an evaluation of the single grain zircon evaporation method in highly discordant samples comparison of results from the isotope dilution and the single grain zircon evaporation methods on highly discordant samples shows that the evaporation method can yield accurate crystallization ages on some zircon populations that display complex discordant patterns three granite gneiss samples from the western gneiss region norway which crystallized at approximately 1660 ma and experienced profound lead loss in zircon during caledonian metamorphism at 400 ma were analyzed by both the isotope dilution tucker et al 1990 and evaporation methods 207ph 206pb evaporation ages on one sample agree with the isotope dilution crystallization age two other samples produced a range of anomalously young 207pb 206pb evaporation ages analyses at the highest evaporation temperatures from one of these two samples were consistent with crystallization ages determined by the isotope dilution method but it would be difficult to interpret them correctly without independent age information the anomalously young results are generally easy to recognize by the pattern of increasing 207ph 206ph ages with increasing evaporation temperature a more serious problem however is that some grains produced 207ph 206pb age plateaus over a range of increasing evaporation temperatures that cannot be correlated with any documented geologic event in the region the evaporation analyses yielding the youngest 207pb 206pb ages also display the most common lead suggesting that common lead may diffuse into the zircon grains during loss of radiogenic lead a pegmatite from massachusetts usa contains zircons with visible xenocrystic cores the 207ph 206pb evaporation ages at lower temperatures agree well with the 299 ma crystallization age determined by the isotope dilution method on the zircon rims r d tucker unpubl data at higher evaporation temperatures a range of older 207pb 206pb ages are produced by mixing pb evaporated from both the zircon rims and xenocrystic cores large changes in the 206pb 208pb ratio accompany the increase in 207pb 206pb ages and indicate mixing of two distinct zircon domains thus in addition to providing useful 207pb 206pb age information the evaporation method may help distinguish between the discordance produced by diffusional lead loss and the mixing of two domains of differing age

space weathered rims found on the surfaces of the itokawa dust particles on the basis of observations using cs corrected stem we identified three types of surface modification probably formed by space weathering on the surfaces of itokawa particles they are 1 redeposition rims 2 3 nm 2 composite rims 30 60 nm and 3 composite vesicular rims 60 80 nm these rims are characterized by a combination of three zones zone i occupies the outermost part of the surface modification which contains elements that are not included in the unchanged substrate minerals suggesting that this zone is composed of sputter deposits and or impact vapor deposits originating from the surrounding minerals redeposition rims are composed only of zone i and directly attaches to the unchanged minerals zone iii zone i of composite and composite vesicular rims often contains nanophase fe mg s the composite rims and the composite vesicular rims have a two layered structure a combination of zone i and zone ii below which zone iii exists zone ii is the partially amorphized zone zone ii of ferromagnesian silicates contains abundant nanophase fe radiation induced segregation and in situ reduction are the most plausible mechanisms to form nanophase fe in zone ii their lattice fringes indicate that they contain metallic iron which probably causes the reddening of the reflectance spectra of itokawa zone ii of the composite vesicular rims contains vesicles the vesicles in zone ii were probably formed by segregation of solar wind he implanted in this zone the textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on itokawa

mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge east pacific rise 17 to 19 s naudur cruise 1993 phase separation processes controlled by volcanic and tectonic activity during the naudur cruise in december 1993 23 dives using the french submersible nautile were conducted on the axis of the east pacific rise between 17 and 19 s where the spreading rate is among the fastest in the ocean 14 to 16 cm yr twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered respectively at 17 25 s 18 15 s and 18 26 s the fluids exhibit a very wide range of temperature chemical and gas compositions on the 17 25 s and 18 26 s segments fluids have quite uniform compositions low chlorinities are gas enriched and are low in dissolved metals relative to fluids from the 18 15 s segment which show high chlorinities are less gas enriched and show high metal concentrations chloride and metal depletion associated with gas enrichment is consistent with phase separation whereas ch 4 end members show large variations between sites the 13 ch 4 data are very similar with 13 c values in a narrow range 22 0 to 23 9 versus pee dee belemnite pdb 13 co 2 measured in fluids within the 18 15 s and 18 26 s segments are respectively 7 9 and 5 8 versus pdb similar to 13 c of co 2 trapped in mid oceanic ridge basalts suggesting a magmatic origin the variability in fluid composition is linked to the variability of the accretion process observed on the three segments the uniform venting of low chlorinity fluids in the 17 25 s and 18 26 s segments is connected with volcanic activity which causes boiling with preferential venting of vapor enriched fluids high salinity fluids are emitted on the 18 15 s segment where the ridge is tectonics dominated and subseafloor circulation controlled by faults phase separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area as previously observed on the juan de fuca ridge or in the north fiji basin ridge

the essence of time fertile skarn formation in the variscan orogenic belt abstract extending from northern america to central china the variscan belt is a paleozoic orogen exceptionally well endowed in magmatic hydrothermal ore deposits including skarn deposits yet the genesis of fertile skarns and their distinction from barren equivalents in orogenic zones is only poorly constrained here we present innovative u pb laser ablation inductively coupled plasma mass spectrometry geochronology of garnet from different skarns in the erzgebirge a classic metallogenic province in central europe garnet ages obtained not only constrain the timing of fertile skarn formation and associated sn w fe zn cu and in mineralization but also clearly distinguish these from barren skarn bodies we show that barren skarns formed during times of peak regional metamorphism at 340 ma whereas mineralized skarns are temporally associated with late orogenic magmatism at 325 313 ma as well as post orogenic magmatism at 308 295 ma the recognition of discrete mineralization events associated with the largest and economically most important skarn deposits provides valuable insight into the punctuated evolution of magmatic hydrothermal systems in ancient collisional orogens on a regional scale this has important implications to direct future mineral exploration

dating multiply overprinted granites the effect of protracted magmatism and fluid flow on dating systems zircon u pb shrimp sims la icp ms ca id tims and rb sr ar ar granites from the western erzgebirge bohemian massif germany abstract the variscan granites from the western erzgebirge were repeatedly dated by various methods but no consensus has been reached about their exact intrusion ages this study presents a multi dating approach for the four largest intrusions from the western erzgebirge aue schwarzenberg bergen eibenstock kirchberg we analysed several samples from each pluton suite with zircon u pb ca id tims chemical abrasion isotope dilution thermal ionization mass spectrometry to obtain robust temporal information on their age and tempo of intrusion these data enable us for the first time to define three intrusive episodes of 1 2 ma each separated by quiet periods of several ma the aue schwarzenberg suite represents the oldest granites that intruded at 323 322 ma followed by the granites from bergen and kirchberg 2 4 ma later the highly evolved ore bearing granites from the eibenstock pluton intruded after a time lag of 5 ma at 315 314 ma the new data show that there is a resolvable age difference between the two known granite groups granite group 2 also assigned as younger igneous complex represented by the eibenstock pluton is 5 ma younger than granite group 1 assigned as older igneous complex represented by granites from aue schwarzenberg bergen and kirchberg protracted magmatism and late post magmatic fluid flow partly reset the u pb system of these granites to variable degrees making a precise and accurate dating of their intrusion ages challenging pb loss in zircons is often combined with high common pb pbc shrimp sims sensitive high mass resolution ion microprobe secondary ion mass spectrometry and la icp ms laser ablation inductively coupled plasma mass spectrometry on non ca zircons document that pb loss and high pbc is quite variable within zircon grains and may be located in micro fractures we demonstrate that chemical abrasion ca clearly minimizes or removes both pb loss and pbc results from prior la icp ms and shrimp dating on non ca zircons from the same samples considerably helped the interpretation of the ca id tims data when pb loss was not completely erased by ca in such cases we often had to choose the oldest analyses for mean age calculation in contrast to the common practice of the ca id tims community to choose the youngest dates rb sr and ar ar dating systems revealed age differences between the older group and the younger ore bearing granites albeit with diverging absolute ages most ar ar ages are identical with ca id tims ages and would imply rapid cooling however samples from the older group have excess ar that could have led to too old ages in contrast rb sr ages for the older granites are 0 7 ma younger than their intrusions fluid induced alteration led to the formation of li mica fluorite and cassiterite greisenization for the youngest granite eibenstock li mica was used to date the first greisenization samples without visible hydrothermal overprint yielded identical ar ar and rb sr ages as severely greisenized samples this implies re equilibration due to the hydrothermal overprint for all ar ar and rb sr ages from the eibenstock pluton according to ar ar dating the first ore formation 315 ma is coeval with the ca id tims intrusion age of the eibenstock granite while it is delayed by 6 3 ma according to rb sr dating 308 3 ma

concordant u pb shrimp ages of u rich zircon in granitoids from the muruntau gold district uzbekistan timing of intrusion alteration ages or meaningless numbers abstract for decades the u pb isotope system of zircon is routinely used in geosciences for the determination of intrusion ages of felsic magmatic rocks it is however well known that zircon and the related u pb isotope system may be influenced by secondary hydrothermal alteration and new hydrothermal zircon growth as a result age determination may yield meaningless or alteration ages an example is given here for intensely altered late paleozoic granitoid rocks from the muruntau area central kyzyl kum uzbekistan pb pb single grain evaporation and u pb shrimp isotope systematics of zircon from six granitoid rocks yield roughly consistent results defining ages ranging between 288 and 303 ma at first glance these ages may be understood as intrusion ages of the magmatic rocks hosting the zircon however there is an apparent contradiction between the sequence of the measured u pb shrimp ages and the relative age sequences established in the field evaluation of data from comprehensive mineralogical and petrological studies reveals that the u pb system of magmatic zircon is complicated by inheritance in i type granitoids recrystallization new hydrothermal zircon growth and subsequent secondary alteration widespread albitization of the granitoids led to the formation of new u rich hydrothermal zircon forming overgrowths on older grains or even new whole single crystals later alteration events caused direct and apparent reverse u pb isotope discordances these late disturbances of the u pb system are mainly due to widespread sericitization consequently precise concordant u pb zircon shrimp ages around 290 294 ma defined for u rich largely undisturbed crystal areas constrain the timing of albitization rather than that of the intrusion the reliability of the concordant u pb shrimp ages measured on u rich zircon is confirmed by a u pb shrimp age of hydrothermal monazite 292 8 ma this monazite associates with extremely u rich zircon u pb shrimp age 291 3 ma in a sample of the murun granite recovered beneath the giant muruntau gold deposit by super deep drilling obviously detailed mineralogical studies are important for a correct interpretation of zircon ages when hydrothermal alteration and late re mobilization of zirconium and uranium were significant as often observed in areas with intense ore mineralization the extensive au mineralization found at muruntau is similar in age to albitization but apparently somewhat younger likewise the intrusion age of permian granites is at least somewhat older than the hydrothermal zircon ages confirmed by u pb ages of monazite it is however difficult to constrain further granite intrusion ages based on presently available data

