the solubility of nb ta zr hf w in granitic melts with li and li f constraints for mineralization in rare metal granites and pegmatites tantalum mineralization is associated with granites and pegmatites that invariably are rich in lithium this in part is due to the extreme fractionation that is necessary for melts to reach saturation in either a ta or an li phase however there may also be chemical interactions in melts that explain this association experiments therefore were conducted to determine the effect of lithium on the solubility of tantalite mnta 2 o 6 in water saturated granitic melts other important elements in rare metal granites include nb zr hf and w consequently the effect of li on the solubilities of columbite mnnb 2 o 6 zircon zrsio 4 hafnon hfsio 4 and hbnerite mnwo 4 in granitic melts was also investigated the haplogranitic melt compositions used in these experiments have constant mole percent si and al and the al li na k ratio was 1 0 for all experiments i e the lithium content was the only compositional variable columbite and tantalite solubilities in the granitic liquids increase by a factor of approximately 2 to 3 with the addition of 2 wt percent li 2 o to haplogranite at 750 degrees to 1 035 degrees c 2 kbars and water saturated conditions the solubility data were extrapolated to a lower temperature more reasonable for columbite tantalite crystallization then compared to compositions representative of evolved liquids from which rare metal granites and pegmatites crystallize it is estimated that at 600 degrees c these melt compositions could be saturated in columbite but not tantalite however tantalite saturation is predicted if the melts contained less li and f therefore the genesis of tantalum mineralization may be explained by ta being retained in the melt because of high li f concentrations tantalite crystallization is delayed until an li or f or p mineral crystallizes which lowers tantalite solubility and results in a general association of ta with li in mineralized granitic rocks in contrast to columbite and tantalite the solubility of wolframite is not affected by the lithium content of the melt with up to 3 8 wt li 2 o in starting glasses and the solubilities of zircon and hafnon decrease with increasing li content of the melt one possible interpretation of the hbnerite solubility data is that the melt contains dominantly w super 6 which behaves as a network former the decrease in zircon hafnon solubility with li may reflect the lower field strength of zr super 4 and hf super 4 compared to nb super 5 and ta super 5 i e zr and hf are less able to compete with li for nonbridging oxygens whereas nb and ta are more able to compete it is also significant that the solubility of hafnon is greater than that of zircon similar to the higher solubility of tantalite compared to columbite this can explain why zr hf and nb ta ratios both decrease with fractionation

distribution of bedrock and alluvial channels in forested mountain drainage basins mountain river networks often consist of both bedrock and alluvial channels1 5 the spatial distribution of which controls several fundamental geomorphological and ecological processes6 7 the nature of river channels can influence the rates of river incision and landscape evolution1 2 as well as the stream habitat characteristics affecting species abundance and aquatic ecosystem structure8 11 studies of the factors controlling the distribution of bedrock and alluvial channels have hitherto been limited to anthropogenic badlands12 here we investigate the distribution of channel types in forested mountain drainage basins and show that the occurrence of bedrock and alluvial channels can be described by a threshold model relating local sediment transport capacity to sediment supply in addition we find that valley spanning log jams create alluvial channels hospitable to aquatic life in what would otherwise be bedrock reaches the formation of such jams depends critically on the stabilizing presence of logs derived from the largest trees in the riverside forests suggesting that management strategies that allow harvesting of such trees can have a devastating influence on alluvial habitats in mountain drainage basins

effects of large floods on sediment transport and reach morphology in the cobble bed sainte marguerite river abstract sediment transport rates were estimated for two flood events on the cobble bed sainte marguerite river in the saguenay region canada morphologic methods were used to derive one set of estimates and a combination of the meyer peter and muller equation with a dimensionless sediment transport ratio after dietrich et al nature 340 1989 215 was used to derive another set of estimates both sets of estimates give consistent results for the first event which had a decade scale return period and for the second event which was the largest flood on record and had a century scale return period the transport occurring during the second event was an order of magnitude greater than that occurring during the first event despite this disparity in the transport intensity of the two events the channel morphology remained qualitatively similar the observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s

