melting of newly formed mafic crust for the formation of neoproterozoic i type granite in the hannan region south china abstract neoproterozoic magmatism in the hannan region at the northwestern margin of the yangtze block is characterized by numerous felsic plutons associated with minor mafic ultramafic intrusions the felsic plutons are either adakitic or normal arc granitic in composition the adakitic plutons are 735 ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust among the normal arc related felsic plutons the tianpinghe pluton is the largest and has a shrimp zircon u pb age of 762 4 ma older than the adakitic plutons in the region rocks from the tianpinghe pluton have relatively high sio2 67 1 70 1 wt and k2o na2o 7 8 8 6 wt and relatively low mgo 0 7 1 3 wt and al2o3 contents 14 5 15 6 wt with al2o3 cao k2o na2o a cnk values ranging from 0 95 to 1 08 they have arc affinity trace element compositions that are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with strong

neoproterozoic adakitic plutons in the northern margin of the yangtze block china partial melting of a thickened lower crust and implications for secular crustal evolution abstract numerous neoproterozoic felsic and mafic ultramafic intrusions occur in the hannan region at the northern margin of the yangtze block among these the wudumen and erliba plutons consist of granodiorites and have shrimp zircon u pb ages of 735 ma the rocks have high k 2 o 0 8 3 6 wt and na 2 o 4 4 6 4 wt and low mgo 0 4 1 7 wt they also have high sr y 32 209 and la yb n ratios 4 4 38 6 their e nd values range from 0 41 to 0 92 and zircon initial 176 hf 177 hf ratios from 0 282353 to 0 282581 these geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust our new analytical results combined with the occurrence of voluminous arc related mafic ultramafic intrusions emplaced before 740 ma lead us to propose that the crustal evolution in the northern margin of the yangtze block during neoproterozoic involved 1 rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from 1 0 to 0 74 ga and 2 partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window producing the 735 ma adakitic wudumen and erliba plutons our model suggests that the crustal thickness was more than 50 km at the northern margin of the yangtze block at 735 ma and rule out the possibility of a mantle plume impact causing the 735 ma magmatism in the region

chalcophile element geochemistry and petrogenesis of high ti and low ti magmas in the permian emeishan large igneous province sw china sulfide poor mafic layered intrusions sills dykes and lava flows in the funing region sw china are part of the 260 ma emeishan large igneous province they belong to either a high ti group tio2 1 6 4 4 wt with elevated ti y ratios 351 1 018 or a low ti group tio2 1 low y pd pm ratios 0 2 7 1 and nearly constant cu pd pm ratios 1 5 3 8 the even and parallel chalcophile element patterns of the high pge low ti subgroup rocks are likely a result of olivine dominated fractionation under s undersaturated condition the low pge low ti group rocks have low mgo 4 5 8 9 wt and very poor pge pt pd 0 5 1 6 ppb ir 0 004 0 02 ppb with low cu zr ratios 0 1 0 5 high y pd pm 26 70 and variable cu pd pm ratios 2 8 14 the trough like chalcophile element patterns of the low pge low ti subgroup rocks indicate that the magmas were sulfide saturation and sulfide melts were extracted from the magmas the extracted sulfide melts might be potential ni cu sulfide ores at depth in the funing region

seafloor alteration of basaltic glass textures geochemistry and endolithic microorganisms 1 the incipient low temperature alteration of glassy margins of recent seafloor lavas from the mohns ridge is characterized by scanning electron microscopy and bulk chemical analyses of major and trace elements these investigations are designed to evaluate the endolithic microbial biomass to identify relations between alteration textures and microorganisms and to identify the chemical processes that take place during alteration the basaltic glass along intersecting fractures and around vesicles is typically altered to concentrically zoned yellow brown amorphous gel palagonite in most fractures microorganisms are observed at the outer surfaces of palagonite rims within porous zones of the rims and frequently also at the glass palagonite interfaces the cells act as nucleation sites for precipitation and become encrusted and embedded in palagonite with time zones of porous palagonite containing numerous hollow fossilized cells alternate with zones of compact palagonite lacking distinct cell structures which together indicate that the microbial growth is discontinuous the palagonite has an average organic carbon content of 0 9 wt 13corg 22 which derive from both living and fossilized biomass the microbial growth and biomineralization are major controls on the porosity and texture of the palagonite and thus likely on the chemical exchange between glass and seawater pit marks in the glass in fractures both with and without microbes indicate that microbial as well as abiotic processes mediate pitting elemental and isotope data show that the transformation of glass to gel palagonite at seafloor conditions with high water rock ratios results in near complete loss of si and alkali elements to seawater formation of fe and al oxyhydroxides complete exchange of alkaline earth elements like sr with seawater retention of v cu y pb th u and trivalent rees accompanied by a strong gain of these and other elements from seawater higher fe ti ratios in the gel palagonite compared to the parental glass suggest that the palagonite represents a mixture of fe ti oxyhydroxides derived from both the glass and seawater

zircon lu hf isotopic constraints on neoproterozoic subduction related crustal growth along the western margin of the yangtze block south china abstract voluminous neoproterozoic igneous rocks along the western margin of the yangtze block south china provide an ideal opportunity for examining the nature of the mantle source petrogenesis of the felsic plutons and processes and timing of crustal growth the dadukou tongde gaojiacun and shaba mafic plutons and datian and dajianshan adakitic plutons were selected for zircon lu hf isotope analysis zircons from the mafic plutons have hf t values ranging from 1 86 to 11 63 with average hf t values of the different samples ranging from 4 78 0 42 to 7 60 1 1 their single stage model ages range from 0 86 to 1 28 ga the two adakitic plutons have hf t values ranging from 2 26 to 11 68 and single stage model ages from 0 86 to 1 24 ga our data further confirm that the mafic plutons were extracted from depleted mantle sources whereas the datian and dajianshan adakitic plutons were products of melting of a subducted oceanic slab the arc geochemical affinity relatively high 143 nd 144 nd ratios and high zircon hf t values as well as the relatively young hf single stage model ages of these neoproterozoic plutons suggest that the continental crustal growth in the region was related to oceanic subduction rather than upwelling of a mantle plume in a rifting setting the neoproterozoic magmatism along the western margin of the yangtze block lasted more than 200 my not show episodic emplacement and contrasting zircon hf isotopes suggesting that continental crustal growth in south china was continuously resulted from subduction

two magma series and associated ore deposit types in the permian emeishan large igneous province sw china the late middle permian 260 ma emeishan large igneous province in sw china contains two magmatic series one comprising high ti basalts and fe rich gabbroic and syenitic intrusions the other low ti basalts and mafic ultramafic intrusions the fe rich gabbros are spatially and temporally associated with syenites each series is associated with a distinctive type of mineralization the first with giant fe ti v oxide ore deposits such as panzhihua and baima the second with ni cu pge sulfide deposits such as jinbaoshan limahe and zhubu new shrimp zircon u pb isotopic data yielded 263 3 ma for the limahe intrusion 261 2 ma for the zhubu intrusion and 262 2 ma for a syenitic intrusion these new age dates together with previously reported shrimp zircon u pb ages suggest that all these intrusions are contemporaneous with the emeishan flood basalts and formed during a major igneous event at ca 260 ma the oxide bearing intrusions have higher al2o3 feo as total iron and total alkalis na2o k2o but lower mgo than the sulfide bearing intrusions all intrusions are variably enriched in lree relative to hree the oxide bearing intrusions display positive nb and ti anomalies and in certain cases negative zr hf anomalies whereas the sulfide bearing intrusions have obvious negative nb and ti anomalies a feature of crustal contamination individual intrusions have relatively small ranges of nd t values all the intrusions however have nd t values ranging from 3 9 to 4 6 and initial 87sr 86sr ratios from 0 7039 to 0 7105 the syenites have very low mgo 2 wt but highly variable fe2o3 2 5 to 13 wt with initial 87sr 86sr ratios ranging from 0 7039 to 0 7089 magmas from both series could have derived by melting of a heterogeneous mantle plume the high ti series from a fe rich more fertile source and the low ti series from a fe poor more refractory source in addition the low ti series underwent significant crustal contamination the two magma series evolved along different paths that led to distinct mineralization styles

chemical processes in buoyant hydrothermal plumes on the east pacific rise near 21 n hydrothermal plume water containing suspended particles of precipitated black smoke has been sampled from three hydrothermal vent fields on the east pacific rise near 21 n the samples were taken within the lower 22 m of the buoyant plumes directly above 273 350 c black smoker vents using go flo bottles mounted on the submersible alvin based on li as a conservative tracer of the high temperature endmember solution mixing ratios of 102 to 104 g seawater g vent water are achieved within the lower 22 m of the plumes the particle concentrations sampled in the lower 13 m of the plumes are much larger than predicted for these mixing ratios suggesting that the particles are settling within the lower part of the plume the major minerals in the smoke are pyrrhotite fe rich sphalerite pyrite unidentified fe s si and fe si s phases chalcopyrite amorphous silica sulfur fe oxyhydroxides including goethite and anhydrite present in trace quantities are barite isocubanite wurtzite covellite marcasite and unidentified silicates and alsilicates organic matter is common the composition of the plume solutions indicates that 35 25 of the hydrothermally injected fe remains in dissolved form i e 0 45 m within the lower 22 m of the plume in spite of a 3 to 9 fold excess of h2s in the vent solutions nearly all of the mn and si and most of the ba also remain in solution h2s by contrast has been largely 70 100 removed by precipitation as sulfides and sulfur but mainly by oxidation to dissolved species with intermediate oxidation states its early removal implies that 50 30 of the injected fe will eventually precipitate as primary oxides rather than as sulfides and that fe can be fractionated from the other chalcophile metals during sulfide precipitation compared with the vent water the particles are enriched in cu co ag cd al zn pb and ni by factors of 4 to 20 relative to fe and depleted in ba s si and mn scavenging from seawater may play a role in co and cu enrichment at the ngs field and for ag cd and ni at all three fields but is negligible for pb and zn these results indicate that two distinct metalliferous components are delivered to the distal regions of a plume as a result of reactions in the lower part of the buoyant plume 1 1 a dissolved component that includes about half the fe and all of the mn and will eventually precipitate as oxides and 2 2 a particulate component consisting of fine grained sulfide minerals that are enriched in ore metals by factors of 4 to 20

