nucleation crystal growth and the thermal regime of cooling magmas crystallization at the margin of a quiet cooling magma has been studied numerically taking into account the kinetics of crystallization the variables are the latent heat value the growth and nucleation functions the initial magma temperature and the thermal contrast between magma and country rock we have investigated a wide range of values for these parameters corresponding to natural conditions we show that after a highly transient stage crystallization tends toward an equilibrium between heat production latent heat release and heat loss given the small diffusivity of country rocks latent heat release is the main factor controlling the temperature evolution in order to minimize the latent heat release crystallization occurs at a temperature where nucleation is small this can be close to either the liquidus or the solidus depending on the initial conditions the main process controlling crystallization is nucleation and not crystal growth nucleation occurs as a series of sharp pulses followed by longer periods of crystal growth the nucleation pulses give birth to thermal oscillations these oscillations can be sustained if the interior magma temperature is above the liquidus independently of the heat loss mechanism we show that the phenomenon occurs on the scale of a few centimeters which corresponds to the inch scale layering of many ultrabasic complexes the model allows us to calculate crystal sizes which are in good agreement with geological observations the crucial parameters which determine crystal size variations near the margins of igneous bodies are the initial thermal conditions as well as the nucleation and growth functions in the main cooling regime close to the liquidus significant size variations can be created by small thermal disturbances

igneous layering through oscillatory nucleation and crystal settling in well mixed magmas oscillatory or competitive nucleation about a binary or perhaps pseudo binary eutectic and ensuing crystal growth and settling is a commonly suggested means of producing layering in magmatic systems a quantitative model is presented of this outwardly relatively simple process of crystal nucleation growth and settling in an otherwise initially crystal free magma avrami style kinetics of crystallization in an always wellmixed body buried in conductive wall rock are coupled to a stokes like formulation of crystal settling in magma whose viscosity depends on temperature and crystallinity two dimensionless numbers se the settling number and av the avrami or kinetic number govern all the results av and se measure the relative importance of crystallization time and settling time respectively relative to the overall cooling time for any value of av which increases strongly with the maximum nucleation and growth rates and cooling time layering is possible only over a range or window of values of se both above and below this window a single layer crystalline below vitric above forms and within this window the number of layers increases systematically with increasing av and se grain size within any single layer generally coarsens upward because the characteristic settling and cooling times both depend on body thickness the lower limit of the settling window is also dependent on sheet thickness within the confines of this model and for nucleation and growth rates set by those observed in natural systems layering is unlikely in sheet like magmas thinner than about 100 m when the body is not always well mixed and crystallization is within inward propagating solidification fronts it is expected that this minimum body thickness will increase

the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

alteration mineralogy and stable isotope geochemistry of paleoproterozoic basement hosted unconformity type uranium deposits in the athabasca basin canada unconformity type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks mineralization may extend up to 400 m into the underlying basement rocks whereas sandstone hosted unconformity type deposits have been well studied deposits hosted primarily in the basement have not this study examines the deposits at rabbit lake dawn lake and mcarthur river in the athabasca basin of canada which are hosted by the metamorphic archean and early paleoproterozoic rocks forming the basement to younger late paleoproterozoic sandstones alteration is similar in the three deposits and is characterized by three distinct paragenetic stages 1 preore alteration involving illitization of plagioclase and amphibole followed by chloritization of biotite and illite which formed at ca 230 c 2 ore stage alteration characterized by uraninite and coarse grained illite which formed at ca 240 c 3 postore alteration comprising spherulitic dravite vein chlorite quartz calcite and fe cu co and pb sulfides which formed at ca 135 c fluid circulation associated with emplacement of later mackenzie dikes initiated partial recrystallization of uraninite a later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca 50 c stable isotope compositions of the alteration minerals in conjunction with their paragenesis indicate that oxidized basinal fluids d 43 to 21 18o 3 8 were derived primarily from evolved seawater and leached uranium from the overlying sandstones of the athabasca formation and transported it into the basement via infiltration along fracture zones associated with reverse faults graphitic units in the basement and preore alteration served as both physical fractured zones and chemical reductants traps for the uranium mineralization the basinal fluids were responsible for the preore illite chlorite synore uraninite illite and the early postore alteration events this differs from many other sandstone hosted deposits where both oxidized basinal and reduced basement derived fluids were responsible for uranium precipitation

measurement of u pb ages of uraninite and davidite by laser ablation hr icp ms laser ablation high resolution inductively coupled plasma mass spectrometry la hr icp ms is a rapid accurate and inexpensive technique for making in situ u pb isotopic measurements of uraninite and davidite the advantages of this method include 1 mineral separation and chemical digestion are not required 2 measurements on complex samples are feasible because significant isobaric interferences can be resolved and 3 accurate u pb and 207pb 206pb dates on 10 25 m spots can be obtained rapidly the la hr icp ms method is applied to u oxide minerals from four deposits and prospects from northern australia and the new dates are compared to previously published conventional thermal ionization mass spectroscopy tims dates and to known ages of geologically important events these comparisons permit us to assess elemental fractionation of u and pb for uraninite and davidite of the new method relative to zircon as well as its geochronological accuracy and precision u pb apparent ages measured previously agree well with our measurements for el sherena palette and mt isa additionally the upper intercept 207pb 206pb dates for adelaide river 701 190 ma 1 and palette 841 94 ma 1 uraninite measured here are similar to those previously obtained for palette 730 ma nabarlek 920 ma and koongarra 870 ma and the upper intercept date for el sherena uraninite 1573 160 ma is within error of that previously determined for ranger 1550 15 and 1472 40 ma such apparent age agreement for uraninite and similarly for davidite indicates that u and pb fractionations are within error of that for zircon whereas the inherent imprecision of our dates and their associated mswd values greater than 2 5 probably indicate that multiple resetting events affected our samples analytically these results demonstrate that the la hr icp ms technique provides excellent spatial resolution while also removing argide phosphide sulfide and halide interferences that can otherwise lead to erroneous data when using quadrupole icp ms geologically the individual 207pb 206pb and upper intercept u pb dates of uraninite from adelaide river and palette are ca 800 ma possibly reflecting recrystallization of uraninite during the break up of rodinia

magmatic graphite inclusions in mn fe rich fluorapatite of perphosphorus granites the belvis pluton variscan iberian belt three mn fe rich fluorapatite types have been found in the highly evolved peraluminous and perphosphorous granites of the belvis pluton one of these apatite types includes abundant graphite microinclusions suggestive of a magmatic origin for the graphite the belvis pluton is a reversely zoned massif composed by four highly fractionated granite units showing a varied accessory phosphate phases u rich monazite u rich xenotime u rich fluorapatite and late eosphorite childrenite the strong peraluminous character of the granites determines an earlier monazite and xenotime crystallization so the three types of fluorapatite record late stages of phosphate crystallization the earlier type 1 apatite is mostly euhedral small and clear type 2 apatite is dusty large 2800 m and mostly anhedral with strong interlobates interfaces with the main granite minerals more abundant in the less fractionated units and absent in the most evolved unit type 3 is subeuhedral to anhedral shows feathery aggregate texture and only appears in the most evolved unit apatite composition was acquired by electron microprobe analyses laser ablation inductively coupled plasma mass spectrometry and electron energy loss spectroscopy type 1 and type 2 apatite display similar broad compositional ranges showing high mno up to 4 30 wt and feo up to 2 88 wt contents without traces of carbon in apatite structure type 2 differs from type 1 by having slightly higher lree and sr contents ree spectra also differ with type 1 displaying both variable lree slope and negative eu anomaly whereas type 2 shows constant lree slope and higher negative eu anomaly although both display similar hree slope type 3 apatite displays higher feo contents up to 5 09 wt positively correlated with higher cl na li be b zn contents and extremely low y ree contents when compared to the other apatite types cation substitution indicates that part of the fe content is as fe graphite has been found exclusively as abundant microinclusions in type 2 apatite parallel or randomly distributed and heterogeneously grouped in clusters within the crystals high resolution electronic images show that graphite occurs with unusual habits filaments of stacked hexagonal flakes up to 15 m length and up to 0 5 m width textural and chemical features suggest a highly crystallized melt which favor compartmentalizing in compositional microdomains where the apatite types would have crystallized the cocrystallization of type 2 apatite and graphite suggests a c f p rich melt sufficiently saturated to acquire an immiscible character with the highly evolved silicate melt in late magmatic stages as type 2 differs from type 1 apatite by higher lree and sr contents we interpret that fluxing components were favorably concentrated in those residual less depleted lree sr fraction melts once monazite and zircon and xenotime would have crystallized type 3 apatite records a change in the ultimate melt stage an increase of the oxygen fugacity coupled with high undercooling by volatile lost of highly fractionated residual cl richer melts the presence of biogenic carbon in granite peraluminous melts derived from metasedimentary sources and its later crystallization as graphite filaments opens a discussion on the carbon behavior during magma evolution especially in highly polymerized melt framework

explosive expansion of a slowly decompressed magma analogue evidence for delayed bubble nucleation 1 while ascending in the plumbing system of volcanoes magma undergoes decompression at rates spanning several orders of magnitude and set by a number of factors internal and external to the volcano slow decompression generally results in an effusive or mildly explosive expansion of the magma but counterexamples of sudden switches from effusive to explosive eruptive behavior have been documented at various volcanoes worldwide the mechanisms involved in this behavior are currently debated in particular for basaltic magmas here we explore the interplay between decompression rate and vesiculation vigor by decompressing a magma analogue obtained by dissolving pine resin into acetone in varying proportions analogue experiments allow direct observations of the processes of bubble nucleation and growth flow dynamics and fragmentation that is not currently possible with magmatic systems our mixtures contain solid particles and upon decompression nucleation of acetone bubbles is observed we find that mixtures with a high acetone content containing smaller and fewer solid particles experience strong supersaturation and fragment under very slow decompressions despite having low viscosity while mixtures with lower acetone content with more and larger solid particles degas efficiently without fragmentation we interpret our results in terms of delayed bubble nucleation due to a lack of efficient nucleation sites we discuss how a similar mechanism might induce violent explosive expansion in volatile rich and poorly crystalline low silica magmas by analogy with the behavior of rhyolitic magmas

antler orogeny a mediterranean type orogeny the late cenozoic thrust belts of the apennine carpathian and hellenic systems of the mediterranean region provide modern analogues for the middle paleozoic antler orogeny each of these young or active mountain belts formed in convergent systems in which thrusting occurred behind a zone of trench retreat while a region of extension developed contemporaneously within the hanging wall of the subduction systems dynamic models that have been recently developed for these tectonic systems provide a basis for interpretation for the antler belt many if not all of the geologic relations observed in the antler belt including the lack of a collided arc and subsidence of a broad extensional region behind the thrust belt are also present within these young mediterranean thrust belt systems if this interpretation is correct then the entire antler belt may have been preserved with no need for other tectonic elements to have been present west of the antler thrust belt and the havallah basin

