considering arenite hosted deposits separately from those in the ore shale it is shown that though the latter appear to contain cobalt within the confines of the copper orebody a marked vertical antipathetic relationship exists the arenite hosted deposits on the other hand tend to display a marked lateral differentiation of the metals in that the cobalt has apparently migrated or been precipitated at or beyond the fringes of the copper orebody compositional variations within the cobalt bearing phases are described which are attributed to corresponding variations in host rock lithology primary porosity the availability of sedimentary or diagenetic sulphate and in particular to the activity of reduced sulphur a genetic model is thus presented which envisages influx of hot hydrothermal solutions into sediments during early diagenesis cooling of these solutions by mixing with connate waters together with the differing solubilities of the various metals results in the precipitation of the metals in a zonal fashion variable bacterial and possibly inorganic reduction of interstitial sulphate and consumption of the reduced sulphur controls not only the quantity but also the composition of the sulphides precipitated in the past the original source of the metals now contained in the orebodies has been dismissed as somewhat irrelevant but generally they were seen as having been derived from erosion of the basement granites or contained mineralisation a case is made that such a source is not compatible with the presence of high cobalt concentrations and that derivation from a differentiated basic magma is a more satisfactory explanation of the facts crustal rifting is the required mechanism for high level penetration of metal enriched hydrothermal fluids

surface modification of olivine by h and he bombardment laboratory simulations of solar wind irradiation of olivine a constituent of ordinary chondritic meteorites and s type asteroids show a dramatic chemical reduction of surface iron and to a lesser extent silicon earth s atmosphere oxidizes any reduced iron instantaneously whereas irradiated material on the surface of an asteroid would remain reduced changes in optical reflectance due to metallization of the surface provide a possible explanation for differences in the near infrared spectra of ordinary chondrite meteorites and their likely parent bodies the s iv asteroids

atomic scale simulation of space weathering in olivine and orthopyroxene classical molecular dynamics was used to study the annealing of anion and cation frenkel defects in olivine and orthopyroxene minerals while it was found that for both minerals reorganization of the si o bonds often accompanied by large si displacements occurs to maintain the fourfold coordination of the sio4 tetrahedra important differences are observed in their annealing behavior specifically cation defects are substantially more mobile in olivine than in orthopyroxene leading to rapid annihilation of cation frenkel defects and formation of extended defects in olivine by contrast the diffusion rate of anion defects in orthopyroxene is much higher than that in olivine and also exhibits large anisotropy consequently it was found that diffusion in orthopyroxene occurs without significant annihilation of anion frenkel defects or trapping of anion interstitials or vacancies into clusters the results obtained here are discussed in the context of space weathering of olivine and orthopyroxene specifically two important observations are made which may explain previous experimental results first ion irradiation experiments that show reduced tolerance for radiation damage in orthopyroxene may be explained by the rapid one dimensional anion mobility which prevents healing of the lattice second laser heating experiments which show that orthopyroxene has enhanced tolerance to reduction and the evolution of nanophase fe inclusions could be due to the observed rapid anion diffusion in orthopyroxene which might allow the bulk to act as a reservoir for the surface

optical effects of space weathering the role of the finest fraction the optical properties of lunar softs are different than those of rocks from which they are derived as a consequence of lunar space weathering soils are darker and exhibit a distinctive red sloped continuum and weaker mineral absorption bands the accumulation of dark glass welded aggregates agglutinates has been thought to account for these optical effects of space weathering on lunar soils spectroscopic analyses of agglutinate separates and size fractions for a suite of lunar softs presented here indicate that the agglutinate paradigm is insufficient to fully account for lunar optical alteration it is the finest fraction of lunar soils 25 gm which constitute 25 wt that dominates the optical properties of the bulk soil unlike size fractions of most silicates for which the finest fraction is the brightest the lunar soil size fractions all have comparable albedos in the short wavelength visible in the near infrared however it is the finest fraction that exhibits the steep red continuum and weak absorption bands the properties of the finest fraction cannot be duplicated by preparing a fine fraction by grinding larger agglutinate rich soil particles these results suggest space weathering on airless bodies is dominated by surface correlated processes perhaps associated with the development of fine grained fe o on or near the surface of grains and the accumulation of the larger agglutinates is not necessarily required to account for lunar optical alteration