metasomatic mantle source and crustal contamination for the formation of the neoproterozoic mafic dike swarm in the northern yangtze block south china abstract in the huangling region of the northern yangtze block south china the 820 ma huangling felsic pluton was intruded by 800 ma mafic dikes these dikes are composed of fine to medium grained diabase porphyritic diabase and olivine gabbro quartz xenocrysts occur locally in the olivine gabbro the rocks are alkaline to sub alkaline in composition and have sio2 ranging from 43 3 to 57 2 wt mgo from 3 7 to 9 2 wt and total alkali from 3 0 to 7 3 wt they exhibit lree enrichment la yb n 3 3 28 5 with slightly negative eu anomalies eu eu 0 6 1 0 their primitive mantle normalized trace element patterns are characterized by enrichment of large ion lithophile elements rb ba th and u depletion of high field strength elements nb ta zr and hf and positive pb and negative ti anomalies they have relatively high initial 87sr 86sr ratios ranging from 0 7052 to 0 7080 and variably negative end t values from 4 2 to 10 9 the rocks have a large range of whole rock ehf values 0 35 to 11 34 and a relatively narrow range of pb isotope ratios 206pb 204pb 16 96 to 17 97 207pb 204pb 15 41 to 15 58 and 208pb 204pb 38 08 to 39 66 initial 87sr 86sr ratios correlate negatively with 206pb 204pb ratios whereas 143nd 144nd and 208pb 204pb ratios and ehf values correlate positively with 206pb 204pb ratios these features are indicative of an em2 source region suggesting a lithospheric mantle source modified by subducted components during the neoproterozoic the quartz xenocrysts and linear isotopic variations indicate strong crustal contamination during magma emplacement the energy constrained assimilation and fractional crystallization ec afc modeling calculations reveal that the parental magmas were contaminated by continental crust including archaean amphibolites granitic gneisses metasedimentary rocks and granitic rocks the mafic dikes are 20 myrs younger than the huangling felsic pluton which was produced by partial melting of a thickened continental crust suggesting that the mafic dikes were a consequence of post orogenic extension the huangling region may have originally been located at the centre of the yangtze block the possible heat source for the generation of the huangling felsic pluton and mafic dikes was asthenospheric wedging resulting from oceanic slab subduction beneath the northern margin of the yangtze block this conclusion rules out the possibility that the yangtze block was located in the central part of the rodinia supercontinent the block was more likely situated along the margin of rodinia

compositional variations of olivine from the jinchuan ni cu sulfide deposit western china implications for ore genesis the jinchuan ni cu sulfide deposit is hosted by an elongated olivine rich ultramafic body that is divided by subvertical strike slip faults into three segments central eastern and western the central segment is characterized by concentric enrichments of cumulus olivine crystals and interstitial sulfides pyrrhotite pentlandite chalcopyrite intergrowth whereas the eastern and western segments are characterized by an increase of sulfides toward the lower contacts in all segments sulfides are concentrated at the expense of intercumulus silicates olivine re crystallization is found to be associated with actinolite alteration in some samples the compositional variations of primary olivine from the sulfide poor samples can be explained by a small degree of olivine crystallization 50 modal and sulfide averaging 5 wt in the rocks are consistent with the interpretation that the jinchuan deposit was formed by olivine and sulfide laden magma successively ascending through a conduit to a higher now eroded level sulfide enrichment toward the center in the central segment and toward the lower contact in the eastern and western segments may have in part resulted from flow differentiation and gravitational settling during magma ascent respectively

temporal and spatial variability of hydrothermal manganese and iron at cleft segment juan de fuca ridge a unique data set for hydrothermal mn and fe was collected at cleft segment on the juan de fuca ridge between 1983 and 1991 the data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting manganese heat and iron heat ratios for plumes from the north end of the cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for mn of 0 36 0 17 mol s 1 and for fe of 0 61 0 34 mol s 1 over 6 years of plume measurements at north cleft segment observed episodic hydrothermal discharge accounted for 15 of the total vented mn and 35 of vented fe the chronic fluxes for mn and fe at a second venting center located at the south end of the cleft segment were estimated to be approximately equal to the fluxes at north cleft segment chronic plumes at north cleft segment are mixtures of focused and diffuse discharge that contribute heat mn and fe in variable proportions similar examination of south cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in mn and fe but contributing substantially to the overall heat temporal and spatial variations in the concentrations of mn and fe and in mn heat and fe heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes manganese heat and iron heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes the accurate assessment of segment scale hydrothermal fluxes of mn and fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge

210pb 230th and 10be in central indian basin seamount sediments signatures of degassing and hydrothermal alteration of recent origin 1 isotopic 210pb 238u 230th 10be major and trace elements and micromorphological and microchemical data were used to identify recent 100 yrs hydrothermal alteration of a 200 kyr sedimentary record from the flank of a seamount in the central indian basin located at the edge of the 75 30 e fracture zone alteration effects are also reflected in 1 the depleted sedimentary organic carbon 2 dissolution features of radiolarian skeletons 3 the presence of altered minerals such as smectite and zeolites and 4 distinctly different magnetic properties in the altered sediments we interpret a predominant influence of neutral chloride type hydrothermal fluids this is the first report of recently occurring sediment alteration by shallow circulating sub surface fluids along the indian ocean intra plate seamount environment

boundary scavenging in the pacific ocean a comparison of 10be and 231pa concentrations of u th 231pa and 10be were measured in holocene sediments from two cores collected off the west coast of south america two cores from the east pacific rise two from the equatorial pacific and one from the south pacific central gyre our results together with data from 5 cores reported in the literature show that boundary scavenging plays a major role in the removal of 10be from the pacific ocean deposition rates of 10be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central pacific deposition of 231pa is 4 to 5 fold greater at the margin sites the residence time of 10be with respect to chemical scavenging defined as its inventory in the water column divided by its rate of removal to the sediments varies regionally from 1000 years at the red clay sites in the deep central pacific to 100 years at the margin sites different factors control boundary scavenging of pa and be for example scavenging of 231pa is enhanced by metal oxide coatings of particles whereas this seems to have little influence on the scavenging of 10be 1990

geochemical constraints on the origin of the permian baimazhai mafic ultramafic intrusion sw china the 260 ma baimazhai mafic ultramafic intrusion is considered to be part of the emeishan large igneous province and consists of orthopyroxenite surrounded by websterite and gabbro the intrusion is variably mineralized with a massive sulfide ore body 20 vol in the core of the intrusion silicate rocks have ni cu ratios ranging from 0 3 to 46 with majority less than 7 and are rich in lree relative to hree and show nb and ta anomalies in primitive mantle normalized trace element patterns with low nb th 1 0 4 5 and nb la 0 3 1 0 ratios their nd t values range from 3 3 to 8 4 uniform pd pt 0 7 3 5 and cu pd 100 000 400 000 ratios throughout the intrusion indicate that all the sulfides in the rocks were formed in a single sulfide saturation event modeling suggests that the baimazhai rocks were formed when an mg rich magma became crustally contaminated in a deep seated staging chamber crustal contamination up to 35 drove the magma to s saturation and forced orthopyroxene opx onto the liquidus the crystal bearing magma forced out of the staging chamber was migrated by flow differentiation and consequently the denser sulfide melt and the opx crystals became centrally disposed in the flowing magma to form the baimazhai intrusion

geology of the northern cleft segment juan de fuca ridge recent lava flows sea floor spreading and the formation of megaplumes geologic mapping and lava sampling were carried out after the discovery of large bursts of hydrothermal fluids megaplumes over the southern juan de fuca ridge in 1986 and 1987 our investigations of the northernmost section of the cleft segment have discovered 1 semicontinuous low temperature venting and one major high temperature vent site along 17 km of the neovolcanic zone and 2 very glassy lightly sedimented sheet flows and pillow mounds superimposed on older terrain over about 24 km along the northern most part the pillow mounds are documented to have erupted between 1981 and 1987 the occurrence of the megaplumes during this same time period strengthens the hypothesis that megaplumes are caused by sea floor extension events although the basalts from the entire length of the neovolcanic zone of the cleft segment appear to have been derived from the same mantle source a systematic northward increase in mg number along the segment within the neovolcanic zone indicates less shallow level differentiation to the north possibly related to the development of new magma chambers during the recent phase of sea floor spreading that has occurred there

siderophile and chalcophile metals as tracers of the evolution of the siberian trap in the noril sk region russia abstract in this study cu ni and platinum group elements pge were determined in a sequence of basaltic and picritic lavas from the siberian trap in the noril sk area of russia to constrain genetic relationships between the basalts and the petrogenesis of ni cu pge sulfide deposits associated with the talnakh and noril sk intrusions in the most primitive basalts 8 19 wt mgo of the tuklonsky tk suite pt and pd concentrations range from 4 13 ppb increasing with decreasing mgo content whereas ir contents decrease with mgo from 0 8 0 05 ppb the contrasting behavior of these elements which all have very high sulfide silicate partition coefficients as well as the primitive mantle like ratios of cu y and pd y suggests that these magmas were not sulfide saturated the high pge abundances imply that their parental magmas were also not sulfide saturated during partial melting in the mantle due to sulfide segregation the overlying basalts of the nadezhdinsky nd series are low in cu and ni 52 and 38 ppm respectively highly depleted in all pge and have very low cu y pd y and pd cu ratios however in stratigraphically higher levels cu ni and pge concentrations increase systematically through the morongovsky mr suite to reach a concentration plateau in the uppermost mokulaevsky mk suite pt 8 ppb pd 9 ppb ir 0 12 ppb rh 0 4 ppb at the same time ratios such as cu y increase and approach primitive mantle values however ratios involving pge such as pd y remain low suggesting the removal of small amounts of sulfide 0 01 0 03 the compositional variations in the basalts and the sulfide liquids can be quantitatively described by fractional segregation of a sulfide liquid in an open or closed system magma chamber the latter model suggests that the basalts represent the eruption products of a zoned magma chamber in which light magma with crustal components contaminated overlies less contaminated denser magma crustal contamination caused sulfide saturation and the resulting sulfide liquids settled through a magma column and accumulated at the bottom of the chamber in this model the sulfide liquid is not in equilibrium with the whole magma mass and sulfide segregation is compared with the zone refining process of metallurgy the sulfides become more enriched as they move through the magma and although the magma left behind is depleted in pge cu and ni their concentrations also increase with depth eventually the magma chamber is emptied from the top to the bottom producing the flood basalt sequence and the associated intrusions and ore deposits in the open system model sulfide saturation was initially caused by assimilation of crustal material by the tuklonsky magma continuous and simultaneous replenishment assimilation and crystallization processes formed the lower nd lavas the concurrent removal of 0 5 1 sulfide strongly depleted these magmas in chalcophile and siderophile metals due to the continuous replenishment of the magma chamber with uncontaminated pge rich magma succeeding lavas mr mk show diminishing signs of crustal contamination and become less sulfide saturated as indicated by the increasing ni cu and pge abundances during the evolution of the chamber the magma remained sulfur saturated and sulfides accumulated at the base the composition of the sulfide ores could be regarded as a mixture consisting of low ni cu and pge sulfides derived with a low silicate sulfide ratio 100 from the tk nd magma and high ni cu and pge sulfides formed with a high ratio 10 000 from the mr mk magma