geochronology of unconformity related uranium deposits in the athabasca basin saskatchewan canada and their integration in the evolution of the basin the importance of geochronology in the study of mineral deposits in general and of unconformity type uranium deposits in particular resides in the possibility to situate the critical ore related processes in the context of the evolution of the physical and chemical conditions in the studied area the present paper gives the results of laser step heating 40ar 39ar dating of metamorphic host rock minerals pre ore and syn ore alteration clay minerals and laser ablation inductively coupled plasma mass spectrometer la icp ms u pb dating of uraninite from a number of basement and sediment hosted unconformity related deposits in the athabasca basin canada post peak metamorphic cooling during the trans hudson orogen of rocks from the basement occurred at ca 1 750 ma and gives a maximum age for the formation of the overlying athabasca basin pre ore alteration occurred simultaneously in both basement and sandstone hosted mineralizations at ca 1 675 ma as indicated by the 40ar 39ar dating of pre ore alteration illite and chlorite the uranium mineralization age is ca 1 590 ma given by la icp ms u pb dating of uraninite and 40ar 39ar dating of syn ore illite and is the same throughout the basin and in both basement and sandstone hosted deposits the mineralization event older than previously proposed as well as several fluid circulation events that subsequently affected all minerals studied probably correspond to far field continent wide tectonic events such as the metamorphic events in wyoming and the mazatzal orogeny ca 1 6 to 1 5 ga the berthoud orogeny ca 1 4 ga the emplacement of the mckenzie mafic dyke swarms ca 1 27 ga the grenville orogeny ca 1 15 to 1 ga and the assemblage and break up of rodinia ca 1 to 0 85 ga the results of the present work underline the importance of basin evolution between ca 1 75 ga basin formation and ca 1 59 ga ore deposition for understanding the conditions necessary for the formation of unconformity type uranium deposits

modelling the lava dome extruded at soufriere hills volcano montserrat august 2005 may 2006 part i dome shape and internal structure lava domes comprise core carapace and clastic talus components they can grow endogenously by inflation of a core and or exogenously with the extrusion of shear bounded lobes and whaleback lobes at the surface internal structure is paramount in determining the extent to which lava dome growth evolves stably or conversely the propensity for collapse the more core lava that exists within a dome in both relative and absolute terms the more explosive energy is available both for large pyroclastic flows following collapse and in particular for lateral blast events following very rapid removal of lateral support to the dome knowledge of the location of the core lava within the dome is also relevant for hazard assessment purposes a spreading toe or lobe of core lava over a talus substrate may be both relatively unstable and likely to accelerate to more violent activity during the early phases of a retrogressive collapse soufriere hills volcano montserrat has been erupting since 1995 and has produced numerous lava domes that have undergone repeated collapse events we consider one continuous dome growth period from august 2005 to may 2006 that resulted in a dome collapse event on 20th may 2006 the collapse event lasted 3 h removing the whole dome plus dome remnants from a previous growth period in an unusually violent and rapid collapse event we use an axisymmetrical computational finite element method model for the growth and evolution of a lava dome our model comprises evolving core carapace and talus components based on axisymmetrical endogenous dome growth which permits us to model the interface between talus and core despite explicitly only modelling axisymmetrical endogenous dome growth our core talus model simulates many of the observed growth characteristics of the 2005 2006 shv lava dome well further it is possible for our simulations to replicate large scale exogenous characteristics when a considerable volume of talus has accumulated around the lower flanks of the dome model results suggest that dome core can override talus within a growing dome potentially generating a region of significant weakness and a potential locus for collapse initiation

on the interaction between convection and crystallization in cooling magma chambers abstract we investigate the interaction of thermal convection and crystallization in large aspect ratio magma chambers because nucleation requires a finite amount of undercooling crystallization is not instantaneous for typical values of the rates of nucleation and crystal growth the characteristic time scale of crystallization is about 10 3 10 4 s roof convection is characterized by the quasi periodic formation and instability of a cold boundary layer its characteristic time scale depends on viscosity and ranges from about 10 2 s for basaltic magmas to about 10 7 s for granitic magmas hence depending on magma viscosity convective instability occurs at different stages of crystallization a single non dimensional number is defined to characterize the different modes of interaction between convection and crystallization using realistic functions for the rates of nucleation and crystal growth we integrate numerically the heat equation until the onset of convective instability we determine both temperature and crystal content in the thermal boundary layer crystallization leads to a dramatic increase of viscosity which acts to stabilize part of the boundary layer against instability we compute the effective temperature contrast driving thermal convection and show that it varies as a function of magma viscosity and hence composition in magmas with viscosities higher than 10 5 poise the temperature contrast driving convection is very small hence thermal convection is weak in low viscosity magmas convective breakdown occurs before the completion of crystallization and involves partially crystallized magma the convective regime is thus characterized by descending crystal bearing plumes and bottom crystallization proceeds both by in situ nucleation and deposition from the plumes we suggest that this is the origin of intermittent layering a form of rhythmic layering described in the skaergaard and other complexes we show that this regime occurs in basic magmas only at temperatures close to the liquidus and never occurs in viscous magmas this may explain why intermittent layering is observed only in a few specific cases

abrupt change in magma liquidus temperature because of volatile loss or magma mixing effects on nucleation crystal growth and thermal history of the magma the liquidus temperature of magma that reaches shallow levels introduction during its ascent may change abruptly as a result of the release of on its way from the source region into the earth s volatiles or through magma mixing or a combination of both crust or even onto the earth s surface magma travelling immediately after this abrupt change of the liquidus temperature through the mantle and crust evolves and one of the occurs a disequilibrium is observed in the melt and here it is shown goals in petrology is to characterize the various processes that the melt rapidly re equilibrates to its thermodynamic equilibrium that occur during this passage of special interest in the a quantitative model for cooling and crystallization of a simple framework of this study are shallow level processes such two component model melt is used to investigate such re equilibration as magma crust interaction magma mixing deprocesses the relaxation time defined as the time required for the volatilization and depressurization that change the geosystem to re equilibrate after varying degrees of liquidus temperature chemical signature of the melt its crystal content its perturbations is found to be 3 or significantly less than the liquidus temperature its flow behaviour and its erupttime it takes to crystallize 40 of the melt regardless of the ability whereas crustal contamination of magma can be amplitude of the perturbation associated with the re equilibration traced through for example trace element and isotope process is an increase in crystal fraction that can easily reach studies e g wilson 1989 and is discussed extensively 10 depending on the amplitude of the perturbation during the in the other papers of this special volume cooling compensation of the liquidus temperature perturbation the temdepressurization and possibly magma mixing strongly perature of the melt remains nearly constant or may even increase slightly depending on the latent heat budget possibly heat of mixing affect the thermal history of magma rising in the mantle and heat absorbed during volatile exsolution which suggests and crust for example any pressure decrease in volatilecrystallization of the melt without cooling saturated systems not uncommon in natural systems especially for more evolved compositions will lead to volatile exsolution and consequent increased liquidus temperature and change in the melting range one of the major consequences of such behaviour is isothermal crystallization which was recognized almost 40 years ago by tuttle bowen 1958 pp 67 and 68 and the

voluminous lava like precursor to a major ash flow tuff low column pyroclastic eruption of the pagosa peak dacite san juan volcanic field colorado abstract the pagosa peak dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous fish canyon tuff 5000 km3 from the la garita caldera at 28 ma the pagosa peak dacite is thick to 1 km voluminous 200 km3 and has a high aspect ratio 1 50 similar to those of silicic lava flows it contains a high proportion 40 60 of juvenile clasts to 3 4 m emplaced as viscous magma that was less vesiculated than typical pumice accidental lithic fragments are absent above the basal 5 10 of the unit thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading despite the very high viscosity of the crystal rich magma resulting in a macroscopic appearance similar to flow layered silicic lava although it is a separate depositional unit the pagosa peak dacite is indistinguishable from the overlying fish canyon tuff in bulk rock chemistry phenocryst compositions and 40ar 39ar age the unusual characteristics of this deposit are interpreted as consequences of eruption by low column pyroclastic fountaining and lateral transport as dense poorly inflated pyroclastic flows the inferred eruptive style may be in part related to synchronous disruption of the southern margin of the fish canyon magma chamber by block faulting the pagosa peak eruptive sources are apparently buried in the southern la garita caldera where northerly extensions of observed syneruptive faults served as fissure vents cumulative vent cross sections were large leading to relatively low emission velocities for a given discharge rate many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits leading to fracture dominated magma fragmentation at low vesicularity subsequent collapse of the 75 35 km 2 la garita caldera and eruption of the fish canyon tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the pagosa peak eruption

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

regional setting geology and paragenesis of the centennial unconformity related uranium deposit athabasca basin saskatchewan canada the centennial unconformity related uranium deposit represents the first significant uranium mineralization along the snowbird tectonic zone in the south central part of the athabasca basin the deposit is associated with a steeply wnw dipping contact between the virgin schist group and mylonitic granite of uncertain age virgin schist group rocks in the deposit area include phyllites impure quartzite and quartzite medium to coarse grained microcline augen porphyroclasts characterize the granite the quartzite being resistant to weathering forms a paleotopographic high flanked by a paleotalus which formed prior to athabasca group sedimentation diabase petrographically and geochemically similar to the 1 27 ga mackenzie dikes intrudes the mineralized trend along brittle structures in the basement rocks and athabasca group a detailed paragenetic study of the deposit area reveals a protracted history that is related to the episodic reactivation of brittle structures and associated fluid movement along this significant structural corridor retrograde metamorphism converted biotite to chamosite at temperatures between 335 to 350 c weathering caused the breakdown of microcline and the dissolution of quartz prior to erosion and deposition of the athabasca group which started at about 1750 ma compaction and diagenesis resulted in quartz overgrowths and development of a diagenetic clay assemblage of illite and sudoite at temperatures between 150 and 200 c which was broadly synchronous with primary mineralization clinochlore euhedral quartz carbonate and pyrite was developed after the intrusion of the diabase under reducing conditions primarily at temperatures between 270 to 320 c uranium remobilization and alteration of uraninite to coffinite appears to have taken place after the emplacement of the diabase dikes kaolinite and uranophane form last in the paragenetic sequence and probably form at relatively low temperatures

mnd microlite number density water exsolution rate meter abstract microlites in effusive or pyroclastic rocks are possible indicators of water exsolution in particular the microlite number density mnd is considered to be a function of the rate of water exsolution from melt in this paper we have constructed a mnd water exsolution rate meter based on the recent results of theory experiments and the natural observation of crystallization kinetics the mnd method accounts for the effects of melt composition and water content on the diffusivity of crytallizing components in melt by using this meter we can estimate the water exsolution rate at the microlite nucleation depth from a mnd value provided the crystal phase plagioclase or clinopyroxene is known we applied the meter to the case of the 1991 1995 dome eruption at unzen and the 1986b subplinian eruption at izu oshima we obtained the water exsolution rates in the range of 6 1 10 6 to 2 8 10 5 wt s approximately at 70 mpa for unzen plagioclase mnd 1014 to 1015 m 3 and 1 1 10 3 to 1 1 10 1 wt s for izu oshima pyroxene mnd 1015 to 1017 m3 under the assumption of equilibrium vesiculation and steady state flow the corresponding decompression rate and ascent velocity are calculated in the range of 240 to 1100 pa s and 0 014 to 0 068 m s for unzen and 1 2 104 to 1 3 106 pa s and 1 3 to 133 m s for izu oshima this contrast in the ascent velocity at the microlite nucleation depth is closely related to the reason why unzen and izu oshima revealed the different eruption styles namely dome growth and explosive eruption respectively