an analysis of the mariner 10 color ratio map of mercury abstract the results of a geological analysis of the mariner 10 orange uv color ratio man of mercury b hapke c christman b rava and j mosher proc lunar planet sci conf 11th 1980 pp 817 822 are given certain errors that occured in reproducing the published version of the 1980 map are pointed out the relationships between color and terrain are distinctly nonlunar there is no correlation between color boundaries and the smooth plains on mercury in contrast to the strong correlation between color and maria highlands contacts on the moon there are no large exposures of low albedo blue material that could be considered to be mercurian analogs of high fe ti lunar maria basalts on any part of mercury imaged by mariner 10 three lines of evidence imply that the crust is low in fe2 and ti4 rays and ejecta blankets are bluer than most areas on mercury the fe2 band in mercury s reflectance spectrum is very weak or nonexistent and the albedo contrasts are smaller than those on the moon there is no evidence in the spectral or albedo data that a lunar type of second wave of melting ever occured on mercury rather the observations are most consistent with the hypothesis that the smooth plains are extrusive landforms derived from local material possibly mobilized by the caloris event in several places correlations between color and topography can be explained if older redder higher fe materials underlie younger bluer lower fe surfaces there is some evidence of late fe rich pyroclastic like activity

reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits 2 application to the pinatubo 1991 eruption the granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic plinian phase of the 1991 eruption at pinatubo the tephra fall deposits are composed of two units layer c1 which corresponds to the first half of the climactic phase and layer c2 the second half the granulometric estimates of the expansion rate of the umbrella cloud for layers c1 and c2 are 7 1010 and 3 1010 m3 s respectively which agree with the observations of satellite images these estimates indicate that the magma discharge rate decreased from 9 108 to 3 108 kg s during the eruption the grain size distribution at the top of the eruption column is characterized by depletion of coarse clasts suggesting a distinct deceleration in the gas thrust region down to several tens of meters per second particularly during the second half of the climactic phase the total amount of layers c1 and c2 is estimated to be 3 1012 kg and the very fine particles which did not deposit in the accessible on land area may occupy up to 60 of the total ejecta the effective duration of the eruption estimated by the present methods a few hours or less is substantially shorter than a previous estimate from the real time observation based on infrasonic data 10 hours this discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous plinian activity and generation of pyroclastic flows it is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time averaged features predicted by the present methods

sulfide bearing veinlets throughout the stratiform mineralization of the central african copperbelt temporal and genetic implications the central african copperbelt of zambia and democratic republic of congo along with its broad time equivalents in botswana and namibia the kalahari copperbelt is the world s largest sediment hosted stratiform copper province the economically dominant stratiform copper cobalt or silver mineralization accepted by many as at least partly early diagenetic in origin is ubiquitously accompanied by sulfide bearing quartz carbonate veins and veinlets traditionally this veining is considered to be the product of either metamorphic lateral secretion or discrete late diagenetic to synorogenic mineralization events this view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies a situation that is taken to imply contemporaneity of the two styles consequently during the mineralization the siltstone sandstone or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis this conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes both the disseminated and veinlet mineralization styles lack ductile deformation features in many congolese deposits but are variably metamorphosed and deformed in some deposits in zambia botswana and namibia in conjunction these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism 765 715 ma most probably spanning peak damara lufilian metamorphism and ductile deformation 530 ma and continuing in places until at least 500 ma thus basin inversion contractional tectonism and associated uplift and exhumation and not the earlier extension and rifting seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion

contrasting grain size and componentry in complex proximal deposits of the 1886 tarawera basaltic plinian eruption the 1886 plinian eruption of tarawera new zealand is a unique basaltic fissure fed eruption with exceptionally well preserved fall deposits to within 200 meters of the source vents these proximal deposits form a series of spatter cinder half cones along the northeastern 8 km long segment of the 1886 fissure here we examine these deposits using grain size and clast componentry techniques we contrast the products of the phreatomagmatic phases i and iii and plinian phase ii stages of the eruption and examine deposit variability as a function of contrasting eruptive intensity within the climactic phase ii of the eruption the opening phreatomagmatic phase i of the eruption involved gas rich magma interacting with water and fragmenting at least 300 meters below the surface the deposits of the climactic phase that followed have relatively uniform grain size but marked contrasts in the relative abundance of juvenile and wall rock lithic clasts deposits linked to vents associated with the high plinian plume are more uniform than those characterized by a weaker cone forming eruption style during the third and closing phase of the eruption magma withdrawal accompanied the onset of decoupling of the exsolved gas phase leading to fragmentation at increasingly greater depths and significant wall rock collapse into the erupting vents variability in eruptive style during phase ii along the fissure appears to be a function of shallow seated controls in particular the variable extent of incorporation of lithic wall rock into the erupting jet as a consequence of vent wall collapse widely dispersed beds centralized around plinian sources along the fissure have very low lithic content cone forming beds at other craters that contain very high lithic contents this incorporation led to a significant reduction of the velocity and stability of the jet at these latter steep walled craters and induced episodicity in the form of vent clearing explosions the result is a large reduction of the physical and thermal ability of these vents to contribute to a stable high eruptive plume instead large volumes of ejecta were sedimented prematurely from shallow heights at rates an order of magnitude greater than for historical strombolian hawaiian and subplinian eruptions this study illustrates that sustained powerful plinian eruptions can be accompanied by heterogeneities and instabilities of the eruptive jet at tarawera the record of complex proximal transport and deposition processes in the eruptive jet cannot be inferred from the eruption products at distances greater than 400 m from the eruptive fissure we suggest that study of ultraproximal deposits as seen at mt tarawera provides the only opportunity to document the complex dynamic behavior of the jet region of plinian eruptions

space weathering on eros constraints from albedo and spectral measurements of psyche crater we present combined multi spectral imager msi 0 95 m and near infrared spectrometer nis 0 8 2 4 m observations of psyche crater on s type asteroid 433 eros obtained by the near earth asteroid rendezvous near shoemaker spacecraft at 5 3 km in diameter psyche is one of the largest craters on eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material at spatial scales of 620 m spectrum psyche crater wall materials exhibit albedo contrasts of 32 40 at 0 946 m associated spectral variations occur at a much lower level of 4 8 2 4 we report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends grain size differences olivine pyroxene and troilite variations and optical surface maturation we find that the albedo contrasts in psyche crater are not consistent with a cause due solely to variations in grain size olivine pyroxene or lunar like optical maturation a grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed olivine and pyroxene variations would produce strong band correlated variations that are not observed a simple lunar like optical maturation effect would produce strong reddening that is not observed the contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite a dark spectrally neutral component simulating optical effects of shock and lunar like optical maturation these results suggest that space weathering processes may affect the spectral properties of eros materials causing surface exposures to differ optically from subsurface bedrock however there are significant spectral differences between eros proposed analog meteorites ordinary chondrites and or primitive achondrites and eros freshest exposures of subsurface bright materials after accounting for all differences in the measurement units of our reflectance comparisons we have found that the bright materials on eros have reflectance values at 0 946 m consistent with meteorites but spectral continua that are much redder than meteorites from 1 5 to 2 4 m most importantly we calculate that average eros surface materials are 30 40 darker than meteorites

the effect of total grain size distribution on the dynamics of turbulent volcanic plumes abstract the impact of explosive volcanic plumes on climate and on air traffic strongly depends on the concentration and grain size distribution gsd of pyroclastic fragments injected into the atmosphere accurate and robust modelling of the evolution of gsd during pyroclast transport from the vent to the ash cloud is therefore crucial for the assessment of major volcanic hazards analysis of field deposits from various recent plinian eruptions shows that their total gsd is well described by a power law as expected from the physics of magma fragmentation with an exponent d ranging from 3 0 to 3 9 by incorporating these measured gsd into the initial conditions of a steady state 1d model of explosive eruption columns we show that they have a first order impact on the dynamical behaviour of explosive eruption columns starting from an initial value of d the model tracks the evolution of gsd in the column and calculates the dynamical consequences of particle sedimentation the maximum height reached by the column one of the first order results relevant to aircraft safety changes by 30 for mass fluxes of 10 7 kg s 1 or larger and by 45 85 for mass fluxes between 105 and 10 7 kg s 1 depending on exponent d we compare our predictions to a specially assembled set of geologic field data and remote sensing observations from 10 plinian eruptions for which maximum column height and mass flux are known independently the incorporation of realistic power law gsd in the model greatly improves the predictions which opens new perspectives for estimation of ash load and gsd in volcanic clouds from near real time measurements available from satellite payloads our results also contribute to the improvement of volcanic source term characterization that is required input for meteorological dispersion models

