sims zircon u pb geochronology and sr nd isotopes of ni cu bearing mafic ultramafic intrusions in eastern tianshan and beishan in correlation with flood basalts in tarim basin nw china constraints on a ca 280 ma mantle plume zircon sims u pb dating of the poshi hongshishan bijiashan and huangshan ni cu bearing and xiangshan ni cu ti fe bearing mafic ultramafic intrusions in the eastern tianshan and beishan rift yields a relatively restricted range of 278 6 ma to 284 0 ma the histogram of compiled age data of basalts in the tarim basin and mafic ultramafic intrusions in the eastern tianshan and beishan rift has a peak of 280 ma which probably represents the time of mantle plume activity the basalts have lower end t values in the range of 9 2 1 7 and mg of 2 wt indicating that they were generated directly from a peripheral zone of the mantle plume by low degree of melting the mafic ultramafic intrusions have higher end t of 1 3 11 2 and mg of 33 90 and lower tio2

the occurrence and significance of biogenic opal in the regolith abstract biogenic opal produced by vascular plants diatoms and siliceous sponges have been found in soils and terrestrial sediments of all continents except antarctica since the middle of the 19th century the opal particles range in size from fine silt to fine sand almost all soils contain detectable opal up to levels of 2 3 and a significant number contain values in excess of 5 even higher values have been found from soils and sediments of all continents in a wide range of soil types the most important factor is poor soil drainage and seasonal to permanent water logging this encourages the proliferation of silica producing organisms such conditions have been found in the soils and aquatic sediments of the monsoonal tropics tropical rain forests temperate forests tropical savanna tropical islands semi arid grasslands and savanna and temperate woodland and grassland the presence of a volcanic substrate also appears favourable in some cases but is not necessary biogenic opal preferentially collects in the a horizon of soils and to a lesser extent in the b horizon this preferential distribution facilitates identification of palaeosols in stacked soil sequences biogenic opal is also a component of windblown dust even in arid environments biogenic opal is significant to regolith processes in a number of ways firstly as in the case in marine environments it is likely to be important in silica cycling and storage because of its greater lability compared to quartz secondly dissolution and reprecipitation of opal a as opal ct or micro quartz may play a role in cementation and silicification of regolith to form silica hardpans and silcrete thirdly the organisms that form biogenic opal can have considerable palaeoenvironmental significance and be valuable in reconstructing regolith evolution finally some forms of biogenic silica in particular sponge spicules can present a health hazard their high abundance in some soils and sediments needs to be considered when assessing the health implications of airborne dust

biogeomorphology revisited looking towards the future although biogeomorphological research is well established with many studies on a range of two way interrelations between organisms and geomorphology in different environments there is little consensus over what constitutes biogeomorphology why it might be useful and where it is heading starting with definitions of core biogeomorphic processes we consider the need for future biogeomorphological studies to evaluate the crucial links between bioprocesses biological community dynamics and inorganic earth surface processes five key applications of biogeomorphological research are identified the roles of organisms in environmental reconstruction trace fossil analysis extraterrestrial geomorphology environmental engineering and the built environment some key research directions and methodological challenges for future biogeomorphological research include expanding the spatial and temporal coverage of datasets investigating the role of bioprocesses in landform development tackling scale issues investigating the relevance of nonlinear dynamical ideas to biogeomorphology and developing better sampling and monitoring techniques for bioprocesses

liquid immiscibility and the evolution of basaltic magma this experimental study examines relationships between alternative evolution paths of basaltic liquids the so called bowen and fenner trends and silicate liquid immiscibility synthetic analogues of natural immiscible systems exhibited in volcanic glasses and melt inclusions were used as starting mixtures conventional quench experiments in 1atm gas mixing furnaces proved unable to reproduce unmixing of ferrobasaltic melts yielding instead either turbid opalescent glasses or crystallization of tridymite and pyroxenes in contrast experiments involving in situ high temperature centrifugation at 1000g g 1 4 9 8 m s did yield macroscopic unmixing and phase separation centrifugation for 3 4 h was insufficient to complete phase segregation and resulted in sub micron immiscible emulsions in quenched glasses for a model liquid composition of the middle zone of the skaergaard intrusion at super liquidus temperatures of 1110 11208c centrifugation produced a thin silicic layer 64 5 wt sio2 and 7 4 wt feo at the top of the main fe rich glass 46 wt sio2 and 21 wt feo the divergent compositions at the top and bottom were shown in a series of static runs to crystallize very similar crystal assemblages of plagioclase pyroxene olivine and fe ti oxides we infer from these results that unmixing of complex aluminosilicate liquids may be seriously kinetically hampered presumably by a nucleation barrier and thus conventional static experiments may not correctly reproduce it in the light of our centrifuge experiments immiscibility in the skaergaard intrusion could have started already at the transition from the lower to the middle zone thus magma unmixing might be an important factor in the development of the fe enrichment trend documented in the cumulates of the skaergaard layered series

geological characteristics of tulargen magmatic cu ni co deposit in eastern xinjiang and its exploration direction the tulargen cu ni deposit a great breakthrough in cu ni exploration in xinjiang in the past ten years is obviously controlled by secondary fractures in the kangguer huangshan regional ductile sheer zone similar to things of the known huangshan and xiangshan cu ni deposits ore bodies are hosted in ultramafic complexes which consist of hornblende peridotite pyroxene peridotite hornblende pyroxenite and gabbro the proved nickel 125000 tons and cobalt reserves 10 000 tons have reached large size and copper reserves have reached medium size the richest massive ore body with an average grade of ni 4 56 and cu 2 52 can reach 37 98 m in thickness in no 1503 drill hole the ore district remains under prospecting now the deposit was formed through deep seated magma segregation and is characterized by bulk mineralization it has experienced a single intrusion with multiple pulsant inburst the m f values are between 3 1 and 4 8 falling into the category of ferrous ultramafic rocks the intrusion has low ti total alkali and al2o3 and contains high sulfur usually higher than 1 suggesting a good ore forming potential ore minerals consist of pyrrhotite pyrite pentlandite chalcopyrite and violarite ni and co elements occur as isolated sulfides and sulfur arsenic mineral components geochemical exploration reveals that the deposit is located in a cu ni co anomaly area with the ni peak value of 133 10 6 and cu value of 83 10 6 geophysical characteristics of this district show that high gravity high polarization and high magnetic anomalies are associated with low electric resistance the exploration direction is also pointed out in this paper new ore bearing ultramafic intrusions will be possibly found along the central weak shear zone between the strong shear zones on both sides and the southwestern extension of no i intrusion with well developed hornblende peridotite is the pipe center for magma upwelling and may be a mineralization enrichment center

the cortlandt complex evidence for large scale liquid immiscibility involving granodiorite and diorite magmas abstract granodiorite and diorite plutons of the rosetown complex n y which are associated with the nearby cortlandt complex have chemical and textural characteristics indicating that large scale liquid immiscibility played a major role in their petrogenesis rare earth element zirconium niobium and phosphorus abundances are much greater in the diorite precluding the possibility that the rosetown diorite and granodiorite are related by fractional crystallization the trace element data also eliminate the possibility that the granodiorite represents 1 a partial melt of crustal rocks including basalt 2 a granitic cumulate or 3 a residue from an aqueous fluid derived either from a silicate melt or crustal rocks liquid immiscibility appears to be viable model for the origin of the rosetown granodiorite and iron rich diorite this model is supported by the following 1 the major element compositions occur in a two liquid field on a greig diagram 2 both bodies have similar sr isotope compositions 3 common phases in the two rock types have overlapping compositions 4 the major and trace element data of the diorite and granodiorite are similar to the experimentally determined partition data of immiscible liquid pairs and 5 possible ocelli of iron rich diorite are found in the granodiorite

