luhf geochronology applied to dating cenozoic events affecting lower crustal xenoliths from kilbourne hole new mexico abstract melt extraction or crystal accumulation events that affected garnet bearing granulite xenoliths from kilbourne hole new mexico have been dated using the lu hf isotope system two garnet bearing granulites from kilbourne hole have extreme 176 lu 177 hf ratios of 0 95 and 1 3 lu hf 28 and 39 chondritic but relatively normal hf values 5 and 12 necessitating either garnet accumulation or melt extraction from a garnet bearing protolith in the cenozoic hf isotope evolution curves for these two samples intersect those of depleted mantle and proterozoic crust at high angles and at similar times demonstrating the potential of hf isotope model ages to yield true age significance even if the initial hf isotope composition is not well constrained a third garnet granulite xenolith ckh63 176 lu 177 hf 0 025 contains zircon which buffered this sample against changes in lu hf during the cenozoic differentiation event the three garnet granulites lie closely about a 25 ma lu hf reference line demonstrating the potential of the lu hf system for detecting garnet controlled differentiation events in the cenozoic given the range in lu hf ratios measured events as young as 5 ma may be detected using the lu hf system conventional u pb zircon data from sample ckh63 reveal both a ca 1 4 ga inherited component and a component of recent pb loss or new zircon growth supporting the cenozoic event documented by hf isotope model ages

franciscan subduction off to a slow start evidence from high precision lu hf garnet ages on high grade blocks lu hf analyses of garnet from metabasic amphibolite glaucophane schist and eclogite facies blocks from the franciscan complex give highly precise ages that allow us to place new constraints on the early thermal history of the franciscan subduction zone garnets yield 176 lu 177 hf ratios ranging from 1 5 to 28 with the highest ratios from garnets with high spessartine pyrope ratio sulphuric acid leaching sal of garnets revealed the presence of inclusions with significantly higher lu hf ratios than those of garnet itself most likely apatite their removal by sal brings the 176 lu 177 hf ratios in garnets down by as much as 40 this suggests that 176 lu 177 hf ratios of apparently pure garnets can be greatly overestimated due to the presence of such inclusions sm nd garnet analyses were dominated by inclusions mainly sphene and failed to provide precise and accurate age information the oldest lu hf ages are 168 7f0 8 and 162 5f0 5 ma on plagioclase bearing garnet amphibolite from panoche pass and the berkeley hills respectively which suggests initiation of the subduction zone at about 169 ma coeval with the formation of the tectonically overlying coast range ophiolite relatively high temperature conditions persisted for about 14 ma as indicated by 153 4f0 8 ma garnet growth recorded in epidote amphibolite and 157 9f0 7 in eclogite from ring mountain and jenner respectively a 146 7f0 7 ma age was obtained from garnet glacuophane schist metamorphosed at around 400 jc the sequence of ages from central and northern california shows a younging trend with decreasing metamorphic grade which supports previous suggestions that the high grade metamorphic blocks and slices resulted from progressive underthrusting and underplating in a cooling subduction system combining geothermometry with geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist facies conditions as ca 15 jc ma the thermal history requires high initial geothermal gradients within both the footwall and the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km ma during the initial stages of franciscan subduction such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre the data also suggest slow exhumation rates and significant residence time at depth of the earliest franciscan rocks

the source region and melting mineralogy of high titanium and low titanium lunar basalts deduced from lu hf isotope data five high ti basalts from the apollo 11 and 17 landing sites have been analyzed for their hafnium isotope composition these data serve to better constrain the hafnium isotope variation of the moon s mantle variations in initial hf and nd values of low and high ti basalts imply that the source region mineral assemblages of these lunar magma types are distinct low ti basalts have higher initial hf values at a given nd value than high ti basalts the differences in the hafnium and neodymium isotopic composition of low and high ti basalts reflect the fact that the source of low ti basalts had a lu hf n ratio approximately four times greater than its sm nd n ratio in contrast the high ti source region had subequal lu hf n and sm nd n ratios if it is assumed that mare basalts are partial melts of the moon s cumulate mantle the differences between low and high ti basalts can only be explained by these mare magma types being generated from melting sources with distinctly different mineral assemblages the large lu hf fractionations relative to sm nd fractionations of low ti basalts can best be produced by an assemblage of olivine and orthopyroxene with trace amount of clinopyroxene that crystallized early in the history of the lunar magma ocean lmo the subequal lu hf n and sm nd n fractionations of high ti basalts can be produced from a variety of ilmenite bearing mineral assemblages low and high ti basalts have similar lu hf ratios approximately 0 6 times chondrite the low lu hf ratios measured for these mare magmas contrast sharply with the high lu hf ratios greater than chondritic calculated for their sources from initial hf values and an assumed chondritic bulk moon initial hf value the difference between the measured lu hf of a lava vs the calculated lu hf of its source implies that during partial melting lu was preferentially retained in the residual source relative to hf in order to explain the extreme fractionation of measured lu hf ratios we suggest mare basalts can best be explained using a polybaric melting model initial melting of a garnet bearing source followed by continued melting in the spinel stability field can produce the required lu hf fractionations and produce a liquid that last equilibrated with a residuum of olivine and orthopyroxene

re os systematics in chondrites and the fractionation of the platinum group elements in the early solar system we have investigated the re os system for samples of whole rock metal and sulfide from ordinary chondrites using closed system analytical techniques we found complete exchange between sample and tracer isotopes for silicate containing samples and obtained precise and reliable re os concentration measurements results on two group iva iron meteorites and on a silicate rich iron steinbach iva an are consistent with the iva ivb isochron and support the previous observation that iva ivb irons may be slightly older than iiab irons data on whole rock fragments and metal rich separates from the st severin chondrite ll6 show a large range in 1870re 188 os and in 187 os 188 os which makes possible in principle the determination of a re os internal isochron on a chondrite for the first time this re os fractionation may be due to partial melting of fenis and macroscopic redistribution of metal and sulfide the st severin data show a good correlation line on a 187 re 187 os evolution diagram if this is considered to represent an internal isochron it gives an age t 4 68 0 15 ae 187 re 1 64 10 11 a 1 and an initial 187 os 188 os 0 0 0953 0 0013 this age is in agreement with but slightly older than the more precise 187 re 187 os age for the iiab irons as well as for irons from other groups t 4 61 0 01 ae a st severin sulfide nodule has very low re and os concentrations and shows a young re os model age 2 3 ae indicating recent element remobilization whole rock and metal rich separates of h group chondrites h3 to h6 yield restricted ranges in 187 re 188 os 0 42 0 47 and 187 os 188 os 0 128 0 133 there is a systematic difference between re os in the metal extracted from a chondrite and the bulk chondrite this shows that there is a small but significant re os fractionation within subsystems contained in the chondrites from whole rock samples of h group chondrites we calculate a mean 187 re 188 os 0 423 0 007 and 187 os 188 os 0 12863 0 00046 which may characterize the evolution of an average chondritic reservoir for re os the ordinary chondrite data plot close to the iiab isochron although the deviations found are larger than found for the irons the re os chronometer in iron meteorites is apparently controlled by the re os fractionation due to fractional crystallization of liquid metal re os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies in contrast the behavior in ordinary chondrites while also dominated by the metal phases must reflect fractionation and transport on a local macroscopic scale within the chondrites between the metal phases after aggregation due to partial melting of fenis or represent variable re os fractionation of the metal phases prior to the accretion of the chondrites however for st severin we attribute the major re os fractionation to early heating of the meteorite above the fe fes eutectic we do not consider that the re os fractionation observed in other chondrites is due to the redistribution of re and os during chondrite metamorphism including shock but it may plausibly represent earlier stages of re os fractionation for the different feni metal constituents prior to accretion

the aouelloul crater mauritania on the problem of confirming the impact origin of a small crater the impact origin of small craters in sedimentary rocks is often difficult to confirm because of the lack of characteristic shock metamorphic features a case in point is the 3 1 ma aouelloul crater mauritania 390 m in diameter which is exposed in an area of ordovician oujeft and zli sandstone we studied several fractured sandstone samples from the crater rim for the possible presence of shock metamorphic effects in thin section a large fraction of the quartz grains show abundant subplanar and planar fractures many of the fractures are healed and are evident only as fluid inclusion trails a few grains showed sets of narrow and densely spaced fluid inclusions trails in one rarely two orientations per grain which could be possible remnants of planar deformation features pdfs although such an interpretation is not unambiguous in contrast an impact origin of the crater is confirmed by re os isotope studies of the target sandstone and glass found around the crater rim which show the presence of a distinct extraterrestrial component in the glass