sequential evolution of sn zn in mineralization at the skarn hosted hammerlein deposit erzgebirge germany from fluid inclusions in ore and gangue minerals skarn hosted deposits can be important high grade resources for a variety of metals but sn skarns are still of subordinate importance for global mining because of their complex mineralogy and evolution as part of recent exploration efforts the economic potential of the sn zn in mineralization at the hammerlein skarn hosted deposit is currently being re evaluated the temporal and spatial evolution of the ore forming hydrothermal system is still debated we analyzed fluid inclusion assemblages fia in ore and gangue minerals using conventional and infrared microthermometry and laser ablation inductively coupled plasma mass spectrometry la icp ms we further estimated alteration temperatures with chlorite thermometry and compared the fluid inclusion fi record at hammerlein with a mineralized greisen sample of the nearby eibenstock granite cassiterite in skarn forms the major mineralization at hammerlein during stage i and hosts fis showing homogenization temperatures of up to 500 c and salinities between 30 and 47 wt nacl eq cassiterite from schists and the additional greisen sample from the eibenstock granite of the later stage ii show lower homogenization temperatures 350 400 c and considerably lower salinities varying from 1 9 to 6 wt nacl eq despite the different homogenization temperatures and salinities the chemical compositions of fis hosted in both generations of cassiterite show that both cassiterite in skarn and in schist and greisen are similar which points to a common source the gangue minerals mainly contain low temperature fia max 330 c and 2 9 wt nacl eq and are interpreted to form during further cooling of the system in stage iii intergrown chlorite has compositions indicating similar temperatures of around 260 c and is thus also related to stage iii fia in sphalerite homogenize around 200 c with salinities between 2 and 6 7 wt nacl eq and show decreasing trace element contents despite having the same salinity range as the gangue minerals indicating dilution of the ore fluid during stage iv as a possible precipitation mechanism stage i inclusions are solely hosted in cassiterite from skarn which shows the importance of fluid inclusion analyses in ore minerals and record remarkable high mineralization temperatures exceeding the typically temperature range reported in other studies by at least 100 c our results suggest that this main ore stage is related to the early expulsion of a high salinity brine phase from an underlying magmatic intrusion at depths greater than 3 km which likely is a relatively short lived event within the evolution of the hydrothermal system

u pb shrimp geochronology of th poor hydrothermal monazite an example from the llallagua tin porphyry deposit bolivia in situ u pb shrimp analysis of hydrothermal monazite virtually free of th and poor in u 0 2 ppm th 40 103 ppm u from the world class llallagua tin porphyry deposit in bolivia defines a mineralization age of 23 4 2 2 ma mswd 0 48 confirming earlier k ar sericite alteration age data these ages are however in contrast with a weighted mean single crystal 207pb 206pb evaporation age of 39 3 6 0 ma and a related pb pb inverse isochron age of 42 4 4 0 ma mswd 0 66 on zircon from a post porphyry dike as well as with an earlier single crystal sm nd apatite isochron age our data points to a significant time gap between emplacement of the ore hosting porphyry intrusion magmatism and its hydrothermal overprint tin mineralization suggesting long lived magmatic hydrothermal activity in this part of the andean back arc crust the decoupling of porphyry magmatism and hydrothermal activity may explain the unusual occurrence of relatively little fractionated felsic rocks together with extensive tin mineralization our study demonstrates the usefulness of the application of the u pb shrimp method to direct age determination of ore mineralization using th poor hydrothermal monazite even when dealing with geological young events the common assumption of synchronous magmatism and hydrothermal ore formation in porphyry systems may not always be warranted

the halite bearing zag and monahans 1998 meteorite breccias shock metamorphism thermal metamorphism and aqueous alteration on the h chondrite parent body zag and monahans 1998 are h chondrite regolith breccias comprised mainly of light colored metamorphosed clasts dark clasts that exhibit extensive silicate darkening and a halite bearing clastic matrix these meteorites reflect a complex set of modification processes that occurred on the h chondrite parent body the light colored clasts are thermally metamorphosed h5 and h6 rocks that were fragmented and deposited in the regolith the dark clasts formed from light colored clasts during shock events that melted and mobilized a significant fraction of their metallic fe ni and troilite grains the clastic matrices of these meteorites are rich in solar wind gases parent body water was required to cause leaching of chondritic minerals and chondrule glass the fluids became enriched in na k cl br al ca mg and fe evaporation of the fluids caused them to become brines as halides and alkalies became supersaturated grains of halite and in the case of monahans 1998 halite with sylvite inclusions precipitated at low temperatures 100 c in the porous regolith in both meteorites fluid inclusions were trapped inside the halite crystals primary fluid inclusions were trapped in the growing crystals secondary inclusions formed subsequently from fluid trapped within healed fractures

neutron capture 36cl 41ca 36ar and 150sm in large chondrites evidence for high fluences of thermalized neutrons we have measured significant concentrations of 36cl 41ca 36ar from decay of 36cl and 150sm produced from the capture of thermalized neutrons in the large chico l6 chondrite activities of 36cl and 41ca corrected for a high energy spallogenic component and a terrestrial age of 50 ka give average neutron capture production rates of 208 atoms min g cl and 1525 atoms min kg ca which correspond to thermal neutron n fluxes of 6 2 n cm2 s and 4 3 n cm2 s respectively if sustained for the 65 ma single stage cosmic ray exposure age of chico these values correspond to thermal neutron fluences of 1 3 1016 and 0 8 1016 n cm2 for 36cl and 41ca respectively stepwise temperature extraction of ar in chico impact melt shows 36ar 38ar ratios as large as 9 the correlation of high 36ar 38ar with high cl ca phases in neutron irradiated chico indicates that the excess 36ar above that expected from spallation is due to decay of neutron produced 36cl excess 36ar in chico requires a thermal neutron fluence of 0 9 1 7 1016 n cm2 decreases in 149sm 152sm due to neutron capture by 149sm correlate with increases in 150sm 152sm for three samples of chico and one of the torino h chondrite the 0 08 decrease in 149sm 152sm shown by chico corresponds to a neutron fluence of 1 23 1016 n cm2 this fluence derived from sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution excess 36ar identified in the arapahoe bruderheim and torino chondrites and the shallowater aubrite suggest exposure to neutron fluences of 0 2 0 6 1016 n cm2 depletion of 149sm in torino and the lew86010 angrite suggest neutron fluences of 0 8 1016 n cm2 and 0 25 1016 n cm2 respectively neutron fluences of 1016 n cm2 in chico are almost as large as those previously observed for some lunar soils consideration of exposure ages suggests that the neutron flux in chico may have been greater than that in many lunar soils neutron capture effects although seldom reported may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic ar combining measurements of radioactive and stable species produced from neutron capture has the potential for identifying large meteorites with complex exposure histories

petrogenesis of the bosworgey granitic cusp in the sw england tin province and its implications for ore mineral genesis the bosworgey granite cusp forms an apical portion of the concealed northern extension of the tregonning godolphin granite ridge it is characterised by unusually high values of b p mn fe as cu nb ta bi sn w u and s which are present largely as tourmaline apatite pyrite arsenopyrite chalcopyrite bismuth columbite cassiterite wolframite and uraninite and low levels of zr hf ti and ree present in zircon ilmenite and monazite the granite is classified as sn and w specialised tischendorf 1974 and it belongs to the ilmenite series of japanese workers the classification of chappell and white 1974 s and i type granites is shown to be inapplicable to cornubian rocks although the bosworgey samples show characteristics of s type granites the accessory mineral assemblages are typical of high temperature lodes cassiterite wolframite arsenopyrite chalcopyrite and the assamblage is concluded to be the cusp analogue of hypothermal lodes produced by extreme differentiation and concentration of volatiles it is speculated that such granites could provide the parent material for the mesothermal crosscourse mineralisation pitchblende bismuth pyrite galena sphalerite

late collisional granites in the variscan erzgebirge germany the late collisional erzgebirge granites 325 318 ma were introduction emplaced at shallow crustal levels in the variscan metamorphic the erzgebirge is a ne sw trending antiformal structure basement shortly after large scale extension caused by orogenic exposing variscan crystalline basement rocks at the northcollapse these granites comprise mildly peraluminous transitional ern margin of the bohemian massif the region is famous i s types and strongly peraluminous s type rocks which can be among economic geologists for its extensive and highly subdivided into three major groups low f biotite granites low f variable metallic ore deposits which have been mined two mica granites and high f high p2o5 li mica granites the since the middle ages e g stemprok seltmann 1994 highest degree of differentiation is reached in the li mica granites tischendorf forster 1994 and for the classic studies which exhibit strongly elevated concentrations of p f li rb cs in the fields of economic geology mining and metallurgy ta sn w and u but very low ti mg co ni sr ba y zr chemistry and mineralogy e g werner 1791 breithaupt hf th and rare earth elements crystal melt fractionation is the 1849 agricola 1974 dominant process controlling the bulk composition of all groups of as in many of the other variscan provinces in central granites however metasomatic processes involving late stage residual europe granitic rocks make up a large proportion of the melts and high t orthomagmatic fluids became increasingly more exposed rocks in the erzgebirge and a great variety of important in highly evolved units and have modified the abundances compositional and textural types occur fig 1 knowof mobile elements p f li rb cs ba sr in the li mica ledge of the processes involved in the generation and granites particularly isotopic and geochemical characteristics suggest evolution of these silicic magmas is essential for underthat the three granite groups cannot be derived from a common standing heat and mass transport during the variscan precursor magma their discrete compositions are source related orogeny and also for the formation of metallic ore and are attributed to melting of quartzo feldspathic and pelitic deposits several lines of evidence indicate that early crustal lithologies in different proportions granites are common in high temperature sn w deposits are associated with the the central european variscides but the erzgebirge is unusual for emplacement of highly evolved volatile rich granitic the predominance of evolved li mica granites associated with melts tischendorf 1986 forster tischendorf 1992 economically important sn w and u deposits the abundance of stemprok 1993 the role of the granites in formation of li mica granites is attributed to a combination of favourable factors later permian u pb zn and younger ore mineralization 1 low degrees of anatectic melting of crustal protoliths 2 wide f ba is controversial but good evidence exists that the distribution of fertile lithologies rich in large ion lithophile elements vein type u deposits owe their uranium to leaching from and ore elements 3 extended magmatic differentiation by crystal the uraninite rich granites e g tischendorf forster melt fractionation and subsequent autometasomatism 1994

geochemical specialization of the tin bearing granitoid massifs of nw bohemia the geochemistry of hercynian tin bearing granitoid massifs of the krusne hory mts erzgebirge slavkovsk les forest kaiserwald and smrciny eastern fichtelgebirge is compared by statistical processing of 270 analyses including a wide spectrum of major and trace elements seven different types of granites are distinguished out of these five types represent the successive differentiation of the largest massif of nw bohemia the karlovy vary karlsbad massif this comprises strongly differentiated peraluminous granites evolving towards extreme li rb cs f and sn enrichment in the youngest members which are albite topaz zinwaldite lithium granites the sixth and seventh types are different from the former by their location in the eastern krusne hory and tectonic setting and they display geochemical features of anorogenic granites they are metaluminous albite zinwaldite granites with marked enrichment of nb y and hree in addition to li rb cs f and sn indicating contamination by sub crustal material sn w mineralizations including flat peri contact greisen bodies steep greisen veins and tourmalinized phyllites are all intimately associated with the most strongly differentiated granites the li granite and the cinovec granite respectively

rb sr isotopic systematics of alkali rich fragments in the yamato 74442 ll chondritic breccia abstract we have undertaken mineralogical petrographical and rb sr isotopic studies on alkali rich igneous rock fragments in the yamato y 74442 ll chondritic breccia the fragments are a few mm in size and are composed mainly of porphyritic olivine and dendritic pyroxene set in alkali rich groundmass glass minor phases include chromite troilite and metallic nickel iron bulk chemical compositions of the fragments are almost identical to the host chondrite except for a depletion of sodium and an enrichment of potassium isotopic analyses of nine fragments from y 74442 yield a rb sr age of 4429 54 ma 2 for 87 rb 0 01402 ga 1 with an initial ratio of 87 sr 86 sr 0 7144 0 0094 2 assuming precursors of the fragments formed 4568 ma with 87 sr 86 sr 0 69889 when the solar system formed a time averaged rb sr weight ratio of the source material for the fragments is calculated to be 2 58 0 91 0 93 the extremely high rb sr value of this source is difficult to interpret by any igneous fractionation or liquid immiscibility but can be explained by mixing of a chondritic component with an alkali rich component formed in the early solar nebula in our preferred model the alkali component with rb sr 30 would have condensed from the residual nebular gas after removal of refractory strontium and must have been isolated for a long time in a region where the temperature was sufficiently low to prevent reaction with other silicates oxides a mixture of the alkali component early nebular condensates and the ferromagnesian component could reflect flash heating induced by impact on an ll chondritic parent body at least 4429 ma ago and further enrichments of rubidium and potassium relative to strontium could have occurred during this event the resulting impact melt rocks could have been fragmented by later impact event s and finally incorporated into the y 74442 parent body thus a remarkable signature of alkali enrichments both in the early solar nebula and later on the ll chondrite parent body is preserved as a minor component of some chondritic breccias such as y 74442

