unraveling the evolution of chondrite parent asteroids by precise u pb dating and thermal modeling u th pb isotopic data are reported for mineral fractions individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite richardton h5 chondrules and milligram sized fractions of pyroxene rich chondrule fragments contain highly radiogenic pb and concordant or nearly concordant u th pb isotopic systems and are suitable for precise pb pb age determinations olivine and sulfide have low u concentrations and contain less radiogenic pb the ages of individual chondrules pyroxene rich and phosphate fractions are determined using u pb and pb pb isochron and model date calculations the pb pb isochron date of 4562 7 1 7 ma of the richardton chondrules and chondrule fragments is resolved from the pb pb isochron date of 4550 7 2 6 ma obtained from multiple phosphate fractions possible biases of the isochron dates due to single stage approximation of multi stage evolution contamination with modern common pb and disturbance to the system by reheating are examined and are found to be insignificant the chondrule and phosphate dates are interpreted as the timing of cessation of pb diffusion during cooling following metamorphism in chondrite parent bodies the difference in estimated closure temperatures 950 1150 k for pyroxenes and 700 800 k for phosphates temperature estimates are based on published diffusion rates for pb in pyroxenes and apatite allows evaluation of the average cooling rate at 26 13 k million years for the richardton parent body over the period of 4563 4551 my thermal modeling of the h chondrite parent body which is assumed to be asteroid 6 hebe heated by decay of 26al suggests a scenario in which accretion initiated at 1 7 m y after formation of calcium aluminum rich inclusions and continued for 3 5 m y

osmium in the rivers there is a large uncertainty in our understanding of the behavior of osmium during weathering and transport into deep oceans and the osmium budget of the oceans the problem stems chiefly from the lack of osmium data on the dissolved load in the rivers and in the estuaries in this study the concentration and isotopic composition of osmium have been determined in three north american rivers the mississippi the columbia and the connecticut and one river draining central europe and flowing into the baltic sea the vistula osmium concentration in the mississippi and the vistula is about 45 femto mol kg 1 it is about 14 and 15 femto mol kg 1 for the connecticut and the columbia respectively the 187 os 186 os ratios estimated for the mississippi and the vistula are 10 4 and 10 7 respectively for the connecticut and the columbia 187 os 186 os 8 8 and 14 4 respectively of all the rivers examined the mississippi is by far the largest supplying 1 6 of the total annual world river flow its osmium isotopic composition is identical to the upper mississippi valley loesses esser and turekian 1993a indicating 1 congruent dissolution of the bedrock and 2 little or no impact of anthropogenic sources on the osmium isotopic composition of the dissolved load the latter observation indicates that the upper limit of the anthropogenic input in the dissolved osmium load of the mississippi outflow is about 250 g yr 1 while the osmium concentration of the vistula is high the isotopic composition does not appear to have been affected by substantial pollution the river data can be used to put limits on the mean residence time of osmium in the oceans t os and on the osmium budget of the oceans if the bulk river influx of dissolved osmium into the oceans is similar to that of the mississippi we get a value for the net riverine inflow of osmium of 1680 mol yr 1 if there were no sequestering of osmium in the estuaries this would give a value of t os of 1 3 10 4 years as the estuaries may trap a significant amount of river osmium this is a lower limit so that t os 1 3 x 10 4 years using the iridium data by anbar et al 1996 the os ir ratio for the vistula is 2 8 we infer that there is no large fractionation between osmium and iridium during the transport of these elements from the continents into the oceans it follows that the high os ir ratio 22 observed in seawater must be related to the different rain out mechanisms of these elements in the estuaries or in the oceans

re os isotope systematics and weathering of precambrian crustal rocks implications for the marine osmium isotope record the effects of weathering of freshly exposed precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western wind river range in wyoming the os budget of the precambrian granitoid source rocks is dominated by os rich trace phases such as magnetite amongst the major silicates biotite or a trace phase in biotite carries most of the os and re and is also the most radiogenic mineral with 187os 186os of 113 and 187re 186os of 2 000 re os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes os with an isotopic composition similar to the isotopic composition of the bulk soils a very radiogenic fraction of os is mobilized through preferential weathering of biotite radiogenic runoff from precambrian shields inferred from osmium isotope analyses of freshwater fe mn nodules suggests that high latitude precambrian shields are important source areas of radiogenic os in seawater we propose that glacial scouring and weathering of glacial tills exposed after deglaciation of precambrian shields surrounding the north atlantic provides a mechanism for the slightly more radiogenic nature of north atlantic seawater compared to other seawater masses glacial interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial interglacial time scales in high latitude shield areas surrounding the north atlantic

further evidence for a low u pb source in the moon uthpb smnd and arar isotopic systematics of lunar meteorite yamato 793169 abstract the coarse grained lunar meteorites yamato 793169 and asuka 881757 represent a new type of low ti mare basalt this paper reports the results of a u th pb sm nd and ar ar isotopic study of yamato 793169 performed as part of a consortium studies of lunar basaltic meteorites the isotopic study was carried out on a small sample 100 mg so that only three density fractions could be separated these fractions were leached with dilute acid in order to eliminate terrestrial pb contamination however the leaching procedure did not completely remove this contamination in some fractions and also apparently caused a fractionation of elements u th pb due to preferential leaching effects producing a secondary disturbance of the systematics furthermore the ar ar analyses indicate that the isotopic systematics in this meteorite might have been disturbed sometime later than 750 ma for these reasons the ages obtained using different isotopic systems disagree with each other and a precise formation age could not be obtained for this meteorite however using what results there are two reasonable interpretations can be made 1 the sm nd system yielded an age of 3 4 ga that could be interpreted as the formation age assuming that this system is possibly the least disturbed during the metamorphic event s that this meteorite experienced at least once and that all other isotopic systems and their corresponding ages were disturbed and 2 the u pb system yielded a nearly concordant age of 3 8 ga that could be interpreted as the formation age and the sm nd isotopic systematics were somehow disturbed we prefer the second interpretation for the reasons discussed below on a concordia diagram the cdt canon diablo troilite corrected u pb isotopic data yield a discordia line similar to the lunar catastrophic array indicating that the source of the meteorite formed during early lunar differentiation 4 4 ga and that the basalt was generated near 3 9 ga total ar ar age on plagioclase is 3 26 ga which seems to be too old if the formation age is 3 4 ga because low temperature fractions lost large amounts of radiogenic 40 ar during the late thermal event if we assume a formation age of 3 8 ga the estimated source 238 u 204 pb is 21 6 3 5 and nd is 3 9 0 3 these results indicate that the source of yamato 793169 is more depleted than apollo 12 and 15 lt basalts but less depleted than asuka 881757 therefore yamato 793169 may represent a new type of lt or vlt like mare basalt that is different from asuka 881757 the wide variety of lead and neodymium isotopic characteristics among lt and vlt mare basalts indicate that the lunar mantle was very heterogeneous with respect to trace element abundances

effects of black shale weathering on the mobility of rhenium and platinum group elements radiogenic os from the continents dominates the late cenozoic marine os budget a variety of proposed radiogenic sources includes organic rich sediments precambrian shields and re rich sulfides identifying the dominant sources of radiogenic os to the oceans has a strong impact on past climate models because weathering of silicates acts as a net sink of co 2 whereas weathering of organic matter acts as a source here we investigate the effects of black shale weathering on the integrity of the re os isotope system and release of re and platinum group elements time correlative pairs of fresh drill core and weathered outcrop black shales of late ordovician age utica shale magnafacies quebec were analyzed for re os ir pt and pd as well as total s n and c as c carb and c org the results indicate that 25 64 of the initial re budget 45 90 of the os budget 14 65 of the ir budget 60 77 of the pt budget 69 86 of the pd budget and 44 96 of the c org budget are lost during weathering we use positive correlations of os pt and pd with c org and n to calculate the respective platinum group element concentrations in the unweathered organic matter end member as 3 ng g os 9 ng g pt and 11 24 ng g pd although black shales make up

search for the proverbial mantle osmium sources to the oceans hydrothermal alteration of mid ocean ridge basalt abstract we present os and sr isotopes and mg os and sr concentrations for ridge crest high temperature and diffuse hydrothermal fluids plume fluids and ridge flank warm spring fluids from the juan de fuca ridge the data are used to evaluate the extent to which 1 the high and low temperature hydrothermal alteration of mid ocean ridge basalts morbs provides os to the deep oceans and 2 hydrothermal contributions of non radiogenic os and sr to the oceans are coupled the os and sr isotopic ratios of the high temperature fluids 265 353 c are dominated by basalts 187 os 188 os 0 2 87 sr 86 sr 0 704 but the concentrations of these elements are buffered approximately at their seawater values the 187 os 188 os of the hydrothermal plume fluids collected 1 m above the orifice of hulk vent is close to the seawater value 1 05 the low temperature diffuse fluids 10 40 c associated with ridge crest high temperature hydrothermal systems on average have os 31 fmol kg 1 187 os 188 os 0 9 and sr 86 mol kg 1 87 sr 86 sr 0 709 they appear to result from mixing of a high temperature fluid and a seawater component the ridge flank warm spring fluids 10 62 c on average yield os 22 fmol kg 1 187 os 188 os 0 8 and sr 115 mol kg 1 87 sr 86 sr 0 708 the data are consistent with isotopic exchange of os and sr between basalt and circulating seawater during low temperature hydrothermal alteration the average sr concentration in these fluids appears to be similar to seawater and consistent with previous studies in comparison the average os concentration is less than seawater by more than a factor of two if these data are representative they indicate that low temperature alteration of morb does not provide adequate non radiogenic os and that another source of mantle os to the oceans must be investigated at present the magnitude of non radiogenic sr contribution via low temperature seawater alteration is not well constrained if non radiogenic sr to the oceans is predominantly from the alteration of morb our data suggest that there must be a different source of non radiogenic os and that the os and sr isotope systems in the oceans are decoupled

the petrology and geochemistry of miller range 05035 a new lunar gabbroic meteorite miller range mil 05035 is a lunar gabbroic meteorite the mineralogy fe mn ratios in olivine and pyroxene bulk rock chemical composition and the bulk oxygen isotope values 17o 2 86 2 97 and 18o 5 47 5 71 are similar to those of other mare basalts and are taken as supporting evidence for a lunar origin for this meteorite the sample is dominated by pyroxene grains 54 61 by area mode of thin section along with large plagioclase feldspar 25 36 by mode and accessory quartz ilmenite spinel apatite and troilite the bulk rock major element composition of mil 05035 indicates that the sample has a very low ti vlt to low ti lunar heritage we measure bulk tio2 to be 0 9 wt and has low bulk incompatible trace element ite concentrations akin to samples from the vlt mare basalt suite to account for these geochemical characteristics we hypothesize that mil 05035 s parental melt was derived from a mantle region dominated by early cumulates of the magma ocean comprised principally of olivine and orthopyroxene mil 05035 is likely launch paired with the asuka 881757 and yamato 793169 basaltic lunar meteorites and the basaltic regolith breccia met 01210 this group of meteorites y a m m therefore may be a part of a stratigraphic column consisting of an upper regolith environment underlain by a coarsening downwards basalt lava flow

