geodynamics magmatism and degassing of the earth abstract in this study terrestrial degassing processes are identified formalized both on local and global scales and combined in a single model of evolution of the upper mantle which constitutes 1 3 of the silicate earth the crust and the atmosphere unknown parameters of these processes are estimated by means of numerical modelling the model envisages early intensive convection and fractionation in the upper mantle resulting in sharp degassing of its matter later relatively slow convection in this reservoir accompanied by partial melting of silicate matter in rift and subduction zones uplift of melts into the oceanic and continental crust and their degassing early formation and subsequent recycling of the continental crust and its degassing due to intracrustal magmatism and metamorphism and accumulation of gases excepting he in the atmosphere magmatic processes are considered the most important for terrestrial degassing two stages of these processes i e partitioning of the gases between melted and solid portions and partial degassing of melt yield a pronounced u shaped fractionation of noble gases helium and neon due to high solubility and accordingly conservation in partially degassed melts as well as xenon due to a high partition coefficient and accordingly conservation in residual portions are released from partially melted matter much more slowly than argon to reconcile calculated and observed noble gas data a high rate of recycling of mantle matter must be provided in the past 74 10 17 g y during the first 0 1 ae about 90 of 36 ar had been released into the atmosphere at that time later this rate decreased sharply down to its contemporary value of about 0 6 10 17 g y less than 0 3 of 36 ar is still conserved in the upper mantle the degassing process resulted in a pronounced u shaped fractionation of noble gases in this reservoir 20 ne 36 ar m 20 ne 36 ar air 3 and 130 xe 36 ar m 130 xe 36 ar air 7 4 he 3 he and 40 ar 36 ar in the upper mantle both depend on the radioactive decay of parent elements and the rate of mantle degassing note that the noble gas radiogenic degassing clock was the only one functional during early stages of the evolution of the earth the complementary reservoir continental crust was formed very early its mass attaining the contemporary value at about 4 ga self consistent concentrations of radioactive elements in the silicate earth upper mantle crust are considerably higher than recent estimates a concentration of k equal to 0 048 is determined by 40 ar amounts in the mantle the crust and mainly in the atmosphere correspondingly u becomes 0 038 ppm and th 0 152 ppm contemporary ratios of k u 13 000 and solth u 4 being assumed for all silicate reservoirs the total contemporary fluxes atoms cm 2 sec of 3 he 4he and 40 ar rad are equal to 4 3 2 0 10 6 and 0 66 10 6 respectively practically the whole 3 he flux and 1 4 of the 4 he flux are produced by upper mantle degassing the residence times for k and 40 ar rad in the upper mantle are 1 1 and 4 1 ga respectively resulting in an enormously high value for its k ar age 4 4 ga the continental crust residence times for both species k and ar are high and the crustal mean k ar age is 1 6 ga

structural evolution of the la trocha fault zone oblique collision and strike slip basins in the cuban orogen 1 the la trocha fault zone acted as a major left lateral transfer zone and is bounded by the la trocha ltf zaza tuinicu ztf cristales ctf and taguasco tgf faults these faults were consistent with the clockwise rotation of convergence and shortening in central cuba from the paleocene to the early eocene 65 48 ma a ssw nne shortening produced transtension in the ltf and transpression in the ztf subsequently during the middle eocene 48 37 ma shortening shifted to a sw ne direction resulting in the normal component of the ltf and transpression in the ztf and ctf since the late eocene 37 ma central cuba has been welded to the north american plate the post welding deformation gave rise to transtension of the ltf and tgf this deformation is consistent with a wsw ene shortening and reflects activity in the transform boundary of the cayman trough both the normal and thrust displacements of these previous faults are corroborated by structural data whereas left lateral displacement is deduced from the concordance between oblique collision and structural features plate kinematics and the structural evolution of the la trocha fault zone indicate that the related central basin is a strike slip polygenetic basin and that the formation of this system i e fault zone strike slip basin was a consequence of the paleogene oblique collision between the caribbean volcanic arc and the bahamas borderland north american plate

mantle heterogeneity beneath oceanic islands some inferences from isotopes radiogenic isotopes in oceanic basalts are extremely useful as tracers of long lived heterogeneities in the earth9s mantle helium isotopes provide unique information in that high 3 he 4 he ratios are indicative of relatively undegassed mantle reservoirs i e mantle with high time integrated 3 he th u ratios an alternative hypothesis is that high 3 he 4 he ratios may have been produced by ancient melting events if the solid melt partition coefficient k d for he is greater than that for th and u i e yielding relatively high he th u in the residue of melting however the distribution of helium within basaltic phenocrysts and olivine glass helium partitioning within mid ocean ridge basalts suggest that helium behaves as an incompatible element during melting k d olivine glass 3 he 4 he ratios are derived from undegassed mantle reservoirs isotopic measurements of he sr and pb in hawaiian volcanoes lavas demonstrate that the mantle sources have changed on extremely short timescales between 100 and 10 000 years before present the preferred explanation for these variations is that they represent heterogeneities within the hawaiian mantle plume combined with late stage melting in the lithosphere for post shield alkali basalts helium isotopic data from kilauea hualalai and mauna loa suggest that the plume is presently located beneath kilauea and loihi seamount and constrain the melting zone of the hawaiian plume to be less than 40 km in radius

paleomagnetism of late cretaceous and eocene limestones and chalks from haiti tectonic interpretations we have conducted a paleomagnetic study of limestones and chalks from haiti which suggests that the island of hispaniola is a composite of at least two tectonically independent blocks sampling sites are distributed among three widely separated localities namely les cayes nine sites 18 3 n latitude 73 6 w longitude and beloc five sites 18 4 n latitude 72 6 w longitude which are located on the southern peninsula of haiti and a northern locality ennery six sites 19 5 n latitude 72 5 w longitude stable magnetization components of dual polarity and moderately high blocking temperature were revealed through progressive thermal demagnetization of samples from all of the beloc and ennery sites while seven of the nine les cayes sites possessed unstable magnetizations mean paleolatitudes calculated from beloc and ennery data show 8 of latitudinal separation that is of at least eocene age pole positions beloc 43 2 n latitude 185 8 e longitude 95 25 1 and ennery 66 1 n latitude 31 3 e longitude 95 19 7 show both colatitudinal and angular discordance with the north american apparent polar wander path based on these disparities we demonstrate that the implied tectonic displacements tend to support some aspects of previously published caribbean plate tectonic models

the earth s missing xenon a combination of early degassing and of rare gas loss from the atmosphere abstract the under abundance of xe in the earth s atmosphere relative to solar system abundances has been long recognised and is usually referred to as the missing xenon problem it is suggested here that this feature of the earth s composition is the result of processes operative during its accretion and earliest degassing history in essence it arises from several competing processes one of these is sorption of rare gases onto the accreting material which favours an excess of xe relative to lighter rare gases another is the subsequent fractional degassing of melts formed during or shortly after accretion which favours preferential degassing of xe over lighter rare gases from the earth s interior these two processes are combined with a major loss of rare gas from the earth s early atmosphere possibly accompanying hydrogen hydrodynamic escape

the use of sem eds pixe and edxrf for obsidian provenance studies in the near east a case study from neolithic catalhoyuk central anatolia in this paper we evaluate the relative analytical capabilities of sem eds pixe and edxrf for characterizing archaeologically significant anatolian obsidians on the basis of their elemental compositions the study involves 54 geological samples from various sources together with an archaeological case study involving 100 artifacts from neolithic catalhoyuk central anatolia with each technique the artifacts formed two compositional groups that correlated with the east gollu dag and nenezi dag sources the non destructive capabilities of these methods are emphasized albeit with certain analytical limitations in the case of sem eds suggesting important new techniques for near eastern obsidian provenance studies

helium distribution in a mantle shear zone from the josephine peridotite a previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high temperature ductile deformation in the mantle in order to test this result this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small 6 m width ductile mantle shear zone from the josephine peridotite oregon usa all measurements were made by coupled in vacuo crushing and melting demonstrating that most of the helium 80 resides within the solid phases rather than fluid or melt inclusions the present study confirms the influence of deformation on helium contents but only at the highest shear strain 20 are helium contents significantly higher the highest helium concentration by roughly a factor of two is found in the center most sample which also has grain size reduction by a factor of 4 dislocations and sub grain boundaries are present in all samples and do not correlate with helium concentrations mineralogy also appears to have a negligible influence in this shear zone as modal mineralogy is relatively homogeneous with all samples being harzburgites these observations suggest that the increase in helium concentration is related to grain size reduction with grain boundaries proposed as an additional storage site for helium in the mantle the present data also characterize the isotopic composition of the josephine peridotite 3he 4he 6 7 0 2 ra n 33 between 6 3 and 7 1 ra the presence of cosmogenic 3he in the matrix is indicated by the helium isotopic composition released by melting 3he 4he 8 5 0 3 ra n 10 from 7 9 to 10 9 this corresponds to an exposure age of 10 kyr which is approximately concordant with the end of the last glacial maximum very little radiogenic helium is present in the samples suggesting extremely low uranium and thorium contents u 0 3 ppb helium isotope measurements in four samples outside the shear zone suggest that mineralogy and melt infiltration are also important factors for understanding helium storage in the mantle