constraining a shrimp u pb age micro scale characterization of zircons from saxonian rotliegend rhyolites we present results of a detailed investigation of zircons from two rhyolites from st egidien and chemnitz saxony using a combination of microprobe techniques shrimp ion probe raman microprobe sem se bse and cl imaging these rhyolites belong to the so called lower volcanics which is the older of two series of late variscan volcanic rocks occurring in the saxonian sub erzgebirge basin germany the purpose of the present contribution is to demonstrate that detailed characterization of zircons as provided by the different micro techniques facilitates soundest interpretation of geochronological data the zircons at most 40 to 80 m in size show oscillatory growth zoning with reversely correlated cl and bse signal intensities these zircons are interpreted to have grown during crystallization of the rhyolite because apart from some cracking they do not appear to have experienced any alteration since the time of their growth the shapes of the zircons and their internal structures revealed by cl and bse imaging appear to be magmatic and neither annealing of the accumulated alpha decay damage nor disturbance of the u pb system is observed the shrimp ion probe measurements on the zircons gave a permian 206pb 238u age of 278 5 ma 95 confidence the concordance of this age is supported by the correlation between the low degrees of metamictization estimated from raman parameters and the accumulated alpha fluxes calculated from shrimp data the 278 ma zircon age is interpreted to represent the age of the lower rhyolites series and with that the age of postkinematic late variscan volcanism in the sub erzgebirge basin which has been related to anorogenic extension and uplift as a result of intracontinental rifting because of genetic association of rhyolites in the sub erzgebirge basin and li f granites and lamprophyres in the neighbouring erzgebirge the rhyolite age also indirectly contributes to the understanding of the geological history of the erzgebirge the 278 ma age is the first absolute dating result for rhyolites from the saxonian sub erzgebirge basin

chloride depletions and enrichments in seafloor hydrothermal fluids constraints from experimental basalt alteration studies na k ca cl fluid was reacted with diabase at 400 and 425 c 400 bars and fluidrock mass ratio of 0 5 to assess the relative mobility of dissolved cl fluids from the present experiments reveal relatively large time and temperature dependent decreases in cl temperature increase enhances cl removal whereas reaction progress has the opposite effect these changes are not due to boiling and or phase separation at 400 and 425 c 400 bars cl removal from solution is accompanied by ca and na fixation respectively both experiments produced mixed layer chlorite smectite clinozoisite and albite olivine was not detected by x ray diffraction or petrographic analysis of run products from either experiment however we propose that the time dependent changes in dissolved cl result from olivine replacement by an fe hydroxy chloride phase followed by hydration effects quenched and washed alteration products however did not reveal anomalous cl which suggests that the cl bearing phase is characterized by retrograde solubility this is consistent with the relative magnitude of cl fixation observed for experiments at both temperatures and from results of an isobaric cooling experiment the experiments provide evidence for the non conservative behavior of cl during hydrothermal alteration of basalt the data are particularly important in light of the cl depleted nature of some ridge crest hot spring fluids and suggest temperatures of formation for these fluids of approximately 410 c assuming sub seafloor pressure of 400 bars in addition the retrograde solubility of the cl bearing phase responsible for cl fixation during high temperature basalt alteration may help to explain cl enrichment in hot spring fluids which have conductively cooled below 350 c

temporal evolution of mineralization events in the bohemian massif inferred from the re os geochronology of molybdenite molybdenite is a common mineral accompanying sn w au and base metal mineralizations located in different geotectonic units of the bohemian massif but it is also widespread in granitoids and or related quartz veins pegmatites forming disseminated mo mineralization thirty re os ages were obtained for molybdenite samples from the bohemian massif to provide constraints on the timing and duration of mineralization event s within the framework of previously published geochronological data for the host and or associated rocks the obtained data for sn w li deposits in the erzgebirge metallogenetic province indicate the predominance of one and or multiple short time mineralization events taking place between 319 and 323 ma with the exception of the krupka deposit associated with the altenberg teplice caldera where the data may suggest prolonged activity until 315 ma the ages of the pb zn au mo h rky u rakovnika and fe cu as ob i d l mineralizations from the exocontacts of the cista pluton and krkonose jizera plutonic complex respectively provide evidence for synchronous emplacement of the ore and the associated granitic rocks in contrast the padr fe as mo mineralization postdates the age of the associated padr granite disseminated mo mineralization in cadomian and variscan granitoids and or related to quartz veins pegmatites provides re os ages that overlap with the previously published geochronological data for the host rocks suggesting coeval evolution molybdenite samples from the sazava suite granites of the central bohemian plutonic complex cbpc have resolvable younger ages than their host granites but similar to the age of spatially related au mineralization which is associated with the latest evolution of the cbpc

depletion of a brine layer at the base of ridge crest hydrothermal systems the variable salinity of fluid venting from mid ocean ridges is indicative of mixing between hydrothermal seawater and fluids that have undergone supercritical phase separation in order to study the stability of a brine saturated layer that may form in the lowermost part of the hydrothermal system we have performed numerical simulations of a system that has returned into the subcritical regime for typical geological parameters it is shown that the interface between the brine layer and the overlying fluids is not very stable but vanishes by one of two dynamical mechanisms convective breakdown or vertical migration this contradicts the conventional picture of a steady layered convective system in which the brine is depleted only by dispersion and diffusion across the interface the depletion mechanism depends on the fluid dynamical stability of the brine layer convection within the brine layer results either in the convective breakdown for low excess salinity of the brine as compared to seawater or the upward migration of the interface for higher excess salinities consequently the depletion times are much shorter than for models with pure dispersion diffusion across the interface if the brine layer is static high chlorinity liquid is entrained slowly by the convecting overlying fluids leading to downward migration of the interface this gradual depletion of the brine layer results in almost constant vent salinities in agreement with measured salinities of chronic high chlorinity vents fl 2000 elsevier science b v all rights reserved

early solar system hydrothermal activity in chondritic asteroids on 1 10 year timescales chondritic meteorites are considered the most primitive remnants of planetesimals from the early solar system as undifferentiated objects they also display widespread evidence of water rock interaction on the parent body understanding this history has implications for the formation of planetary bodies the delivery of water to the inner solar system and the formation of prebiotic molecules the timescales of water rock reactions in these early objects however are largely unknown here we report evidence for short lived water rock reactions in the highly metamorphosed ordinary chondrite breccia villalbeto de la pena l6 an exotic clast d 2cm has coexisting variations in feldspar composition and oxygen isotope ratios that can only result from hydrothermal conditions the profiles were modeled at t 800 c and p h2o 1 bar using modified grain boundary diffusion parameters for oxygen self diffusion and reaction rates of nasica 1al 1 exchange in a fumarole the geochemical data are consistent with hydrothermal activity on the parent body lasting only 1 10 y this result has wide ranging implications for the geological history of chondritic asteroids

internal structures and dating of complex zircons from meissen massif monzonites saxony abstract u pb zircon ages were determined for accessory zircons from two monzonites from the meissen massif northern bohemian massif germany by shrimp ion microprobe bse and cl imaging revealed that the monzonite zircons showing oscillatory growth zoning often contain inherited cores raman microprobe measurements showed that zoned rims u 277 1426 ppm th 144 910 ppm are slightly disordered to moderately metamict whereas cores have a wide spectrum of structural states from well crystallized to highly metamict the radionuclide u th content of cores varies from a few hundred to more than 20 000 ppm shrimp ion probe measurements on zoned rims interpreted as zircon growth during crystallization of the monzonite magma gave concordant carboniferous 206 pb 238 u ages of 326 6 ma monzonite from leuben and 330 5 ma monzonite from heidenschanze 95 confidence respectively these ages are consistent with recently published 40 ar 39 ar ages of 329 1 2 8 to 330 4 2 8 ma 2 on amphiboles from the same rocks in contrast shrimp measurements on cores range from slightly discordant proterozoic relic ages in low u and th cores to highly discordant data in the most metamict cores previously obtained 207 pb 206 pb ages single zircon evaporation technique in the range 340 16 to 357 16 ma 1 are probably mixed ages between magmatic and inherited zircon components the u pb ages of the monzonite zircons postdate the upper devonian lower carboniferous subduction and collision events in the area by about 20 ma due to their shoshonitic chemical signature which is typical of rocks from destructive plate boundary settings the meissen massif monzonites might be interpreted as originating during these 350 ma events and the earlier 207 pb 206 pb ages appeared to confirm this interpretation however both the 40 ar 39 ar ages and our current results show that only the enrichment of the lithospheric mantle source but not the monzonite intrusion itself was directly related to these events the monzonite intrusion itself took place 330 ma ago in the framework of strike slip tectonics in the elbe valley zone widely occurring zircon cores are indicative of crustal contamination of the magma causing considerable modification of chemical signatures trace element content of the monzonites

autometasomatic rb enrichments in highly evolved granites causing lowered rbsr isochron intercepts abstract whole rock samples of the younger members of a suite of the erzgebirge granites yield rb sr isochrons with remarkably low intercepts 0 689 0 702 whereas the older granites yield initial sr isotope ratios which are higher and rather uniform 0 70677 22 a model is proposed to explain this observation by a two stage evolution of the younger granites after a time span t 1 following the intrusion autometasomatic processes caused by residual magmatic solutions increase the rb contents of the younger granites by more or less the same amount this rb addition lowers the intercepts of the isochrons but it leaves their slopes undisturbed the rb amounts added during autometasomatosis can be derived by a petrogenetic model of fractional crystallization the time t 1 may be calculated approximately on the basis that the true initial ratio is at least as high as those of the older granite suite t 1 is in the order of 10 ma and agrees well with the difference in age between whole rock data and the rb sr mineral ages of metasomatically formed dark micas in the rocks 10 20 ma and with k ar data it is concluded that decreased granite isochron intercepts may be more common than hitherto expected

geochemistry of north cleft segment vent fluids temporal changes in chlorinity and their possible relation to recent volcanism hydrothermal vent fluids from the north cleft segment of the juan de fuca ridge between 44 54 and 45 00 n were sampled in 1988 1990 1991 and 1992 in 1988 chloride depleted relative to seawater diffuse fluids with low dissolved metal concentrations relative to other mid ocean ridge mor fluids were sampled over a distance of 10 km along axis in 1990 and 1991 both high temperature 200 c and diffuse vents were consistently metal and chloride enriched the end member compositions of high temperature vents and nearby diffuse vents were very similar there were small but significant correlated changes in diffuse and high temperature composition from 1990 to 1991 the large scale but temporary venting of low chlorinity fluids over the entire north cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride enriched fluids it is possible that a volcanic event along the north cleft segment prior to 1987 for which there is firm geologic evidence initially caused a boiling event which resulted in the preferential venting of vapor enriched fluids through 1988 followed by a transition to brine enriched fluids by 1990 high iron low sodium and low sr ca ratios in the high chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event the absence of chloride depleted fluids from 1990 onward and the systematics of lithium boron and manganese with chloride in the high temperature fluids from north cleft suggest that the evolution toward lower chlorinity at monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