mineralogy and u pb pb pb and sm nd geochronology of the key lake uranium deposit athabasca basin saskatchewan canada the key lake deposit is one of several large high grade unconformity related uranium deposits located at the eastern margin of the athabasca basin in northern saskatchewan canada the deposit consists of the gaertner orebody now mined out and the deilmann orebody which is presently being mined in the past radiometric dating efforts yielded an age of oldest ore forming event of 1250 34 ma at the gaertner orebody and 1350 4 ma at the deilmann orebody this unlikely age difference called for further investigation innovative preparation techniques were used to separate the paragenetically oldest u mineral an anisotropic uraninite ore microscopy and u pb isotopic data show that the oldest event of uranium emplacement occurred simultaneously at the two orebodies at 1421 49 ma the primary ore forming phase was followed by younger generations of u mineralization and periods of remobilization sm nd data of key lake uraninite form an isochron corresponding to an age of 1215 ma this is interpret

petrogenesis of the proterozoic athabasca basin northern saskatchewan canada and its relation to diagenesis hydrothermal uranium mineralization and paleohydrogeology abstract in the athabasca basin proterozoic sandstones unconformably overlie archean and aphebian rocks multiple fluid events involving isotopically and chemically distinct fluids which have migrated laterally for considerable distances and along fault zones have produced a paragenetically identifiable assemblage of clay silicate and oxide minerals in the basin and basement rocks altered sandstones and the underlying metasedimentary rocks proximal to fault zones host high grade u ni co unconformity type uranium ore deposits and contain an alteration assemblage of highly crystalline 2m 3t illite 1m kaolinite clinochlore and sudoite hematite dravite and euhedral quartz in the sandstones 18o values of coeval quartz hematite quartz illite quartz dravite illite hematite and kaolinite hematite fluid inclusion temperatures in diagenetic quartz and d values of 1m kaolinite 2m 3t illite dravite and in fluid inclusions in the quartz indicate formation from a basinal fluid that had d values of 60 10 and which underwent salinity temperature and 18o increases from 5 to 34 wt nacl 120 to 240 c and 5 to 3 respectively as a result of increasing burial depth and sustained water rock interactions with the basinal sediments over substantial time periods 18 o 16 o and d h ratios of sudoite and clinochlore in meta sedimentary basement rocks within the alteration haloes indicate formation from a basement derived fluid with 18o and d values of 4 4 and 30 10 respectively that was isotopically distinct from the basinal fluids 18o and d values of late kaolinites in reactivated fault zones indicate formation mainly from low temperature 50 25 c fluids having 18o and d values of 16 and 130 respectively similar to relatively modern meteoric waters in the area some kaolinites have 18o and d values which suggest they formed from meteoric waters at mid latitudes and have experienced varying degrees of oxygen and hydrogen isotope exchange with relatively modern meteoric waters in the re activated faults illite sudoite and dravite have d values that vary by 100 without concurrent changes to their 18o values indicating preferential hydrogen isotope exchange with relatively modern meteoric waters at low temperatures and under conditions of high integrated water rock ratios rb sr isochron and model ages of 1477 57 and 952 ma respectively for the crystalline 2m 3t illites indicate the timing of the high temperature basinal fluid events associated with fluid mixing rb sr and k ar ages on illites with the lowest d values are as young as 300 ma and indicate late stage resetting of their radiogenic isotope systematics as a result of substantial interactions with relatively modern meteoric waters the evolution of sr isotopic compositions of fluids in the basin and basement rocks as reflected by the 87 sr 86 sr ratios of chlorite illite and dravite indicates that mixing of two isotopically distinct fluids in the vicinity of faults was the process by which uranium was precipitated similar to that suggested by oxygen and hydrogen isotope systematics in the clays and silicate minerals nd ratios in the high temperature diagenetic and hydrothermal clay and silicate minerals and the cogenetic uranium ores suggest they all have formed from similar fluids which have interacted substantially with the basin sediments in general the isotopic chemical microthermometric and petrologic data indicate that the athabasca basin has had a protracted fluid history comparable to that documented for more modern sedimentary basins the data clearly show that the major sandstone aquifers have been affected by widespread lateral flow of diagenetic fluids over distances of hundreds of kilometers and also that these fluid migration paths have been modified by cross formational fluid flow near active fault zones

characteristics of arctic sea ice thickness variability in gcms abstractskillful arctic sea ice forecasts may be possible for lead times of months or even years owing to the persistence of thickness anomalies in this study sea ice thickness variability is characterized in fully coupled gcms and sea ice ocean only models ioms that are forced with an estimate of observations derived from atmospheric reanalysis and satellite measurements overall variance in sea ice thickness is greatest along arctic ocean coastlines sea ice thickness anomalies have a typical time scale of about 6 20 months a time scale that lengthens about a season when accounting for ice transport and a typical length scale of about 500 1000 km the range of these scales across gcms implies that an estimate of the number of thickness monitoring locations needed to characterize the full arctic basin sea ice thickness variability field is model dependent and would vary between 3 and 14 models with a thinner mean ice state tend to have ice thickness anomalies that are generally shorter lived and s

halogen variations in the paleoproterozoic layered mafic ultramafic intrusions of east kimberley western australia implications for platinum group element mineralization a number of paleoproterozoic layered mafic ultramafic intrusions in the central part of the halls creek orogen of east kimberley western australia have been explored for platinum group elements chromium nickel copper cobalt and gold here we report on the halogen geochemistry of apatite and biotite in a number of these intrusions interstitial apatite is ubiquitous in these intrusions and in most samples tends to be relatively enriched in f and oh end members and relatively poor in cl 20 mole fluorapatite occurs in the more evolved igneous rocks and in samples from the margins of the intrusions that apparently have been contaminated by metamorphic country rock cl f ratios generally increase with bulk rock molar mg mg fe ratios as observed in other intrusions only a few samples show cl enrichment as high as that seen in the stillwater and bushveld complexes beneath the major stratabound platinum group element deposits the most cl rich compositions observed occur in the upper part of the springvale intrusion where it is associated with troctolite and in a single sample from the mcintosh intrusion for the former intrusion it is suggested that volatiles migrating out of the lower part of the mafic stratigraphy stabilized olivine at the expense of pyroxene associated biotite tends to contain little no more than 10 mole cl or f it is concluded that the east kimberley intrusions contained a low to moderate volatile component that during the combined processes of crystallization degassing and fractionation of interstitial halogen bearing minerals produced a late mobile interstitial silicate liquid or volatile rich fluid phase of variable cl f content that in turn caused most of the observed variations within any given intrusion the exceptions include the margins of the intrusions that appear to have been affected by country rocks either during emplacement assimilation or during later metamorphism apatite s generally low cl f ratio the lack of primary amphibole and the high background sulfur concentrations of the east kimberley intrusions suggest that these magmas were relatively volatile poor the development of high grade platinum group element enriched horizons by late stage hydrothermal processes that could have mobilized significant amounts of the platinum group elements and sulfur is considered to be unlikely

numerical simulation of plagioclase rim growth during magma ascent at bezymianny volcano kamchatka abstract slow caal nasi interdiffusion in plagioclase crystals preserves chemical zoning of plagioclase in detail which along with strong dependence of anorthite content in plagioclase on melt composition pressure and temperature make this mineral an important source of information on magma processes a numerical model of zoned crystal growth is developed in the paper the model is based on equations of multicomponent diffusion with diagonal cross component diffusion terms and accounts for mass conservation on the melt crystal interface and growth rate controlled by undercooling the model is applied to the data of plagioclase rim zoning from several recent bezymianny volcano kamchatka eruptions we show that an equilibrium growth model cannot explain crystallization of naturally observed plagioclase during magma ascent the developed non equilibrium model reproduced natural plagioclase zoning and allowed magma ascent rates to be constrained matching of natural and simulated zoning suggests ascent from 100 to 50 mpa during 15 20 days magma ascent rate from 50 mpa to the surface varies from eruption to eruption plagioclase zoning from the december 2006 eruption suggests ascent to the surface in less than 1 day whereas plagioclase zoning from march 2000 and may 2007 eruptions are better explained by magma ascent over periods of more than 30 days based on comparison of diffusion coefficients for individual elements a mechanism of atomic diffusion during plagioclase crystallization is proposed

in situ measurements of plagioclase growth using sims depth profiles of 7li 30si a means to acquire crystallization rates during short duration decompression events numerous petrologic studies have attempted to determine crystal growth rates in volcanic systems through several methods including analyses of crystal size distributions in natural samples and decompression experiments on hydrous magmas experiments have revealed that rim growth on existing crystals will occur under a wide range of decompression conditions and is favored under conditions of low to moderate undercoolings over microlite nucleation which has typically been the focus of decompression induced crystallization studies for this study samples of eruptive clasts were collected from a vulcanian explosion that occurred following the july 12 13 2003 dome collapse of soufriere hills volcano montserrat plagioclase phenocrysts were extracted and examined with secondary ion mass spectrometry sims depth profiling lithium inflection depths within the profiles along with the observed time interval between the peak in the collapse decompression and the explosion that ejected the examined samples were used to calculate the growth rates as a result of magma devolatilization with an average of 8 3 x 10 8 mm s anorthite content of the plagioclase rim growth indicates an average decompression magnitude of 40 mpa inducing an undercooling of 45 c that favors crystal growth over microlite nucleation however variability in the final anorthite contents suggests that not all phenocrysts recorded an equilibrium composition reflecting accurate pressure conditions in such events occurring over short timescales 10 h lithium is a more reliable indicator of decompression induced growth than changing anorthite content due to lithium s rapid diffusivity

vesiculation and crystallization under instantaneous decompression numerical study and comparison with laboratory experiments abstract vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior and they interplay each other through the water exsolution process we conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions decompression history is unknown and in laboratory experiments the amount of decompression is constant with time the numerical results which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water viscosity and the amount of decompression provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression the bubble nucleation in the homogeneous nucleation can be divided into two regimes the diffusion control regime and viscosity control regime depending on the modified peclet number and the effective supersaturation in the cases of both homogeneous and heterogeneous nucleations the bubble growth is controlled by diffusion or viscosity depending on the modified peclet number and bubble number density the water exsolution rate which is controlled by the modified peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime acts as an effective cooling rate in a decompression induced crystallization process a comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion limited growth of crystal the several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate

evolution of crystal sizes in the series of dissolution and precipitation events in open magma systems article i nfo we propose a model that describes the evolution of crystal sizes and crystal size distributions csd of igneous phenocrysts in a sequence of dissolution and crystallization events this model is based on the assumption that crystal dissolution is rate limited by diffusion in melt while crystal growth is controlled by the slower kinetic of new nucleation and growth as a result the dissolution rate is inversely proportional to crystal size coming into effect through the curvature of the crystal s surface but the growth rate does not depend on the crystal size closed form analytical solution of equation for csd is obtained we apply results of modeling to quartz and zircon two prime minerals in silicic igneous systems that are widely used in geochemical and isotopic investigations the time series of multiple solution reprecipitation episodes generate concave downward csds and this result fits well with experimental and natural observations on the abundant concave down csd in silicic igneous rocks we suggest that maturation of crystal populations with sizes above several micrometers can not be caused by a size effect on the solubility of the crystals ostwald ripening but is rather driven by thermal oscillations in experiments and in nature the model predicts that mean crystal size increases with time proportionally to t 0 20 which is very close to the published experimental results for quartz maturation with the exponent of 0 19 0 22 our proposed model gives an opportunity to use natural csds for interpretation of pre eruptive magma history when solubilities and diffusion data are available for constituent elements of the dissolving mineral in particular we present time estimates for maturing zircon populations in large volume ignimbrites and estimate that it takes 100 1000 yrs to mature an initially exponential csd to a lognormal csd

u pb geochronology of minerals from the midwest uranium deposit northern saskatchewan analyses of u pb ratios in 30 microsamples of pitchblende and coffinite from the midwest uranium deposit in northern saskatchewan as well as two altered zircons from host rocks indicate an age of

sr isotopic study of helikian sediment and diabase dikes in the athabasca basin northern saskatchewan the athabasca group overlies 1 8 ga or older basement of the churchill province and predates uranium mineralization at 1 3 ga cross cutting diabase dikes have been dated by rb sr mineral isochrons

the influence of viscous and latent heating on crystal rich magma flow in a conduit the flow dynamics of crystal rich high viscosity magma is likely to be strongly influenced by viscous and latent heat release viscous heating is observed to play an important role in the dynamics of fluids with temperature dependent viscosities the growth of microlite crystals and the accompanying release of latent heat should play a similar role in raising fluid temperatures earlier models of viscous heating in magmas have shown the potential for unstable thermal runaway flow as described by a gruntfest number using an arrhenius temperature dependence for the viscosity but have not considered crystal growth or latent heating we present a theoretical model for magma flow in an axisymmetric conduit and consider both heating effects using finite element method techniques we consider a constant mass flux in a 1 d infinitesimal conduit segment with isothermal and adiabatic boundary conditions and newtonian and non newtonian magma flow properties we find that the growth of crystals acts to stabilize the flow field and make the magma less likely to experience a thermal runaway the additional heating influences crystal growth and can counteract supercooling from degassing induced crystallization and drive the residual melt composition back towards the liquidus temperature we illustrate the models with results generated using parameters appropriate for the andesite lava dome forming eruption at soufriere hills volcano montserrat these results emphasize the radial variability of the magma both viscous and latent heating effects are shown to be capable of playing a significant role in the eruption dynamics of soufriere hills volcano latent heating is a factor in the top two kilometres of the conduit and may be responsible for relatively short term days transients viscous heating is less restricted spatially but because thermal runaway requires periods of hundreds of days to be achieved the process is likely to be interrupted our models show that thermal evolution of the conduit walls could lead to an increase in the effective diameter of flow and an increase in flux at constant magma pressure

influence of extrusion rate and magma rheology on the growth of lava domes insights from particle dynamics modeling abstract lava domes are structures that grow by the extrusion of viscous silicic or intermediate composition magma from a central volcanic conduit repeated cycles of growth are punctuated by collapse as the structure becomes oversized for the strength of the composite magma that rheologically stiffens and strengthens at its surface here we explore lava dome growth and failure mechanics using a two dimensional particle dynamics model the model follows the evolution of fractured lava with solidification driven by degassing induced crystallization of magma the particle dynamics model emulates the natural development of dome growth and rearrangement of the lava dome which is difficult in mesh based analyses due to mesh entanglement effects the deformable talus evolves naturally as a frictional carapace that caps a ductile magma core extrusion rate and magma rheology together with crystallization temperature and volatile content govern the distribution of strength in the composite structure this new model is calibrated against existing observational models of lava dome growth results show that the shape and extent of the ductile core and the overall structure of the lava dome are strongly controlled by the infusion rate the effects of extrusion rate on magma rheology are sensitive to material stiffness which in turn is a function of volatile content and crystallinity material stiffness and material strength are key model parameters which govern magma rheology and subsequently the morphological character of the lava dome and in turn stability degassing induced crystallization causes material stiffening and enhances material strength reflected in non newtonian magma behavior the increase in stiffness and strength of the injected magma causes a transition in the style of dome growth from endogenous expansion of a ductile core to stiffer and stronger intruding material capable of punching through the overlying material and resulting in the development of a spine or possibly inducing dome collapse simulation results mimic development of a megaspine upon the influx of fresh magma which leads to the re direction of magma flow creating a new shear zone and the switching of dome growth from one side to the other our model shows similar dome growth dynamics as observed at soufriere hills volcano montserrat indicating a strong correlation between extrusion rate and its subsequent effect on mechanical properties and variations in magma rheology

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

the halogen geochemistry of the bushveld complex republic of south africa implications for chalcophile element distribution in the lower and critical zones solidification of the interstitial liquid the stratigraphic distribution halogen bearing minerals especially apatite are minor but ubiof s cu and the pge in the critical zone cannot readily be quitous phases throughout the bushveld complex interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end member chlorapatite below the merensky reef lower a function of trapped liquid abundance evidence from potholes and and critical zones and has increasingly fluorian compositions with the pge rich driekop pipe of the bushveld complex imply that increasing structural height above the reef main and upper zones migrating cl rich fluids mobilized the base and precious metal cl f variations in biotite are more limited owing to crystal chemical sulfides we suggest that the distribution of sulfide minerals and controls on halogen substitution but are also consistent with a the chalcophile elements in the lower and critical zones reflects a decrease in the cl f ratio with structural height in the complex general process of vapor refining and chromatographic separation of a detailed section of the upper lower zone to the critical zone is these elements during the evolution and migration of a metalliferous characterized by an upward decrease in sulfide mode from 0 01 cl rich fluid phase 0 1 to trace 0 001 cu tends to correlate with other incompatible elements in most samples whereas the platinum group elements pge can behave independently particularly in the critical zone the decrease in the cl f ratio of apatite in the

water transfer during magma mixing events insights into crystal mush rejuvenation and melt extraction processes many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal rich mushes to investigate water transfer processes in such systems following thermal equilibration we conducted 24 h experiments to establish the petrological evolution of a water undersaturated 4 wt h 2 o in the interstitial melt quartz bearing dacite crystal mush 0 5 0 8 in crystal fraction intruded by a water saturated 6 wt h 2 o initially crystal free andesite magma at 950 c and 4 kbar 12 km depth our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content specifically mafic samples dramatically crystallize during water escape into the felsic end members and consequent increase in liquidus temperature conversely the addition of water to the felsic mush reduces the liquidus temperature leading to an increase in melt fraction the experiments provide insights into how volatiles contribute to crystal mush rejuvenation i e increase of melt fraction however h 2 o diffusion alone is not sufficient to promote melt extraction from short and long lived mushes in the earth s crust

morphologic variation of an evolving dome controlled by the extrusion of finite yield strength magma abstract degassing induced crystallization in volatile rich intermediate composition magmas results in material stiffening and strengthening that prior to solidification is reflected in non newtonian rheology we explore the effects of a spectrum of such rheological regimes on eruptive style and morphologic evolution of lava domes using a two dimensional 2d particle dynamics model for a spreading viscoplastic bingham fluid we assume that the ductile magma core of a 2d synthetic lava dome develops finite yield strength and that deformable frictional talus evolves from a carapace that caps the magma core our new model is calibrated against an existing analytical model for a spreading viscoplastic lava dome and is further compared against observational data of lava dome growth results indicate that a degassing induced increase in strength of the injected magma causes a transition in the lava dome morphology from a dome with low surface relief evolving endogenously with apparent bulk yield strength 104 105 106 pa extruded through the dome carapace the virtual lava dome with 0a 0 6 mpa shows good agreement with the observed dome heights observed at the soufriere hills volcano montserrat during a period of endogenous growth the calculated apparent flow viscosity 1 36 1011 pa s for 0a 0 6 mpa is in the range of estimated viscosities 109 to 1012 pa s for andesitic dacitic crystal rich lavas our model results indicate a strong correlation between apparent yield strength and dome morphology with both controlled by degassing induced crystallization and extrusion rate

modelling the lava dome extruded at soufriere hills volcano montserrat august 2005 may 2006 part ii rockfall activity and talus deformation abstract during many lava dome forming eruptions persistent rockfalls and the concurrent development of a substantial talus apron around the foot of the dome are important aspects of the observed activity an improved understanding of internal dome structure including the shape and internal boundaries of the talus apron is critical for determining when a lava dome is poised for a major collapse and how this collapse might ensue we consider a period of lava dome growth at the soufriere hills volcano montserrat from august 2005 to may 2006 during which a 100 10 6 m 3 lava dome developed that culminated in a major dome collapse event on 20 may 2006 we use an axi symmetrical finite element method model to simulate the growth and evolution of the lava dome including the development of the talus apron we first test the generic behaviour of this continuum model which has core lava and carapace talus components our model describes the generation rate of talus including its spatial and temporal variation as well as its post generation deformation which is important for an improved understanding of the internal configuration and structure of the dome we then use our model to simulate the 2005 to 2006 soufriere hills dome growth using measured dome volumes and extrusion rates to drive the model and generate the evolving configuration of the dome core and carapace talus domains the evolution of the model is compared with the observed rockfall seismicity using event counts and seismic energy parameters which are used here as a measure of rockfall intensity and hence a first order proxy for volumes the range of model derived volume increments of talus aggraded to the talus slope per recorded rockfall event approximately 3 10 3 13 10 3 m 3 per rockfall is high with respect to estimates based on observed events from this it is inferred that some of the volumetric growth of the talus apron perhaps up to 60 70 might have occurred in the form of aseismic deformation of the talus forced by an internal laterally spreading core talus apron growth by this mechanism has not previously been identified and this suggests that the core hosting hot gas rich lava could have a greater lateral extent than previously considered