a meteorological overview of the second pacific exploratory mission in the tropics meteorological conditions over the central pacific basin are summarized during nasa s second pacific exploratory mission in the tropics pem b which was conducted during february april 1999 mean flow patterns during pem b are described important features near the surface include subtropical anticyclones the south pacific convergence zone spcz and the intertropical convergence zone itcz the itcz is found to exhibit a double structure with branches at 5 n and 5 s both the itcz and spcz are areas of widespread cloudiness and convection extensive lightning occurs over the land masses surrounding the pacific basin and over the central south pacific ocean itself pem b occurs during a la nina period of relatively cold sea surface temperatures in the tropical pacific compared to climatology the pem b period exhibits deep convection located west of its typical position stronger than normal easterly trade winds a relatively strong weak northern southern hemispheric jet stream the spcz located west of its normal position and an upper tropospheric cyclonic wind couplet that straddles the equator circulation patterns during pem b are compared with those of pem a which occurred during august september 1996 pem b is found to exhibit a less organized itcz a comparatively weak jet stream in the southern hemisphere a relatively strong jet stream in the northern hemisphere and enhanced convection over the central pacific finally meteorological conditions for selected flights are discussed utilizing streamlines 10 day backward trajectories thermodynamic soundings and satellite imagery air parcels sampled by the aircraft are found to originate or pass over diverse regions including asia south america southern africa and australia some parcels remain over the pacific ocean during the preceding 10 day period

rhenium osmium and samarium neodymium isotopic systematics of the stillwater complex isotopic data for the stillwater complex montana which formed about 2700 ma million years ago were obtained to evaluate the role of magma mixing in the formation of strategic platinum group element pge ore deposits neodymium and osmium isotopic data indicate that the intrusion formed from at least two geochemically distinct magmas ultramafic affinity u type magmas had initial e nd of 0 8 to 3 2 and a chondritic initial 187 os 186 os ratio of 0 88 whereas anorthositic affinity a type magmas had e nd of 0 7 to 1 7 and an initial 187 os 186 os ratio of 1 13 these data suggest that u type magmas were derived from a lithospheric mantle source containing recycled crustal materials whereas a type magmas originated either by crustal contamination of basaltic magmas or by partial melting of basalt in the lower crust the nd and os isotopic data also suggest that os and probably the other pges in ore horizons such as the j m reef was derived from a type magmas the nd and os isotopic heterogeneity observed in rocks below the j m reef also suggests that a type magmas were injected into the stillwater u type magma chamber at several stages during the development of the ultramafic series

detection of a meteoritic component in ivory coast tektites with rhenium osmium isotopes measurement of rhenium re and osmium os concentrations and os isotopic compositions in ivory coast tektites natural glasses with upper crustal compositions that are ejected great distances during meteorite impact and rocks from the inferred source crater lake bosumtwi ghana show that these tektites incorporate about 0 6 percent of a meteoritic component analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals the re os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons i the isotopic compositions of the target rocks and the meteoritic impactor are significantly different ii closed system mixing of target rocks and meteorites is linear on re os isochron diagrams which thus permits identification of the loss of re or os iii osmium isotopic compositions are not likely to be altered during meteorite impact even if re and os are lost

identifying regional dust transport pathways application of kinematic trajectory modelling to a trans tasman case pi kinematic trajectory model is used to investigate potential pathways of dust transport from australia to new zealand historically these have been assumed to follow rather direct west east trajectories spanning 2 to 3 days often resulting in red snow events in the southern alps of new zealand however results from the present study which examined the route taken by air parcels originating in southern australia during dust storms on 24 and 25 may 1994 indicate that trans tasman dust transport trajectories are more diverse than previously thought and display considerable variation during single events these mon divergent pathways tie in more closely with aeolian dust sedimentation patterns identified by ocean coring in the tasman sea and may account for the deposition of australian dust on sub antarctic islands located well south of the australian continent copyright 2000 john wiley sons ltd

evidence for protracted prograde metamorphism followed by rapid exhumation of the zermatt saas fee ophiolite major and trace element zoning in garnet in combination with rb sr sm nd and lu hf geochronology provide evidence for a protracted garnet growth history for the zermatt saas fee zsf ophiolite western alps four new lu hf ages from pfulwe c 52 46 ma and one from chamois c 52 ma are very similar to a previously published lu hf age from lago di cignana overall the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history commensurate with their similar structural position near the top of the zsf samples from the lower part of the zsf at saas fee and st jacques however produced much younger lu hf ages c 41 38 ma neither differences in whole rock geochemistry which might produce distinct garnet growth histories nor rare earth element zoning in garnet can account for the age differences in the two suites this suggests a much later prograde history for the lower part of the zsf supporting the idea that it was subducted diachronously such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions where early oblique subduction which produced long prograde garnet growth changed to more orthogonal subduction which corresponds to shorter prograde garnet growth six new rb sr phengite ages range from c 42 to 39 ma and in combination with previously published rb sr ages constrain the timing of the transition from eclogite to upper greenschist facies p t conditions the proximity of the zsf in the saas fee region to the underlying continental monte rosa unit and the similarity of peak metamorphic ages suggest these two units were linked for part of their tectonic history this in turn indicates that the monte rosa may have been partly responsible for rapid exhumation of the zsf unit

meteorological conditions and anomalies during the intercontinental chemical transport experiment north america 1 meteorological conditions are described during the intercontinental chemical transport experiment north america intex na that was conducted over the united states during july and august 2004 relatively zonal flow dominated the contiguous united states during the first 2 weeks of the mission while a series of large amplitude troughs traversed the eastern half of the country during the final 4 weeks these troughs were accompanied by cold fronts reaching the gulf of mexico an uncommon occurrence during august frontal passages over the northeast were somewhat above average but the short time interval between passages precluded the formation of stagnant high pressure centers containing abundant pollution atmospheric chemistry during intex na was heavily influenced by record breaking fires over alaska and western canada persistent high pressure over alaska provided ideal conditions for the wildfires and for transporting their burning by products southeastward toward the intex domain where they were sampled frequently by intex aircraft forward trajectories and satellite imagery showed that the plumes later were carried to parts of europe africa and even the arctic deep convection and lightning were important factors during intex na cloud to ground cg lightning data show that horizontal patterns and numbers of lightning flashes during intex na are similar to those of 2003 and 2005 statistics derived from 10 day backward trajectories indicate that the dc 8 often sampled lightning influenced air

a meteorological overview of the trace a period general meteorological conditions over the south atlantic basin during the transport and atmospheric chemistry near the equator atlantic trace a experiment period are described trace a occurred during one of the longest continuous el nino southern oscillation enso events of the twentieth century effects of this enso event included warmer than normal temperatures concurrent with a severe drought in southern africa and warm dry conditions over northeastern brazil large scale flow patterns are described and the trace a period is compared to climatological normals significant meteorological features affecting the trace a flights included a northward moving middle latitude cold frontal system over brazil eastward moving middle latitude frontal waves crossing the south atlantic basin and southern africa and deep convection over central africa the brazilian cold front helped induce organized convective systems which led to vertical transports of biomass burning related chemical species a well defined middle latitude frontal wave was traversed during the transit flight from rio de janeiro to johannesburg along with the penetration of a tropopause fold of stratospheric air deep surface based mixed layers and frequent diurnal convection over central africa were responsible for vertical transports over that continent middle latitude frontal waves that crossed the southern region of africa slightly displaced the shear axes and col zones that often were present off the west coast of central africa

hf isotopic measurements on barberton komatiites effects of incomplete sample dissolution and importance for primary and secondary magmatic signatures komatiites from a single lava flow carl s flow in the schapenburg greenstone remnant near barberton in south africa yielded very different hf isotope compositions depending on the manner in which the samples were treated analytically data from samples dissolved in savillex beakers define a statistically significant mswd 0 77 isochron with an age of 3 4 0 2 ga which is identical within error to the probable eruption age and an initial ehf of 1 3 analyses of samples dissolved in bombs on the other hand plot on a linear array which if interpreted as an isochron gives an age of 2 9 ga their initial 176hf 177hf calculated using the probable eruption age of 3 45 ga are positive and variable ranging from ehf of 1 3 to 6 3 hf contents are much lower and lu hf ratios systematically higher in the savillex beaker dissolved batch of komatiites indicating that a hf rich phase zircon or baddeleyite remained undissolved in these samples the negative ehf of the savillex beaker dissolved samples is attributed to interaction with crust derived fluids that affected the komatiite soon after its eruption the anomalously low isochron age of the bomb dissolved samples is due to the presence of a single olivine cumulate sample that appears to have lost hf during post emplacement alteration without this sample initial ehf values cover a much narrower range between 4 3 and 6 3 which is believed to be that of the mantle source of the komatiites the combination of highly positive ehf and lower than chondritic lu hf indicates that the mantle source of the carl s flow was garnet enriched for a substantial period of time prior to komatiite formation and that garnet was retained in the source during the melting event that eventually produced the komatiite magma