re os depositional age for archean carbonaceous slates from the southwestern superior province challenges and insights abstract rhenium osmium re os dating of carbonaceous rocks using h 2 so 4 cro 3 dissolution provides a powerful new geochronologic tool for archean terranes here we document the oldest re os depositional ages yet reported for carbonaceous sedimentary rocks carbonaceous slates are intercalated with graywackes and mafic tuffs of the joy lake sequence in the western wawa subprovince of the superior province well developed d 2 cleavage is sub parallel to bedding pyrite occurs in nodules disseminations and bedding parallel stringers samples from two drill holes dh26503 and dh26506 yield isochron ages of 2695 14 ma and 2684 16 ma and initial 187 os 188 os ratios os i of 0 15 0 16 and 0 29 0 20 respectively the ages identical within uncertainty provide the first temporal constraints for the westernmost wawa subprovince greenstone sequence the ages are consistent with cessation of mafic ultramafic volcanism before ca 2695 ma in the western wawa subprovince and d 2 deformation before 2685 ma farther east the joy lake sequence is likely the temporal equivalent of turbiditic graywackes deposited further east both sourced from arc volcanoes during collision of the wawa and wabigoon arcs along the southern margin of the superior province a chondritic os i 0 109 at 2 69 ga is expected for archean shales but the negative os i for dh26506 requires explanation re which is significantly more mobile than os may have been released from intercalated high re os mafic tuffs by local oxidation during recent or cretaceous exposure to meteoric waters the adjacent carbonaceous rocks are a ready sink for mobile re a small re addition can shift isochron data points to higher 187 re 188 os ratios with minimal slope change this increases scatter and uncertainty for the os i shifts the isochron to the right and produces a spuriously low os i intercept if re addition is recent the age is not significantly affected notably the slates in dh26506 which yield the negative os i are intercalated with relatively permeable and reactive lithic wackes and mafic tuffs whereas the slates in dh26503 are in contact with less permeable massive intrusive rocks this process can be detected only if the true os i is approximately chondritic and is more likely where reactive mafic rocks are prevalent both the case for archean greenstone belts

osmium isotopes in hydrothermal fluids from the juan de fuca ridge we present os data for axial high temperature and off axial low temperature hydrothermal solutions from the juan de fuca ridge the high temperature h s bearing axial fluids have unradiogenic os isotopes pointing to a nearly complete domination of osmium isotopes from the basalts during hydrothermal circulation the ridge axis fluids typically do not show large enrichment in osmium concentration over seawater although one high temperature fluid has an os concentration enhanced by a factor of 4 above seawater it appears that the os concentration of high temperature hydrothermal fluids is typically buffered at roughly the seawater concentration we suggest that subseafloor precipitation of pyrite from the high temperature hydrothermal fluids controls the osmium transportation the axial hydrothermal activity does not supply significant amounts of unradiogenic osmium to the deep oceans in contrast a low temperature off axis fluid is enriched in non radiogenic osmium over seawater by a factor of 9 showing much less precipitation of osmium at low temperature because a large fraction of the cooling of oceanic lithosphere occurs on ridge flanks os from low temperature ridge flank hydrothermal circulation may be a significant contributor to the balance of os in the oceans a detailed balance between os contributions from dissolution of cosmic dust and hydrothermal fluids is still not possible the low temperature hydrothermal sample gives os os 0 110 0 001 this extremely unradiogenic osmium can only come from a source that underwent depletion of re over 2 6 ga ago assuming no contamination during sample collection this result suggests that the convecting upper mantle contains ancient depleted material that imparted unradiogenic osmium to intruding basaltic melts

ion microprobe u th pb dating and ree analyses of phosphates in the nakhlites lafayette and yamato 000593 000749 u th and pb isotopes and rare earth elements rees in individual phosphate grains from martian meteorites lafayette and yamato 000593 000749 were measured using a sensitive highresolution ion microprobe shrimp observed u pb data of 12 apatite grains from yamato y 000593 000749 are well represented by linear regressions in both conventional 2d isochron plots and the 3d u pb plot total pb u isochron indicating that the formation age of this meteorite is 1 53 0 46 ga 2 on the other hand the data of nine apatite grains from lafayette are well represented by planar regression rather than linear regression indicating that its formation age is 1 15 0 34 ga 2 and that a secondary alteration process slightly disturbed its u pb systematics as discussed in the literature regarding nakhla the observed ree abundance patterns of the apatites in lafayette and yamato 000749 normalized to ci chondrites are characterized by a progressive depletion of heavy rees hrees a negative eu anomaly similarity to each other and consistency with previously reported data for nakhla considering the extensive data from other radiometric systems such as sm nd rb sr ar ar and trace elements our results suggest that the parent magmas of the nakhlites including the newly found y 000593 000749 are similar and that their crystallization ages are 1 3 ga

diffusion of osmium in pyrrhotite and pyrite implications for closure of the re os isotopic system abstract in order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite experiments consisted of either 1 isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered os bearing fe sulfide or 2 relaxation of initially high near surface osmium concentrations produced in the latter experiments pyrite only osmium penetration into samples was characterized by depth profiling using rutherford backscattering spectroscopy rbs pyrite or electron microprobe analyses across sectioned run products pyrrhotite results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces with the extent of penetration uncorrelated with run duration images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near surface occurred by solute uptake during step growth and not by volume diffusion prolonged 1000 h relaxation experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for rbs a maximum diffusion coefficient of 2 5 10 23 m2 s at 500 c was calculated experiments involving pyrrhotite over the temperature range of 950 1100 c showed extensive osmium uptake and osmium concentration gradients that conform with fickian diffusion behavior we found that pyrrhotite fe s could be varied by changes in the composition of the starting material and osmium source and over the range of fe s produced in experiments molar fe s 0 83 0 90 we observed no systematic variation in the osmium diffusion coefficient diffusion coefficients measured parallel to the a crystallographic axis were on average 1 4 higher than values measured parallel to c and regression of the c axis data yielded the arrhenius relation d t 1 31 0 16 10 5 m 2 s exp 211 5 14 7 kj mol rt the application of these diffusion data to simple models of diffusive exchange during static or polythermal time temperature histories is used to assess the conditions under which radiogenic osmium will be retained during isothermal annealing calculations indicate that the cores of millimeter sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in 0 5 ma at temperatures exceeding 400 c pyrite undergoing the same process at 500 c requires in excess of 10 ma before crystal cores are affected the relatively short core retention time scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events thus possibly accounting for the scatter that commonly occurs in re os isochrons generated from massive sulfide samples calculated closure temperatures tc for osmium exchange in pyrrhotite yielded values of 300 400 c for grain sizes ranging from 10 to 1000 m these values of tc are similar to those calculated for ar retention in biotite and considerably lower than for sr in apatite and plagioclase for example such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late stage hydrothermal fluids this latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage

ion microprobe u pb dating of apatite abstract to illustrate in situ sensitive high resolution ion microprobe shrimp analysis of apatites pb isotopes and pb u ratios of five apatite samples with different ages have been measured using shrimp ii ion microprobe at hiroshima university the prap apatite from a carbonatite complex at prairie lake ontario with a 207 pb 206 pb age of 1156 45 ma 2 was used as the standard for 206 pb 238 u 238 u 16 o 238 u calibration apatites from the poohbah lake alkaline complex give 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages of 2726 145 ma 2 and 2701 86 ma 2 in agreement with a conventional tims 207 pb 206 pb age of 2667 10 ma 2 and a k ar isochron age of 2706 46 ma 2 for the remaining three apatite samples the 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages obtained by shrimp are consistent with radiometric and geological ages in the literature

primitive os and 2316 ma age for marine shale implications for paleoproterozoic glacial events and the rise of atmospheric oxygen abstract re os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the rooihoogte and timeball hill formations transvaal supergroup south africa provides a precise isochron age of 2316 7 ma 4 statistical uncertainty if error on decay constant is excluded and a chondritic initial 187os 188os ratio of 0 1121 0 0012 these units were deposited between what are interpreted as the second and third of three paleoproterozoic global glacial events and thus provide minimum and maximum ages respectively for these events the rooihoogte formation is correlative with the duitschland formation which records previously undated carbon isotope excursions because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative 34s values the rise of atmospheric oxygen most likely began prior to 2 32 ga the chondritic initial 187os 188os ratio requires that primitive hydrothermal sources of os dominated the os budget of seawater at 2 32 ga os is mobile only under oxidizing conditions therefore the chondritic initial 187os 188os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic os to offset the primitive hydrothermal magmatic flux even if atmospheric oxygen levels had increased significantly by 2 32 ga anoxic conditions throughout the archean most likely limited os enrichment in archean marine shales so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic os to paleoproterozoic seawater

a possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter the os isotopic composition of leachable os from a north pacific pelagic clay sequence pegram et al 1992 and bulk sediment samples of metalliferous carbonates deposited near the east pacific rise ravizza 1993 display similar patterns of temporal variation fig 1 this similarity indicates that the composite record of t87os 186os ratio variations reflects changes in the os isotopic composition of seawater temporal changes in the sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration the os isotopic composition of seawater should be influenced by similar processes in both the rb sr and re os systems the continental crust is characterized by large time integrated parent daughter ratios compared to the deep earth imparting relatively radiogenic sr and os signatures to continental material however the seawater sr and os records are decoupled from one another over the past 27 ma from 27 to 15 ma 87sr 86sr increased rapidly while the 18vos 186os ratio of seawater remained nearly constant at 15 ma the 87os ts6os ratio of seawater began to increase rapidly but the rate