unusual rare earth element fractionation in a tin bearing magmatic hydrothermal system normalized rare earth element patterns of rock and mineral samples from evolved granitic systems and associated hydrothermal tin deposits frequently show an unusual split into four consecutive curved segments referred to as tetrads in the present contribution the simultaneous occurrence of complementary convex and concave tetrads is described for the first time for vein fluorite suggesting that this unusual trace element signature may develop during a single evolutionary stage within the hydrothermal environment based on geochemical analysis of vein fluorite fluid inclusion studies and thermodynamic modeling it is shown that fractionation of the rare earth elements can be linked to fluid immiscibility and preferential partitioning of these elements between vapor and coexisting liquid the findings provide the first direct evidence constraining the geological conditions responsible for the occurrence of the tetrad effect in tin bearing magmatic hydrothermal systems and require a reassessment of the current understanding of the origin of tin deposits

tetrad effect in rare earth element distribution patterns a method of quantification with application to rock and mineral samples from granite related rare metal deposits abstract in some geological environments the tetrad effect can be observed as a split of rare earth element ree patterns into four rounded segments a new method is proposed to quantify the sizes of the individual segments and for the first time the significance of observed tetrad effects is evaluated by taking analytical errors into account the outlined method was applied to lanthanide patterns of whole rock and fluorite samples collected from granite related rare metal deposits the ree patterns of the granite and greisen samples investigated exhibit significant tetrad effects that may not be accounted for by analytical uncertainties it is shown that the study of whole rock samples is insufficient to determine whether this effect is developed during fractional crystallization or is due to other processes such as fluid rock interaction a concave tetrad effect mirroring the pattern of the whole rock samples was not observed in the ree patterns of related vein fluorite samples therefore it is unlikely that the convex tetrad effect in the samples from the magmatic environment can be explained by removal of a respective complementary ree pattern by a coexisting hydrothermal fluid as previously suggested it is proposed that the tetrad effect formed within the magma fluid system before emplacement in the subvolcanic environment where phase separation caused a split of this system into fluid and magma subsystems alternatively the tetrad effect may also be inherited from an external fluid influencing the system during or after the emplacement of the magma on the basis of the fluorite data it is shown that the behavior of eu in the fluids is not related to the tetrad effect consequently different physico chemical factors control the occurrence of both phenomena y was found to be strongly enriched in samples precipitating from hydrothermal fluids that experienced prolonged interaction with the wall rocks whereas the tetrad effect in the fluids vanished with time and increasing distance from the ore bearing granite thus these different geochemical parameters can be used to reconstruct different aspects of the fluid evolution within this type of deposit

the application of single zircon evaporation and model nd ages to the interpretation of polymetamorphic terrains an example from the proterozoic mobile belt of south india the technique of single zircon dating from the thermal evaporation of 207pb 206pb kober 1986 1987 provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages in contrast nd model ages may provide a measure of the period of crustal residency for the sample or its protolith these two techniques have been combined to elucidate the tectonic history of the proterozoic mobile belt of southern india exposed south of the palghat cauvery shear zone that marks the southern boundary of the archaean craton of karnataka the two main tectonic units of this mobile belt comprise the madurai and trivandrum blocks both of which are characterised by massive charnockite uplands and low lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the proterozoic evidence for early palaeoproterozoic magmatism is restricted to the madurai block where single zircon evaporation ages from a metagranite 2436 4 ma are similar to model nd ages from a range of lithologies suggesting crustal growth at that time the trivandrum block to the south of the achankovil shear zone is comprised of the kerala khondalite belt the nagercoil charnockites and the achankovil metasediments single zircon evaporation ages together with conventional zircon and garnet chronometry suggest that all three units underwent upper amphibolite facies metamorphism at 1800 ma an event unrecorded in the metagranite from the madurai block this implies that the madurai and trivandrum blocks represent distinct terrains throughout the palaeoproterozoic model nd ages from the achankovil metasediments are much younger 1500 1200 ma than those from the adjacent kerala khondalite belt and madurai blocks 3000 2100 ma but there is no evidence for zircon growth in these metasediments during the mesoproterozoic hence the comparatively young model nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth zircon overgrowths from the madurai block 547 17 ma and achankovil metasediments 530 21 ma suggest that all tectonic units of the proterozoic mobile belt of south india shared the same metamorphic history from the early palaeozoic this event has been recognised in the basement lithologies of sri lanka and east antarctica confirming that the constituent terrains of east gondwana had assembled by this time

dating multiply overprinted sn mineralized granites examples from the erzgebirge germany granites and primary tin mineralization in the erzgebirge were dated using 1 conventional u pb dating of uraninite inclusions in mica 2 rb sr dating of inclusions in quartz that represent highly evolved melts 3 re os dating of magmatic hydrothermal molybdenite and 4 chemical th u pb dating of uraninite conventional isotope dilution and thermal ion mass spectrometry and chemical th u pb dating of uraninite in granites from the ehrenfriedersdorf mining district provide ages of 323 9 3 5 ma 2 greifenstein granite and 320 6 1 9 and 319 7 3 4 ma 2 both sauberg mine in agreement with u pb apatite ages of 323 9 2 9 and 317 3 1 6 ms 2 both sauberg mine rb sr analysis of melt inclusions from zinnwald gives highly radiogenic sr isotopic compositions that with an assumed initial sr isotopic composition permit calculation of precise ages from single inclusions the scatter of the data indicates that some quartz hosted melt inclusions have been affected by partial loss of fluid exsolved from the melt inclusion re os dating of two molybdenite samples from altenberg provides ages of 323 9 2 5 and 317 9 2 4 ma 2 together with age data from the literature our new ages demonstrate that primary tin mineralization and the emplacement of the large sn specialized granites in the erzgebirge fall in a narrow range between 318 and 323 ma primary sn mineralization occurred within a short interval during post collisional collapse of the variscan orogen and was essentially synchronous over the entire erzgebirge in contrast to earlier claims no systematic age difference between granites of the eastern and western erzgebirge was established furthermore our data do not support a large age range for late variscan granites of the erzgebirge 330 290 ma as has been previously suggested

trace element geochemistry of sphalerite in contrasting hydrothermal fluid systems of the freiberg district germany insights from la icp ms analysis near infrared light microthermometry of sphalerite hosted fluid inclusions and sulfur isotope geochemistry the historic silver mining district of freiberg germany comprises hydrothermal vein style mineralization of permian and cretaceous age we compare sphalerite compositions with associated ore forming fluids and constrain the behavior of critical metals such as in ge and ga in contrasting hydrothermal environments fluid inclusion studies reveal that the permian veins formed due to boiling and cooling of a low salinity 0 to 6 eq w nacl magmatic hydrothermal fluid at 350 to 230 c in contrast cretaceous veins formed by mixing of highly saline 17 to 24 eq w nacl cacl2 and variable na na ca ratios brines at low temperatures 120 c sulfides of the permian ore stage have a narrow range of 34svcdt from 2 3 to 0 9 while the sulfides of the cretaceous stage have a large scatter and significantly more negative 34svcdt values 30 9 to 5 5 supporting the different nature of the hydrothermal systems contrasting fluid systems and ore forming mechanisms correspond to markedly different trace element systematics in sphalerite permian sphalerite is significantly enriched in in up to 2500 g g in relative to two sphalerite generations of cretaceous veins the latter have higher ge up to 2700 g g ge and ga up to 1000 g g ga concentrations the observed trace element systematics of different sphalerite generations imply that in is enriched in high temperature low to intermediate salinity fluids with a significant magmatic hydrothermal fluid component while ge and ga are more concentrated in low temperature high salinity crustal fluids with no obvious magmatic hydrothermal affiliation

time of formation and peak of variscan hp ht metamorphism of quartz feldspar rocks in the central erzgebirge saxony germany the variscan erzgebirge represents an antiform with a core of gneisses and mica schists surrounded by a phyllitic mantle the gneiss eclogite unit geu in the central part is a composite tectonometamorphic assemblage characterized by a hp ht imprint and comprises migmatitic para and orthogneisses ht mylonites hp granulites eclogites and garnet peridotites it is tectonically sandwiched between two major units with distinctly lower pt histories the geu experienced a characteristic kinked retrograde pt path after hp ht equilibration with 1 strong near isothermal decompression at high temperatures 2 extensive re equilibration at medium pressures followed 3 by rapid cooling during continued uplift we dated zircons pb pb evaporation from granitoid orthogneisses and metapelites of the geu the orthogneisses contain euhedral long prismatic zircons of igneous origin that provided protolith ages between 470 and 524 ma metapelites retain well preserved granulite facies mineral assemblages and contain spherical multifaceted metamorphic zircons that grew near the peak of hp ht metamorphism inclusions of prograde hp phengite 15 kbar and rutile are included in one such zircon metamorphic zircons of three samples from different localities yielded identical 207pb 206pb ages of 340 5 0 7 ma 341 2 0 5 ma and 341 6 0 5 ma respectively consideration of these zircon ages with published 39ar 40ar white mica ages suggests fast cooling and uplift rates in excess of 50 c ma and 4 km ma this is typical for large scale extensional tectonic unroofing of the ultra deep part of a fossil thickened variscan continental crust 60 km during continuing continental collision and orogenic collapse

a time scale of formation of the first solids we revisit an attempt to establish a unified chronology of the first ca 20 myr of the solar system in the light of recent data this period encompasses the formation of the first solids and the earliest planetary processes we investigate the consistency of calibrations of the chronometers based on the extinct radionuclides 26 al 53 mn and 129 i with one another and with the pb pb chronometer we find the most persuasive chronology to be that based on identifying isotopic closure of the three short lived systems in ste marguerite with a common event in effect this modifies the calibration of the i xe system based on acapulco phosphate this can be tied in turn to an absolute time scale based on the pb pb and mn cr ages of the angrite lew86010 the result is a common chronology agreement between i xe and mn cr is particularly striking although ages derived for calcium aluminium rich inclusions cais are discordant this discordancy may be explained by nuclear effects in the cai formation region

carboniferous permian volcanic evolution in central europe u pb ages of volcanic rocks in saxony germany and northern bohemia czech republic nine shrimp u pb ages on zircon and two pb pb single zircon ages have been determined from late paleozoic volcanic rocks from saxony and northern bohemia samples came from the teplice altenberg volcanic complex the meissen volcanic complex the chemnitz basin the dohlen basin the brandov olbernhau basin and the north saxon volcanic complex the teplice altenberg volcanic complex is subdivided into an early namurian phase mikulov ignimbrite 326 8 4 3 ma thus older than assumed by previous studies and a late caldera forming phase teplice ignimbrite 308 8 4 9 ma the age of the latter however is not well constrained due to a large population of inherited zircon and possible hydrothermal overprint the leutewitz ignimbrite product of an early explosive volcanic episode of the meissen volcanic complex yielded an age of 302 9 2 5 ma stephanian a volcanic rocks intercalated in the brandov olbernhau basin bob 302 2 8 ma chemnitz basin cb 296 6 3 0 ma dohlen basin db 296 3 0 ma and the north saxon volcanic complex nsvc c 300 290 ma yielded well constrained stephanian to sakmarian ages the largest late paleozoic ignimbrite forming eruption in central europe the rochlitz ignimbrite has a well defined middle asselian age of 294 4 1 8 ma ages of palingenic zircon revealed that the namurian westphalian magmatism assimilated larger amounts of crystalline basement that formed during previous paleozoic geodynamic phases the precambrian inherited ages support the chronostratigraphic structure assumed for the saxo thuringian zone of the variscan orogen the present results help to improve the chronostratigraphic allocation of the late paleozoic volcanic zones in central europe at the same time the radiometric ages have implications for the interbasinal correlation and for the geodynamic evolution of the variscan orogeny