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

assessment of the 1 na2co3 technique to quantify the phytolith pool amorphous silica asi pool and fluxes have become parameters of a growing interest in the understanding of the biogeochemical cycle of si and the modeling of anthropogenic impacts extraction by 1 na2co3 followed by a correction for crystalline si demaster 1981 has recently become widely used and is proposed as the standard technique for quantifying amorphous silica asi pools in soil and continental aquatic environments however the 1 na2co3 method was developed to quantify diatom frustules and not phytoliths phsi which constitute the most common fraction of asi in soils the aim of this study was to assess the 1 na2co3 method for fresh and aged phytoliths we founded a significant positive correlation between si extracted by the 1 na2co3 method and si extracted by other standard procedures guntzer et al 2010 for various shoot samples elm horsetail fern and larch because the si shoot samples are mainly composed of phytoliths we concluded that the 1 na2co3 method was well adapted for the determination of phsi of fresh phytoliths containing less than 70 mg si g 1 to assess the 1 na2co3 method for phsi of aged phytoliths we used artificial mixtures of soil phytoliths from la reunion meunier et at 1999 and quartz our results showed that the 1 na2co3 method underestimated phsi for aged phytoliths by a factor of 3 7 combining the 1 na2co3 method and a physical method of extraction using samples of various origins showed a dispersion of the data that can be explained by a combination of several factors including loss during physical extraction and the presence of resistant phytoliths in the absence of standard technique we recommend to check the nature of the particles using physical extraction in combination with the 1 na2co3 in order to provide a more careful analysis of asi c 2013 elsevier b v all rights reserved

a comparison of stream bank erosion processes on forested and moorland streams in the balquhidder catchments central scotland stream bank erosion rates measured over a two year period on a moorland and a forested stream in the institute of hydrology s balquhidder paired catchments in central scotland were compared bank erosion rates are generally higher on the mainstream of the moorland catchment and highest in wintger on both streams bank erosion is correlated with the incidence of frost minimum temperatures measured on stream banks of the forested stream were an average of 3 7 c higher than on stream banks both outside the forest and on the moorland stream this makes the incidence of frost on forested stream banks half as frequent volumes of material eroded from the mainstreams were combined with bulk density measurements and it is estimated that erosion of the mainstream banks is contributing 1 5 and 7 3 per cent of the sediment yield of the forested and moorland catchments respectively analysis of the vertical distribution of erosion on the banks of both streams suggests an undercutting mechanism which is more pronounced in the moorland stream the influence of trees on bank erosion and possible implications for the management of forest streams are discussed 1997 by john wiley sons ltd

understanding seismic heterogeneities in the lower mantle beneath the americas from seismic tomography and plate tectonic history we combine results from seismic tomography and plate motion history to investigate slabs of subducted lithosphere in the lower mantle beneath the americas using broadband waveform cross correlation we measured 37 000 differential p and s traveltimes 2000 pcp p and scs s times along a wide corridor from alaska to south america we invert the data simultaneously to obtain p and s wave velocity models we interpret slab structures and unravel subduction history by comparing our v s tomographic images with reconstructed plate motion from present day up to 120 myr convergence of the pacific with respect to the americas is computed using either 1 the pacific and indo atlantic hot spot reference frames or 2 the plate circuit passing through antarctica around 800 km depth four distinctive fast anomalies can be associated with subduction of the nazca cocos and juan de fuca plates beneath south central and north america respectively and of the pacific plate beneath the aleutian island arc the large fast anomalies in the lowermost mantle which are most pronounced in the s wave models can be associated with late cretaceous subduction of the farallon plate beneath the americas near 2000 km depth the images record the post 80 myr fragmentation of the proto farallon plate into the kula plate in the north and the farallon plate in the northeast near 1000 km depth we infer separate fast anomalies interpreted as the kula pacific juan de fuca and farallon slabs this interpretation is consistent with the volume and length of slabs estimated from the tomographic images and the plate history reconstruction

u pb ages and hf o isotopes of zircons from late paleozoic mafic ultramafic units in the southern central asian orogenic belt tectonic implications and evidence for an early permian mantle plume abstract u pb age and hf o isotope data of zircons from the mafic ultramafic complexes and their related rhyolites and granites in the eastern tianshan and beishan regions were obtained by sims and la icpms the hf o isotopic composition and hf model age of the zircons are closely related to their tectonic setting and formation age the zircons from the mafic ultramafic complexes in the bogeda haerlike and jueluotage belts have higher ehf t 8 17 and lower 18o 4 6 whereas most of the zircons in the gabbros granites and rhyolite from the middle tianshan massif and beishan rift display lower ehf t 0 8 and higher 18o 5 8 the positive ehf t and relatively lower 18o values suggest that these mafic ultramafic complexes were derived from depleted mantle which was subjected to subduction related modification processes the hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the central asian orogenic belt during the paleozoic the hf model ages of the analyzed zircons together with the regional geology suggest that the beishan area had a northward subduction possibly from ca 900 ma to ca 400 ma whereas the eastern tianshan had a south directed subduction most likely from ca 600 ma to ca 310 ma additionally zircons with ca 280 ma u pb ages display wider and more scattered ehf t and 18o variations than the relatively older and younger ones which further support the early permian mantle plume model

phytolith transport in soil a field study using fluorescent labelling abstract soil phytoliths have been widely applied to reconstruct vegetational history to date however the transport behaviour of phytoliths in soil is poorly understood causing uncertainties in the interpretation of phytolith data the present study was therefore designed 1 to determine transport rates of phytoliths in loamy and sandy soils under field conditions and 2 to elucidate the effect of phytolith size and shape on their transport behaviour in sandy and loamy soils for this purpose we adopted a fluorescent labelling technique from veterinary science the phytoliths were extracted from common reed phragmites australis labelled with the fluorescent dye fluorescein isothiocyanate and applied to a loamy sand soil haplic cambisol and a silty loam soil stagnic luvisol in southern germany one year after application the soils were sampled to analyse phytolith distribution with soil depth the weighted mean transport distance of phytoliths after one year was 3 99 1 21 cm for the cambisol and 3 86 0 56 cm for the luvisol phytolith size significantly affected transport behaviour indicating a preferential translocation of small sized phytoliths our study provides direct evidence for a significant downward mobility of phytoliths in sandy and loamy soils under natural conditions this should be taken into account when using phytoliths as palaeoenvironmental tracers quantifying phytoliths in soil with fluorescent labelling makes it possible to identify artificially applied phytoliths without using the conventional extraction method

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

the influence of riparian vegetation on stream width eastern pennsylvania usa we surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches while deposition is localized on active floodplains on the insides of bends our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches two dimensionless parameters and e explain our observations represents the influence of grassy vegetation on rates of active floodplain deposition it is 5 times higher in nonforested reaches than in forested reaches e is proportional to rates of cutbank migration it is 3 times higher in nonforested reaches than in forested reaches differences in width between forested and nonforested reaches are proportional to e in forested reaches channels are wide with banks that are difficult to erode dense tree roots create a low value of e and the channel migrates slowly e is high however because is very low shade from trees inhibits the growth of grass on active floodplains in nonforested reaches channels are narrow with banks that are easy to erode e is high and the channel migrates rapidly e is low however due to a very large value of grass grows readily on nonforested convex bank floodplains thus differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation mediated rates of deposition on active flood plains these results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes rather than focusing on these processes in isolation from one another