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

fluxes to sediments underlying the rainbow hydrothermal plume at 36 14 n on the mid atlantic ridge abstract a geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the rainbow hydrothermal field as well as a large biogenic component 80 caco3 these sediments record clear enrichments of the elements fe cu mn v p and as from hydrothermal plume fallout but only minor detrital background material systematic variations in the abundances of hydrothermal elements are observed at increasing distance from the vent site consistent with chemical evolution of the dispersing plume further pronounced ni and cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near field locations radiocarbon dating reveals mean holocene accumulation rates for all four cores of 2 7 to 3 7 cm kyr 1 with surface mixed layers 7 to 10 cm thick from which a history of deposition from the rainbow hydrothermal plume can be deduced deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced e g fe mn cu or scavenged from seawater via the hydrothermal plume e g v p as holocene fluxes into to the cores surface mixed layers are presented which typically are an order of magnitude greater than background authigenic fluxes from the open north atlantic one core collected closest to the vent site indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14c kyr ago the preferred explanation is that this variation reflects the initiation intensification of hydrothermal venting at the rainbow hydrothermal field at this time perhaps linked to some specific tectonic event in this fault controlled hydrothermal setting

mantle source magma differentiation and sulfide saturation of the 637ma zhouan mafic ultramafic intrusion in the northern margin of the yangtze block central china abstract the sill like zhouan mafic ultramafic intrusion in the northern margin of the yangtze block formed at 637 4 ma and consists of a lherzolite unit and an olivine gabbronorite unit with a marginal zone between the lower margin of the olivine gabbronorite unit and the underlying country rocks composed of marble and schist of the meso to neoproterozoic zhujiashan formation rocks from the lherzolite unit have 41 4 44 0 wt sio2 and 29 0 38 4 wt mgo and contain olivine with fo values ranging from 80 to 85 mole indicating crystallization from a moderately evolved magma they commonly have we propose that moderately evolved magmas may have reached sulfide saturation due to fractionation of olivine in a deep seated staging magma chamber forming an olivine laden crystal mush with minor sulfide droplets in the lower part and more evolved pge depleted magmas in the upper part of the chamber the evolved pge depleted magmas were forced out of the staging chamber and entered the shallow magma chamber to form the olivine gabbronorite unit the lherzolite unit formed from the emplacement of the olivine laden crystal mush into the same chamber because of paucity of extensive crustal contamination only minor amounts of sulfide segregation occurred in the olivine gabbronorite unit and the marginal zone the zhouan intrusion thus may not be a good target for economic ni cu sulfide deposit

hydrothermal scavenging at the mid atlantic ridge radionuclide distributions we have carried out a study of u series radionuclide distributions in the neutrally buoyant plume overlying the tag hydrothermal vent field at 26 n mid atlantic ridge limited uptake of u accompanies the formation of fe oxyhydroxide particulate material within the hydrothermal plume the particulate u concentrations are too small for measurable dissolved u anomalies to be produced within the water column at plume height however such uptake followed by deposition in the underlying sediments could account for 10 of the u enrichment that has been reported previously for metalliferous ridge crest sediments particulate 234th 230th and 228th activities in the tag hydrothermal plume represent 65 of total th activities typical of the deep ocean low dissolved th activities coincide with the high particulate th activities indicating that hydrothermal scavenging strongly affects th isotope distributions in the water column close to hydrothermal vents much of the scavenging occurs during the initial formation of fe rich hydrothermal precipitates but some further scavenging continues as the material is dispersed away from its source distributions of particulate 210pb and 231pa activities in the neutrally buoyant plume are also extremely high 7 12 dpm 102 l and 30 160 dpm 106 l respectively like th 210pb and231pa show evidence for scavenging within the hydrothermal plume total th activities at plume height are 50 higher than background this focusing of th into the plume is consistent with the entrainment and re emplacement at plume height of particulate material recycled from beneath the neutrally buoyant plume

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

the global average production rate of10be precipitation collected in continuously open containers for about a year at seven sites around the united states was analyzed for10be 90sr 210pb and238u based on these data and long term precipitation 90sr and210pb delivery patterns the stratospheric tropospheric and recycled10be components in the collections were estimated and the global10be production rate was assessed single station production rate estimates range from 0 52 106 atoms cm 2 yr 1 to 2 64 106 atoms cm 2 yr 1 the mean value is 1 21 106 atoms cm 2 yr 1 with a standard error of 0 26 106 atoms cm 2 yr 1

removal of230th and231pa from the open ocean concentrations of230th and231pa were measured in particulate matter collected by sediment traps deployed in the sargasso sea site s2 the north equatorial atlantic site e and the north equatorial pacific site p as well as in particles collected by in situ filtration at site e concentrations of dissolved th and pa were determined by extraction onto manganese dioxide adsorbers at site p and at a second site in the sargasso sea site d dissolved230th 231pa activity ratios were 3 6 at sites p and d in contrast for all sediment trap samples from greater than 2000 m unsupported230th 231pa ratios were 22 35 average 29 7 ratios were lower in particulate matter sampled at shallower depths particles filtered at 3600 m and 5000 m at site e had ratios of 50 and 40 results show that suspended particulate matter in the open ocean preferentially scavenges th relative to pa most of the230th produced by decay of234u in the open ocean is removed by adsorption to settling particulate matter in contrast less than 50 of the231pa produced by decay of235u is removed from the water column by this mechanism mixing processes transport the remainder to other sinks

geochemistry of the siberian trap of the noril sk area ussr with implications for the relative contributions of crust and mantle to flood basalt magmatism the sequence investigated of the siberian trap at noril sk ussr consists of at least 45 flows that have been divided into six lava suites the lower three suites consist of alkalic to subalkalic basalts the ivakinsky suite overlain by nonporphyritic basalts the syverminsky suite and porphyritic and picritic basalts the gudchikhinsky suite the upper three suites are tholeiitic the uppermost 750 m of dominantly non porphyritic basalt belong to the mokulaevsky suite and are characterized by a nearly constant mg number 0 54 0 56 sio2 48 2 49 1 wt ce 12 18 ppm and ce yb 5 8 the underlying 1100 m of dominantly porphyritic basalt belong to the morongovsky and nadezhdinsky suites there is a continuous increase in sio2 48 1 55 2 wt ce 12 41 ppm and ce yb 5 18 from the top of the mokulaevsky to the base of the nadezhdinsky with little change in the mg number 0 53 0 59 mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid ocean ridge basalts the change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by mokulaevsky magma the nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the noril sk talnakh mining camp the mokulaevsky and morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the noril sk region these magmas may have risen up out of a different conduit system

beryllium 10 thorium 230 and protactinium 231 in galapagos microplate sediments implications of hydrothermal activity and paleoproductivity changes during the last 100 000 years biogenic particle fluxes from highly productive surface waters boundary scavenging and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the galapagos microplate eastern equatorial pacific in order to evaluate the importance of these three processes throughout the last 100 kyr concentrations of the radionuclides 10be 230th and 231pa and of mn and fe were measured at high resolution in sediment samples from two gravity cores klh 068 and klh 093 high biological productivity in the surface waters overlying the investigated area has led to 10be and 231pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr however during periods of high productivity at the up welling centers off peru and extension of the equatorial high productivity zone a relative loss of 10be and 231pa may have occurred in these sediment cores because of boundary scavenging the effects of hydrothermal activity were investigated by comparing the 230thex concentrations to the mn fe ratios and by comparing the fluxes of 230th and 10be which exceed production the results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core klh 093 during isotope stages 2 and 3 the hydrothermal supply of mn was deposited elsewhere probably because of changes in current regime or deep water oxygenation a strong increase of the mn fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230thex concentration is interpreted to be an effect of mn remobilization and reprecipitation in the sediment

oxyanions in metalliferous sediments tracers for paleoseawater metal concentrations abstract the concentration of carbon fe mn s p v cr as mo cd u and re were determined in sediment core y 71 7 53p near an active hydrothermal location on the southern east pacific rise crest 10 s the goal was to determine the usefulness of hydrothermal iron oxides as carriers of elemental tracers of seawater concentrations the sediments are 70 carbonate and about 1 3 detrital material with the remainder having a hydrothermal origin rhenium concentrations are at detrital levels suggesting little anaerobic diagenesis beyond manganese remobilization however u cd and mo concentrations all indicate alteration after deposition oxyanion fe ratios of p v and as in surface sediments are lower than the ratios in plume particles and covary with fe content downcore the probable explanation for these variations is variable efficiency of incorporation of these metals during iron oxide formation or early diagenetic processes after deposition the p fe ratio in the core sediment is about half that in the plume particles among the oxyanion ratios only the v as ratio in the sediments is relatively constant at the present plume particle ratio during the past 60 70 kyr changes in sediments older than this could reflect variability in the incorporation efficiency or changes in v as ratio in seawater this study demonstrates the difficulties of using hydrothermal sediment element enrichments as tracers of past seawater chemistry changes in this core it appears that all oxyanion ratios change for reasons other than seawater metal concentration changes with the possible exception of the v as ratio

examination of hydrothermal influences on oceanic beryllium using fluids plume particles and sediments from the tag hydrothermal field we have carried out a comprehensive study of the distributions of9be and the cosmogenic radionuclide10be in fluids nearfield sediments and neutrally buoyant plume particles associated with the tag hydrothermal field these results coupled with previously published uranium decay series radionuclide distributions 1 2 suggest that the level of mno2 in iron oxide rich sediments and particles may play a role in be scavenging in addition the data are used to quantify the net effects of nearfield coprecipitation with fes and of scavenging by iron oxyhydroxide plume particles on the oceanic balance of the be isotopes these calculations suggest that hydrothermal input of9be is nearly balanced by nearfield and plume removal processes and that hydrothermally induced scavenging has a negligible effect on the oceanic10be budget

composition and dissolution of black smoker particulates from active vents on the juan de fuca ridge during two atlantis ii alvin cruises to the juan de fuca ridge in 1984 active high temperature 140 284 c vents were sampled for black smoker particulates using the grassle pump individual mineral phases were identified using standard x ray diffraction and petrographic procedures in addition elemental compositions and particle morphologies were determined by x ray energy spectrometry and scanning electron microscope x ray energy spectrometry techniques the vent particulates from the southern juan de fuca ridge vent sites were highly enriched in s si fe zn and cu and were primarily composed of sphalerite wurtzite pyrite pyrrhotite barite chalcopyrite cubanite hydrous iron oxides and elemental sulfur two additional unidentified phases which were prevalent in the samples included an fe si phase and a ca si phase the grain sizes of the individual particle phases ranged from 100 m for the fe si particles grain size and current meter data were used in a deposition model of individual phase dispersal for many of the larger sulfide and sulfate particles the model predicts dispersal to occur over length scales of only several hundreds of meters the high temperature black smokers from the more northerly endeavour segment vents were highly enriched in fe s ca cu and zn and were primarily composed of anhydrite chalcopyrite sphalerite barite sulfur pyrite and other less abundant metal sulfide minerals the grain sizes of the individual particles ranged from 10 m to slightly larger than 500 m the composition and size distributions of the mineral phases are highly suggestive of high temperature mixing between vent fluids and seawater a series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals the dissolution rates ranged over more than 3 orders of magnitude from 3 2 10 8 cm s 1 for anhydrite to 1 2 10 12 cm s 1 for chalcopyrite the results indicate that for some minerals particularly anhydrite and marcasite total dissolution occurs within a few hours to a few weeks of their formation for other more stable minerals including pyrite sphalerite and chalcopyrite the time required for total dissolution is much longer and consequently individual crystals may be expected to persist in the sediments for considerable periods of time after deposition