variscan silicic magmatism and related tin tungsten mineralization in the erzgebirge slavkovský les metallogenic province integration of geochemical mineralogical isotopic and geochronological data with geodynamic considerations suggests that the variscan granites in the erzgebirge slavkovsk les domain originated from repeated melting events and were emplaced over a period of about 40 ma 330 290 ma several lines of evidence exist supporting the idea that erzgebirge granites assigned to different types biotite granites two mica granites strongly peraluminous p rich li mica granites and slightly peraluminous p poor granites are in most cases not genetically related via continuous fractional crystallization from a common magmatic reservoir the genesis of the slavkovsky les granites however might be discussed in terms of an uninterrupted fractionation series geological models of sn w deposits based upon geochemical and structural results imply that the main ore depositional events followed immediately the emplacement and solidification processes of melt via fluid melt immiscibility breccia pipe formation and or pervasive rock fluid interactions

gases and helium isotopes in high temperature solutions sampled before and after odp leg 158 drilling at tag hydrothermal field 26 n mar hydrothermal fluids were sampled for dissolved gases at tag 26 n mar during two alvin dive series in april may 1993 and march 1995 respectively 17 months before and 4 months after ocean drilling program leg 158 total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude even if some increase in h2s c02 and ch4 was noticeable in the 1995 samples no significant difference was observed in helium concentration and helium isotopic ratio 3 he 4 he 8 2 0 1 ra the ch4 3 he ratio found in black smokers sampled in 1993 is around 9 x 10 6 close to ratio found in mar basalts in 1995 this ratio is around 4 times higher due to the ch4 increase the 13 c in c02 measured on two black smokers sampled in 1993 is uniform at 8 4 to 8 8 versus pdb while a 13 c value of 13 is measured at a new site sampled in 1995 located 40 meters east of the main black smokers complex bsc ch4 3 he and 13 c values both point to a mainly magmatic abiogenic origin of the carbon species in the system overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium c02 and ch4 of magmatic origin even if some thermogenic ch4 contribution seems likely in the 1995 samples collected in the new active area we can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the tag mound and temporarily enhanced the hydrothermal circulation by opening new pathways favouring transient input of ch4 rich fluids before coming back to a new steady state

inferring protoliths of high grade metamorphic gneisses of the erzgebirge using zirconology geochemistry and comparison with lower grade rocks from lusatia saxothuringia germany protoliths of highly metamorphosed gneisses from the erzgebirge are deduced from the morphology age and chemistry of zircons as well as from whole rock geochemistry and are compared with lower grade rocks of lusatia gneisses with similar structural appearance and or geochemical pattern may have quite different protoliths the oldest rocks in the erzgebirge are paragneisses representing meta greywackes and meta conglomerates the youngest group of zircon of meta greywackes that did not undergo pb loss represents the youngest igneous component for source rocks about 575 ma similar ages and zircon morphology reflect the subordinate formation of new zircon grains or only zircon rims in the augengneiss from barenstein and wolkenstein which probably represent metamorphic equivalents to lower cambrian two mica granodiorites from lusatia bulk rock chemistry intense fracturing and high u and th concentrations of zircons suggest deformation induced and fluid enhanced recrystallisation of zircon grains temperatures during tectonic overprinting too low to reset zircon ages indicate mid or upper crustal levels for shearing recorded in these augengneisses lower cambrian 540 ma granodiorites are widespread in lusatia but are exclusively represented by the freiberg gneiss dome in the eastern erzgebirge ordovician protolith ages were recorded by zircons from the augengneisses of the reitzenhain catherine dome and the schwarzenberg dome western erzgebirge documenting significant regional differences between the eastern and the western erzgebirge 540 vs 490 ma in the western erzgebirge most meta volcanic rocks muscovite gneisses and meta granites mainly red augengneisses yield ordovician zircon ages whereas in the eastern part similar rocks mainly recorded lower cambrian protolith ages zircon overprinting was highest within discrete tectonic zones where the combination of fluid infiltration and deformation induced variable degrees of recrystallisation and formation of a new augengneiss structure variable degrees of pb loss caused age shifts that do not correspond to changes in zircon morphology but may be associated with u and th enrichments major changes in bulk rock composition appear to be restricted to discrete zones and to u hp nappes whereas gneisses with a mp mt metamorphic overprint basically show no geochemical modifications

evolution of the cornubian ore field southwest england part i batholith modeling and ore distribution the cornubian ore field of southwest england is associated with an s type ilmenite series high heat production hhp two mica tourmaline bearing monzogranite batholith which formed between 270 to 295 ma base metal deposits are mainly in the form of steeply dipping lodes emplaced in the batholith roof and a hornfelsic sequence of upper paleozoic mud rocks sandstones and mafic volcanics the steep sided batholith has a gently sloping base 10 to 15 km below the present surface three dimensional modeling of gravity data indicates that it has a total volume of about 68 000 km 3 and is divisible into distinct western and eastern parts computer aided evaluation of mineral production statistics shows that most of the sn w cu and as production is located within the axial trace of the batholith with the western end enriched in sn cu and zn relative to the eastern end by contrast most of the fe as sb f ba and china clay production is located in the eastern end while pb and ag production is concentrated in the country rock filled south facing embayments on the flank of the batholith all significant wolframite production is from within the granite or within a 700 m distance of an observed or projected granite contact whereas 90 to 100 percent of the sn cu and as production occurs within 1 500 m of a contact by contrast 30 percent of the zn production and over 80 percent of the pb and ag production is from distances of more than 1 500 m from a granite contact pre syn and postbatholith faults and extensional fractures exert a fundamental control on ore distribution with most of the sn w cu as zn ores hosted by fractures trending parallel to the axial trace of the batholith and most of the pb sb ag ba and fluorite mineralization hosted by faults trending approximately normal to the axial trace of the batholith the nature of the sedimentary sequence also influenced metal distribution with most of the sn cu as production coming from areas underlain by devonian argillaceous rocks an important feature of the granite which has a bearing on its metallogenic potential is its high internal heat production due to its high total content of u th and k modeling of the cooling history of the granites reveals that temperatures in the deeper parts of the batholith were maintained close to or above the solidus for 10 to 20 ma after initial emplacement at 280 to 295 ma residual volatile and metal enriched magmas trapped within the batholith provided source materials for younger magmatism 270 280 ma and the main episode of lode formation which was related to the late magmatic events continued high heat flow throughout the mesozoic and cainozoic promoted thermally driven ground water convection and epithermal mineralization

origin of hydrogen nitrogen gas seeps oman six gas samples were collected from five thermal springs in the semail nappe ophiolite and the calcareous calcite and dolomite hajar formation northern oman the3he 4he 4he 20ne 40ar 36ar and38ar 36ar ratios chemical compositions h2 n2 co2 ch4 o2 ar and he and stable isotope compositions dh2 dh2o 13cco2 13cch4 and 15nn2 are reported samples from the ophiolite region are significantly anoxic with major constituents of h2 ch4 and n2 while those from calcite and dolomite regions are ordinary gas seeps consisting of n2 co2 and or o2 the former h2 rich gas is characterized by relatively high3he 4he ratio 0 4 0 8 ratm with low he content 300 ppm slight radiogenic40ar 36ar ratio high n2 ar ratio 77 97 and low 15nn2 value 0 observed dh2 value of 536 in h2 rich gas is distinguished from the literature value of 699 in the ophiolite region giving discrepant isotope formation temperatures

pb pb and u pb zircon dating of subvolcanic rhyolite as a time marker for hercynian granite magmatism and sn mineralisation in the eibenstock granite erzgebirge germany considering effects of zircon alteration tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo and exocontact of the eibenstock granite erzgebirge germany the same structures which cut the granite control the rhyolite emplacement and the location of ore bearing greisen bodies albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock comprehensive zircon studies by scanning electron microscopy secondary and backscattered electron as well as cathodoluminescence imaging edx measurements reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity including distortion of the u pb isotope system when affected by greisenisation single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a 207pb 206pb age of 290 5 ma whereas zircon from little altered granite gave a 207pb 206pb age of 320 8 ma the single evaporation age of zircon from rhyolite is confirmed by 238u 206pb shrimp ages of 297 8 ma the significant time gap of at least 20 ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the eibenstock granite cannot be regarded as a source for tin ore forming fluids as previously assumed

devgaon h3 chondrite classification and complex cosmic ray exposure history the devgaon meteorite fell in india on february 12 2001 and was immediately collected it is an ordinary chondrite having a number of sio2 rich objects and some ca al rich inclusions olivines fa17 19 are fairly equilibrated while pyroxenes fs4 20 are unequilibrated occasionally shock veins are visible but the bulk rock sample is very weakly shocked s2 chondrules and chondrule fragments are abundant based on chemical and petrological features devgaon is classified as an h3 8 group chondrite several cosmogenic radionuclides ranging in half lives from 5 6 d 52mn to 7 3 x 10 5 yr 26al noble gases he ne ar kr and xe and particle track density have been measured the track density in olivines from five spot samples varies between 4 6 to 9 x 10 6 cm 2 showing a small gradient within the meteorite the light noble gases are dominated by cosmogenic and radiogenic components large amounts of trapped gases ar kr and xe are present in addition n products from br and i are found in kr and xe respectively the average cosmic ray exposure age of 101 8 ma is derived based on cosmogenic 38ar 83kr and 126xe the track production rates correspond to shielding depths of about 4 9 to 7 8 cm indicating that the stone suffered type iv ablation low 60co high 22ne 21ne c and large neutron produced excesses at 80kr 82kr and 128xe indicate a complex exposure history of the meteoroid in the first stage a meter sized body was exposed for nearly 108 yr in the interplanetary space that broke up in 50 cm sized fragments about a million years ago stage 2 before it was captured by the earth

skarn formation and tin enrichment during regional metamorphism the hammerlein polymetallic skarn deposit abstract the hammerlein sn rich polymetallic skarn deposit erzgebirge germany which is hosted by variscan schists and gneisses includes garnet amphibole pyroxene and magnetite skarns that formed during regional metamorphism at 340 ma the skarn deposit was overprinted during the intrusion of the ca 320 ma eibenstock granite tin was introduced into the system during regional metamorphism at 340 ma sn mostly hosted in silicate minerals and after the intrusion of the eibenstock granite at c 320 ma sn hosted in cassiterite the emplacement of late stage melts of the eibenstock granite also resulted in the formation of cassiterite quartz veins in the schists hosting the hammerlein skarns and at some distance in the formation of several cassiterite greisen major trace and rare earth element data of the skarn units and their hosts rocks demonstrate that skarns formed from impure carbonates with variable proportions of silicate minerals depending on the original modal compositions of the skarns and their host rocks the mineralizing fluid selectively added and removed elements which resulted for the skarns and the schist ore in a general enrichment in sn in cu and w and a noticeable depletion in al and sr the sr nd and pb isotopic composition of the skarns schists and gneisses resembles those of the metamorphic rocks of the erzgebirge rather than those of the granites possibly indicating that the geochemical signature of granite derived fluids had been severely altered during fluid rock interaction before these fluids reached the skarns schists and gneisses similarly the li and b isotopic composition of the skarn samples reflects the likely range of possible source rocks for metamorphic fluids rather than the composition of the granite derived fluid in contrast the li and b signature of greisen samples shows a variable mixture of material derived from the granite and the metasedimentary rocks respectively our data show that a first sn enrichment took place when the paleozoic sedimentary rocks were affected by regional metamorphism at ca 340 ma fluids derived from these siliciclastic sedimentary protoliths mobilized and transported sn and fe these fluids reacted with the impure carbonates resulting in decarbonation and the formation of calc silicate minerals and biotite in the various skarn types the changing fluid composition as well as the incorporation of fe in skarn silicates and formation of fe oxides resulted in the incorporation of sn into silicate minerals during or after the emplacement of the eibenstock granite the original skarn was to a variable extent retrogressed and sn was released from sn bearing calc silicates and biotite during this event additional sn was added to the skarn and the hosting schists released sn and newly added sn precipitated as cassiterite