woody vegetation and channel morphogenesis in low gradient gravel bed streams in the ozark plateaus missouri and arkansas woody vegetation affects channel morphogenesis in ozark streams of missouri and arkansas by increasing local roughness increasing bank strength providing sedimentation sites and creating obstructions to flow variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area in upstream reaches streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees in downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions at drainage areas of greater than 100 200 km2 ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates referred to as disturbance reaches and stable reaches respectively whereas stable reaches can develop stable forested floodplains if they are not farmed disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes disturbance reaches can be subdivided into low gradient and high gradient longitudinal zones low energy zones are characterized by incremental unidirectional lateral channel migration and deposition of gravel and sand bars the bars are characterized by prominent bands of woody vegetation and ridge and swale topography channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload transporting events the same floods caused up to 20 m of erosion of adjacent cutbanks scoured non vegetated areas between vegetation bands and increased thalweg depth and definition in high energy or riffle zones channel movement is dominantly by avulsion in these zones vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them woody vegetation on islands creates steep root defended banks that contribute to narrow channels with high velocities calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age mainly through decreasing stem density if all other factors remain constant the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age

copper solubility in a basaltic melt and sulfide liquid silicate melt partition coefficients of cu and fe the solubility of copper in a sulfur saturated basaltic melt has been determined at 1245 c as a function of fo2 and fs2 copper solubilities at log fo2 values between 8 and 11 fall into two distinct populations as a function of fs2 at log fs2 values 1 65 sulfide liquid that coexists with the basaltic glass quenches to sulfur poor bornite solid solution at log fs2 values in excess of 1 65 the sulfide liquid quenches to a complex intergrowth of sulfur rich bornite and intermediate solid solution copper solubilities in the low fs2 population range from 594 to 1550 ppm whereas those in the high fs2 population range from 80 to 768 ppm sulfide liquid silicate liquid partition coefficients d for cu and fe range from 480 to 1303 and 0 7 to 13 6 respectively metal sulfur complexing in the silicate liquid is shown to be insignificant relative to metal oxide complexing for fe but permissible for cu at high fs2 values on log dfe sulfide silicate and log dcu sulfide silicate vs log fs2 log fo2 diagrams both fs2 populations show distinct but parallel trends the observation of two d values for any fs2 fo2 ratio indicates nonideal mixing of species involved in the exchange reaction the two distinct trends observed for both cu and fe are thought to be due to variations in activity coefficient ratios e g feo fes and cuo0 5 cus0 5 results of the experiments suggest that accurate assessments of fs2 fo2 ratios are required for the successful numerical modeling of processes such as the partial melting of sulfide bearing mantle and the crystallization of sulfide bearing magmas as well as the interpretation of sulfide mineralogical zoning in addition the experiments provide evidence for oxide or oxy sulfide complexing for cu in silicate magmas and suggest that the introduction of externally derived sulfur to mafic magma may be an important process for the formation of cu rich disseminated magmatic sulfide ore deposits

subduction induced mantle heterogeneity beneath eastern tianshan and beishan insights from nd sr hf o isotopic mapping of late paleozoic mafic ultramafic complexes abstract extensive late paleozoic mafic ultramafic complexes in the eastern tianshan and beishan regions in the southern central asian orogenic belt caob represent an ideal opportunity to assess the characteristics of the mantle in this area whole rock nd sr and zircon hf o isotopic data from the mafic ultramafic complexes of the jueluotage belt middle tianshan massif and beishan rift were compiled for isotopic mapping end t and 87sr 86sr i values range from 5 5 to 9 5 and 0 702 to 0 706 respectively in the jueluotage belt and from 2 to 11 and 0 703 to 0 710 respectively in the middle tianshan and beishan zircon ehf t and 18o values are in the range of 6 0 17 0 and 4 14 8 00 respectively in the jueluotage belt 1 8 and 5 17 6 30 respectively in the middle tianshan and show large variations of 1 17 and 4 12 respectively in the beishan spatially the 87sr 86sr i ratios of mafic ultramafic complexes increase from the jueluotage belt and beishan to the middle tianshan while the end t values show an opposite trend and reflect a mixing character between depleted mantle and em ii reservoirs zircon ehf t and 18o values exhibit an apparent southward decreasing and increasing trends respectively from the jueluotage belt through middle tianshan to beishan rift their corresponding hf model ages increase continuously from north to south these observations demonstrate that the mantle sources beneath the eastern tianshan and beishan are compositionally heterogeneous which presumably result from significant and variable degrees of subduction related modification we therefore suggest that the late paleozoic subcontinental lithospheric mantle beneath the jueluotage belt was modified by slab derived melts and fluids during the subduction of the junggar ocean whereas the mantle beneath the beishan rift was influenced mainly by slab derived fluids during the subduction of the south tianshan ocean the unique similarities of the middle tianshan mafic ultramafic complexes reflect a two stage modification of their mantle source namely the sequential subduction of the early south tianshan ocean followed by subduction of the junggar ocean thus the subduction induced mantle heterogeneity beneath the caob is probably regional in scope

chalcophile elemental compositions of morbs from the ultraslow spreading southwest indian ridge and controls of lithospheric structure on s saturated differentiation article i nfo sulfide globules in mid ocean ridge basalts morbs from the ultra slow spreading southwest indian ridge swir are present in olivine crystals and in the matrix glass indicating s saturated fractionation during mag matic evolution most of the sulfides were in equilibrium with the magmas based on the ni contents of the sul fides the major element trends of morbs from zone c 48 51 e of the swir define two distinct liquid lines of descent lld including one which is composed of morbs depleted in hree the morbs have variable pge con centrations with pd contents ranging from 0 056 to 1 572 ppb they are pge depleted relative to ni and cu with high cu pd ratios 55 000 1 785 000 quantitative modeling using pd vs ni correlations yields relatively low amounts of segregated sulfides for both groups assuming complete equilibration between the sulfides and the melt however the correlation of calculated sulfur contents with mgo suggests much higher amounts of segre gated sulfides indicating that the segregated sulfides are incompletely equilibrated with the magma there are no obvious correlations between pge concentrations and spreading rates for morbs globally morbs from the swir show slowly decreasing pge contents during magmatic evolution but exhibit higher sulfide seg regation rates compared to morbs from faster spreading ridges largely due to the poor equilibration between the segregated sulfides and the magma such incomplete equilibration could result from thicker lithosphere and an absence of steady magma chambers and conduits beneath ultra slow spreading ridges

platinum group element geochemistry of cu ni mineralization in the basal zone of the babbitt deposit duluth complex minnesota the babbitt cu ni sulfide deposit located in the duluth complex of minnesota contains associated low grade platinum group element pge mineralization interelement correlations show that for both massive and disseminated ore zones rh ru os re and ir are positively correlated to s abundance and suggest that they have behaved as immobile elements however pt pd and au are strongly decoupled from the rh ru os re ir group and show no significant correlation with s cu or ni a pge enriched zone up to 3 000 ppb pt and 1 100 ppb pd is located laterally adjacent to the highest grade cu ni ore the pge enriched zone is characterized by abundant bornite lower sulfide delta 34 s values 1 4 to 8 9ppm than those from other sulfide bearing zones at babbitt and oxide assemblages that include cr bearing hercynite and magnetite rich spinels as well as ilmenite sperrylite ptas 2 is the only discrete pge mineral identified in the babbitt deposit synchrotron x ray fluorescence microprobe analyses indicate that some rocks with a high bulk pge content are characterized by ppm quantities 16 132 of pd in cubanite chalcopyrite maucherite and mackinawire although cl contents of biotite and apatite from mineralized units range from 0 10 to 0 71 and 0 87 to 1 43 wt percent respectively no direct relationship between pge enrichment and cl content of volatile rich phases has been observed geochemical data suggest that both magmatic and hydrothermal processes have been important in the localization of pge at the babbitt deposit the distribution of rh ru os re and ir reflects magmatic concentration processes whereas pt pd and au have been locally mobilized and redistributed during hydrothermal alteration