the relationship between p fe and v fe ratios in hydrothermal precipitates and dissolved phosphate in seawater a strong positive relationship between the molar p fe ratio in fresh hydrothermal fe ferrihydrite precipitates and seawater dissolved phosphate has been demonstrated using samples from the atlantic and pacific oceans we ascribe this relationship to scavenging of dissolved phosphate from seawater during the formation of hydrothermal fe rich particles above hydrothermal vents in contrast molar v fe ratios in hydrothermal particles are inversely correlated with dissolved phosphate these results indicate that inter ocean variations in dissolved phosphate control the p fe and v fe ratios in hydrothermal fe precipitates one of the precursors of metalliferous sediment

geochemistry and morphology of metalliferous sediments and oxyhydroxides from the endeavour segment juan de fuca ridge abstract we present first data on the geochemistry mineralogy and morphology of near vent sediments 35 and 200 m from active vent and ridge flank sediments approximately 3 km from the vent field as well as oxyhydroxide deposits from the endeavour segment juan de fuca ridge the purpose of the study was to understand better the origin and characteristic features of metalliferous sediments associated with base and precious metal massive sulfides in volcanic terrains hydrothermal components in sediments are fe si s rich and mn fe si rich phases sulfides and barite which were exclusively derived from plume fallout sulfides are only a minor constituent of near vent sediments 2 4 wt and were not detected in ridge flank sediments the study suggests that the distribution of hydrothermal phases and associated elements in near vent and ridge flank sediments is affected mainly by processes of agglomeration dissolution absorption and settling that take place within a plume and to a lesser extent post depositional processes rapid deposition of sulfides in the vicinity of the vents is reflected in a sharp drop of the cu concentrations in sediments with increasing distance from the vents besides sulfides important carriers of pb cu zn and co in near vent sediments are fe mn oxyhydroxides that occur together with silica as aggregates of gel like material and flaky particles and as coatings on filaments away from the vents trace metals are mostly in fe mn oxyhydroxides and authigenic fe rich montmorillonite oxyhydroxides at the main endeavour field are interpreted to have originated from oxidation of mound sulfides accompanied by precipitation of primary fe oxyhydroxide silica from low temperature fluids at the mothra field seafloor deposits and chimney crusts composed of fe oxyhydroxide mn silica are considered to be direct precipitates from hydrothermal fluids that have been less diluted with seawater oxyhydroxide deposits exhibit unique microtextures that resemble mineralized microorganisms and may indicate existence of diverse microbial communities

neoproterozoic arc related mafic intrusions along the northern margin of south china implications for the accretion of rodinia abstract south china has been considered as part of the rodinian supercontinent during neoproterozoic time although its paleogeographic position within this supercontinent is still a matter of debate the wangjiangshan and bijigou complexes along the northern margin of south china are among the largest mafic intrusions in china new shrimp zircon u pb results indicate that these two intrusions have crystallization ages of 820 ma and 780 ma respectively enrichment of large ion lithophile and light rare earth elements and depletion of high field strength elements in these intrusions suggest derivation from an active arc along a continental margin this interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a neoproterozoic mantle plume that initiated the breakup of rodinia the presence of a neoproterozoic magmatic arc suggests that rodinian oceanic lithosphere was subducted beneath the present northern margin of south

the effect of hydrothermal processes on midwater phosphorus distributions in the northeast pacific abstract the distributions of dissolved particulate and sedimentary phosphorus were measured in the region of the juan de fuca ridge to determine the impacts of hydrothermal processes on the phosphorus cycle in the oceans significant negative dissolved phosphate anomalies ranging from 0 to 60 nmol l were observed in the water column at depths between 1900 and 2300 db the largest anomalies

sulfur saturation and second stage melts application to the bushveld platinum metal deposits a model is presented for the formation of platinum group element ores by precipitation from sulfur deficient platinum group element rich second stage magmas numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption mantle derived peridotites residual from midocean ridge basalt magma genesis have pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation they imply the presence of an accessory immiscible sulfide melt to host the platinum group elements the very high sulfide melt silicate melt partition coefficients of the platinum group elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event the extreme platinum group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing pd and pt abundances at the low ppb level keays et al 1981 magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation these first stage magmas become impoverished in platinum group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase e g midocean ridge basalt glasses average

sedimentary and geomorphic responses to ignimbrite emplacement readjustment of the waikato river after the a d 181 taupo eruption new zealand abstract the posteruption readjustment of landscapes to major explosive volcanic eruptions is one of the most dramatic processes in physical sedimentology and geomorphology large ignimbrite emplacing eruptions from rhyolitic caldera volcanoes can form new landscapes over 104 km2 create new depocenters in calderas and bury existing topography and drainage systems beneath hundreds of meters of pyroclastic material subsequent remobilization of this debris constitutes a widespread and enduring hazard in the central north island of new zealand the a d 181 taupo eruption destroyed hydrologic systems over 20 000 km2 when emplacement of the climactic ignimbrite filled valleys and depressions with up to 70 m of pyroclastic material and blocked the waikato river outlet to the intracaldera lake basin posteruption remobilization of this material in the waikato catchment was associated with axial stream reestablishment vertical lateral and proximal distal changes in volcaniclastic lithofacies preserved al

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

hydrothermal scavenging at the mid atlantic ridge modification of trace element dissolved fluxes abstract we have carried out an icp ms study of the distributions of trace elements fe ca mg al mn cu zn v as cr co pb y mo ag sn cd and be in particulate material collected from the tag hydrothermal plume 26 n mid atlantic ridge to examine the extent to which fluxes of dissolved trace elements emitted from hydrothermal vent fluids are balanced by the precipitation of associated particulate phases comparison of particulate trace element distributions with particulate fe concentrations within the neutrally buoyant plume reveals that the distribution of trace elements within the plume is controlled by the following processes i co precipitation from vent fluids ii scavenging from seawater and iii preferential settling from the plume and oxidative dissolution within the water column for y and be continuous scavenging from seawater appears to be important whereas for v as and cr it appears that conditions suitable for uptake into particles may be restricted to the buoyant portion of the hydrothermal plume from a comparison of trace element fe ratios in the particles and associated mid atlantic ridge vent fluids it appears that for many elements e g cu co ag pb al cd be and zn the flux of dissolved material from vent fluids is matched closely by the precipitation of hydrothermal particulate phases of the various elements studied here hydrothermal activity only appears to act as a significant source for dissolved ca and dissolved mn

u pb ages and lu hf isotopes of detrital zircons from the southern qinling orogen implications for precambrian to phanerozoic tectonics in central china abstract the qinling orogen central china was constructed during the mesozoic collision between the north china and yangtze continental plates the orogen includes four tectonic units from north to south the huaxiong block reactivated southern margin of the north china craton north qinling accretion belt south qinling fold belt or block and songpan fold belt evolved from the northernmost paleo tethys ocean separating the gondwana and laurentia supercontinents here we employ detrital zircons from the early cretaceous alluvial sediments within the qinling orogen to trace the tectonic evolution of the orogen the u pb ages of the detrital zircon grains from the early cretaceous donghe group sediments in the south qinling fold belt cluster around 2600 2300 ma 2050 1800 ma 1200 700 ma 650 400 ma and 350 200 ma corresponding to the global kenorland columbia rodinia gondwana and pangaea supercontinent events respectively the distributions of ages and ehf t values of zircon grains show that the donghe group sediments have a complex source comprising components mainly recycled from the north qinling accretion belt and the north china craton suggesting that the south qinling fold belt was a part of the united qinling north china continental plate rather than an isolated microcontinent during the devonian triassic the youngest age peak of 350 200 ma reflects the magmatic event related to subduction and termination of the mian lue oceanic plate followed by the collision between the yangtze craton and the united qinling north china continent that came into existence at the triassic jurassic transition the interval of 208 145 ma between the sedimentation of the early cretaceous donghe group and the youngest age of detrital zircons was coeval with the post subduction collision between the yangtze and the north china continental plates in jurassic

sulphide segregation in ferropicrites from the pechenga complex kola peninsula russia parental ferropicritic magma in differentiated flows and intrusions the palaeoproterozoic ni cu sulphide deposits of the pechenga the sulphide liquids segregated and accumulated at the base of these complex kola peninsula occur in the lower parts of ferropicritic bodies but because of a low silicate sulphide mass ratio the sulphide intrusions emplaced into the phyllitic and tuffaceous sedimentary liquids had a low pge tenor and pt ir and cu ir ratios similar unit of the pilgujarvi zone the intrusive rocks are comagmatic to the parental silicate melts during cooling the sulphide liquid with extrusive ferropicrites of the overlying volcanic formation crystallized 40 50 of monosulphide solid solution mss and the massive lavas and chilled margins from layered flows and intrusions residual sulphide liquid became enriched in cu pt and pd and contain 100 most of the massive and breccia ores represent during crystallization of the parental magma compared with the mixtures of mss and residual cu rich sulphide liquid whereas pge depleted central zones of differentiated flows spinifex and chalcopyrite rich veins formed when the cu rich sulphide liquids clinopyroxene cumulate zones the olivine cumulate zones at the were squeezed out into the surrounding sediments base contain elevated pge abundances up to 10 ng g pd and pt a similar pattern is displayed in intrusive bodies such as the kammikivi sill and the pilgujarvi intrusion the olivine cumulates at the base of these bodies contain massive and disseminated ni cu

particle geochemistry in the rainbow hydrothermal plume mid atlantic ridge we report the analysis of 18 large volume 500 1500 l in situ filtered samples of particulate material from the largest hydrothermal plume on the mid atlantic ridge overlying the ultramafic hosted rainbow hydrothermal field at 36 deg 14 n measured particulate iron concentrations reach 614 nm high concentrations of particulate fe oxyhydroxides result from the extremely high fe concentration 24 mm and fe h2s ratio 24 of the vent fluids and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along axis flow two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume these samples are characterized by anomalously compared to neutrally buoyant plume samples high concentrations of mg u and chalcophile elements and low concentrations of mn ca v y and the rare earth elements ree within the neutrally buoyant plume elemental distributions are largely consistent with previously observed behaviors preferential removal of chalcophile elements conservative behavior of oxyanions p v and u and continuous scavenging of y and the ree this consistency is particularly significant in light of the underlying differences in fluid chemistry between rainbow and other studied sites chalcophile elements are preferentially removed from the plume in the order cd zn co cu phosphorus iron and vanadium iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the rainbow plume cave et al 2002 indicates however that while p fe ratios are indeed constant v fe ratios increase progressively from early stage plume particles to sediments ree distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column