mobility of bed material in harris creek 1 the mobility of bed material is investigated in a small gravel bed river in which the bed is weakly bimodal in texture and relatively stable the bed surface remained intact throughout the freshet and scour was limited to small areas of the bed observations of bedload caught in pit traps and displacement of magnetically tagged particles during two spring freshets permit analysis of the mobility of each size fraction fractional transport rates plot up to three orders of magnitude below the reference transport rate suggested in the literature at low flows no size fraction was fully mobile at intermediate flows shear stress 27 pa sand was fully mobile while larger material remained partially mobile at high flows 43 pa the division between the full and partial mobility regimes occurred around 16 mm which remains finer than the median size of the subsurface material the largest material scarcely moves at mean annual flood magnetic tracers confirmed results obtained from the pit traps three distinct zones of mobility can be defined in fractional transport plots partial mobility full mobility and overpassing suspended critical shear stress for incipient motion varied over an order of magnitude a qualitatively similar result for bimodal sediment was reported by wilcock and mcardell 1993 incipient motion analysis based on the largest grain observed to move yielded an upper envelope implying transport similarity for sizes found on the bed surface and a lower envelope that corresponds with our nonsimilar entrainment criterion

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small

bed load transport in managed steep gradient headwater streams of southeastern alaska 1 bed load transport was investigated in four headwater streams in southeastern alaska subjected to different management and disturbance regimes bed load yield was positively correlated to peak discharge during the fall 1999 monitoring period fine bed load materials 1 11 mm that were supplied from hillslope sources were equally mobilized during most storm events medium sized bed materials 11 200 m were only partially mobilized even during large storms whereas large particles 200 mm were immobile and often formed interlocking channel structures the transport distances of medium size materials depended on amount of channel obstructions e g woody debris and sediment supply conditions both of these factors were influenced by the occurrence of mass movement timber harvesting and the related recovery processes the highest total bed load yield was observed in a channel affected by a debris flow in 1993 total sediment yields are similar among channels with old growth clear cut logged 4 years before monitoring and young alder affected by landslides and debris flows in 1961 riparian stands by comparing the old growth and young alder channels it appears that bed load yield recovers from debris flow disturbances in about 40 years however recovery of channel conditions e g reach types and woody debris may take much longer effects of timber harvesting on bed load transport are controlled by sediment linkages between hillslopes and channels related to the occurrence of mass movement

formation of extremely f rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin granite magmas a melt fluid inclusion study quartz crystals from topaz zinnwaldite albite granites from zinnwald erzgebirge germany contain in addition to primary and secondary fluid inclusions fis abundant crystalline silicate melt inclusions mis with diameters up to 200 m these mis represent various stages of evolution of a highly evolved melt system that finally gave rise to granite related sn w mineralization the combination of special experimental techniques with confocal laser raman microprobe spectroscopy and empa permits precise measurement of elevated contents of h2o f and b in re homogenized mis the contents of h2o and f were observed to increase from 3 to 30 and 1 9 to 6 4 wt respectively during magma differentiation however there is a second mi group very rich in h2o with values up to 55 wt h2o and an f concentration of approximately 3 wt ongoing enrichment of volatiles h2o f b and cl and of cs and rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus is suggested to reflect elemental abundances in natural magmas and not boundary layer melts partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements particularly sn the coexistence of two types of mis with primary fis indicates fluid saturation early in the history of magma crystallization connected with a continuous sequestration of sn f and b the results of this study provide additional evidence for the extraordinary importance of the interplay of h2o f and b in the enrichment of sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid

the storage of bed material in mountain stream channels as assessed using optically stimulated luminescence dating abstract a detailed understanding of channel forming and maintenance processes in mountain streams requires some measurement and or prediction of bed load transport and sediment mobility traditional field based measurements of such processes are problematic because of the high formative discharges characteristic of such streams the application of optically stimulated luminescence osl dating is proposed here as a new way of determining actual residency times of fine sediments and consequently validating selected predictions for the entrainment of sediment in these streams model predictions of sediment mobility for selected step pool and plane bed channels in a mountain catchment in south eastern australia are initially calculated using equations of hydraulic competence and the one dimensional hec ras model results indicate that floods exceeding bankfull with recurrence intervals up to 13 years are competent to mobilise the maximum overlying surface grain sizes at both sites osl minimum age model results from 7 samples of well bleached quartz in the fine matrix particles indicate general agreement with selected competence equations the apparent long 100 1400 y burial age of most of the mineral quartz however suggests that competent flows are not able to flush all subsurface fine bed material the depth of maximum bed load exchange flushing was limited to twice the depth of the overlying d 90 grain size application of osl in this study provides important insight into the nature of storage and flushing of matrix material in mountain streams

