multichronometric evidence for an in situ origin of the ultrahigh pressure metamorphic terrane of dabieshan china abstract u pb zircon data on gneisses indicate a late proterozoic protolith at 700 ma at bixiling and a late archean to early proterozoic for the shuanghe sheet within the ultrahigh pressure metamorphic terrane of dabieshan china u pb zircon and sm nd dates on the gneisses of the shuanghe sheet constrain a metamorphic peak at 230 ma rb sr and 40ar 39ar analyses of phengite from five gneisses in the bixiling complex yield ages at 198 212 ma overlapping the 190 220 ma age on eclogites the host gneisses record a similar metamorphic evolution as the coesite bearing eclogites since both the peak and the retrograde evolution were contemporaneous this argues for an in situ tectonic relationship

a middle silurian early devonian magmatic arc in the qinling mountains of central china a discussion lerch et al 1995 documented an early to middle paleozoic magmatic arc in the qinling orogenic belt of central china by detailed field mapping geochemistry and geochronology in the heihe area of north qinling their u pb analyses of zircons from i type granites developed in the heihe area suggested short lived arc activity from middle silurian to early devonian time this result is very interesting the termination of calc alkaline magmatism in the devonian is plausible and consistent with our studies on geochemistry and geochronology of a mafic pluton developed in lerch et al s 1995 zone 2 we reported a sm nd mineral isochron pyr 2 plag wr age of 402 6 17 4 ma for the lajimiao noritegabbro that intruded into the metabasaltic lava developed in the south of the shangxian area li et al 1989 then demonstrated its island arc origin using trace elements and sr nd isotopes li et al 1994a along the south marigin of zone 2 metamorphic andesite interbedded with metabasaltand intruded by mafic and granitic plutons was found recently in the heihe area their major and trace element compositions show island arc andesite characteristics w sun et al 1995 the 40ar 39ar age of 426 0 1 3 ma li et al unpub age for an amphibole from a metabasalt sample shows that these rocks underwent low amphibolite facies metamorphism in the middle silurian the caledonian metamorphic age was regarded as the time of collision between this volcanic arc and the north china craton li et al 1992 the tectonic system of the north qinling area between 426 and 400 ma might be similar to that of the andes in south america i e an island arc that had already accreted to the continent while the subduction of oceanic crust continued

germanium silicon fractionation in the weathering environment abstract we present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the ge si system as a tracer of silicate weathering processes in both modern and ancient environments intensely weathered soils developed on hawaiian basalts have bulk soil ge si ratios 2 to 10 times higher than fresh basalt e g 10 to 25 mol mol vs 2 5 mol mol soil ge concentrations increase with si and decrease with fe suggesting that ge sequestration is related to accumulation of secondary soil silicates rather than retention in soil fe oxy hydroxides sequential extractions of these soils suggest that ge si fractionation takes place by ge sequestration during the initial precipitation of secondary soil aluminosilicates principally allophane further si loss and changes in mineralogy as these soils age result in little additional ge si fractionation ge si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals kaolinite has a ge si ratio 5 9 mol mol higher than the plagioclase from which it forms 3 1 mol mol whereas accumulation of primary quartz ge si 0 5 mol mol prevents granitic soils from attaining high ge si ratios laboratory synthesis of allophane confirms that ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution

a high precision u pb age of metamorphic rutile in coesite bearing eclogite from the dabie mountains in central china a new constraint on the cooling history this paper first reports a high precision u pb age of 218 1 2 ma for rutile in coesite bearing eclogite from jinheqiao in the dabie mounteins east central china this work shows that the u pb mineral rutile omphacite isochron age of 218 2 5 ma and conventional rutile u pb concordia age of 218 1 2 ma obtained by common pb correction based on the pb isotopic composition of omphacite in the same eclogite sample are consistent proving that the omphacite with low u pb ratio 2 8 can be used for common pb correction in u pb dating of rutile oxygen isotope analysis of rutile aliquots gave the consistent 18o values of 6 1 0 1 demonstrating oxygen isotope homogenization in the rutile of different grains as inclusion in garnet and grain in matrix oxygen isotope thermometry yields temperatures of 695 35 and 460 15 c for quartz garnet and quartz rutile pairs respectively these oxygen isotopic observations suggest that the diffusion of oxygen in rutile as inclusion in garnet is not controlled by garnet according to field based thermochronological studies of rutile an estimate of the tc of about 460 c for u pb system in rutile under rapid cooling conditions 20 c ma was advised based on this u pb age as well as the reported chronological data with their corresponding metamorphic and or closure temperature an improved t t path has been constructed the t t path confirms that the uhpm rocks in south dabie experienced a rapid cooling following the peak metamorphism before 220 ma and a long isothermal stage from 213 to 180 ma around 425 c

magnetic remanence in the murchison meteorite the murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules various inclusions and matrix with occasional porphyroblasts of olivine and or pyroxene it also contains amino acids that may have served as the necessary components for the origin of life magnetic analyses of murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence the rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids the level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition we support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination when using this approach 1 2 of the natural remanence left in terrestrial rocks with trm and or crm determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral with the exception of hematite the same method is applied to the murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200 2000 nt during and or after the formation of the parent body

evolution of the sanbagawa and shimanto high pressure belts in sw japan insights from k ar ar ar geochronology abstract the sanbagawa belt sensu lato in japan is one of the world s classical high pressure hp metamorphic belts and has long been considered to be a coherent hp belt of the cretaceous age however recent progress in detrital zircon geochronology together with the accumulation of abundant phengites k ar ar ar ages revealed that the belt is comprised of two petrotectonic belts sanbagawa sensu stricto hp belt and shimanto hp belt these two belts have distinctly different ages of peak metamorphism and different p t conditions of metamorphism the sanbagawa schists sensu stricto were metamorphosed in the conditions of the pumpellyite actinolite facies through the epidote blueschist to epidote amphibolite facies and up to the eclogite facies during a prograde stage in a subduction zone at 120 115 ma phengite k ar geochronology revealed that the subsequent exhumation after the peak metamorphism took placed in a manner that the higher grade rocks exhumed more rapidly in comparison with the lower grade rocks this is supported by a positive correlation between age and apparent metamorphic gradient that have formed during the post metamorphic peak hydration recrystallization moreover the schists have experienced intense ductile deformation and long term exhumation it took longer than 31 m y to reset the phengite k ar system in contrast the shimanto hp schists were metamorphosed in the epidote blueschist greenschist transitional facies to the epidote amphibolite facies and have experienced a short term deformation for less than 13 m y this short deformation formed a negative correlation between k ar age and apparent metamorphic gradient these two contrasting age temperature relationships suggest different exhumation processes between the two hp belts most likely due to a change in subducting oceanic plates in the cretaceous along the paleo japanese convergent margin

interpreting high pressure phengite 40 ar 39 ar laserprobe ages an example from saih hatat ne oman new single grain fusion and core rim 40ar 39ar laserprobe phengite data from the saih hatat high pressure terrane in ne oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane the majority of the determined ages are older than the previously reported u pb zircon peak metamorphic age core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores or rarely older rims some samples manifest all three patterns numerical diffusion modelling shows that due to the peak temperature of ca 550 c the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system the age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm cm scale coupled with pervasive and heterogeneously distributed excess argon anomalously old eclogite phengite 40ar 39ar ages are due either to internally derived 40ar inherited from a k bearing precursor or externally derived 40ar distributed by grain boundary fluids mica rich schists within the eclogite boudins yield younger phengite ages suggesting excess argon was absent or diluted pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist facies overprint and yield very variable ages commonly with apparently older rims on younger cores the grain and sample scale age variations measured in saih hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle

triassic polyphase deformation in the feidong zhangbaling massif eastern china and its place in the collision between the north china and south china blocks the feidong zhangbaling massif is located between the dabieshan and sulu areas along the tan lu fault in eastern china five tectonic metamorphic events are distinguished there the earliest deformation d1 corresponds to a southward compression that occurred during subduction of the south china block below the north china block top to the south shearing is coeval with late permian early triassic blueschist facies metamorphism and possibly with the development of south verging recumbent folds in the neoproterozoic paleozoic sedimentary cover of the south china foreland the main ductile deformation d2 is an extensional one characterized by top to the north shearing coeval with the early stage of exhumation of the high pressure rocks a top to the nw ductile shearing and microfolds overturned to the northwest belong to a second deformational phase of exhumation d3 which is distinct from the main event d2 previous 40ar 39ar mica dates ranging between 245 and 212 ma suggest late permian early triassic ages for the d1 to d3 events the d4 event produced ne sw trending folds in the sedimentary cover interpreted as gravity collapse structures a late cretaceous brittle extensional event d5 controls the opening and infill of continental half grabens in the study area the tan lu fault is a cretaceous brittle normal fault the lack of ductile deformation presented along the tan lu fault suggests that it did not play a significant role in the exhumation of high pressure metamorphic rocks

germanium isotopic variations in igneous rocks and marine sediments a new technique for the precise and accurate determination of ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal the analyses were performed using a continuous flow hydride generation system coupled to a mc icpms samples have been purified through anion and cation exchange resins to separate ge from matrix elements and eliminate potential isobaric interferences variations of 74ge 70ge ratios are expressed as 74ge values relative to our internal standard and the long term external reproducibility of the data is better than 0 2 for sample size as low as 15 ng of ge data are presented for igneous and sedimentary rocks and the overall variation is 2 4 in 74ge representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of ge is not unique co variations of 74ge 70ge 73ge 70ge and 72ge 70ge ratios follow a mass dependent behaviour and imply natural isotopic fractionation of ge by physicochemical processes the range of 74ge in igneous rocks is only 0 25 without systematic differences among continental crust oceanic crust or mantle material on this basis a bulk silicate earth reservoir with a 74ge of 1 3 0 2 can be defined in contrast modern biogenic opal such as marine sponges and authigenic glauconite displayed higher 74ge values between 2 0 and 3 0 this suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the 74ge of the seawater to 3 0 this suggests that seawater is isotopically heavy relative to bulk silicate earth and that biogenic opal may be significantly fractionated with respect to seawater deep sea sediments are within the range of the bulk silicate earth while mesozoic deep sea cherts opal and quartz have 74ge values ranging from 0 7 to 2 0 the variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz further work is now required to determine ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification

late jurassic blueschist facies pebbles from the western carpathian orogenic wedge and paleostructural implications for western tethys evolution in spite of the absence of ophiolitic slices at the surface some traces of the lost tethys ocean are recorded along the pieniny klippen belt pkb a narrow decollement thrust system sutured at the transpressive boundary between the outer and inner carpathians the enigmatic precollisional evolution of western carpathians can be deciphered from some late albian to campanian flysch conglomerates which display chrome spinel grains ophiolitic detritus and pebbles of blueschist facies tholeiitic metabasalts yielding a 40ar 39ar plateau age of 155 4 0 6 ma other detrital components are represented by extrabasinal pebbles of limestones arc volcanics and igneous to metamorphic basement rocks from southern sources our results suggest a markedly northward extension of the sublongitudinal triassic vardar meliata ocean and its subduction since the late middle jurassic supposedly balanced westward by coeval spreading in the ligurian piedmont basin of the apennine western alpine tethys a lateral kinematic connection between these diachronous and roughly parallel tethys branches was provided on the north by a left lateral east west trending shear zone running from the swiss austrian penninic domain to the northern carpathians this reconstruction replaces the classic model of two paired north penninic and south penninic oceanic basins and eastern homologues with the brianconnais hochstegen and czorstin microcontinents in between the late jurassic early cretaceous evolution of the carpathian active margin was characterized by subduction metamorphism and accretion of a wide orogenic wedge in this time the shallowing to deeply subsiding basins inferred from facies analyses on the sedimentary units of the pkb were likely floored by individual sections of the growing wedge later some exhuming blueschist ophiolitic units of the wedge were uplifted to the surface and functioned in the albian campanian as an exotic ridge supplying clasts to the forearc basin finally the colliding wedge became a cryptic paleostructure when since the latest cretaceous it disappeared beneath the inner carpathian orogenic lid and was incorporated within the eastward moving infrastructure of the carpathian orocline

diagenetic fractionation of ge and si in reducing sediments the missing ge sink and a possible mechanism to cause glacial interglacial variations in oceanic ge si the average ge si ratio in the ocean is determined by the budgets for each of these elements previous budget formulations have assumed that the only important sink for both elements is burial as opal based on studies of the si cycle and the close oceanic coupling observed between inorganic ge and si distributions however these budgets implied two paradoxes 1 hydrothermal flow through ocean ridges is smaller than predicted by other tracers and 2 the lower ge si ratio of opal deposited during glacial times compared to that deposited during interglacial times required enhanced weathering during cooler drier climates both paradoxes could be resolved if a significant sedimentary sink for ge other than opal burial could be identified and the objective of this study was to search for one two pore water profiles collected in equatorial pacific sediments show that ge and si behave similarly in the upper 10 cm of sediment indicating no evidence for a significant non opal sink for ge in oxic sediments by contrast profiles in several cores from the california margin demonstrate that in reducing sediments ge diagenesis is poorly coupled to si diagenesis significant ge removal is evident both downcore and sometimes in the near surface benthic flux chamber measurements at three continental slope stations all with an oxic layer less than 1 cm thick and large iron gradients in near surface pore waters showed that 55 9 of the ge released by opal dissolution is sequestered however at two locations with anoxic sediments but little pore water fe 2 in the upper 2 cm flux measurements indicated little fractionation from the oceanic ratio during diagenesis implicating the importance of iron for fractionating ge from si during diagenesis if the ge sequestration observed in the iron rich ca margin sediments is typical of all slope sediments using a depth range of 200 1000 m then the ge sink is sufficient to bring the hydrothermal budget based on ge into concurrence with that based on other tracers the temporal variation in oceanic ge si could be explained if ge and si inputs remain constant and the effective diagenetic fractionation of ge increases by a factor of 2 3 during glacial times increased fractionation would require that glacial periods are characterized by increased opal dissolution in iron rich reducing sediments this could be caused by 1 thinning of the oxygenated sediment layer in response to decreased bottom water oxygen concentrations or increased rain of organics to the sea floor 2 increased rain of iron rich detrital sediments in areas receiving high opal rain 3 increased rain of opal to sediments in margin areas if the oceanic ge si ratio reflects increased rain of diatom opal or organic carbon in margin areas during glacial periods it may indicate an increase in the efficiency of the biological pump for co2 during glacial times

dating the subduction of the arabian continental margin beneath the semail ophiolite oman the presence of high pressure p eclogites along the northeast margin of the arabian continental plate beneath the semail ophiolite demonstrates that subduction of thinned continental crust to pressures of 20 kbar 80 km depth has occurred current debate centers on whether this high p metamorphism occurred prior to during or following ophiolite emplacement which is known to have started ca 95 93 ma five concordant u pb zircon ages from a garnet clinopyroxene phengite crossite eclogite from as sifah northeast oman precisely constrain the age of high p metamorphism at 79 1 0 3 ma this age demonstrates that high p metamorphism occurred during continental subduction 15 m y after formation of the ocean floor that subsequently became the semail ophiolite

40ar 39ar constraints on the tectonic history and architecture of the ultrahigh pressure sulu orogen new 40ar 39ar ages are presented from the giant sulu ultrahigh pressure uhp terrane and surrounding areas combined with u pb ages sm nd ages rb sr ages inclusion relationships and geological relationships they help define the orogenic events before during and after the triassic collision between the sino korean and yangtze cratons in the qinling microcontinent tectonism occurred between 2 0 and 1 4 ga the uhp metamorphism occurred in the yangtze craton between 240 and 222 ma its thermal effect on the qinling microcontinent was limited to partial resetting of k feldspar 40ar 39ar ages subsequent unroofing at rates of 5 25 km myr 1 brought the uhp terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism the end of that metamorphism is marked by 40ar 39ar ages in the 219 210 ma range implying cooling at crustal depths at rates of 50 200 c myr 1 ages in the 210 170 ma range may reflect protracted cooling or partial resetting by jurassic or cretaceous magmatism jurassic 166 149 ma plutonism was followed by cooling at rates of c 15 c myr 1 suggesting relatively deep crustal conditions whereas cretaceous 129 118 ma plutonism was succeeded by cooling at rates of c 50 c myr 1 suggesting relatively shallow crustal depths

mechanisms controlling the silicon isotopic compositions of river waters it has been proposed that silicon si isotopes are fractionated during weathering and biological activity leading to heavy dissolved riverine compositions in this study the first seasonal variations of stable isotope compositions of dissolved riverine si are reported and compared with concomitant changes in water chemistry four different rivers in switzerland were sampled between march 2004 and july 2005 the unique high resolution multi collector icp ms nu1700 has been used to provide simultaneous interference free measurements of 28si 29si and 30si abundances with an average limiting precision of 0 04 on 30si this precision facilitates the clarification of small temporal variations in isotope composition the average of all the data for the 40 samples is 30si 0 84 0 19 1 sd despite significant differences in catchment lithologies biomass climate total dissolved solids and weathering fluxes the averaged isotopic composition of dissolved si in each river is strikingly similar with means of 0 70 0 12 for the birs 0 95 0 22 for the saane 0 93 0 12 for the ticino and 0 79 0 19 for the verzasca however the 30si undergoes seasonal variations of up to 0 6 comparisons between 30si and physico chemical parameters such as the concentration of dissolved si and other cations the discharge of the rivers and the resulting weathering fluxes permits an understanding of the processes that control the si budget and the fate of dissolved si within these rivers the main mechanism controlling the si isotope composition of the mountainous verzasca river appears to be a two component mixing between the seepage of soil ground waters with heavier si produced by clay formation and superficial runoff associated with lighter si during high discharge events a biologically mediated fractionation can be excluded in this particular river system the other rivers display increasing complexity with increases in the proportion of forested and cultivated landscapes as well as carbonate rocks in the catchment in these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of si contributed by biological processes as opposed to abiotic weathering the presence of seasonal variations in si isotope composition in mountainous rivers provides evidence that extreme changes in climate affect the overall composition of dissolved si delivered to the oceans the oceanic si isotope composition is very sensitive to even small changes in the riverine si isotope composition and this parameter appears to be more critical than plausible changes in the si flux therefore concurrent changes in weathering style may need to be considered when using the si isotopic compositions of diatoms sponges and radiolaria as paleoproductivity proxies

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

two stage exhumation of ultrahigh pressure metamorphic rocks from the western dabie orogen central china abstract zircon trace element u pb age and lu hf isotope composition were determined for two eclogite samples from the xinxian ultrahigh pressure uhp unit in the western dabie orogen to constrain its exhumation processes cathodoluminescence imaging reveals that most zircons are metamorphic while some have inherited magmatic cores in one sample the magmatic cores have high th u and lu hf ratios high trace element contents and a clear negative eu anomaly consistent with their igneous genesis they yield an upper intercept age of ma and a positive ehf t value of 8 0 arguing for reworking of early mesoproterozoic crust during the middle neoproterozoic in the western dabie orogen the metamorphic zircons contain mineral inclusions of garnet omphacite rutile and quartz they are characterized by low nb ta y and heavy rare earth elements hree contents a nearly flat hree pattern and an insignificant negative eu anomaly these indicate that the metamorphic zircons formed under high pressure

extraneous argon in high pressure metamorphic rocks distribution origin and transport in the cycladic blueschist unit greece the 40ar 39ar geochronological technique has significantly contributed to the study of crustal deformation however the frequent occurrence of extraneous argon in high pressure terrains can make the interpretation of 40ar 39ar ages problematic this contribution attempts to i characterize the distribution and origin of extraneous argon in the cycladic blueschist unit cbu greece by applying the 40ar 39ar dating technique on three cycladic islands syros tinos and ios which span a complete transect of the cbu and ii discuss regional implications for the tectonometamorphic evolution of the cbu our results combined with ages available in the literature show that periods of peak burial conditions 55 49 ma and syn 45 38 ma to post orogenic exhumation 37 21 ma are well constrained in the cbu by the 40ar 39ar method moreover variscan apparent ages are preserved in the cycladic basement of ios indicating that the 40ar 39ar system was not completely reset during the eocene metamorphic episode additionally our 40ar 39ar data show that the occurrence of extraneous argon is heterogeneously distributed in the cbu while high pressure rocks of syros and tinos are only locally affected extraneous argon is clearly evidenced in the cbu of ios yielding geologically meaningless ages up to 900 ma on each island the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets we suggest that the cycladic basement a polymetamorphic continental unit behaved as an argon reservoir that has been mobilized by fluid circulation at the eocene these argon enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the cbu finally we claim that a minimum closure temperature of 550 c must be accepted for the phengites of the cbu and conclude that volume diffusion is not the main process resetting the argon isotopic system in high pressure rocks compare to recrystallization enhanced by fluid circulation and deformation

