two pulses of mineralization and genesis of the zhaxikang sb pb zn ag deposit in southern tibet constraints from fe zn isotopes abstract zhaxikang is one large sb pb zn ag deposit located in the north himalaya of southern tibet to date the genesis of this deposit still remains controversial here we present new pyrite fe and sphalerite zn isotopic data for the first three stages of mineralization fe zn isotopic data for mn fe carbonate that formed during the first two stages of mineralization and zn isotopic data for the slate wall rocks of the jurassic ridang formation to discuss the genesis of the zhaxikang deposit the overall 56 fe and 66 zn values range from 0 80 to 0 43 and from 0 03 to 0 38 respectively the 56 fe values of mn fe carbonates are lighter than those of associated pyrite in six mineral pairs indicating that the iron carbonates are preferentially enriched in light fe isotopes relative to pyrite the sphalerite has lighter 66 zn values than associated mn fe carbonates in three mineral pairs the 56 fe values of pyrite that formed during the first three stages of mineralization gradually increase from stage 1 0 33 to 0 09 through stage 2 0 30 to 0 19 to stage 3 0 16 0 43 in comparison the sphalerite that formed during these stages has 66 zn values that gradually decrease from stage 1 0 16 0 35 through stage 2 0 09 0 23 to stage 3 0 03 to 0 22 these data in conjunction with the observations of hand specimens and thin sections suggest that the deposit was overprinted by a second pulse of mineralization this overprint would account for these fe zn isotopic variations as well as the kinetic rayleigh fractionation that occurred during mineralization the temporally increasing 56 fe and decreasing 66 zn values recorded in the deposit are also coincident with an increase in alteration again supporting the existence of two pulses of mineralization the 56 fe values of the first pulse of ore forming fluid were calculated using theoretical equations yielding values of 0 54 to 0 34 that overlap with those of submarine hydrothermal solutions 1 to 0 however the 56 fe values of the stage 3 pyrite are heavier than those of typical submarine hydrothermal solutions which suggests that the second pulse of mineralization was probably derived from a magmatic hydrothermal fluid in addition the second pulse of ore forming fluid has brought some fe and taken away parts of zn which results the lighter 66 zn values of sphalerite and heavier 56 fe values of pyrite from the second pulse of mineralization overall the zhaxikang deposit records two pulses of mineralization and the overprint by the second pulse of mineralization causes the lighter 66 zn values and heavier 56 fe values of modified samples

crustal structure of the india asia collision zone southern tibet from indepth mt investigations abstract project indepth was initiated in 1992 to develop a better understanding of the deep structure and mechanics of the himalaya tibet region and magnetotelluric surveying was added during indepth phase ii in 1995 broadband and long period data were acquired in 1995 along a n s transect crossing the india asia collision zone the yarlung zangbo suture at 90 e longitude however these data failed to penetrate to the deep crustal and mantle depths due to low solar activity and the presence of the thick conductive crust deep crustal and upper mantle information is key to test the model of subcretion to southern tibet by the stiff indian mantle lid in order to try to obtain such information and to test along strike variability of the yarlung zangbo suture zone a magnetotelluric experiment was performed in 2001 roughly along longitude 92 e containing an ultra long period component mt data were acquired at 28 locations along the new profile with long period data at 15 of these and ultra long period data at five of these 15 initial comparisons of the models generated from the 1996 and 2001 datasets indicate little variation in electrical conductivity structure along strike

significant late neogene east west extension in northern tibet field mapping in northern tibet reveals that the normal slip along late cenozoic northsouth trending faults is comparable to that estimated for equivalent structures in southern tibet the orientation of fault striations in two north south trending rifts suggests an eastnortheast west northwest direction of extension in northern tibet which in turn implies that northeast striking active faults in northern tibet have significant left slip components initiation of rifting in northern tibet postdates the early oligocene and possibly occurred after 4 ma the broad similarities in the magnitude of slip and the direction of extension for normal faults in both northern and southern tibet imply that the entire plateau has been extending this precludes significant eastward extrusion of north tibet relative to south tibet and requires a regional boundary condition as the cause of east west extension for the entire tibet plateau