petrogenesis and chronology of lunar meteorite northwest africa 4472 a kreepy regolith breccia from the moon northwest africa nwa 4472 is a polymict lunar regolith meteorite the sample is kreep rich high concentrations of potassium rare earth elements and phosphorus and comprises a heterogeneous array of lithic and mineral fragments these clasts and mineral fragments were sourced from a range of lunar rock types including the lunar high magnesian suite the high alkali suite kreep basalts mare basalts and a variety of impact crater environments the kreep rich nature of nwa 4472 indicates that the sample was ejected from regolith on the nearside of the moon in the procellarum kreep terrane and we have used lunar prospector gamma ray remote sensing data to show that the meteorite is most similar to and most likely sourced from regoliths adjacent to the imbrium impact basin u pb and pb pb age dates of nwa 4472 phosphate phases reveal that the breccia has sampled pre nectarian 4 35 ga rocks related to early episodes of kreep driven magmatism some younger phosphate u pb and pb pb age dates are likely indicative of impact resetting events at 3 9 4 ga consistent with the suggested timing of basin formation on the moon our study also shows that nwa 4472 has sampled impact melts and glass with an alkali depleted incompatible trace element rich high sc low rb th ratios low k compositional signature not related to typical apollo high k kreep or that sampled by kreepy lunar meteorite sayh al uhaymir sau 169 this provides evidence that there are numerous sources of kreep rich protoliths on the moon

ion microprobe u th pb dating of phosphates in martian meteorite alh 84001 phosphates in martian meteorites are important carriers of trace elements although they are volumetrically minor minerals po4 also has potential as a biomarker for life on mars here we report measurements of the u th pb systematics of phosphates in the martian meteorite alh 84001 using the sensitive high resolution ion microprobe shrimp installed at hiroshima university japan eleven analyses of whitlockites and 1 analysis of apatite resulted in a total pb u isochron age of 4018 81 ma in the 238u 206pb 207pb 206pb 204pb 206pb 3 d space and a 232th 208pb age of 3971 860 ma these ages are consistent within a 95 confidence limit this result is in agreement with the previously published ar ar shock age of 4 0 0 1 ga from maskelynite and other results of 3 8 4 3 ga but are significantly different from the sm nd age of 4 50 0 13 ga based on the whole rock and pyroxene taking into account recent studies on textural and chemical evidence of phosphate our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 81 ma and that since then alh 84001 has not experienced a long duration thermal metamorphism which would reset the u pb system in phosphates

the isotopic record of lunar volcanism abstract the timing and extent of lunar volcanism was determined primarily by the internal thermal evolution of the moon a late heavy bombardment produced most of the observable lunar basins easily recognized stratigraphic markers and also may have influenced lunar volcanism most of the nearside basins seem to have been produced 3 8 4 0 ga ago during a period often referred to as the lunar cataclysm the pre cataclysm period was characterized by formation of the lunar crust plutonic rocks within the crust and pre mare volcanism isotopic analysis of breccia clasts pre dating the cataclysm has yielded an outline of this period the isotopic record of post cataclysm mare volcanism is clear but incomplete because samples from the youngest mare areas are lacking specifically the isotopic data show that some lunar anorthosites formed 4 45 ga ago consistent with early formation of an anorthositic lunar crust some mg suite norites troctolites and dunites also formed approximately contemporaneously with the oldest anorthosites but others are up to 200 300 ma younger more evolved rocks granites and quartz monzodiorites also formed between 4 4 and 4 0 ga ago the earliest known pre mare volcanism at 4 25 ga ago included aluminous mare like basalts and an olivine gabbronorite similar to apollo 12 basalts kreep basalt volcanism began at least 4 1 ga ago approximately contemporaneously with that of the ancient mare like basalts and continued during the cataclysm recorded instances of high ti mare basalt volcanism began during or immediately after the cataclysm low ti mare basalt volcanism is the most recent for which samples are available the isotopic data record an early primary lunar differentiation during which mare basalt sources formed probably by crystallization of cumulus minerals from a lunar magma ocean the isotopic record of post cataclysm basalts indicates a heterogeneous lunar mantle and can be explained by partial melting of cumulate sources but more complex petrogenesis also has been suggested more complex petrogenetic processes including assimilation and assimilation fractional crystallization afc seem to be required for pre mare volcanics the isotopic data contain a hint that petrogenesis of the oldest pre mare volcanics may be linked to formation of the so called procellarum basin possibly the oldest and largest lunar basin such a basin would provide a geologic context for the pre mare volcanics and its formation may have been a defining moment in lunar history

middle proterozoic ocean chemistry evidence from the mcarthur basin northern australia we report on the iron sulfur systematics and s isotopic compositions of upper paleoproterozoic carbonaceous shales from the wollogorang and lower reward formations mcarthur basin northern australia iron speciation in these rocks is similar to that of modern euxinic black sea sediments and distinct from sediments depositing from oxygenated water providing persuasive evidence of euxinic depositional environments in a mid proterozoic marine basin high values of degree of pyritization dop also indicate anoxic bottom water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides the isotopic composition of the sulfides is however quite enriched in 3 4 s and for the lower reward formation similar to contemporaneous seawater sulfate these 3 4 s enriched sulfides are consistent with extensive or even complete sulfate depletion within the euxinic basins in which the sediments were deposited based on analogy with modern euxinic environments and further constrained by model results middle proterozoic seawater sulfate concentrations were probably within the range of 0 5 to 2 4 mm taken together our results indicate a relatively low sulfate late paleoproterozoic ocean and sulfidic deep waters in the mcarthur seaway a condition that could have encompassed much larger regions of the ocean

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

tectonic evolution and metallogenesis of north china craton the north china craton ncc is one of the oldest cratons in the world with the age of the old crust up to 3 8 ga it has a complicated evolution history and has recorded almost all the important geological events of the earth the main early precambrian geological events and key tectonic issues are as follows old continental nuclei and main crustal growth in the ncc took place at 2 7 2 9 ga by 2 5 ga the micro blocks had amalgamated to form a coherent craton the 2300 1950 ma paleoproterozoic mobile belts came into being the 1800 ma lower crust uplifted as a whole and there occurred mafic dyke swarm continental rifting and intrusion of orogenic magmatic association in the period of neoproterozoic and paleozoic the ncc was tectonically inactive but was affected by the central asian orogenic belt the north china craton probably experienced disruption and reconstruction in mesozoic and belonged to the circling pacific orogenic belt in cenozoic the north china craton has abundant ore deposits corresponding in age to the above important geological events mesozoic metallogenesis was closely related to mesozoic tectonic inversion and lithospheric thinning the main ore forming types include porphyry mo polymetallic deposits and large scale metallogenic gold explosion related to partial melting of the basement and granitic intrusion

holocene climate change inferred from stratigraphy and osl chronology of aeolian sediments in the qaidam basin northeastern qinghai tibetan plateau abstract paleoclimatic reconstruction based on aeolian sediments in the eastern qaidam basin qb has been hindered by the limited chronological data here we present 61 optically stimulated luminescence osl ages on the basis of these osl ages and the lithologic stratigraphy we propose the effective moisture index emi for aeolian sediments to reconstruct the effective moisture change based on the emi from twelve sections the effective moisture change moisture sources and relevant mechanisms for paleoclimatic change in the eastern qb are discussed the results indicate that 1 aeolian deposition started at least before 12 4 0 7 ka during the deglaciation the paleosols developed at the early and mid holocene and aeolian sand and loess accumulated at mid and late holocene 2 effective moisture history was hyper arid at 12 8 11 6 ka humid and variable at 11 6 8 3 ka moderately humid and stable at 8 3 3 5 ka and increasingly arid at 3 5 0 ka 3 the effective moisture change was mainly controlled by the asian summer monsoon asm which mainly followed the change of northern hemispheric summer insolation and the westerlies strengthened and increased the aridity in the qb when the asm shrank

deposition of osmium and other platinum group elements beneath the ultramafic hosted rainbow hydrothermal plume osmium and other platinum group elements pges have been measured in sediments accumulating directly under the rainbow hydrothermal plume at 36 deg n on the mid atlantic ridge 187os 188os and pge concentrations in the sediments of four cores taken 2 25 km from the active vent site reveal evidence of both radiogenic os scavenged from seawater and importantly unradiogenic os released by hydrothermal venting the majority of analyses n 16 yield 187os 188os ratios that fall in a narrow range 0 99 1 06 close to that of modern seawater in these samples os concentrations correlate linearly with hydrothermal v enrichments which are also derived from seawater scavenging a smaller subset of samples characterized by unusually high concentrations of ni and cr derived from ultramafic rock fragments exhibit distinctly lower 187os 188os ratios their ir content is also higher and their pge ratios are more similar to mantle material than the pge patterns of the larger set of samples dominated by seawater scavenged os although the two most proximal cores lack evidence for any similar ultramafic detritus in their surficial sediments 187os 188os ratios in these samples are also lower than that of ambient seawater suggesting they are likely to have been influenced by unradiogenic os from vent fluid incorporated into the plume samples from the most distal core by contrast exhibit 187os 188os ratios indistinguishable from that of seawater compared with background ne atlantic sediments os is enriched four fold in the seawater dominated samples while pd pt and ir are twice the background value burial fluxes of os scavenged by hydrothermal fe in these sediments are 200 400 pg cm 2 kyr 1 up to twice that found on the epr and several times the background bg of 50 pg cm 2 kyr 1 for pt burial fluxes are 1800 3000 pg cm 2 kyr 1 bg ca 1000 pg cm 2 kyr 1 and for pd 2700 4600 pg cm 2 kyr 1 bg ca 800 pg cm 2 kyr 1

the contribution of the sensitive high resolution ion microprobe shrimp to lunar geochronology abstract the sensitive high resolution ion microprobe shrimp developed at the australian national university anu was the first of the high resolution ion microprobes the impact of this instrument on geochronological research over the last twenty years has been immense this is particularly so for lunar geochronology where it has opened up avenues of research that were not possible using conventional tims techniques the great advantage of shrimp is that it provides a means for determining precise u pb isotopic ratios on selected micron size areas on polished grains of zircon and other u bearing minerals one of the first projects undertaken on the newly invented shrimp i was an investigation of u pb ages of lunar zircon using shrimp multiple analyses could be made on areas of individual zircons to test the stability of u pb systems in shocked grains also by analysing grains in situ textural relationships between the analysed zircon and the components of the sample breccia could be used in the interpretation of the shrimp data as a result of this research it was realised that most lunar zircons have ages up to 500 ma older than the imbrium and serenitatis impacts at ca 3 9 ga demonstrating that the zircons have not been affected by the these impact events although heating and shock effects have profoundly disturbed other dating systems this has opened the way for research into the early lunar magmatic and bombardment record for example recent shrimp results have revealed profound differences in the ages of zircons from breccias from the apollo 14 and apollo 17 sample sites raising new questions about the evolution of lunar magmatism also multiple shrimp analyses on complex lunar zircons have shown that these grains can record u pb disturbance by later impact events shrimp u pb age determinations on phosphates in lunar meteorites has identified lunar events not recognised in samples from the apollo program shrimp based research on lunar materials is ongoing and in combination with other chemical and structural evidence continues to stimulate new ideas on the early evolution of the moon