composition and sedimentation of hydrothermal plume particles from north cleft segment juan de fuca ridge in 1990 and 1991 particles from buoyant and neutrally buoyant hydrothermal plumes above hydrothermal vents at the north cleft segment of the juan de fuca ridge were sampled to study their changing composition and fluxes away from the vent field in the rising buoyant plume 75 of the p v cr and as scavenging from seawater by hydrothermal precipitates occurs in the first 50 m above the vent cu and zn are most enriched in buoyant plume particles collected from the first few meters above the vent however the degree of enrichment decreases very rapidly with increased height above the vents due to sedimentation of the more dense cu and zn rich sulfide phases using the plume data coupled with the results of our analysis of sediment trap samples we estimated that more than 99 and 99 9 respectively of the total hydrothermal fe and mn produced at the vent field are transported beyond the vent field and dispersed in the open ocean

piston core record of late paleogene 31 ma to recent seafloor hydrothermal activity in the southwest pacific basin a large diameter piston core containing 8 35 m of metalliferous sediment has been recovered from a small abyssal valley in the remote southwest pacific basin 31 42 194 s 143 30 331 w 5082 m water depth providing unique insight into hydrothermal activity and eolian sedimentation there since the early oligocene a combination of fish teeth sr isotope stratigraphy and inaa geochemical data reveals an exponentially decreasing hydrothermal flux 31 ma to the present although hydrothermal sedimentation related to seafloor spreading explains this trend a complex history of late eocene early oligocene ridge jumps propagating rifts and plate tectonic reorganization of south pacific seafloor could have also played a role a possible hiatus in deposition as recorded by changes in core composition just below 2 m depth is beyond the resolution of the fish teeth sr isotope dating method employed here however the timing of this interval may be coincident with extinction of the pacific farallon ridge at 20 ma a low flux eolian component accumulating at this site shows an increase relative to the hydrothermal component above 2 m depth consistent with dust generating continental sources far to the west australia new zealand this is the first long term paleoceanographic record obtained from within the south pacific bare zone rea et al 2006 an anomalous region where pacific seafloor has largely escaped sediment accumulation since the late cretaceous

volcaniclastic resedimentation in distal fluvial basins induced by large volume explosive volcanism the ebisutoge fukuda tephra plio pleistocene boundary central japan the ebisutoge fukuda tephra plio pleistocene boundary central japan has a well recorded eruptive style history magnitude and resedimentation styles despite the absence of a correlative volcanic edifice this tephra was ejected by an extremely large magnitude and complex volcanic eruption producing more than 400 km 3 total volume of volcanic materials volcanic explosivity index 7 which extended more than 300 km away from the probable eruption centre remobilization of these ejecta occurred progressively after the completion of a series of eruptions resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins more than 100 km from the source facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins the distal tephra 100 300 km from the source comprises two different lithofacies primary pyroclastic fall deposits and reworked volcaniclastic deposits the resedimented volcaniclastic succession shows five distinct sedimentary facies interpreted as debris flow deposits facies a hyperconcentrated flow deposits facies b channel fill deposits facies c floodplain deposits with abundant flood flow deposits facies d and floodplain deposits with rare flood deposits facies e resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic large ignimbrite forming eruption factors controlling inter and intrabasinal facies changes are 1 temporal change of introduced volcaniclastic materials into the basin 2 proximaldistal relationship and 3 distribution pattern of pyroclastic flow deposits relative to drainage basins thus studies of the ebisutoge fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large magnitude eruptions an aspect of volcaniclastic deposits that has often been ignored or poorly understood

uptake and fractionation of rare earth elements on hydrothermal plume particles at 9 45 n east pacific rise abstract particulate samples 0 45 m from a neutrally buoyant hydrothermal plume at 9 45 n on the northern east pacific rise were collected using large volume in situ filtration and analyzed for fe al mn ni and fourteen rare earth elements ree the sm fe ratio a proxy for overall ree fe and nd er light heavy ree fractionation increased moderately with decreasing particulate fe chemically the sense of these relationships matched that documented previously in the tag plume on the mid atlantic ridge german et al 1990 although particulate fe was about 10 fold lower at 9 45 n spatial trends relative to the vent source however were opposite of expectation because slow fe ii oxidation and fe iii colloid aggregation over this interval led to increased particulate fe 10 26 nm with distance from source field and sherrell submitted after subtraction of non plume background particle composition plume particles at 9 45 n and tag had indistinguishable ranges of light ree enriched fractionation relative to ambient seawater and had very similar sm fe therefore kd for fe oxyhydroxides demonstrating that plume particles in both oceans reflect to a first degree the local seawater ree composition within plume ree variations at 9 45 n were investigated using a simple mixing model which accounts for the bulk fe al mn variations in the plume using two endmembers fresh hydrothermal oxyhydroxide precipitates and ridge crest background particles composed largely of locally resuspended sediment sm fe and nd er plot linearly with mixing ratio r 0 96 implying that the observed ree trends result from mixing of these two endmembers extrapolation to the composition of pure hydrothermal precipitates suggests that nd er is fractionated relative to seawater by a factor of 1 8 during adsorption onto fresh fe oxyhydroxide particles the ridge crest background particles are 5 fold higher in sm fe and nd er is 2 49 relative to seawater partly a result of enriched terrigenous component in the resuspended matter a reinterpretation of ree at tag reveals that positive curvature in ree vs fe plots argued previously to reflect continuous ree uptake i e increasing kd german et al 1990 may result from local depletion of the dissolved ree pool by partitioning onto fe particles at fe 100 nm similar drawdown effects could contribute to the variable degrees of curvature observed for all seawater source particle reactive species in plumes that are sampled at high particulate fe concentration in sum ree behavior in hydrothermal plumes is more consistent with equilibrium adsorption and mixing of distinct particle types than with kinetic uptake control precise measurements of rees in modern ridge crest metalliferous sediments could be compared to the endmember composition calculated from the plume data to evaluate long term changes in ree of the hydrothermal component

pb sr and nd isotopes in basalts and sulfides from the juan de fuca ridge pb sr and nd isotopes of seven basalt glasses collected by the submersible alvin from the southern juan de fuca ridge sjfr are almost identical 206pb 204pb 18 45 207pb 204pb 15 47 208pb 204pb 37 81 87sr 86sr 0 70249 143nd 144nd 0 51315 whereas all basalts appear cogenetic four of the samples have uniform abundances of u th rb nd sm pb and sr indicating that they are also comagmatic two basalt glasses dredged previously at the sjfr have similiar isotopic compositions but higher concentrations of u th and pb the 206pb 204pb ratios are intermediate between generally less radiogenic ridge basalts from south of the juan de fuca ridge jfr and often more radiogenic basalts from the northern jfr and ne pacific seamounts sr and nd isotopic compositions closely resemble data of other ridge basalts from the northernmost east pacific rise and are intermediate between isotopically more diverse seamount basalts produced nearby eight sulfides collected from three hydrothermal vent sites at sjfr have very similar pb isotopic compositions to that of the associated basalts sr and nd isotopic ratios of a single sulfide sample lie on a mixing curve between seawater and basalt data indicating that the sulfide precipitated after mixing of hydrothermal solutions and ambient seawater

global environmental effects of large volcanic eruptions on ocean chemistry evidence from hydrothermal sediments odp leg 185 site 1149b a sr nd pb isotope investigation has been carried out on sediments overlying the cretaceous oceanic crust at odp leg 185 site 1149b the sediments biogenic carbonate and siliceous clays contain two horizons with high excess concentrations of fe zn cu and pb these horizons are both characterized by rare earth element patterns that are concave upwards and exhibit both negative ce and positive eu anomalies comparable to those from modern hydrothermal plume particles and sediments however pb isotope signatures differ significantly between the two metal rich horizons metal rich sediments in the lower part of unit iv have pb isotopes that lie on a mixing line between seawater and local oceanic crust end members which is consistent with a hydrothermal fall out originating from a local oceanic spreading centre in contrast sediments from the upper part of unit iv have much more radiogenic pb isotope ratios that cannot be related readily to local end members off axis hydrothermal vent clays oceanic crust or large igneous provinces lip their age corrected pb isotope compositions do however overlap with basalts from the parana etendeka lip evidence for related environmental effects include a drastic biotic change higher oceanic 13c values and more radiogenic seawater 87sr 86sr we estimate that the parana etendeka lip released 1 1019 g of carbon into the atmosphere as co2 corresponding to pb and s degassing fluxes of 1 109 g a 1 and 15 1012 g a 1 respectively the study demonstrates that pb isotopes when combined with other geochemical parameters are ideal tools for detecting and tracing lip volcanism in the marine geological records

geomorphic and sedimentary evidence of a gigantic outburst flood from towada caldera after the 15 ka towada hachinohe ignimbrite eruption northeast japan abstract this study reports on geomorphic and sedimentary evidence for a gigantic breakout flood from towada caldera honshu island northeast japan as observed along the oirase river downstream from the outlet of the caldera lake a number of features of the oirase river are inconsistent with its present discharge including the occurrence of 1 hanging valleys and a horseshoe shaped waterfall in the upstream gorge area 2 boulder bars and scattered flood boulders 3 a dry valley at a high elevation in the gorge and 4 a large fan the sanbongi fan in the downstream area of the river the sanbongi fan is composed of thick lithic rich hyperconcentrated flow deposits that include pumice clasts derived from the 15 ka towada hachinohe ignimbrite and well rounded meter sized and outsized boulders derived from bedrock of welded ignimbrite the deposits are entirely aggradational with no major channels indicating the absence of a major hiatus during sedimentation the depositional facies also indicate that a single sheet like flood event deposited the sediment within the sanbongi fan area based on the age of the sanbongi fan the flood occurred between 15 and 12 ka after eruption of the towada hachinohe ignimbrite the most probable water source for the flood is towada caldera lake as suggested by landforms and sediments along the lake outlet a paleohydrological analysis indicates that at least 6 km 3 of water was released from the caldera at a peak discharge of 2 10 4 to 3 10 5 m 3 s 1 during the breakout flood although the sanbongi fan was previously considered to be a normal alluvial fan that formed during a stage of low stand sea level the present results show that the formation of the fan was closely related to a catastrophic flood from towada caldera rather than perennial fluvial activity climate change or a change in relative sea level