hf w sm nd and rb sr isotopic evidence of late impact fractionation and mixing of silicates on iron meteorite parent bodies abstract we report the first sm nd and rb sr isotopic analyses of silicate inclusions in four iie iron meteorites miles weekeroo station a and b and watson we also report the hf w isotopic composition of a silicate inclusion from watson and 182 w 184 w of the host feni metal in all four iies the host metal in watson has a negative w value 2 21 0 24 similar to or higher than other iron meteorites 1 35 and consistent with segregation of metal from silicate early in solar system history however the large silicate inclusion in the watson iie iron yielded a chondritic w value 0 50 0 55 thus indicating a lack of equilibration with the feni host within the practical lifetime of activity of the parent 182 hf 50 ma one of the silicate inclusions in miles is roughly chondritic in major element composition has a present day nd of 10 3 relatively non radiogenic 87 sr 86 sr 0 714177 13 and a t chur age of 4270 ma two silicate inclusions from weekeroo station and one from watson exhibit fractionated sm nd and rb sr ratios and more radiogenic 87 sr 86 sr 0 731639 12 to 0 791852 11 and non radiogenic nd values 5 9 to 13 4 the silicate inclusion in watson has a t chur age of 3040 ma in agreement with previously determined 4 he and 40 ar gas retention ages indicative of a late thermal event a later event is implied for the two silicate inclusions in weekeroo station which yield indistinguishable t chur ages of 698 and 705 ma silicate inclusions in iie iron meteorites formed over a period of 3 billion yr by impacts involving an h chondrite parent body and an feni metal parent body the lile enriched nature of some of these silicates suggests several stages of melting mixing and processing however there is little evidence to suggest that the silicates in the iie irons were ever in equilibrium with the host feni metal

a chemical method for hafnium separation from high ti whole rock and zircon samples abstract a chemical separation method of hf from rocks is presented a procedure involving three stages of ion exchange columns and sulfuric acid as an eluant produces consistent zr and ti free hf separations the methods described are well suited for high ti samples such as mafic rocks in addition the method provides a very efficient means for hf separation from zircons

heat and moisture budgets of an intense midlatitude squall line abstract rawinsonde data from ok pre storm are used to calculate the heat and moisture budgets during the late mature through dissipating stages of an intense squall line system with a trailing stratiform region budgets have been computed at three separate times using composited data from approximately 30 rawinsondes over three hour intervals data spacing is marginally adequate to resolve features within the broad 200 km stratiform region but not the narrow 40 km leading convective line low level radar data and surface accumulated rainfall are used to partition the system into convective line and stratiform regions and to cheek the accuracy of the heat and moisture budgets the squall line had a pronounced descending rear inflow jet that extended toward the front of the system with time and strong front to rear flow above and beneath this jet the diagnosed vertical motion showed deep ascent in the leading convective line and upper level ascent above lower level descent in the trailing stratif

high precision lu and hf isotope analyses of both spiked and unspiked samples a new approach 1 the functional form of instrumentally produced mass fractionation associated with mc icp ms analysis is not accurately known and therefore cannot be fully corrected by traditional approaches of internal normalization using power linear or exponential mass bias laws we present a method for robust correction of instrumentally produced mass fractionation of both spiked and unspiked samples that can be applied to mass analysis of hf as well as nd sr os etc correction of 176hf 177hf for unspiked samples follows a traditional approach of internal normalization using an exponential law followed by normalization to a standard of known composition such as jmc 475 for spiked samples standards are used to characterize a linear instrumental mass bias coefficient the mass bias coefficient is defined by the slope of a tie line between measured and true values of a standard this approximation results in identical precision and accuracy of measurements for spiked and unspiked samples 0 005 2 external reproducibility the effects of the spike on the 176hf 177hf ratio and calculation of the molar spike sample ratio is determined by a closed form solution modified from the double spike approach used for fe isotope analysis by tims johnson and beard 1999 the measured 176lu 175lu ratios are corrected by doping the sample with er and using the 167er 166er ratio to externally normalize the 176lu 175lu ratio using an exponential law finally spike sample equilibration is confirmed for our sample dissolution protocol through analysis of varying physical mixtures of 1 ga garnet and hornblende where all the data lie on a mixing line within error on a 176lu 177hf 176hf 177hf diagram precision of 176lu 177hf ratios is determined to be 0 2 2 for standards and for physical mixtures of garnet and hornblende

isotopic composition of osmium in terrestrial samples determined by accelerator mass spectrometry interest has revived in the rhenium osmium system due to significant advances in detection methods1 4 and because of its use for the identification of extraterrestrial material at the cretaceous tertiary boundary5 another perhaps more important application is its use as a tracer for the origin of crustal material this application is however severely restricted by the very low concentrations typical of crustal rocks we present here the first results for the isotopes 187os 188os and 189os measured in sub p p b parts per 109 level samples by accelerator mass spectrometry ams our first exploratory measurements on extraterrestrial and crustal material with os concentrations between 2 p p m parts per 106 and 0 006 p p b show a detection limit of 0 01 p p b and a precision of better than 10 the 187os 186os ratios measured for samples for the canyon diablo meteorite and from the east clearwater crater in quebec canada were close to 1 typical of extraterrestrial material in contrast ratios in samples from the ries crater in germany were around 10 which is evidence of predominantly crustal osmium in these rocks

an investigation of dissolution methods for lu hf and sm nd isotope studies in zircon and garnet bearing whole rock samples 1 the distinct chemical behavior of lu and hf can limit complete sample dissolution and spike sample equilibration here we investigate the effects that four dissolution methods hotplate low temperature microwave high temperature microwave and steel jacketed parr bomb have on the sm nd and lu hf isotope compositions of rocks any of the dissolution methods produce high quality sm nd isotope data and hotplate dissolution of young volcanic rocks produces adequate lu hf data however low temperature microwave and hotplate dissolution methods produce significantly different lu hf and 176hf 177hf ratios as compared to parr bomb dissolution data for old samples or those that contain garnet rutile or zircon a characteristic feature of incomplete spike sample equilibration and sample dissolution for garnet bearing samples is anomalously high 176lu 177hf ratios relative to 176hf 177hf ratios our results show that for lu hf isotope studies of rocks of any significant age the errors in calculated initial hf values may exceed 10 hf units if complete dissolution and spike sample equilibration are not attained for garnet or zircon bearing rocks high temperature and pressure dissolution such as that which can be achieved through parr bomb dissolution is critical to ensure complete dissolution of refractory phases such as garnet or zircon

rhenium osmium isotope systematics in meteorites i magmatic iron meteorite groups iiab and iiiab abstract using resonance ionization mass spectrometry rims re and os abundances were determined by isotope dilution id and 187 os 186 os ratios measured in nineteen iron meteorites eight from group iiab ten from group iiiab and treysa iiib anomalous abundances range from 1 4 to 4800 ppb re and from 13 to 65000 ppb os and generally agree well with previous id and neutron activation naa results the re and os data suggest that abundance trends in these iron groups may be entirely explained by fractional crystallization addition of late formed metal to produce re os variation in the b subgroups is not essential but cannot be excluded whole rock isochrons for the iiab and iiiab groups are statistically indistinguishable pooled data yield an initial 187 os 186 os of 0 794 0 010 with a slope of 7 92 0 20 10 2 corresponding to a magmatic iron meteorite age of 4 65 0 11 ga using a decay constant of 1 64 10 11 a 1 given the errors in the slope and half life this age does not differ significantly from the canonical chondrite age of 4 56 ga but could be as young as 4 46 ga

metalliferous asteroids as potential sources of precious metals recent discoveries of near earth asteroids neas and chemical analyses of fragments of asteroids meteorites suggest that there may be a gold mine literally in near earth space judged from meteorite analyses two types of asteroids offer particularly bright prospects for recovery of large quantities of precious metals defined as au pt ir os pd rh and ru the ordinary ll chondrites which contain 1 2 5 3 fe ni metal containing 50 220 ppm of precious metals and metallic asteroids which consist almost wholly of fe ni phases and contain variable amounts of precious metals up to several hundred ppm the pulverized regolith of ll chondrite asteroids could be electromagnetically raked to separate the metallic grains suitable metallic asteroids could be processed in their entirety statistically there should be approximately six metallic neas larger than 1 km in diameter that contain over 100 ppm of precious metals successful recovery of 400 000 tons or more of precious metals contained in the smallest and least rich of these metallic neas could yield products worth 5 1 trillion us at recent market prices if marketed over 20 years this would represent a 10 fold increase over the recent global production rate of all precious metals combined the market response to the hypothetical introduction of such large quantities of precious metals is difficult to predict because these metals historically have not obeyed normal economic laws of the market an empirical model suggests that the effects on market prices of a hypothetical increase in production of a given metal can be predicted given existing knowledge of the metal s present market value and production rate this model suggests that the total value of 400 000 metric tons of precious metals if marketed over 20 years would decline to about 320 billion 16 billion per year except for au for which production and prices would be marginally affected the market prices of precious metals may decline by 1 to 2 orders of magnitude if one of the six asteroids were to be mined to depletion over 20 years less conservatively there is a 50 chance that the richest metallic nea contains at least 1 9 million metric tons of precious metals this quantity if marketed over 40 years may be worth approximately 900 billion at collapsed market prices 22 5 billion per year the actual economic and technological impact of asteroidal metals may be considerably greater due to the increased availability and reduced prices of these resources despite this great potential first order technological scientific and economic uncertainties remain before the feasibility of exploitation of asteroids for precious metals can be ascertained