dioritic intrusions of the slavkovský les kaiserwald western bohemia their origin and significance in late variscan granitoid magmatism mafic and intermediate intrusions occur in the slavkovsk les as dykes sills and minor tabular bodies emplaced in metamorphic rocks or enclosed in late variscan granites near the sw contact of the western krusne hory erzgebirge granite pluton they are similar in composition and textures to the redwitzites defined in ne bavaria single zircon pb evaporation analyses constrain the age of a quartz monzodiorite at 323 4 4 4 ma and of a granodiorite at 326 1 5 6 ma the p t range of magma crystallization is estimated at 1 4 2 2 kbar and 730 870 c and it accords with a shallow intrusion level of late variscan granites but provides lower crystallization temperatures compared to the bavarian redwitzites we explain the heterogeneous composition of dioritic intrusions in the slavkovsk les by mixing between mafic and felsic magmas with a minor effect of fractional crystallization increased k ba rb sr and ree contents compared to tholeiitic basalts suggest that the parental mafic magma was probably produced by melting of a metasomatised mantle the melts being close to lamprophyre or alkali basalt composition diorites and granodiorites originated from mixed magmas derived by addition of about 25 35 and 50 vol respectively of the acid end member granite to lamprophyre or alkali basalt magma our data stress an important role of mafic magmas in the origin of late variscan granitoids in nw bohemian massif and emphasize the effect of mantle metasomatism on the origin of k rich mafic igneous rocks

phosphate minerals in the h group of ordinary chondrites and fluid activity recorded by apatite heterogeneity in the zag h3 6 regolith breccia phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids we have carried out a petrographic study of the phosphate minerals merrillite and apatite in metamorphosed h group ordinary chondrites of petrologic type 4 6 to understand development of phosphate minerals and associated fluid evolution during metamorphism in unbrecciated chondrites apatite is cl rich and shows textural evolution from fine grained apatite merrillite assemblages in type 4 toward larger uniform grains in type 6 the cl f ratio in apatite shows a similar degree of heterogeneity in all petrologic types and no systematic change in compositions with metamorphic grade which suggests that compositions in each meteorite are dictated by localized conditions possibly because of a limited fluid rock ratio the development of phosphate minerals in h chondrites is similar to that of l and ll chondrites despite the fact that feldspar equilibration resulting from albitization is complete in h4 chondrites but not in l4 or ll4 chondrites this suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions and chemical equilibrium was not achieved throughout the h chondrite parent body or bodies during the late stages of metamorphism a relict igneous clast in the h5 chondrite oro grande has apatite rims on relict phenocrysts of possibly diopside that have equilibrated with the host chondrite apatite in the zag h3 6 regolith breccia records a complex fluid history which is likely related to the presence of halite in this meteorite the porous dark h4 matrix of zag where halite is observed has a high apatite merrillite ratio and apatite is extremely cl rich one light h6 clast contains similarly cl rich apatite in a second light h6 clast apatite compositions are very heterogeneous and more f rich apatites in both h4 matrix and h6 clasts have very low h 2o contents heterogeneous apatite compositions in zag record multiple stages of regolith processing and shock at the surface of the h chondrite parent body and apatite records either the passage of fluids of variable compositions resulting from different impact related processes or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies with implications for physical and mechanical properties of chondritic asteroids the behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general if impact melt materials close to the surface of a chondritic asteroid are readily degassed the volatile inventories of terrestrial planets could be considerably more depleted than the ci carbonaceous chondrite abundances that are commonly assumed

helium and methane measurements in hydrothermal fluids from the mid atlantic ridge the snake pit site at 23 n methane and helium isotopes concentrations have been measured in hydrothermal fluids of black smoker vents at the snake pit area mar 23 n as part of the hydrosnake cruise with the submersible nautile the samples contain large quantities of helium and methane with endmember values of 4 6 10 5 cm3 stp g for helium and 1 38 10 3 cm3 stp g for methane the helium isotopic ratio 3he 4he is 8 4 times the atmospheric ratio these values are strikingly similar to those already obtained on the east pacific rise the ch4 3he ratio is also quite similar ch4 3he 2 6 106 suggesting a non biogenic origin of this gas in the fluids assuming that the range of variation of the 3he concentration data obtained so far from various tectonic settings is close to the genuine range of values encountered on mid ocean ridges the global mass flow involved in the hydrothermal circulation at the ridge s axis is estimated between 1000 and 3000 m3 of water sec leading to a residence time of the global ocean with respect to the hydrothermal circulation between 15 and 45 million years this estimate implies that in addition to the individual vents smokers there is an extensive diffuse circulation of hydrothermal fluids along the ridges in agreement with recent observations concerning heat budgets at hydrothermal sites in the eastern pacific

serpentinization and the origin of hydrogen gas in kansas hydrogen gas occurs in ten kansas wells near the mid continent rift system since 1982 two wells have yielded small amounts of gases containing an average of 29 37 mole h2 the remainder being chiefly n2 with only traces of hydrocarbons isotopic compositions for hydrogen dgrd 740 to 836 pmil imply near ambient about 10 c equilibration temperatures for the gases which are among the most deuterium depleted in nature and resemble the h2 rich gases described from ophiolites in oman isotopic values for the kansas n2 differ slightly from those of the atmosphere but not enough to rule out an atmospheric origin because they are low in ch4 and co2 expected byproducts of biogenic activity the gases are probably abiogenic in origin the existence of such gases near a major rift system containing mafic rocks and not far from known kimberlites is consistent with an origin from reactions involving fe 2 oxidation for example during serpentinization because the gases may be associated with kimberlites and deep seated rifting mantle outgassing is possible but such an origin would be difficult to reconcile with the low isotopic temperatures the h2 gases from kansas and elsewhere seem to be too low in pressure to have commercial value however neither the kansas gases nor those from other h2 occurrences have been adequately examined to assess their importance as potential resources

interactions between magma chambers and hydrothermal systems oceanic and ophiolitic constraints a compilation of published exit temperatures and salinities for seafloor hydrothermal fluids along with fluid inclusion data on oceanic and ophiolitic rocks shows that ridge axes hydrothermal systems are generally characterized by maximum temperatures around 300 360 c thus clearly being in the one phase field they overlie a lower system marked by the generation of highly saline brines at high temperatures the brines may have originated either when seawater approached the top of the magma chamber or more probably as exsolution products of the differentiated melts that typify the roof of oceanic magma chambers in ophiolites the brines typically occur within the transition zone between the sheeted dike complex and the plutonic sequence i e on top of magma chambers this transition zone relatively well known from ophiolite studies oman troodos bay of islands josephine is characterized by mutually intrusive relationships between gabbro plagiogranite and dikes and xenoliths of altered diabase within plagiogranite and gabbro in fast to medium spreading ophiolites oman it is the locus of a major downward decrease in the density of hydrothermal veins and alteration the vein system is strongly anisotropic and shows a well marked preferred vertical along strike orientation the underlying cumulates are almost unaltered and exhibit only a diffuse net of amphibole veins while the overlying sheeted dike complex is densely veined and pervasively altered this zone is marked by major gradients in the physical parameters over short distances temperature several degrees celsius per meter pressure several hundred kilopascals transition from brittle to ductile in steady state systems direct interactions between the magma chamber and the hydrothermal system are restricted to this zone characterized by exsolution of magmatic volatiles into the hydrothermal system and the incorporation of hydrothermal fluids into the magma chamber by digestion of altered roof rocks complications to this steady state model resulting from a non steady state magma chamber e g josephine ophiolite are mainly the development of late stage faulting and more pronounced retrogressive alteration

reduction of co2 during serpentinization of olivine at 300 c and 500 bar co 2 reduction processes occurring during experimental serpentinization of olivine at 300 c and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas data reveal that conversion of fe ii in olivine to fe iii in magnetite during serpentinization leads to production of h 2 and conversion of dissolved co 2 to reduced c species including methane ethane propane and an amorphous carbonaceous phase hydrocarbon gases generated in the process fit a schulz flory distribution consistent with catalysis by mineral reactants or products magnetite is inferred to be the catalyst for methanization during serpentinization because it has been previously shown to accelerate fischer tropsch synthesis of methane in industrial applications involving mixtures of h 2 and co 2 the carbonaceous phase was predominantly aliphatic but had a significant aromatic component although this phase should ultimately be converted to hydrocarbon gases and graphite if full thermodynamic equilibrium were established its formation in these experiments indicates that the pathway for reduction of co 2 during serpentinization processes is complex and involves a series of metastable intermediates

sylvite and halite on particles recovered from 25143 itokawa a preliminary report we observed cross sectional ultra thin sections near the surface of 12 particles recovered from the s type asteroid itokawa by the hayabusa spacecraft in 2010 using spherical aberration corrected stem and conventional tem although their mineralogy is almost identical to the equilibrated ll chondrites and therefore basically anhydrous micrometer to submicrometer sized sylvite was identified on the surface of itokawa particle ra qd02 0034 separately micrometer sized halite was also identified on the surface of itokawa particle ra qd02 0129 detailed inspection of the sample processing procedures at the jaxa s planetary materials sample curation facility and textural observation of the sylvite and halite indicate that they were clearly present on two itokawa particles before they were removed from clean chamber 2 at jaxa however there is no direct evidence for their extraterrestrial origin at present if the sylvite and halite are extraterrestrial their presence suggests that they may be more abundant on the surface of s type asteroids than previously thought

the chemical composition of ree y th u rich accessory minerals in peraluminous granites of the erzgebirge fichtelgebirge region germany part i the monazite ce brabantite solid solution series peraluminous granites of the erzgebirge fichtelgebirge germany are hosts of various members of the monazite group of minerals that display an unprecedented compositional diversity the eibenstock s type granite constitutes the third reported occurrence worldwide of brabantite and the first occurrence of this mineral in a granite many new occurrences of cheralite ce as well as a monazite group mineral intermediate between monazite ce and huttonite for which the term huttonitic monazite is proposed were discovered even common monazite ce may show extreme ranges of actinide and lanthanide element concentrations the granites that host brabantite and cheralite ce are highly differentiated strongly peraluminous low temperature residual melts of s type affinity which are rich in fluorine and other volatile constituents but depleted in thorium and the light rare earth elements such highly evolved volatile rich compositions resemble rare element pegmatites and appear favorable for the precipitation of cheralite ce and brabantite but not of monazite with large amounts of huttonitic substitution instead these minerals occur preferentially in f poor biotite and f rich li mica granites of a type affinity irrespective of the level of uranium in silicate melts which may exceed that of thorium the substitution of uranium in monazite remains limited the compositional data reported here are consistent with complete miscibility in the monazite ce brabantite solid solution series under magmatic conditions these granites contain monazites that span almost the entire compositional range reported for monazite group minerals worldwide and therefore granites appear to be ideal rocks in which to study the crystal chemistry of this mineral group in general

kinetics of pb release during the zircon evaporation technique abstract the release of silica and pb from zircon by the thermal evaporation technique is shown to relate to the conversion of zircon to a porous baddeleyite structure the zircon baddeleyite reaction occurs at a sharp reaction front which progressively migrates towards the centre from the grain margin significant reaction in non metamict zircon commences in the mass spectrometer at temperatures of the order of 1500 c the progressive migration of zircon baddeleyite transformation towards grain interiors explains why the evaporation technique with its inherent advantage of not requiring chemical separation of pb and u progressively samples pb in zones towards the centre of the grain and gives 207 pb 206 pb ages as accurate as conventional u pb dating methods the rates of pb and silica loss are primarily determined by the rate of advance of this de silicification reaction front towards the interior of the grain most estimates of pb diffusion coefficients in zircon imply that diffusion distances in the unreacted zircon are negligible 10 2 m at the temperatures and duration of the pb evaporation experiments isothermal experiments demonstrate the rate of zircon baddeleyite transformation with time and temperature and provide an estimate of 130 10 1 kcal mol 1 for the activation energy of the process significantly higher than estimates of pb diffusion activation energy