geochemical evidence for proterozoic continental arc and continental margin rift magmatism along the northern margin of the yangtze craton south china abstract middle to late proterozoic volcanic intrusive magmatism of continental island arc and continental margin rift nature is well developed in the xixiang beiba area shaanxi province along the northern margin of the yangtze craton in the xixiang area volcanism evolved temporally from submarine low k arc tholeiite through calc alkaline to subaerial alkali or shoshonitic this was accompanied by gradational enrichment of lree and zr an increasing negative eu anomaly as well as strong depletion of compatible elements cr co v and sc the associated hannan intrusive complex evolved from gabbro through tonalite trondhjemite granodiorite to granite with the distribution of basic to felsic plutons suggesting arc polarity there is also a systematic lateral increase in k 2 o content k 2 o na 2 o ratio and la n yb n ratio towards the interior of the yangtze craton in these rocks this spatial framework and compositional variation resembles that found in circum pacific island arcs or active continental margins in contrast both volcanic and intrusive rocks in the beiba area are characterized by alkaline compositions and a bimodal habit they are composed of tholeiite gabbro and alkali rhyodacite rhyolite alkali granite and lack rocks of intermediate composition they may have formed in a continental margin rift setting geological and geochemical evidence further suggests that the xixiang paleo arc was a mature continental arc built on older sialic basement the emplacement of voluminous tonalites and trondhjemites thickened the crust and may have marked an important continent building event at the northern margin of the yangtze craton in the middle to late proterozoic

particle recycling in the tag hydrothermal plume abstract analysis of th isotope distributions in the tag hydrothermal plume yields evidence for pronounced 234 th 230 th isotope fractionation this fractionation can be attributed to a combination of four processes 1 particle ageing 2 differential scavenging of 234 th and 230 th 3 differential focusing of 234 th and 230 th and 4 recycling of old particles we present arguments for the importance of the last of these processes particle recycling particles in the tag hydrothermal plume exhibit grain sizes which fall in the range 1 10 m applying the range of densities reported for amorphous iron oxyhydroxide and iron oxide mineral phases stokes law predicts that these particles should achieve terminal settling velocities ranging between 4 and 8 m d 1 for coarse grained 5 m radius particles and 0 2 0 3 m d 1 for fine grained particles 1 m radius these velocities have been used in a physical model of re entrainment and recycling in neutrally buoyant plumes our results indicate that particle recycling and re entrainment in hydrothermal plumes may be important over length scales of 1 10 km the model predicts that the proportion of particles re entrained is independent of particle size but increases with plume strength for plumes with volumetric fluxes greater than 10 3 m 3 s 1 the proportion of particles re entrained exceeds 50 in agreement with previous calculations based upon 230 th distributions sampling strategies in the past have typically concentrated upon processes operating within 1 km of active vent fields and will need to be revised accordingly in the future

metalliferous sediments associated with presently forming volcanogenic massive sulfides the susu knolls hydrothermal field eastern manus basin papua new guinea short sediment cores were examined from the active susu knolls hydrothermal field in the eastern manus back arc basin papua new guinea in order to explain the origin of the currently accumulating metalliferous sediments the mineralogy and geochemistry of the sediments were studied by x ray diffraction scanning electron microscopy energy dispersive x ray fluorescence and instrumental neutron activation analysis sediments of the suzette site originated from the deposition of volcaniclastic material with the addition of a hydrothermal component from the mass wasting of old oxidized chimneys a strong mass wasting event was recognized at the base of the studied sedimentary succession from the occurrence of chalcopyrite pyrite and barite aggregates similar to those found in chimneys as well as the presence of atacamite the metalliferous component is characterized by high concentrations of cu up to 2 3 and au up to 3 5 ppm elevated concentrations of as ba zn and fe as well as a positive eu anomaly the material derived from eroded chimneys was deposited together with abundant glass fragments eroded from the volcanic edifice the mass wasting was succeeded by deposition of volcaniclastic sediment containing dacite fragments ca plagioclase pyroxene glass shards cristobalite si rich amorphous material alunite pyrite barite and magnetite the volcaniclastic sediment may have originated from hydrothermal eruptions at north su and south su the present day plume at suzette is a likely source of rare barite aggregates amorphous cu containing aggregates and cu fe sulfides creating geochemical anomalies of ba and cu in the uppermost sediments at suzette interbedded volcaniclastic and hemipelagic sediments were retrieved east and west of susu knolls the enrichment of mn in these sediments is considered to be a result of settling of mn oxides from a hydrothermal plume

tracing dust input to the mid atlantic ridge between 14 45 n and 36 14 n geochemical and sr isotope study abstract a geochemical and sr isotope study was carried out on sediment cores collected from the logatchev hydrothermal vent site 14 45 n on the mid atlantic ridge with the aim of identifying aeolian inputs into the overlying water column the logatchev vent site lies beneath the ne trade winds which transport large amounts of detrital material across the north atlantic sequential dissolution of the sediments has been used to discriminate the detrital fraction from other types of sediment ree patterns for the acetic acid fraction at the logatchev site record contributions from biogenic carbonate seawater like ree pattern and oxy hydroxides which exhibit positive eu and negative ce anomalies typical of hydrothermal origin the logatchev site leach fractions have similar major element compositions to terrigenous al fe mn oxides whose ree patterns show strong positive ce and negative eu anomalies that are comparable to those of preformed fe mn oxides of continental origin based on major element and ree data and 87sr 86sr ratios 0 7197 0 7236 the residue fractions at the logatchev site contain a significant contribution from aeolian sources whose chemical composition is virtually identical to oligotrophic settling particles in the equatorial atlantic ocean in comparison residues from the rainbow hydrothermal site 36 14 n have geochemical composition controlled by hydrothermal anhydrite chavagnac v german c r milton j a palmer m r 2005 sources of ree in sediment cores from the rainbow vent site 36 14 n mar chem geol 216 329 352 they are characterised by 87sr 86sr ratios of 0 7087 0 7093 which are significantly less radiogenic than those of the saharan dust hence the data suggest 1 a significant contribution of aeolian material to the sediments at 14 45 n that is not present at 36 14 n 2 that this material is most likely of north african origin and 3 that this source of aeolian material has remained relatively constant over the last 13 kyr

u series isotopes and destructive plate margin magma genesis in the lesser antilles high precision tims230th232th isotope ratios are presented from a suite of lavas which span the length of the lesser antilles island arc compositions vary from low k tholeiitic lavas with low87sr86sr in the north to high k calc alkaline to alkalic lavas with higher87sr86sr in the south the northern lavas are characterised by 230th238u 1 a three component model is required for magma genesis in the lesser antilles sediments from the subducting slab melt and variably enrich the mantle wedge while the subducting oceanic crust dehydrates releasing fluids enriched in u ba sr and k but with negligible th and low87sr86sr which promote partial melting in the wedge ta zr increases with87sr86sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment mass balance calculations are consistent with a relatively constant fluid flux except for u and little variation in the degree of partial melting 10 15 along the arc and they suggest that the sediment contribution to the arc lavas increases from 2 in the north to 15 in the south in the northern parts of the arc the addition of u by these fluids overprints any increases inth u that might have been caused by partial melting and results in lavas with 230th238u 1 m yr 1 which imply channelled flow through the mantle wedge

10be and 9be in marine sediments and their potential for dating using sequential leaching experiments we have measured the fraction of the cosmogenic isotope 10be half life 1 5 my and its stable isotope 9be co extracted with the following phases in marine sediments exchangeable calcium carbonate fe and mn oxyhydroxides organic opal and detrital the extraction of the authigenic phases typically release 70 of the 10be but only 40 of the 9be in order to test its potential for dating marine sediments we have measured the authigeniccally associated 10be 9be in 17 surface sediment samples from different ocean basins the observed spatial variability which appears to be influenced by proximity to continental input of 9be precludes the general and precise applications of leached 10be 9be for dating however the relatively constant value observed in surface sediments over large areas of the pacific indian and southern atlantic oceans shows considerable promise for approximate dating of open oceans sediments in order to test temporal variability a profile of the authigenically associated 10be 9be ratio has been measured in the independently dated central pacific core rc12 65 the observed variation in 10be 9be over the past nine my is approximately twice that expected on the basis of 10be decay alone several possible explanations for such behavior and their potential consequences for the use of leached 10be 9be for dating are discussed

removal of 230th and 231pa at ocean margins abstract uranium thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the panama basin and by in situ filtration of large volumes of seawater in the panama and guatemala basins concentrations of dissolved th and pa isotopes were determined by extraction onto mno 2 adsorbers placed in line behind the filters in the in situ pumping systems concentrations of dissolved 230 th and 231 pa in the panama and guatemala basins are lower than in the open ocean whereas dissolved 230 th 231 pa ratios are equal to or slightly greater than ratios in the open ocean particulate 230 th 231 pa ratios in the sediment trap samples ranged from 4 to 8 in contrast to ratios of 30 or more at the open ocean sites previously studied particles collected by filtration in the panama basin and nearest to the continental margin in the guatemala basin contained 230 th 231 pa ratios similar to the ratios in the sediment trap samples the ratios increased with distance away from the continent suspended particles near the margin show no preference for adsorption of th or pa and therefore must be chemically different from particles in the open ocean which show a strong preference for adsorption of th ocean margins as typified by the panama and guatemala basins are preferential sinks for 231 pa relative to 230 th furthermore the margins are sinks for 230 th and to a greater extent 231 pa transported by horizontal mixing from the open ocean

iridium ruthenium and rhodium in komatiites evidence for iridium alloy saturation abstract a large body of new data over 400 samples on platinum group element pge concentrations in yilgarn craton komatiites is combined with all available high quality literature data to reveal trends in a spectrum of komatiite lithologies ranging from spinifex textured rocks to pure olivine and olivine chromite adcumulates the data set is restricted to carius tube isotope dilution analyses and high precision nickel sulfide fire assay icp ms analyses and we demonstrate good agreement between the two methods only s poor samples are considered and the data set is filtered to eliminate the effects of addition of cumulus chromite and addition or segregation of magmatic sulfide liquid the residual trends indicate that ir ru and rh are decoupled from one another and from pt and pd rhodium behaves more like pt and pd whereas ru is intermediate between ir and rh rhodium and ru both show indications of olivine control with partition coefficients less than unity the maximum ir whole rock content is a roughly linear positive function of mgo but the minimum is constant sub 1 ppb and independent of mgo ruthenium shows some evidence for limited control by chromite but anomalous ru enrichment is also found in some olivine cumulate samples with no evidence for accumulated chromite or sulfide neither rh nor ir shows evidence for any detectable control by chromite the distribution of ir and to a lesser degree ru and rh cannot be adequately explained by olivine control and a major influence of an ir rich phase is strongly indicated the pattern of ir variance in relation to mgo bears some important similarities to that of cr this similarity arises from the fact that both elements are controlled by a temperature dependent solubility limit the chromite saturation surface in the case of cr and saturation in ir or ir os alloy in the case of ir combining this effect with moderate compatibility in olivine allows us to successfully model the entire range of ir data the upper limit of ir concentration in olivine cumulates in an exact analogy to that for cr is interpreted as the cotectic ratio of olivine to alloy as determined by the average slope of the tangent to the alloy solubility curve sulfide undersaturated mantle melts of a wide range of compositions are likely to be ir alloy saturated experimental predictions of very low ir solubilities in magmas are borne out with the further implication that true thermodynamic sulfide silicate liquid partition coefficients at least for ir and by inference for the other pges are of the order of 107 or even higher