long range transport of sulfur dioxide in the central pacific 1 long range transport of sulfur dioxide so2 from east asia to the central north pacific troposphere was observed on transit flights during the nasa transport and chemical evolution over the pacific mission a series of so2 enhanced layers above the boundary layer was observed during these flights the significant features included enhanced so2 layers associated with low water vapor and low turbulence that were usually dynamically isolated from the marine boundary layer this study shows that atmospheric dynamics were very important in determining the so2 distributions in the central pacific during march and april 2001 trajectory studies revealed that so2 enhanced layers could be connected to both volcanic and anthropogenic sources in east asia these trajectory studies also showed that the air parcels usually were lifted 2 km above the source regions and then progressed to the east in the midlatitudes 30 60 n the air parcels arrived in the central pacific within 2 3 days sulfur dioxide transported at altitudes of 2 4 km dominated the so2 distribution in the central pacific a comparison of so2 observations and results of chemical transport models indicated that so2 was removed primarily by cloud processes therefore in the absence of cloud so2 can be transported long distances if the trajectory is decoupled from the boundary layer another important observation was that the miyake jima volcano made a major contribution to the so2 concentrations in the central pacific troposphere during march and april 2001 at times the volcanic so2 had more influence in the central pacific than the six largest anthropogenic so2 source regions in east asia

a simulation of the cerro hudson so2 cloud an isentropic trajectory model is used to simulate the evolution of the southern hemisphere so2 cloud associated with the eruption of cerro hudson by matching the parcel trajectories with total ozone mapping spectrometer so2 retrievals the principal stratospheric injection region is determined to be between 11 and 16 km in altitude this region is characterized by weak wind shears and is located just poleward of the subtropical jet in the outer fringe of the stratospheric polar vortex the lack of wind shear in the injection region explains the slow zonal dispersal of the so2 cloud which was still clearly observed 19 days after the eruption the trajectory model simulation of the so2 cloud shows good agreement with observations for 7 days after the eruption using the potential vorticity and potential temperature estimates of the initial eruption cloud the cloud position relative to the polar night jet is shown to be nearly fixed up to september 2 1991 which was as long as the cloud was observed this result suggests that the lower stratospheric polar and mid latitude regions are nearly isolated from each other during the late august period

re os systematics in pallasite and mesosiderite metal re os data on pallasite metal yield a sufficient range in re os to permit the determination of a whole rock isochron if pallasites from different groups are considered together the results are indistinguishable from the data obtained on groups iab iiab iid iiiab iva and ivb iron meteorites pallasite data were obtained on samples of the main group which have been considered related to group iiiab irons and on samples of the distinctive pallasites eagle station finmarken and marjalahti it appears that iron meteorites and pallasites crystallized within a time interval of 20 ma this result is consistent with the evidence from the short lived chronometers 107 pd 107 ag and 53 mn 53 cr it is also consistent with the evidence from 182 hf 182 w that the time interval over which molten feni metal from different iron meteorites equilibrated with and became segregated from silicates was 10 ma the well defined 187 re 187 os whole rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated feni metal and fes and then all crystallized within a 20 ma time interval this conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic the short time scale requires that the irons and pallasites were formed in small parent planets of 10 km radius or near the surface of larger bodies in contrast to the pallasites we find that feni samples from mesosiderites have a narrow range in re os so that it is not possible to determine a whole rock isochron for these samples the mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi stage evolution based on sm nd data on mesosiderite silicate clasts the re os results on mesosiderite metal are compatible with the model of melted or partially molten feni cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 ma after early formation and the feni was splashed onto the surfaces of other small differentiated planetesimals

geochemical evidence of saharan dust parent material for soils developed on quaternary limestones of caribbean and western atlantic islands abstract most previous workers have regarded the insoluble residues of high purity quaternary limestones coral reefs and oolites as the most important parent material for well developed clay rich soils on caribbean and western atlantic islands but this genetic mechanism requires unreasonable amounts of limestone solution in quaternary time other possible parent materials from external sources are volcanic ash from the lesser antilles island arc and saharan dust carried across the atlantic ocean on the northeast trade winds soils on quaternary coral terraces and carbonate eolianites on barbados jamaica the florida keys united states and new providence island bahamas were studied to determine which if either external source was important caribbean volcanic ashes and saharan dust can be clearly distinguished using ratios of relatively immobile elements al 2 o 3 tio 2 ti y ti zr and ti th comparison of these ratios in 25 soils where estimated ages range from 125 000 to about 870 000 yr shows that saharan dust is the most important parent material for soils on all islands these results indicate that the northeast trade winds have been an important component of the regional climatology for much of the quaterary saharan dust may also be an important parent material for caribbean island bauxites of much greater age

geochemical mapping of carbonate sediments in the makgadikgadi basin botswana using moderate resolution remote sensing data this paper maps the carbonate geochemistry of the makgadikgadi pans region of northern botswana from moderate resolution 500 m pixels remotely sensed data to assess the impact of various geomorphological processes on surficial carbonate distribution previous palaeo environmental studies have demonstrated that the pans have experienced several highstands during the quaternary forming calcretes around shoreline embayments the pans are also a significant regional source of dust and some workers have suggested that surficial carbonate distributions may be controlled in part by wind regime field studies of carbonate deposits in the region have also highlighted the importance of fluvial and groundwater processes in calcrete formation however due to the large area involved and problems of accessibility the carbonate distribution across the entire makgadikgadi basin remains poorly understood the modis instrument permits mapping of carbonate distribution over large areas comparison with estimates from landsat thematic mapper data show reasonable agreement and there is good agreement with estimates from laboratory analysis of field samples the results suggest that palaeo lake highstands reconstructed here using the srtm 3 arc second digital elevation model have left behind surficial carbonate deposits which can be mapped by the modis instrument copyright c 2006 john wiley sons ltd

biogeochemistry of iron in australian dust from eolian uplift to marine uptake 1 dust is an important vector for iron supply to the ocean which subsequently impacts ocean productivity atmospheric co2 concentrations and hence global climate here we synthesize the processes influencing the biogeochemistry of australian dust and compare them with those from other southern hemisphere dust sources our observations range from soil and dust physical properties to abrasion and cloud chamber chemistry experiments to dust storms and their dispersion and deposition we then present satellite observations of the impact of episodic dust deposition events on the productivity of low iron oceanic waters north i e low nitrate low chlorophyll lnlc and south i e high nitrate low chlorophyll hnlc of australia dust deposition from the largest dust storm in over 40 years did not result in iron mediated algal blooms in either oceanic region a comparison of australia with other southern hemisphere source regions reveals that the relatively well sampled australian system is a poor generic model furthermore there are marked distinctions between southern and northern hemisphere iron dust biogeochemistry that must be recognized by modelers and included in future simulations better information is required on the relative role of the atmosphere and ocean on influencing iron biogeochemistry and how their relative influences might change in the future due to climate change

multiple sources supply eolian mineral dust to the atlantic sector of coastal antarctica evidence from recent snow layers at the top of berkner island ice sheet the sr and nd isotopic composition of dust extracted from recent snow layers at the top of berkner island ice sheet located within the filchner ronne ice shelf at the southern end of the weddell sea enables us for the first time to document dust provenance in antarctica outside the east antarctic plateau eap where all previous studies based on isotopic fingerprinting were carried out berkner dust displays an overall crust like isotopic signature characterized by more radiogenic 87sr 86sr and much less radiogenic 143nd 144nd compared to dust deposited on the eap during glacial periods differences with eap interglacial dust are not as marked but still significant indicating that present day berkner dust provenance is distinct at least to some extent from that of the dust reaching the eap the fourteen snow pit sub seasonal samples that were obtained span a two year period 2002 2003 and their dust sr and nd isotopic composition reveals that multiple sources are at play over a yearly time period southern south america patagonia in particular likely accounts for part of the observed spring summer dust deposition maxima when isotopic composition is shifted towards younger isotopic signatures in the spring possible additional inputs from australian sources would also be supported by the data most of the year however the measured isotopic signatures would be best explained by a sustained background supply from putative local sources in east antarctica which carry old crust like isotopic fingerprints whether the restricted east antarctic ice free areas produce sufficient eolian material has yet to be substantiated however the fact that large 5 mu m particles represent a significant fraction of the samples throughout the entire time series supports scenarios that involve contributions from proximal sources either in patagonia and or antarctica possibly including snow free areas in the antarctic peninsula and other areas as well this also indicates that additional dust transport which does not reach the eap must occur at low tropospheric levels to this coastal sector of antarctica