rhenium osmium isotope systematics of an early mesoproterozoic sedex polymetallic pyrite deposit in the north china craton implications for geological significance and the marine osmium isotopic record abstract the rhenium osmium re os isotopic system preserved in ancient seafloor hydrothermal sulfide deposits can be used to gain insight into the os composition of ancient seawater in particular the age of sedimentary exhalative sedex deposits can be determined by measuring re os isotopes in syn depositional pyrite a common constituent of sedex deposits here we report on the gaobanhe sediment hosted polymetallic pyrite deposit which is hosted within the gaoyuzhuang formation in the central part of the northern margin of the north china craton ncc re os isotopic data from syn depositional pyrite yield an isochron age of 1505 55 ma constraining the timing of primary mineralization as well as the minimum age of the host sediments to the mesoproterozoic analyzed pyrite grains indicate an initial 187os 188os osi value of 0 181 0 066 suggesting that the mineralizing fluids were an admixture of seawater and submarine hydrothermal fluids with the os being primarily derived from seawater the osi value is slightly higher than that of paleoproterozoic seawater 0 13 suggesting that there was a radiogenic riverine flux to the oceans and that atmospheric oxygen must have been sufficiently high at that time to drive this increased continental radiogenic os input to the oceans we propose that the re os isotopic system as recorded by these sedex deposits may provide key insights when examining changes in the os isotopic composition of seawater throughout the precambrian

pre 4 2 ae mare basalt volcanism in the lunar highlands abstract the concept that the plutonism of the lunar highlands and the mare type volcanism are two separate problems in both time 4 4 ae versus apollo 14 breccia 14305 contains a clast 122 which is an olivine gabbronorite that is texturally and mineralogically similar to several apollo 12 basalts e g 12005 12035 12040 it consists of cumulus olivine 40 modal fo 62 70 and ti chromite 2 5 modal post cumulus phases include low ca pyroxene 29 modal wo 7 13 en 68 75 augite 10 modal wo 31 40 en 47 50 plagioclase 15 modal an 82 93 and ilmenite 4 modal 5 7 mgo the tio2 content of this rock 4 3 cao al2o3 1 0 cao 5 1 mgo feo 1 0 mgo 21 9 the ree pattern normalized to chondritic abundances is approximately 30 ch and hump shaped with a pronounced eu depletion and a non kreepy signature a four point rb sr isochron reveals an age of 4 23 0 05 ae the sample has a low initial 87sr 86sr 0 69911 3 the data presented here show that non kreepy mare type volcanism commenced at least as early as 4 2 ae in the fra mauro region and probably across much of the lunar surface massive bombardment during the terminal cataclysm and the subsequent veneer of younger mare basalts has obliturated most of the evidence for these ancient volcanic events these old mare type volcanics may be related to basin forming events such as made procellarum i e impact triggered igneous activity

microfabrics in situ trace element and sulfur isotope compositions of pyrite from the jinjiwo copper deposit in chengmenshan orefield northern yangtze block syngenetic stratabound mineralization and hydrothermal remobilization abstract the jinjiwo copper deposit is located in the jiurui ore district of the middle lower yangtze metallogenic belt the deposit has stratiform orebodies hosted by late carboniferous carbonate units pyrite is the dominant metallic mineral in the ores here we report on an integrated study on the microfabrics raman spectroscopy in situ la mc icp ms trace element and sulfur isotope of pyrite with a view to understand the geochemical variation genesis of copper mineralization and ore forming process textural observations suggested four types of pyrite corresponding to three metallogenic episodes syngenetic sedimentation pyi metamorphism or deformation pyii and skarn hydrothermal mineralization including the late skarn stage pyiii and hydrothermal stage pyiv raman spectra of pyrite reveal subtle differences in band position and widths fwhm and display the increasing formation temperature of from pyi through pyiv and pyiii to pyii colloform and laminated pyrite pyi share similarities in behavior of pairs of elements and have low co ni ratios and the former contains the highest concentrations of bi cu pb zn ag au mn that suggest rapid precipitation from highly saturated fluids pyii shows high co ni and as concentrations and co ni ratios of 0 03 to 6 19 pyiii and pyiv have greatly varying co ni ratios of 1 07 to 29 and typically low au concentrations suggesting a hydrothermal source pyiii is rich in co and se but is depleted in as cu pb zn ag and au in comparison with pyiv there are two kinds of sulfur one is that 34s values are near zero suggesting that the pyrite sulfur in the ores was derived from a mixed source in deep crust and the other is that 34s values are less than 39 1 indicating the sedimentary pyrite sulfur was originated from bacterial sulfate reduction the combined textural and compositional data of pyrite suggest that the formation of the jinjiwo copper deposit might have recorded the submarine exhalation sedimentation and the mesozoic tectonic transition deformation and hydrothermal overprint

h and cl isotope systematics of apatite in brecciated lunar meteorites northwest africa 4472 northwest africa 773 sayh al uhaymir 169 and kalahari 009 we have investigated the h and cl systematics in apatite from four brecciated lunar meteorites in northwest africa nwa 4472 most of the apatites contain 2000 6000 ppm h2o with d between 200 and 0 except for one grain isolated in the matrix which contains 6000 ppm h2o with d of 500 900 this low d apatite contains 2500 7500 ppm cl associated with 37cl of 15 20 while the high d grain contains 2500 ppm cl with 37cl of 7 15 in nwa 773 apatites in a first group contain 700 2500 ppm h2o with d values averaging around 0 100 while apatites in a second group contain 5500 16500 ppm h2o with d 250 50 in sayh al uhaymir sau 169 and kalahari kal 009 apatites are similar in terms of their h2o contents 600 3000 ppm and d values 100 to 200 in sau 169 apatites contain 6000 10 000 ppm cl characterized by 37cl of 5 12 overall most of the analyzed apatite grains have d within the range reported for carbonaceous chondrites similar to apatite analyzed in ancient 3 9 ga lunar magmatic one grain in nwa 4472 has h and cl isotope compositions similar to apatite from mare basalts with an age of 4 35 ga this grain could be a representative of the oldest known lunar volcanic activity finally since numerous evolved clasts in nwa 773 formed through silicate liquid immiscibility the apatite grains with extremely high h2o contents reaching pure hydroxylapatite composition could provide insights into the effects of such process on the evolution of volatiles in lunar magmas

depositional age of the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa and implications for low level re os geochronology and paleoproterozoic global correlations abstract we present new re os isotope data from the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa to further evaluate low level re os geochronology of organic rich sedimentary rocks and improve global correlations of paleoproterozoic sedimentary rocks the klipputs member contains re 0 3 0 8 ppb total os 34 177 ppt and common os 192 os 10 66 ppt abundances that overlap upper crustal abundances of 0 2 2 ppb 30 50 ppt and 12 21 ppt respectively five samples yield a model 1 re os date of 2479 20 ma 22 ma if the 187 re decay constant uncertainty is included 2 mean square of weighted deviates mswd 0 01 using the cr vi h 2 so 4 digestion protocol dissolution of the same sample powders using inverse aqua regia yields a model 3 re os date of 2862 190 ma 2 mswd 148 comparison of re os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital os whereas the cr vi h 2 so 4 medium did not the re os date of 2479 22 ma for the klipputs member is consistent with published geochronological ages from underlying and overlying stratigraphic units suggesting the cr vi h 2 so 4 protocol yields accurate re os depositional ages from ors containing very low levels of hydrogenous re and hydrogenous radiogenic os hence there is potential for widespread application of low level re os ors geochronology including sedimentary successions deposited before the great oxidation event the initial 187 os 188 os 0 12 0 01 from the klipputs member cr vi h 2 so 4 regression indicates that dissolved os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and or cosmic dust rather than oxidative weathering of the radiogenic upper crust in light of the re os depositional age for the klipputs member we propose that the stratigraphic units in the upper hamersley group above the brockman iron formation western australia are younger than the ghaap group diamictite from the doradale formation in the ghaap group is constrained to be older than 2479 22 ma and thus may not be correlative with glacial deposits assigned to the oldest paleoproterozoic glaciation

evolved mare basalt magmatism high mg fe feldspathic crust chondritic impactors and the petrogenesis of antarctic lunar breccia meteorites meteorite hills 01210 and pecora escarpment 02007 antarctic lunar meteorites meteorite hills 01210 and pecora escarpment 02007 are breccias that come from different regolith lithologies on the moon met 01210 is composed predominantly of fractionated low ti basaltic material and is classified as an immature predominantly basaltic glassy matrix regolith breccia pca 02007 is a predominantly feldspathic regolith breccia consisting of metamorphosed feldspathic noritic troctolitic and noritic anorthosite clasts agglutinate and impact glasses as well as a number of basaltic clasts with mare and possible non mare affinities the basalt clasts in met 01210 have undergone fenner trend enrichments in iron and may also have witnessed late stage crystallization of zircon or a zirconium rich mineral some of the features of met 01210 are similar to other basaltic lunar breccia meteorites e g northwest africa 773 elephant moraine 87521 96008 yamato 793274 981031 but it is not paired with them the presence of metamorphic anorthositic clasts as well as agglutinates indicates a small regolith component similarities with previously discovered evolved e g lapaz icefield 02205 northwest africa 032 and ferroan e g asuka 881757 yamato 793169 basaltic lunar meteorites suggest a similar mare source region for met 01210 despite lack of evidence for pairing pca 02007 shares many features with other feldspathic regolith breccias e g yamato 791197 queen alexandra range 94281 including a high mg fe whole rock composition glass spherules agglutinate fragments and a diverse clast inventory spanning the range of ferroan anorthosite and high magnesium suite rocks some of the basalt fragments in this sample are fractionated and have an igneous origin however the majority of the basalt fragments are impact melt clasts pca 02007 supports previous studies of feldspathic lunar meteorites that have suggested an aluminous crust for the moon with compositions more similar to magnesium granulite breccias than ferroan anorthosites a chondrule like fragment found in pca 02007 and unlike any previously described lunar material is described and tentatively identified as the remnants of a chondritic lunar impactor this clast is porphyritic with equant olivines that have forsterite rich cores fo 98 extreme normal zonation to more fayalitic rims fo 44 and a mineral assemblage with rare earth element abundances distinct from described lunar material and more similar to chondrules found in ordinary or carbonaceous chondrites its discovery and description is significant for understanding the composition of lunar impactors previously the main evidence for chondritic lunar impactors was from chondritic relative abundances and near chondritic ratios of highly siderophile elements in lunar impact melt breccias however the presence of this clast along with two other chondritic clasts from apollo soils 12037 and 15602 provides clues to the identity of ancient meteorite impactors on the moon