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

evidence for zn isotopic fractionation at merapi volcano abstract fumarolic gases rocks and condensate samples from merapi volcano indonesia have been analyzed by multiple collector inductively coupled plasma mass spectrometry mc icp ms for their zn isotopic compositions the variation observed expressed as 66 zn 66 zn 66 zn 64 zn sample 66 zn 64 zn jmc 3 049l 1 10 3 ranges from 0 05 to 1 68 the overall reproducibility was found to be better than 0 10 2sd the 66 zn of andesitic rock samples are homogeneous with a mean value of 0 24 which is comparable to previously reported basalt data 66 zn in the woro fumerolic field are ranging from 0 05 to 0 85 and 1 48 to 1 68 in fumarolic gases and condensates respectively upon cooling from 590 to 297 c gaseous zn concentrations decrease and become increasingly enriched in the lighter zn isotopes while the condensates become increasingly enriched in the heavier isotopes gas and condensate isotopic compositions can be explained with a rayleigh condensation model with temperature dependent isotope fractionation factor solid vapor over the 574 c to 297 c range of observations 1000 ln solid vapour c 1 c 2 t c 3 t 2 with c 1 0 c 2 0 88 10 3 c 3 1 00 10 6 and t in degrees kelvin which corresponds to solid vapor of 1 00033 to 1 00153 respectively at the indicated temperatures

evidence of zn isotopic fractionation in a soil plant system of a pristine tropical watershed nsimi cameroon abstract the objective of this study is to determine for the first time the zinc zn isotopic composition of different soil horizons parent rocks litter and plants from a single pristine watershed three soil profiles from nsimi zoetele site south cameroon africa have been investigated the 66zn 66zn 66zn 64zn sample 66zn 64zn jmc 3 0749l 1 103 measured in the soils and rocks range from 0 05 to 0 64 in a typical hillslope soil profile developed in situ from a granitic parent rock we observe two important and systematic trends i a weathering regime in the saprolite horizon that involves strong zn depletion with no or slight fractionation compared to the fresh igneous rock and ii a shallower weathering regime with significant depletion in heavy zn isotopes in the most superficial soil horizons in contrast the soil profile of the swamp zone does not exhibit isotopic fractionation between the soil and the parent rock six plants and two litter samples were analyzed for their zn isotopic compositions 66zn varied from 0 75 to 0 91 among all samples plant roots and shoots are generally enriched in heavy isotopes 66zn of 0 42 to 0 76 relative to the litter layer 0 12 to 0 25 and most superficial soils similar to the controlled plant growth study of weiss et al weiss d j mason t f d zhao f j kirk g j d coles b j and horstwood m s a 2005 isotopic discrimination of zinc in higher plants new phytologist 165 703 710 negative 66zn values were obtained for tree leaves the most aerial part of the tree based on biomass data for the tropical forest vegetation is on average enriched in light isotopes compared to the litter and the superficial soil horizons which are the nutrients reservoirs due to the age of these soils and the complexity of pedogenetic processes it is difficult to explain the isotopic composition of the soils at present however the transformation of the saprolite horizon into a ferruginous horizon is associated with an enrichment in light zn isotopes this observation may be due to the adsorption of zn onto clays surfaces the uptake of zn by roots in the soils leads to an enrichment in heavy isotopes once the zn is in the xylem it will be transported within the plant during this transport both diffusion and active uptake of heavy isotopes by cells out of the xylem favour the mobility of light isotopes to the most aerial parts of the plants the difference in zn isotopic composition between the deep horizons and the superficial horizons of soils suggests that zn isotopes can be used to discriminate the source of zn and other metals leaving the catchment by river water

integrated fe and s isotope study of seafloor hydrothermal vents at east pacific rise 9 10 n abstract in this study we report on coupled fe and s isotope systematics of hydrothermal fluids and sulfide deposits from the east pacific rise at 9 10 n to better constrain processes affecting fe isotope fractionation in hydrothermal environments we aim to address three fundamental questions 1 is there significant fe isotope fractionation during sulfide precipitation 2 is there significant variability of fe isotope composition of the hydrothermal fluids reflecting sulfide precipitation in subsurface environments 3 are there any systematics between fe and s isotopes in sulfide minerals the results show that chalcopyrite precipitating in the interior wall of a hydrothermal chimney displays a limited range of 56fe values and 34s values between 0 11 to 0 33 and 2 2 to 2 6 respectively the 56fe values are on average slightly higher by 0 14 relative to coeval vent fluid composition while 34s values suggest significant s isotope fractionation 0 6 0 2 during chalcopyrite precipitation in contrast systematically lower 56fe and 34s values relative to hydrothermal fluids by up to 0 91 and 2 0 respectively are observed in pyrite and marcasite precipitating in the interior of active chimneys these results suggest isotope disequilibrium in both fe and s isotopes due to s isotopic exchange between hydrothermal h2s and seawater so42 followed by rapid formation of pyrite from fes precursors thus preserving the effects of a strong kinetic fe isotope fractionation during fes precipitation in contrast 56fe and 34s values of pyrite from inactive massive sulfides which show evidence of extensive late stage reworking are essentially similar to the hydrothermal fluids multiple stages of remineralization of ancient chimney deposits at the seafloor appear to produce minimal fe isotope fractionation similar affects are indicated during subsurface sulfide precipitation as demonstrated by the lack of systematic differences between 56fe values in both high temperature fe rich black smokers and lower temperature fe depleted vents