neon diffusion kinetics in olivine pyroxene and feldspar retentivity of cosmogenic and nucleogenic neon abstract we performed stepwise degassing experiments by heating single crystals of neutron or proton irradiated olivine pyroxene and feldspar to study diffusion kinetics of neon this is important in evaluating the utility of these minerals for cosmogenic 21 ne measurements and potentially for ne thermochronometry degassing patterns are only partially explained by simple arrhenius relationships most samples do not exhibit a precisely determined activation energy in an individual diffusion domain regardless we find clear differences in diffusion kinetics among these minerals based on sub selected data our estimates for neon diffusion kinetics activation energy e a and pre exponential factor d o assuming the analyzed fragments approximate the diffusion domain in each mineral are as follows for the feldspars e a ranges from 65 to 115 kj mol and d o from 3 9 10 3 to 7 1 10 2 cm 2 s 1 for the pyroxenes e a ranges from 292 to 480 kj mol and d o from 1 6 10 2 to 2 9 10 11 cm 2 s 1 for the olivines e a ranges from 360 to 370 kj mol and d o from 1 5 10 6 to 5 0 10 6 cm 2 s 1 differences in these parameters are broadly consistent with the expected effect of structural differences between feldspar and olivine and pyroxene these results indicate that cosmogenic 21 ne will be quantitatively retained within olivine and pyroxene at earth surface temperatures over geological timescales the diffusion kinetics for feldspars on the other hand predicts that 21 ne retention at earth surface temperatures will vary significantly with domain size crystal microtexture surface temperature and exposure duration quantitative retention is expected only in favorable conditions this conclusion is reinforced by additional measurements of cosmogenic 21 ne in coexisting quartz and feldspar from naturally irradiated surface samples sanidine from a variety of rhyolitic ignimbrites exhibits quantitative retention whereas alkali feldspar from several granites does not

extra terrestrial noble gases in deep sea sediments abstract magnetic fractions separated from four deep sea sediments from the pacific ocean were analysed for elemental abundances and isotopic ratios of noble gases by stepwise heating a 3 he 4 he ratio of 1 8 0 4 10 4 was observed in most samples in the high temperature fractions in all the samples 20 ne 22 ne ratios are constant 11 6 0 6 in most temperature fractions these ratios are different from he a ne a and he b ne b and appear to come from a unique component the 40 ar 36 ar ratios in high temperature fractions are lower than the atmospheric ratio indicating the existence of extra terrestrial ar while the 38 ar 36 ar ratios are indistinguishable from atmospheric planetary and solar ar indicating negligible spallogenic ar the isotopic ratios of kr and xe are indistinguishable from atmospheric ratios from the 20 ne 22 ne ratios and from thermal release patterns of he we conclude that the extra terrestrial noble gases are implanted solar flare particles the energy range of the solar flares is estimated from the relation between the grain size of idps interplanetary dust particles and the penetration depth of particles impinging on to a grain which gives less than 1 mev n the uniformity of isotopic ratios of he and ne can be explained by a grain size sieving effect of idps during atmospheric entry and by the subsequent diffusion loss of shallowly implanted solar wind noble gases

u th he dating of volcanic phenocrysts with high u th inclusions jemez volcanic field new mexico we measured u th he ages of fayalite and hedenbergite phenocrysts with britholite and chevkinite inclusions from the 1 6 ma otowi lower member of the bandelier tuff jemez volcanic field u and th the major parent nuclides of radiogenic 4 he are concentrated in britholite and chevkinite inclusions and are essentially absent from the host grains we analyzed 36 fayalite 21 non abraded 15 abraded and 13 hedenbergite 7 non abraded 6 abraded phenocrysts and applied alpha ejection corrections assuming uniform intracrystalline distributions of inclusions and parent nuclides in the phenocrysts a subset of phenocrysts was subjected to abrasion to remove the alpha ejection affected regions the alpha ejection correction factors ft of the non abraded samples range between 0 90 and 0 96 whereas those of the abraded samples are virtually unity the fayalite and hedenbergite crystals yield mean ages of 1 60f0 10 ma s d and 1 69f0 13 ma respectively the overall mean age of 1 62f0 11 ma s d is indistinguishable from previously published 40 ar 39 ar ages of 1 61f0 01 ma 1j our results suggest that other inclusion

he and sr isotopic constraints on subduction contributions to woodlark basin volcanism solomon islands in order to assess the nature and spatial extent of subduction contributions to arc volcanism sr and he isotopic compositions are measured for dredged volcanic rocks from the woodlark basin in the western pacific sup 87 sr sup 86 sr ratios increase geographically from ocean ridge values 7025 7029 at the woodlark spreading center to island arc ratios 7035 7039 in the solomon islands forearc with intermediate values near the triple junction where the woodlark spreading center subducts beneath the solomon islands sup 3 he sup 4 he ratios are also more radiogenic in the forearc 6 9 plus minus 2 r sub a at active kavachi volcano than along the spreading center where values typical of major ocean ridges were found 8 2 9 3 r sub a very low sup 3 he sup 4 he ratios occur in many triple junction rocks 1 to 5 r sub a but consideration of he isotopic differences between crushing and melting analyses suggests that the low ratios were caused by atmospheric 1 r sub a and radiogenic approx 0 2 r sub a helium addition after eruption variations in unaltered magnetic sup 3 he sup 4 he and sup 87 sr sup 86 sr ratios are best explained by subduction related fluid or silicate melt contributions to the magma source region perhaps from ancient pacific lithosphere however more mantle volatiles dominate the generation of woodlark basin rocks despite extensive subduction in the region less

search for the mantle nitrogen in the ultramafic xenoliths from san carlos arizona abstract mineral separates olivine ol clinopyroxene cp and orthopyroxene op of ultramafic xenoliths from san carlos have been analyzed for nitrogen and noble gases in an effort to evaluate the isotopic signatures for the sub continental mantle the concentrations of noble gases are generally small but in one clinopyroxene separate cp1 they are about an order of magnitude higher the gases seem mostly to be sited in fluid inclusions elevated isotopic ratios of 20 ne 22 ne 40 ar 36 ar 129 134 136 xe 132 xe with respect to air clearly show the presence of mantle noble gas components in san carlos xenoliths isotopic ratios of xe and ar show that the noble gas systematics in san carlos xenoliths can be explained as a mixture of morb and air like components nitrogen contents in these separates range from 0 1 to 1 5 ppm the 15 n varying from 2 to 5 in totals and from 9 to 10 in the temperature fractions the nitrogen data can be explained by the presence of at least three components a a light nitrogen component 15 n 9 of mantle origin b a heavy nitrogen component 15 n 15 from recycled materials and c an air like nitrogen component 15 n 0 which is incorporated during the emplacement of the host magma while the noble gas isotopic ratios cannot clearly distinguish between an atmospheric and a recycled component 15 n clearly resolves these two using the noble gas and nitrogen isotopic systematics and the end member values from the literature a contribution of 30 to 60 recycled nitrogen component has been estimated in the mantle source region of san carlos xenoliths

paleomagnetic evidence for tertiary anticlockwise rotation in southwest puerto rico late cretaceous and eocene paleomagnetic results from calcareous mudstone limestone and volcaniclastic rocks in southwest puerto rico yield a significant anticlockwise discordance in magnetization direction relative to north american reference data progressive thermal demagnetization has revealed stable characteristic high blocking temperature magnetization components that display an intrasite bipolarity the direction of this stable magnetization in tilt corrected coordinates is consistently either northwest and down or southeast and up the pole position 39 4 n lat 209 6 e long dp 9 3 dm 16 7 n 10 suggests post eocene anticlockwise rotation of the sampling region relative to north america by 45 about a vertical axis a rotating block domain model in which north south compression is superimposed on wnw ese strike slip faults in sw puerto rico can explain the inferred rotation

volcanic degassing of argon and helium and the history of crustal production on venus we develop a new methodology linking 40ar and 4he degassing to crustal production on venus in order to examine two different scenarios for the history of magmatism on that planet both consistent with recent analyses of the cratering record of the venus surface the first scenario includes episodic global resurfacing events and modest levels of magmatism between such events while the second scenario invokes different rates of steady magmatism before and after a given transition time our degassing models include distinct mantle crustal and atmospheric reservoirs diffusive transfer of 40ar and 4he from the crust to the atmosphere is also taken into account the 40ar abundance in the present atmosphere reflects the integrated degassing and magmatism over most of planetary history in contrast helium escapes from the planetary atmosphere within a characteristic residence time of 200 myr to 1 8 gyr so the present atmospheric 4he abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first scenario and to the two steady rates of magmatism and the transition time under the second unfortunately large uncertainties in mineral partition coefficients and in the ar and he mixing ratios in the lower atmosphere of venus presently prevent the use of our degassing models to distinguish among crustal formation history models we therefore explore the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements that will most strongly constrain crustal production history as an important step in the development of the ar degassing model the k budget in the bulk silicate fraction of venus is re examined on the basis of venera and vega ray measurements of k u and th concentrations in surface materials for u and th concentrations in the bulk silicate planet of 18 29 ppb and 64 94 ppb respectively the degree of mantle melting that formed the surface materials at the venera 9 and 10 and vega 1 and 2 landing sites is calculated to lie between 0 02 and 0 16 the degree of melting calculated for materials at each landing site is combined with measurements of surface k concentration to estimate the k concentration in the bulk silicate portion of the planet at between 100 and 300 ppm these results suggest that venus and earth have similar heat production

the atmospheric inventory of xenon and noble gases in shales the plastic bag experiment noble gases in five shales have been examined in the traditional protocol for trapped gas analysis and also in a new protocol in which the shales are sealed in air prior to analysis the second protocol eliminates the possibility of gas loss in the preanalysis laboratory vacuum exposure characteristic of the traditional protocol this experiment is designed to further test the shale hypothesis which stipulates that atmospheric noble gases occur in the same proportions as planetary gases in meteorites and that the factor of approximately 23 deficiency of air xe relative to planetary xe is made up by xe stored in shales or other sedimentary rocks previously available data do not support this hypothesis but could be criticized as possibly seriously underestimating shale xe contents because of experimental limitations the procedure used in this study circumvents these limitations and the results still do not support the shale hypothesis from these results it appears quite unlikely that sedimentary rocks can account for the hypothesized missing xe and unless some other atmospheric reservoir can be found to contain it it correspondingly appears that the best working hypothesis is that atmospheric noble gases are simply not planetary if so either the chemical history of the earth must account for preferential storage of xe in the earth s interior or solar nebula process responsible for noble gas incorporation in solids were significantly different for meteorites and the earth