re os geochronology of the iimori besshi type massive sulfide deposit in the sanbagawa metamorphic belt japan abstract besshi type cu deposits are strata bound volcanogenic massive sulfide deposits usually associated with mafic volcanic rocks or their metamorphic equivalents although there are numerous besshi type deposits in the sanbagawa metamorphic belt japan their tectonic settings and depositional environments remain controversial because of a lack of depositional age constraints we report re os data for the iimori deposit one of the largest besshi type deposits in western kii peninsula in order to examine the robustness of the re os isotope system for dating sulfide minerals in the high p t metamorphic belt and to elucidate the primary depositional environment of the iimori sulfide ores an 11 point re os isochron plot yields an age of 156 8 3 6 ma as this re os isochron age is significantly older than the timing of the sanbagawa peak metamorphism 110 120 or 90 ma and a well defined isochron was obtained the re os age determined here is most likely the primary depositional age despite high grade metamorphism at up to 520 25 c and 8 9 5 kbar the re os isotope system of the iimori sulfides was not disturbed hence we consider that the whole rock re os closure temperature for the iimori sulfide ores was probably higher than 500 c as the accretion age of the sanbagawa metamorphic belt is considered to be 120 130 or 65 90 ma on the basis of radiolarian and radiometric ages we estimated the time from the iimori sulfide deposition on the paleo seafloor to its accretion at the convergent plate boundary to be greater than 25 myr consequently the depositional environment of the iimori sulfide ores was not in the marginal sea but was truly pelagic

pb pb ages and initial pb isotopic composition of lunar meteorites nwa 773 clan nwa 4734 and dhofar 287 4constraining the duration of magmatic activity on the moon is essential to understand how the lunar mantle evolved chemically through time determining age and initial isotopic compositions of mafic lunar meteorites is a critical step in defining the periods of magmatic activity that occurred during the history of the moon and to constrain the chemical characteristics of mantle components involved in the sources of the magmas we have used the in situ pb pb sims technique to investigate eight lunar gabbros and basalts including six meteorites from the northwest africa nwa 773 clan nwa 2727 nwa 2700 nwa 3333 nwa 2977 nwa 773 and nwa 3170 nwa 4734 and dhofar 287a these samples have been selected as there is no clear agreement on their age and they are all from the dominant low titanium chemical group we have obtained ages of 2981 12 ma for nwa 4734 and 3208 22 ma for dhofar 287 for the nwa 773 clan four samples the fine grained basalt nwa 2727 and the three gabbros nwa 773 nwa 2977 nwa 3170 out of six yielded isochron calculated ages that are identical within uncertainties and yielding an average age of 3086 5 ma the age obtained for the fine grained basalt nwa 2700 is not precise enough for comparison with the other samples the gabbroic sample nwa 3333 yielded an age of 3038 20 ma suggesting that two distinct magmatic events may be recorded in the meteorites of the nwa 773 clan the present study aims to identify and assess all potential issues that are associated with different ways to date lunar rocks using u pb based methods to achieve this we have compared the new ages with the previously published data set the entire age data set from lunar mafic meteorites was also screened to identify data showing analytical issues and evidence of resetting and terrestrial contamination the data set combining the ages of mafic lunar meteorites and apollo rocks suggests pulses of magmatic activity with two distinct phases between 3950 and 3575 ma and between 3375 and 3075 ma with the two phases separated by a gap of approximately 200 ma the evolution of the pb initial ratios of the low ti mare basalts between approximately 3400 and 3100 ma suggests that these rocks were progressively contaminated by a kreep like component

the osmium riverine flux and the oceanic mass balance of osmium the osmium concentration os and isotopic composition were determined in a set of 17 of the largest rivers of the world os varies between 4 6 and 52 1 pg kg and the 187os 188os ratio varies between 0 64 and 2 94 measurement of rainwater samples shows that there is no input of oceanic os to the continent through rain assuming a negligible anthropogenic os input in the dissolved load the natural average river water has an os concentration of 7 9 pg kg and a mean 187os 188os ratio of 1 54 the total riverine flux of os to the ocean is estimated to be 295 kg yr the dissolved os flux from island arcs and oceanic islands represents less than 5 of the total riverine flux and is not further considered the continental os flux to the ocean is then represented by the riverine flux as dissolved os from eolian dust and glacial sediments is negligible assuming steady state it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg yr cosmic dust or mantle derived and an oceanic residence time between 2 5 104 and 5 4 104 with a mean of 3 5 104 year the estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater os would be 14 of the contribution of the unradiogenic component which means that the mantle derived flux should contribute a major part the first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and or of the ultramafic exposures contributes dominantly to the input of unradiogenic os to the seawater we show that it would be necessary to leach 1 3 of the os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic os to the ocean in which case the riverine input represents 94 of the total dissolved flux to the ocean instead of 70 the modern global dissolved os flux to the ocean would then have a 187os 188os ratio of 1 44 instead of 1 06 and the system would be far from steady state

pb pb geochronology petrography and chemistry of zr rich accessory minerals zirconolite tranquillityite and baddeleyite in mare basalt 10047 abstract in situ u pb isotopic measurements were carried out by ion microprobe on the zr rich accessory minerals zirconolite cazrti2o7 tranquillityite fe82 zry 2ti3si3o24 and baddeleyite zro2 in low k high ti mare basalt 10047 collected during the apollo 11 mission the analysed minerals are concentrated in pockets of late stage mesostasis that comprises an intergrowth of silica barian k feldspar and si al k glass from a phaneritic subophitic basalt comprising mainly pyroxene plagioclase ilmenite cristobalite and troilite most zr rich minerals are unaltered however some tranquillityite is replaced by a complex intergrowth of zirconolite baddeleyite ilmenite and fayalite suggesting that the mineral became unstable during crystallization several baddeleyite crystals have also undergone alteration to secondary zircon zirconolite was analysed in thin section 10047 11 and tranquillityite and baddeleyite in 10047 227 using a 6 m primary ion beam both zirconolite and tranquillityite have significant u and low initial pb contents and are highly suitable for pb pb dating fifteen analyses of zirconolite give a 207pb 206pb age of 3708 7 ma 207pb 206pb 204pb 206pb isochron 95 confidence including renormalisation of ratios and twenty five analyses of tranquillityite give 3710 6 ma the 207pb 206pb dates are consistent with each other and refine results from an earlier study baddeleyite data were less precise mainly due to lower secondary ionisation efficiency our results show that zirconolite and tranquillityite can provide precise isotopic dates and given their presence in other samples they represent important u pb chronometers for refining lunar geology

40ar 39ar chronology of lunar meteorites northwest africa 032 and 773 the 40ar 39ar dating technique has been applied to the lunar meteorites northwest africa 032 nwa 032 an unbrecciated mare basalt and northwest africa 773 nwa 773 composed of cumulate and breccia lithologies to determine the crystallization age and timing of shock events these meteorites may have experienced stepped heating analyses of several different samples of nwa 032 gave complex age spectra but indistinguishable total ages with a mean of 2 779 0 014 gyr possible causes of the complex age spectra obtained from nwa 032 include recoil of 39ar or the presence of pre shock 40ar incorporated into shock melt veins the effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins the laser ages show a narrow age distribution between 2 61 2 86 gyr and a mean of 2 73 0 03 gyr identical to the total age of 2 80 gyr obtained for the bulk sample diffusion calculations based on the stepped heating data indicate that ar release can be reconciled by release from feldspar and possibly shock veins at low temperatures followed by pyroxene at higher temperatures the exposure age of nwa 032 is 212 11 myr and it contains low trapped solar ar stepped heating of cumulate and breccia portions of nwa 773 also give a relatively young age of 2 91 gyr the presence of trapped ar in the breccia makes the age determination of this component less precise but release of ar appears to be from the same mineral phase assumed to be plagioclase in both lithologies a marked difference in exposure age between the 2 lithologies also exists with the breccia having spent 81 myr longer at the lunar surface this finding is consistent with the higher trapped ar content of this lithology assuming that 2 80 gyr and 2 91 gyr are the crystallization ages of nwa 032 and nwa 773 respectively these two meteorites are the youngest lunar mare basalts available for study

northwest africa 032 product of lunar volcanism mineralogy major element compositions of minerals and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite northwest africa 032 nwa 032 an unbrecciated basalt found in october 1999 the rock consists predominantly of olivine pyroxene and chromite phenocrysts set in a crystalline groundmass of feldspar pyroxene ilmenite troilite and trace metal whole rock shock veins comprise a minor but ubiquitous portion of the rock undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro faults in groundmass and phenocrysts also are attributed to shock several geochemical signatures taken together indicate unambiguously that nwa 032 originated from the moon the most diagnostic criteria include whole rock oxygen isotopic composition and ratios of fe mn in the whole rock olivine and pyroxene a lunar origin is documented further by the presence of fe metal troilite and ilmenite zoning to extremely fe rich compositions in pyroxene the ferrous oxidation state of all fe in pyroxene and the rare earth element pattern with a well defined negative europium anomaly this rock is similar in major element chemistry to basalts from apollo 12 and 15 but is enriched in light rare earth elements and has an unusually high th sm ratio some apollo 14 basalts yield a closer match to nwa 032 in rare earth element patterns but have higher concentrations of al2o3 ar ar step release results are complex but yield a whole rock age of ca 2 8 ga suggesting that nwa 032 was extruded at 2 8 ga or earlier this rock may be the youngest sample of mare basalt collected to date noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites in short the geochemical and petrographic features of nwa 032 are not matched by apollo or luna samples nor by previously identified lunar meteorites indicating that it originates from a previously unsampled mare deposit detailed assessment of petrographic features olivine zoning and thermodynamic modelling indicate a relatively simple cooling and crystallization history for nwa 032 chromite spinel olivine and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 c hr based on the polyhedral morphology of olivine comparison of olivine size with crystal growth rates and preserved fe mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days plumose textures in groundmass pyroxene feldspar and ilmenite and fe rich rims on the phenocrysts formed during rapid crystallization cooling rates 20 to 60 c hr after eruption