sedimentary history of the tethyan basin in the tibetan himalayas after an epicontinental phase the sedimentary rocks in the tibetan himalayas document a complete wilson cycle of the neo tethyan tethys ill evolution between the gondwana supercontinent and its northward drifting margin lhasa block from the late permian to the eocene during the triassic rift stage the basin was filled with a huge clastic dominated sediment wedge with up to 5 000 m of flysch in the northern zone widespread deltaic clastics and shallow water carbonates of late norian to earliest jurassic age in the southern zone mark in conjunction with decreasing tectonic subsidence the transition to the drift stage some 4 500 m of jurassic and early cretaceous shallow water carbonates and siliciclastics accumulated on the tethyan indian passive margin deepening upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments extensive abyssal sediments with basaltic volcanics in the northern deep water zone reflect continued ocean spreading and thermal subsidence paleomagnetic data gained separately for the northern indian plate and the lhasa block indicate that the neo tethys reached its maximum width about 110 ma ago with a spreading rate of 4 8 cm year before it commenced to close again during the remnant basin stage in the late cretaceous and paleogene a shallowing upward megasequence capped by a carbonate platform developed in the southern inner shelf realm in the northern slope basin plain zone turbidites and chaotic sediments derived from both the acretionary wedge and the steepening slope of the passive margin accumulated the depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting the sediments from the triassic to the paleogene are tentatively subdivided into five mega sequences which are controlled mainly by regional tectonics climatic influence e g carbonate deposition due to northward plate motion is partially subdued by terrigenous input and or increased water depth during the oligocene and miocene crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins

two mica and tourmaline leucogranites from the everest makalu region nepal tibet himalayan leucogranite genesis by isobaric heating in the higher himalaya of the region from cho oyu to the arun valley northeast of makalu the miocene leucogranites are not hosted only in the upper high himalayan crystallines hhc a network of dykes also cuts the lower hhc and the lesser himalayan crystallines lhc the plutons and dykes are mainly composed of two mica muscovite biotite tourmaline cordierite andalusite sillimanite leucogranite with tourmaline 2 6 and biotite 1 5 modal and tourmaline muscovite tourmaline biotite sillimanite garnet kyanite andalusite spinel corundum leucogranite with tourmaline 2 2 and biotite 1 5 modal both leucogranite types were produced by partial melting in the andalusite sillimanite facies series under lp ht conditions constrained by the occurrence of peritectic andalusite and cordierite the geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite rich metapelites at p 350 mpa and t 640 c whereas two mica leucogranite was produced by biotite dehydration melting in biotite rich metapelites at p 300 mpa and t 660 710 c melting in fertile muscovite rich metapelites of the top of both the hhc and lhc produced magmas which were emplaced at the same structural level in which they had been generated melting in the biotite rich gneiss of both the hhc and lhc produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper hhc a similar process also produced a network of two mica granite at the top of the lhc in the ama drime nyonno ri range northeast of makalu the prograde character of leucogranite melt producing reactions in the everest makalu area suggests that here the generation of miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression

iron isotope fractionation during skarn type alteration implications for metal source in the han xing iron skarn deposit abstract the han xing iron mineralization in the central north china craton is a typical fe skarn deposit associated with altered diorites here we report the fe isotopic compositions of whole rocks and mineral separates from this deposit with a view to evaluate the fe isotope fractionation during the formation of fe skarn deposit and to constrain the metal source the fe isotopes show a large variation both in whole rocks and mineral separates altered diorites show a wide range in 56 fe values 0 07 to 0 21 relative to the fe isotope standard irmm 014 which positively correlate with their tfe 2 o 3 tio 2 ratios fe 2 o 3 and feo calculated as tfe 2 o 3 the positive correlation indicates that heavy fe isotopes were preferentially leached from diorites during the skarn type alteration among the metallic minerals pyrite and pyrrhotite are isotopically heavier 0 12 to 0 48 than the magnetite 0 07 to 0 21 fe isotope fractionation between mineral pairs demonstrates that magnetite did not attain fe isotopic equilibrium with pyrite and pyrrhotite whereas pyrite and pyrrhotite might have attained isotopic equilibrium petrological observations and major element data also suggest that iron was leached from the diorites during the skarn type alteration if the leached iron provides the main fe budget of the han xing fe skarn deposit magnetite in ores would be isotopically heavier than the unaltered diorite however our results are in contrast with the magnetite being isotopically lighter than the unaltered diorite this suggests that the major fe source of the han xing fe skarn deposit is not from the leaching of diorites and might be from magmatic fluid which is isotopically lighter than the silicate melt our data demonstrate that fe isotopes can be used as important tracers in deciphering the metal source of fe skarn deposits