strong tin enrichment in a pegmatite forming melt to investigate processes of magmatic tin enrichment and cassiterite deposition we studied the abundances of major trace and volatile elements in a large number of rehomogenized silicate melt inclusions in quartz and topaz from a pegmatite body at the ehrenfriedersdorf sn w deposit this deposit is associated with evolved variscan granites of the central erzgebirge southeast germany the melt inclusions are peraluminous the molar aluminum saturation index asi ranges from 1 15 to 2 0 and many inclusions are characterized by a very high content of fluxing components and volatiles some inclusions contain more than 20 wt of h2o f cl and p2o5 plus li as well as very high levels of sn some rare highly evolved fractions of late stage pegmatite forming liquid at ehrenfriedersdorf contained up to 7000 ppm sn the presence of hydrogen and methane in addition to water and carbon dioxide in the vapor phase of the melt inclusions suggests a very low oxygen fugacity for some fractions of magma the extreme levels of tin volatiles and fluxing components in this magma had an important influence on processes of melt movement and cassiterite precipitation melts like these that are high in volatiles and alkalis sum of li2o na2o k2o rb2o and cs2o is 8 wt have low densities 1 8 g cm3 low viscosities 10 pa s at 700 c facilitate relatively rapid diffusion of ions through melts and hence are excellent solvents for extracting and transporting ore forming elements

critical specific stream power in gravel bed rivers experiments with marked pebbles were carried out on different sized rivers of the belgian ardenne catchment areas varying from less than 1 km2 to 2700 km2 specific stream power required to cause bedload movement was evaluated and critical values were obtained three types of relationship between critical specific stream power 0 and grain size d were established the values for 0 in the largest river the ourthe were the lowest and were close to the values obtained for mountainous rivers carrying large boulders in medium sized rivers catchment area between 40 and 500 km2 the critical unit stream power was higher it is likely that it is due to the bedform s greater resistance this resistance would use up some of the energy that can cause movement and transport of bedload the amount of resistance of the bedform can be expressed as bedform shear stress determined by the relationship between grain shear stress that determines movement and transport of the bedload and the total shear stress this ratio varies between 0 4 and 0 5 in the medium sized rivers compared to 0 7 in the ourthe in headwater streams less than 20 km2 there is greater loss of energy due to bedform resistance 0 3 critical specific stream power is higher in this third type of river than in the other two

velocity characteristics along a small step pool channel this paper summarizes measurements of velocity along three reaches of a small mountain channel with step pool bedforms a one dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph measurement cross sections were located upstream from a bed step at the step lip downstream from the step and in a uniform gradient run data analyses focused on characteristics of velocity profiles and on correlations between velocity characteristics and the potential control variables bedform type reach gradient and flow depth to test the hypothesis that velocity characteristics are related to channel bedform types anova and ancova tests were performed for the average velocity and coefficient of variation of point velocity data results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge velocity became more variable as stage increased particularly at low gradient reaches with less variable bed roughness velocity profiles suggest that locations immediately downstream from bed steps are dominated by wake turbulence from mid profile shear layers locations immediately upstream from steps at step lips and in runs are dominated by bed generated turbulence adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation whereas favourable pressure gradients at steps are suppressing turbulence the bed generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake generated turbulence and form drag of step pool bedforms copyright 2000 john wiley sons ltd