re os isotope systematics in black shales from the lesser himalaya their chronology and role in the 187os 188os evolution of seawater abstract re and os abundances and os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines maldeota and durmala in the lesser himalaya the re and os concentrations in these samples exhibit wide range 0 2 to 264 ng g and 0 02 to 13 5 ng g respectively with a mean re os of 25 the 187 os 188 os of these samples also show a broad range 1 02 to 11 6 with many of them far more radiogenic than the currently eroding continental crust the black shales from the maldeota and durmala mines collected a few meters above the krol tal pc c boundary and occurring in the same stratigraphic horizon yield 187 re 187 os isochron ages of 554 16 and 552 22 ma respectively consistent with fossil evidences and those assigned for the pc c boundary at various other locations these results indicate closed system behaviour of re and os in these mine samples and reaffirm the validity of 187 re 187 os pair to date organic rich sediments the age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary re os age of 839 138 ma for black shales from theog collected from the inner belt these initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer belt however more analyses of black shales are needed to confirm this the role of weathering black shales from the himalaya in contributing to the os isotope evolution of seawater over the past 25 ma was assessed using a simple budget model for seawater os and assuming values for os fluxes and 187 os 188 os based on the available limited data on os concentration and its 187 os 188 os in rivers and oceans these calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic 187 os 188 os over the past 16 ma is difficult to satisfy by varying only the os flux from htp rivers 187 os 188 os kept constant at 1 324 or their 187 os 188 os 188 os flux constant at today s value 131 moles yr the requirements to balance os isotope budget in ocean through weathering of black shales however is easier to accommodate if both the os flux from htp himalayan tibetan plateau rivers and their 187 os 188 os are allowed to vary measurements of os abundance in htp rivers and its isotopic composition are needed to test the validity of the model results

late cretaceous metallogeny in the zhongdian area constraints from re os dating of molybdenite and pyrrhotite from the hongshan cu deposit yunnan china abstract the zhongdian area in yunnan southwestern china located at the southern end of the yidun volcano magmatic arc that was formed during the triassic westward subduction of the gaze litang ocean hosts numerous triassic large porphyry and skarn deposits the arc suffered jurassic to cretaceous arc continental orogenic collision and cenozoic intracontinental strike slip shearing the hongshan cu mo pb zn deposit is potentially a large deposit and contains two ore types 1 predominant layered skarn cu pb zn ores along marble hornfels contacts and 2 minor crosscutting vein type cu mo mineralization previous research forwards a two stage genetic model without sufficient dating evidence supposing the skarn mineralization is related to the triassic calc alkalic intrusions and the vein type mineralization related to cretaceous quartz monzonite porphyries re os dating of molybdenite from vein type ores and quartz monzonite porphyries and that of pyrrhotite from skarn ores are presented here to constrain the mineralization age and rebuild the genetic model analyses of eight molybdenite samples yield an isochron age of 79 7 3 1 ma mswd 9 2 for the vein type mineralization and a model age of 81 9 1 1 ma for the quartz monzonite porphyries isotope data on seven pyrrhotite samples from the skarn ores yield an isochron age of 79 16 ma z mswd 8 4 the re os ages for the two ore types are concordant within analytical errors indicating that the hongshan deposit was formed in the late cretaceous elevated re contents in molybdenite 13 65 to 63 91 g g and extremely radiogenic initial 187 os 188 os ratios in pyrrhotite 0 7673 to 0 8184 weighted average 0 796 0 038 together with elevated os values in pyrrhotite 507 to 547 average 528 imply a significant crustal component in the ore forming materials that was likely derived from a lower crustal reservoir combined with the tectonic evolution of the zhongdian area and geochemical characteristics of corresponding intrusions the ages of mineralization obtained in this study indicate that the hongshan deposit was formed in a post collision setting with a genetic relationship to the emplacement of the quartz monzonite porphyry these results provide significant new information for the study and exploration of the late cretaceous metallogeny in the zhongdian area

ion microprobe u pb dating of phosphates in lunar basaltic breccia elephant moraine 87521 1 we report ion microprobe u pb dating of phosphates in lunar meteorite elephant moraine 87521 eet87521 which is a fragmental breccia consisting of very low ti vlt basaltic clasts and a small component of highland derived materials the observed pb pb age of phosphates in eet87521 is 3503 140 ma which is distinct from the results from previous chronological studies on vlt mare basalt of 3 2 3 3 ga for luna 24 this suggests that the vlt basalt volcanism appears to have been prolonged on the moon moreover the age is apparently different from those of other vlt meteorites 3 8 ga for que94281 and 4 0 4 4 ga for yamato793274 which are proposed to have been launched by a single impact event based on the similarity of launching ages mineralogical and geochemical signatures this evidence questions the validities of bulk age analyses for the yamato que meteorites in the literature and or the hypothesis of a single crater origin

pge re os and mo isotope systematics in archean and early proterozoic sedimentary systems as proxies for redox conditions of the early earth abstract re os data and pge concentrations as well as mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the barberton greenstone belt south africa fig tree and moodies groups 3 25 3 15 ga the belingwe greenstone belt zimbabwe manjeri formation ca 2 7 ga and shales from the witwatersrand ventersdorp and transvaal supergroups south africa ranging from 2 95 to 2 2 ga moderately oxidizing conditions are required to mobilize re and mo in the environment mo fractionation only occurs in solution and these parameters thus have potential use as paleoredox proxies for the early earth pge re abundance patterns of barberton greenstone belt sediments are uniform and very similar in shape to those of komatiites this indicates 1 that the pge came from a source of predominantly ultramafic composition and 2 that pge were transported and deposited essentially in particulate form sediments from the younger belingwe greenstone belt show more fractionated pge re patterns and have re os ratios 10 to 100 higher than those of barberton sediments their pge abundance patterns and re os ratios are intermediate between those of the mid archean shales and neoproterozoic to recent black shales they reflect scavenging of re from solution in the sedimentary environment 98 95 mo values of black shales of all ages correlate with their concentrations the barberton greenstone belt samples have 1 3 ppm mo similar to a granitoid basaltic source this mo has 98 95 mo between 1 9 and 2 4 relative to present day mean ocean water molybdenum momo and is thus not isotopically fractionated relative to such a source similar to the pge this indicates transport in solid form sediments from the belingwe greenstone belt show in part enhanced mo concentrations up to 6 ppm and mo isotope fractionation 98 95 mo up to 1 4 relative to momo the combined pge re and mo data show mainly reducing conditions in the mid archean and suggest that by 2 7 ga the atmosphere and oceans had become more oxidizing substantially younger samples from the transvaal supergroup to ca 2 2 ga surprisingly have mainly low mo concentrations around 1 ppm and show no significant mo isotope fractionation relative to the continental source among possible explanations for this are a return to reducing atmospheric conditions after 2 7 ga reservoir effects or mo removal by sulfide precipitation following sulfate reduction in early proterozoic oceans

history of lunar meteorites queen alexandra range 93069 asuka 881757 and yamato 793169 based on noble gas isotopic abundances radionuclide concentrations and chemical composition we investigated the characteristics and history of lunar meteorites queen alexandra range 93069 yamato 793169 and asuka 881757 based on the abundances of all stable noble gas isotopes the concentrations of the radionuclides 10be 26al 36cl and 81kr and the abundances of mg al k ca fe cl sr y zr ba and la based on the solar wind and cosmic ray irradiations que 93069 is the most mature lunar meteorite studied up to now the 40ar 36ar ratio of the trapped component is 1 87 0 16 this ratio corresponds to a time when the material was exposed to solar and lunar atmospheric volatiles 400 ma ago on the other hand yamato 793169 and asuka 881757 contain very little or no solar noble gases which indicates that these materials resided in the top layer of the lunar regolith only briefly or not at all for all lunar meteorites we observe a positive correlation of the concentrations of cosmic ray produced with trapped solar noble gases the duration of lunar regolith residence for the lunar meteorites was calculated based on cosmic ray produced 21ne 38ar 78kr 83kr and 126xe and appropriate production rates that were derived based on the target element abundances and the shielding indicator 131xe 126xe for que 93069 yamato 793169 and asuka 881757 we obtained 1000 400 ma 50 10 ma and 1 ma respectively both asuka 881757 and yamato 793169 show losses of radiogenic 4he from u and th decay and yamato 793169 also 40ar loss from k decay for asuka 881757 we calculate a k ar gas retention age of 3100 600 ma and a 244pu 136xe fission age of 4240 170 ma this age is one of the oldest formation ages ever observed for a lunar basalt the exposure history of que 93069 after ejection from the moon was derived from the radionuclide concentrations ejection 0 16 0 03 ma ago duration of moon earth transit 0 15 0 02 ma and fall on earth 0 015 ma ago this ejection event is distinguished temporally from those which produced the other lunar meteorites we conclude that six to eight events are necessary to eject all the known lunar meteorites

nanosims pb pb dating of tranquillityite in high ti lunar basalts implications for the chronology of high ti volcanism on the moon in this study we carried out pb pb dating of tranquillityite in high ti mare basalts 10044 75055 and 74255 using a cameca nanosims 50 at a spatial resolution of 3 m the analyses yielded 207pb 206pb dates of 3722 11 ma for sample 10044 3772 9 ma for sample 75055 and 3739 10 ma for sample 74255 at 95 confidence level these dates are consistent with previously determined crystallization and emplacement ages of these samples using different radiogenic systems these high precision ages allow refinement of the timing of some of the high ti basaltic volcanism on the moon crystallization ages of three different high ti basalt units integrating these new pb pb ages with previous rb sr and sm nd age determinations are consistent with previous estimates but associated with uncertainties 3 to 5 times lower in addition the data obtained in this study confirm that tranquillityite contains very low amounts of initial common pb and has a high pb ionization efficiency making it an excellent candidate for pb pb dating by ion microprobe the higher spatial resolution afforded by nanosims 50 and the recent discovery of tranquillityite in several terrestrial mafic rocks opens up a new area of research allowing an independent and rapid age dating of basaltic rocks in polished sections