iron isotope fractionation during supergene weathering process and its application to constrain ore genesis in gaosong deposit gejiu district sw china abstract fe isotope compositions of bulk samples from gaosong ore deposit in gejiu district sw china were investigated and an overall variation in 56 fe values from 0 33 to 0 20 has been observed in this study the 56 fe values of primary sulfide ores are lower than the oxidized ores while the iron isotope compositions of gossan samples which represent highest oxidation degree are heavier than the oxidized ore samples these data indicate that iron isotopes experienced fractionation during supergene weathering process furthermore heavier iron isotopes enriched in more oxidized products another highlight is the fe isotope compositions that are affected by mineralogy which is considered as the primary mechanism of the large fe isotopic variations in the primary ores enrichment of light iron isotopes in primary sulfide ores suggests that iron isotopes fractionated during fluid exsolution from its parental magma and the fe source of skarn ores is mainly the magmatic source this observation that significant fe isotope fractionation occurs in supergene weathering process and fluid exsolution process provides an example in using fe isotopes to trace supergene geochemical cycling and ore deposit formation evolution processes

zinc isotope variations in deep sea carbonates from the eastern equatorial pacific over the last 175 ka the carbonate fraction of sediment core odp 849 leg 138 located in the eastern equatorial pacific mostly consisting of coccoliths was separated and analyzed for its zn isotopic composition the overall variation in zn isotopic composition as determined by multiple collector magnetic sector inductively coupled plasma mass spectrometry was found to be on the order of 1 expressed in 66zn where xzn xzn 64zn sample xzn 64zn standard 1 103 and x 66 67 or 68 over the last 175 ka the analytical precision was 0 04 and the overall reproducibility was usually better than 0 07 the zn isotopic composition signal exhibits several marked peaks and a high frequency variability a periodogram of the 66zn signal showed two periodicities of 35 2 and 21 2 ka we suggest that the latter is caused by the precession of the earth s axis of rotation the periodogram exhibits a minimum at 41 1 ka thus showing that the zn isotopic composition is independent of the obliquity in the eastern equatorial pacific the range of 66zn values observed for the carbonate fraction of odp 849 overlaps with the range observed for fe mn nodules in the world s oceans which suggests that seawater carbonate zn isotope fractionation is weak we therefore assume that most of the zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean the odp 849 66zn record seems to follow the changes in the insolation cycles changes in the late summer fall equatorial insolation modulate the intensity of the equatorial upwelling hence the mixing between deep and surface waters we propose that during decreased summer fall equatorial insolation when a steep thermocline can develop el nino like conditions the surface waters cannot be replenished by deep waters and become depleted in the lighter zn isotopes by biological activity thus resulting in the progressive increase of the 66zn values of the carbonate shells presumably in equilibrium with surface seawater

disequilibrium iron isotopic fractionation during the high temperature magmatic differentiation of the baima fe ti oxide bearing mafic intrusion sw china abstract iron isotopic compositions of olivine clinopyroxene and titanomagnetite of the baima fe ti oxide bearing layered mafic intrusion sw china are used to investigate fe isotopic fractionation during the formation of the fe ti oxide ore bodies and to constrain the origin of silicate minerals and fe ti oxides the baima intrusion comprises the lower zone of interlayered troctolite clinopyroxenite and oxide ores and the upper zone of gabbros significant differences of fe isotope values between olivine fe 56 0 01 to 0 11 clinopyroxene fe 56 0 11 to 0 22 and titanomagnetite fe 56 0 20 to 0 31 were observed in rocks and ores of the lower zone iron isotopic fractionation between olivine and clinopyroxene olivine and titanomagnetite and clinopyroxene and titanomagnetite exhibits large variations of 0 06 to 0 22 0 12 to 0 27 and 0 00 to 0 20 respectively suggesting disequilibrium fractionation this disequilibrium cannot be explained by thermal or chemical gradient induced kinetic fractionation or by possible subsolidus exsolution of granular ilmenite from titanomagnetite instead it could be attributed to crystallization of silicates and titanomagnetite from two immiscible si rich and fe rich liquids continuous segregation of the fe rich liquid changed the fe isotopic composition of the si rich liquid and thus the fe isotopic compositions of olivine and clinopyroxene crystallized from it resulting in disequilibrium fe isotopic fractionation between them the effect of oxygen fugacity on the crystallization order of titanomagnetite and ilmenite from the fe rich melts on the other hand gave rise to the disequilibrium fe isotope fractionation between titanomagnetite and olivine clinopyroxene