evidence for a meteoritic component in impact melt rock from the chicxulub structure abstract the chicxulub structure in yucatan mexico has recently been recognized as a 200 km diameter multi ring impact crater of k t boundary age crystalline impact melt rocks and breccias from within the crater which have compositions similar to those of normal continental crustal rocks and which show shock metamorphic effects have been studied for trace element and re os isotope compositions re os isotope systematics allow the sensitive and selective determination of an extraterrestrial component in impact derived rocks a melt rock sample shows elevated indium concentrations an osmium concentration of 25 ppb and a low 187 os 188 os ratio of 0 113 which are incompatible with derivation from the continental crust even though the 187 os 188 os ratio is slightly lower than the range so far measured in meteorites a mantle origin seems unlikely for mass balance reasons and because the cratering event is unlikely to have excavated mantle material the data support the hypothesis of a heterogeneously distributed meteoritic component in the chicxulub melt rock a sample of impact glass from the haitian k t boundary at beloc yielded about 0 1 ppb osmium and an 187 os 188 os ratio of 0 251 indicating the presence of a small meteoritic component in the impact ejecta as well

phosphate lu hf geochronology abstract igneous metamorphic and biogenic apatite contains lu and hf in proportions that make this mineral suitable for dating by the 176lu 176hf method we present a new method for separation of lu and hf from apatite that involves a single extraction column step for hf and a second exchange column step for lu this procedure allows rapid sample processing prior to analysis by mc icp ms results from the igneous gardiner skaergaard and khibina intrusions indicate that the closure temperature for the lu hf isotopic system in apatite is relatively high the lu hf isochron age of 60 ma for the skaergaard intrusion east greenland is older than the generally accepted emplacement age but does not appear to be a product of mixing a centimeter sized apatite from the otter lake area grenvillian province yields high and variable 176lu 177hf ratios showing that metamorphic apatite may in some cases allow for mono mineral dating the most important application of the method is undoubtedly its potential use for dating sedimentary formations although we find biogenic apatite in porous matrix highly susceptible to post depositional resetting of the lu hf isotopic system the preservation of a near depositional age for a shark tooth from the impermeable london clay and recent work on crystalline authigenic phosphorites earth planet sci lett 201 2002 203 show the lu hf system to be a promising tool for direct dating of sedimentary formations

re os isotope systematics of ni cu sulfide ores sudbury igneous complex ontario evidence for a major crustal component sudbury igneous complex sublayer ores from the levack west falconbridge and strathcona mines were analyzed for their re and os concentrations and os isotopic compositions the re os isotope systematics of three ores from the different mines give isochron ages of1 840 60 ma 1770 60 ma and 1780 110 ma suggesting that the re os system became closed at the time of or soon after the 1850 1 ma crystallization age of the complex the os isotopic compositions of different portions of the complex at the time of crystallization varied considerably with initial187os 186os1850 ranging from 4 64 at levack west to 7 55 at strathcona these heterogeneities require that the os and probably also the other platinum group elements contained in the ores were derived from at least two sources in addition the high initial187os 186os ratios indicate that the os was derived predominantly from ancient crust previous studies have suggested that the complex either crystallized from a mixture of mantle derived basaltic melt and ancient continental crust or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact results of modelling suggest that if a contemporaneous mantle derived basaltic melt was involved in the origin of the sic it likely contributed 50 of the os to all three ores the large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact

horizontal and vertical transport of air over southern africa tropospheric air trajectories that occurred during the southern african fire atmosphere research initiative safari in august october 1992 are described in terms of a circulation classification scheme and the vertical stability of the atmosphere three major and frequently occurring stable discontinuities are found to control vertical transport of aerosols in the subtropical atmosphere at the end of the dry season of these the main subsidence induced feature is a spatially ubiquitous and temporally persistent absolutely stable layer at an altitude of about 5 km 3 5 km above the interior plateau elevation this effective obstacle to vertical mixing is observed to persist without break for up to 40 days below this feature an absolutely stable layer at 3 km 1 5 km above the surface prevails on and off at the top of the surface mixing layer for up to 7 days at a time being broken by the passage of regularly occurring westerly wave disturbances above the middle level discontinuity a further absolutely stable layer is frequently discerned at an altitude of about 8 km it is shown that five basic modes can be used to describe horizontal aerosol transportation fields over southern africa dominating these is the anticyclone mode which results in frequent recirculation at spatial scales varying from hundreds to thousands of kilometers in exiting the anticyclonic circulation transport on the northern periphery of the system is to the west over the atlantic ocean via a semistationary easterly wave over the western part of the subcontinent on the southern periphery wave perturbations in the westerly enhance transports which exit the subcontinent to the east into the indian ocean independently derived data suggest that during safari only 4 of the total transport of air from three locations south of 18 degrees 8 was into the atlantic ocean over 90 of the transport was into the indian ocean across 35 degrees e this result reflects circulation fields typical of the extremely dry conditions prevailing in 1992 the integrated effect of the control exerted by atmospheric stability on vertical mixing on the one hand and the nature of the horizontal circulation fields on the other is to produce a distinctive suite of transport patterns that go a long way to explain the observed high concentrations of tropospheric aerosols and trace gases observed over the subcontinent in winter and spring as well as over the tropical south atlantic and southwestern indian oceans

low level wind maxima and the transport of pyrogenic products over southern africa low level jets llj over various regions of southern africa are shown to occur mostly as a result of a thermal gradient established over gently sloping terrain and have been observed to overlie the nocturnal temperature inversions strong low level winds associated with plain mountain and mountain plain circulations have also been observed over the central namib desert and natal interior with the exception of the namib and natal where no temperature profiles accompanied wind profile observations the llj is seen to increase in strength as the nocturnal surface temperature inversion intensifies through the night the height of the llj above the surface also increases as the inversion deepens wind speeds in all regions exceed 10 m s 1 in the jet core which typically is located between 200 and 300 m above ground level agl over the central namib the summertime plain mountain wind reaches similar wind speeds in a jetlike core located about 200 to 600 m agl the natal mountain plain winds may exceed 10 m s 1 in a layer of up to 1000 m deep it is suggested that all these regional scale wind systems transport pyrogenic products in excess of 400 km during a single night subsequent daytime convective lifting of these ozone precursors coupled with synoptic scale anticyclonic circulation provides a transport mechanism for pyrogenic products over southern africa toward the atlantic and indian oceans

do hf isotopes in magmatic zircons represent those of their host rocks abstract lu hf isotopic system in zircon is a powerful and widely used geochemical tracer in studying petrogenesis of magmatic rocks and crustal evolution assuming that zircon hf isotopes can represent initial hf isotopes of their parental whole rock however this assumption may not always be valid disequilibrium partial melting of continental crust would preferentially melt out non zircon minerals with high time integrated lu hf ratios and generate partial melts with hf isotope compositions that are more radiogenic than those of its magma source dissolution experiments with hotplate bomb and sintering procedures of zircon bearing samples demonstrate this disequilibrium effect where partial dissolution yielded variable and more radiogenic hf isotope compositions than fully dissolved samples a case study from the neoproterozoic jiuling batholith in southern china shows that about half of the investigated samples show decoupled hf isotopes between zircons and the bulk rocks this decoupling could reflect complex and prolonged magmatic processes such as crustal assimilation magma mixing and disequilibrium melting which are consistent with the wide temperature spectrum from 630 c to 900 c by ti in zircon thermometer we suggest that magmatic zircons may only record the hf isotopic composition of their surrounding melt during crystallization and it is uncertain whether their hf isotopic compositions can represent the primary hf isotopic compositions of the bulk magmas in this regard using zircon hf isotopic compositions to trace crustal evolution may be biased since most of these could be originally from disequilibrium partial melts