stability of phytoliths in the archaeological record a dissolution study of modern and fossil phytoliths opaline phytoliths are important microfossils used in archaeological and ecological research relatively little is known about the stability of phytoliths after burial under alkaline ph conditions they can dissolve and mechanical disturbances can cause a loss of their more delicate appendages here we present an experimental study of phytolith stability combination of solubility and abrasion modern and fossil phytoliths were extracted from wheat using new methods to minimize dissolution and by burning in an oven these assemblages were placed in a solution buffered to ph 10 and maintained under constant temperature and shaking conditions the silicon concentrations in the solution were monitored once a week for 5 weeks the phytolith morphologies in each assemblage were determined at the outset of the experiment and after 5 weeks the results show that there are differences in stability between various assemblages modern inflorescence wheat phytolith assemblages are more unstable than those from leaves stems burnt assemblages are less stable than unburnt assemblages and a fossil phytolith assemblage about 3000 years old is more stable than the modern wheat assemblages the results also show that individual phytolith morphotypes have different stabilities and as a result of dissolution and abrasion some morphotypes may resemble others this study further shows that archaeological and or paleo environmental interpretation of phytolith assemblages may change with the assemblage s state of preservation

segregation and fractionation of magmatic ni cu pge sulfides in the western jinchuan intrusion northwestern china insights from platinum group element geochemistry the jinchuan ni cu pge deposit is one of the world s largest magmatic sulfide deposits recent studies have suggested that the jinchuan intrusion originally comprised two individual intrusions the eastern and the western intrusions the western intrusion consists of an upper and a lower unit the upper unit is largely free of sulfides with weakly disseminated sulfides only at the base of the unit these sulfides are characterized by moderate cu pd ratios of 5 500 to 14 900 and relatively high pge tenors pge in 100 sulfides in the range of 510 to 1 000 ppb irsul 420 to 1 200 ppb rusul and 2 900 to 8 000 ppb pdsul disseminated net textured and minor massive sulfides in the lower unit comprise the no 24 orebody the third largest at jinchuan the disseminated ores have remarkably high cu pd ratios 24 200 85 600 and low pge tenors in the range of 120 to 560 ppb irsul 100 to 480 ppb rusul and 430 to 3 600 ppb pdsul the net textured and massive ores contain high ipge contents 240 820 ppb irsul and 170 630 ppb rusul and ni cu ratios 1 7 11 3 low pd ru ratios 0 53 3 1 and exhibit significant pt depletion 300 ppb ptsul additionally small cu rich orebodies occur at the base of the no 24 orebody or in small intrusions within footwall schists marbles and granites beneath the western intrusion the cu rich sulfides have very low ipge tenors 1 9 16 ppb irsul and 6 4 21 ppb rusul and ni cu ratios 0 3 2 8 high pd ru ratios 37 3 378 and highly variable ptsul between 43 and 11 000 ppb the pge geochemistry and model calculations indicate that the disseminated sulfides of the upper and lower units segregated from pge undepleted and depleted parental magmas respectively the pge depleted magma had undergone a minor amount of earlier sulfide liquid extraction at depth furthermore this pge depleted parental magma had slightly higher pge contents than the magma from which the sulfides of the nos 1 and 2 orebodies in the eastern intrusion segregated the significant variations in base and precious metal ratios of the net textured and massive sulfide ores are attributed to fractionation of the sulfide melts that formed these ores this led to both migration of the residual cu rich sulfide melts away from the early formed monosulfide solid solution mss to form cu rich ores as well as back intrusion of the cu rich ores into the mss the negative pt anomalies of the net textured and massive sulfides resulted from preferential extraction of pt into the cu rich residual melts due to the presence of as te and bi

crustal contamination and sulfide immiscibility history of the permian huangshannan magmatic ni cu sulfide deposit east tianshan nw china abstract the huangshannan mafic ultramafic intrusion is a permian ni cu sulfide bearing intrusion in the southern margin of the central asian orogenic belt the intrusion consists of an ultramafic unit which is composed of lherzolite and olivine websterite and a mafic unit which is composed of olivine gabbronorite gabbronorite and leuco gabbronorite this intrusion was formed by two separate pulses of magma a more primitive magma for the early ultramafic unit and a more evolved magma for the late mafic unit u pb isotope geochronology of zircon from the mafic unit yields an age of 278 2 ma according to its olivine and cr rich spinel compositions the estimated parental magma of lherzolite for the huangshannan intrusion has 12 4 wt mgo indicating picritic affinity fractional crystallization modeling results and the presence of rounded sulfide inclusions in an olivine crystal fo 86 7 indicate that sulfide immiscibility was achieved at the beginning of olivine fractionation co magmatic zircon crystals from gabbronorite have a 18o value close to 6 5 which is 1 2 higher than the typical mantle value and suggests significant crustal contamination 20 the positive ehf t values of co magmatic zircon which vary from 9 2 to 15 3 and positive whole rock end t values which vary from 4 7 to 7 8 also indicate that the parental magma was derived from a depleted mantle source and contaminated by 5 20 juvenile arc crust and then by 5 upper crustal materials however modeling results of sulfur content at sulfide saturation reveal that such a large amount of crustal contamination is not sufficient to trigger sulfide saturation in the parental magma which strongly suggests that external sulfur addition probably during contamination has played a critical role in causing sulfide immiscibility furthermore the arc magmatism geochemical signatures of the huangshannan intrusion such as significant nb and ta depletion relative to la and low ca contents in olivine are interpreted as the result of slab derived fluid modification during previous subduction we speculate that the contamination of juvenile arc crust may have occurred in the lower crust because of picritic magma underplating and that partial melting in the mantle was triggered by the impingement of a mantle plume or lithosphere delamination in a post subduction environment

regolith mass balance inferred from combined mineralogical geochemical and geophysical studies mule hole gneissic watershed south india the aim of this study is to propose a method to assess the long term chemical weathering mass balance for a regolith developed on a heterogeneous silicate substratum at the small experimental watershed scale by adopting a combined approach of geophysics geochemistry and mineralogy we initiated in 2003 a study of the steep climatic gradient and associated geomorphologic features of the edge of the rifted continental passive margin of the karnataka plateau peninsular india in the transition sub humid zone of this climatic gradient we have studied the pristine forested small watershed of mule hole 4 3 km2 mainly developed on gneissic substratum mineralogical geochemical and geophysical investigations were carried out i in characteristic red soil profiles and ii in boreholes up to 60 m deep in order to take into account the effect of the weathering mantle roots in addition 12 electrical resistivity tomography profiles ert with an investigation depth of 30 m were generated at the watershed scale to spatially characterize the information gathered in boreholes and soil profiles the location of the ert profiles is based on a previous electromagnetic survey with an investigation depth of about 6 m the soil cover thickness was inferred from the electromagnetic survey combined with a geological pedological survey taking into account the parent rock heterogeneity the degree of weathering of each of the regolith samples has been defined using both the mineralogical composition and the geochemical indices loss on ignition weathering index of parker chemical index of alteration comparing these indices with electrical resistivity logs it has been found that a value of 400 ohm m delineates clearly the parent rocks and the weathered materials then the 12 inverted ert profiles were constrained with this value after verifying the uncertainty due to the inversion procedure synthetic models based on the field data were used for this purpose the estimated average regolith thickness at the watershed scale is 17 2 m including 15 2 m of saprolite and 2 m of soil cover finally using these estimations of the thicknesses the long term mass balance is calculated for the average gneiss derived saprolite and red soil in the saprolite the open system mass transport function indicates that all the major elements except ca are depleted the chlorite and biotite crystals the chief sources for mg 95 fe 84 mn 86 and k 57 biotite only are the first to undergo weathering and the oligoclase crystals are relatively intact within the saprolite with a loss of only 18 the ca accumulation can be attributed to the precipitation of caco3 from the percolating solution due to the current and or the paleoclimatic conditions overall the most important losses occur for si mg and na with 286 106 mol ha 62 of the total mass loss 67 106 mol ha 15 of the total mass loss and 39 106 mol ha 9 of the total mass loss respectively al fe and k account for 7 4 and 3 of the total mass loss respectively in the red soil profiles the open system mass transport functions point out that all major elements except mn are depleted most of the oligoclase crystals have broken down with a loss of 90 the most important losses occur for si na and mg with 55 106 mol ha 47 of the total mass loss 22 106 mol ha 19 of the total mass loss and 16 106 mol ha 14 of the total mass loss respectively ca al k and fe account for 8 6 4 and 2 of the total mass loss respectively overall these findings confirm the immaturity of the saprolite at the watershed scale the soil profiles are more evolved than saprolite but still contain primary minerals that can further undergo weathering and hence consume atmospheric co2