zinc isotopes in sphalerite from base metal deposits in the red dog district northern alaska analyses of sphalerite samples from shale hosted massive sulfide and stratigraphically underlying vein breccia deposits in the red dog district in northern alaska show a range of 66zn values from zero to 0 60 per mil the lowest values are observed in the vein breccia deposits and the stratigraphically overlying but structurally displaced shale hosted massive sulfide deposits show a systematic trend of increasing 66zn values from south to north main aqqaluk paalaaq anarraaq the 66zn values are inversely correlated with sphalerite fe mn ratio and also tend to be higher in low cu sphalerite consistent with precipitation of lower 66zn sphalerite closer to the principal hydrothermal fluid conduits the most likely control on isotopic variation is rayleigh fractionation during sulfide precipitation with lighter zinc isotopes preferentially incorporated in the earliest sphalerite to precipitate from ore fluids at deeper levels vein breccias and close to the principal fluid conduits in the orebodies followed by precipitation of sulfides with higher 66zn values in shallower and or more distal parts of the flow path there is no systematic variation among the paragenetic stages of sphalerite from a single deposit suggesting an isotopically homogeneous zinc source and consistent transport deposition conditions and or dissolution reprecipitation of earlier sphalerite without significant fractionation decoupled zn and s isotope compositions are best explained by mixing of separate metal and sulfur bearing fluids at the depositional site the results confirm that zn isotopes may be a useful tracer for distinguishing between the central and distal parts of large hydrothermal systems as previously suggested and could therefore be of use in exploration

abundance of zinc isotopes as a marine biogeochemical tracer 1 the zn isotopic compositions of ferromanganese nodules sediment trap samples sediments and organic reference samples have been analyzed by multiple collector inductively coupled plasma mass spectrometry icp ms the range of isotopic variations 1 for 66zn 64zn is significant with respect to the analytical precision 0 04 at the 95 confidence level marine argillaceous sediments show rather constant values consistent with those of continental shales and loess and with the value of a basalt from reunion island we infer that the depletion of the light zn isotopes in marine particles and in ferromanganese nodules presumably equilibrated with seawater results from biological activity in the upper water column the 66zn values of sediment trap samples collected near the upwelling off the coast of mauritania central atlantic show a seasonal isotopic fluctuation consistent with biological pumping during the high productivity period higher values of 66zn in nodules appear to be associated with the amplitude of seasonal variations rather than with the mean values of the biological productivity

fractionation of cu fe and zn isotopes during the oxidative weathering of sulfide rich rocks abstract we measured the fe cu and zn isotopic compositions of the fluids generated during leaching experiments with pyrite chalcopyrite and sphalerite rich rocks and with a sphalerite mineral separate our study demonstrates that the oxidative weathering of sulfide rich rocks can produce substantial variations in fe 1 75 to 1 0 56 fe solution pyrite rock and cu 0 0 to 2 0 65 cu solution chalcopyrite rock isotopes and small variations in zn isotopes 0 0 to 0 2 66 zn solution sphalerite in the fluid phase relative to the rock for the fe and cu systems we suggest that isotopic fractionation is caused by electron exchange driven e g fe ii fe iii and cu i cu ii redox reactions at the surfaces of the sulfide minerals that occur during air and aqueous chemical reactions under acidic conditions these reactions tend to enrich the fluid phase in the heavier fe and cu isotopes however under circumneutral ph conditions the fe isotopic composition in solution was controlled by the precipitation of fe iii oxide phases which enriched the solution in the lighter fe isotopes this investigation provides a preliminary framework for interpreting the impact of sulfide oxidation reactions on the distributions of stable fe cu and zn isotopes in natural waters

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

origin of siderite mineralization in western guizhou sw china constrains from rees c o sr and s isotopes abstract numerous siderite deposits in western guizhou of sw china are mainly hosted in the middle devonian dolostones the larger caiyuanzi cyz and xiongxiongjia xxj deposits have an ore reserve in excess of 300 mt with an average ore grade of 35 total iron there are banded and massive siderite ores and sulfide siderite ores the major ore mineral is magnesian siderite and gangue minerals are ankerite dolomite kaolinite quartz and carbon there are also metallic minerals including pyrite chalcopyrite and minor galena tetrahedrite and enargite rare earth elements rees and carbon oxygen and strontium isotope data indicate a marine origin of the host dolostones and a hydrothermal metasomatic origin of the siderite mineralization the variations of rees and carbon and oxygen isotopic compositions of siderite from the cyz and xxj deposits reflect the variable mineralization processes and different fluid rock ratios the lree depletion of siderite from the cyz deposit could be due to crystallographic control whereas the lree depletion along with mree enrichment and positive eu anomalies of siderite from the xxj deposit was possibly controlled by both crystallography and fluid composition siderite and calcite have 13 c pdb and 18 o smow values ranging from 8 to 4 01 and from 13 42 to 20 07 respectively the variations in c and o isotopic compositions of siderite and calcite may reflect metasomatic replacement of marine carbonates by hydrothermal fluids at different fluid rock ratios dolostones have 87 sr 86 sr initial values of 0 70844 0 70920 whereas siderite and calcite have values ranging from 0 71396 to 0 71708 negative correlations of initial sr isotope ratios versus carbon and oxygen isotopes of the host dolostones siderite and calcite indicated that carbonate minerals were metasomatic products of marine carbonates by hydrothermal fluids leaching from the proterozoic basement rocks iron was possibly derived from the basement leaching rather than from the continental weathering the 34 s cdt values 19 9 to 13 6 of sulfides associated with siderite indicated that the sulfur was probably derived from the marine sulfates through either bacterial sulfate reduction or organic mediated thermochemical reduction