argon oxygen and boron isotopic evidence documenting 40 ar e accumulation in phengite during water rich high pressure subduction metasomatism of continental crust abstract the luliang shan area of the north qaidam high pressure hp to ultrahigh pressure uhp metamorphic terrane in northwestern china features thick garnet and phengite rich metasomatic selvages that formed around gneiss hosted mafic eclogite blocks during hp conditions here we present new 40 ar 39 ar 18 o and 11 b results from a previously studied 30 m 18 sample traverse that extends from the host gneiss into a representative eclogite block previous thermobarometry and new mica quartz oxygen isotope thermometry from the traverse reveal that the phengite rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure quartz and white mica 18 o data from the selvage cannot be explained by simple mixing of gneiss and eclogite and indicate a fluid rock ratio 1 during regional scale infiltration of high 18 o ca 14 fluids heavy 18 o overgrowths of metamorphic zircon over lighter 18 o detrital grains indicate that the gneiss was similarly affected starkly contrasting boron content and 11 b compositions for the host gneiss and the selvage also cannot be explained by local scale devolatilization of the gneiss to form the selvage instead the boron systematics are best attributed to two distinct phases of fluid infiltration 1 low boron selvage phengite with 11 b from 10 to 30 grew under hp conditions and 2 tourmaline and boron rich muscovite with generally positive 11 b crystallized in the host gneiss under subsequent lower pressure epidote amphibolite facies conditions as the luliang shan gneiss terrane was exhumed past shallower portions of the subduction channel consistent with observations made worldwide we were able to identify uptake of excess argon 40 ar e in phengite as a high pressure phenomenon phengite 40 ar 39 ar ages from massive eclogite exceed the ca 490 ma zircon u pb age of eclogite metamorphism by a factor of 1 5 however phengite ages from the more permeable schistose selvage were even older exceeding the time of eclogite formation by a factor of 1 7 in contrast lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40 ar 39 ar ages that were younger than the time of hp metamorphism and consistent with regional cooling age patterns our observation of high 40 ar e concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at hp conditions runs contrary to widely held expectations conventional wisdom dictates that low phengite fluid partition coefficients for argon d phg fluid ar 10 3 to 10 5 coupled with the dry closed systems conditions that are widely reported to characterize hp metamorphism of continental crust explains why high concentrations of 40 ar e partitions are able to accumulate within phengite we alternatively propose that phengite fluid partition coefficients for argon increase linearly with pressure to values as high as 10 2 to allow phengites to accumulate large amounts of 40 ar e from aqueous fluids under hp to uhp conditions

exposure history of the regolithic chondrite fayetteville ii solar gas free light inclusions abstract noble gases and cosmic ray tracks were determined on light solar gas free inclusions of the solar gas rich h4 chondrite fayetteville 10 be 26 a1 major and trace element concentrations as well as oxygen isotope ratios complete the data set one exceptional inclusion is an l chondrite xenolith identified by its oxygen isotope signature and the concentration of metallic fe ni oxidized fe of this inclusion equilibrated with the host early in the history of fayetteville s parent body track density variations indicate that the majority of fayetteville s inclusions were irradiated as pebbles in the parent body by solar flare particles for about 10 4 10 6 years the l chondritic inclusion contains an unequivocal excess of gcr produced he and ne relative to the remaining inclusions this clast as well as two more solar gas free samples suffered a gcr exposure on the parent body lasting some 10 million years or longer this exposure is comparable in length to the residence times of solar gas rich matrix samples in the gcr active zone of the fayetteville parent regolith probably the other inclusions also were irradiated by gcr during at least several million years prior to the separation of the meteoroid from the parent body

sorption and diffusion of helium in garnet implications for volatile tracing and dating abstract we present data on he sorption in garnet obtained by loading garnet samples with he at 250 10 bar in the temperature range 800 990 c for this temperature range we find a three fold decrease in he sorption from 2 1 10 5 800 c to 0 72 10 5 cm 3 stpg 1 bar 1 990 c we use the sorption data of partially saturated samples to obtain the activation energy e a 660 120 190 kj mol and frequency factor d 0 10 15 6 8 5 2 cm 2 s for he diffusion in garnet the results of our study suggest that garnet is probably one of the most retentive silicate minerals for he which is in agreement with its very low ion porosity the high retentivity results in a high closure temperature t c for the u th he system for cooling rates as low as 0 1 1 c ma t c lies between 590 and 630 c accordingly u th he dating of garnet may provide information about the medium to high temperature cooling path of a rock at the lower end of the temperature range of currently used isotopic systems in garnet in addition the high retentivity for he makes garnet a potentially useful mineral in the field of exposure age dating currently it is the only known common silicate mineral occurring in upper crustal rocks which probably retains cosmogenic noble gases under all climatic conditions for geochemical studies using noble gases as tracers for geofluids garnet is a perfect container if temperatures never exceed 600 c that is all originally trapped noble gases are quantitatively retained

diffusion of cosmogenic3he in olivine and quartz implications for surface exposure dating abstract thein situ production of3he in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool to evaluate helium loss problems in this technique cosmogenic3he diffusivities were measured in quartz and olivine by incremental heating at 150 600 c arrhenius temperature dependences were observed in both minerals with similar activation energies ea 25 4 kcal mole logd0 3 7 0 9 in olivine andea 25 2 0 9 logd0 0 2 0 4 in quartz and imply very low diffusivities when extrapolated to environmental temperatures less than 10 18 cm2 s in quartz and 10 22 cm2 s in olivine at 20 c these low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 106 years in quartz and 109 years in olivine provided large 2 mm sample grains are used exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose cosmogenic3he diffuses more than 100 times faster than trapped magmatic3he in olivine or radiogenic4he in quartz the mechanisms responsible for these differences have yet to be determined but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage therefore it is possible that helium diffusivities may depend on a sample s cosmic ray exposure or radiogenic production history whatever the precise mechanism the higher mobility of3he suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies comparison of the diffusivity results with total cosmogenic3he contents of different quartz size fractions for an antarctic quartzite suggests that he loss may have occurred more rapidly than predicted by the laboratory measurements nonetheless retention of cosmogenic3he is quite high equivalent to more than a million years of exposure and cosmogenic3he geochronology is feasible in this and other quartz containing rocks

investigation of the he solubility in h2oâco2 bearing silicate liquids at moderate pressure a new experimental method we have designed the first available experimental method capable to investigate the solubility of inert gases in h2o co2 bearing silicate melts in a large range of pressures the method overcomes the difficulties imposed by the physical state of volatiles at room conditions experiments were done by using an internally heated pressure vessel where sealed capsules containing the sample are introduced the peculiarity of the method consists in the capability of loading in accurately known proportions even lower than ppm volatiles in a gaseous state at room conditions gas is loaded as a weighed amount of a gas bearing glass which was previously prepared by using the same gas as a pressure medium of a superliquidus experimental run by analyzing the gas concentration in the prepared glass the glass itself can be used as a gas source we applied this method in order to investigate the helium solubility in silicate melts having dissolved volatile mixtures mostly composed of h2o and co2 in a range of pressure suitable to provide useful information regarding pre eruptive magma degassing our results indicate that helium solubility is strongly affected by the h2o content in silicate liquid displaying an increase by about a factor three as a consequence of the addition of 3 wt h2o to the melt taking into account the physical mechanism of he dissolution the sites of the silicate melt structure where noble gases can be accommodated drastically increase by adding modest aliquots of h2o the influence of dissolved co2 on helium solubility is not certain although few signs would suggest an opposite effect with respect to h2o at least in basalt melts on the basis of the obtained results we could reasonably expect that the helium behavior during the degassing of h2o co2 bearing magmas is sensibly different with respect to a h2o co2 free silicate liquid therefore the effect of the major volatiles on noble gas solubility in magmas should be taken into account in the investigation of several geological processes such as magma genesis ascent and degassing or mantle evolution

noble gas isotopic systematics of fe ti v oxide ore related mafic ultramafic layered intrusions in the panxi area china the role of recycled oceanic crust in their petrogenesis olivine and clinopyroxene grains have been separated from four large fe ti v oxide ore bearing intrusions panzhihua hongge baima and taihe in the panxi area emeishan large igneous province southwest china for he and ar isotope studies the samples examined revealed extremely low 3 he 4 he ratios 0 078 4 34 ra with the mean value 0 78 ra for gases extracted by stepwise heating this feature combined with low 40 ar 36 ar ratios can be interpreted as due to addition of subduction related fluids and melts that had been stored in the lithospheric mantle for long periods considering the regional geologic history such addition can be attributed to the paleo subduction that occurred along the western margin of the yangtze block during the neoproterozoic the subducted oceanic crust beneath the panxi area underwent eclogite facies metamorphism and subsequent exhumation the infiltration of subduction related melts and fluids into the lithospheric mantle led to enriched isotopic signatures from that of the slightly depleted asthenopheric mantle which has been suggested by the sr nd and pb isotopic data of the emeishan basalts and picrites in addition considerable amounts of eclogitic melts produced by partial melting of eclogite facies oceanic crust extensively contaminated the lithospheric mantle during the late permian partial melting of an upwelling mantle plume that contained an eclogite or pyroxenite component generated the parental fe rich magma that supplied the ore bearing intrusions the combination of these factors may have been the crucial reason that many world class fe ti v oxides deposits are clustered in the panxi area

plume ridge interaction revisited evidence for melt mixing from he ne and ar isotope and abundance systematics abstract submarine volcanic glass he ne and ar data from the pacific antarctic rise at its intersection with the foundation seamount chain have been obtained to further elucidate one of the main outstanding problems in plume ridge interaction studies the physical state in which mantle flow and mixing occur in those settings mixing of plume mantle and mid ocean ridge basalt source material may either be in the form of two solids of solid melt or of two melt phases only the positive correlation of 4 he 40 ar with 206 pb 204 pb as well as the negative correlation of 4 he 40 ar with 40 ar and 4 he concentrations combined with a lack of correlation between 4 he 40 ar and ridge depth or mgo concentration indicate that the enriched mantle plume material has been extensively degassed prior to mixing with the mid ocean ridge source material degassing of the plume source and mid ocean ridge source material prior to mixing is also suggested by the he versus pb isotope systematics however the occurrence of degassing processes requires the existence of melts implying that mixing between the plume and the mid ocean ridge material occurs in the physical form of melts