lu hf and sm nd evolution in lunar mare basalts lu hf and sm nd data for mare basalts combined with rb sr and total ree data taken from the literature suggest that the mare basalts were derived by small or 10 degrees of partial melting of cumulate sources but that the magma ocean from which these sources formed was light ree and hf enriched calculated source compositions range fromm lherzolite to olivine websterite nonmodal melting of small amounts of ilmenite or 3 in the sources seems to be required by the lu hf data a comparison of the hf and nd isotopic characteristics between the mare basalts and terrestrial oceanic basalts reveals that the epsilonhf epsilonnd ratios of low ti mare basalts are much higher than in terrestrial oceanic basalts the results are qualitatively consistent with the hypothesis that terrestrial basalt sources are partial melt residues whereas mare basalt sources are cumulates alternatively the results may imply that the terrestrial mantle has evolved in two or more stages of evolution and that the net effect was depletion of the mantle during the first approx 1 3 b y followed by enrichment during the last 1 2 b y or simply that there is a difference in lu hf crystal liquid partitioning relative to sm nd between the lunar and terrestrial mantles

re os and pd ag systematics in group iiiab irons and in pallasites using improved analytical techniques which reduce the re blanks by factors of 8 to 10 we report new re os data on low re and low pge pallasites pal anom and iiiab irons the new pallasite samples nearly double the observed range in re os for pallasites and allow the determination of an isochron of slope 0 0775 0 0008 t 4 50 0 04 ga using the adjusted 187 re 1 66 10 11 a 1 and initial 187 os 188 os 0 0 09599 0 00046 if the data on different groups of pallasites including the anomalous pallasites are considered to define a whole rock isochron their formation would appear to be distinctly younger than for the iron meteorites by 60 ma five iiiab irons acuna bella roca chupaderos grant and bear creek with re contents ranging from 0 9 to 2 8 ppb show limited re os fractionation and plot within errors on the iiab iron meteorite isochron of slope 0 07848 0 00018 t 4 56 0 01 ga and initial 187 os 188 os 0 0 09563 0 00011 many of the meteorites were analyzed also for pd ag and show 107 ag enrichments correlated with pd ag requiring early formation and fractionation of the feni metal in a narrow time interval after injection of live 107 pd t 1 2 6 5 ma into the solar nebula based on pd ag the typical range in relative ages of these meteorites is 10 ma the pd ag results suggest early formation and preservation of the 107 pd 107 ag systematics both for iiiab irons and for pallasites while the younger re os apparent age for pallasites suggests that the re os system in pallasites was subject to re equilibration the low re and low pge pallasites show significant re os fractionation higher re os as the re and pge contents decrease by contrast the iiiab irons show a restricted range in re os even for samples with extremely low re and pge contents there is a good correlation of re and ir contents the correlation of re and os contents for iiiab irons shows a similar complex pattern as observed for iiab irons morgan et al 1995 and neither can be ascribed to a continuous fractional crystallization process with uniform solid metal liquid metal distribution coefficients

kalkkop crater cape province south africa confirmation of impact origin using osmium isotope systematics the kalkkop structure a circular depression with a diameter of 640 m is situated in the eastern cape province south africa in 1992 a bore hole was drilled in the center of the crater to a depth of 151 8 m to assess a possible impact origin of this structure the abundances and isotopic ratios of osmium and rhenium were measured in breccias and in sandstones and shales derived from the basement the basement rocks show rhenium and osmium abundances and isotopic compositions of osmium and neodymium 187os188os 0 61 to 0 92 and nd 6 to 7 that are typical for continental crust one of the breccia samples shows a significantly elevated osmium content and a much lower 187os188os ratio of about 0 215 187os188os ratio 1 79 this ratio is much closer to meteoritic compositions than to crust indicating the presence of about 0 05 of an extraterrestrial component in the breccia this is the first time that osmium isotope systematics have been used to confirm the impact origin of a crater structure and demonstrates their sensitivity for impact studies

saltpan impact crater south africa geochemistry of target rocks breccias and impact glasses and osmium isotope systematics abstract the pretoria saltpan crater is a well preserved 220 000 year old 1 13 km diameter simple impact crater the crater was formed in nebo granites of the bushveld complex some minor intrusions thought to be younger than the nebo granite are present at the crater and have earlier been believed to support a volcanic origin of the structure but recent geological studies showed them to be part of the regional geology and of proterozoic age we studied the petrology and geochemistry of fourteen target granite samples three suevitic breccias nine intrusive rocks as well as melt agglutinates handpicked impact glass fragments and sulfide spherules from the saltpan impact crater unconsolidated suevitic breccias recovered from different depths in the crater were found to contain abundant evidence of shock metamorphism the target rock granites show only limited compositional variability the major and trace element composition of the bulk breccia is very similar to that of average basement granite impact glass fragments recovered from the unconsolidated suevitic breccia have a cipw normative composition similar to that of the basement granites no evidence for admixture from any of the minor intrusions was found the similarity of trace element abundances and ratios and ree patterns between impact glasses and granites favors derivation of the glasses from the granites the impact glass fragments show considerable enrichments of mg cr fe co ni and ir compared to the basement granites the abundances of these elements in the glasses after correction for indigenous concentrations can be explained by admixture of about 10 of a chondritic component high ir concentrations 100 ppb have been found in sulfide spherule samples which may complement the lower ir abundances in the glasses and could indicate some fractionation during impact re os isotopic studies were applied to further investigate the presence of a meteoritic component in the suevitic breccia the target granite shows very low osmium abundances of about 7 ppt and high 187 os 188 os ratios of about 0 72 that would be expected for old continental crust in contrast the breccia samples were found to have much higher osmium abundances 80 ppt and lower 187 os 188 os ratios of about 0 205 these values can be explained by mixing of target rocks with a chondritic component

a routine high precision method for lu hf isotope geochemistry and chronology a method for chemical separation of lu and hf from rock meteorite and mineral samples is described together with a much improved mass spectrometric running technique for hf this allows i geo and cosmochronology using the176lu 176hf decay scheme and ii geochemical studies of planetary processes in the earth and moon chemical yields for the three stage ion exchange column procedure average 90 for hf chemical blanks are 0 2 ng for lu and hf from 1 g of hf a total ion current of 0 5 10 11 ampere can be maintained for 3 5 h yielding 0 01 0 03 precision on the ratio176hf 177hf normalisation to179hf 177hf 0 7325 is used extensive results for the johnson matthey hf standard jmc 475 are presented and this sample is urged as an international mass spectrometric standard suitable aliquots prepared from a single batch of jmc 475 are available from denver lu hf analyses of the standard rocks bcr 1 and jb 1 are given the potential of the lu hf method in isotope geochemistry is assessed

pt re os systematics of group iiab and iiiab iron meteorites the pt re os isotopic and elemental systematics of 13 group iiab and 23 group iiiab iron meteorites are examined as has been noted previously for iron meteorite groups and experimental systems solid metal liquid metal bulk distribution coefficients d values for both iiab and iiiab systems show dos dre dpt 1 during the initial stages of core crystallization assuming closed system crystallization the latter stages of crystallization for each core are generally characterized by dpt dre dos the processes governing the concentrations of these elements are much more complex in the iiiab core relative to the iiab core several crystallization models utilizing different starting parameters and bulk distribution coefficients are considered for the re os pair each model has flaws but in general the results suggest that the concentrations of these elements were dominated by equilibrium crystallization and subsequent interactions between solid metal and both equilibrium and evolved melts late additions of primitive metal to either core were likely minor or nonexistent the 187re 187os systematics of the iiab and iiiab groups are consistent with generally closed system behavior for both elements since the first several tens of ma of the formation of the solar system consistent with short lived chronometers the re os isochron ages for the complete suites of iiab and iiiab irons are 4530 50 ma and 4517 32 ma respectively and are similar to previously reported re os ages for the lower ni endmembers of these two groups both isochrons are consistent with but do not require crystallization of the entire groups within 10 30 ma of the initiation of crystallization the first high precision 190pt 186os isochrons for iiab and iiiab irons are presented the pt os isochron ages for the iiab and iiiab irons calculated using the current best estimate of the for 190pt are 4323 80 ma and 4325 26 ma respectively the re os and pt os ages do not overlap within the uncertainties the younger apparent ages recorded by the pt os system likely reflect error in the 190pt decay constant the slope from the pt os isochron is combined with the age from the re os isochron for the iiiab irons to calculate a revised of 1 415 10 12 a 1 for 190pt although additional study of this decay constant is still needed

comparative reos smnd and rbsr isotope and trace element systematics for archean komatiite flows from munro township abitibi belt ontario abstract a series of komatiite flows from munro township ontario have re and os concentrations that range from 0 545 to 1 27 ppb and 1 00 to 2 20 ppb respectively the187re 186os in the suite range from 14 05 to 52 37 and the187os 186os range from 1 49 to 3 22 giving a crystallization age of2726 93ma and an of initial187os 186os of0 873 0 035 this age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems the re os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks the initial os isotopic composition is consistent with the derivation of the melts from a mantle source s that originated with a chrondritic 187os 186os i and evolved with an approximately chondriticre os sm nd data for the same suite cluster about anend value at 2 72 ga of 2 to 3 rb sr isotope data indicate significant mobilization of these elements by the greenschist grade metamorphism that has altered the rocks comparisons of re and os concentrations with those of mgo feo ni cr and s suggests that re was incompatible d 1 os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization and not by the modally predominant silicate minerals