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

high precision iron isotope measurements of terrestrial and lunar materials we present the analytical methods that have been developed for the first high precision fe isotope analyses that clearly identify naturally occurring mass dependent isotope fractionation a double spike approach is used which allows rigorous correction of instrumental mass fractionation based on 21 analyses of an ultra pure fe standard the external precision 1 sd for measuring the isotopic composition of fe is 0 14 mass for demonstrated reproducibility on samples this precision exceeds by at least an order of magnitude that of previous attempts to empirically control instrumentally produced mass fractionation dixon et al 1993 using the double spike method 15 terrestrial igneous rocks that range in composition from peridotite to rhyolite 5 high ti lunar basalts 5 fe mn nodules and a banded iron formation have been analyzed for their iron isotopic composition the terrestrial and lunar igneous rocks have the same isotopic compositions as the ultra pure fe standard providing a reference fe isotope composition for the earth and moon in contrast fe mn nodules and a sample of a banded iron formation have iron isotope compositions that vary over a relatively wide range from 56fe 0 9 to 1 2 this range is 15 times the analytical errors of our technique these natural isotopic fractionations are interpreted to reflect biological vital effects and illustrate the great potential fe isotope studies have for studying modern and ancient biological processes

zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys many of the heaviest and lightest natural zinc zn isotope ratios have been discovered in hydrothermal ore deposits however the processes responsible for fractionating zn isotopes in hydrothermal systems are poorly understood in order to better assess the total range of zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation we have measured zn isotopes in seafloor hydrothermal fluids from numerous vents at 9 10 n and 21 n on the east pacific rise epr the tag hydrothermal field on the mid atlantic ridge and in the guaymas basin fluid 66zn values measured at these sites range from 0 00 to 1 04 of the many physical and chemical parameters examined only temperature was found to correlate with fluid 66zn values lower temperature fluids 250 c had both heavier and more variable 66zn values compared to higher temperature fluids from the same hydrothermal fields we suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite zn sulfide and that this process is a primary cause of zn isotope variation in hydrothermal fluids thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest zn found in fluids that were calculated to be saturated with respect to sphalerite we have also measured zn isotopes in chimney sulfides recovered from a high temperature 383 c and a low temperature 203 c vent at 9 10 n on the epr and in both cases found that the 66zn of chimney minerals was lighter or similar to the fluid 66zn the first measurements of zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid 66zn and suggest that subsurface zn sulfide precipitation is a primary factor in causing variations in fluid 66zn by understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid 66zn zn isotopes may be used as a tracer for studying hydrothermal processes

fe isotope fractionation on fes formation in ambient aqueous solution iron sulfide phases are the ultimate repository of iron and reduced sulfur in sediments the sulfur isotope geochemistry of pyrite has had much to tell us about modern and ancient earth environments and it is likely that fe isotopes will too once fractionations for key processes are known we report the results of an experimental study of fe isotope fractionation on precipitation of fes synthetic mackinawite from excess aqueous fe ii solutions by addition of sodium sulfide solution at 2 40 c the results show a significant kinetic isotope effect in the absence of a redox process no detectable effect of temperature on the fractionation factor was observed the fe isotope fractionation for zero age fes is fe ii fes 0 85 0 30 across the temperature range studied giving a kinetic isotope fractionation factor of fe ii fes 1 0009 0 0003 on ageing the fes in contact with aqueous fe ii becomes progressively isotopically heavier indicating that the initial fractionations are kinetic rather than equilibrium from published reaction mechanisms the opportunity for fe isotope fractionation appears to occur during inner sphere ligand exchange between hexaqua fe ii and aqueous sulfide complexes fe isotope fractionation on mackinawite formation is expected to be most significant under early diagenetic situations where a readily available reactive fe source is available since fes aq is a key reactive component in natural pyrite formation kinetic fe isotope fractionations will contribute to the fe isotope signatures sequestered by pyrite subject to the relative rate of fes2 formation versus fes fe ii aq isotopic equilibration