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

obsidian from pulo di molfetta bari southern italy provenance from lipari and first recognition of a neolithic sample from monte arci sardinia twenty four obsidian artefacts of the neolithic age were found at the large karstic doline of molfetta near bari along the southern italian adriatic coast non destructive chemical analyses of the glass by sem eds allowed their source rock regions to be determined twenty three of the studied artefacts had been traded from the island of lipari only one sample had a glass composition and microphenocrysts biotite pyroxene and feldspar that indicate provenance from the monte arci area of sardinia in particular from the perdas urias sc outcrops the attribution of this artefact to sardinia enlarges the geographical pattern of monte arci obsidian exploitation and distribution from island to southern italy during the middle to late neolithic

experimental evidence for high noble gas solubilities in silicate melts under mantle pressures abstract the solubilities of ar and xe in fe free synthetic haplogranitic and tholeiitic melts were experimentally determined in the pressure range of 1 11 gpa and at temperatures between 1500 and 2000 c experiments were performed in a piston cylinder apparatus 1 3 gpa and in a multi anvil apparatus 2 11 gpa the noble gas concentrations in the quenched glasses were determined with electron microprobe as a function of pressure ar solubility increases linearly up to about 4 5 gpa where it reaches about 4 0 and 0 8 wt for the haplogranitic and tholeiitic melt respectively at higher pressure the amount of dissolved ar remains constant suggesting that some threshold concentration is reached the xe solubility in tholeiite melt exhibits a very similar pattern it increases linearly up to about 6 gpa where a threshold concentration of 0 8 wt is reached a further increase of pressure up to 11 gpa does not result in changes in xe solubility the leveling off in noble gas solubility at high pressures may imply that the interstitial sites in the melt structure suitable for the accommodation of noble gas atoms are fully occupied indeed the experimental data can be successfully reproduced with the langmuir isotherm implying a solubility model in which the gas atoms occupy a certain population of interstitial sites however the data can be equally well described by a model assuming mixing of the noble gas atoms with the oxygen atoms of the silicate melt from a thermodynamic point of view the constant noble gas solubility at high pressures simply implies that the partial molar volumes of the respective noble gas in the fluid and in the melt are equal our results differ from those of chamorro perez et al earth planet sci lett 145 1996 97 107 nature 393 1998 352 355 who reported an abrupt order of magnitude drop of ar solubility in silica and olivine melt at around 5 gpa suggesting that melt densification results in an abrupt decrease of the hole size distribution a geochemical consequence of our results is that noble gases remain incompatible elements at pressure conditions covering most of the upper mantle therefore partial melting remains an efficient process in extracting noble gases and other volatiles from the earth s mantle

combined u th he and 40ar 39ar geochronology of post shield lavas from the mauna kea and kohala volcanoes hawaii late quaternary post shield lavas from the mauna kea and kohala volcanoes on the big island of hawaii have been dated using the sup 40 ar sup 39 ar and u th he methods the objective of the study is to compare the recently demonstrated u th he age method which uses basaltic olivine phenocrysts with sup 40 ar sup 39 ar ages measured on groundmass from the same samples as a corollary the age data also increase the precision of the chronology of volcanism on the big island for the u th he ages u th and he concentrations and isotopes were measured to account for u series disequilibrium and initial he single analyses u th he ages for hamakua lavas from mauna kea are 87 40 ka to 119 23 ka 2 sigma uncertainties which are in general equal to or younger than sup 40 ar sup 39 ar ages basalt from the polulu sequence on kohala gives a u th he age of 354 54 ka and a sup 40 ar sup 39 ar age of 450 40 ka all of the u th he ages and all but one spurious sup 40 ar sup 39 ar ages conform to the previously proposed stratigraphy and published sup 14 c and k ar ages the ages also compare favorably to u th whole rock olivine ages calculated from sup 238 u sup 230 th disequilibria the u th he and sup 40 ar sup 39 ar results agree best where there is a relatively large amount of radiogenic sup 40 ar 10 and where the sup 40 ar sup 36 ar intercept calculated from the ar isochron diagram is close to the atmospheric value in two cases it is not clear why u th he and sup 40 ar sup 39 ar ages do not agree within uncertainty u th he and sup 40 ar sup 39 ar results diverge the most on a low k transitional tholeiitic basalt with abundant olivine for the most alkalic basalts with negligible olivine phenocrysts u th he ages were unattainable while sup 40 ar sup 39 ar results provide good precision even on ages as low as 19 4 ka hence the strengths and weaknesses of the u th he and sup 40 ar sup 39 ar methods are complimentary for basalts with ages of order 100 500 ka

40ar 39ar and u pb evidence for late proterozoic grenville age continental crust in north central cuba and regional tectonic implications renne p r mattinson j m hatten c w somin m onstott t c millfin g and linares e 1989 4 ar 39ar and u pb evidence for late proterozoic grenville age continental crust in north central cuba and regional tectonic implications precambrian res 42 325 341 central cuba is composed of fault bounded tectonostratigraphic terranes juxtaposed and deformed during plate collision and subsequent transform motion between the caribbean and north american plates in the late mesozoiccenozoic one of these the las villas terrane contains crystalline basement rocks thought to be pre upper jurassic on stratigraphic grounds the socorro complex occurs in the northwestern las villas terrane and consists of marbles and siliciclastic metasedimentary rocks and the rio carla granite an 4 ar 39ar plateau date of 903 5 7 1 ma for phlogopite from a marble corroborates previous k ar dates from this unit and establishes unambiguously a late proterozoic age for high grade metamorphism discordance of the 4 ar agar age spectrum can be reliably attributed to diffusive ar loss which if modeled as an episodic thermal event implies a reheating age that closely coincides with the late cretaceous paleogene collision between the caribbean and north american plates u pb zircon data indicate an intrusive age of 172 4 ma for the rio carla granite and reveal an inherited zircon component with an age of 900 ma radio isotopic data from the socorro complex display no evidence of pan african age thermal overprinting these observations combined with constraints provided by published results from nearly pangean landmasses suggest that the complex lay substantially to the southwest present day co ordinates during the early paleozoic following its genesis in the mid mesozoic the caribbean plate evidently transported fragments of an extensive grenville age belt that spanned the americas during the late proterozoic

cenozoic tectonic history of the north america caribbean plate boundary zone in western cuba structural studies of well dated jurassic to lower miocene rocks in western cuba constrain the sequence of structural events affecting this oblique collisional zone between the late cretaceous island arc and the jurassic cretaceous north america passive margin in the southeastern gulf of mexico and straits of florida results of detailed mapping and collection of fault slip data at 34 sites define a regionally consistent five phase tectonic model for the period from the late paleocene to the post early miocene during the late paleocene to the early eocene the cuban island arc collided with the north american passive margin bahamas platform northwest ward overthrusting during the collision defines tectonic phase i a nne ssw compression concurrent with early eocene left lateral strike slip faulting along the pinar fault zone defines phase ii this result is consistent with structural mapping showing sinistral shear within the 065 striking pinar fault zone an ene wsw to e w compression defining phase iii overprinted phase ii faults in the lower eocene and older rocks post early miocene normal faulting characterizes phase iv inversion of fault slip data indicates two contemporaneous directions of tension of 120 and 170 strike slip faults that overprint phase iv normal faults yield a 120 compression phase v the direction of compression associated with the arc continent collision rotates clockwise from nw se in the late paleocene early eocene phase i to nne ssw phase ii and to ene wsw by the middle eocene phase iii the rotation in the compression direction occurred because the arc turned toward an oceanic area in the present day area of central and eastern cuba progressive collision led to complete subduction of the remnant oceanic crust by middle to late eocene time

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

characterization of the monte arci sardinia obsidian sources in the western mediterranean obsidian from four volcanic island sources was used beginning in the neolithic period the geological sources on lipari palmarola and pantelleria have been located and chemically characterized until now however the monte arci source in sardinia was better known from the analysis of archaeological rather than geological specimens the results of a comprehensive field survey and characterization study of the obsidian sources in sardinia are presented here and finally allow all obsidian artefacts of western mediterranean origin to be attributed to specific sources using minimally destructive and relatively inexpensive methods of analysis five chemically distinct sources in the monte arci region were exploited with their location and the physical properties of the obsidian itself contributing to chronologically and spatially diverse frequency of use patterns

combining 3 he cosmogenic dating with u th he eruption ages using olivine in basalt this paper presents new u th he crystallization and 3 he exposure ages on olivine for a suite of basalt samples from the snake river plain idaho the results provide the first demonstration that u th he geochronology can be successfully applied to late quaternary basalts using measurements of olivine phenocrysts as opposed to u rich minerals the approach employs abrasion to remove the outer rinds of olivine crystals to avoid problems with 4 he implantation from the groundmass and a specially designed extraction system that allows u th and he concentrations and isotopic compositions to be measured on the same olivine grains we demonstrate the viability of the u th he technique on basalts with ages of 373 ka and 3400 ka previously dated by ar ar and show that concurrent measurements of u th 4 he ages and 3 he cosmogenic exposure ages improve the accuracy of both methods the combinedu th 4 he 3 hetechniqueisappliedtoaseriesofbouldersandflowsfromboxcanyonnearhagerman idaho toevaluate the ages of the basalts and to use exposure ages to make inferences about the age and mode offormation of the canyon the u th he ages of the box canyon basalts are between 86 12 ka and 130 12 ka 1 sigma errors and suggest at least two periods of eruption the ages are similar to a ca 95 ka ar ar age previously assigned and distinguish the box canyon basalts from nearby ca 395 ka basalt exposure ages of boulders within the canyon at the mouth head and middle of box canyon are 21 1 19 3 and 48 3 ka scoured bedrock at the canyon head has an exposure age of 45 5 ka these data indicate that the canyon was carved before the