87sr 86sr ca sr and ge si ratios as tracers of solute sources and biogeochemical cycling at a temperate forested shale catchment central pennsylvania usa plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems hydrologic and other losses are offset by inputs from atmospheric deposition and weathering this multi tracer study investigates these effects using 87sr 86sr ca sr and ge si ratios from solid soil profiles and bedrock biological leaves and sap waters and water pore ground stream waters samples to study ca sr ge and si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central pennsylvania leaves and sap waters were found to have similar ge si ratios 1 mol mol which is consistent with biological fractionation occurring at the root soil water interface ge si ratios in soil porewaters were higher near the surface and increased over the growing season suggesting plant preferential uptake of si over ge ca sr ratios in oak leaves 1360 40 mol mol were significantly higher than those in maple leaves 650 20 mol mol ca sr ratios were also generally higher in leaves than in sap waters which are consistent with preferential sr adsorption or sr uptake during transpiration 87sr 86sr ratios in both leaves and sap waters were similar for a given site implying that trees access similar pools of sr and ca although there are site to site differences additionally 87sr 86sr ratios in plant tissues were most similar to shallow pore waters 0 50 cm suggesting that mineral nutrients are obtained by trees from the near sub surface 87sr 86sr ratios in soil solutions from ridgetop and swale sites are best explained by mixing sr derived from shale and atmospheric deposition valley bottom soil reservoirs and stream and groundwater samples include sr and ca derived from dissolution of two isotopically distinct generations of carbonates a preliminary estimate of the sr and ca stream fluxes and isotopic mass balances imply propagation of a carbonate weathering front of ca 300 m myr 1 a value larger than previously reported for regolith weathering advance rates calculated based on cosmogenic nuclides and u series isotopes the data for ca sr si and ge in soil soil solutions and stream waters reflect the interaction of slower weathering processes with rapid biologically driven cycling between soils and biomass

glacial effects on weathering processes new insights from the elemental and lithium isotopic composition of west greenland rivers greenland is by far the dominant source of glacial runoff to the oceans but the controls on the chemical and isotopic composition of this runoff are poorly known to better constrain glacial effects on weathering processes we have conducted elemental and lithium isotope analyses of glacial and non glacial rivers in gneiss catchments in west greenland the glacial rivers have high total suspended solids 0 5 g l 1 and low total dissolved solids 12 scm 1 relative to the non glacial rivers and they contain a higher proportion of dissolved ca2 and k because of subglacial preferential weathering of trace carbonates and biotite the glacial rivers also have high so42 because of the oxidation of trace sulphides under the ice both glacial and non glacial rivers have high 7li respectively not vert similar 26 and not vert similar 30 relative to the rocks from which the li is derived not vert similar 8 saturation state modelling suggests that this is due to the formation of fe oxyhydroxides in the non glacial rivers with preferential uptake of 6li during inner sphere sorption of li on the fe oxyhydroxide surface glacial rivers however are undersaturated with respect to clay minerals and fe oxyhydroxides nevertheless leaching of suspended sediments indicates that not vert similar 65 of the li in these sediments is associated with fe oxyhydroxide phases and the 7li value of this li is low not vert similar 5 these results suggest that these fe oxyhydroxides formed under the ice as a product of sulphide oxidation with preferential uptake of 6li onto the mineral surface solubilisation of li from these fe oxyhydroxide phases is unlikely to represent a significant flux of li to the oceans moreover because the difference between the 7li values of glacial vs non glacial rivers is small glaciation has not had a significant impact on the li isotopic composition of the riverine flux delivered to the oceans in the past even at the height of the last deglaciation

clinoenstatite exsolution in diopsidic augite of dabieshan garnet peridotite from depth of 300 km clinoenstatite exsolution lamellae have been discovered in clinopyroxene grains of garnet peridotite from the bixiling massif of the ultrahigh pressure metamorphic uhpm terrane in the dabie mountains china using transmission electron microscopy we show here that although the lamellae are low clinoenstatite lclen with p 2 1 c space group symmetry they contain nanometer scale antiphase domains indicating that the originally precipitating phase had c 2 c space group symmetry the lamellae are oriented parallel to 010 and 18 to 001 of the host approximately parallel to 401 both the very cold exhumation path of the dabie uhpm terrane and the orientation of the lamellae preclude the possibility that the originally precipitating phase could have been high temperature clinoenstatite htclen we conclude that the lamellae precipitated as high pressure clinoenstatite hpclen and subsequently inverted to lclen analysis of the incomplete high pressure crystallographic data available for compositions approximating those of the lamellae and host suggests a minimum pressure of precipitation of 9 gpa for dabie and about 12 gpa for similar exsolution previously observed in the alpe arami peridotite switzerland the bixiling complex is a crustal cumulate hence this result extends the minimum depth of subduction of continental rocks in the dabie sulu orogen to 300 km p 9 gpa

low δ18o zircons u pb dating and the age of the qinglongshan oxygen and hydrogen isotope anomaly near donghai in jiangsu province china zircons from metamorphosed granites exposed near qinglongshan have 18ovsmow values of 7 to 0 in both grain rims and cores the concordant 238u 206pb ages of zircon cores are 684 to 754 ma with rims at 221 ma discordant 238u 206pb ages range from 242 to 632 ma results demonstrate a neoproterozoic age for the origin of the qinglongshan oxygen and hydrogen isotope anomaly the low 18o values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate groundwater circulation was driven by heat from cooling granitic magma the geologic age of the hydrothermal system correlates with that of the nantuo tillite in the sinian strata of the south china block suggesting that qinglongshan s cold climate may be a manifestation of neoproterozoic snowball earth

positive correlation between li and mg isotope ratios in the river waters of the mackenzie basin challenges the interpretation of apparent isotopic fractionation during weathering during chemical weathering magnesium mg is released by the dissolution of both carbonate and silicate sources the degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs or cycling by a series of processes associated with weathering is poorly constrained in the river waters of the mackenzie basin canada the mg content is high and mg isotope ratios 26mg 24mg expressed as mg 26 show in excess of one per mil variability part of this variability is attributed to the 3 range in the carbonate and silicate rocks drained despite this inherent lithological control on river water mg 26 values there is also evidence for a fractionation control a linear positive covariation between lithium 7li 6li expressed as li 7 and mg isotope ratios in the river waters of the mackenzie basin is reported this covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with mg isotope ratios

grain size control of river suspended sediment geochemistry clues from amazon river depth profiles residual solid products of erosion display a wide range of size density shape mineralogy and chemical composition and are hydrodynamically sorted in large river channels during their transport we characterize the chemical and isotopic variability of river sediments of the amazon basin collected at different water depths as a function of grain size absolute chemical concentrations and sr and nd isotopic ratios greatly varies along channel depth the al si ratio tightly linked to grain size distribution systematically decreases with depth mostly reflecting dilution by quartz minerals a double normalization diagram is proposed to correct from dilution effects elements define fan shaped patterns and can be classified in three different groups with respect to hydrodynamic sorting during transport in the amazon 1 poorly sorted insoluble elements like al fe th and rees 2 well sorted insoluble elements like zr and ti mainly carried by heavy minerals and 3 alkali na to cs and alkali earth elements mg to ba for which a large variety of patterns is observed related for alkali to their variable affinity for phyllosilicates sr isotopes show that the amazon river at the mouth is stratified the madeira and solimoes derived sediments being preferentially transported near the channel surface and at depth respectively the comparison between the solimoes and madeira rivers shows how the interplay between grain sorting weathering and crustal composition controls the composition of the suspended river sediments

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

silicon isotope variations accompanying basalt weathering in iceland abstract this study presents the si isotope compositions of the dissolved phase of twenty rivers from across iceland the high resolution nu plasma 1700 mc icp ms was used to provide interference free 30 si 28 si ratios with an average limiting precision of 0 04 30 si the si isotope composition ranges from 0 08 to 1 46 30 si averaging 0 63 0 38 1 sd this is only slightly lighter than the recently reported mean of 0 84 0 19 for high precision measurements of swiss rivers but is significantly lighter than compositions reported for some other rivers the si isotope composition varies slightly with the amount of glacial cover which provides evidence that the continental si input to the oceans may be climatically controlled and thus likely to vary over glacial interglacial cycles after correction for the dissolution of trace amounts of calcite in one region a correlation is found between 30 si and ca si this is consistent with the measured basaltic composition and indicates isotopic fractionation during si removal from the dissolved phase by calculating the fraction of si that is removed from the dissolved phase into secondary phases it is possible to use the riverine si flux si denudation rate to calculate the initial dissolution rate of si within these catchments from this study we estimate that the total amount of si being dissolved by primary weathering of iceland is 3 million tons per yr however only 50 of this si is released into the rivers and contributes to denudation using the si isotope composition and the si fraction removed it is possible to model the si isotope fractionation as either a rayleigh type fractionation or a steady state system both yield similar fractionation factors of about 1 5 30 si implying only one underlying fractionation process that is controlled by external parameters such as runoff furthermore rivers that follow a steady state fractionation model are associated with higher si dissolution rates and lower 30 si values indicating an inverse relationship between weathering rates and dissolved riverine si isotope composition

stable isotope characteristics of eclogites from the ultra high pressure metamorphic terrain east central china abstract a stable isotope study was made for the ultra high pressure uhp metamorphic rocks in the dabie and sulu terrains east central china 18 o values of minerals in eclogites cover a wide range from 4 to 6 the o isotope fractionations between minerals i e omphacite phengite and or amphibole and garnet in eclogites are mostly not in isotopic equilibrium the large range of o isotope composition of garnet may approximate the 18 o variations of eclogites during uhp metamorphism and must have been largely inherited from their pre subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water rock ratios before uhp metamorphism the o isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions the fluids required for these hydration reactions must have low or even negative o isotope compositions it is postulated that such fluids were either originated from meteoric deep infiltration or were derived from low 18 o para gneisses