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

sensitivity of trajectory calculations to the temporal frequency of wind data abstract a mesoscale primitive equation model is used to create a 36 h simulation of the three dimensional wind field of an intense maritime extratropical cyclone the control experiment uses the simulated wind field every 15 min in a trajectory model to calculate back trajectories from various horizontal and vertical positions of interest relative to synoptic features of the storm the latter trajectories are compared to trajectories that were calculated with the simulated wind data degraded in time to 30 min 1 h 3 h 6 h and 12 h various error statistics reveal significant deterioration in trajectory accuracy between trajectories calculated with 1 and 3 h data frequencies trajectories calculated with 15 min 30 min and 1 h data frequencies yielded similar results while trajectories calculated with data time frequencies 3 h and greater yielded results with unacceptably large errors

direct laboratory determination of the 187 re half life the long lived naturally occurring radionuclide 187re is important in geochemistry and cosmology as a nucleochronometer until now there have been no direct laboratory measurements which have avoided the difficulties of both low energy counting and dependence on radiometric ages of rocks and meteorites we report here a half life of 4 35 0 13 1010 yr based on the growth of 187os over a 4 yr period into a large source of osmium free rhenium since our result agrees with the best geochemically determined values no significant revision of the present galactic age limits based on the geochemical values is necessary the agreement at the 5 level of the decay rates determined on laboratory and meteorite age timescales places a tight constraint on cosmological models in which fundamental constants are allowed to vary in particular the fine structure constant must have changed by less than one part in 105 over the past 4 5 109 yr

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

burial rates during prograde metamorphism of an ultra high pressure terrane an example from lago di cignana western alps italy abstract estimation of burial rates and duration of prograde metamorphism of ultra high pressure uhp rocks t 590 630 c p 2 7 2 9 gpa of the zermatt saas ophiolite from lago di cignana italy may be made through combined lu hf and sm nd garnet geochronology in conjunction with petrologic estimates of the prograde p t path we report a lu hf garnet omphacite whole rock isochron age of 48 8 2 1 ma from the uhp locality at lago di cignana which stands in contrast to the sm nd age of 40 6 2 6 ma amato et al earth planet sci lett 171 1999 425 438 obtained from the same sample and mineral material the sm nd and lu hf ages as well as other ages determined on metamorphic garnet zircon and white mica amato et al earth planet sci lett 171 1999 425 438 mayer et al eur union geosci 10 1999 abstr 809 rubatto et al contrib mineral petrol 132 1998 269 287 dal piaz et al int j earth sci 90 2001 668 684 from lago di cignana and elsewhere in the zermatt saas ophiolite lie within a range of 50 38 ma which we interpret to encompass the duration of prograde metamorphism and possibly the duration of garnet growth the difference in measured sm nd and lu hf ages from cignana can be accounted for by expected core to rim variations in lu hf sm and nd the measured yttrium content in garnet which may be a proxy for lu is highest in garnet core and lowest in the mineral rim generally following a profile that is predicted by rayleigh fractionation preferential enrichment of lu in the core produces a lu hf age that is weighted toward the older garnet core sm nd ages as predicted by rayleigh fractionation of sm and nd during garnet growth however reflect later grown garnet as compared to lu hf ages the difference in sm nd and lu hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as sm nd and lu hf blocking temperatures were not exceeded for a long period of time based on the 12 myr duration of prograde garnet growth estimated in this study burial rates for rocks at lago di cignana were on the order of 0 23 0 47 cm yr these values correlate with continuous shortening rates of 0 4 1 4 cm yr between the european plate and the african adriatic promontory between 50 and 38 ma which is on the order of that calculated for plate velocities from plate reconstructions suggesting that the zermatt saas ophiolite may have remained well coupled to the down going slab to uhp conditions

prograde mica 40ar 39ar growth ages recorded in high pressure rocks syros cyclades greece abstract phengites from the eclogite and blueschist facies sequences of the cycladic island of syros greece have been dated by the in situ uv laser ablation 40 ar 39 ar method a massive phengite rich eclogite and an omphacite rich metagabbro were investigated the phengites are euhedral and coarse grained several 100 m strain free and exhibit no evidence for late brittle deformation or recrystallization apparent ages in these samples range from 43 to 50 ma for the phengite rich eclogite and 42 to 52 ma for the omphacitic metagabbro this large spread of ages is visible at all scales within individual grains as well as in domains of several 100 m and across the entire sample ca 2 cm such variations have been traditionally attributed to metamorphic cooling or the incorporation of excess argon however the textural equilibrium between the phengites and other high pressure phases and the subtle compositional variations within the phengites especially the preservation of growth textures alternatively suggest that the observed range in ages may reflect variations of radiogenic argon acquired during phengite formation and subsequent growth thus dating a discrete event on the prograde path this implies that the oldest phengite 40 ar 39 ar ages provide the best estimate of a minimum crystallization age which is in agreement with recently reported u pb and lu hf geochronological data our results are consistent with available stable isotope data and further suggest that under fluid restricted conditions both stable and radiogenic isotopic systems can survive without significant isotopic exchange during subduction and exhumation from eclogite facies p t conditions

comparison of eastern atlantic atmospheric trajectories for present day and last glacial maximum abstract for three deep sea core positions in the eastern atlantic ocean off africa gik 15669 34 53 n 7 49 w gik 16415 9 34 n 19 06 w and geob 1016 11 46 s 11 41 e 4 day isobaric back trajectories calculated on the 850 hpa level are presented the trajectories were computed for present day pd and for the last glacial maximum lgm cluster analysis was used for a concise assessment of trajectories the trajectories are clustered by year to investigate the interannual variability of the flow patterns and also by month for investigating the seasonal cycle the results indicate that for gik 15669 and geob 1016 the flow patterns are the same for both time slices but the wind speed was higher during the lgm for gik 16415 the flow pattern changed significantly from the lgm to pd and the wind speed was lower during the lgm

assessment of the 187re decay constant by cross calibration of re os molybdenite and u pb zircon chronometers in magmatic ore systems the past decade has seen renewed interest in 187re 187os geochronology using a variety of matrices including sulfide minerals shales and meteorites the most widely used value of the 187re decay constant 187re is 1 666 0 005 10 11 a 1 0 31 which is based on cross calibration of re os and pb pb chronometers for certain meteorites smoliar m i walker r j and morgan j w 1996 re os isotope constraints on the age of group iia iiia iva and ivb iron meteorites science 271 1099 1102 however other recent studies have yielded alternate values of 187re based upon either direct counting experiments or analysis of meteorites here we provide an independent assessment of 187re using methodology sample materials and preparation of os standard solutions different from those of smoliar et al 1996 combining re os age data for molybdenite formed in magmatic ore deposits with the u pb zircon age of the magmatic rocks a refined 187re value is determined by averaging 11 individual cross calibration experiments spanning ca 2700 ma of earth history using the u decay constants of jaffey jaffey a h flynn k f glendenin l e bentley w c and essling a m 1971 precision measurement of half lives and specific activities of 235u and 238u phys rev 4 1889 1906 a value for 187re of 1 6668 0 0034 10 11 a 1 is determined using the 238u value of jaffey et al 1971 and 235u value of schoene schoene b crowley j l condon d j schmitz m d and bowring s a 2006 reassessing the uranium decay constants for geochronology using id tims u pb data geochim cosmochim acta 70 426 445 a value for 187re of 1 6689 0 0031 10 11 a 1 is determined these values are nominally higher ca 0 1 and ca 0 2 than the value determined by smoliar et al smoliar m i walker r j and morgan j w 1996 re os isotope constraints on the age of group iia iiia iva and ivb iron meteorites science 271 1099 1102 but within calculated uncertainty further refinement of 187re by cross calibrating the molybdenite and u pb zircon chronometers should be possible by utilizing high precision single grain chemical abrasion zircon u pb analyses

smnd systematics of a silicate inclusion in the caddo iab iron meteorite we report the first sm nd data from an iron meteorite silicate inclusion mineral separates from an inclusion in the caddo iab iron define a 147 sm 143 nd age of 4 53 0 02 ga and an initial 146 sm 144 sm of 0 0086 0 0021 a plagioclase separate from the inclusion yields the lowest 142 nd 144 nd ratio measured to date which provides an absolute maximum initial solar system 142 of 2 1 0 4 when the 146 sm 142 nd data are combined with the 147 sm 143 nd data we obtain our preferred initial solar system 142 value of 3 2 u which corresponds to an initial 146 sm 144 sm of 0 009 rare earth element data from the caddo silicate inclusion suggest that it was derived by partial melting of a chondritic source prior to incorporation into the metal therefore differentiation of the silicate portion of the parent body must have started approximately within the first 30 ma of solar system history the data reported here clearly show that iab meteorites preserve a record of very early planetary differentiation processes modeling of element diffusion indicates that if the silicate material was incorporated into its feni host at a time much later than 4 53 ga then the mixing must have taken place at shallow levels i e 50 m in the parent body in order to preserve the sm nd systematics it is more likely that incorporation of the silicate took place at depths greater than 2 km and that it occurred within the first 30 ma of solar system history this is consistent with estimates of cooling rates determined for iab irons