palaeomagnetism of the central cuban cretaceous arc sequences and geodynamic implications abstract a detailed palaeomagnetic study of cretaceous age volcanic and sedimentary arc rocks from central cuba has been carried out samples from 32 sites 12 localities were subjected to detailed demagnetisation experiments nineteen sites from the los paso matagua provincial and cabaiguan formations yielded high unblocking temperature dual polarity directions of magnetisation which pass the fold tests with confidence levels of 95 or more and are considered to be primary in origin the palaeomagnetic inclinations are equivalent to palaeolatitudes of 9 n for the aptian 18 n for the albian a synfolding remanence identified in 5 sites from the younger hilario formation indicates a late cretaceous remagnetisation at a palaeolatitude of 16 n our results are in good agreement with previous palaeogeographic models and provide the first high quality palaeomagnetic data demonstrating the gradual northward movement of the cretaceous volcanic arc throughout the cretaceous the declination values obtained all indicate significant and similar amounts of anticlockwise rotation from the oldest sequences studied through to the late cretaceous remagnetisation this rotation is most likely related to collision of the arc with the north american plate and transpressional strike slip movement along the northern margin of the caribbean plate as it progressed eastwards into the large proto caribbean basin

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

solubility and partitioning of ne ar kr and xe in minerals and synthetic basaltic melts we have measured the solubilities of ne ar kr and xe in natural samples of anorthite diopside forsterite spinel and in synthetic basaltic melts the minerals and melts represent equilibrium pairs in the fo an di sio2 system samples were suspended in individual crucibles in a one bar flowing mixed noble gas atmosphere at 1300 or 1332 c for times ranging from 7 18 days the solubilities in the minerals increase with increasing noble gas atomic number and typical solubility values are surprisingly high samples of a particular mineral i e anorthite that came from different localities yield distinctly different and reproducible solubility values this indicates that properties intrinsic to individual samples can influence solubility noble gases are likely to be sited in lattice vacancy defects in contrast the solubilities in the melts decrease with increasing atomic number our data overlap the low end of the range defined previously for natural basalts however the dynamic range of the values from ne to xe is not as great as for the natural melts solubilities correlate well with melt molar volume but poorly with density the partition coefficients increase with increasing noble gas atomic number for all mineral melt pairs such trends imply that the terrestrial planet atmospheres were not derived from partial melting of chondritic source material significant fractionation of kr relative to xe is not observed ruling out an origin for earth s missing xe via magmatic fractionation partition coefficient absolute values are frequently near or greater than unity for example the ranges for five diopside measurements are the following dnel 0 013 0 37 darl 0 15 0 84 dkrl 0 31 2 4 and dxel 3 2 47 this indicates that magmatic transport is not an efficient mechanism for degassing the terrestrial planets our results favor a catastrophic origin for the atmospheres

he ar o sr and nd isotope constraints on the origin and evolution of mount etna magmatism abstract the 0 5 ma history of the origin and evolution of mount etna sicily has been investigated by analysing the isotopic composition of he ar o sr and nd in 21 selected lava samples the near constancy of the isotopic compositions of oxygen 18 o 5 4 0 3 and of helium trapped in olivine phenocrysts 6 7 0 4 ra is interpreted as evidence of a single mantle source despite drastic petrological changes during the volcano s history he analyses performed by crushing cogenetic pyroxene and olivine phenocrysts show a tendency to lower 3 he 4 he ratios in pyroxenes this is best explained by crystallization of pyroxenes at a depth shallower than that of olivines and or by exchange of helium trapped in pyroxenes with atmospheric or radiogenic he before eruption 87 sr 86 sr ratios of recent lavas tend to increase with time and to correlate with rb th ratios and for historical lavas these variations are tentatively attributed to shallow selective contamination from underlying sediments based on the similarity of the 3 he 4 he ratios at etna to those of european mantle xenoliths 1 we propose that the baseline geochemical signature of isotopic tracers at etna reflects the composition of the subcontinental mantle comparison to other southern italian active volcanoes etna vulcano ischia campi flegrei vesuvius shows gradual dilution of the predominantly mantle etnean end member by more radiogenic sr and he and material with higher 18o and c 3 he which is reasonably explained by the progressively important influence of subducted continental crust

reconnaissance paleomagnetic results from western cuba abstract a paleomagnetic study of mesozoic rocks from the sierra de los organos and sierra del rosario fold belts of western cuba revealed postfolding magnetisation in diabases of the late jurassic el sabalo formation and carbonates of the middle cretaceous pons and the late cretaceous carmita and moreno formations steep components with inclinations of about 70 were isolated from all three formations at the same time postfolding shallow components were also found in a few samples of the pons limestones we rule out a possibility to account for these results by either horizontal movements or non dipole field anomaly neither very appealing is a hypothesis of a post remagnetization tilt of the entire region all the components appear to be confined to a plane perpendicular to the main structural trends we hypothesize that the remanences might have been distorted or re aligned during deformation this assumption however is far from being proven in contrast well defined characteristic components were isolated from basalts of the aptian albian encrucijada d i 247 23 k 14 a95 9 0 and the late cretaceous orozco d i 228 22 k 110 a95 4 7 formations from the bahia honda zone in the north of western cuba the remanence in the encrucijada formation is shown to predate deformation mean inclinations in both formations match those in cretaceous volcanics from central cuba and all the results show lower latitudes than expected from the reference data for the north american plate thus implying that volcanic domains of cuba were displaced northward by about 1000 km prior to the middle eocene cretaceous declinations in western and central cuba differ by about the same amount as the major structural trends of these two areas suggesting oroclinal bending of cuba at the same time both areas are rotated counterclockwise with respect to north america thus implying movements on a broader scale

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

evidence for subduction related components in the subcontinental mantle from low 3he 4he and 40ar 36ar ratio in mantle xenoliths from far eastern russia abstract by applying both vacuum crushing and stepwise heating methods for the extraction of the noble gases we have discovered 3he 4he ratios much lower than the atmospheric ratio 0 3 ra ra is the atmospheric 3he 4he ratio of 1 4 10 6 and relatively low 40ar 36ar ratios spectroscopic and petrographic observations confirm that there are at least two compositionally distinct fluids in these xenoliths liquid co2 inclusions and melt inclusions with shrinkage bubbles based on the crushing experiments it is inferred that the inclusions of liquid co2 have a 3he 4he ratio similar to that of morb and the component with the low 3he 4he ratio is derived from the shrinkage bubbles in the melt inclusions for the present samples the 40ar 36ar ratios obtained by crushing were less than 1000 suggesting incorporation of atmospheric components in the source materials since low 40ar 36ar ratios were observed irrespective of the occurrence of the liquid co2 inclusions the atmospheric component exists in the melt inclusions ne and xe isotopes are also consistent with incorporation of atmospheric components since the far eastern russia area was located at a subduction zone in the jurassic early cretaceous period it is most likely that the melt inclusions displaying atmospheric noble gas characteristics together with low 3he 4he ratios have been derived from the jurassic cretaceous subducted slab although we cannot exclude the possibility that low 3he 4he ratios are due to the existence of minor u bearing minerals in the lithospheric continental mantle caused by metasomatism we have no petrographical evidence for such minerals in this area on the other hand 3he 4he ratios observed in the liquid co2 inclusions which are similar to the morb like value might reflect the general character of the upper mantle the far eastern russian mantle may therefore be a morb like source that has been partly infiltrated by subduction related fluids

4he 3he thermochronometry using classical diffusion theory we present a mathematical technique for the determination of 4he concentration profiles in minerals this approach should prove useful for constraining the low temperature cooling histories of individual samples and for correcting u th he ages for partial diffusive loss the calculation assumes that the mineral of interest contains an artificially produced and uniform distribution of 3he obtained by proton irradiation shuster et al earth planet sci lett 217 2004 19 32 in minerals devoid of natural helium this isotope allows measurement of he diffusion coefficients in minerals with measurable radiogenic he it permits determination of 4he profiles arising during ingrowth and diffusion in nature the 4he profile can be extracted from stepwise degassing experiments in which the 4he 3he ratio is measured the evolution of the 4he 3he ratio as a function of cumulative 3he released can be compared with forward models to constrain the shape of the profile alternatively we present a linear inversion that can be used to directly solve for the unknown 4he distribution the inversion incorporates a standard regularization technique to filter the influence of random measurement errors on the solution using either approach we show that stepwise degassing data can yield robust and high resolution information on the 4he profile profiles of radiogenic he are a sensitive function of the time temperate t t path that a cooling sample experienced thus by step heating a proton irradiated sample it is possible to restrict the sample s acceptable t t paths the sensitivity of this approach was explored by forward modeling 4he profiles resulting from a range of realistic t t paths using apatite as an example results indicate that 4he profiles provide rich information on t t paths especially when the profiles are coupled with u th he cooling ages on the same sample samples that experienced only moderate diffusive loss have 4he concentration profiles that are rounded at the edge but uniform in the core of the diffusion domain such profiles can be identified by nearly invariant 4he 3he ratios after the first few to few tens of percent of 3he have been extracted by step heating we show how such data can be used to correct u th he ages for partial diffusive loss

discordant mid cretaceous paleomagnetic pole from the zaza terrane of central cuba thermal demagnetization of samples from five sites in mid cretaceous volcaniclastic and carbonate rocks of south central cuba reveals two or more components of magnetization a pre folding magnetite borne component is recognized in all but was difficult to isolate from secondary components in many specimens remagnetization circle analysis using data for 42 specimens from all five sites yields a pole at 30 7 degree n lat 193 3 degree e long this pole is discordant with respect to the north american apwp indicating 43 degree plus minus 16 degree of anticlockwise rotation and 8 degree plus minus 6 degree of northward displacement since the mid cretaceous this result suggests that part or all of cuba was transported on the caribbean plate before accretion to north america in the eocene