iodine xenon analysis of ordinary chondrite halide implications for early solar system water we report the results of iodine xenon analyses of irradiated halide grains extracted from the h chondrite monahans 1998 and compare them with those from zag whitby et al 2000 to address the timing of aqueous processing on the h chondrite parent body xe isotopic analyses were carried out using the relax mass spectrometer with laser stepped heating the initial 129i 127i ratio in the monahans halide was determined to be 9 37 0 06 10 5 with an iodine concentration of 400 ppb significant scatter especially in the zag data indicates that a simple interpretation as a formation age is unreliable instead we propose a model whereby halide minerals in both meteorites formed 5 ma after the enstatite achondrite shallowater at an absolute age of 4559 ma this age is in agreement with the timing of aqueous alteration on the carbonaceous chondrite parent bodies and ordinary chondrite metamorphism and is consistent with the decay of 26al as a heat source for heating and mobilisation of brines on the h chondrite parent body post accretion surface impact events may have also contributed to the heat source

applications of single grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites abstract 207 pb 206 pb ages determined by evaporation of single zircon grains from rocks of the new england appalachians in the usa permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age whose u pb upper intercept zircon ages obtained through isotope dilution methods show profound post crystallization discordance demonstrate that the evaporation method is successful in recovering protolith ages uncertainties are 10 15 my for rocks of paleozoic age or older preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics 207 pb 206 pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the new england appalachians reveal the distinct provenances of these rocks the missisquoi formation in southeastern vermont usa is comprised in part of detritus derived from the 1 0 1 2 ga grenvillian province of laurentia whereas this component is conspicuously absent from the plainfield formation in easternmost connecticut the detrital zircon age data suggest that the missisquoi formation accumulated peripheral to the laurentian craton during the early paleozoic whereas the plainfield formation detritus was supplied by non laurentian ossibly west african sources during the late proterozoic or early paleozoic in southern vermont the evaporation method successfully distinguishes between two groups of augen gneisses with ages of 960 ma and 1120 ma which were first recognized through isotope dilution methods collectively these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

variscan sn w mo metallogeny in the gravity picture of the krusne hory erzgebirge granite batholith central europe abstract late variscan wolframite molybdenite and cassiterite wolframite greisen skarn and vein deposits occur in a close spatial association with the granites of the krusne hory erzgebirge batholith kheb in central europe we examined the distribution of the deposits in relation to the gravity field affected by late variscan granites using the data from previous gravity and metallogenic studies late variscan granites are differentiated into earlier biotite monzogranites low f granites and later biotite or lithium mica syenogranites high f granites in accordance with the previous classifications all the outcrops of granites in the kheb region and their hidden continuation are confined to the bouguer anomaly contour of 20 mgal the sn w mo rare metal deposits and occurrences are within the gravity contour of 30 mgal with the exception of the grossschirma stratiform tin deposit in the freiberg polymetallic ore district we constructed a geological model based on the gravity data along two profiles across the kheb showing the position of some rare metal deposits and of outcropping and hidden granite bodies the models show that the overlapping of earlier and later granites is in the areas of the most intense regional gravity minima these coincide with the eastern volcano plutonic complex altenberg minimum which encloses large volumes of felsic extrusives microgranite dikes and granites and the western plutonic complex eibenstock minimum with small volumes of felsic dikes and predominance of earlier and later granites with no extrusives preserved there is no distinct relationship between the masses of late variscan granites and the distribution and the sizes of associated w mo and sn w deposits we prefer the idea that rare metal mineralization was formed by hydrothermal fluids derived from outside of presently outcropping granites it originated in two cycles one connected with the formation of earlier granites producing w mo associations and the other one associated with later granites connected with sn w mineralization mineralizing fluids were probably generated by mantle crustal interaction in the crust near the mantle crust boundary as also indicated by lamprophyric intrusions coeval with the late variscan granitic magmatism

the age of the latest thermal overprint of tin and polymetallic deposits in the erzgebirge germany constraints from fluorite u th sm he thermochronology the erzgebirge region of germany records two major episodes of hydrothermal activity which includes a late carboniferous to early permian event associated with significant sn w mineralization that is related to late variscan granite magmatism and a mesozoic episode of polymetallic vein mineralization in contrast to the first event the age of the younger hydrothermal activity is poorly constrained for the latter various geochronological methods yielded a wide age range from permian to early tertiary here we apply fluorite u th sm he thermochronology fhe on both types of mineralization with a twofold goal 1 to investigate the sensitivity and applicability of the new fhe method evans et al 2005 and 2 to constrain the thermal history of ore deposits in the erzgebirge region two hundred and thirty three aliquots from seven mineralization localities have been dated fluorite from six deposits yielded cretaceous fhe ages between 112 to 79 ma which are independent of their paragenesis in contrast fluorite from the sadisdorf sn w deposit yielded an age of 234 ma the younger ages are interpreted as cooling ages indicating the time of the last thermal overprint including possible hydrothermal activity in the erzgebirge the oldest triassic fhe ages at sadisdorf indicate that the mesozoic thermal overprint only partially reset the u th sm he system of the late variscan mineralization thermal modeling based on fhe ages and he diffusion parameters in fluorite results in thermal histories comparable to the results from the well established apatite based thermal modeling this study emphasizes the applicability of fluorite u th sm he thermochronology which is especially suited for ore deposits where apatite is lacking

isotopic search for live 135cs in the early solar system and possibility of 135cs 135ba chronometer abstract here we report the isotopic excess of 135 ba studied from ba isotopic measurements of acid leachates of allende calcium aluminum rich inclusions cais and two primitive chondrites beardsley h5 and zag h6 clasts from the excess 135 ba abundances of cais after subtraction of the r process nucleosynthetic component in a model dependent manner a value for 135 cs 133 cs 4 8 0 8 10 4 is proposed as the initial ratio in the early solar system a correlation between excess 135 ba abundances and cs ba ratios observed in the beardsley and the zag meteorites reveals the possible existence of primordial 135 cs in the two meteorite parent bodies on the assumption of two cases for 135 cs 133 cs initial in the early solar system the 135 cs 135 ba isochron suggests that primitive aqueous alteration in the beardsley and zag meteorite bodies occurred at 8 2 11 9 ma and 13 9 17 6 ma after cai formation respectively our results are apparently consistent with previously reported data from pb pb model age determinations and 53 mn 53 cr chronometry for chondrites this is the first report of isotopic excess of 135 ba in meteorites which may relate to live 135 cs in the early solar system and the 135 cs 135 ba chronological application

cassiterite u pb geochronology constrains magmatic hydrothermal evolution in complex evolved granite systems the classic erzgebirge tin province saxony and bohemia

the hammerlein skarn hosted polymetallic deposit and the eibenstock granite associated greisen western erzgebirge germany two phases of mineralization two sn sources the hammerlein polymetallic deposit is hosted in skarn schists and gneisses that reached their metamorphic peak at 340 ma during the variscan orogeny the deposit is spatially closely associated with one of the most voluminous granites of the erzgebirge the eibenstock granite which intruded the metamorphic units at 320 ma and locally also developed greisen mineralization cassiterite is the main ore mineral in the hammerlein skarn and in greisen mineralizations associated with the eibenstock granite the age of skarn formation is bracketed by multi mineral rb sr isochron ages of the gneisses 340 ma and the end of ductile deformation 330 ma the dated calc silicate minerals of the skarn have elevated sn contents which implies that some sn was present in the system during regional metamorphism i e well before the emplacement of the eibenstock granite tin in the 330 ma old skarn silicates possibly was mobilized from the metamorphic wall rocks retrogression of the skarn mineral assemblage may have released some sn that formed cassiterite in an assemblage with chlorite and fluorite the sr isotope signatures of fluorite indicate that this late assemblage is not related to cooling of the metamorphic rocks but to the emplacement of the eibenstock granite which introduced additional sn into the skarn thus mineralization in the hammerlein deposit includes sn that was introduced during two different events from different sources

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

precise electron microprobe age determination in altered uraninite consequences on the intrusion age and the metallogenic significance of the kirchberg granite erzgebirge germany an integral method of data evaluation for chemical dating of accessory uraninite by electron microprobe analysis and electron microscopy is proposed the internal structure of uraninite grains the contents of si ca and fe and tests for loss or gain of pb and u are suitable criteria to decide whether or not a given crystal area was affected by secondary alteration the method was applied to uraninite from the kirchberg granite erzgebirge germany and an age of 330 5 ma 2 was constrained this result supports earlier assumptions that there were at least two events of hercynian granite magmatism separated by a time gap of about 40 ma and that the magmatic evolution of the kirchberg granite is not related to the formation of the w mineralisation developed in the endocontact of this granite secondary alteration of uraninite is probably closely related to the formation of u deposits in the vicinity of the kirchberg massif which was extensively mined in the past

widespread hydrothermal alteration minerals in the fine grained matrices of the tieschitz unequilibrated ordinary chondrite mineralogic textural and compositional studies of black and white matrices in the unequilibrated ordinary chondrite tieschitz h l 3 6 show for the first time in an ordinary chondrite the presence of widespread randomly distributed geode like voids and veins scanning electron microscope sem and transmission electron microscope tem studies show that these voids and veins are partially or completely filled by sodic calcic amphiboles winchite and barroisite the occurrence of amphiboles provides unequivocal evidence of the involvement of fluids in the metamorphic evolution of the parent body of tieschitz the presence of amphiboles as the main hydrous phases rather than phyllosilicates indicates that aqueous fluids were present at or close to the peak of thermal metamorphism rather than during the waning stages of the cooling history of the parent body in addition ferrous olivine crystals in association with the amphibole also establish an important link between thermal metamorphism and hydrous phases formed at high temperatures mineralogic and textural evidence suggests that the white matrix and amphibole formed contemporaneously from the same hydrous fluid prior to the formation of ferrous olivine crystals additionally a dark inclusion identified in the host chondrite has mineralogic petrologic and bulk chemical characteristics that are similar to the black matrix of host tieschitz suggesting that this dark inclusion was emplaced before or during parent body metamorphism

the search for and analysis of direct samples of early solar system aqueous fluids we describe the current state of the search for direct surviving samples of early inner solar system fluids fluid inclusions in meteorites meteoritic aqueous fluid inclusions are not rare but they are very tiny and their characterization is at the state of the art for most analytical techniques meteoritic fluid inclusions offer us a unique opportunity to study early solar system brines in the laboratory inclusion by inclusion analyses of the trapped fluids in carefully selected samples will in the immediate future provide us detailed information on the evolution of fluids as they interacted with anhydrous solid materials thus real data can replace calculated fluid compositions in thermochemical calculations of the evolution of water and aqueous reactions in comets asteroids moons and the terrestrial planets this article is part of the themed issue the origin history and role of water in the evolution of the inner solar system