pt and pd mobility in hydrothermal fluids evidence from komatiites and from thermodynamic modelling abstract komatiite host rocks and ores provide a natural laboratory for the study of pt and pd during hydration and carbonation of ultramafic rocks the degree of correlation between pt and pd and of those elements with lithophile elements such as ti in komatiitic rocks and ores is strongly dependent on the magmatic sulfide content of the rock pt and pd are typically very strongly correlated within s poor komatiites and in disseminated ores containing 5 volume or less magmatic sulfides in contrast they are almost completely decoupled in sulfide rich ore samples thermodynamic modelling based on the existing body of published experimental data on pt and pd solubilities suggests that pd and pt can be readily transported as bisulfide complexes in acidic neutral solutions under reduced and moderate oxidation condition at 300 c and chloride complexes are only important under extremely acidic oxidised conditions pd is much more soluble than pt in the case of fluids in sulfide rich environments in equilibrium with the characteristic assemblages found within altered and metamorphosed komatiites solubilities of both elements are much lower in sulfide poor environments especially where ph values are constrained to neutral values by the near ubiquitous presence of carbonate minerals in the alteration assemblage this accounts for the evident near immobility of pt and pd in even the most altered silicate host rocks as compared with their wide dispersion in sulfide rich ores

immiscible silicate liquid partition coefficients implications for crystal melt element partitioning and basalt petrogenesis this study investigates partitioning of ele ments between immiscible aluminosilicate and boro silicate liquids using three synthetic mixtures doped with 32 trace elements in order to get a good spatial separation of immiscible liquids we employed a high temperature centrifuge experiments were performed at 1 050 1 150 c 1 atm in sealed fe and pt containers quenched products were analysed by electron micro probe and la icp ms nernst partition coefficients d s between the fe rich and si rich aluminosilicate immiscible liquids are the highest for zn 3 3 and fe 2 6 and the lowest for rb and k 0 4 0 5 the plots of d values against ionic potential z r in all the composi tions show a convex upward trend which is typical also for element partitioning between immiscible silicate and salt melts the results bear upon the speciation and structural position of elements in multicomponent silicate liquids the ferrobasalt rhyolite liquid immis cibility is observed in evolved basaltic magmas and may play an important role in large gabbroic intrusions such as skaergaard and during the generation of unusual lavas such as ferropicrites

the role of vegetation and bed level fluctuations in the process of channel narrowing abstract a catastrophic flood ire 1965 on plum creek a perennial sandbed stream in the western great plains removed most of the bottomland vegetation and transformed the single thalweg stream into a wider braided channel following eight years of further widening associated with minor high flows a process of channel narrowing began in 1973 narrowing continues today the history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5 km reach of plum creek near louviers colorado sixty three of these plants were excavated in order to determine the age and elevation of the germination point the reconstructed record of channel change was verified from historical aerial photographs and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach channel narrowing at plum creel occurs in two ways first during periods of high flow sand and fine gravel are delivered to the channel temporarily raising the general bed level subsequently several years of uninterrupted low flows incise a narrower channel second during years of low flow vegetation becomes established on the subaerial part of the present channel bed in both cases surfaces stabilize as a result of vegetation growth and vertical accretion of sediment

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

a modified genetic model for the huangshandong magmatic sulfide deposit in the central asian orogenic belt xinjiang western china the huangshandong ni cu deposit is the largest magmatic sulfide deposit discovered to date in the central asian orogenic belt in northern xinjiang western china the host intrusion is a 274 ma composite mafic ultramafic intrusion consisting of four separate intrusive units a large layered gabbroic sequence phase i a sheet like ultramafic body phase ii a dyke like gabbronorite body phase iii and an irregular ultramafic unit phase iv important sulfide mineralization is present in the last three intrusive units predominantly as disseminated and net textured sulfides pyrrhotite pentlandite and chalcopyrite the huangshandong mafic ultramafic intrusive rocks are characterized by arc like geochemical signatures such as low ca content in olivine and negative nb ta anomalies in whole rocks this together with a post subduction setting for the east tianshan in the permian suggests that the source mantle was modified previously by slab derived fluids in the carboniferous the mantle derived magma was ponded in a staging chamber in the lower part of the newly formed arc crust the first batch of magma to arrive at huangshandong was most fractionated and depleted in ni crystallizing fe rich and ni depleted olivine fo67 300 ppm ni the second batch of magma was more primitive crystallizing more primitive olivine fo81 84 the third batch of magma was also highly fractionated and depleted in ni crystallizing fe rich and ni depleted olivine fo72 600 ppm ni the final batch of magma became more primitive again crystallizing the most primitive olivine fo81 86 the occurrence of rounded sulfide inclusions in olivine primocrysts in the huangshandong ultramafic rocks indicates that immiscible sulfide liquid droplets were present during olivine crystallization the ni tenors of disseminated sulfide ores in the gabbronorite dyke vary mainly between 5 and 8 wt which are too high to have been produced by the parental magma of the dyke the ni cu and platinum group elements pge tenors of disseminated sulfide ores in the dyke phase iii and the ultramafic sheet phase ii are remarkably similar these observations together with the sequence of magma emplacement suggest that the sulfide liquids entrapped in the magma of the dyke formed at depth by a previous pulse of more primitive magma the estimated parental magma for the most primitive lherzolites in the huangshandong intrusion contains 10 wt mgo modeling shows that sulfide saturation in the parental magma of the huangshandong lherzolites could have resulted from fractional crystallization significant pge depletions relative to ni and cu in the disseminated sulfide ores of the huangshandong deposit may be due to sulfide retention in the source mantle

evaluation of a linear bank erosion equation field surveys and aerial photographs were used to measure the rate of migration of a bend of the brandywine creek near chadds ford pennsylvania velocities near the bank were measured in the field during high flows and also estimated by solving a form of the st venant equations engineering properties of the bank soils and the spatial density of trees growing along the bank were also measured the data indicate that rates of bank migration are linearly related to the magnitude of the near bank velocity if the st venant equations are used to estimate the near bank velocity the data also suggest that erosion rates are proportional to ub u where ub is the near bank velocity and u is the reach averaged velocity in addition rates of bank migration are reduced where the density of silver maples acer saccharinum is high boxelder acer negundo and white ash fraxinus americana are also found along the bend but these species do not noticeably influence rates of bank migration

use of alloformations for definition of holocene meander belts in the middle amite river southeastern louisiana geomorphological studies in drainage basins of the northern gulf of mexico coastal plain have indicated similarities in flood plain character and comparable influences on basin evolution alluvial morphology and stratigraphy can be linked to flood plain evolution through the use of alloformations unconformity bounded stratigraphic units this procedure extends classical terrace mapping concepts to conform with formally defined stratigraphic procedures a meander belt segment of the middle amite river valley in southeastern louisiana was selected to test the applicability of alloformation mapping in part of a regional scale 5 000 km 2 drainage basin three alloformations identified as magnolia bridge mag denham springs ds and watson wat can be differentiated by unconformable boundaries landscape morphology and relative pedogenic development the valley fill consists of a lower sandy and upper silty facies preserved deposits consist of channel lag point bar and scroll bar channel fill and overbank sequences lithofacies and sedimentary environments are dependent on fluvial process and a suite of meander belt environments can be identified within each alloformation surface horizons of alluvial soils are all similar ochric epipedons but subsoil horizons are chiefly stratified parent material on the mag entisols cambic horizons on the ds inceptisols and minimally developed argillic horizons on the wat alfisols allostratigraphy provides an objective method for definition of geologic units that contain genetically related but heterogeneous sedimentary deposits alluvial sediments are differentiated in a manner that successfully integrates geomorphic sedimentologic and pedologic data into significant stratigraphic models this approach is applicable to a wide variety of quaternary settings where prior stratigraphic techniques have resulted in ambiguity