a noble gas profile across a hawaiian mantle xenolith coexisting accidental and cognate noble gases derived from the lithospheric and asthenospheric mantle beneath oahu a noble gas profile across a garnet pyroxenite xenolith from salt lake crater oahu hawaii provides information about the scale and origin of noble gas heterogeneities within such rocks variations in both absolute and relative noble gas concentrations are large and comparable to those observed between individual salt lake crater pyroxenite xenoliths 3he 4he varies from 7 7 to 9 4 times the atmospheric value ra and correlates inversely with 40ar 36ar which ranges between 4100 and 9700 neon krypton and xenon isotopes are uniform and indistinguishable from air with the exception of excess 129xe overall the observed noble gas compositions reflect a derivation from depleted morb type mantle sources the spatial distribution of noble gas signatures within the xenolith and the observed correlation between helium and argon isotopes suggest the presence of two different noble gas components which are trapped in different phases and are unevenly distributed within the xenolith 40ar 36ar and 1 36ar correlate inversely indicating that atmospheric contamination is insignificant hence the observed isotopic variations reflect mixing of two mantle derived noble gas components correlations between hear isotopes and co2 h2o in different pyroxenites from salt lake crater including our sample reveal that the first component is characterized by highly radiogenic helium and argon isotopes and related to abundant secondary co2 rich fluid inclusions given the high diffusivity of he at mantle temperatures hart 1984 the observed helium isotope heterogeneities on a sub mm scale require that the fluids were introduced concurrently with eruption this interpretation is supported by the low entrapment depths of fluid inclusions in salt lake crater pyroxenites 30 km murck et al 1978 this implies that the fluids are genetically related to the host magma itself and reflect its composition the second noble gas component is interpreted as being magmatic i e cognate to the basaltic magma from which the pyroxenite precipitated within the mantle it is proposed that this component resides inside the mineral lattices and was trapped during magmatic crystallization our results indicate that the depleted lithospheric mantle source of the post erosional host magma honolulu volcanic series is characterized by 40ar 36ar 10000 r ra 7 5 and excesses in129xe noble gas signatures of the asthenospheric mantle source parental to the pyroxenite producing magma are less radiogenic with respect to these isotopes combining noble gas and strontium neodymium and lead isotope evidence we propose that the asthenosphere beneath oahu was originally similar to depleted morb type mantle but became slightly modified by noble gases and other incompatible trace elements derived from the hawaiian mantle plume prior to partial melting this study provides evidence that noble gas isotopes and solid radiogenic isotopes are coupled in the mantle sources of basalts e g allegre et al 1983 but decoupled in xenoliths e g vance et al 1989

morb type neon in an enriched mantle beneath etna sicily abstract noble gas elemental and isotopic compositions were determined for five co 2 ch 4 samples collected around etna sicily to investigate the geochemical features of the mantle beneath the volcano the samples contain mantle derived noble gases the measured helium isotopic ratios 3 he 4 he vary between 5 9 and 6 4 times atmospheric ratio r a 1 4 10 6 which are comparable to the ratios of olivines 6 1 8 2 r a in the lavas of the same volcano 1 neon in the samples is enriched in both 20 ne and 21 ne 20 ne 22 ne 9 95 10 7 21 ne 22 ne 0 030 0 037 indicating derivation from the mantle the 20 ne 22 ne 21 ne 22 ne values are identical with that of mid ocean ridge basalts morb indicating a similarity in the time integrated u ne ratio between the etnean magma source and the depleted upper mantle morb source argon in the samples has 40 ar 36 ar ratios up to of 1800 which are higher than in atmosphere these ratios are positively correlated with the 20 ne 22 ne ratios indicating a mantle origin of the radiogenic argon compared with olivines from the etnean lavas 1 the argon in our natural gas samples is less contaminated by atmospheric argon two samples from a co 2 well show small but resolvable excess of 129 xe and 134 136 xe the volcanic rocks of etna ranging from tholeiites to alkaline are enriched in incompatible elements nd sr and pb isotopes of the volcanic rocks indicate that the magma source is isotopically heterogeneous and contain a component with high sr pb and low nd isotopic ratios derived from a mantle region which has been enriched in incompatible elements for a few million years 2 3 4 relative to the depleted upper mantle the helium isotopic ratios of the gas samples are lower than those of morb and are consistent with geochemical signatures of the solid elements in the etnean volcanic rocks however the observed morb type neon is on a morb correlation array without accumulation of nucleogenic 21 ne this apparent decoupling may be explained by a recent mixing of the depleted upper mantle morb source with fluid enriched in both incompatible elements and radiogenic 4 he when the fluid was formed with a small degree of partial melting fractionation of radiogenic 4 he from nucleogenic 21 ne could have occurred because of smaller partition coefficient of he than ne

cosmogenic 3he production rate in ilmenite and the redistribution of spallation 3he in fine grained minerals cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of 3he in olivine or pyroxene however geochemical investigations using 3he have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals or where such crystals were present but have been chemically weathered fine textured basalts often contain small grains of ilmenite a weathering resistant mineral that is a target for cosmogenic 3he production with good he retention and straightforward mineral separation but with a poorly constrained production rate here we empirically calibrate the cosmogenic 3he production rate in ilmenite by measuring 3he concentrations in basalts with fine grained 20 m cross section ilmenite and co existing pyroxene or olivine from the columbia river and snake river plain basalt provinces in the western united states the concentration ratio of ilmenite to pyroxene and olivine is 0 78 0 02 yielding an apparent cosmogenic 3he production rate of 93 6 7 7 atom g 1 yr 1 that is 20 30 greater than expected from prior theoretical and empirical estimates for compositionally similar minerals the production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and 3he and the associated long stopping distances that cause them to redistribute within a rock fine grained phases with low cosmogenic 3he production rates like ilmenite will have anomalously high production rates owing to net implantation of 3he from the surrounding higher 3he production rate matrix semi quantitative modeling indicates implantation of spallation 3he increases with decreasing ilmenite grain size leading to production rates that exceed those in a large grain by 10 when grain radii are 150 m the modeling predicts that for the ilmenite grain size in our samples implantation causes production rates to be 20 greater than expected for a large grain and within uncertainty resolves the discrepancy between our calibrated production rate theory and rates from previous work the redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number as they do between whole rock basalt and ilmenite

preliminary uvlamp determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts an ultra violet laser ablation microprobe uvlamp has been applied for the first time to investigate argon partition coefficients for olivines and clinopyroxenes grown from silicate melts at 1 bar argon pressure these preliminary measurements yield crystal melt partition coefficients ranging from 0 138 0 01 to 0 013 0 003 for olivine and 0 589 0 003 to 0 0016 0 0005 for clinopyroxene the higher values may indicate sub microscopic melt inclusions or some other heterogeneous distributions of non equilibrium argon in the crystals the lower values are probably more representative of true partition coefficients and fall at least an order of magnitude below the previously reported experimental data the possibility of anomalous high argon contents for crystals in previous studies is discussed in terms of surface adsorption trapped argon and early partial melting

tectonic implications of radiogenic noble gases in planetary atmospheres abstract studies of the radiogenic isotopes 4 he and 40 ar in planetary atmospheres provide important constraints on tectonic processes within planetary interiors for mantle k u and th u mass ratios r k u 12 700 and r th u 3 8 and a urey number ur 0 7 the 40 ar in the earth s atmosphere can be obtained by complete outgassing of the crust and upper mantle and a 7 outgassing of the lower mantle or by a 23 to 33 outgassing of the whole mantle and crust the flux of 4 he from the mantle can be derived by complete steady state outgassing of the upper mantle or by partial outgassing 9 of the entire mantle the 40 ar in the venus atmosphere can originate from the outgassing of only the upper 6 km of an enriched crust 8 5 of the entire mantle or 28 of the upper mantle the low concentration of 36 ar in the earth s atmosphere relative to venus strongly supports the collision hypothesis for the origin of the moon such a major collision would have eroded a large fraction of the earth s atmosphere the 40 ar on mars requires that only the upper 640 m of the crust be outgassed however the presence of large volcanoes on mars indicates greater degassing thus supporting the hypothesis of past erosion of the martian atmosphere by collisions with planetesimals and large meteorites the 40 ar flux through the lunar atmosphere requires the outgassing of the upper 9 km of the lunar crust

he sr and nd isotopes in xenoliths from hawaii and other oceanic islands abstract isotopic analyses on he whole rock and sr nd separated clinopyroxenes are reported for ultramafic and gabbroic xenoliths from the hawaiian islands canary islands kerguelen gough and madeira the he isotopic compositions in samples of varied petrologic type from hawaii show a remarkably narrow range r ra 8 2 9 5 are within the previously reported range for hawaiian xenoliths and are indistinguishable from the range of 3he 4he ratios seen in morb in contrast sr and nd isotopic compositions show a significant range samples from the canary islands kerguelen gough and madeira considered as a group show a similar trend with a restricted range in 3he 4he 5 2 7 ra contrasted by sr and nd isotopic compositions extending over almost the entire range of oceanic basalts the relationship between he and sr nd isotopes of ocean island xenoliths are strikingly similar to that observed for continental ultramafic xenoliths by porcelli et al 1986 1 these systematics suggest metasomatic introduction of lithophile sr and nd and volatile he elements into the lithosphere beneath ocean islands overprinting depleted mantle isotopic characteristics the addition of volatiles must be accomplished by a different process to that of lithophiles probably due to selective transport most likely migration of a lithophile poor co2 h2o fluid consideration of he sr data for oceanic basalts suggests that similar fluid related processes may operate prior to or during mantle melting if so the interpretation of he sr relationships of oceanic basalts in terms of simple mixing between isotopically well defined sources is likely to be invalid