particle segregation of an eruption plume as revealed by a comprehensive analysis of tephra dispersal theory and application abstract a new way to deduce particle segregation in an uprising eruption column is developed this method subdivides an eruption column into horizontal slices and particle dispersal from each column slice is calculated using an advection diffusion model named tephra2 a grid search is employed to obtain the particle segregation from each column slice and other unknown parameters vent position diffusion coefficient and particle density the initial conditions for the calculation were mass loading data collected on the ground wind profile and minimum empirical parameters this method is applied to the 1986b eruption of izu oshima volcano japan which was a sub plinian eruption with a vertically uprising column of 10 km high a previous study of this eruption employed a gravity current model which assumes that most of the tephra segregation took place at the column umbrella transition and at the base of the umbrella cloud the gravity current model was used to calculate the base height of the umbrella cloud from the mass loading distribution on the ground and the calculated base height 7 9 km is consistent with the observed column height however the present study shows that most of the particles segregated from the lower parts of the column 1 3 km high and no significant fallout from the upper part of the column was observed even though the segregation pattern is different between the present and past studies the total mass and total grain size distribution are similar this coincidence is attributed to the use of the same decay function of mass loading in both models both assume the exponential decay of mass loading as a function of the area enclosed by the isopleth although the physical meaning of the decay rate is different in the two models this conclusion also implies that the shape of the decay function itself cannot be used to determine which model is appropriate in describing the source column however the applicability of the gravity current model should be justified when the decay rate is low and when an unreasonably high diffusion coefficient is obtained by inversion techniques that use advection diffusion models the new approach of this study also enables the calculation of the total mass and total grain size distribution without hand drawn isopleth maps and uses all of the point data with error evaluations for each point data the calculation results e g the erupted mass are shown with associated errors

high calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites our analyses of high quality spectra of several s type asteroids 17 thetis 847 agnia 808 merxia and members of the agnia and merxia families reveal that they include both low and highcalcium pyroxene with minor amounts of olivine 0 4 high calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies the silicate mineralogy inferred for thetis and the merxia and agnia family members requires that these asteroids experienced igneous differentiation producing broadly basaltic surface lithologies together with 4 vesta and its smaller vestoid family members and the main belt asteroid 1489 magnya these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies based on this analysis of a small subset of the near infrared asteroid spectra taken to date with spex at the nasa irtf we expect that the number of known differentiated asteroids will increase consistent with the large number of parent bodies inferred from studies of iron meteorites

a chemostratigraphic study of the campanian ignimbrite eruption campi flegrei italy insights on magma chamber withdrawal and deposit accumulation as revealed by compositionally zoned stratigraphic and facies framework abstract petrochemical analyses of juvenile samples from twenty stratigraphic sections of the campanian ignimbrite medial deposits located from 30 to 79 km from the vent are presented here sampling has accurately followed a well defined stratigraphic framework and the new component facies scheme the campanian ignimbrite succession is formed by a basal plinian pumice fall deposit overlain by a complex architecture of pyroclastic density current deposits emplaced from a single sustained pyroclastic density current through a mechanism of vertical and lateral accretion the deposit is broadly zoned from more evolved trachyte at its base to less evolved trachyte at its top and is similarly less evolved with increasing distance from the area of emission irregular chemical trends are locally observed and interpreted to represent only a limited patchy record of the entire vertical geochemical trend the petrochemical variation observed horizontally was ascribed to changes in the flow dynamics and interaction between the advancing flow and the underlying topography the results of this study were used to propose a unified volcanological petrological model for the campanian ignimbrite eruption taking into account the emplacement of both the proximal i e the breccia museo formation and medial deposits