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

a morpho statistical classification of mountain stream reach types in southeastern australia abstract the concepts of sediment transport capacity q c and sediment supply q s have shown promise in broadly differentiating mountain streams the important role of lithology in determining reach characteristics is also noted but as yet not fully included in existing process domain frameworks this study uses topographic and grain size surveys undertaken over 42 channel reaches with slopes between 0 01 and 0 20 m m to describe mountain stream morphology in southeastern australia reaches with two different lithologies are surveyed to specifically address the role of lithology in resultant channel morphology spatial autocorrelation analysis based on moran s i is used to detect the frequency and occurrence of in stream features such as bars and pools these data are combined with the grain size data to describe eight dominant reach morphologies most channel morphologies display broad characteristics similar to previous accounts of mountain streams although a number of intermediate morphologies affecting subtle variations in form and process where evident there was no significant longitudinal arrangement of the observed morphologies particularly with respect to catchment area and slope this is related to the strongly segmented longitudinal profiles characteristic of these streams where increasing catchment area does not result in a commensurate decrease in slope five of the eight channel morphologies identified are lithology dependent with respect to grain size and shape regime diagrams applied to quantify the physical controls on the different reach morphologies identified different q c q s domains for most morphologies however some lithology dependent morphologies could not be differentiated supporting the need to incorporate the effects of dominant grain shape or sphericity in assigning dimensionless critical shear stress values in determining q s overall the differences in mountain stream reach morphologies described in this study reflect the characteristic segmentation of the longitudinal profiles of these streams and contrasts in the supply and breakdown of parent material

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

contrasting fluid inclusion characteristics of staniferous and non staniferous pegmatites of southeast bastar central india abstract tin and rare metal bearing granitic pegmatites in the bastar malkangiri pegmatite belt of central india are hosted by metabasic and metasedimentary country rocks fluid inclusion studies were conducted in spatially associated two mica granite and the staniferous and non staniferous pegmatites to characterize the physicochemical environment of mineralization to distinguish different pegmatites in terms of their fluid characteristics and to envisage a possible genetic link between the pegmatites and spatially associated granite three different types of primary inclusions were identified the type i aqueous bi phase l v inclusions are the most abundant and ubiquitous type ii polyphase l v s inclusions are rare type iii monophase l and metastable aqueous inclusions though less abundant than type i inclusions are ubiquitous the fluid evolution trends indicate that mixing of two different fluids of contrasting salinities one of high salinity 20 30 wt nacl equivalent and another of low salinity 0 10 wt nacl equivalent was responsible for precipitation of the bulk of the cassiterite this mixing is the single most important characteristic that distinguishes the staniferous pegmatites from their non staniferous counterparts the non staniferous pegmatites on the other hand are typified by the presence either of a high saline or a low saline fluid that evolved through simple cooling the minimum pressure temperature of entrapment estimated from the intersections of the halide liquidus with the corresponding inclusion isochores of type ii inclusions range between 2 1 2 2 kb and 300 325 c the similar p t range of fluid entrapment of the staniferous and non staniferous pegmatites indicates that they were possibly emplaced within a similar physical environment type i inclusions from granite recorded only the high salinity fluid the salinity of which compares well with that of the highly saline fluid component of type i inclusions in the pegmatites this is a possible indication of a genetic link between the pegmatites and spatially associated granite

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

columbite solubility in granitic melts consequences for the enrichment and fractionation of nb and ta in the earth s crust the behaviour of niobium and tantalum in magmatic processes has been investigated by conducting mnnb2o6 and mnta2o6 solubility experiments in nominally dry to water saturated peralkaline aluminium saturation index a s i 0 64 to peraluminous a s i 1 22 granitic melts at 800 to 1035 c and 800 to 5000 bars the attainment of equilibrium is demonstrated by the concurrence of the solubility products from dissolution crystallization mn doped and nb or ta doped experiments at the same pressure and temperature the solubility products of mnnb2o6 ksp nb and mnta2o6 ksp ta at 800 c and 2 kbar both increase dramatically with alkali contents in water saturated peralkaline melts they range from 1 2 10 4 and 2 6 10 4 mol2 kg2 respectively in subaluminous melt a s i 1 02 to 202 10 4 and 255 10 4 mol2 kg2 respectively in peralkaline melt a s i 0 64 this increase from the subaluminous composition can be explained by five non bridging oxygens being required for each excess atom of nb5 or ta5 that is dissolved into the melt the ksp nb and ksp ta also increase weakly with al content in peraluminous melts ranging up to 1 7 10 4 and 4 6 10 4 mol2 kg2 respectively in the a s i 1 22 composition columbite tantalite solubilities in subaluminous and peraluminous melts a s i 1 02 and 1 22 are strongly temperature dependent increasing by a factor of 10 to 20 from 800 to 1035 c by contrast columbite tantalite solubility in the peralkaline composition a s i 0 64 is only weakly temperature dependent increasing by a factor of less than 3 over the same temperature range similarly ksp nb and ksp ta increase by more than two orders of magnitude with the first 3 wt h2o added to the a s i 1 02 and 1 22 compositions whereas there is no detectable change in solubility for the a s i 0 64 composition over the same range of water contents solubilities are only slightly dependent on pressure over the range 800 to 5000 bars the data for water saturated sub and peraluminous granites have been extrapolated to 600 c conditions at which pegmatites and highly evolved granites may crystallize using a melt concentration of 0 05 wt mno 70 to 100 ppm nb or 500 to 1400 ppm ta are required for manganocolumbite and manganotantalite saturation respectively the solubility data are also used to model the fractionation of nb and ta between rutile and silicate melts predicted rutile melt partition coefficients increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions it is demonstrated that the nb2o5 ta2o5 activity coefficient ratio in the melt phase depends on melt composition this ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compositions accordingly all the relevant accessory phases in subaluminous to peraluminous granites are predicted to incorporate nb preferentially over ta this explains the enrichment of ta over nb observed in highly fractionated granitic rocks and in the continental crust in general