fluid kinematics fluid residence times and rock degassing in oceanic crust determined from noble gas contents of deep sea drilling project pore waters the 3he 4he ne and ar in sedimentary pore fluids extracted in situ were measured on samples from deep sea drilling project dsdp sites 398 410 419 420 424 436 and 438 earlier results from dsdp leg 15 are also discussed where regional sediment cover prevents direct penetration of seawater into basement rocks at sites 149 436 and 438 he concentration profiles indicate slow vertical and or horizontal advection of sedimentary pore fluids suggesting that fluids can ventilate through the sediment cover either in diffuse bulk flow or in aquifers confined to regions of relatively high permeability such as ash beds sand silt layers or fracture channels in lithified sediments at site 410 where basement outcrops are prevalent no advection was detectable in the sediments however the basement was well flushed by seawater on 1 5 108 years apparent age in sediments on the active continental margin off ne japan

the effect of channel spacing during magma migration on trace element and isotopic ratios an analytical mathematical model is discussed for the transport of trace element and isotopic ratios during magma migration when no major dissolution or precipitation of minerals occurs but diffusive exchange of atoms between magma and solid is important this model is then used to elucidate the effect of channel spacing on the transport of trace element and isotopic ratios during the migration of basaltic magma it is found that when total reequilibration occurs as it will in migration through closely spaced channels on grain boundaries isotopic ratios are transmitted to the surface unchanged but trace element ratios may vary wildly when channels are far enough apart so that only partial reequilibration occurs variations in both isotopic and trace element ratios are reduced or eliminated during magma migration finally when channels are sufficiently distant so that negligible reequilibration occurs both trace element and isotopic ratios are transmitted to the surface unchanged

helium isotopic composition of the tabar lihir tanga feni island arc papua new guinea helium abundances and isotopic ratios have been measured in samples of geothermal gases submarine ultramafic xenoliths and subaerial mafic phenocrysts and xenoliths from the tabar lihirtanga feni tltf arc in the bismarck archipelago of papua new guinea these unusual volcanoes are produced by adiabatic decompression melting of subduction modified upper mantle and apparently carry an exceptionally large slab derived component they therefore provide an ideal setting for studying the role of subducted helium in arc volcanoes helium isotopic ratios in the geothermal fluids submarine xenoliths and most helium rich subaerial samples indicate the mantle source regions beneath the tabar island group and lihir island have 3he 4he ratios of about 7 2 times the atmospheric ratio r a whereas the mantle source for ambitle island feni group in the more southern tltf arc has a lower ratio of about 6 6 r a these ratios are only slightly lower than typical depleted upper mantle values of about 8 5 r a indicating that even severely slab modified mantle wedge carries a relatively minor slab derived helium component the systematically lower 3he 4he ratios of the ambitle samples are interpreted to reflect a slightly greater slab derived helium component helium isotopic ratios as low as 2 r a which are correlated with low total helium abundances were observed in subaerial augitic clinopyroxene phenocrysts coexisting olivine and sodium rich acmitic clinopyroxene have higher helium abundances and isotopic ratios this isotopic disequilibrium between coexisting mineral phases is interpreted as crustal level addition of a low 3he 4he component to an actively outgassing magma from which different mineral phases trap helium sequentially

argon behavior in basaltic melts in presence of a mixed h2o co2 fluid at upper mantle conditions abstract the behavior of argon in h2o co2 bearing basaltic melts was experimentally investigated in the pressure range 1 5 gpa and at temperatures between 1350 and 1600 c our experimental data simulate the partitioning of argon between an ar h2o co2 bearing fluid and a silicate melt occurring during magma ascent and degassing the experimental results show several features at a pressure of 1 gpa the variation of h2o from 0 35 to about 2 wt at constant co2 content does not induce any systematic variation of argon dissolved in basaltic melt for higher water contents 2 wt we observe a positive effect of water on argon solubility in the range of 3 5 gpa we did not observe any systematic variation of the argon content in the basalt melt for water concentration from 0 35 to 5 3 wt and at co2 content 0 5 wt argon concentration in basalt melt decreases from about 3800 to 2400 ppm when the co2 content increases from 0 5 to 0 8 wt at all pressures investigated in the present study a negligible effect of co2 for concentration more importantly despite all these variations it seems that the effect of pressure in the range of 1 to 5 gpa is the dominant parameter on argon solubility in basaltic melt pressure has a positive effect on argon incorporation in the h2o co2 bearing basaltic melt reaching at 3 gpa a concentration of 0 38 wt this maximum of 0 38 wt corresponds to 6 8 10 5 cm3 g 1 bar 1 at standard temperature and pressure a value of the same order of magnitude as that derived from volatiles free basaltic melts equilibrated with argon the experimental data can be well described by a thermodynamic model assuming mixing of volatile species and oxygen in the silicate melt the results can be applied for a better understanding of the fractionation noble gas noble gas and noble gas co2 occurring in degassing processes during magma ascent

ar diffusion and solubility measurements in plagioclases using the ultra violet laser depth profiling technique we describe the first direct measurements of ar diffusion and solubility in plagioclases using ultra violet uv laser ablation depth profiling and noble gas mass spectrometer analyses of experimentallytreated 599 1000 8c 50 200 mpaofar crystalfragmentsoflabradoriteandoli goclase labradorite 40 ar gain diffusion profiles were measured yielding an activation energy of 26 72 4 58 kcal mol 21 118 0 19 16 kj mol 21 and a frequency factor of 9 77 10 29 8 79 10 28 28 79 10 29 c m 2 s 21 95 confidence the ar solubility in labradorite was measured yielding a value of 0 2 ppb bar 21 which is similar to or lower than many rock forming minerals the labradorite diffusion parameters indicate ar closure temperatures of 211 8c for a spherical diffusion geometry and 243 8c for a planar diffusion geometry for 100 mm diameter grains with cooling rates of 10 8 cm a 21 the data indicate that labradorite is less ar retentive than k feldspar at low temperatures but more ar retentive than k feldspar at high temperatures corroborating previous work on plagioclase the relatively slow ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems supplementary material details of the uv laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at www geolsoc org uk sup18608

argon solubility in silicate melts at very high pressures experimental set up and preliminary results for silica and anorthite melts a co2 laser heated diamond anvil cell was used for performing argon solubility experiments in silicate melts this technique allowed solubility experiments to be carried out at much higher pressures than in a piston cylinder type apparatus when the beam of the co2 laser is focused on the silicate sample argon acting as a pressure transmitting medium melts by conductive heating from the molten sample and can disserve into the melt preliminary results for argon solubility in silica and anorthite melts up to 10 gpa are presented in anorthite melt the ar content levels up at 0 5 wt above 5 gpa for silica melt ar contents increase up to nearly 5 wt at 5 gpa and decrease below 1 at higher pressures this behaviour is interpreted as resulting from a profound change in the structure of the melt above 5 gpa probably related to an increase in the proportion of 4 and 3 membered sio4 rings

a reconnaissance paleomagnetic study of cretaceous rocks from central cuba we applied thermal demagnetization to 12 sites of mid and upper cretaceous volcanics and sediments of the zaza terrane central cuba consistent directions of low and or high temperature components were obtained from eight sites the fold test performed on a short wave fold in the upper cretaceous arimao formation is negative as well as the conglomerate test on lava boulders from the same formation whereas site means for the entire data set are better grouped after tilt correction we conclude that the entire cretaceous succession of the zaza terrane was remagnetized in the campanian the measured latitude is lower by 15 6 with respect to the north american apwp indicating ca 1600 600 km northward displacement of the zaza terrane since the campanian also discordant is the measured declination implying 37 11 counterclockwise rotation of the zaza terrane

on the siting of gases shock emplaced from internal cavities in basalt abstract noble gases were extracted by stepped combustion and crushing from basalts which contained gas filled cavities of controlled sizes prior to shock at 40 gpa analysis of fractions enriched and depleted in shock glass from a single sample gave a factor of 2 higher gas abundances in the glass rich separate release patterns were nearly identical suggesting similar siting in glass in both fractions crushing of a sample released 45 of implanted noble gases but only 17 of n 2 indicating that most or all of the noble gas was trapped in vesicles analysis by sem eds confirmed the presence of vesicles in glassy areas with an average diameter of 10 m samples with relatively large pre shock cavities were found to consist of up to 70 80 glass locally and generally exhibit greater local shock effects than solid and densely packed paniculate targets at the same shock pressure though the latter give higher gas emplacement efficiencies the petrographic results indicate that in situ production of glassy pockets grossly similar to those in the shergottite eeta 79001 is possible from shock reverberations in the vicinity of a vug however the siting of the gases points to a more complex scenario in which spb gas and melt material were probably injected into eeta 79001

argon diffusion and solubility in silicic glasses exposed to an ar he gas mixture we have determined argon solubility and diffusivity in sio2 naalsi3o8 and rhyolitic composition glasses exposed to an equimolar ar he gas mixture at 1225 2500 bars and 500 850 c the diffusivity results from the mixedar he experiments are almost identical to those from experiments with pure ar activation energies for diffusion ea kj mol 1 1 uncertainties are for silica 110 9 2 1 he ar mix vs 100 8 5 4 pure ar for albite 157 3 3 8 he ar mix vs 137 2 6 3 pure ar and for rhyolite 161 9 1 7 he ar mix vs 144 3 6 3 pure ar argon solubilities determined from these experiments show slight negative deviations from henry s law far xar k the henry s law constant as observed in pure ar experiments at ar fugacities greater than about 1500 bars the predominant effect of he on ar solubility in the mixed gas experiments is one of dilution of ar activity in the gas phase solubilities in mixed gas experiments are similar to those of pure ar experiments performed at equivalent ar fugacities these observations combined with measured noble gas concentrations in natural glasses suggest that there should be no significant interactions between different dissolved noble gas atoms in natural melts