formation and evolution of the centennial unconformity related uranium deposit in the south central athabasca basin canada the centennial u deposit is situated in the south central athabasca basin canada and straddles the unconformity between early paleoproterozoic to archean metasedimentary and metavolcanic rocks and granitoids and the clastic sediments of the paleoproterozoic athabasca group although it has most characteristics of an unconformity related uranium deposit the centennial deposit is atypical in that it is not directly associated with an electromagnetic conductor there is a paucity of graphite in the basement or with a major reverse fault zone it is distal from a major fluid conduit ca 300 to 400 m from the dufferin lake fault has low ni cu co zn and pb contents and contains an unusually large amount up to 5 of secondary uranyl minerals additionally a network of diabase dikes and sills is observed at centennial seemingly intruding the main u mineralization of massive uraninite based on the relatively sharp contacts between the diabase dike and the high grade u mineralization the pre u alteration assemblage at centennial includes kaolinite illite and sudoite which have been formed by fluids with isotopic and chemical compositions that are comparable with those from other sandstone hosted unconformity type u deposits in the athabasca basin pre u illite related fluids have 18o of ca 3 and d of ca 40 whereas pre u chlorite related fluids have 18o between 1 7 and 4 3 and d between 18 and 1 laser ablation inductively coupled plasma mass spectrometry la icp ms u pb dating of the various u phases indicates that initial mineralization represented by disseminated uraninite found directly to the north northeast of the centennial deposit occurred at ca 1 6 ga the main u mineralization represented by massive and strongly altered uraninite followed at an unknown time a minor 5 unaltered uraninite formed from the local remobilization of the main massive uraninite at ca 380 ma the main uranyl mineral uranophane formed last at ca 2 ma the recurrence of local u remobilization might have been facilitated by the persisting high permeability of the sandstones in the area due to the nearby dufferin lake fault and to the emplacement of the diabase dikes the usefulness of pb isotopes for exploration is demonstrated at the centennial deposit with strongly radiogenic pb isotope signatures close to the deposit and a common pb signature observed at a distance of a few km from the deposit

rift related magmatism of the central atlantic magmatic province in algarve southern portugal abstract lower jurassic volcanic sedimentary successions of the algarve basin 198 ma are associated with the central atlantic magmatic province camp that was one aspect of pangean intracontinental rifting volcanic products of the algarve camp include subaerial lava flows pyroclastic deposits and peperites and contemporaneous sedimentation is dominated by mudstones and conglomerates often containing volcanic fragments intercalated in the volcanic sequence these lithological characteristics and associations are compatible with a facies model typical of continental basaltic magmatism volcanic rocks are fractionated low ti tholeiites enriched in large ion lithophile elements relative to high field strength elements the compositional spectrum e g ba nb zr nb is consistent with derivation of primary magmas by partial melting of heterogeneous sources initially continental lithospheric mantle progressively replaced by an asthenospheric source fractional crystallization processes occurred in magma chambers located at depths close to the crust mantle boundary from density and geobarometric constraints reflecting magma underplating during the initial stages of continental rifting sporadic occurrence of crustal xenoliths widespread evidence for basement carbonate assimilation as well as sr 87sr 86sr 0 0 7050 and oxygen 18o 7 40 isotope ratios that are higher than those typical of continental lithospheric mantle indicate that crustal contamination also contributed to the observed geochemical variations the carnian norian 228 0 to 203 6 ma or older age estimated for the basal silves sandstones suggests that the inception of rift related sedimentation preceded volcanism by 20 to 30ma these results lead us to propose a passive rifting model for the algarve sector of camp magmatism the orientation of which was conditioned by strain localization along pre existing lithospheric mechanical anisotropies

crystallization calculations for a binary melt cooling at constant rates of heat removal implications for the crystallization of magma bodies abstract we report on thermodynamic non equilibrium crystallization calculations for a unit volume of a binary melt subject to a constant prescribed rate of heat loss crystallization histories and crystal size distributions for both melt components were calculated by accounting for the nucleation and growth of crystals the crystal sizes were found to decrease with increasing rates of heat loss the crystallization time defined as the time to crystallize 99 of the unit volume also decreased strongly with increasing rates of heat loss up to a critical rate the critical rate was found to be somewhat smaller than the heat loss rate for the beginning of glass formation at larger than critical rates crystallization time increased again and for rates larger than the glass formation rate crystallization time became infinite the residual melt composition was found to increasingly deviate from the equilibrium composition with increasing rates of heat loss but as long as the loss rate was less than the critical rate the crystallization path reverted to the eutectic composition during the final crystallization for supercritical rates no such reversion was observed we compared the critical rate with estimates of the rates of heat loss in magmatic intrusions based on the stefan solution for a freezing half space it was found that rates of heat loss should be supercritical at distances of up to 0 5 m from the margin of an intrusion in this region non equilibrium effects are expected to dominate and the texture of the crystallized rock should be characterized by small crystals and by glass the glass and the crystals should be of non equilibrium composition non equilibrium effects should be negligible only at distances of more than 5 m from the margin where the rates of heat loss are less than 10 2 times critical at these distances the crystallized rock should have an equigranular texture and an equilibrium composition

stable isotope geochemistry of alteration associated with the key lake uranium deposit canada the key lake uranium deposit occurs within a shear zone that intersects the unconformity between early proterozoic basement gneiss and overlying middle proterozoic athabasca group sandstone uranium is concentrated near the unconformity and is associated with intense illite chlorite and kaolinite alteration the delta d and delta 18 o values of the clay minerals suggest that several different fluids have affected the ores and surrounding host rocks alteration of the basement gneisses associated with the uranium mineralization consists mainly of mg chlorite that equilibrated about 200 degrees c with a fluid having a delta d value near 20 per mil and a delta 18 o value near 5 per mil an illite rich sandstone alteration halo preserved only locally above the deposit equilibrated about 200 degrees c with a basinal brine having a delta d value near 65 per mil and a delta 18 o value near 5 per mil the uranium was concentrated near the unconformity by the mixing of these two distinct fluids the delta 18 o values of coexisting quartz and illite sampled from the shear zone hosting the uranium mineralization yield equilibration temperatures of 190 degrees to 230 degrees c which is postulated to be the temperature of the ore forming event fracturing in the sandstone above the deposit controlled late low temperature 50 degrees c kaolinitization of preexisting hydrothermal illite and chlorite by meteoric waters having delta d values of 50 or 30 per mil and delta 18 o values of 7 or 3 per mil the youngest event to affect the key lake deposit is related to the influx of modern meteoric waters that have lowered the delta d values but not greatly affected the delta 18 o values of some of the clay minerals associated with the shear and breccia zones this latest event has recrystallized much of the hydrothermal mg chlorite associated with the uranium to produce an fe mg chlorite with delta d values as low as 170 per mil

kinetics of crystallization of igneous rocks the geochemistry of igneous rocks is discussed with the primary objectives of bringing together the theories underlying the kinetics of crystallization of igneous rocks and illustrating the use of these theories in understanding experimental and observational data the primary purpose of the chapter is to introduce current thinking about the kinetics of igneous rocks and to provide a basis for understanding other work a basic assumption made in the discussion is that the rate of any chemical reaction including the crystallization of igneous rocks is zero at equilibrium and proceeds at a finite rate only at a finite deviation from equilibrium as such an understanding of the processes operating in igneous rocks requires an understanding of how deviation from equilibrium affects the rates and mechanisms of the processes occurring during crystallization these processes are detailed with special emphasis given to nucleation and crystal growth jmt

controls on conduit magma flow dynamics during lava dome building eruptions 1 lava dome eruptions commonly display fairly regular alternations between periods of high and low or no activity with timescales typically of weeks to years and sudden transitions between effusive and explosive activity we develop a transient model of the magma flow in a volcanic conduit from an open system magma chamber with continuous replenishment the model incorporates gas exsolution bubble growth gas escape through the magma and decompression induced crystallization and considers variations in magma temperature water content conduit diameter phenocryst size chamber volume and magma rheology calculations show the presence of periodic variations in discharge rate due to the transition from a stable regime when discharge rate is low and crystals grow efficiently leading to high magma viscosity to another stable state when discharge rate is high and crystallization is negligible the difference in discharge rates between these regimes can be several orders of magnitude periods are similar to the observed timescales and mainly depend on the chamber volume the system shows strongly nonlinear responses to the variation of governing parameters if magma has a bingham rheology pauses in discharge rate occur between peaks of discharge and the peaks are much higher than for the case of newtonian rheology large changes in discharge rate and eruptive behavior can occur as the consequence of small changes in magma temperature water content phenocryst size distribution or conduit diameter the system can fluctuate between low and high discharge rates with transitions to explosive activity

zoning of phosphorus in igneous olivine we describe p zoning in olivines from terrestrial basalts andesites dacites and komatiites and from a martian meteorite p2o5 contents of olivines vary from below the detection limit 0 01 wt to 0 2 0 4 wt over a few microns with no correlated variations in fo content zoning patterns include p rich crystal cores with skeletal hopper or euhedral shapes oscillatory zoning structures suggesting replacement of p rich zones by p poor olivine and sector zoning melt inclusions in olivines are usually located near p rich regions but in direct contact with low p olivine crystallization experiments on basaltic compositions at constant cooling rates 15 30 c h reproduce many of these features we infer that p rich zones in experimental and natural olivines reflect incorporation of p in excess of equilibrium partitioning during rapid growth and zoning patterns primarily record crystal growth rate variations occurrences of high p phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling most cases of oscillatory zoning in p likely reflect internal factors whereby oscillating growth rates occur without external forcings but some p zoning in natural olivines may reflect external forcings e g magma mixing events eruption that result in variable crystal growth rates and or p contents in the magma in experimental and some natural olivines al cr and p concentrations are roughly linearly and positively correlated suggesting coupled substitutions but in natural phenocrysts cr zoning is usually less intense than p zoning and al zoning weak to absent we propose that olivines grow from basic and ultrabasic magmas with correlated zoning in p cr and al superimposed on normal zoning in fe mg rapidly diffusing divalent cations homogenize during residence in hot magma al and cr only partially homogenize and delicate p zoning is preserved because p diffuses very slowly this interpretation is consistent with the fact that zoning is largely preserved not only in p but also in al cr and divalent cations in olivines with short residence times at high temperature e g experimentally grown olivines komatiitic olivines groundmass olivines and the rims of olivine phenocrysts grown during eruption p zoning is widespread in magmatic olivine revealing details of crystal growth and intra crystal stratigraphy in what otherwise appear to be relatively featureless crystals since it is preserved in early formed olivines with prolonged residence times in magmas at high temperatures p zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma s history study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks especially because these features can be observed with standard electron microprobe techniques