channel flow competence and sediment transport in upland streams in southeast australia bedload transport data from planebed and step pool reach types are used to determine grain size transport thresholds for selected upland streams in southeast australia morphological differences between the reach types allow the effects of frictional losses from bedforms microtopography and bed packing to be incorporated into the dimensionless critical shear stress value local sediment transport data are also included in a regime model and applied to mountain streams to investigate whether empirical data improve the delineation of reach types on the basis of dimensionless discharge per unit width q and dimensionless bedload transport q b instrumented planebed and step pool sites are not competent to transport surface median grains d50s at bankfull discharge qbf application of a locally parametrized entrainment equation to the full range of reach types in the study area indicates that the majority of cascades cascade pools step pools and planebeds are also not competent at qbf and require a 10 year recurrence interval flood to mobilize their d50s consequently the hydraulic parameters of the regime diagram which assume equilibrium conditions at bankfull are ill suited to these streams and provide a poor basis of channel delineation modifying the diagram to better reflect the dominant transported bedload size equivalent to the d16 of surface sediment made only slight improvements to reach delineation and had greatest effect on the morphologies with smaller surface grain sizes such as forced pool riffles and planebeds likewise the corey shape factor was incorporated into the regime diagram as an objective method for adjusting a base dimensionless critical shear stress c50b to account for lithologically controlled grain shape on bed packing and entrainment however it too provided only minor adjustments to reach type delineation

solubility of tin tungsten and molybdenum oxides in felsic magmas saturation versus undersaturation of granitic melts in tin tungsten and molybdenum oxides is discussed on the basis of experimental data results of dry and hydrothermal experiments are evaluated under the assumption of ideal solubility of sn w and mo oxides in granitic melts a conservative interpretation arrives at concentration levels of 1000 ppm sno2 wo3 and moo3 respectively considered as the maximum solubility of these components in granitic melts at 750 c 800 c such values are never reached in natural granites unaffected by hydrothermal alteration and therefore even highly evolved granites are expected to be undersaturated in these metals consequently cassiterite and scheelite are neither common liquidus minerals of ore bearing granites nor restite minerals from partial melting events

phase shifts in shear stress as an explanation for the maintenance of pool riffle sequences the stability of the pool rif e sequence is one of the most fundamental features of alluvial streams for several decades the process of velocity or shear stress reversal has been proposed as an explanation for an increase in the amplitude of pool rif e sequence bars during high ows offsetting gradual scour of rif es and deposition in pools during low ows despite several attempts reversal has rarely been recorded in eld measurements we propose that instead of being reversed maxima and minima in shear stress are phase shifted with respect to the pool rif e sequence bedform pro le so that maximum shear stress occurs upstream of rif e crests at high ow and downstream at low ow such phase shifts produce gradients of shear stress that explain rif e deposition and pool scour at high ow in accord with sediment continuity the proposal is supported by results of a one dimensional hydraulic model applied to the surveyed bathymetry of a pool rif e sequence in a straight reach of a gravel bed river in the sequence studied the upstream phase shift in shear stress at high ow was associated with variations in channel width with width minima occurring upstream of rif e crests approximately coincident with shear stress maxima and width maxima occurring downstream of rif e crests assuming that the width variation is itself the result of ow de ection by rif e crests at low ow and associated bank toe scour downstream low and high ow can be seen to have complementary roles in maintaining alluvial pool rif e sequences copyright 2004 john wiley sons ltd