geochemistry and si o fe isotope constraints on the origin of banded iron formations of the yuanjiacun formation lvliang group shanxi china abstract banded iron formations bifs within the lvliang region of shanxi province china are hosted by sediments of the yuanjiacun formation part of the paleoproterozoic lvliang group these bifs are located in a zone where sedimentation changed from clastic to chemical deposition indicating that these are superior type bifs here we present new major trace and rare earth element ree data along with fe si and o isotope data for the bifs in the yuanjiacun within the fe deposits at yuanjiacun jianshan and hugushan when compared with post archean australian shale paas these bifs are dominated by iron oxides and quartz contain low concentrations of al 2 o 3 tio 2 trace elements and the ree and are light rare earth element lree depleted and heavy rare earth element hree enriched the bifs also display positive la y and eu anomalies high y ho ratios and contain 30 si depleted quartz with high 18 o values that are similar to quartz within siliceous units formed during hydrothermal activity these data indicate that the bifs within the yuanjiacun formation were precipitated from submarine hydrothermal fluids with only negligible detrital contribution none of the bif samples analyzed during this study have negative ce anomalies although a few have a positive ce anomaly that may indicate that the bifs within the yuanjiacun formation formed during the great oxidation event goe within a redox stratified ocean the positive ce anomalies associated with some of these bifs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed ho lree and ce 4 these deposits formed during reductive dissolution at the oxidation reduction transition zone or in deeper level reducing seawater the loss of ce lree and ho to seawater and the deposition of these elements with iron hydroxides caused the positive ce anomalies observed in some of the bif samples although the limited oxidizing ability of surface seawater at this time meant that y ho and lree hree ratios were not substantially modified unlike similar situations within stratified ocean water during the late paleoproterozoic magnetite and hematite within the bifs in the study area contain heavy fe isotopes 56 fe values of 0 24 1 27 resulting from the partial oxidation and precipitation of fe 2 to fe 3 in seawater in addition mass independent fractionation of sulfur isotopes within pyrite indicates that these bifs were deposited within an oxygen deficient ocean associated with a similarly oxygen deficient atmosphere even though the bifs within the yuanjiacun formation formed after initiation of the goe

zinc isotope variation in hydrothermal systems preliminary evidence from the irish midlands ore field little is known about the range or controls on the zinc isotope composition of terrestrial materials and no systematic studies have been carried out on ore forming systems we have obtained zinc isotope data from 19 sphalerite samples formed over a range of well constrained precipitation conditions from the irish zn pb ore field the results reveal variation in 66zn where 66zn 66zn 64zn sample 66zn 64zn standard 1 1000 from 0 17 to 1 33 per mil relative to the lyon jmc 3 0749l zinc standard this variation is significant compared to the external reproducibility 0 12 2 and the data show very good mass dependency with 67zn and 68zn values thus natural variations in the zinc isotope composition of these ore minerals can be resolved our results span the entire range of 66zn values measured on terrestrial geologic samples to date the data suggest that variations in the primary source rock composition or precipitation temperature are unlikely to be important controls on the zinc isotope composition of sphalerite in the ore field we suggest that the variation is most likely due to a kinetic fractionation involving the preferential incorporation of light zinc isotopes in sphalerite precipitated rapidly under disequilibrium conditions however we cannot rule out the possibility of mixing of zinc derived from two isotopically distinct sources the significant variation in zinc isotope compositions we have observed in the irish ore field confirms that such fractionations can provide new insights into mineralizing processes in the earth s crust