sulfur isotope composition of carbonate associated sulfate from the mesoproterozoic jixian group north china implications for the marine sulfur cycle abstract the mesoproterozoic has been traditionally viewed as a period of prolonged stability in terms of environmental and biological evolution yet growing body of evidence suggests subtle yet dynamic changes in marine biogeochemical cycles in this study we present a high resolution analysis of carbonate associated sulfate cas from the mesoproterozoic jixian group 1 6 1 4 ga north china combined with previously reported carbon isotopes from the same sections sulfur isotope data provide insight into both oceanic chemistry and biospheric evolution during this period the sulfur isotopic composition of cas in the jixian group displays high amplitude variations with a total range from 3 7 to 51 9 stratigraphic variation in 34scas is most noted in the wumishan formation and records shifts of 10 over stratigraphic thickness of 100 200 m indicating a short residence time resulting from a limited oceanic sulfate reservoir differences in the pattern of isotopic variability between the gaoyuzhuang and wumishan formations suggest complex behavior of the marine sulfur cycle based on proximity of depositional environments to a local or regional chemocline and the degree of mixing between adjacent environments deposition under predominantly oxic conditions favored a stable sulfate reservoir whose isotopic composition was controlled primarily by pyrite burial by contrast depositional environments proximal to euxinic waters were subject to rapid changes in both sulfate reservoir size and isotopic composition resulting from growth and subsequent oxidation of a reactive hydrogen sulfide reservoir together these units demonstrate the difficulty of interpreting marine sulfur cycling in a low oxygen low sulfate world subject to a range of environmental controls data also reveal a long term positive trend in average 34scas and 13ccarb from approximately 10 to 20 and 0 5 to 0 5 respectively which is interpreted as reflecting prolonged organic carbon and pyrite burial burial of reduced carbon and sulfur may have resulted in a slow but steady release of oxygen to the biosphere ultimately driving a gradual increase in oceanic sulfate levels our study combined with previous contributions from the late mesoproterozoic supports hypotheses of protracted biospheric oxygenation marked by a modest increase in oceanic sulfate concentrations through the mesoproterozoic

uranium lead systematics of low ti basaltic meteorite dhofar 287a affinity to apollo 15 green glasses article i nfo dhofar 287 is a lunar meteorite found in oman in 2001 which consists of a major portion 95 of low ti mare basalt dho 287a and a minor attached part 5 of regolith breccia dho 287b here we report the u pb systematics of dho 287a using data collected with a sensitive high resolution ion microprobe shrimp in situ analyses of five merrillite and three apatite grains which are resistant to secondary petrologic events resulted in a total pb u isochron age of 3 34 0 20 ga in 238 u 206 pb 207 pb 206 pb 204 pb 206 pb 3 d space 95 confidence level the observed pb pb isochron of these eight phosphates coupled with four plagioclase grains alsoyieldeda 207 pb 206 pbageof3 35 0 13ga thisformationage whenconsideredasthecrystallizationageof

cryptomare magmatism 4 35 gyr ago recorded in lunar meteorite kalahari 009 the origin and evolution of the moon remain controversial one notable point of controversy being the timing of the beginning and end of basaltic volcanism in the lunar mares or plains lunar research over the past three decades has suggested that mare volcanism occurred mainly after the late heavy bombardment that ended 3 9 billion years ago because most of known mare basalts lunar meteorites and returned apollo and luna samples and remote sensing data indicate crystallization ages younger than 3 9 billion years but a new u pb dating of minerals in the lunar meteorite kalahari 009 gives an age of roughly 4 35 billion years for a lunar basalt this suggests that the volcanism started as early as 4 35 billion years ago relatively soon after lunar formation and differentiation and that kalahari 009 is our first sample of a cryptomare from the moon the result of u pb dating of minerals in a lunar meteorite kalahari 009 is reported analyses of five grains associated with basaltic clasts give an age of 4 35 0 15 billion years these are thought to represent the crystallization ages of parental basalt magma making kalahari 009 one of the oldest known mare basalt this suggests that mare basalt volcanism on the moon started as early as 4 35 billion years ago the origin and evolution of the moon remain controversial1 2 with one of the most important questions for lunar evolution being the timing and duration of basaltic mare magmatism1 3 4 5 6 7 8 here we report the result of ion microprobe u pb dating of phosphates in a lunar meteorite kalahari 009 which is classified as a very low ti mare basalt breccia in situ analyses of five phosphate grains associated with basaltic clasts give an age of 4 35 0 15 billion years these ancient phosphate ages are thought to represent the crystallization ages of parental basalt magma making kalahari 009 one of the oldest known mare basalts we suggest that mare basalt volcanism on the moon started as early as 4 35 gyr ago relatively soon after its formation and differentiation and preceding the bulk of lunar volcanism which ensued after the late heavy bombardment around 3 8 3 9 gyr refs 7 and 8 considering the extremely low abundances of incompatible elements such as thorium and the rare earth elements in kalahari 009 ref 9 and recent remote sensing observations illustrating that the cryptomaria tend to be of very low ti basalt type10 11 12 we conclude that kalahari 009 is our first sample of a very low ti cryptomare from the moon

os isotopes as tracers in seafloor hydrothermal systems metalliferous deposits from the tag hydrothermal area 26 n mid atlantic ridge abstract os isotopic analyses of a suite of samples from the tag hydrothermal area 26 n on the mid atlantic ridge reveal evidence of release of ocean crust derived os to seawater and removal of seawater derived os to the seafloor 187 os 186 os ratios of sulfide samples from the active tag mound vary between 8 7 and 1 3 the highest ratios are indistinguishable from the inferred 187 os 186 os ratio of seawater the incorporation of seawater derived os in these samples is interpreted as evidence of both entrainment of seawater into hydrothermal fluids and the addition of seawater os to samples during sulfide oxidation several sulfide samples from the active tag mound have 187 os 186 os ratios lower than the inferred seawater value with the lowest ratio approaching values expected for pristine oceanic crust these low 187 os 186 os ratios indicate that os derived from alteration of the oceanic crust is released to seawater in association with venting of high temperature hydrothermal fluids precipitates from low temperature hydrothermal fluids fe and mn oxide deposits from the east wall of the axial rift valley also have low 187 os 186 os ratios 4 2 6 8 relative to seawater these data suggest os isotopes may be sensitive tracers of low temperature hydrothermal circulation on the seafloor and that supply of ocean crust derived os to seawater is not strictly limited to high temperature hydrothermal vents the os concentration and isotopic composition of metalliferous sediment samples from a push core collected 125 m from the base of the active tag mound are also reported in this core metalliferous sediment horizons derived from mass wasting of altered sulfides exhibit variable 187 os 186 os ratios 4 8 8 7 reflecting mixing of seawater derived and ocean crust derived os in contrast sediments in which the metalliferous component is derived from fall out of hydrothermal material from the neutrally buoyant hydrothermal plume have 187 os 186 os ratios which vary over a narrow range 8 6 8 7 and appear to accurately record the os isotopic composition of seawater

ion microprobe uranium thorium lead dating of shergotty phosphates we report ion microprobe u th pb dating of shergotty phosphates by means of the sensitive high resolution ion microprobe shrimp recently installed at hiroshima university japan ten analyses of whitlockite merrillite and three analyses of apatite indicate a 238u 206pb isochron age of 225 200 ma and a tera wasserburg concordia constrained linear three dimensional isochron age of 217 110 ma in the 238u 206pb 207pb 206pb204pb 206pb diagram these ages agree well with the 232th 208pb age of 189 83 ma which suggests that primary crystallization or a shock metamorphic event defined the formation age of the phosphate minerals the average of the later two ages 204 68 ma is consistent with the previously published rb sr age of 165 11 ma and u th pb age of 200 ma these show marginal agreement with the 40ar 39ar age of 254 10 ma but are significantly different from the sm nd age of 360 16 ma taking into account the closure temperature of the u pb system in apatite we suggest the time that shergotty last experienced a temperature of 900 c was 204 68 ma

re os dating of sulfides associated with gold mineralization in central victoria australia we report re os isotope ages that are the first for sulfides closely associated with gold mineralization at the bendigo and maldon goldfields in central victoria two arsenopyrite samples from bendigo contain 11 ppb re and 0 08 ppb highly radiogenic os llhr samples stein et al 2000 and have model ages of 444 7 and 447 7 ma with a mean model age of 446 5 ma a five point isochron based on these arsenopyrite analyses plus three pyrite analyses including one replicate gives an age of 438 6 ma molybdenite associated with au te cu and sb mineralization at maldon gives an re os age of 376 2 ma similar to the age of the nearby har court granite these results support previous 40 ar 39 ar data indicating that initial emplacement of gold mineralization in central victoria occurred in the late ordovician during the earliest stages of accretionary development of the southern lachlan fold belt the molybdenite data are also consistent with the mobilization and or introduction of gold during late devonian magmatism

rhenium osmium abundance and isotopic compositions of massive sulfides from modern deep sea hydrothermal systems implications for vent associated ore forming processes studies of rhenium re and osmium os concentrations and isotopic compositions in seafloor hydrothermal sulfides are an important tool for understanding the evolution of hydrothermal systems allowing the determination of both metal sources and reconstructing the physicochemical conditions of their deposition the re os concentrations and isotopic compositions of 38 massive sulfide samples have been studied in different hydrothermal fields from the east pacific rise epr mid atlantic ridge mar central indian ridge cir southwest indian ridge swir and back arc basin bab the majority of the sulfides possess os 187 os 188 that span a narrow range 1 004 to 1 209 which is most easily explained as a seawater derived component this may suggest that those initial os 187 os 188 isotope compositions of ancient seafloor hydrothermal sulfide deposits which were formed by the mixing process between seawater and hydrothermal fluid are possible for analysing ancient seawater os components only two of samples mar05 tvg1 10 2 and mar05 tvg1 21 from the logatchev hydrothermal field lhf mar possess moderately less radiogenic os 187 os 188 0 645 to 0 730 which may reflect the lesser extent of hydrothermal fluid seawater mixing during hydrothermal ore forming process

structural characteristics and re os dating of quartz pyrite veins in the lewisian gneiss complex nw scotland evidence of an early paleoproterozoic hydrothermal regime during terrane amalgamation in the archaean basement rocks of the assynt and gruinard terranes of the mainland lewisian complex in nw scotland a regional suite of quartz pyrite veins cross cut regional palaeoproterozoic badcallian ca 2700 ma inverian ca 2480 ma fabrics and associated scourie dykes the quartz veins are overprinted by amphibolite greenschist facies laxfordian deformation fabrics ca 1760 ma and later brittle faults the hydrothermal mineral veins comprise a multimodal system of tensile hybrid hydraulic fractures which are inferred to have formed during a regional phase of nw se extension the almost orthogonal orientation of the quartz veins ne sw to the scourie dykes nw se are incompatible and must result from distinct paleostress regimes suggesting they are related to different tectonic events this hypothesis is supported by rhenium osmium dating of pyrite that yields an age of 2249 77 ma placing the vein hosted mineralisation event after the oldest published dates for the scourie dykes 2420 ma but before the youngest ages 1990 ma sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopically indistinguishable from primitive mantle the presence of the ca 2250 ma quartz pyrite veins in both the assynt and gruinard terranes confirms that these crustal units were amalgamated during or prior to inverian deformation the absence of the veins in the rhiconich terrane is consistent with the suggestion that it was not finally amalgamated to the assynt terrane until the laxfordian