dispersal patterns for hydrothermal plumes in the south pacific using manganese as a tracer abstract manganese profiles over the spreading axis of the east pacific rise epr between 22 15 s and 19 25 s disclose a series of plumes which are formed by buoyant emanations of hot hydrothermal vent fluids the plume ceilings or anomaly maxima can occur several hundred meters above the sea floor these plumes are dispersed laterally and are detectable as mid depth manganese anomalies 2000 km from the spreading axis along this section the plumes are dispersed mainly westward but at 30 s the plumes are advected eastward the asymmetrical dispersal pattern inferred from these data is coincident with the asymmetrical distribution of metalliferous sediments about the epr which suggests that plumes generated by high temperature venting play a dominant role in the formation of these sediments

beryllium isotope systematics of submarine hydrothermal systems abstract both 10 be and 9 be concentrations have been measured in a hydrothermal fluid from the sediment starved east pacific rise at 21 n as well as in fluids and associated sediments from the sediment covered guaymas basin and escanaba trough hydrothermal systems the results show that while the 9 be concentrations in fluids from both types of systems are comparable about three orders of magnitude higher than that of the seawater supplying the systems their 10 be concentrations are significantly different the 10 be concentration in the solution emanating from the studied sediment starved hydrothermal system is at least two times lower than that of the pacific bottom water by contrast the 10 be concentration in hydrothermal solutions from sediment hosted environments are enriched over ambient seawater and are at least one order of magnitude higher than that measured in the 21 n fluid the sedimentary cover being the only available source of 10 be in these systems these data demonstrate that 10 be is an unambiguous tracer of interactions between hydrothermal fluids and recent sediments as an example this study examines hydrothermal solutions from the sediment starved mariana trough back arc basin which have compositions similar in many respects to those from sediment hosted environments however these fluids are depleted in 10 be relative to ambient seawater which precludes interaction with recent sediment as the cause of their peculiar chemical composition

rare earth element geochemistry of standard sediments a study using inductively coupled plasma spectrometry the rare earth elements ree and y have been determined in 15 international standard sediments from france german democratic republic canada and the u s a samples were prepared using conventional rock dissolution techniques and the ree and y were separated from other constituents using cation exchange chromatography the ree la ce pr nd sm eu gd dy ho er yb lu and y were analysed simultaneously by inductively coupled plasma atomic emission spectrometry results indicate that u s g s standard shale sco 1 cody shale is a good reference material to use for the normalisation of the ree distributions of sediments data normalised to sco 1 are more readily interpreted than those normalised to existing estimates of average shale the ree patterns of standard sediments vary considerably in shape span an order of magnitude in absolute concentration and provide valuable reference values for the ree analysis of other sedimentary materials

u th and pb isotopes in hot springs on the juan de fuca ridge the concentrations and isotopic compositions of u th and pb in three hydrothermal fluids from the juan de fuca ridge were determined the samples consisted of 10 2 57 6 of the pure hydrothermal end members based on mg contents the pb contents of the samples ranged from 34 to 87 ng g u from 1 3 to 3 0 ng g and th from 0 2 to 7 7 pg g these samples showed large enrichments of pb and th relative to deep sea water and some depletion of u they did not show coherent relationships with mg however indicating nonideal mixings between the hot hydrothermal fluids and cold ambient seawater particles filtered from these hydrothermal fluids contained significant amounts of th and pb which may effectively increase the concentration of these elements in the fluids when acidified the sup 234 u sup 238 u values in all samples show a sup 234 u enrichment relative to the equilibrium value and have a seawater signature the pb isotopic composition of the juan de fuca hydrothermal fluids resembles that of 21 sup 0 n east pacific rise and has a uniform mid ocean ridge basalt signature the hydrothermal systems at oceanic spreading ridges have circulated through a large volume of basalts therefore pb in these fluids may representmore the best average value of the local oceanic crust from the effects of u deposition from seawater to the crust and pb extraction from rock to the ocean the u pb ratio in the hydrothermally altered oceanic crust may be increased significantly copyright american geophysical union 1987 less

pb and sr isotopic systematics of some basalts and sulfides from the east pacific rise at 21 n project rita abstract tholeiitic basalts and sulfide deposits from the cyana and alvin diving programs rita project on the east pacific rise were analyzed for pb and sr isotopes the basalt data plot within the field defined previously by other east pacific rise basalts 206 pb 204 pb 18 35 18 58 207 pb 204 pb 15 48 15 53 208 pb 204 pb 37 8 38 1 87 sr 86 sr 0 7022 0 7025 pb u and sr contents 0 5 0 05 and 110 ppm respectively and values 6 are typical of morb whereas th u ratios 3 5 are significantly higher the pb isotopic ratios of the sulfide samples are very homogeneous 206 pb 204 pb 18 47 207 pb 204 pb 15 49 208 pb 204 pb 37 90 and plot in the middle of the basalt field this indicates that 1 the sulfide pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments and 2 the solutions from which the sulfides were deposited had uniform pb isotopic composition the pb contents of three sulfide samples is relatively high 170 1310 ppm the sr contents of five sulfide samples are widely scattered from 12 to 210 ppm with 87 sr 86 sr ratios intermediate between basaltic and seawater values 0 70554 0 00005 to0 70795 0 00011 leaching experiments show that both basalt derived sr and seawater sr were present in the solutions which deposited the sulfides in some cases sr was also adsorbed from seawater onto the sulfides following their deposition basalt derived sr and seawater sr are also present in associated non sulfide phases

hydrodynamic behaviour of taupo 1800a pumice implications for the sedimentology of remobilized pyroclasts large scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments on land erosion remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run off conversely in the submarine realm both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium despite this and the number of studies devoted to volcaniclastic sedimentation relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves which ought to underpin any assessment of transport or depositional process this is crucial as many volcanic particles exhibit variable density according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and or gaseous phases during transport and deposition investigation of the physical and hydrodynamic properties of taupo 1800a pumice with reference to sedimentary facies developed during the eruption aftermath shows that although buoyant when dry when sufficiently waterlogged cool pumice clasts will sink and behave more like quartzo feldspathic material saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air water exchange in smaller pores low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent sized quartzo feldspathic or crystal lithic particles fine grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles these unusual properties of temporary buoyancy variable saturation low density and size dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies these findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments but also for primary depositional processes during subaqueous explosive volcanism

geochemistry of a hydrothermal sediment core from the obs vent field 21 n east pacific rise abstract we report a geochemical investigation of near vent hydrothermal sediment from the east pacific rise collected in a short 11 cm core by submersible from the obs vent site at 21 n the sediment has high concentrations of fe 23 25 cu 10 22 zn 4 13 and s 22 30 but low mn concentrations 108 279 ppm indicating a much fresher sulfidic input than in previously reported near vent sediments from the mid atlantic ridge further studies are required to fully characterise near field hydrothermal sediments but the data in this study allow the first comparisons with similar near vent material collected previously from the mid atlantic ridge as well as ridge flank data from the epr and juan de fuca ridge compared to ridge flank sediments the near field sediments exhibit high ratios of fe mn cu fe and zn fe unlike cu and zn pb is not so significantly enriched pb isotope compositions are indistinguishable from local morb indicating a dominant hydrothermal source for this element in these sediments this may not necessarily be the case for ridge flank sediments however where scavenging from seawater may also be important ree distributions throughout the core exhibit low concentrations and characteristics typical of vent fluids with lree enrichments and pronounced positive eu anomalies these patterns are similar to those of relatively fresh sulfides but there is also evidence for progressive oxidation downward into the core by ingressing seawater

seawater metasomatism in hydrothermal sediments escanaba trough northeast pacific abstract hydrothermal sediments from the nesca vent field escanaba trough are enriched in a number of typical vent fluid metals for which fluid data for the same sediment hosted site exhibit anomalous depletion these results are consistent with cooling of rising fluids and precipitation of various metal sulphide sulphate and oxide phases in situ within the sediment column prior to expulsion of the fluids from the seabed significant enrichments of mg and u elements which are quantitatively removed from high temperature vent fluids are also observed in the escanaba trough sediments however enrichments of these elements cannot derive from simple conductive cooling of the rising vent fluids but instead are believed to result from extensive seawater metasomatism as relatively fresh unreached seawater is heated from below by hot sediments and or percolating vent fluids in the upper levels of the circulation system calculated seawater sediment interaction ratios range from 5 1 to 2500 l of seawater per gram of sediment pb sr and nd isotope distributions for the escanaba sediments are consistent with combined sediment fluid and seawater sediment interactions pb isotope ratios for hydrothermal and background sediments coincide with local sulphide deposits but are much more radiogenic than local morb this indicates not only that complete isotopic re equilibration must occur between the hot rising fluids and the host sediments but in addition that sediment composition appears to dominate the pb isotopic composition of these fluids sr and nd isotope ratios in the hydrothermally altered sediments which are distinct from background ratios coincide closely with modern seawater values providing confirmatory evidence that extensive interactions must also have occurred between these hydrothermal sediments and relatively fresh unreacted seawater