ni cu au and platinum group element contents of sulphides associated with intraplate magmatism a synthesis the tectonic setting of intraplate magmas typically a plume intersecting a rift is ideal for the development of ni cu platinum group element bearing sulphides the plume transports metal rich magmas close to the mantle crust boundary the interaction of the rift and plume permits rapid transport of the magma into the crust thus ensuring that no sulphides are lost from the magma en route to the crust the rift may contain sediments which could provide the sulphur necessary to bring about sulphide saturation in the magmas the plume provides large volumes of mafic magma thus any sulphides that form can collect metals from a large volume of magma and consequently the sulphides will be metal rich the large volume of magma provides sufficient heat to release large quantities of s from the crust thus providing sufficient s to form a large sulphide deposit the composition of the sulphides varies on a number of scales i there is a variation between geographic areas in which sulphides from the noril

microgranitoid enclaves in granites globules of hybrid magma quenched in a plutonic environment microgranitoid enclaves autoliths cognate xenoliths mafic inclusions are common in high level granitoid plutons they have relatively fine grained igneous microgranitoid microstructures1 and many show evidence of having flowed in a magmatic condition2 these features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3 5 or restite6 11 an alternative interpretation is that the enclaves represent globules of mafic magma that have mingled commingled and quenched in the granitoid host magma12 16 a detailed review of the literature occurrence morphology and composition of micro granitoid enclaves strongly supports this view hut indicates that the enclave magmas range in composition from mafic to felsic and that magma mixing may be involved in their formation17 i summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid

late holocene phytolith and carbon isotope record from a latosol at salitre south central brazil abstract the reliability of paleovegetation records inferred from soil phytolith assemblages and som soil organic matter carbon isotope analysis was examined in light of previous pollen and charcoal reconstructions the sampled latosol is located in south central brazil salitre at a boundary between forest and cerrado the derived mean age of phytoliths and som at each level is the result of a balance between continuous translocation and selective dissolution it increases with depth in a regular quantifiable fashion that allows paleoenvironmental interpretation phytoliths and som tracers first record a savanna phase associated with the last holocene long dry period occurring between ca 5500 and 4500 yr b p two periods of tree community development followed between ca 4000 and 3000 and after ca 970 yr b p leading to the present cerrado forest association the dry spell interrupted this trend about 970 60 yr b p the second development of woody elements was contemporaneous with an increase in anthropogenic fires therefore climate was more important than fires and human activities in constraining the growth of vegetation during the last nine centuries at salitre more generally despite pedogenic processes soil phytoliths and 13c values of the som may be accurate tracers of vegetation changes

nickel copper occurrences in the belleterre angliers belt of the pontiac subprovince and the use of cu pd ratios in interpreting platinum group element distributions within the belleterre angliers greenstone belt of the pontiac subprovince there are a number of ni cu sulfide occurrences associated with synvolcanic gabbros the stratigraphy and geochemistry of the belt suggest that it is a rifted island arc and that the gabbros which contain the ni cu sulfides were derived from the mantle plume that rifted the island arc the conditions under which the sulfides formed may be modeled either by recalculating the sulfide compositions to 100 percent sulfide or by using the cu pd ratios both approaches suggest that most of the sulfide occurrences formed from an enriched morb magma with r factors of 200 to 1 000 one of the occurrences la force is depleted in platinum group elements pge relative to cu and this depletion may be the result of the removal of sulfides from the magma prior to emplacement at this stratigraphic level the advantage of using the cu pd ratio rather than recalculating to 100 percent sulfide is that it is not necessary to have s analyses to model the conditions of formation of the sulfides further the cu pd ratio may be used as a prospecting tool rocks with ratios greater than mantle values formed from magmas depleted in pd possibly due to removal of sulfide from the magma this implies the potential for a pge bearing sulfide deposit at stratigraphically lower levels such an approach has been applied to pge targets in the eastern abitibi subprovince samples from four ni cu occurrences with cu pd ratios close to mantle ratios have the potential to produce pge as a by product samples from seven ni cu sulfide occurrences and the layered intrusions lac dore bell river and cummings complex have cu pd ratios much greater than mantle ratios indicating that these intrusions and sulfides are not good targets for pge at this stratigraphic level although there may be pge rich sulfides present at depth

mafic felsic magma mixing limited by reactive processes a case study of biotite rich rinds on mafic enclaves article history mafic enclaves in felsic plutons are often used to argue that intermediate magmas are formed by mafic felsic magma mixing but the extent and nature of mixing remains unclear here we examine biotite rich rinds on mafic enclaves from the cretaceous bernasconi hills pluton in the peninsular ranges batholith of southern california to gain insight into magma mixing processes rinds differ from the enclave interior and the host monzogranite in being more fine grained and more mafic and potassic rinds are also 2 5 times more enriched in rare earth elements than the host monzogranite and up to 3 times more enriched than enclave interiors these observations indicate that the rinds were not generated by isochemical quenching binary mixing between enclave and host monzogranite or in situ magmatic differentiation instead rinds appear to have been formed by chemical reaction between the solidified enclave and a hydrous k rich residual melt or fluid formed after progressive crystallization and cooling of the host magma body transforming amphibole in the enclave into biotite rich rinds field observations show snapshots of biotite rich rinds being eroded away and new rinds simultaneously forming on freshly eroded surfaces of enclaves consistent with rinds being formed by chemical reaction instead of as quenching products deformation of enclaves is accommodated primarily by ductile attenuation of the thin rind while the enclave as a whole tends to rotate as a rigid body with minimal internal deformation other than localized brittle failure a comparison of the aspect ratios and cross sectional areas of mafic bodies in the pluton shows that those with high aspect ratios indicating greater accumulated strain are systematically more biotite rich and have smaller cross sectional areas than those with lower aspect ratios which are amphibole rich these relationships not only confirm that biotite rich lithologies are more deformable but also indicate that the high aspect ratio biotite rich bodies also known as schlieren derive from small parent bodies consistent with a derivation from eroding enclave rinds rather than from the enclave itself finally geochemical and thermodynamic modeling indicates that the biotite rich rinds formed when the host felsic magma had cooled to a low melt fraction state f 0 15 0 3 700 760 c suggesting that such reactions occur late in the lifespan of a magma body thus mafic felsic mixing may not be an efficient process for making intermediate magmas unless the magma body can reside at this low temperature range long enough to permit rind formation and subsequent deformation