multiphase solid inclusions in zoisite bearing eclogite evidence for partial melting of ultrahigh pressure metamorphic rocks during continental collision abstract multiphase solid ms inclusions in both garnet and omphacite were investigated for zoisite bearing eclogite from the ultrahigh pressure uhp metamorphic zone in the sulu orogen the results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust there are three types of ms inclusions 1 plagioclase quartz 2 plagioclase quartz k feldspar and 3 barite plagioclase k feldspar zoisite epidote the host minerals mostly exhibit radial fractures surrounding the ms inclusions the first and second types of ms inclusions were analyzed for their bulk compositions yielding high sio 2 and na 2 o but very low feo mgo tio 2 with variable k 2 o for the second type trace element analyses of representative ms inclusions yield generally very low concentrations except such large ion lithophile elements as sr ba and rb these features suggest different origins for the three types of ms inclusions the first type of ms inclusions would be primarily derived from dehydration melting of paragonite whereas the second type of ms inclusions would be derived from dehydration melting of both paragonite and phengite the third type of ms inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz providing components for growth of garnet during the exhumation therefore the anatexis of zoisite bearing uhp eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite the major and trace element compositions of ms inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel

individualization of textural and reactional microdomains in eclogites from the bergen arcs norway consequences for rb sr and ar ar radiochronometer behavior during polymetamorphism 1 rb sr 40ar 39ar and sm nd isotopic data are reported in caledonian eclogites from the lindas nappe bergen arcs norway in order to investigate processes controlling isotopic equilibrium at mineral scale in polymetamorphic rocks the bergen arcs exposes sveconorwegian 950 930 ma granulites partially overprinted by caledonian eclogite facies metamorphism at 425 ma and amphibolite facies metamorphism at 410 ma geochemical and rb sr data from more than 10 phengite fractions separated from one sample reflect the composition of the microdomain a few hundred microns in size in which phengite crystallized phengite crystallized after garnet or plagioclase by dissolution precipitation processes yield apparent age between 700 and 600 ma at the time of their crystallization these phengites inherited the isotopic composition of their precursor minerals at a microdomain scale phengite from quartz veins which crystallized from elements mobilized by the circulating fluid yield an age closer to the eclogite facies metamorphic age the closed system evolution of the eclogitizing fluid the segregation of textural and reactional microdomains the high sr content of the studied phengite and the short duration of the recrystallization processes 1ma are interpreted as the main factors responsible for the lack of a rb sr isotopic equilibrium at the scale of hand samples such equilibrium is nevertheless reached in quartz veins where the crystallization of minerals implies that the fluid circulation acted as a factor of isotopic homogenization the in situ single mineral ar ar datings revealed that both eclogite and amphibolite facies minerals are characterized by an excess of radiogenic argon ages between 425 and 520 ma the excess of argon has been inherited from the previous granulite and has been only partially evacuated from the system by the circulating fluid the apparent sm nd ages from garnets inherited from the granulite facies metamorphism c 930 ma are in agreement with previous estimates this result confirms that the diffusion of nd in garnet does not occur at temperatures lower than 700 c this study highlights the complexity of radiochronometer behavior during hp metamorphism and demonstrates that coupling different radiochronometers such as ar ar and rb sr does not always guarantee the validity of the geochronological results

the behaviour of magnesium and its isotopes during glacial weathering in an ancient shield terrain in west greenland the magnesium mg isotope composition of rivers is sensitive to changes in the balance of primary mineral dissolution and secondary mineral formation as these processes are regulated by climate then changes to the earth s climate system such as the onset of glaciation could potentially alter the isotopic behaviour of mg in rivers to investigate how mg isotopes behave during glacial weathering we have determined the mg concentration and mg isotope ratio for the dissolved suspended and bedload phases of glacial and non glacial rivers in west greenland this region is essentially monolithological and there is little biological activity hence isotope variations largely result from differences in weathering processes dissolved mg in rivers is always enriched in the light mg isotopes relative to the bedload 26mg 0 4 and the glacial rivers have lighter mg isotope compositions 26mg 1 to 1 3 than the non glacial rivers 26mg 0 6 enrichment of light mg in the dissolved load is consistent with the preferential uptake of heavy mg from solution during the formation of secondary silicate minerals however evidence from saturation state modelling suggests that little secondary mineral formation is likely to have occurred in the glacial rivers because the concentration of dissolved solids is very low moreover the glacial derived suspended sediment has identical mg isotope ratios to the bedload these observations suggest that the formation of secondary weathering minerals is not controlling the behaviour of mg isotopes in these rivers rather variations in the 26mg value of the dissolved load are attributed to incongruent weathering of the solid phase preferential dissolution of carbonate minerals which have low 26mg 1 1 to 5 2 imparts a more negative 26mg signal to the glacial river waters although our study indicates that the mg isotopic composition of the dissolved load is sensitive to changes in weathering congruence it is unlikely that glaciation will result in major changes in the 26mg composition of continental runoff

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

ge and si isotope signatures in rivers a quantitative multi proxy approach abstract solutes derived from the dissolution of silicate minerals play a key role in earth s climate via the carbon and other biogeochemical cycles silicon si is a unique constituent of silicate minerals and a biologically important nutrient so tracing its behavior in near surface environments may provide important insights into weathering processes however si released by weathering is variably incorporated into secondary mineral phases and biota obscuring signals derived from primary weathering processes due to chemical similarities germanium ge may help better understand the si cycle and its relationship to chemical weathering with this aim we report new measurements of the concentration and isotopic composition of ge for both the dissolved and particulate phases of a variety of global rivers these measurements are combined with analyses of concentration and isotopic ratio of si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems with this dataset we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers i e ge si 74 ge and 30 si this multi proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean dissolved 74 ge composition of rivers studied thus far range from 0 9 to 5 5 with a discharge weighted global average of 2 6 0 5 the ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks which is consistent with mass balance expectations the multi proxy modeling suggests that among the watersheds studied here the isotopic fractionation of si during secondary mineral phase precipitation 30 si s e c ranges from 2 7 to 0 2 which removes between 19 79 of the initial dissolved si while between 12 54 is incorporated by biota for ge modeling indicates that 79 98 of the dissolved load is incorporated into secondary mineral phases with a 74 ge s e c ranging from 4 9 to 0 3 the fractionation induced by biological uptake is calculated to range from 2 6 to 1 3 for 30 si b i o and 0 7 0 7 for 74 ge b i o in addition to improving our understanding of the coupled ge and si cycles our study provides a framework for using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

metamorphic rocks seek meaningful cooling rate interpreting 40ar 39ar ages in an exhumed ultra high pressure terrane 40ar 39ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and or exhumation of metamorphic terranes in theory so long as the dated mineral originally crystallised above its closure temperature the yielded 40ar 39ar ages should not vary significantly within or between samples and should vary only due to grain size 3 ma variation depending on the estimated cooling rate infra red laser single grain fusion and ultra violet laser spot muscovite and or phengite data from mafic eclogite bodies and their host gneisses in the nordfjord region of the western gneiss complex wgc norway show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284 ma ages yielded within the same pelitic gneiss sample vary by up to 28 ma overall the 40ar 39ar phengite dates from mafic eclogites range from 750 to 440 ma and are significantly older than the previously reported 405 400 ma u pb zircon age constraining peak metamorphism in the nordfjord wgc the felsic host gneiss samples which recrystalised during a post peak exhumation related amphibolite facies overprint yield single grain fusion and laser spot 40ar 39ar white mica dates spanning 420 to 385 ma encompassing previous estimates of the timing of the entire metamorphic cycle these data suggest that ar is heterogeneously distributed both within grains and between grains on the cm to sub mm scale in all samples and imply restricted ar transport and inefficient removal during the metamorphic cycle open system zero grain boundary ar concentration behaviour is an inherent assumption in the commonly applied closure temperature formulation but unless this can be demonstrated linking 40ar 39ar ages to temperature is no longer trivial the high spatial and chronological precision afforded by uv laser ablation mean that intra grain ar concentration distributions may be used to assess open vs closed system behaviour the data presented here suggest that open system behaviour cannot be assumed in metamorphic systems even those in which the pressure temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle high spatial resolution 40ar 39ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales

germanium silicon fractionation in a river influenced continental margin the northern gulf of mexico abstract in this study we have sampled the water column and sediments of the gulf of mexico to investigate the effects of high riverine terrigenous load and sediment redox conditions on the cycling of ge and si water column ge si ratios across the gulf of mexico continental shelf range from 1 9 to 25 mol mol which is elevated compared to the global ocean value of 0 7 mol mol the ge enrichment in the gulf of mexico seawater is primarily due to anthropogenic contamination of the mississippi river which is the main ge and si source to the area and to a smaller extent due to discrimination against ge during biogenic silica bsi production ge si 1 2 1 8 mol mol especially by radiolarians and siliceous sponges ge si 0 6 1 1 mol mol most sediment pore waters ge si 0 3 4 5 mol mol and sediment incubation experiments benthic flux ge si 0 9 1 2 mol mol indicate precipitation of authigenic phases that sequester ge from pore waters non opal sink this process appears to be independent of oxidation reduction reactions and suggests that authigenic aluminosilicate formation reverse weathering may be the dominant ge sink in marine sediments compilation of previously published data shows that in continental margins non opal ge burial flux is controlled by bsi supply while in open ocean sediments it is 10 100 times lower and most likely limited by the supply of lithogenic material we provide a measurement based estimate of the global non opal ge burial flux as 4 32 mmol yr 1 encompassing the 2 16 mmol yr 1 needed to keep the global marine ge cycle at steady state