thermal effects of magma degassing abstract the thermodynamics of diffusive bubble growth is dominated by latent heat of water exsolution vaporization and the work of gas expansion p dv work a numerical assessment of the effects of these on cooling of bubbly magma water in albite indicates that a magma can exsolve volatiles at equilibrium or with varying degrees of oversaturation depending on decompression history and degassing kinetics heat of water exsolution vaporization from magmatic melts has not previously been experimentally or analytically determined and we have evaluated it from others thermodynamic functions known for the albite water system heat of exsolution is small for pressures over 100 mpa but can reach 20 kj mole at 1 2 mpa 10 20 bar oversaturation degassing at 10 20 bar can cause cooling of albite by 8 k wt of exsolved water the thermal effect of equilibrium degassing depends on the starting pressure of decompression because it follows the solubility curve for a saturated melt at 100 mpa 4 wt water it can cool a magma at least by 35 k before it disrupts into spray as gas volume fraction exceeds 0 8 although we do not specifically consider the dynamics of bubble interactions in this study our results suggest that the dynamic cooling rate could be significant at the vent of an erupting magma column and under extreme eruption conditions can lead to glass formation around fast growing bubbles of magmatic foam this could cause fragmentation into fine ash by brittle failure conversely bubble wall cooling even before glass formation can serve to reduce diffusive volatile flux into bubbles decreasing the overall cooling of the system and lead to solidification of oversaturated magmas as occasionally observed in eruption products

an origin for harrisitic and granular olivine in the rum layered suite nw scotland a crystal size distribution study dendritic crystal morphologies occur in a number of igneous rocks and are thought to originate from the rapid growth of crystals yet many examples of dendritic morphologies are found in plutonic igneous rocks where cooling rates should be low results from crystal size distribution csd measurements on harrisitic olivines from rum scotland combined with estimated olivine growth rates suggest that the characteristic skeletal hopper and branching olivines of harrisitic cumulates that are up to centimetres long may have exceptionally short crystal growth times several hours to several hundreds of days this together with very low calculated nucleation densities for harrisitic olivine supports the interpretation of harrisite being a disequilibrium texture developed in response to supersaturation of the magma in olivine we propose that this supersaturation arose through undercooling of thin picrite sheets emplaced along the rum magma chamber floor beneath cooler resident magma it is envisaged that the picrite sheets were largely free of suspended olivine crystals coupled with the olivine enriched composition of the melt and the increasing cooling rate this allowed homogeneous nucleation of olivine to set in at deeper undercooling and greater olivine supersaturation than if there had been plentiful suspended olivines to act as heterogeneous nuclei the enhanced supersaturation caused rapid growth of olivine once nucleation began with skeletal and dendritic shapes it is suggested that the observed interlayered sequences of harrisite and cumulus peridotite found throughout the rum layered suite are a result of multiple episodes of harrisite crystallization resulting from picrite emplacement that alternated with periods of crystal growth and accumulation in the main body of magma at lesser degrees of undercooling

on the distribution and separation of crystals in convecting magma abstract fractionation of crystals from magma stands squarely as the central principle of igneous petrology and the critical balance is the competition between crystal settling or rising velocity and magmatic convective velocity a ratio we call s the convective velocity is intimately related to the rate of heat transfer from the system although the theory of sediment distribution in rivers establishes the critical parametric balance and it has been often applied to igneous systems this method turns out to be valid only for small concentrations of crystals and it is not clear if it is accurate for other than turbulent flows where the prandtl number is near unity a generalized theory of particle crystal or bubble behavior is presented scaling of the generalized result shows for the magmatic conditions of small crystallinities small particle reynolds number and newtonian viscosity that the exact distribution of crystals in a convecting magma can be found analytically regardless of the flow field critical retention occurs only for 0 s s 0 for sinking crystals retention is centered in the region of maximum updraft for floating particles retention occurs in regions of downflow which for a simple cellular convective pattern produces a torus shaped retention region against the walls about the midsection of the body for a stack of flattened convective cells which in the crudest sense is analogous to double diffusive convection the zones of retention form depending on the exact flow field either a stack or staggered stack of pillows through the central region crystals settling from top to bottom around this stack take a tortuous path and may exhibit a complex but regular compositional zoning additional effects of fluids possessing a yield strength turbulence particle shape and wall and particle dispersive effects are more qualitatively evaluated for each flow field the amount of retention and the effective settling velocity of all crystals is found as a function of s the critical parameter s controlling retention can be estimated from a parametric representation involving stokes s law and the rayleigh number this shows that some retention probably occurs in all magmas and it increases with approach of the magma to the earth s surface although both crystal settling rate and convection decrease with increasing magmatic viscosity the overall effect is to increase retention decrease s magmas having a viscosity near and beyond about 10 4 poise and chambered in the nearsurface environment should show large degrees of crystal retention retention is however never complete for magmas and the flux of settling crystals leading to differentiation varies as a function of crystallinity this flux or flux probability p f is zero both at the liquidus and the point of maximum packing of crystals which for basalts is about 60 the importance of differentiation is regulated by the amount of time spent in the high crystal flux state and a relative measure of this time is given by the thermal probability p t introduced previously thus the probability of differentiation p d is given by the product p f p t which shows that differentiation must occur over a restricted range of crystallinities from about 20 to 45 for basalts and from about 10 to 30 for rhyolitic magmas the most basic lessons learned here are that the full range of crystallinities is not equally accessible to crystal fractionation and the most fundamental information describing the dynamic state and behavior of magma may well be the distribution of crystal sizes found in lavas

petrology and geochemistry of the orbicular granitoid of sierra de velasco nw argentina and implications for the origin of orbicular rocks the velasco orbicular granitoid is a small 65 15 m irregularly shaped body that crops out within the huaco granite central sierra de velasco nw argentina it consists of ellipsoid shaped orbicules of 3 to 15 cm length immersed in an aplitic to pegmatitic matrix the orbicules are formed by a core made up of a k feldspar megacryst partially to totally replaced by plagioclase an inner shell of radial and equant plagioclase crystals a layer of tangentially oriented biotite laths and an outer shell of plumose plagioclase crystals containing diffuse rings of tangentially oriented biotite the orbicular granitoid formed in situ in a pocket of evolved and volatile rich melt segregated from the surrounding partially crystallized huaco granite possibly via a filter pressing mechanism the segregated melt entrained relatively few k feldspar megacrysts into the pocket leaving behind a concentration of megacrysts around the pocket high water concentration caused effective superheating of the melt and destruction of nuclei with only the large megacrysts surviving as solids sudden water pressure loss and exsolution of the volatile phase perhaps related to a volcanic eruption or fracturing of the surrounding granite caused rapid undercooling of the melt the orbicules grew in the undercooled melt by heterogeneous nucleation on the megacrysts which acted as nucleation seeds and crystallization of reversely zoned radial plagioclase and sporadic crystallization of tangential biotite rings according to fluctuations in its saturation orbicular growth gave way to crystallization of the equiaxial inter orbicular matrix in two stages when sufficient polymerization of the melt was attained the time scale of formation of the orbicular granitoid was fast possibly a matter of a few weeks or months

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

the millennium uranium deposit athabasca basin saskatchewan canada an atypical basement hosted unconformity related uranium deposit the millennium uranium basement hosted unconformity related deposit is one of the most recent discoveries of basement hosted deposits and may hold as much as 46 8 million pounds 18 000 metric tonnes u at an average grade of 4 53 percent u3o8 the alteration zone associated with the deposit is atypical as it consists predominantly of muscovite rather than the chlorite and muscovite commonly found in the other basement hosted deposits of the athabasca basin the preore stage consists of early minor clinochlore alteration at temperatures of about 275 c interpreted to result from retrograde metamorphism of the basement rocks prior to the deposition of the athabasca basin this clinochlore event is followed by an extensive 1mc muscovite alteration of the basement rocks by na k fe rich basinal brines at temperatures of about 250 c creating up to 25 percent void space in the basement rocks subsequently a weak chamoisite alteration resulted from fe mg ca rich fluids with temperatures of about 300 c that evolved from reactions between the basinal brines and the basement rocks ore stage uraninite has an extrapolated chemical age of 1590 ma and is contemporaneous with synore 1mc muscovite and iron oxide deposited at temperatures near 250 c from fluids with similar 18o but lower d values to preore basinal fluids the low d values indicate postdepositional exchange of h isotopes with cenozoic fluids postore alteration consists of dravite and sudoite precipitated from mg ca rich fluids from the same evolved basinal brines that precipitated preore chamoisite at temperatures of about 185 c filling voids created during the preore stage our results indicate that the pre and synore minerals were formed dominantly by basinal fluids with no contribution from basement fluids which differ from some other basement hosted deposits in the athabasca basin where basement derived fluid influx was more significant several u pb and ar ar resetting events recorded on preore muscovite and on uraninite are coincident with fluid flow induced by distal orogenies remobilizing radiogenic pb from the deposit the radiogenic pb remobilization events were not widespread as the presence of radiogenic pb is restricted to basement rocks proximal to the deposit and along the unconformity

direct observations of degassing induced crystallization in basalts magma degassing can trigger crystal growth by increasing the magma liquidus temperature as crystallization greatly increases magma viscosity this process can strongly influence eruptive dynamics we use a microscope and heated stage to obtain the first direct observations of degassing driven crystal growth in natural basaltic melts at magmatic temperatures samples from mount etna italy 0 39 wt h2o and kilauea volcano hawaii 0 18 wt h2o were heated in air at 1 bar and held isothermally for 0 5 17 h between 1190 c and 1270 c before cooling to solidus temperatures on heating bubble growth at 900 c indicated volatile exsolution in the hydrous etna sample isothermal conditions produced numerous new plagioclase crystals that grew to 160 m at maximum rates of 5 2 18 10 6 cm s 1 growth rates and crystal morphologies tabular to spherulitic depended on dwell temperature growth slowed dramatically after 20 min as equilibrium was approached in the h2o poor kilauea sample few new crystals appeared they grew at maximum rates of 1 7 6 5 10 6 cm s 1 on cooling crystal nucleation and growth were strongly influenced by preexisting crystal textures highlighting the importance of studying natural samples our results document rapid crystal growth triggered by melt devolatilization when the h2o content of the glass is sufficiently high resultant swift substantial changes in magma texture are a key control on lava rheology at etna and elsewhere

effect of h2o on liquidus relationships in mgo al2o3 sio2 at 30 kilobars dehydrated gels with compositions corresponding to mixtures of pyrope enstatite pyrope quartz and pyrope quartz corundum were sealed in platinum capsules and reacted in half inch piston cylinder apparatus at 30 kbar samples were pyrope composition with variable h2o and other compositions with 7 5 wt h2o highly aluminous metastable orthopyroxenes nucleated from and persisted in mixtures with compositions near pyrope there is no univariant fusion curve for pyrope h2o pyrope dissolves incongruently in h2o subsolidus and a trace of liquid is produced near 1000 c a univariant reaction pyrope forsterite vapor spinel liquid was located at 1150 20 c the liquidus for pyrope with excess h2o is at 1310 c orthopyroxenes coexisting with sio2 rich liquids contain 0 8 5 7 excess sio2 compared with mgsio3 al2o3 liquidus diagrams dry with 7 5 h2o and with excess h2o illustrate the effect of h2o on the liquidus relationships with increasing h2o the liquidus volume for opx expands greatly at