noble gases and halogens in graves nunataks 06129 the complex thermal history of a felsic asteroid crust the meteorite graves nunataks 06128 06129 is a rare example of felsic asteroidal crust knowledge of its history can help shed light on the evolution processes of planetesimals the noble gases can be used to constrain both the chronology of meteorites and the processes that result in movements of volatile elements on asteroidal bodies we have examined the i xe and ar ar systems of the plagioclase rich achondrite graves nunataks 06129 by high resolution laser step heating of irradiated samples iodine and 129xe are both present but are released at different temperatures and do not show a correlation therefore the i xe system in gra 06129 has no chronological significance we propose that radiogenic 129xe was lost from primary phases and parentless 129xe was later introduced into the rock by interaction with a fluid sourced from a reservoir that evolved with a high i xe ratio this could have been the same halogen rich fluid that induced the conversion of merrillite and pyroxene into chlorapatite inherited 40ar i e not generated by in situ decay of 40k is also present in one of three fragments studied here and may have been introduced at the same time as parentless 129xe

the source and significance of argon isotopes in fluid inclusions from areas of mineralization argon isotopes in fluid inclusions in quartz veins associated with granite hosted tungsten mineralization in the southwest and north of england have been investigated in detail by the40ar 39ar technique the natural argon is present as a number of discrete components which can be identified through correlations with39ar 38ar and 37ar induced by neutron bombardment of potassium chlorine and calcium the potassium correlated component arises principally from in situ decay of potassium in solid phases in the inclusions in the case of the hemerdon tungsten deposit of southwest england the phases responsible are small 25 m captive authigenic micas which are shown to have been deposited from a fluid 268 20 ma ago shortly after the emplacement of the host granite the chlorine correlated component is present in the brines which constitute the fluid phase of the inclusions the argon in these hydrothermal fluids is made up in part of parentless or excess 40ar leached from surrounding crustal rocks and in part of dissolved ancient atmospheric argon absolute concentrations of both atmospheric and excess components in the brine can be estimated from 40arcl ratios and independent determinations of the salinity of the inclusions the absolute concentrations of the atmospheric argon are close to those found in modern meteoric water while those of the excess component can be interpreted in terms of the degree of interaction betwen the circulating fluids and country rock a calcium correlated component with a much higher ratio of excess to atmospheric argon than that in the brine was found to be a dominant phase in one sample from the hemerdon deposit indicating the presence of a solid phase probably a caso4 daughter mineral inclusions of this composition represent fluids which have had a more prolonged interaction with crustal rocks the results obtained from this study provide a systematization and a framework for future multi component argon studies of fluid inclusions together with an indication of the wide range of information which can be inferred

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

extinct 129i in halite from a primitive meteorite evidence for evaporite formation in the early solar system halite crystals from the zag h3 6 chondrite contain essentially pure monoisotopic xenon 129 129xe produced in the early history of the solar system by the decay of short lived iodine 129 129i half life 15 7 million years correlated release of 129xe and 128xe produced artificially from 127i by neutron irradiation corresponds to an initial 129i 127i ratio of 1 35 0 05 10 4 close to the most primitive early solar system value if the 129xe was produced by in situ decay then the halite formed from an aqueous fluid within 2 million years of the oldest known solar system minerals

discrimination of protolithic versus metamorphic zircon ages in eclogites constraints from the erzgebirge metamorphic core complex germany abstract protolithic ages of meta basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans however in high grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint the mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature zircon from eclogites from the eastern erzgebirge metamorphic complex yielded both protolithic 540 ma 5 samples and metamorphic 340 ma 4 samples 207 pb 206 pb ages detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole plagioclase symplectite rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of zr nb lree and p eclogites with omphacite breakdown are very dark in colour in difference lighter coloured eclogites contain k feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite this supports additional new growth of zircon grains in some samples idiomorphic zircon grains with a lot of micro inclusions indicate a coupled dissolution reprecipitation process additional rounded resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth therefore scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth in contrast protolithic zircon ages were found in eclogites where no water or melt infiltration occurred this emphasizes the importance of water and or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies in the eastern erzgebirge most eclogites and amphibolites were derived from morb type or arc related basalts while the plagioclase rich amphibolites formed mainly from crustal sources the protolith ages of meta basites from the eastern erzgebirge 540 ma document a bimodal magmatic activity in a cadomian back arc basin

halite and stable chlorine isotopes in the zag h3 6 breccia zag is an h3 6 chondrite regolith breccia within which we have studied 14 halite grains 3mm the purity of the associated nacl h2o brine is implied by freezing characteristics of fluid inclusions in the halite and epma analyses together with a lack of other evaporite like phases in the zag h3 6 component this is inconsistent with multistage evolution of the fluid involving scavenging of cations in the zag region of the parent body we suggest that the halite grains are clastic and did not crystallise in situ halite and water soluble extracts from zag have light cl isotopic compositions 37 cl 1 4 to 2 8 i previously reported bulk carbonaceous chondrite values are approximately 37 cl 3 to 4 i this difference is too great to be the result of fractionation during evaporation and instead we suggest that cl isotopes in chondrites are fractionated between a light reservoir associated with fluids and a heavier reservoir associated with higher temperature phases such as phosphates and silicates extraterrestrial carbon released at 600 degrees celsius from the h3 4 matrix has 13 c 20 i consistent with poorly graphitised material being introduced into the matrix rather than indigenous carbonate derived from a brine we have also examined 28 other h chondrite falls in order to ascertain how widespread halite or evaporite like mineral assemblages are in ordinary chondrites we did not find any more to add to zag h3 6 and monahans h5 which suggests that such highly soluble phases were not usually preserved on the parent bodies

the monahans chondrite and halite argon 39 argon 40 age solar gases cosmic ray exposure ages and parent body regolith neutron flux and thickness abstract the monahans h chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite we made detailed noble gas analyses of each of these phases the 39ar 40ar age of monahans light is 4 533 0 006 ma monahans dark and halite samples show greater amounts of diffusive loss of 40ar and the maximum ages are 4 50 and 4 33 ga respectively monahans dark phase contains significant concentrations of he ne and ar implanted by the solar wind when this material was extant in a parent body regolith monahans light contains no solar gases from the cosmogenic 3he 21ne and 38ar in monahans light we calculate a probable cosmic ray space exposure age of 6 0 0 5 ma monahans dark contains twice as much cosmogenic 21ne and 38ar as does the light and indicates early near surface exposure of 13 18 ma in a h chondrite regolith the existence of fragile halite grains in h chondrites suggests that this regolith irradiation occurred very early large concentrations of 36ar in the halite were produced during regolith exposure by neutron capture on 35cl followed by decay to 36ar the thermal neutron fluence seen by the halite was 2 4 1014 n cm2 the thermal neutron flux during regolith exposure was 0 4 0 7 n cm2 s the monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils but the neutron flux is lower by a factor of 5 comparison of the 36arn 21necos ratio in monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an h chondrite regolith suggests that irradiation of monahans dark occurred under low shielding in a regolith that may have been relatively shallow late addition of halite to the regolith can be ruled out however irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded

precise upb chronometry of 345 340 ma old magmatism related to syn convergence extension in the southern vosges central variscan belt abstract the southern vosges host a large volcano sedimentary basin which contains clastic sediments of famennian late devonian ca 365 ma to late visean ca 340 335 ma age the basin is bordered by an exhumed high grade basement complex and intruded by several granitoid plutons a chronology for basin evolution and magmatism was established dating volcanic rocks of the basin as well as cross cutting granitoid rocks by the u pb technique applied to zircon and titanite the basin evolution started with olistolithic deposits and overlying marine shales that have a famennian fauna ca 365 ma and contain detrital zircons with an age of 386 ma the lowermost rhyolite flows interlayered with the basin sediments and tholeiitic basalts yielded an age of 345 2 ma following a tectonic event at the bottom of the upper visean the rates of sedimentation and volcanism were strongly enhanced leading to basin emersion and were concluded by the extrusion of the uppermost ignimbrite molkenrain rhyolite emplaced at 340 2 ma the whole volcano sedimentary association was subsequently intruded by melts of the ballons and cre tes granites as well as by small monzodioritic and dioritic satellite intrusions along the border of the ballons granite within 3 2 myr 342 1 to 339 5 2 5 ma the zircons of most of the volcanic and plutonic rocks investigated contain inherited lead of 0 6 and 2 1 ga age the differences in initial nd isotopic composition of the ballons and cre tes granites can be explained by differential uptake of crustal material during magma genesis the precise age data prove that magmatism and basin formation in the southern vosges were extremely short lived and coeval to the exhumation of adjacent high grade gneiss terrains revealing an episode of extension at the end of the lower carboniferous between 345 and 340 ma visean the tectonic regime in the internal part of the variscan orogen is that of extension and strike slip while convergent tectonics were still ongoing in the external part of the variscan orogen

conventional u pb dating versus single grain pb evaporation dating of complex zircons from a pegmatite in the high grade gneisses of southwestern sweden u pb multi grain dating of zircons from a pegmatite dyke in southwestern sweden yield analyses which define a discordia with upper and lower intercept ages of 1510 32 ma and 631 64 ma respectively dating by the single zircon evaporation method on prismatic zircons from the same dyke indicates a primary crystallization age of 1409 20 ma in addition four oval zircon grains yield a 207pb206pb age of 969 27 ma the older single zircon evaporation age is interpreted as representative of pegmatite emplacement whereas the younger age is suggested to correspond to isotopic disturbance and new zircon growth during a late sveconorwegian metamorphic event to explain the false intercept ages in the u pb concordia diagram this study suggests a three stage model including 1 crystallization of zircons during pegmatite intrusion 2 episodic lead loss and new zircon growth during high grade metamorphism and 3 a recent lead loss event as the dykes are strongly deformed the protolith age of the pegmatite provides an upper age limit for the gneiss forming event the single zircon age of 969 27 ma is interpreted to be associated with this event which took place during high grade metamorphic conditions

salty old rocks small grains of halite in meteorites are generally interpreted as a sign of the former presence of water in his perspective ott 1 discusses the unexpected discovery of halites in relatively highly metamorphosed meteorites according to the age determined by whitby et al 2 for halite crystals in one of these meteorites they are among the oldest known materials in the solar system 1 http www sciencemag org cgi content full 288 5472 1761 2 http www sciencemag org cgi content short 288 5472 1819

magmatic flare up at the carboniferous permian boundary in the ne german basin revealed by shrimp zircon ages shrimp ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the carboniferous permian boundary throughout much of the ne german basin rhyolitic crystal rich samples have been taken from quarries in the halle volcanic complex hvc and in the flechtingen ignimbrite and from drill cores of the kotzen mirow friedland and penkun areas ten samples yielded 206pb 238u ages between 302 and 297 ma 3 ma respectively which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the ne german basin two hvc samples have ages of 307 and 294 ma this remarkably synchronous magmatic activity occurred during the initial phase of the basin development considering published volume estimates 48 000 km3 extrusion rates during this time period probably were in the order of 0 01 km3 a 1 implying magma production rates of 0 1 km3 a 1 the calc alkaline sio2 rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra continental transtensional setting the basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes the dated volcanic rocks occur in three geotectonic provinces namely the a mid german crystalline rise which forms the northern part of the internal variscides the b external variscides and the variscan foreland which is considered as c eastern avalonia many of the old zircons found in the hvc samples reflect the magmatic activity of the mid german crystalline rise 325 400 ma however cadomian 500 650 ma and older gondwanian elements 1674 2373 ma are also present although the flechtingen and kotzen areas are located within the external variscides it seems more likely that the middle to lower crust that experienced anatexis formed part of the eastern avalonia plate overridden by the variscan front thus the old zircons found in flechtingen 350 and 538 ma and kotzen 345 ma but certainly those present in the penkun drill cores 1483 ma represent eastern avalonia tectonic history the anatectic magmas in the friedland area probably formed partly from baltic crust thrusted below eastern avalonia during the caledonian orogeny among the old zircons from the friedland samples in addition to proterozoic ages 1456 ma we found testimony of caledonian 443 ma and of clearly post caledonian 387 ma magmatism