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

petrogenesis and ni cu co sulfide formation of mafic enclaves in tulaergen mafic ultramafic intrusive rocks eastern tianshan northwest china implications for liquid immiscibility and hydrothermal remobilization of platinum group elements

large scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the daly gap the dearth of intermediate magmatic compositions at the earth s surface referred to as the daly gap remains a major issue in igneous petrology the initially favored explanation invoking silicate liquid immiscibility during evolution of basalt to rhyolite has lost support because of the absence of any firm geological evidence for separation of fe and si rich liquids in igneous rocks this work presents a record of large scale magmatic differentiation due to immiscibility in the tholeiitic sept iles intrusion canada one of the largest layered plutonic bodies on earth gabbroic cumulate rocks from the critical zone of this intrusion show a bimodal distribution in density and p2o5 content despite identical major element chemistry of the principal magmatic phases immiscibility is supported by the presence of contrasting fe rich and si rich melt inclusions trapped in cumulus apatite phase diagrams and well documented occurrences of small scale immiscibility confirm that liquid liquid unmixing and the separation of fe rich and si rich liquids may contribute significantly to the daly gap along the tholeiitic liquid line of descent

large scale silicate liquid immiscibility a possible example from northeastern brazil abstract ultrapotassic peralkalic silica saturated plutons 580 ma are widespread in the cachoeirinha salgueiro foldbelt northeastern brazil they consist of alkali feldspar syenites with pyroxenite as co magmatic inclusions and syn plutonic or late stage dikes pyroxenite and syenite have the same mineral phases aegirine augite microcline sphene apatite blue amphibole magnetite but only in different proportions rare inclusions of a mixed rock about 60 syenite 40 pyroxenite in an emulsion like texture are also present pyroxenes in the three units are all only slightly zoned silica saturated and extremely low in al2o3 0 2 1 4 amphiboles are mostly k rich richterite characterized by high sio2 low al2o3 and tio2 contents and low mg the three rock types have similar ree chondrite normalized patterns with negative slopes and lack of eu anomaly with the total ree in the pyroxenite greater than that of the syenite trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs the rocks have similar high 18o values avg w r 8 smow corrected from pyroxene high initial 87 sr 86 sr ratios about 0 710 and low 143 nd 144 nd avg 0 51104 field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite the mixed rock representing the original magma composition

width and depth of self formed straight gravel rivers with bank vegetation the influence of bank vegetation on the stable channel cross sectional geometry of stable straight gravel rivers is studied theoretically singular perturbation and matched asymptotic expansion techniques are employed to derive the lateral distribution of depth averaged fluid velocity and the bed shear stress a condition of sediment incipient motion is imposed at the junctions of the bed and the banks to derive the stable depth and a formula for resistance to flow is used to obtain the stable width the theory reveals that a thicker vegetation yields a larger depth and a smaller width increasing discharge increases the effect of vegetation the analysis agrees reasonably well with available field data

stable channels with mobile gravel beds regime type equations for mobile gravel bed rivers are presented based on data obtained from 62 stable gravel bed river reaches in the united kingdom multiple regression techniques are applied to derive equations relating reach average and riffle values of width mean and maximum depth slope velocity sinuosity and riffle spacing to bankfull discharge bed and bank material characteristics valley slope bank vegetation type and an independent estimate of bankfull bed load transport rates although discharge has a dominant control on channel geometry these equations indicate that bed load discharge also has a significant influence particularly with regard to channel slope bank vegetation has a major control on width and velocity while depth velocity and slope are storngly affected by bed material size reasons for these results are considered in terms of the physical processes controlling channel adjustment the application of these regime equations is discussed and particular consideration is given to the design of sinuous channels with a pool riffle bed topograph

biogenic silica storage in soils the role of plants in soil development has become of crucial interest in quantifying global weathering processes on the volcanic reunion island indian ocean a 15 cm thick phytolith rich horizon biogenic opal a developed at the expense of trachytic ashes between 3820 85 and 335 90 yr b p then 97 138 t km 2 yr of sio 2 were biogeochemically recycled from the weathering of parent rocks through a past forest of bamboos nastus borbonicus this rate is in the same range as the present chemical weathering rates of silica in the area considered to be among the highest in the world these results demonstrate that the storage of biogenic silica in soils may be significant and may retard the output of silica to rivers and ocean

plant impact on the biogeochemical cycle of silicon and related weathering processes abstract the contribution of plants to the biogeochemical cycle of si and related weathering processes was studied in an equatorial rainforest ecosystem congo where the biologic turnover of si is high 58 to 76 kg ha y litterfall leaves a soil profile and groundwaters were analysed phytoliths and organic matter have a similar distribution with depth in the soil profile the model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92 of the biogenic silica input is rapidly recycled while about 8 of the biogenic silica input supplies a stable pool of phytoliths with a lower turnover reprecipitation of silica was observed at the base of the soil profile indicating a local geochemical environment that is oversaturated with respect to amorphous silica a balance in biogeochemical cycle of si requires that the vegetation absorb dissolved silicon released from weathering of minerals which otherwise would be available for mineral neoformation or export from the profile towards regional drainages plant uptake of si increases the chemical weathering rate without increasing the denudation rate this study shows that the uptake storage and release of si by the vegetation have to be taken into account when using dissolved si for tracing chemical weathering dynamics

ontogeny of a flood plain the ontogeny of five flood plain segments is described for a period of 18 yr following a major flood in 1978 on the powder river in southeastern montana the flood plains developed on relatively elevated sand and gravel deposits left within the channel by the 1978 flood in cross section the flood plains resemble benches with well developed natural levees flood plain growth occurred as sediment was draped onto preexisting surfaces in layers of sand and mud a few centimeters to decimeters thick resulting in some lateral but mostly vertical accretion annual and biannual measurements indicated that as the flood plain segments grew upward the annual rate of vertical accretion decreased as the partial duration recurrence interval for the threshold or bankfull discharge increased from 0 16 to 1 3 yr it is clear that a constant recurrence interval for overbank flow cannot be meaningfully assigned to this type of flood plain ontogeny these flood plains did not grow on migrating point bars and vertical accretion at least initially occurred within the channel rather than across the valley flat during extensive overbank flows sediments of these flood plains define narrow elongated stratigraphic units that border the active channel and onlap older flood plain deposits these characteristics are considerably different from those of many facies models for meandering river deposits facies similar to those described in this paper are likely to be preserved thereby providing important evidence in the geologic record for episodes of periodic channel expansion by ancient rivers

the chromite deposits of the troodos complex cyprus evidence for the role of a fluid phase accompanying chromite formation the troodos ophiolite of cyprus hosts chromite deposits both as podiform bodies within the basal harzburgite and as lesser layers and schlieren within the overlying dunites and clinopyroxene dunites of the metacumulate sequence all chromitite types have been studied from both field and mineralogical aspects with particular emphasis on the chemistry of the ubiquitous silicate inclusions within chromite grains two distinct inclusion assemblages have been documented chromites from podiform bodies contain abundant hornblende serpentine ortho and clinopyroxene and lesser olivine while those from the metacumulates host pargasite albite serpentine and rare pyroxenes this implies that no single silicate inclusion assemblage in chromite can be considered typically ophiolitic the presence of an alkalirich fluid phase during chromite crystallization as evidenced by the unusual mineralogy of these inclusions has already been widely discussed in the literature however the marked stratigraphic control of inclusion types in chromite as reported here enables more complete modelling of the evolution of this phase to be made

the role of woody vegetation in stabilizing slopes in the cincinnati area ohio tree roots markedly stabilize colluvium on steep hillsides in the cincinnati area ohio according to detailed analysis of a landslide complex on the south side of rapid run creek a tributary of the ohio river the shear surface of the slide developed near the interface between bouldery silty clay colluvium and underlying calcareous ordovician shale and it broke woody roots that penetrated through the colluvium and into the underlying bedrock measurements of positions sizes and tensile strengths of the roots indicate that the average shear strength contributed by the roots was about 5 7 kn m 2 of the shear surface the average strength contributed by residual friction of the soil was about 0 7 kn m 2 the tree roots increased the factor of safety against sliding nine fold root strength allows forested colluvium mantled hillslopes in the cincinnati area to resist sliding at slope angles as high as 35 whereas similar slopes devoid of trees are subject to sliding at slope angles of 12 to 14 the contribution of tree roots to slope stability should be evaluated before removal of trees for development of hillsides