lava dome growth and evolution with an independently deformable talus subduction zone volcanism occurs due to partial melting from subducting slabs which generally results in high viscosity magma containing a large amount of volatiles such volcanic eruptions can form a lava dome from which collapse events are a common and important part of their evolution collapse events can have devastating consequences in the form of block and ash avalanche deposits pyroclastic ows surges and the generation of tsunamis if they enter the sea in addition to this once a mass of lava dome has been removed during a collapse event this results in a drop in pressure in the remaining volatile rich magma which may then erupt explosively the internal structure of a lava dome and the extent of the surrounding carapace talus is unknown but likely to be critical for hazard assessment and is the motivation for this research presented is a computational model for the growth and evolution of an endogenous lava dome including an independently deformable talus using the finite element method dome growth is modelled to occur under two time scales continuous dome expansion via the addition of new magma into the molten core interior and relatively instantaneous talus readjustments due to rockfalls and the disintegration of the solid surface the continuous deformation of the dome is modelled as a uid with a yield strength in the talus region while talus deformation is modelled as a granular material that rests at angles below its angle of repose both surfaces dome and core talus interface are displaced using the level set method the model is axi symmetric and assumes that solidi cation and therefore talus growth occurs due to gas exsolution which promotes crystallisation rather than from surface cooling appropriate for intermediate composition lava ows for the purpose of this paper we consider and apply the model to the soufriere hills volcano montserrat but the techniques used are generic allowing the model to be applied to other dome forming eruptions the model provides information on the shape of the dome with the growth and extent of the talus and core found to be predominantly governed by the lava extrusion rate degree of solidi cation i e a solidus pressure the friction angle associated with the talus and lava dome viscosity

transient effects of magma ascent dynamics along a geometrically variable dome feeding conduit abstract the transient dynamics of magma ascent during dome forming eruptions were investigated and the effects of magma chamber pressure perturbations on eruption rate are illustrated the numerical model domeflow developed by the authors for this work is applied to the problem domeflow is a transient 1 5d isothermal two phase flow model of magma ascent through an axisymmetric conduit of variable radius which accounts for gas exsolution bubble growth crystallization induced by degassing permeable gas loss through overlying magma and through conduit walls as well as viscosity changes due to crystallization and degassing for runs in which chamber pressure increases the time required to reach the new steady state transition time is a complex function of the pressure perturbation while for decreasing chamber pressure transition time is a monotonic function of the magnitude of the pressure perturbation the transition to the new steady state is mainly controlled by magma compressibility travel time time required for one parcel of magma to travel from chamber to surface and the time over which the pressure perturbation occurs results of many runs 300 were analyzed using dimensional analysis to reveal a general relationship which predicts the temporal evolution of magma effusion rate for a given sudden increase in chamber pressure the product of the change in steady state extrusion rate and the time required to reach the new steady state is linearly proportional to the normalized change in chamber pressure the volume of the conduit and the ratio of top and bottom conduit radii and inversely proportional to the cubic root of volatile fraction this relationship is used to interpret observed variations in two ongoing dome building eruptions the soufriere hills volcano montserrat and merapi volcano indonesia

the effects of phase transition kinetics on subducting slabs the effects of kinetics on the non equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically a one dimensional model consisting of the kinetic equations and the heat diffusion equation with latent heat release has been constructed numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume for slow slab velocities less than 6 cm yr near equilibrium conditions are found finger like structures emanating from the phase boundaries are obtained in this regime these phase boundary prolusions may cause earthquakes for slab velocities of around 10 cm yr the metastable olivine region may be pushed down to a depth of about 600 km with a sharp phase boundary a direct transition from olivine to perovskite may be feasible

field evidence of magma mixing from microgranular enclaves hosted in palaeoproterozoic malanjkhand granitoids central india abstract the malanjkhand granitoids mg pluton about 1500 sq km occurs in the balaghat district of madhya pradesh the mg 2400 ma represent an episode of palaeoproterozoic felsic magmatism in central india and hosts potential cu mo au deposits the enclaves hosted in mg can be broadly classified into two categories microgranular enclaves dark coloured fine grained magmatic and xenoliths of country rocks the microgranular enclaves me may be rounded ellipsoidal discoid elongated lenticular or tabular and their size commonly reaches up to 2 metres across the me have sharp and in places diffuse contacts with their host granitoids the shape and size of me indicate contemporaneous flow and mingling of partly crystalline felsic mafic magmas some me exhibit dark crenulated margins giving them a pillow like form that has been attributed to undercooling of a me magma as globules intruded into a granitoid magma the presence of corroded felsic and mafic minerals xenocrysts in me is interpreted as the result of mechanical transfer during the mafic felsic magma interaction and mixing event mafic minerals biotite rim the quartz xenocrysts giving rise to ocellar texture which exhibit signatures of resorption under hybrid enclave magma conditions all these features suggest an origin for the calc alkaline intermediate granitoid magma in malanjkhand involving a magma mixing process

critical geochemical and mineralogical factors for the formation of unconformity related uranium deposits comparison between barren and mineralized systems in the athabasca basin canada the spring point barren alteration system and the maurice bay subeconomic uranium deposit situated on the northwest rim of the athabasca basin northern saskatchewan are hosted by the sandstones and conglomerates of the fair point formation of the athabasca group macroscopic observations indicate two different styles of alteration for the two systems chlorite dominated partly reduced alteration at spring point and illite dominated strongly oxidized alteration at maurice bay the paragenetic sequences developed for each area are similar but the crystal chemistry of the alteration minerals differs markedly indicating that the alteration fluids at spring point were richer in mg and ca than at maurice bay this is consistent with a predominantly basement origin for the former stable isotope analyses of clay alteration minerals indicate that in particular the chlorite related fluids from spring point have a basement signature 18 o 3 5 and d 13 whereas chlorite from maurice bay is more consistent with a mixture of basement and basinal fluids 18 o 5 2 and d 28 the absence of uranium transporting basinal fluids at the critical time when reducing basement fluids were producing chlorite alteration is likely the single most important cause for the lack of uranium mineralization at spring point

formation of the enigmatic matoush uranium deposit in the paleoprotozoic otish basin quebec canada the matoush uranium deposit is situated in the paleoproterozoic otish basin northern quebec canada and is hosted by the indicator formation sandstones its sheet like ore bodies are closely associated with the steeply dipping matoush fracture which hosts mafic dykes and minor quartz feldspar tourmaline pegmatites regional diagenesis involving oxidizing basinal fluids 2h 15 18o 8 produced mostly illite and possibly leached u from accessory phases in the indicator formation sandstones the bimodal matoush dyke intruded the indicator formation along the matoush fracture and the related metasomatism produced cr rich dravite and muscovite in both the dyke and the proximal sandstones uraninite formed when u6 in the basinal brine was reduced to u4 in contact with the mafic dyke and by fe2 in cr dravite and cr muscovite and precipitated together with eskolaite and hematite because of its unique characteristics the matoush deposit cannot be easily classified within the generally accepted classification of uranium deposits two of its main characteristics unusual reduction mechanism structural control do not correspond to the sandstone hosted group of deposits unconformity type tabular roll front in spite of uranium being derived from the otish group sandstones

the age of unconformity related uranium mineralization in the athabasca basin northern saskatchewan age data are presented for major athabasca basin uranium deposits at cigar lake cluff lake collins bay dawn lake eagle point mcarthur river midwest and rabbit lake as well as for several minor or undeveloped deposits including hughes lake and nisto the best constrained data indicate that almost all the deposits formed in a restricted time interval between about 1330 and 1380 ma this range of ages is believed to be real and not the result of uncertainties in the calculation of ages based on discordant data the one major exception is the recently discovered nias free deposit at mcarthur river for which a well determined age of 1514 18 ma 2 has been obtained even this deposit yields an age in the1330 1380 ma range for some material periods of reworking redeposition occurred at 1280 1000 575 and 225 ma these may be basin wide affecting to some degree all the deposits that we have studied other times of redeposition are less well determined but may be present as well no ages tha

geochemical variations in late stage growth of volcanic phenocrysts revealed by sims depth profiling in the petrologic and geochemical study of volcanic rocks microlites have become an important tool with which to determine magma decompression rate and ascent path prior to eruption however studies in experimental petrology and kinetic modeling indicate that growth of pre existing phenocrysts will occur over a much wider range of decompression induced undercoolings than microlite nucleation consequently we have developed a method using secondary ion mass spectrometry sims depth profiling to measure geochemical trends recorded during the final stage of phenocryst growth to test and demonstrate the new method we examined explosive and effusive eruptive products collected from soufriere hills volcano montserrat plagioclase feldspar crystals were removed from clasts of pyroclastic deposits phenocrysts were individually selected on the basis of euhedral morphology and a relatively homogeneous distribution of surface contamination such as volcanic glass crystals were cleaned embedded in in and analyzed by sims in depth profiling mode we used an o 2 primary ion beam which provides a faster sputtering rate than the typically utilized o primary beam a normal incidence electron gun is used for charge compensation ten isotopes were examined over 2 10 h periods and the resulting crater depth was determined data show variable trends in an k and fe that we interpret to represent late stage physical variations within the magma prior to eruption the continuous chronology of geochemical data obtained from the sims analyses presented here provide information with unprecedented 0 1 m resolution allowing researchers to determine physical conduit conditions during magma ascent from the chamber to eruption at the surface

formation and evolution of the cigar lake uranium deposit based on u pb and k ar isotope systematics a systematic geochronological study was conducted on the cigar lake uranium deposit u pb and pb pb systematics were applied to different types of uranium ore and various associated sheet silicates were dated by the k ar method these two approaches define a widespread retrograde metamorphic event in the basement which occurred at about 1780 ma during the hudsonian orogeny and a four stage evolutionary model for the ore deposit this model is compatible with the history of other unconformity type uranium deposits in the athabasca basin the first stage in the evolution of the cigar lake uranium deposit is marked by uraninite crystallization at 1341 17 ma which in association with arsenides and sulpharsenides constitutes the mineralization of economic interest main orebody the second stage characterized by a fe kaolinite and fe illite paragenesis occurred locally at about 900 ma the third stage involved uranium mobilization at about 325 ma and is characterized by i preferential loss of radi

new insight into explosive volcanic eruptions connecting crystal scale chemical changes with conduit scale dynamics explosive volcanic eruptions may be triggered by processes in the conduit including p t variations volatile loss and crystal growth these unobserved processes may be inferred via textural and chemical examination of eruptive products juvenile clasts were examined from vulcanian explosions of the soufriere hills volcano here we show that secondary ion mass spectrometry sims depth profiling into plagioclase phenocrysts provides a nearly continuous record of magma ascent from chamber to surface via high resolution 10 nm chemical profiles of decompression induced crystal growth changes in lithium concentration allow quantification of crystal growth rates which range from 2 10 8 mm s to 7 10 8 mm s anorthite variations suggest conduit temperature increases of 50 to 100 c assuming contemporaneous overpressures approaching 20 mpa these data demonstrate a new means of constraining conduit conditions preceding explosive volcanic eruptions