oldest lamproites from peninsular india track the onset of paleoproterozoic plume induced rifting and the birth of large igneous province abstract potassic and ultrapotassic magmatism from deep lithospheric sources in intra cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of large igneous provinces lips triggered by mantle plumes here we report for the first time precise zircon u pb age data from a suite of lamproites in the bastar craton of central india that mark the onset of paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major lip events during the precambrian evolution of the indian shield the lamproites from the nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile apatite and zircon the rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite geochemical features of the lamproites correlate with their counterparts in peninsular india and other similar suites elsewhere in the world related to rift settings and also indicate oib like magma source the associated syenite shows subduction related features possibly generated in a post collisional setting magmatic zircon grains with high th u ratios in the syenite from the nuapada lamproite form a coherent group with an upper intercept age of 2473 8 ma representing the timing of emplacement of the magma zircon grains in three lamproite samples yield four distinct age groups at ca 2 4 ga 2 2 ga 2 0 ga and 0 8 ga the 2 4 ga group corresponds to xenocrysts entrained from the syenite whereas the 2 2 ga group is considered to represent the timing of emplacement of the lamproites the ca 2 0 ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern bastar and the adjacent dharwar cratons a few young zircon grains in the syenite and lamproites show a range of early to middle neoproterozoic ages from 879 to 651 ma corresponding to younger thermal event s as also represented by granitic veins cutting across these rocks and extensive silicification zircon lu hf isotope data suggest magma derivation from a refertilized paleo mesoarchean sub continental lithospheric mantle or oib type sources the differences in hf isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale a combination of the features from geochemical and zircon hf isotope data is consistent with asthenosphere lithosphere interaction during the lamproite magma evolution the timing of lamproite emplacement in central india correlates with the global 2 2 ga record of lips we link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction accretion history prior to the neoarchean cratonization as well as the thermal blanket effect of the earth s oldest supercontinent pulsating plumes and continued rifting generated voluminous dyke swarms across the bastar and dharwar cratons forming part of a major global rifting and lip event

metallogenic significance of trace element and u pb isotope data for uraninite rich mineral separates from the labrador central mineral belt thirteen concentrates of uraniferous material were prepared from uranium occurrences in the central mineral belt of labrador host rocks to these occurrences include granitoid rocks of the archean basement ca 2000 ma metasedimentary rocks of the lower aillik group and 1860 ma felsic volcanic rocks of the upper aillik group common lead corrected pb isotope data from inductively coupled plasma mass spectrometry analyses define 207pb 206pb ages ranging from 1805 to 1697 ma for all but one sample with a mean age of 1752 27 ma 1 ages calculated for individual samples are similar to those derived by previous workers using standard analytical techniques eleven of these samples define linear trends that intersect the u pb concordia at 1741 23 ma and a tera wasserburg curve at 1740 21 ma respectively these data suggest that the occurrences are epigenetic with respect to host rocks and possibly related to a common metallogenic event therefore resolving a long standing controversy about the timin

high pressure melting of metapelite and the formation of ca rich granitic melts in the namche barwa massif southern tibet abstract a detailed geochemical and geochronological study of anatectic migmatites from the namche barwa massif nbm southern tibet has been carried out to place important constraints on the thermal and tectonic evolution of the eastern himalayan syntaxis shrimp zircon u pb dating indicates that the granulite facies metapelite underwent metamorphism at 21 8 0 7 ma and 24 5 0 7 ma respectively the latter is similar to the timing of partial melting and the formation of ca rich leucosomes at 24 25 ma these leucosomes are characterized by 1 high cao na 2 o and na k ratios 2 radiogenic sr 87 sr 86 sr t 0 7407 0 7904 but unradiogenic nd e nd t 7 0 to 21 2 isotope compositions 3 depleted hfse and 3 variable but depleted hree relative to their host pelites some of the leucosomes show large degrees of nd isotopic disequilibrium up to 10 epsilon units with respect to their hosts these high cao and na 2 o leucosomes were derived from fluxing melting of metapelite at high pressures a similar process could have operated during the formation of the himalayan leucogranites and contributes to the heterogeneities in such granites

stable isotope chemical and mineral compositions of the middle proterozoic lijiaying mn deposit shaanxi province china abstract the lijiaying mn deposit located about 250 km southwest of xian is a high quality ore characterized by low p and fe contents and a mean mn content of about 23 the ore deposit occurs in shallow water marine sedimentary rocks of probable middle proterozoic age carbonate minerals in the ore deposit include kutnahorite calcite mn calcite and mg calcite carbon 0 4 to 4 0 and oxygen 3 7 to 12 9 isotopes show that with a few exceptions those carbonate minerals are not pristine low temperature marine precipitates all samples are depleted in rare earth elements rees relative to shale and have negative eu and positive ce anomalies on chondrite normalized plots the fe mn ratios of representative ore samples range from about 0 034 to

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

nyainqentanglha shear zone a late miocene extensional detachment in the southern tibetan plateau a major low angle ductile shear zone containing s c mylonites involves metamorphosed granitic rocks at the southeastern edge of the nyainqentanglha mountain range southern tibet prominent triangular facet geomorphology is developed by valley erosion of the detachment surface defined by the top of the southeast dipping mylonitec shear zone kinematic criteria consistently indicate a top to southeast sense of shear the ductile shearing deformation is inferred from isotopic cooling ages to have occurred during the interval 11 5 ma late miocene we interpret the shear zone as a regional extensional detachment its development indicates when the extensional tectonics in this area started which in turn may mark the time when the maximum sustainable surface elevation and perhaps crustal thickness was reached in southern tibet