the gardnos impact structure norway petrology and geochemistry of target rocks and impactites abstract the gardnos structure norway is an approximately circular area of anomalously fractured and brecciated rock about 4 5 km in diameter emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite the original recognition of gardnos as a deeply eroded impact structure between 900 and 400 ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks impactites deep erosion of the structure has erased the original rim removed much of the crater fill deposits and exposed large areas at or near the original crater floor however a wide variety of distinctive impactites fractured and blackened quartzites in the sub crater basement rocks lithic breccias and melt bearing breccias are still preserved these impactites show petrographic and chemical characteristics that confirm an impact origin distinctive planar deformation features pdfs in quartz and feldspar incipient melting of feldspar clasts in the melt bearing breccias close matches between the chemical composition of the breccias and mixtures of the target lithologies and the detection of an extraterrestrial component a minor extraterrestrial component 0 15 was detected in the melt bearing breccias based on significantly elevated ir and os contents and lower 187os 188os ratios compared to those in the target rocks the gardnos impactites are significantly enriched in c 5 10x over the exposed target rocks this may reflect the presence of a c rich shale overlying the metamorphic basement at the time of impact this idea is supported by 13c values of 28 1 to 31 5 measured in the impactites mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60 90 wt granite gneiss 0 30 wt amphibolite 0 12 wt quartzite and 3 19 wt of a c rich shale component the deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult one possible scenario involves the impact of a 300 m diameter stony meteorite that released 1019 j of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m the original crater was filled with at least 0 3 km3 of allochthonous melt matrix breccias containing about 0 06 km3 of impact melt the structure underwent low grade greenschist metamorphism in caledonian time about 400 ma ago and was subsequently eroded to its present appearance the value of the gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone in the preservation of a possibly complete sequence of crater fill breccias beneath a cap of elastic sediments and in the unusual carbon enrichment of its impactites

lu hf and sm nd isotopic systematics in chondrites and their constraints on the lu hf properties of the earth sm nd and lu hf isotopic data are presented for 19 chondritic meteorites six carbonaceous chondrites five l chondrites seven h chondrites and a single enstatite chondrite the primary goal of the study is to better define the bulk silicate earth bse reference values for hf isotopes except for one sample with lower sm nd the sm nd data define a cluster around the accepted reference values for chondrites and terrestrial planets giving a mean 147sm 144nd of 0 1960 0 0005 and a mean 143nd 144nd of 0 512631 0 000010 uncertainties are two standard errors it seems appropriate to retain the presently accepted sm nd reference parameters 147sm 144nd 0 1966 and 143nd 144nd 0 512638 when fractionation corrected to 146nd 144nd 0 7219 lu hf isotopic data are not clustered but spread along an approximate 4 5 ga isochron trend with a range of 176lu 177hf from 0 0301 to 0 0354 the data are similar to many of the samples of chondrites presented by bizzarro et al nature 421 2003 931 but lack the range to lower lu hf shown by those authors our chondrite data define a regression line of 4 44 0 34 ga when 1 867 10 11 year 1 is used for the decay constant of 176lu science 293 2001 683 earth planet sci lett 219 2004 311 324 combining our data with the main population of analyses from bizzarro et al nature 421 2003 931 yields 4 51 0 24 ga unless samples of eucrite meteorites and deviating replicates of chondrites with 176lu 177hf less than 0 030 are employed no combination of the main population of chondrite lu hf data yields a regression with sufficiently low error to constrain the decay constant of 176lu sample heterogeneity seems to hinder the acquisition of reproducible lu hf analyses from small manually ground pieces of chondrites and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the lu hf isotope systematics of chondritic reservoirs and of bse our results for carbonaceous chondrites show higher average 176lu 177hf and 176hf 177hf than ordinary chondrites and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite the allende powder from the smithsonian institution use of the carbonaceous chondrite mean for bse lu hf characteristics results in a bse hf nd point that lies well within the array of terrestrial compositions and leads to plausible initial ehf values for precambrian rocks an improved objective resolution of meteorite data and of meteoritic models for the earth needs to occur before bse can be established for lu hf

tectonic evolution of the grenville orogen in the central appalachians abstract remnants of the grenville orogeny a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent rodinia between ca 1 3 and 1 0 ga are exposed throughout eastern north america for ca 3000 km from labrador to alabama although tectonic models for the development of the grenville have historically been based on observations from the large and well exposed canadian grenville province recent investigations suggest that initial continental collision occurred proximal to the appalachian blue ridge mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during paleozoic orogenesis to better understand the tectonic history of this critical region we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho and paragneiss from the appalachian blue ridge province the resultant dates together with new results from trace element thermobarometry define a prolonged record of high temperature metamorphism coincident with the timing of continent continent collision as recognized in the northern portion of the orogen initial garnet and monazite growth commenced between 1100 and 1065 ma with peak granulite facies conditions 750 900 c 0 55 0 85 gpa occurring from 1050 to 1010 ma burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring 20 30 myr after deposition of the sedimentary protoliths local hydrothermal alteration followed peak metamorphism and occurred at 1000 990 ma with later cooling through 660 c between 980 and 935 ma 3 2 c myr the metamorphic history outlined here together with insights from previous studies is consistent with formation of the blue ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the himalayan tibetan orogen

trace a trajectory intercomparison 2 isentropic and kinematic methods kinematic and isentropic trajectories are compared quantitatively during a single 5 day period october 13 18 1992 when several flights for the transport and atmospheric chemistry near the equator atlantic trace a experiment were conducted off the west coast of africa european centre for medium range weather forecasts ecmwf data are used to compute the 5 day backward trajectories arriving at locations over the south atlantic ocean and nearby parts of south america and southern africa two versions of kinematic trajectories are examined one version employs vertical motions supplied with the ecmwf data these trajectories often differ greatly from those based on the isentropic assumption the kinematic trajectories usually undergo considerably greater vertical displacements than their isentropic counterparts however most diabatic rates are consistent with those of synoptic scale systems ratios of acetylene to carbon monoxide are related to backward trajectories at various locations along a trace a flight a second version of kinematic trajectories employs vertical motions diagnosed from ecmwf horizontal wind components using the continuity equation these vertical motions are stronger than those supplied with the ecmwf data causing many of the trajectories to have larger vertical displacements and considerably different paths than the original kinematic versions many of these kinematic trajectories undergo diabatic rates that exceed generally accepted values on the synoptic scale this occurs in part because the diagnosed vertical motions are inconsistent with the ecmwf data the research indicates that the kinematic procedure yields realistic 5 day backward trajectories when the three dimensional wind data are available from a numerical model or other dynamically consistent data set such as provided by ecmwf

the structure of the polar vortex reconstruction of the airborne antarctic ozone experiment and airborne arctic stratosphere expedition aircraft constituent observations radiative heating rate computations and trajectory calculations are used to generate comparative pictures of the 1987 southern hemisphere sh late winter and 1989 northern hemisphere nh mid winter lower stratospheric polar vortices overall both polar vortices define a region of highly isolated air where the exchange of trace gases occurs principally at the vortex edge through erosional wave activity aircraft measurement showed that 1 between 50 and 100 mbar horizontally stratified long lived tracers such as n20 are displaced downward 2 3 km on the cyclonic poleward side of the jet with the meridional tracer gradient sharpest at the jet core 2 eddy mixing rates computed using parcel ensemble statistics are an order of magnitude or more lower on the cyclonic side of the jet compared to those on the anticyclonic side 3 poleward zonal mean meridional flow on the anticyclonic side of the jet terminates in a descent zone at the jet core despite the similarities between the sh and nh winter vortices there are important differences during the aircraft campaign periods the sh vortex jet core was located roughly 8 o 10 o equatorward of its nh counterpart after pole centering as a result of the arger size of the sh vortex the dynamical heating associated with the jet core descent zone is displaced further from the pole the sh polar vortex can therefore approach radiative equilibrium temperatures over a comparatively larger area than the nh vortex the subsequent widespread formation of polar stratospheric clouds within the much colder sh vortex core gives rise to the interhemispheric differences in the reconstructed h20 noy c10 and 03 species which are affected by polar stratospheric clouds