variation in the reference shields stress for bed load transport in gravel bed streams and rivers 1 the present study examines variations in the reference shear stress for bed load transport r using coupled measurements of flow and bed load transport in 45 gravel bed streams and rivers the study streams encompass a wide range in bank full discharge 1 2600 m3 s average channel gradient 0 0003 0 05 and median surface grain size 0 027 0 21 m a bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate w versus the reach average dimensionless shear stress the reference dimensionless shear stress r was then estimated by selecting the value of corresponding to a reference transport rate of w 0 002 the results indicate that the discharge corresponding to r averages 67 of the bank full discharge with the variation independent of reach scale morphologic and sediment properties however values of r increase systematically with average channel gradient ranging from 0 025 0 035 at sites with slopes of 0 001 0 006 to values greater than 0 10 at sites with slopes greater than 0 02 a corresponding relation for the bank full dimensionless shear stress bf formulated with data from 159 sites in north america and england mirrors the relation between r and channel gradient suggesting that the bank full channel geometry of gravel and cobble bedded streams is adjusted to a relatively constant excess shear stress bf r across a wide range of slopes

coarse sediment transport in mountain streams in colorado and wyoming usa since the early 1990s us forest service researchers have made thousands of bedload measurements in steep coarse grained channels in colorado and wyoming usa in this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes including step pool plane bed pool riffle and near braided channels this effort builds upon previous work where we applied a piecewise regression model to 1 relate flow to rates of bedload transport and 2 define phases of transport in coarse grained channels earlier the model was tested using bedload data from eight sites on the fraser experimental forest near fraser colorado the analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied the earlier results were however derived from data collected at sites that for the most part have quite similar geology and runoff regimes in this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions the results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs the transition from primarily low rates of sand transport phase i to higher rates of sand and coarse gravel transport phase ii occurs on average at about 80 per cent of the bankfull 1 5 year return interval discharge a comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase i transport moreover the movement and capture of the d16 to d25 grain size of the bed surface seems to correspond with the onset of phase ii transport particularly in systems with largely static channel surfaces however while there were many similarities in observed patterns of bedload transport at the 19 studied sites each had its own bedload signal in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system published in 2005 by john wiley sons ltd

vapor undersaturated experiments with macusani glass h 2 o at 200 mpa and the internal differentiation of granitic pegmatites vapor undersaturated fractional crystallization experiments with macusani glass macusanite a peraluminous rhyolite obsidian at 200 mpa yield mineralogical fabrics and zonation and melt fractionation trends that closely resemble those found in zoned granitic pegmatites and other granitoids of comparable composition typically peraluminous li be ta rich deposits the zonation from the edge of charges inward is characterized by 1 fine grained sodic feldspar quartz border zones 2 a fringe of very coarse grained graphic quartz feldspar intergrowths that flair radially toward melt and terminate with nearly monophase k feldspar 3 cores of very coarse grained nearly monominerallic quartz or virgilite lialsi5o12 mica and 4 late stage fine grained albite mica intergrowths that are deposited from alkaline na rich interstitial melt at vapor saturation similar experimental products have been observed in compositionally simpler less evolved systems liquid lines of descent from initially h2o undersaturated runs are marked by a decrease in sio2 and increases in na k b p f h2o and a variety of trace lithophile cations these trends are believed to be governed by three factors 1 disequilibrium growth of feldspars quartz via metastable supersaturation 2 fractionation of melt toward sio2 depleted na rich compositions due to increases in b p and f and 3 changes in nucleation and growth rates mostly as a function of the h2o content of melt x w m in contrast experiments that are cooled below the liquidus from the field of melt aqueous vapor london et al 1988 fail to replicate pegmatitic characteristics in most respects on the basis of these and other experiments we suggest that the formation of pegmatite fabrics stems primarily from fractional crystallization in volatile rich melts and that enrichments in normally trace lithophile elements result from melt differentiation trends toward increasingly alkaline silica depleted compositions although vapor saturation at near solidus and subsolidus conditions may promote extensive recrystallization an aqueous vapor phase does not appear to be necessary for the generation of most of the salient characteristics of pegmatites