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

re os isotopic systematics of the voisey s bay ni cu co magmatic ore system labrador canada abstract re and os concentrations and os isotopic compositions have been obtained for massive matrix and disseminated sulphide ores from three environments within the voisey s bay intrusion the ovoid eastern deeps and discovery hill zone in order to assess the role of crustal contamination in the genesis of this large cu ni co deposit these samples have high re concentrations 148 to 288 ppb in 100 sulphide for their common os concentrations 4 8 to 24 ppb in 100 sulphide yielding high re os ratios 12 to 33 these data confirm that the magma parental to the voisey s bay ore system was broadly basaltic in major element chemistry rather than picritic consistent with the low ni cu ratio of the ores 1 5 re os isotopic data exhibit a limited spread in 187 re 188 os 57 to 157 and define an imprecise 1323 135 ma model 3 isochron likely the result of small r factor variations within the ore system the re os isochron age is within error of 1334 ma u pb ages obtained for baddeleyite from the ore bearing troctolites demonstrating that whole rock re os isotopic systematics have remained closed since crystallisation the initial os isotopic composition of the isochron os 1040 200 implies significant magma interactions with radiogenic os that most likely resides in the nain churchill province crust these data are therefore consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components with further addition of base and precious metals as a function of r factor analyses of sulphide separates from the proterozoic tasiuyak churchill and archaean nain gneisses confirm that both units contained significant os 8 6 ppb and 0 38 ppb respectively that was very radiogenic at 1334 ma os 1908 and 5202 respectively yielding crustal residence tchur model ages of 2200 to 2400 ma however these model ages may have been affected by re and or os mobility during the 1 85 ga torngat and 1 1 ga grenville orogeny re os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the nain plutonic suite nps with either tasiuyak 2 contamination or nain 16 contamination gneiss followed by an r factor process 200 to 1000 which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active replenished voisey s bay intrusion magma chamber however the dominance of plagioclase rich high al2o3 magmas in the mafic members of the nps troctolites and anorthosites may indicate that other deeper forms of lithospheric interaction potentially involving mafic lower crust may be crucial to understanding this style of magmatic sulphide ore system

millennial scale reworking of tephra in alluvial to shallow marine settings distinguishing pseudo isochrons from genuine ones abstract primary tephra layers can provide isochrons in a stratigraphic sequence such isochrons are powerful tools in terms of correlation and chronology of strata and they constrain a chronological framework for geology geomorphology archaeology and palaeo environmental studies in the neogene to quaternary however reworking processes can re distribute tephra materials in both time and space so that a given tephra can form a new discrete layer later in the depositional sequence or it can be distributed in low concentrations no visible layers i e crypto tephras over a range of different stratigraphic horizons the present study based on sedimentological petrographical and geochemical analyses of reworked tephras in japan evaluates the duration and persistence of reworking and the tephra mobilisation and delivery to watersheds furthermore it discusses how a unique isochron can be constructed from tephra distribution in a sedimentary sequence examples presented here are from fluvial and deltaic sequences in the japanese quaternary which contain several visible tephra layers and a certain amount of tephra materials from multiple sources at different stratigraphical horizons in the niigata plain 5 4 ka numazawako tephra material continued to be reworked for more than 4000 years after the eruption of numazawa volcano and was re deposited as reworked pumice grains consisting of a visible tephra layer in delta front deposits mixed tephra materials originating from an earlier eruption of numazawa volcano and eruptions from other calderas were also identified in the tsugaru plain post 2 5 ka subsurface deposits have 10 glass shards content throughout the sequence glass shards were derived from several different tephra materials such as those from the most recent eruption of towada volcano ad 915 and earlier caldera forming eruptions 30 ka 15 ka some of the glass shards probably originated from erosion of exposed pliocene pyroclastic bedrock thus tephra reworking processes in alluvial to shallow marine settings can continue long after an eruption has ended and can persist even during inter eruptive i e background sedimentation periods as tephra reworking involves complicated processes careful observation and interpretations are necessary to evaluate whether tephra materials can truly provide an isochron for the strata in which they are found to exist

volcanic ash layers facilitate high resolution sequence stratigraphy at convergent plate margins an example from the plio pleistocene forearc basin fill in the boso peninsula japan abstract in this paper a method is presented for the definition of high resolution siliciclastic depositional sequences in onshore outcrop belts in a forearc basin setting and the development of high frequency depositional sequences in the infill of the plio pleistocene kazusa forearc basin in the boso peninsula of japan in terms of the interaction between forearc tectonics and glacio eustasy is discussed volcanic ash layers are commonly intercalated in siliciclastic successions at convergent plate margins and provide physically defined chronostratigraphic markers the mapping of lateral termination of volcanic ash layers together with the interpretation of depositional facies discontinuities permits the identification of onlap offlap and downlap stratal termination patterns in onshore outcrop belts in particular this type of analysis is useful to identify bounding surfaces in offshore muddy sediments the relative abundance of intercalated volcanic ash layers and their associated pyroclastic volcaniclastic fragments in marine siliciclastic successions is interpreted to correspond to temporal variation in the relative rate of supply of coarse grained terrigenous sediments to a marine environment during a relative sea level cycle in particular transgressive stages are characterized by the reduction of active supply of coarse grained terrigenous sediments to a marine elastic environment and this results in the increase of relative abundance of intercalated volcanic ash layers and their associated volcanogenic lithofacies this kind of sediment is hierarchically identified in response to the development of marine flooding surfaces transgressive systems tracts and transgressive sequence sets in general boundaries of depositional sequences and their component building blocks are interpreted to provide a chronostratigraphic framework for local and regional correlations of siliciclastic successions the mapping of volcanic ash layers can identify diachronous evolution of bounding surfaces and component building blocks of a high frequency depositional sequence in relation to spatial variations in sedimentation rates and basin subsidence which are superimposed by eustatic sea level changes

hydrothermal scavenging on the juan de fuca ridge 23othxs 10be and rees in ridge flank sediments abstract we have investigated the geochemistry of a hydrothermally enriched sediment core recovered from the western flank of the n cleft segment juan de fuca ridge 8km west of the megaplume area previously identified near 45 n the core contains varying biogenic lithogenic and hydrothermal components as reflected in caco3 al and fe contents respectively horizons of pronounced hydrothermal input in core top sediments and at depth exhibit increased concentrations of fe cu zn and pb and shifts in pb isotopic compositions toward nonradiogenic morb hydrothermal values ree concentrations co vary with hydrothermal fe down core and shale normalised ree distribution patterns exhibit both negative ce anomalies and positive eu anomalies indicative of input from plume particle fall out unsupported 230thxs activities down core are consistent with continuous slow sediment accumulation rates of 0 54 cm ky for 200 ky since the deposition of the deeper fe rich horizon 10be 0 and 9be isotope concentrations also co vary with hydrothermal fe down core and exhibit 10 be o 9 be ratios which approach that of pacific ocean deep water indicative of a seawater scavenging source 10 be 0 230 oth xs 0 ratios throughout most of core gc88 6 are greater than mean pacific ocean values indicating that hydrothermal scavenging can lead to significant net removal of dissolved 10be into ridge flank sediments

late neoproterozoic early paleozoic evolution of the south china block as a retroarc thrust wedge foreland basin system the early paleozoic wuyi yunkai fold belt of south china represents a major orogenic belt in east asia however its formation is not well understood this paper evaluates the origins of the neoproterozoic early paleozoic basin and the southern margin of the south china block scb based on a compilation of the spatiotemporal evolution of the late neoproterozoic early paleozoic strata and their depositional environments these data are combined with high resolution structural geochronologic metamorphic and petrologic data that are published from the region a new model of a retroarc thrust wedge foreland basin system is proposed for the succession based on the following 1 the nature of the basal unconformity 2 the wedge shaped basin fill geometry bounded by the mountain belt to the south 3 basinward propagation of the thrust front with time and 4 the presence of a typical underfilled stratigraphic trinity of the foreland basin the proposed model suggests two major cycles of evolution of the retroarc thrust wedge foreland basin system 725 635 and ca 564 446 ma which record two periods of northwestward subduction of the oceanic slab beneath the southern margin of the scb terrane accretion is inferred to have taken place during the late stage of these cycles

ree fe variations in hydrothermal sediments implications for the ree content of seawater seafloor hydrothermal vent solutions exhibit rare earth element ree enrichments ranging between one to three orders of magnitude greater than average seawater to assess the impact of these hydrothermal inputs on ocean chemistry we have examined the behavior of rees for hydrothermal sediments collected adjacent to two pacific spreading ridge sites the east pacific rise at 19 s and the southern juan de fuca ridge at 4s n in general the ree fe ratios for both proximal and distal hydrothermal sediments are greater than vent solutions by a factor of 2 to 500 and these ratios increase with increasing distance away from the ridge axis an evaluation of these results in the context of previous models of ree behavior indicates that in fact seawater experiences a net depletion in rees as a result of hydrothermal activity this is due primarily to the large scavenging capacity of iron oxyhydroxides which precipitate from these solutions such an interpretation explains why the ree content of seawater collected in the vicinity of hydrothermal vents is anomalously lower than normal seawater sampled from a comparable depth

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

hydrothermal deposition on the juan de fuca ridge over multiple glacial interglacial cycles abstract hydrothermal systems play an important role in modern marine chemistry but little is known about how they may have varied on 100 000 year timescales here we present high resolution records of non lithogenic metal fluxes within sediment cores covering the last 500 000 years of hydrothermal deposition on the flanks of the juan de fuca ridge six adjacent gridded cores were analyzed by x ray fluorescence for fe mn and cu concentrations corrected for lithogenic inputs with ti and normalized to excess initial 230 th to generate non lithogenic metal flux records that provide the longest orbitally resolved reconstructions of hydrothermal activity currently available fe fluxes vary with global sea level over the last two glacial cycles suggesting higher hydrothermal deposition during interglacial periods the observed negative relationship between fe and mn indicates variable sediment redox conditions and diagenetic remobilization of sedimentary mn over time thus mn fluxes may not be a reliable indicator for hydrothermal activity in the juan de fuca ridge sediment cores cu fluxes show substantial high frequency variability that may be linked to changes in vent temperature related to increased magmatic production during glacial periods deglacial hydrothermal peaks on the juan de fuca ridge are consistent with previously published records from the mid atlantic ridge and the east pacific rise moreover on the juan de fuca ridge the deglacial peaks in hydrothermal activity are followed by relatively high hydrothermal fluxes throughout the ensuing interglacial periods relative to the previous glacial period

tracking the dispersal of hydrothermal plumes from the juan de fuca ridge using suspended matter compositions in 1988 and 1989 particles from on and off axis neutrally buoyant plumes over the juan de fuca ridge were sampled to study their changing composition with distance from the ridge crest plume depth and particle composition in off axis hydrothermal plumes were used to identify plume sources and trajectories iron and phosphorus concentrations and in particular p fe molar ratios were found to be sensitive indicators of hydrothermal phases in the suspended matter phosphorus vanadium and arsenic are scavenged from solution where dissolved iron from hydrothermal vents oxidizes in seawater and forms submicrometer sized particles of fe oxyhydroxide the p fe molar ratio 0 23 is essentially the same for vent fields along cleft and endeavour segments indicating that the scavenging processes are the same for both regions high p fe ratios are observed over the south and north cleft segment vent fields and to the west along the vance seamount chain the seamounts evidently function as a barrier bathymetrically steering the hydrothermal plumes to the west away from the ridge crest since particles from the endeavour vent field are greatly enriched in copper relative to hydrothermal plumes from other vent fields along the juan de fuca ridge copper concentrations in the plumes can be used to distinguish plume sources based on water column depth and elemental composition hydrothermal plume particles from specific vent fields on the juan de fuca ridge may be tracked as much as several tens of kilometers from their source using particle compositions