s saturation history of nain plutonic suite mafic intrusions origin of the voisey s bay ni cu co sulfide deposit labrador canada the voisey s bay deposit is hosted in a 1 34 ga intrusion composed of troctolite olivine gabbro and ferrogabbro the sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites and also occurs as veins along structures in adjacent paragneiss a dyke is connected to the base of the north wall of the eastern deeps intrusion and the entry point of this dyke into the chamber is the locus of the eastern deeps nickel sulfide deposit a detailed exploration in the area between the eastern deeps and the ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke the eastern deeps deposit is surrounded by a halo of moderately to weakly mineralized variable textured troctolite vtt that reaches a maximum thickness above the axis of the eastern deeps deposit along the northern wall of the eastern deeps intrusion the massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the vtt and there is a marked break in ni tenor of sulfide between the two sulfides hosted in the dyke tend to have low metal tenors ni 100 2 5 3 5 sulfides in eastern deeps massive and breccia ores have intermediate ni tenors ni 100 3 5 4 and disseminated sulfides in overlying rocks have high ni tenors ni 100 4 8 four principal processes control the compositions of the voisey s bay sulfides coarse grained loop textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the pd ir ratio which is due to mineralogical variations where pentlandite is enriched in pd and ir is dispersed throughout the mineral assemblage a decrease in ir and rh from the margin of the ovoid toward cubanite rich parts at the central part of the ovoid is consistent with fractionation of the sulfide from the margins toward the center of the ovoid the ovoid ores have higher ni and pd tenor than the eastern deeps massive sulfides this is consistent with both a higher r factor and greater degree of silicate parental magma evolution in the ovoid than the eastern deeps the disseminated sulfides surrounding the eastern deeps deposit have some of the highest ni and pd tenors at voisey s bay which are indicative of not only more primitive magmas but also higher r factors than the ovoid or the eastern deeps vtt and normal textured troctolite of the eastern deeps that contain trace sulfide have 0 1 3 ppb pt and 0 1 3 ppb pd whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of pt and pd troctolites and olivine gabbros from other parts of the voisey s bay intrusion and other nain plutonic suite intrusions including the kiglapait newark bay barth island mushua and nain bay south intrusion also have low platinum group element abundances although it is possible that this is a signature of a widespread sulfide saturation event that pre dated ore formation at voisey s bay it is more likely that platinum group element pge depletion is a product of the source melting process where low degrees of melting resulted in the retention of pge in the mantle source if so this indicates that pge depletion should be used with caution as an exploration tool in the nain plutonic suite

iridium and palladium as discriminants of volcanic exhalative hydrothermal and magmatic nickel sulfide mineralization neutron activation of pd au and ir in samples from 1 economically significant ni sulfide deposits of western australia 2 the host rocks of these deposits 3 hydrothermal and or metasomatic ni sulfide mainly millerite mineralization in peridotite at nullagine western australia 4 hydrothermal replacement volcanic exhalative ni cu sulfide mineralization at sherlock bay western australia and 5 volcanic exhalative sulfides in inter flow sediments at kambalda are used to show that ir and pd can be used to discriminate between the various origins magmatic volcanic exhalative hydrothermal metasomatic and metamorphic ascribed to ni sulfide deposits all the economically significant ni sulfide deposits in western australia are strongly enriched in the precious metals relative to other crustal rocks and have similar pd ni and ir ni ratios to each other these ratios persist regardless of whether the nickel sulfides occur at the base of volcanic ultramafic piles e g kambalda or as centrally disposed interstitial sulfides in intrusive dunite bodies the strong similarities of the pd ir ratios of the ni sulfide ores and of the spinilex textured komatiites which represent ultramafic liquids from which the ni sulfide liquids were derived together with other features of the precious metal geochemistry of the ores and host rocks of the ni sulfide deposits are used to show that the deposits had magmatic origins in contrast the pd ir and ir ni ratios of the hydrothermal and or metasomatic ni sulfide mineralization at nullagine are distinctly different from those of the major ni deposits the former contains high au moderate but variable pd and exceptionally low ir similarly the volcanic exhalative sulfides at kambalda and sherlock bay contain negligible quantities of pd and ir all the ir in both the interflow sediments and the hydrothermal metasomatic mineralization is contributed by the host rocks and not by the sulfides the western australian data together with precious metal data for other examples of hydrothermal ni cu sulfide mineralization strongly suggest that although ni au and pd may be transported in aqueous solution ir cannot hence the relative abundance of ir can be used as a discriminant of magmatic sulfides

bank erodibility of shallow sandbed streams cross sections were surveyed at straight reaches of 16 sandbed streams in the midwestern u s two stratigraphic horizons are found in the banks at each site an upper cohesive unit usually composed of silt and clay and a lower unit composed of sand bank erosion on these rivers occurs when the upper cohesive unit is undercut by scour at bends the overhanging cohesive block fails by toppling forward into the channel during failure the soil is primarily in tension rather than compression or shear analysis of this failure mechanism leads to a field method for measuring the tensile strength of riverbanks measured values of the tensile strength are not correlated with the channel geometry thus the erodibility of the cohesive bank sediments does not influence the geometry of the rivers studied

stream channel erosion and change resulting from riparian forests forested stream banks compared to grassed ones can destabilize stream channels by promoting erosion four reaches of coon creek wisconsin each with long term grassed and forested subreaches were examined grassed reaches were narrower and had smaller channels bankfull cross sections than forested reaches suggesting that grassed channel reaches stored about 2100 to 8800 m 3 more sediment per kilometre than forested reaches available evidence suggests that conversion of riparian forests to grass would allow storage of sediment along channels possibly decreasing downstream sediment yields these findings are important as many grassed riparian corridors are rapidly reverting to forest because of economic conditions and governmental policies

analysis of petrologic hypotheses with pearce element ratios pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity in basaltic systems the incompatible elements ti k and p are often conserved the slope of the trend on a pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases a judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions in basaltic rocks diagrams with ti k vs p k can provide a test of the comagmatic hypothesis diagrams with 0 5 mg fe k vs si k have trends that are distinct for each comagmatic suite and different mineral assemblage different suites are distinguished by the intercepts in diagrams whereas mineral assemblages are recognized by the slopes of the trends for example if olivine is the sole crystallizing and segregating phase the trend will have a slope of 1 diagrams with 2ca na k vs al k distinguish plagioclase from augite assemblages and in conjunction with 0 5 mg fe k diagrams unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation analyses from the uwekahuna laccolith kilauea the 1955 and 1967 68 eruptions of kilauea diamond craters volcanic field oregon and experimental data on morb glasses provide illustrations of the interpretations that can be obtained from pearce element ratios

estimated storage of amorphous silica in soils of the circum arctic tundra region we investigated the vertical distribution storage landscape partitioning and spatial variability of soil amorphous silica asi at four different sites underlain by continuous permafrost and rep

biogenic silica dissolution in the oceans reconciling experimental and field based dissolution rates 1 on average 50 60 of diatomaceous opal produced in the euphotic zone redissolves within the upper 100 m of the water column high specific silica dissolution rates in the surface ocean contrast with order of magnitude lower values in deep sea sediments the lack of a mechanistic understanding of these large variations in dissolution kinetics is a major source of uncertainty in models of nutrient silicon cycling in the oceans here we show that the observed variability in biogenic silica reactivity is consistent with results of experimental dissolution studies differences in aluminum content and specific surface area of the diatom skeletons plus differences in temperature and degree of undersaturation all contribute to lowering silica dissolution rates in deep sea sediments relative to the euphotic zone when variations in these material and environmental properties are accounted for the predicted specific opal dissolution rate in average surface ocean water is over 2 orders of magnitude faster than the rate measured in recently deposited biosiliceous oozes of the southern ocean the predicted specific dissolution rate however is approximately 5 times higher than the value obtained from global estimates of the depth integrated dissolution rate and biogenic silica concentration in the upper water column this discrepancy likely reflects the inhibitory effect of protective organic coatings on fresh diatom shells in surface waters the experimental data further imply that aluminum incorporation in the silica matrix during biomineralization or after death of the organisms enhances the preservation efficiency of biogenic opal in marine sediments