decompression induced crystallization in hydrated silica rich melts empirical models of experimental plagioclase nucleation and growth kinetics isothermal and isobaric crystallization of plagioclase in a water saturated synthetic rhyolitic melt is investigated through time series of decompression experiments the experimental variables are the rate at which samples are initially decompressed 30 150 and 1200 mpa h from 200 mpa and 875 c final pressure 25 to 160 mpa and holding time at final pressure up to 17 days through textural measurements of the crystals plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth plagioclase crystallization is markedly dependent on effective undercooling teff and holding time at crystallization pressure with teff increasing from 55 to 110 c i nucleation lag decreases from 1 2 days to 15 min ii maximum nucleation rates increase from 10 3 to 10 2 mm 2 s 1 and iii maximum growth rates decrease from 10 6 to 5x10 7 mm s 1 the initial decompression rate 30 150 and 1200 mpa h has no systematic control on crystallization at final pressure except for the 1200 mpa h series in which samples show nucleation difficulties from the experimental data of teff constrained plagioclase number density proportion and morphology we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly ascended rhyolitic melts through the determination of plagioclase liquidus curve and teff prevailing during crystallization

degassing driven crystallisation in basalts abstract syn eruptive crystallisation can drastically increase magma viscosity with profound implications for conduit dynamics lava emplacement and volcanic hazards there is growing evidence that crystallisation is not only cooling driven but can also occur almost isothermally during decompression induced degassing on ascent from depth here we review field and experimental evidence for degassing driven crystallisation in a range of magma compositions we then present new results showing for the first time experimental evidence for this process in basaltic magma our experiments use simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry tga dsc ms to monitor degassing patterns and thermal events during heating and cooling of porphyritic basaltic samples from mt etna italy the partly degassed samples which contained 0 39 0 81 wt total volatiles in the glass fraction were subjected to two cycles of heating from ambient to 1250 c on the first heating tga data show that 30 60 of the total volatiles degassed slowly at

from evaporated seawater to uranium mineralizing brines isotopic and trace element study of quartz dolomite veins in the athabasca system abstract stable isotope o h c radiogenic isotope sr nd and trace element analyses have been applied to quartz dolomite veins and their uranium u bearing fluid inclusions associated with proterozoic unconformity related uo 2 uraninite ores in the athabasca basin canada in order to trace the evolution of pristine evaporated seawater towards u mineralizing brines during their migration through sediments and basement rocks fluid inclusion data show that quartz and dolomite have precipitated from brines of comparable chemistry excepted for relatively small amounts of co 2 found in dolomite hosted fluid inclusions however 18 o values of quartz veins 18 o 11 to 18 and dolomite veins 18 o 13 to 24 clearly indicate isotopic disequilibrium between quartz and dolomite hence it is inferred that this isotopic disequilibrium primarily reflects a decrease in temperature between the quartz stage 180 c and the dolomite stage 120 c the 13 c values of co 2 dissolved in dolomite hosted fluid inclusions 13 c 30 to 4 and the 13 c values of dolomite 13 c 23 5 to 3 5 indicate that the co 2 dissolved in the mineralizing brines originated from brine graphite interactions in the basement the resulting slight increase in the fluid partial pressure of co 2 p co 2 may have triggered dolomite precipitation instead of quartz 18 o values of quartz veins and previously published 18 o values of the main alteration minerals around the u ores illite chlorite and tourmaline show that quartz and alteration minerals were isotopically equilibrated with the same fluid at 180 c the ree concentrations in dolomite produce paas normalized patterns that show some similarities with that of uo 2 and are clearly distinct from that of the other main ree bearing minerals in these environments monazite zircon and aluminum phosphate sulfate aps minerals the radiogenic isotope compositions of dolomite 87 sr 86 sr i 0 7053 to 0 7161 and e nd t 8 8 to 20 3 differ from one deposit to another reflecting both heterogeneity in the basement geology and variable preservation of the original composition of brines the previously published 87 sr 86 sr i and e nd t values of uo 2 compare with the most evolved dolomites i e dolomites precipitated from brines that exchanged the most with the basement this reinforces a close genetic link between dolomites and uo 2 deposition and implies that uo 2 deposition occurred in a cooling system during the transition from quartz to dolomite formation the 18 o and d values of the mineralizing brines 18 o 1 to 8 and d 150 to 50 are considerably shifted from that of their theoretical original values acquired during evaporation of seawater 18 o 3 and d 40 the positive 18 o shift is explained by protracted fluid rock interaction within the basin and basement rocks the negative d shift is attributed to incomplete mixing between the u mineralizing brines and low d water this low d water was likely produced during the abiogenic synthesis of bitumen by fisher tropsch like reactions involving co 2 derived from brine graphite interaction in the basement and radiolytic h 2 the resulting low d brines have been equilibrated with alteration minerals this may explain why some alteration minerals yield anomalously low d values whose significance has long been debated

crystallization history of obsidian dome inyo domes california samples obtained by u s department of energy research drilling at the 600 year old obsidian dome volcano provide the rare opportunity to examine the transition from volcanic dome to plutonic intrusion textures in a silicic magma system textures in the lavas from obsidian dome record multiple periods of crystallization initiated in response to changes in undercooling t related to variable degassing in the mag ma phenocr ysts formed first at low t a drastic increase in t related to loss of a vapor phase during initial stages of eruption caused nucleation of microlites all of the lavas thus contain phenocrysts and microlites extrusion and subsequent devitrification of the dry 0 1 wt h2o magma crystallized spherulites and fine grained rhyolite at high t a granophyric texture representing crystallization at a moderate t formed in the intrusions beneath obsidian dome textures in the intrusion apparently represent crystallization of hydrous 1 2 wt h2o rhyolitic magma at shallow depths

controls of conduit geometry and wallrock elasticity on lava dome eruptions abstract many lava dome building eruptions show periodic to complex non periodic pulsatory activity typical time scales associated with this activity range from hours to decades previous studies modelled the ascent of magma using a set of transient 1 d transport equations accounting for degassing induced crystallization kinetics gas exsolution and viscosity increase due to crystal growth these models assumed flow in a cylindrical conduit with a fixed cross section area since several observations suggest that extrusions are mainly fed by dykes with cylindrical geometries developing only at shallow levels here we generalised the model to the flow geometry represented by an elliptical dyke with major and minor semi axes changing with depth quasi static elastic deformation of the dyke is accounted by an analytical solution that couples cross section area with the magmatic overpressure the effects of the main dyke geometrical parameters and boundary conditions on the eruption dynamics were investigated the presence of a deformable dyke can lead to a more complex periodic behaviour with a wider range of time scales and cyclicity patterns with respect to a uniform cylindrical conduit there is a regime where the period of pulsations is controlled by the elasticity of the dyke and a regime where the period is controlled by the volume of the magma chamber intermediate regimes are possible periodic variations in discharge rate are also possible for both fixed pressure in dyke source region and fixed influx rate into the dyke our study emphasizes the strong non linearities and complex behaviours of lava dome eruptions from a forecasting and hazard perspective intrinsic uncertainties in governing parameters may make volcanic systems in some circumstances unpredictable on the other hand lava dome systems may also develop episodic and systematic behaviours so that behaviour becomes predictable for a while

magma mixing during the 2001 event at mount etna italy effects on the eruptive dynamics abstract during the 2001 eruptive episode three different magmas were erupted on the southern flank of mount etna volcano from distinct vent systems major and minor element chemistry of rocks and minerals shows that mixing occurred and that the mixed magma was erupted during the last eruptive phases the space time integrated analysis of the eruption supported by geophysical data together with major and trace element bulk chemistry xrf icp ms and major and trace mineral chemistry epma lam icp ms support the following model 1 trachybasaltic magma rises through a nnw sse trending structure connected to the main open conduit system 2 ascent of an amphibole bearing trachybasaltic magma from a 6 km deep eccentric reservoir through newly open n s trending fractures 3 just a few days following the eruption onset the two tectonic systems intersect at the laghetto area 4 at the laghetto vent a mixed magma is erupted mixing occurred between the amphibole bearing trachybasaltic magma and an inferred deep more basic end member the most relevant aspect in the eruptive dynamics is that the eruption of the mixed magma at the laghetto vent was highly explosive due to volatile content in the magma the gas phase formed mainly because of the decreased volatile solubility due to rapid fractures opening and increased t related to mixing and partially because of the amphibole breakdown

the 2001 2004 dome forming eruption of shiveluch volcano kamchatka observation petrological investigation and numerical modelling there have been three episodes of lava dome growth at shiveluch volcano kamchatka since the plinian explosive eruption in 1964 the episodes in 1980 1981 1993 1995 and 2001 2004 have discharged at least 0 27km 3 of silicic andesite magma a time averaged mean extrusion rate of 0 2m 3 s is thus estimated for the last 40years here the 2001 2004 activity is described and compared with the earlier episodes the recent activity involved three pulses in extrusion rate and a transition to ongoing lava extrusion estimated magma temperatures are in the range 830 to 900 c with 850 c as the best estimate using the plagioclase amphibole phenocryst assemblage and fe ti oxides melt inclusions in amphibole and plagioclase have maximum water contents of 5 1wt implying a minimum pressure of 155mpa for water saturated conditions the magma chamber depth is estimated to be about 5 6km or more a result consistent with geophysical data the thicknesses of opx mt amph reaction rims on olivine xenocrysts are used to estimate the residence time of olivine crystals in the shallow chamber in the range 2 months to 4 years suggesting replenishment of deeper magma into the shallow chamber contemporaneous with eruption the absence of decompression driven breakdown rims around amphiboles indicates ascent times of less than 7days volcanological observations of the start of the 2001 2004 episode suggest approximately 16 days for the ascent time and a conduit equivalent to a cylinder of diameter approximately 53 71m application of a conduit flow model indicates that the magma chamber was replenished during the 2001 2004 eruption consistent with the results of olivine reaction rims and that the chamber has an estimated volume of order 7km 3

magma residence and eruption at the taupo volcanic center taupo volcanic zone new zealand insights from rhyolite melts geobarometry diffusion chronometry and crystal textures the taupo volcanic center tvc taupo volcanic zone tvz new zealand is home to the two most recent silicic caldera forming eruptions in the tvz the giant 530 km3 dre oruanui supereruption and the large 35 km3 dre taupo eruption this offers a unique opportunity to investigate similarities and differences between large and supersized volumes of rhyolitic magma at the tvc and more generally we combine information from crystal textures crystal size distributions crystal compositions ti in quartz diffusion chronometry and glass compositions rhyolite melts geobarometry to investigate the crustal storage conditions longevity and eruption of these magmas results show differences between the storage depths of the oruanui and taupo magmas consistent with the differences in their glass silica contents but strikingly similar short crustal residence times for the final crystal poor eruptible magma bodies decades to centuries differences in groundmass textures between the two systems also likely reflect differences in ascent durations decompression paths and or amounts of undercooling experienced by these systems due to the differences in their storage depths finally comparing the evolution of storage depths and magma compositions at the tvc with those in a preceding central tvz ignimbrite flare up supports the idea that the modern tvc may be flaring up and highlights the evolution of magma storage depths with increasing system maturity in this volcanic region