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

the riverine silicon isotope composition of the amazon basin we present here the first large scale study of riverine silicon isotope signatures in the amazon basin the amazon and five of its main tributaries were studied at different seasons of the annual hydrological cycle the 30si signature of the dissolved silicon dsi exported to the estuary weighted for dsi flux for the period considered is estimated at 0 92 a river cross section shows the homogeneity of the amazon river regarding dsi concentration and isotope ratio the biogenic silica bsi concentration measured in surface water from all rivers is generally small compared to the dsi reservoir but large variations exist between rivers very low isotope signatures were measured in the upper rio negro 30si 0 05 0 06 which we explain both by an equilibrium between clay formation and dissolution and by gibbsite formation the si isotope fractionation in the andean tributaries and the amazon main stem can be explained by clay formation and follow either a rayleigh or a batch equilibrium fractionation model our results also suggest that the formation of 2 1 clays induces a fractionation factor similar to that of kaolinite formation

stable isotope geochemistry of marbles from the coesite uhp terrains of dabieshan and sulu china abstract marbles from dabieshan and sulu china suffered ultra high pressure uhp metamorphism in the coesite eclogite facies at approximately 700 c and 30 kbars during triassic continental collision and subduction the marbles range in isotopic composition from 7 to 25 18 o vsmow and from 0 to 6 13 c vpdb high 13 c values are representative of unmodified protoliths and are similar to those of 13 c enriched sinian carbonate rocks from the yangtze craton high oxygen isotope ratios reflect pristine protoliths but the low values may have been caused by infiltration of low 18 o meteoric water during diagenesis and dolomitization by fracture controlled infiltration of water during subduction by metamorphic mineral reactions or by a combination of these processes no evidence of regional isotopic transport during uhp metamorphism has been found sampling on scales of 1 to 100 m shows marbles to be inhomogeneous in both carbon and oxygen isotopes only samples separated by less than 10 cm have equilibrated oxygen and carbon isotope compositions limited isotopic equilibration between adjacent rocks is consistent with the preservation of unaltered uhp minerals and indicates that the metamorphic fluid rock system was rock dominated during and following peak metamorphism a freely flowing pervasive fluid phase was not present during uhp metamorphism there is no evidence of isotopic exchange between marble and the upper mantle into which it was subducted correlation of geochemical similarities of uhp marbles with sinian limestones implies that the subducted edge of the yangtze craton extends at least as far north as the coesite eclogite facies rocks of dabieshan deposition of protolith carbonates may have taken place in a cold climate either preceding or following but not coincident with neoproterozoic glaciation

oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at shuanghe in the dabie mountains the oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss biotite gneiss and granitic gneiss associated with ultrahigh pressure uhp eclogites in the shuanghe district of the eastern dabie mountains there are significant differences in 18o between the two gneisses the uhp biotite gneiss varying from 4 3 to 10 6 similar to the associated eclogites whereas the non uhp granitic gneiss ranges only from 3 8 to 1 2 the d values are similar in the two gneisses with 37 to 64 for epidote zoisite 92 to 83 for amphibole and 63 to 109 for biotite phengite hydrogen isotope disequilibrium among the coexisting hydroxyl bearing minerals is ascribed to retrograde exchange subsequent to amphibolite facies metamorphism oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock 18o oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals the metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss the isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite facies metamorphism however high temperatures of 550 to 650 c are obtained from those minerals resistant to retrograde oxygen isotope exchange implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers the 18o depletion of both gneisses is interpreted to result from meteoric hydrothermal exchange before during plate subduction therefore the measured 18o values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the uhp metamorphism it is inferred that the uhp unit is in foreign contact with the non uhp unit like a tectonic melange but both of them experienced the two common stages of geodynamic evolution 1 18o depletion prior to the uhp metamorphism 2 uplifting since the amphibolite facies metamorphism

stable silicon isotopes of groundwater feldspars and clay coatings in the navajo sandstone aquifer black mesa arizona usa abstract we present some of the first analyses of the stable isotopic composition of dissolved silicon si in groundwater the groundwater samples were from the navajo sandstone aquifer at black mesa arizona usa and the si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed silicon isotope compositions were measured using high resolution multi collector inductively coupled mass spectrometry hr mc icp ms nu1700 nuplasma hr the quartz dominated bulk rock and feldspar separates have similar 30 si of 0 09 0 04 and 0 15 0 04 95 sem respectively and clay separates are isotopically lighter by up to 0 4 compared to the feldspars from isotopic mass balance considerations co existing aqueous fluids should have 30 si values heavier than the primary silicates positive 30 si values were only found in the shallow aquifer where si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals however 30 si decreases along the flow path from 0 56 to 1 42 representing the most negative dissolved si isotope composition so far found for natural waters we speculate that the enrichment in 28 si is due to dissolution of partly secondary clay minerals and low temperature silcretes in the navajo sandstone the discovery of the large range and systematic shifts of 30 si values along a groundwater flow path illustrates the potential utility of stable si isotopes for deciphering the si cycling in sedimentary basins tracing fluid flow and evaluating global si cycle

primary study of germanium isotope composition in sphalerite from the fule pb zn deposit yunnan province abstract the sichuan yunnan guizhou metallogenic province in southwest china is one of the most important low temperature metallogenic domains in the country and more than 400 pb zn deposits and or mineralization points are located in this area the fule deposit is better known that the other pb zn deposits in the region because it is extremely highly enriched with dispersed elements including ge ga and cd based on information obtained from our previous studies the contents of ge and its isotopic compositions in sphalerite are investigated herein results show that the ge content ranges from 60 ppm to 141 5 ppm and 74 70ge values vary from 6 57 to 0 97 in this respect 6 57 is the most negative ge isotope composition value reported in any sphalerite or seafloor sulfides to date the cd and ge contents in sphalerite have a good negative relationship and it is suggested that the mechanisms of cd and zn substitution are responsible for the low incorporation of ge and relate to the high cd content up to 3 of the sphalerite in addition there is a good positive correlation between the ge contents and 34s values which was likely triggered by variations of fluid temperature in the same hand specimen light ge isotope is more likely to be enriched in dark sphalerite rather than light sphalerite similarly to cd isotope distribution in dark and light colored sphalerite in addition cd and ge isotope compositions measured in an ore profile from sbfl22 to sbfl26 show a good positive relationship suggesting that the fractionation mechanisms between cd and ge isotopes may be similar and they were controlled by kinetic fractionation during sphalerite precipitation based upon previous limited studies we conclude that ge in the fule deposit is derived from mixing sources and sediment is likely the dominant one

polyphase growth of accessory minerals during continental collision geochemical evidence from ultrahigh pressure metamorphic gneisses in the sulu orogen abstract an integrated study of petrology geochronology and geochemistry was performed for ultrahigh pressure metamorphic gneisses in the sulu orogen the results are used to elucidate the polyphase growth of such metamorphic minerals as zircon titanite and garnet in response to pressure temperature changes and fluid melt action during continental collision this provides insights into the property of metamorphic fluid melt and their effects on trace element mobility a combined result from ree patterns mineral inclusions and ti in zircon temperatures indicates three stages of zircon growth prograde growth occurred at 237 ma primarily at eclogite facies retrograde growth at 222 ma mostly at eclogite facies and the last growth at 205 ma at granulite facies the three stages of zircon growth are deciphered by distinct ree patterns and trace element compositions reflecting the differences in the property of metamorphic fluid melt the episodic growth of metamorphic zircon is primarily dictated by the episodic concentration of zr and si in metamorphic fluid melt relict domains of magmatic titanite are distinguished from metamorphosed and metamorphic domains by their distinctive ree patterns and trace element compositions the metamorphic titanite exhibit variably elevated nb contents and nb ta ratios suggesting nb ta fractionation due to breakdown of amphibole and or biotite during metamorphism polyphase growth of garnet is suggested by an integrated analysis of mineral inclusions and major and trace elements in large garnet grains trace element abundances vary in different zones of garnet which is ascribed to changes in the paragenesis and composition of matrix minerals involved in garnet forming reactions at different p t conditions therefore the metamorphic growth of zircon titanite and garnet would have occurred not only during prograde subduction but also during retrograde exhumation in the continental collision zones both metamorphic dehydration and partial melting would have taken place episodically during the collisional orogeny the breakdown of hydrous minerals at high to ultrahigh pressure conditions is a key to fluid liberation and element supply for the growth of these accessory minerals

ultrarapid exhumation of ultrahigh pressure diamond bearing metasedimentary rocks of the kokchetav massif kazakhstan the diamond bearing ultrahigh pressure kokchetav massif recrystallized at eclogite facies conditions deep in the mantle at 180 km depth at 535f3 ma and yet new 40 ar 39 ar ages suggest that it may have been exhumed to crustal depths as indicated by closure of mica to ar loss by f529 ma these data indicate a possible exhumation rate of tens of kilometers per million years i e a rate that is comparable to rates of horizontal plate motion and subduction d 2003 elsevier b v all rights reserved

silicon isotope compositions of dissolved silicon and suspended matter in the yangtze river china abstract silicon isotope compositions of main channel samples of the yangtze river were systematically investigated along with their chemical compositions the concentration of suspended matter in the yangtze river tends to decrease from the upper reaches to the lower reaches corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow the silica contents of suspended matter vary from 52 1 to 56 9 and their 30si values vary from 0 to 0 7 both similar to those of shales from the upper to lower reaches the silica contents of suspended matter tend to increase whilst their 30si values tend to decrease both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter the concentrations of dissolved silicon vary from 97 to 121 mol l and their 30si values vary over a wide range from 0 7 to 3 4 from the upper to lower reaches dissolved silica concentrations tend to decrease and their 30si values tend to increase these trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches the major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass in wetlands and rice in paddy fields in drainage areas of the middle and lower reaches there is no correlation between 30si of dissolved silicon and that of suspended matter the 30sidiss spm values vary over a wide range of 1 0 3 7 indicating that 1 they are out of isotopic equilibrium 2 dissolved silicon and the associated suspended matter do not belong to one physico chemical system and 3 isotopic exchange rate between them is very slow the 30si value of dissolved silicon output from the yangtze river to the east sea is estimated to be 3 0 much higher than the values reported for the amazon and congo rivers this increases the 30si range of dissolved silicon in the world s rivers from 0 4 1 2 to 0 4 3 4

excess argon in phengite from eclogite evidence from dating of eclogite minerals by smnd rbsr and 40ar39ar methods the existence of excess ar in phengites from coesite bearing ultrahigh pressure eclogites of the dabie mountains and su lu terrain china has been confirmed by dating of minerals using sm nd rb sr and 40ar39ar methods 40ar39ar ages of phengites for two samples studied are much older than the sm nd and rb sr mineral isochron ages sample ql 1 has rb sr age of 219 5 0 5 ma sm nd age of 226 3 4 5 ma and phengite 40ar39ar age of 877 5 15 9 ma the corresponding ages for sample zb 4 are 223 9 0 9 ma rb sr 228 4 6 0 ma sm nd and 943 3 12 4 ma 40ar39ar phengite an age of 225 ma based on the sm nd and rb sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the north china and yangtse blocks and formation of these eclogites we call for greater caution in 40ar39ar dating of phengites from other occurrences of ultrahigh pressure environments