space weathering on the moon patina on apollo 17 samples 75075 and 76015 we studied patinas on lunar rocks 75075 and 76015 from the apollo collection using a multi disciplinary approach including scanning electron microscopy sem energy dispersive x ray spectrometry eds transmission electron microscopy tem wavelength dispersive x ray wds mapping mossbauer spectroscopy spectral reflectance and microspectrophotometry based on sem petrography we have defined three textural types of patina glazed fragmental and classic cratered the presence of classic patina is diagnostic of lunar samples that have been exposed directly to the space weathering environment it is characterized by the presence of microcraters and glass pancakes and is the patina type studied by earlier workers classic patina is found on 76015 but not on 75075 glazed patina is found on both 76015 and 75075 whereas fragmental patina is found only on 75075 the glazed and fragmental patinas on 75075 were probably formed as a result of relatively large nearby impacts and although these two types of patina are not strictly the result of direct exposure to the space weathering environment they are important because they affect the optical properties of the rocks field emission gun sem fe sem of classic patina on 76015 shows evidence of possible solar wind sputtering erosion transmission electron microscope studies of 76015 reveal the presence of impact generated deposits and solar flare particle tracks which like microcraters and pancakes are diagnostic of direct exposure to space weathering processes the outermost surface of the 76015 patina consists of an amorphous rim very much like the rims found on individual lunar soil grains this amorphous patina rim probably formed by similar processes of impact generated vapor condensation and possible sputter deposition wavelength dispersive x ray element maps of polished thin sections of 75075 and 76015 indicate that patina compositions are poor indicators of the compositions and mineralogies of the rocks underlying them on average the reflectance spectra of patinas on both samples are slightly darker than those of their unweathered equivalents microreflectance measurements show that a thick patina can dramatically alter the optical properties of the rock on which it forms the backscatter mossbauer bams spectrum of a patina covered surface of 76015 is very similar to that of an unweathered surface indicating that the mossbauer signal is generated from beneath the patina because bams sees through surface space weathering effects to the underlying rock this technique has great potential for use in robotic missions to other planetary bodies

sedimentation of tephra by volcanic plumes i theory and its comparison with a study of the fogo a plinian deposit sao miguel azores sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the fogo a plinian deposit sao miguel azores the fogo a trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour abundance of syenite accessory lithic clasts and distribution the lower syenite poor part was dispersed to the south and was clearly influenced by wind the upper syenite rich part is coarsegrained and has a nearly symmetrical distribution around the vent elongation of isopachs to the east indicate a weak wind influence the grain size variations of lithic and crystal components in the upper coarse part were studied total accumulation and accumulation per unit area expressed in kg m2 show good fits to a gaussian function at distances greater than 7 km for grain diameters less than 2 cm these results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid the gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column the coefficient is therefore also a function of column height the column height deduced from these data is 21 km which is in broad agrrement with the column height of 27 km deduced from maximum clast dispersal using the method of carey and sparks 1986 the accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts these calculations constrain the wind speed during the later part of the fogo a eruption to be at most a few metres per second the study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns

clementine images of the lunar sample return stations refinement of feo and tio2 mapping techniques clementine uvvis images of the lunar sample return sites have been processed and used to produce refined calibrations for the iron and titanium determination algorithms of lucey et al 1995 1996 the high spatial resolution of the clementine data permits individual sampling stations to be resolved at the apollo 15 16 and 17 landing sites we find an excellent linear correlation between the spectral fe and ti parameters and the average feo and tio 2 contents of soils sampled at each site or station this correlation demonstrates that these techniques can confidently be applied to other areas of the moon the luna 24 site does not fit the ti relation found for other sites suggesting that either its sample is nonrepresentative or the reported landing coordinates are incorrect

multispectral images of mercury from the first messenger flyby analysis of global and regional color trends abstract we have analyzed messenger multispectral images of mercury in order to gain information on the nature of materials present on the planet s surface the quantitative analysis tools of lucey which are built on the hapke conceptual framework for the reflectance of a silicate regolith allow us to derive spectral parameter images that are sensitive to the abundance of ferrous iron in silicates the abundance of opaque phases and the state of maturity of the surface consistent with previous estimates we conclude that the silicate portion of mercury s surface is low in ferrous iron with a small range of variation the spectral changes that accompany regolith maturation on mercury are consistent with lunar style space weathering via the accumulation of nanophase iron the spectral character of mercury s major surface units high reflectance plains intermediate terrain and low reflectance material can be attributed to variations in the abundance of an opaque phase s a class of minor color unit the red spots occurs in at least three morphological varieties two of which are associated with deposits interpreted to be of pyroclastic origin the bright crater floor deposits a second type of minor unit have extremely shallow blue spectral slopes that are unrelated to the presence of immature material