re os age constraints and new observations of proterozoic glacial deposits in the vazante group brazil abstract a new re os radiometric age date for an organic rich shale horizon from the vazante group in brazil coupled with geological observations provide evidence for late mesoproterozoic glacial episodes conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time field observations of a reverse fault juxtaposing older mesoproterozoic sedimentary rocks above younger neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of u pb detrital zircon ages through the succession combined the geochronological geochemical and stratigraphic evidence suggest that the vazante group sediments accumulated along a passive margin of the sao francisco craton and are correlative with the neighboring paranoa group biomarker sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post glacial transgression and deposition of one of the bituminous shale horizons the relationship of the glaciogenic vazante group to other late mesoproterozoic successions such as the non glacial atar group in west africa and bylot supergroup in arctic canada however remains enigmatic

ion microprobe u pb dating and ree analyses of phosphates in h4 chondrite yamato 74371 1 u and pb isotopes and rare earth elements rees in individual phosphate grains from h4 chondrites y 74371 were measured by using an ion microprobe nine analyses of whitlockite merrillite and five analyses of apatites indicate a total pb u isochron age of 4533 40 ma 2 sigma which are consistent with the results from previous using thermal ionization mass spectrometer tims the observed apatites age of 4525 30 ma 2 sigma however is younger than those of previous works on the other hand the time difference between the whitlockite age of 4601 94 ma 2 sigma and those of apatites are not observed because of the large uncertainties the ree abundance patterns of the whitlockites normalized to ci chondrite are characterized by an almost constant enrichment of 200 800 times ci abundance and a striking negative eu anomaly the ree abundances of the apatites are lower than those of whitlockite grains and vary appreciably among different grains the remarkable correlation between the eu anomaly eu eu and total concentration of ree is confirmed but significant correlation with eu anomaly and formation age of individual grains are not observed

an extremely low source in the moon uthpb smnd rbsr and isotopic systematics and age of lunar meteorite asuka 881757 abstract we have undertaken u th pb sm nd rb sr and 40 ar 39 ar isotopic studies on asuka 881757 a coarse grained basaltic lunar meteorite whose chemical composition is close to low ti and very low ti vlt mare basalts the pb pb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 28 ma which is similar to the u pb 3850 150 ma and th pb 3820 290 ma internal isochron ages the sm nd data for the mineral separates yield an internal isochron age of 3871 57 ma and an initial 143 nd 144 nd value of 0 50797 10 the rb sr data yield an internal isochron age of 3840 32 ma 87 rb 1 42 10 11 yr 1 and a low initial 87 sr 86 sr ratio of 0 69910 2 the 40 ar 39 ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 12 ma we interpret these ages to indicate that the basalt crystallized from a melt 3 87 ga ago the sm nd age and an impact event disturbed the rb sr system and completely reset the k ar system at 3 80 ga the slightly higher pb pb age compared to the sm nd age could be due to the secondary pb from terrestrial and or lunar surface pb contamination that remained in the residues after acid leaching alternatively the following interpretation is also possible the meteorite crystallized at 3 94 ga the pb pb age and the sm nd rb sr and k ar systems were disturbed by an impact event at 3 80 ga the crystallization age obtained here is older than those reported for low ti basalts 3 2 3 5 ga and for vlt basalts 3 4 ga but similar to ages of some mare basalts indicating that the basalt may have formed from a magma related to a basin forming event imbrium the age span for vlt basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least 500 million years the impact event that thermally reset the k ar system of asuka 881757 must have been post imbrium perhaps orientale in age the lead isotopic composition of asuka 881757 is nonradiogenic compared with typical apollo mare basalts and the estimated 238 u 204 pb value for the basalt source is 10 3 this source value is the lowest so far measured for lunar rocks a large positive nd value 7 4 0 5 and the time averaged 147 sm 144 nd ratio for the basalt source are similar to those for some apollo 12 15 and 17 basalts suggesting a lree depleted mantle which is consistent with the global magma ocean hypothesis the u th pb sm nd and rb sr data on asuka 881757 suggest that the basalt was derived from a low u pb low rb sr and high sm nd source region mainly composed of olivine and orthopyroxene with minor amounts of plagioclase or clinopyroxene and with sulfides enriched in volatile chalcophile elements the basalt source may be deep in origin and different in chemistry from those previously estimated from studies of apollo and luna mare basalts indicating heterogeneous sources for mare basalts

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

the geochemical cycle of rhenium a reconnaissance rhenium re is one of a suite of elements including uranium and molybdenum that display conservative behavior in seawater and are enriched in anoxic sediments the decay of187re to187os provides a geochronometer in ancient sedimentary rocks and gives rise to os isotopic variations in nature in order to better characterize its sources to seawater re was measured in three major rivers amazon orinoco ganges brahmaputra and some of their tributaries re concentrations span four orders of magnitude from 0 02 to 400 pmol kg with the highest concentrations found in rivers draining black shales in the venezuelan andes mainstream re levels in the three rivers are between 1 and 10 pmol kg with a flux weighted average of 2 3 pmol kg the residence time for re in the oceans is estimated to be 750 000 yr with respect to river inputs re profiles from the atlantic and pacific oceans confirm that re behaves conservatively in seawater with no significant uptake onto particles and or recycling within the water column this is also true in the anoxic water column of the black sea re removal into anoxic sediments occurs at or below the sediment water interface as demonstrated in sediment pore waters from chesapeake bay in oxic sediments re is not cycled with manganese oxides and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component or in manganese nodules burial of re in anoxic sediments which accumulate on 0 3 of the ocean floor removes approximately 50 of the riverine re flux to the oceans hence oceanic re concentrations may be very sensitive to changes in the area of anoxic sedimentation

uranium lead systematics of phosphates in lunar basaltic regolith breccia meteorite hills 01210 chronological studies of brecciated lunar meteorites have proved difficult because they are mixtures of materials from various sources and the radiometric clocks are sometimes affected by the subsequent impact events on the moon here we report the in situ u pb dating of phosphates in lunar meteorite meteorite hills met 01210 which is a regolith breccia consisting of low ti mare basalt clasts and mineral fragments with a minor anorthositic component in situ analyses of four merrillite and four apatite grains in met 01210 which are resistant to secondary events resulted in a 207pb 206pb 204pb 206pb isochron age of 3904 85 ma 95 confidence limit this phosphate formation age when considered as the crystallisation age of this low ti basalt is similar to crystallization ages of 3 8 3 9 ga for unbrecciated low ti basalt meteorites asuka 881757 and yamato 793169 this result reinforces the hypothesis that all these three meteorites originated from the same area on the moon and were launched by a single impact event consistent with the similarity of launch ages mineralogical and geochemical signatures

correlation of sturtian diamictite successions in southern australia and northwestern tasmania by re os black shale geochronology and the ambiguity of sturtian type diamictite cap carbonate pairs as chronostratigraphic marker horizons recent geochronological studies have raised concerns regarding the accuracy of regional and global correlations of neoproterozoic sedimentary successions using relative dating methods such as carbon isotope chemostratigraphy and glaciogenic diamictite cap carbonate lithological and geochemical characteristics precise and accurate radio isotopic age constraints for neoproterozoic sedimentary rocks thus remain a high priority for establishing a reliable neoproterozoic chronostratigraphy here we present a new re os age of 640 7 4 7 ma from black shales of the upper black river dolomite togari group northwestern tasmania that represents a minimum age for deposition of the immediately underlying diamictite bearing julius river member the upper black river dolomite re os age is statistically identical to a re os age of 643 0 2 4 ma previously determined for the sturtian post glacial tindelpina shale member umberatana group adelaide rift complex southern australia consistent with previous lithostratigraphic and stromatolite biostratigraphic evidence the re os ages suggest broadly coeval deposition of the julius river member and sturtian diamictites and overlying organic rich marine shales consideration of the neoproterozoic geochronological database suggests that sturtian type diamictite cap carbonate pairs cannot be used as chronostratigraphic marker horizons for global correlation of the neoproterozoic rock record

thorite in an apollo 12 granite fragment and age determination using the electron microprobe abstract we present the first quantitative compositional analysis of thorite in a lunar sample the sample a granitic assemblage also contains monazite and yttrobetafite grains all with concentrations of u th and pb sufficiently high to determine reliably with the electron microprobe the assemblage represents the first documented occurrence of these three minerals together and only the second reported occurrence of thorite in a lunar rock sample 12023 147 10 is a small monomict rock fragment recovered from an apollo 12 regolith sample it comprises graphic intergrowths of k feldspar and quartz and plagioclase and quartz along with minor or accessory hedenbergite fayalite ilmenite zircon yttrobetafite thorite monazite and fe metal thorite ideally thsio4 occurs in the assemblage adjacent to quartz and plagioclase and includes a 12 xenotime y hree po4 component from quantitative electron probe microanalysis epma of th u and pb in thorite assuming that all of the measured pb is radiogenic we calculate an age of 3 87 0 03 ga yttrobetafite and monazite which contain lesser concentrations of u th and pb than the thorite yield ages of 3 78 0 06 ga and 3 9 0 3 ga respectively these dates are consistent with formation of the granitic material around 3 8 3 9 ga possibly associated with or after the formation of the imbrium basin this age falls within a group of younger ages for granitic samples measured mainly by ion microprobe analysis of zircon compared to a suite of older ages ca 4 20 4 32 ga also from zircons meyer et al 1996 a 3 8 3 9 ga age may reflect an origin following the imbrium event whereby granitic melt formed as a result of heating and melting and was mobilized and emplaced along an imbrium related ring fracture system silicic volcanic or exposed intrusive materials occur in several circum imbrium locations such as the mairan and gruithuisen domes and in ejecta excavated by aristarchus crater perhaps sample 12023 147 10 and some of the other granitic materials sampled at the apollo 12 site represent rocks similar to the rocks that make up these large silicic rock occurrences

variations in the osmium isotope composition of sea water over the past 200 000 years the first osmium isotope data are presented for marine sediments spanning the past 200 000 years two full glacial cycles and establish that there have been ocean wide variations in sea water osmium isotope composition over this time while the osmium isotope composition of sea water has been relatively invariant over much of this interval 187os 186os of 8 5 to 8 6 excursions to lower isotope ratios 187os 186os 8 1 occur late in the last two glacial periods at 20 ka and 160 ka these are times of high global aridity low global temperature and maximum ice extent while various interpretations are possible a likely explanation is that the low sea water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity there is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation the response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12 000 years