the silicon isotope composition of the upper continental crust the upper continental crust ucc is the major source of silicon si to the oceans and yet its isotopic composition is not well constrained in an effort to investigate the degree of heterogeneity and provide a robust estimate for the average si isotopic composition of the ucc a representative selection of well characterised continentally derived clastic sediments have been analysed using high precision mc icpms analyses of loess samples define a narrow range of si isotopic compositions 30si 0 28 to 0 15 this is thought to reflect the primary igneous mineralogy and predominance of mechanical weathering in the formation of such samples the average loess 30si is 0 22 0 07 2 s d identical to average granite and felsic igneous compositions therefore minor chemical weathering does not resolvably affect bulk rock 30si and loess is a good proxy for the si isotopic composition of unweathered crystalline continental crust the si isotopic compositions of shales display much more variability 30si 0 82 to 0 00 shale si isotope compositions do not correlate well with canonical proxies for chemical weathering such as cia values but do correlate negatively with insoluble element concentrations and al si ratios this implies that more intensive or prolonged chemical weathering of a sedimentary source with attendant desilicification is required before resolvable negative si isotopic fractionation occurs shale 30si values that are more positive than those of felsic igneous rocks most likely indicate the presence of marine derived silica in such samples using the data gathered in this study combined with already published granite si isotope analyses a weighted average composition of 30si 0 25 0 16 2 s d for the ucc has been calculated

the iron isotopic composition of subglacial streams draining the greenland ice sheet abstract in this study we present the first measurements of iron fe stable isotopic composition 56 fe of subglacial streams draining the greenland ice sheet gis we measure the 56 fe values 56 fe 56 fe 54 fe sample 56 fe 54 fe standard 1 10 3 of both dissolved and suspended sediment fe in subglacial outflows from five distinct land terminating glaciers suspended sediments have 56 fe values that lie within the crustal array 56 fe 0 in contrast the 56 fe values of dissolved fe in subglacial outflows are consistently less than 0 reaching a minimum of 2 1 in the outflow from the russell glacier the 56 fe values of dissolved fe vary geographically and on daily time scales major element chemistry and mineral saturation state modeling suggest that incongruent silicate weathering and sulfide oxidation are the likely drivers of subglacial stream fe chemistry and that the extent of chemical weathering influences the 56 fe of dissolved fe the largest difference in 56 fe between dissolved and suspended load is 2 1 and occurs in the subglacial system from the russell glacier southwest gis major element chemistry indicates this outflow to be the least chemically weathered while more mature subglacial systems i e that exhibit greater extents of subglacial weathering have dissolved loads with 56 fe that are indistinguishable from suspended sediments 56 fe suspended dissolved 0 ultimately the dissolved fe generated in some subglacial systems from the gis is a previously unrecognized source of isotopically light fe into the hydrosphere the data illustrate that the dissolved fe supplied by subglacial weathering can have variable 56 fe values depending on the degree of chemical weathering thus fe isotopes have potential as a proxy for subglacial chemical weathering intensity or mode finally based on our regional fe concentration measurements from each glacial outflow we estimate a flux weighted continental scale dissolved iron export of 2 1 gg fe yr 1 to the coastal ocean which is within the range of previous estimates

a global ge isotope budget we present measurements of ge isotope composition and ancillary data for samples of river water low and high temperature hydrothermal fluids and seawater the dissolved 74ge composition of analyzed rivers ranges from 2 0 to 5 6 which is significantly heavier than previously determined values for silicate rocks 74ge 0 4 0 7 escoube et al geostand geoanal res 36 2 2012 from which dissolved ge is primarily derived an observed negative correlation between riverine ge si and 74ge signatures suggests that the primary 74ge fractionation mechanism during rock weathering is the preferential incorporation of light isotopes into secondary weathering products high temperature 150 c hydrothermal fluids analyzed in this study have 74ge of 0 7 1 6 most likely fractionated during fluid equilibration with quartz in the reaction zone low temperature 25 63 c hydrothermal fluids are heavier 74ge between 2 9 and 4 1 and most likely fractionated during ge precipitation with hydrothermal clays seawater from the open ocean has a 74gesw value of 3 2 0 4 and is indistinguishable among the different ocean basins at the current level of precision this value should be regulated over time by the isotopic balance of ge sources and sinks and a new compilation of these fluxes is presented along with their estimated isotopic compositions assuming steady state non opal ge sequestration during sediment authigenesis likely involves isotopic fractionation 74gesolid solution that is 0 6 1 8

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

incremental heating of hornblende in vacuo implications for 40ar 39ar geochronology and the interpretation of thermal histories increasing evidence of the incompatibility between the geologic thermal histories deduced from the sup 40 ar sup 39 ar incremental heating technique and those derived form other methods have necessitated a careful evaluation of the assumptions inherent in the former approach sup 40 ar sup 39 ar age spectra are frequently used to infer the spatial distribution of ar in hydrous minerals and as such require that the mineral remains stable while heated in vacuo aliquots of mmhb 1 and inter laboratory hornblende standard used for k ar and sup 40 ar sup 39 ar dating were heated in vacuo at temperatures of 500 700 930 and 1 050c for 30 min up to 700c no physical changes in the hornblende were observed with the scanning electron microscope although x ray diffraction data indicate that it had dehydroxylated to oxyhornblende at this temperature the onset of the structural decomposition of the oxyhornblende as revealed by backscattered and secondary electron images occurs at approximately 930c and is directly correlated with the initial release of 6 of its radiogenic ar sup 40 ar the breakdown of the oxyhornblende by the internal nucleation of incongruent melt is almost 93 complete by 1 050c the reaction products being titano magnetite melt a pyribole phase and internal voids because ar is released simultaneously frommore these products and is subsequently homogenized in vacuo this breakdown can yield plateau age spectra regardless of any original ar spatial gradients in the mineral consequently independent evidence may be required to establish whether a plateau date from hornblende is truly a geologically significant age less

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh pressure metagranite in the sulu orogen

the acid and alkalinity budgets of weathering in the andes amazon system insights into the erosional control of global biogeochemical cycles the correlation between chemical weathering fluxes and denudation rates suggests that tectonic activity can force variations in atmospheric pco 2 by modulating weathering fluxes however the effect of weathering on pco2 is not solely determined by the total mass flux instead the effect of weathering on pco 2 also depends upon the balance between 1 alkalinity generation by carbonate and silicate mineral dissolution and 2 sulfuric acid generation by the oxidation of sulfide minerals in this study we explore how the balance between acid and alkalinity generation varies with tectonic uplift to better understand the links between tectonics and the long term carbon cycle to trace weathering reactions across the transition from the peruvian andes to the amazonian foreland basin we measured a suite of elemental concentrations na k ca mg sr si li so4 and cl and isotopic ratios 87 sr 86 sr and 34 s on both dissolved and solid phase samples using an inverse model we quantitatively link systematic changes in solute geochemistry with elevation to downstream declines in sulfuric acid weathering as well as the proportion of cations sourced from silicates with a new carbonate system framework we show that weathering in the andes mountains is a co 2 source whereas foreland weathering is a co 2 sink these results are consistent with the theoretical expectation that the ratio of sulfide oxidation to silicate weathering increases with increasing erosion altogether our results suggest that the effect of tectonically enhanced weathering on atmospheric pco 2 is strongly modulated by sulfide mineral oxidation

germanium sequestration by soil targeting the roles of secondary clays and fe oxyhydroxides abstract germanium is a trace element that behaves similarly to silicon and shows promise as a tracer of silicate weathering processes sequestration of ge by clay minerals is thought to be the dominant mechanism responsible for ge si fractionation during weathering in some environments ge may be sequestered by iron oxyhydroxides we examine ge behavior in soils along a well characterized climate gradient on the hawaiian island of maui to clarify the relationship between fe geochemistry and soil ge si ratios all of the sites are developed on basaltic lavas and tephras approximately 400 ky old are highly si depleted and have ge si ratios higher than parent material sites experiencing less than 3300 mm rain yr host oxidized highly weathered basaltic soils in which fe is present as secondary fe oxyhydroxide minerals in contrast the wetter sites 3340 mm yr show extensive fe reduction mobilization and loss we show that ge sequestration is independent of fe redox behavior along the gradient and thus precipitation of fe oxyhydroxides is not a major factor contributing to ge si fractionation during weathering instead elevated soil ge si ratios reflect partitioning of ge into secondary clay minerals plus additional retention of ge by refractory non silicates

geomorphic regime modulates hydrologic control of chemical weathering in the andes amazon abstract the interplay between the hydrologic processes that supply store and route water in catchment systems and the chemical weathering reactions that add and remove solutes acts as an important control on chemical weathering fluxes in this study we use paired measurements of solute chemistry and runoff in four nested catchments that span the transition from the andes mountains to the amazonian foreland floodplain in peru in order to investigate the links between hydrology and weathering processes and to determine how these links change across a geomorphic gradient all of the sites show variation in elemental concentrations and ratios with runoff consistent with hydrologically driven changes in lithologic sources the extent of secondary mineral precipitation and potentially fluid flow paths in the andean sites solute concentrations are relatively constant despite large changes in runoff this is in direct contrast to the foreland floodplain site where solute concentrations are diluted as runoff increases in the andes amazon the concentration runoff behavior is correlated with the mean catchment slope angle which suggests that erosional processes by modulating the timescales over which weathering reactions occur within the critical zone can be an underlying control on solute production and therefore on chemical weathering due to the co variation between the geomorphic and hydrologic controls on chemical weathering weathering fluxes in andean sites are more sensitive to seasonal changes in runoff than in the foreland floodplain site

timing of synorogenic granitoids in the south qinling central china constraints on the evolution of the qinling dabie orogenic belt abstract the ca 400 km long granitoid belt in the south qinling is believed to be a synorogenic product of the collision between the north and south china blocks along the qinling dabie orogenic belt in central china single and multigrain zircon u pb dating of six of these granitoid bodies indicate that the granitoids were formed between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 220 pm 1 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage m