impact induced thermal effects in the lunar and mercurian regoliths while the moon and mercury are similar objects in a geomorphologic sense they also possess important differences particularly in the context of small scale impact phenomena not only is the surface of mercury notably hotter than that of the moon but the impact flux is also more intense at mercury due to higher impact velocities and a greater spatial density of micrometeoroids by extrapolating the terrestrial micrometeoroid flux to the moon and mercury it is found that the impact rate at mercury is 5 5 times greater and the mean impact velocity more than 60 percent higher than at the moon a model of impact melting and vaporization applied to the lunar and mercurian environments indicates that almost 14 times more impact melt and over 20 times more vapor are generated per unit time in the mercurian regolith although the surface temperature plays a role in determining the melt and vapor volumes the difference in impact velocity appears to be markedly more important average craters formed by identical projectiles under the two velocity distributions on the two planets should be very similar in size because differences in the kinetic energy of the average impactors will be offset by the differences in gravitational acceleration as a result variations in regolith mixing should depend more on the actual impact rates than on disparities in excavated volumes each average impact on mercury however will produce more than twice as much impact melt as its lunar counterpart introducing more glass into the mercurian regolith although substantial quantities of impact vapor are produced on both planets mixing of the regolith should occur quickly enough in each case to spread the deposited vapor over large surface areas the resulting vapor coatings should be negligibly small for most purposes a much larger agglutinate population should exist on mercury and individual agglutinates should be similar to those of the lunar highlands reflectance spectra and visual albedos indicate that the mercurian crust possesses relatively small quantities of fe perhaps similar to those of apollo 16 light and dark matrix breccias laboratory studies have demonstrated that as the magnetic fractions which are correlated with agglutinate abundances of soils derived from these breccias increase the differences in 0 565 mm visible light reflectivity between the soils diminish should the regional variations in fe content of the mercurian crust be similar to those of these apollo 16 samples then the intense agglutination environment could account for the lack of optical contrast across the mercurian surface at the same time the very large fraction of impact glass in the regolith would imply that most pyroxenes have been fused and incorporated into these glasses seriously attenuating the fe2 crystal field absorption near 0 9 mm supporting this picture are the most recently published spectral observations of mercury which provide little or no evidence of such an absorption feature

the breccia museo formation campi flegrei southern italy geochronology chemostratigraphy and relationship with the campanian ignimbrite eruption the breccia museo is one of the most debated volcanic formations of the campi flegrei volcanic district the deposit made up of six distinctive stratigraphic units has been interpreted by some as the proximal facies of the major caldera forming campanian ignimbrite eruption and by others as the product of several more recent independent and localized events new geochemical and chemostratigraphical data and ar ar age determinations for several units of the breccia museo deposits 39 ka correlate well with the campanian ignimbrite forming eruption the chemical zoning of the breccia museo deposits is interpreted here to be a consequence of a three stage event that tapped a vertically zoned trachytic magma chamber

genesis of the ores of the zambian copperbelt abstract the theories for the origin of the ores of the copperbelt of zambia and zaire are reviewed following an account of the historical development of ideas from epigenetic through syngenetic to syngenetic diagenetic and ultimately to neohydrothermalist theories models for the genesis of these deposits are critically assessed in the light of the data currently available this examination is conducted with particular regard to the questions of the sources of copper and cobalt the timing of mineralizing events and the physicochemical conditions of metal sulphide precipitation although accepting that the origin of these ores remains controversial it is concluded that the most likely sources of both copper and cobalt are the rocks of the basement complex that metals were probably transported both in surface and ground waters and that the main mineralizing event took place during early diagenesis with sulphide precipitation occurring via bacterial reduction of seawater sulphate the precipitation of sulphides probably occurred at temperatures around 20 60 c and at near neutral ph with minor fluctuations in eh and variations in the solubilities of the metals involved causing mineral zonation the burial and compaction of these sediments probably resulted in the expulsion of metal rich fluids and formation of sulphides in the immediate footwall and hanging wall of the main ore horizon with continued burial resulting in the formation of sulphide bearing pods and veins of lateral secretion origin tectonic deformation and metamorphism in the lufilian orogeny although resulting in a set of cross cutting high temperature mineralized veins appears not to have resulted in significant remobilization of the sulphides