riverine li isotope fractionation in the amazon river basin controlled by the weathering regimes we report li isotope composition 7li 7li of river borne dissolved and solid material in the largest river system on earth the amazon river basin to characterize li isotope fractionation arl a continental scale the 7li 7li in the dissolved load 1 2 to 32 is fractionated toward heavy values compared to the inferred bedrock 1 to 5 and the suspended sediments 6 8 to 0 5 as a result of the preferential incorporation of 66li into secondary minerals during weathering despite having very contrasted weathering and erosion regimes both andean headwaters and lowland rivers share similar ranges of dissolved 7li 7li 1 2 to 18 correlations between dissolved 7li 7li and li na and li mg ratios suggest that the proportion of li incorporated in secondary minerals during weathering act as the main control on the 7lidiss 7lidiss across the entire amazon basin a batch steady state fractionation model for andean and lowland rivers satisfactorily reproduces these variations with a fractionation factor between weathering products and dissolved load sec dis sec dis of 0 983 two types of supply limited weathering regimes can be identified for the lowlands clearwaters with dominant incorporation of li in secondary minerals and black waters e g rio negro where dissolution of secondary minerals enhanced by organic matter produces low 7li 7li apart from the black waters the 7li 7li of andean and lowland rivers is negatively correlated to the denudation rates with the lowest 7li 7li corresponding to the rivers having the highest denudation rates in contrast the main tributaries draining both the andes and the lowlands have higher 7li 7li compared to other rivers we propose that part of the dissolved li derived from weathering in the andes is re incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved 7li 7li along the course of these rivers unlike other rivers the dissolved 7li 7li in the main tributaries is best described by a rayleigh fractionation model with a fractionation factor sec dis sec dis of 0 991 altogether the control imposed by residence time in the weathering zone and floodplain processes results in i a non linear correlation between dissolved 7li 7li and the weathering intensity defined as w d and ii a positive relationship between the dissolved li flux and the denudation rate these results have important implications for the understanding of past ocean 7li 7li and its use as a paleo weathering proxy

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

modeling novel stable isotope ratios in the weathering zone when rock is converted to weathering products the involved processes can be fingerprinted using the stable isotope ratios of metals for example li mg ca fe sr and metalloids b si here we construct a framework for interpreting these novel stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context the approach is applicable to any novel stable isotope system and is based on a simple steady state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays and uptake of nutrients by plants the model results show that the isotope composition of a given element in the weathering zone compartments depends on 1 the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material consisting of secondary precipitates and organic matter 2 the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants a relationship is established between isotope ratios isotope fractionation factors and indexes for chemical weathering such as chemical depletion fractions cdf and elemental mass transfer coefficients derived from simple elemental concentration measurements from this relationship isotope fractionation factors can be calibrated from chemical and isotope data measured on field material furthermore we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock water and sediment this calculation can be applied to samples from soils from rivers and from the sedimentary record and does not require knowing the isotope fractionation factors involved in the reactions finally we apply the model to the oceanic li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the cenozoic

ge si ratios indicating hydrothermal and sulfide weathering input to rivers of the eastern tibetan plateau and mt baekdu abstract concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals formation and weathering of secondary clay minerals hydrothermal input and biological cycling formation and dissolution of opal phytoliths and growth of diatoms we applied the ge si ratio to assess the different sources of dissolved si in rivers hailing from the eastern tibetan plateau the salween mekong chang jiang yangtze hong red and huang he yellow and from mt baekdu the duman elevated riverine ge si ratios were observed in arid regions with high geothermal activity in the salween chang jiang and mt baekdu streams in the huang he and hong river basins geothermal influence was not as pronounced but weathering of sulfide and coal bearing minerals may be responsible for the high ge si ratios in rivers where inputs from hydrothermal and sulfide weathering are minimal our data mostly fall in the weathering limited regime of high riverine si concentrations and low ge si ratios

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

crustal structure and continental dynamics of central china a receiver function study and implications for ultrahigh pressure metamorphism abstract the qinling tongbai hong an dabie sulu orogenic belt records the tectonic history of paleozoic convergence between the south china and north china blocks in this study the distribution of crustal thickness and p and s wave velocity ratio vp vs is obtained by using the h k stacking technique from the dabie sulu belt in central china our results show marked differences in the crustal structure between the dabie and sulu segments of the ultrahigh pressure uhp orogen the lower crust in the dabie orogenic belt is dominantly of felsic intermediate composition whereas the crust beneath the sulu segment is largely intermediate mafic the crust of the dabie orogenic belt is thicker by ca 3 5 km as compared to that of the surrounding region with the presence of an orogenic root the crustal thickness is nearly uniform in the dabie orogenic belt with a generally smooth crust mantle boundary a symmetrically thickened crust in the absence of any deep structural features similar to that of the yangtze block suggests no supportive evidence for the proposed northward subduction of the yangtze continental block beneath the north china block we propose that the collision between the yangtze and north china blocks and extrusion caused crustal shortening and thickening as well as delamination of the lower crust resulting in asthenospheric upwelling and lower crustal uhp metamorphism along the dabie orogen our results also reveal the presence of a se to nw dipping moho in the north china block beneath the tran north china orogen and eastern block suggesting the fossil architecture of the northwestward subduction of the kula plate

the silicon isotopic composition of fine grained river sediments and its relation to climate and lithology abstract the 30si stable isotopic composition of silicon in soils and fine grained sediments can provide insights into weathering processes on continents with important implications on the si budget of modern and past oceans to further constrain the factors controlling the distribution of si isotopes in sediments we have analysed a large number n 50 of separate size fractions of sediments and suspended particulate materials collected near the mouth of rivers worldwide this includes some of the world s largest rivers e g amazon congo mackenzie mississippi murray darling nile yangtze and rivers from the case study areas of the congo river basin and northern ireland silt size fractions exhibit a mean si isotopic composition 30si 0 21 0 19 2 s d similar to that previously inferred for the upper continental crust in contrast clay size fractions display a much larger range of 30si values from 0 11 to 2 16 which yield a global 30siclay of 0 57 0 60 2 s d representative of the mean composition of the average weathered continental crust overall these new data show that the si isotopic signature transported by river clays is controlled by the degree of chemical weathering as inferred from strong relationships with al si ratios at a global scale the clay bound si isotopic composition of the world s largest river systems demonstrates a link with climate defining a general correlation with mean annual temperature mat in corresponding drainage basins while the distribution of si isotopes in river sediments also appears to be influenced by the tectonic setting lithological effects and sediment recycling from former sedimentary cycles our results pave the way for their use as paleo weathering and paleo climate proxies in the sedimentary record

the isotope composition of inorganic germanium in seawater and deep sea sponges abstract although dissolved concentrations of germanium ge and silicon si in modern seawater are tightly correlated uncertainties still exist in the modern marine ge cycle germanium stable isotope systematics in marine systems should provide additional constraints on marine ge sources and sinks however the low concentration of ge in seawater presents an analytical challenge for isotopic measurement here we present a new method of pre concentration of inorganic ge from seawater which was applied to measure three ge isotope profiles in the southern ocean and deep seawater from the atlantic and pacific oceans germanium isotopic measurements were performed on ge amounts as low as 2 6 ng using a double spike approach and a hydride generation system coupled to a mc icp ms germanium was co precipitated with iron hydroxide and then purified through anion exchange chromatography results for the deep i e 1000 m depth pacific ocean off hawaii nearby loihi seamount and the deep atlantic off bermuda bats station showed nearly identical 74 70ge values at 3 19 0 31 2sd n 9 and 2 93 0 10 2sd n 2 respectively vertical distributions of ge concentration and isotope composition in the deep southern ocean for water depth 1300 m yielded an average 74 70ge 3 13 0 25 2sd n 14 and ge si 0 80 0 09 mol mol 2sd n 12 significant variations in 74 70ge from 2 62 to 3 71 were measured in the first 1000 m in one station of the southern ocean near sars seamount in the drake passage with the heaviest values measured in surface waters isotope fractionation by diatoms during opal biomineralization may explain the enrichment in heavy isotopes for both ge and si in surface seawater however examination of both oceanographic parameters and 74 70ge values suggest also that water mass mixing and potential contribution of shelf derived ge also could contribute to the variations combining these results with new ge isotope data for deep sea sponges sampled nearby allowed us to determine a ge isotope fractionation factor of 0 87 0 37 2sd n 12 during ge uptake by sponges although ge has long been considered as a geochemical twin of si this work underpins fundamental differences in their isotopic behaviors both during biomineralization processes and in their oceanic distributions this suggests that combined with si isotopes ge isotopes hold significant promise as a complementary proxy for delineating biological versus source effects in the evolution of the marine silicon cycle through time

river fluxes of dissolved silica to the ocean were higher during glacials ge si in diatoms rivers and oceans two centric marine diatom species thalassiosira oceanica and thalassiosira antarctica were grown in batch cultures to determine the incorporation of germanium ge and silicon si into siliceous shells opal the results were modeled as ge si isotope fractionation during exponential growth diatoms take up and incorporate ge si from solution without major discrimination against ge during stationary phase growth near silica limitation the antarctic species t antarctica discriminates slightly against ge but integrated ge si opal produced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio these results confirm experiments using radioactive 68ge that showed absence of fractionation during diatom silica uptake azam and volcani 1981 in contrast to two box ocean models that invoke 50 ge discrimination by diatoms to explain the observed excess surface ocean germanium concentration murnane and stallard 1988 froelich et al 1989 and late pleistocene ocean sediment ge si opal records mortlock et al 1991 runs of a 10 box ocean ge and si model pandora with 50 discrimination reproduce the excess surface ocean ge but introduces curvature into the deep ocean ge versus si relationship that is not observed in the oceans thus 50 fractionation is not supported by either cultures or models if diatoms do not fractionate ge si then late pleistocene ge si opal variations in piston cores are caused not by changes in local biosiliceous production and silica utilization mortlock et al 1991 but rather by whole ocean changes in ge si seawater the marine ge si opal record of the last 450 kyr can be modeled as transient oceanic responses to instantaneous continental climate transitions consistent with the chemical weathering model of murnane and stallard 1990 glacial periods are characterized by lower continental weathering intensity lower ge si riv and two fold higher dissolved silica river fluxes marine ge si opal records thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering

carboniferous and triassic eclogites in the western dabie mountains east central china evidence for protracted convergence of the north and south china blocks shrimp u pb dating and laser ablation icp ms trace element analyses of zircon from four eclogite samples from the north western dabie mountains central china provide evidence for two eclogite facies metamorphic events three samples from the huwan shear zone yield indistinguishable late carboniferous metamorphic ages of 312 5 307 4 and 311 17 ma with a mean age of 309 3 ma one sample from the hong an group 1 km south of the shear zone yields a late triassic age of 232 10 ma similar to the age of ultra high pressure uhp metamorphism in the east qinling dabie orogenic belt ree and other trace element compositions of the zircon from two of the huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase namely in the eclogite facies an interpretation supported by the presence of garnet omphacite and phengite inclusions zircon also grew during later retrogression zircon cores from the huwan shear zone have ordovician to devonian 440 350 ma ages flat to steep heavy ree patterns negative eu anomalies and in some cases plagioclase inclusions indicative of derivation from north china block igneous and low pressure metamorphic source rocks cores from hong an group zircon are neoproterozoic 780 610 ma consistent with derivation from the south china block in the western dabie mountains the first stage of the collision between the north and south china blocks took place in the carboniferous along a suture north of the huwan shear zone the major triassic continent continent collision occurred along a suture at the southern boundary of the shear zone the first collision produced local eclogite facies metamorphism in the huwan shear zone the second produced widespread eclogite facies metamorphism throughout the dabie mountains sulu terrane and a lower grade overprint in the shear zone

cretaceous isochron ages from k ar and 40ar 39ar dating of eclogitic rocks in the tso morari complex western himalaya india abstract the tso morari complex which is thought to be originally the margin of the indian continent is composed of pelitic gneisses and schists including mafic rock lenses eclogites and basic schists eclogites studied here have the mineral assemblage grt omp ca amp zo phn pg qtz rt they also have coesite pseudomorph in garnet and quartz rods in omphacite suggesting a record of ultrahigh pressure metamorphism they occur only in the cores of meter scale mafic rock lenses intercalated with the pelitic schists small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism the garnet omphacite thermometry and the univariant reaction relations for jadeite formation give 13 21 kbar at 600 c and 16 18 kbar at 750 c for the eclogite formation using the jadeite content of clinopyroxene x jd 0 48 phengites in pelitic schists show variable si al and na k ratios among grains as well as within single grains and give k ar ages of 50 87 ma the pelitic schist with paragonite and phengite yielded k ar ages of 83 5 ma k 4 9 wt for paragonite phengite mixture and 85 3 ma k 7 8 wt for phengite and an isochron age of 91 13 ma from the two dataset the eclogite gives a plateau age of 132 ma in ar ar step heating analyses using single phengite grain and an inverse isochron age of 130 39 ma with an initial 40 ar 36 ar ratio of 434 90 in ar ar spot analyses of phengites and paragonites the cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature 500 c for phengite k ar system the phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic 40 ar from the crystals the argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions

lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion abstract the erosion of major mountain ranges is thought to be largely cannibalistic recycling sediments that were deposited in the ocean or on the continents prior to mountain uplift despite this recognition it has not yet been possible to quantify the amount of recycled material that is presently transported by rivers to the ocean here we have analyzed the li content and isotope composition li 7 of suspended sediments sampled along river depth profiles and bed sands in three of the largest earth s river systems amazon mackenzie and ganga brahmaputra rivers the li 7 values of river sediments transported by these rivers range from 5 3 to 3 6 and decrease with sediment grain size we interpret these variations as reflecting a mixture of unweathered rock fragments preferentially transported at depth in the coarse fraction and present day weathering products preferentially transported at the surface in the finest fraction only the finest surface sediments contain the complementary reservoir of li solubilized by water rock interactions within the watersheds li isotopes also show that river bed sands can be interpreted as a mixture between unweathered fragments of igneous and sedimentary rocks a mass budget approach based on li isotopes li al and na al ratios solved by an inverse method allows us to estimate that for the large rivers analyzed here the part of solid weathering products formed by present day weathering reactions and transported to the ocean do not exceed 35 li isotopes also show that the sediments transported by the amazon mackenzie and ganga brahmaputra river systems are mostly sourced from sedimentary rocks 60 rather than igneous rocks this study shows that li isotopes in the river particulate load are a good proxy for quantifying both the erosional rock sources and the fingerprint of present day weathering processes overall li isotopes in river sediments confirm the cannibalistic nature of erosion and weathering

pb sr nd isotopic behavior of deeply subducted crustal rocks from the dora maira massif western alps italy ii what is the age of the ultrahigh pressure metamorphism pb nd and sr isotope data are reported from two localities on mineral separates from mg rich metapelites and associated rocks that have been subducted to depths of at least 100 km for which metamorphic conditions are estimated at 28 33 kilobars pressure and 700 800 c and then returned to the surface initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites the initial ratios indicate predominantly recycled aged granitic crustal materials for the sources of all of the samples five zircon samples 4 from pyrope megacrysts and 1 from fine grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304 10 and 38 0 1 4 ma in a concordia diagram zircon from the country rock also plots along the chord the zircon data together with initial nd and sr data indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic hercynian rocks ellenbergerite from pyrope megacrysts and monazite from the fine grained ground mass yield slightly younger ages of 30 34 ma apparently reflecting lower blocking temperatures than that of zircon sm nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 ma in agreement with the zircon date a major question concerns the timing of the ultrahigh pressure metamorphism experimental data suggest that pyrope and quartz coesite as well as ellenbergerite formed by various metamorphic reactions if as generally assumed the ultrahigh pressure metamorphism occurred ca 100 ma ago our data require that the zircon did not experience measurable lead loss at that time but lost major amounts of lead 38 ma ago during late alpine metamorphism estimates of diffusion rates for nd in pyrope further suggest that the apparent sm nd age of 38 ma for the megacryst is not consistent with that model those problems are resolved if the ultrahigh pressure metamorphism occurred 38 40 ma ago but problems remain from ar ar dates of 100 ma on phengite an inferred 120 ma age for zircon lead loss from another study and possibly by the very rapid uplift required if the metamorphism is that young

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

sims u pb rutile age of low temperature eclogites from southwestern chinese tianshan nw china article i nfo the geological evolution in the southwestern chinese tianshan orogen in nw china is poorly understood because of controversial geochronological results of the low temperature eclogites it is likely due to sm nd and rb sr isotopic disequilibrium in different mineral fractions excess 40 ar in the high pressure k bearing minerals and multi stage growths of zircon in u pb system rutile as a typical eclogite facies mineral with high u pb ratio is a feasible candidate to constrain the eclogite facies metamorphic event in this study we measured u pb rutile age data using sims cameca ims 1280 for low temperature eclogites from southwestern chinese tianshan orogen nw china we demonstrate that sims is a powerful technique for in situ u pb dating of rutile with u content as low as 1 ppm sims u pb rutile dating yielded an age of 318 7 ma which may record the time when the rocks cooled down to the closure temperature of pb in rutile ca 500 c but should be very close to the time of eclogite facies metamorphism in terms of common fast exhumation of coesite bearing eclogite this new proof strongly suggests a late carboniferous collision between kazakhstan yili block and central tianshan microcontinent

coupled ge si and ge isotope ratios as geochemical tracers of seafloor hydrothermal systems case studies at loihi seamount and east pacific rise 9 50 n germanium ge and silicon si exhibit similar geochemical behavior in marine environments but are variably enriched in seafloor hydrothermal fluids relative to seawater in this study ge isotope and ge si ratio systematics were investigated in low temperature hydrothermal vents from loihi seamount pacific ocean 18 54 n 155 15 w and results were compared to high temperature vents from the east pacific rise epr at 9 50 n loihi offers the opportunity to understand contrasting ge and si behavior in low temperature seafloor hydrothermal systems characterized by abundant fe oxyhydroxide deposition at the seafloor the results show that both ge si and 74 70ge in hydrothermal fluids are fractionated relative to the basaltic host rocks the enrichment in ge vs si relative to fresh basalts together with ge isotope fractionation 74 70gefluid basalt up to 1 15 at epr 9 50 n and 1 64 at loihi are best explained by the precipitation of minerals e g quartz and fe sulfides during higher temperature seawater rock reactions in the subsurface the study of fe rich hydrothermal deposits at loihi largely composed of fe oxyhydroxides shows that ge isotopes are also fractionated upon mineral precipitation at the seafloor we obtained an average ge isotope fractionation factor between fe oxyhydroxide ferrihydrite and dissolved ge in the fluid of 2 0 0 6 2sd and a maximum value of 3 6 0 6 2sd which is consistent with recent theoretical and experimental studies the study of a hydrothermal chimney at bio 9 vent at epr 9 50 n also demonstrates that ge isotopes are fractionated by approximately 5 6 0 6 2sd during precipitation of metal sulfides under hydrothermal conditions using combined ge si and estimated ge isotope signatures of ge sinks and sources in seawater we propose a preliminary oceanic budget of ge which reveals that an important sink referred as the missing ge sink may correspond to ge sequestration into authigenic fe oxyhydroxides in marine sediments this study shows that combining ge si and 74 70ge systematics provides a useful tool to trace hydrothermal ge and si sources in marine environments and to understand formation processes of seafloor hydrothermal deposits

tracing weathering regimes using the lithium isotope composition of detrital sediments lithium li isotopes are a promising tracer of chemical weathering processes for both modern and ancient times in order to improve the use of li isotopes in the sedimentary record here we calibrate the relationship between weathering intensity and detrital li isotope composition 7li using the fine fraction of modern large river sediments through independent estimates for sediment provenance to calculate the li isotope signature of the rock from which the sediments derive through weathering we show that source rock variability in particular the relative contribution of sedimentary versus igneous rocks must be corrected for before using li isotopes as a weathering proxy we also show that for rivers draining mountain ranges the contribution to river sediments of particles derived from sedimentary rocks is correlated to their li al ratio making it possible to use li contents to estimate the average source rock composition once corrected for bedrock variability the li isotope signature defines a negative relationship with the weathering intensity ratio between silicate weathering rate and total denudation rate with highest li isotope fractionation for the highest weathering intensity altogether we propose a set of new relationships between weathering erosion provenance and li isotopes that can be used to quantify present day and paleo weathering using detrital sediment