partition of noble gases between olivine and basalt melt abstract the distribution coefficients ki of noble gases were determined for synthesized olivine basalt melt pairs seven samples bh series were synthesized at 1370 1300 c under a one atmospheric pressure 105 pa noble gas mixture and four samples hpp and hpg series at 1360 1050 c at high pressures 0 2 1 5 gpa the two different experiments gave consistent results with each other some bh series olivine samples were apparently contaminated with gas rich inclusions or the glass phase and the effects of such interferences on the calculated distribution coefficients were evaluated the possible ranges of the distribution coefficients of noble gases thus obtained are khe 0 07 kne 0 006 0 08 kar 0 05 0 15 kkr 0 15 and kxe 0 3 these show remarkable features compared to solid element distribution coefficients the distribution coefficients of noble gases are rather insensitive to or even positively correlated with the atomic size of noble gases the striking contrast between the partition of noble gases and that of solid elements may be attributed to the electrical neutrality of noble gases and the increasing electronic polarizability from he to xe

early neolithic obsidians in sardinia western mediterranean the su carroppu case all the obsidians from the undisturbed early neolithic cardial ware phase i layer of the su carroppu rock shelter sardinia island were studied their elemental composition and that of obsidians from the monte arci sardinia volcanic complex was determined by ion beam analysis pixe a comparison between the composition of su carroppu artefacts analysed non destructively and that of western mediterranean analysed in the same conditions shows that the archaeological material belongs to the sa sb2 and sc monte arci types to the exclusion of the sb1 type the typological technological study of this industry allowed us to reconstruct two chaines operatoires for the production of blades using predominantly sc obsidians and of flakes based exclusively on sa and sb2 obsidians respectively but on the whole assemblage blade bladelet production was performed somewhat preferably with sa and sb2 types thus in the earliest en culture known on the island ancient man had for the making of its obsidian toolkit a highly adaptive behaviour applied to the reduction of different useful obsidian sources

a 3100 year history of argon isotopic and compositional variation at el chichon volcano abstract el chichon volcano has produced at least 11 eruptive events in the past 8000 years all of which display a relatively constant trachyandesitic composition the constancy of the eruptive products attests to the tapping of a long lived magma chamber and suggests a system held in steady state between the influx of basaltic magma and differentiation of resident magma we have sampled eruptive products from eight of these events subjecting their dominant phenocryst phases plagioclase and hornblende to argon isotopic analysis in order to investigate the dynamics of a steady state magmatic system plagioclase from the older eruptions 1500 yr bp 1600 yr bp 2000 yr bp and 3100 yr bp display variable enrichment of 40 ar excess argon whereas hornblende from the same eruptions shows little or no enrichment in contrast both plagioclase and hornblende phenocrysts from the younger eruptions in 1982 550 yr bp 900 yr bp and 1250 yr bp have argon isotopic ratios near atmospheric values isochron analysis of these mineral phases rules out xenocrystic contamination as the source of excess argon while mafic enclaves and isotopic and compositional zoning evidence frequent recharge events suggesting the source of this argon is most likely the same as the source of this basaltic magma correlation with cl points to melt vapor inclusions as the primary host for ar within the phenocrysts argon isotopic variations point to a disturbance of the system between 1500 yr bp and 1250 yr bp while compositional evidence for a major mafic input is present in the 900 yr bp eruption indicating a complex relationship between recharge and isotopic signature of eruptive products the amount of excess argon within a plagioclase phenocryst are a function of variations in melt and vapor inclusion abundance time elapsed between melt vapor inclusion entrapment and eruption variations in ar abundance in melt itself a function of vapor exsolution timing and time variations in ar isotopic composition of the melt subdued behavior of hornblende is due to slower diffusion and minimal inclusions

noble gas state of the ancient mantle as deduced from noble gases in coated diamonds cores and coats of five coated diamonds one from botswana and four from zaire were separately analyzed for their noble gases noble gases in the diamonds are essentially of a trapped origin including radio and nucleogenic components such as4he 40ar 21neexcess and excesses in xe isotopes 129 131 136 the fairly precise elemental and isotopic abundances allow us to infer the noble gas state in the ancient mantle 20ne 22ne ratios are fairly constant 11 8 0 4 and very close to that of sep solar energetic particle ne but distinctly different from the atmospheric ratio 21ne 22ne ratios range from 0 028 to 0 06 which is attributed to nucleogenic 21ne from 18o n 21ne and 24mg n 21ne reactions the difference in 20ne 22ne between atmosphere and mantle can be attributed to the hydrodynamic escape of hydrogen from the primitive atmosphere during the very early stage in the earth s history 38ar 36ar and kr isotopic ratios are identical to the atmospheric values within 1 after correction for 238u or 244pu fission xe the 131 136xe abundance ratios are indistinguishable from atmospheric ratios lighter xe isotopes 124 128xe are also likely to be atmospheric but a final conclusion must wait until better data are obtained in a 136xe 130xe 129xe 130xe diagram diamond data lie on the same line as defined for morb the observed identical correlation for both diamonds and morb s appears to suggest that the progenitor of the excess131 136xe is 244pu but not238u though the direct xe isotopic measurements was not precies enough to decide unanimously the progenitor

noble gas distribution between basalt melt and crystals we studied the partition of ar and kr between synthetized crystals clinopyroxene olivine and a basalt melt clinopyroxene glass pairs were synthetized in a platinum capsule containing air with a piston cylinder type high pressure apparatus at 1 5 gpa at1250 1175 c olivine glass pairs were synthesized at1350 1300 c in an electric furnace with ar flow distribution coefficients noble gas content in crystal noble gas content in melt measured for these crystal melt pairs are0 2 0 5 for clinopyroxene glass for ar and kr and0 08 0 18 for olivine glass for ar

petrogenesis of ultramafic xenoliths from the 1800 kaupulehu flow hualalai volcano hawaii the 1800 kaupulehu flow on hualalai volcano hawaii contains abundant xenoliths of dunite wehrlite and olivine clinopyroxenite with minor gabbro troctolite anorthosite and websterite the petrography and mineral compositions of 41 dunite wehrlite and olivine clinopytoxenite xenoliths have been studied and clinopyroxene separates from eight of these have been analyzed for ba k rb sr rare earth elements 87sr 6sr and 143nd 14 nd temperatures of equilibration obtained by olivine spinel and pyroxene geothermometry range from 1000 to 1200 c mineralogical data combined with published fluid inclusion data indicate depths of origin in the range of 8 30 km the rarity of orthopyroxene the presence of fe rich olivine fog 89 and clinopyroxene fs3 2x and the occurrence of high tio2 in spinel 0 9 2 8 wt and clinopyroxene 0 35 1 33 wt all indicate that the xenoliths are cumulates not residues from partial fusion the separated clinopyroxenes have 87sr 6sr 0 70348 0 70367 and nd nd o 51293 o 51299 values that are different from sr and nd isotope ratios of pacific abyssal basalts 0 5130 respectively also clinopyroxenes and spinels in the xenoliths have generally higher tio2 contents o 35 and 0 91 wt respectively than their counterparts in abyssal cumulates 0 40 and 0 70wt respectively these differences indicate that the xenoliths are not a normal component of oceanic crust because the xenoliths and alkalic to transitional hualalai lavas have similar values for cr cr al

fe ni exchange between olivine and sulphide liquid implications for oxygen barometry in sulphide saturated magmas abstract in order to better understand the behaviour of nickel in magmatic processes we have measured the apparent equilibrium constant k d for the exchange of fe and ni between coexisting olivine and sulphide liquid at controlled oxygen and sulphur fugacities f o 2 10 8 10 10 and f s 2 10 2 10 4 over the temperature range 1200 to 1400 c and with 5 to 50 wt nickel in the sulphide liquid measured values of k d are independent of temperature and sulphur fugacity but increase linearly with the nickel content of the sulphide liquid and follow a power law increase with oxygen fugacity behaviour that is consistent with previous measurements of k d under controlled conditions of f o 2 and f s 2 the variation of k d with melt nickel content and f o 2 is most likely the result of nonideal mixing in the sulphide liquid which results in a decrease in nis fes with melt metal sulphur ratio as a consequence of the systematic dependence of k d on f o 2 a new oxygen barometer is proposed for estimating oxygen fugacity in igneous rocks that were cosaturated in olivine and sulphide liquid application of the experimental results to natural samples shows that the relatively large variations that exist in k d values from different olivine sulphide saturated rock suites can be interpreted as arising from variations in f o 2 and or the nickel content of the sulphide liquid oxygen fugacities calculated for oceanic basalt samples using the proposed fe ni exchange oxybarometer are found to be relatively high 10 8 5 10 10 4 which is in accord with the range of values determined using glass ferric ferrous ratios moreover the very low f o 2 10 14 calculated for the mafic dike from disko island is consistent with the presence of native iron in these samples and is in quantitative agreement with indicators of f o 2 based on chromite and olivine melt partitioning of vanadium consideration of the f o 2 exhibited by olivine sulphide saturated intrusive suites reveals a range from relatively oxidized i e similar to oceanic basalts to values at or below that of disko island field petrographic and geochemical evidence suggests that the production of the low f o 2 suite of samples is probably the result of reduction accompanying the assimilation of carbonaceous country rock

argon solubility in sio2 melt under high pressures a new experimental result using laser heated diamond anvil cell abstract we have measured the solubility of argon in sio2 melt under high pressure by melting a sio2 crystal at the argon pressure as high as 19 gpa using laser heated diamond anvil cell lhdac sio2 was successfully melted at the pressure of about 12 gpa and recovered with a completely glassy phase above 15 gpa the sample contained a large amount of crystalline sio2 although it was completely melted and spherical due to the surface tension this may arise from the change in the coordination number of si in sio2 melt at around 12 15 gpa which is close to the triple point of coesite stishovite and liquid sio2 argon content in the recovered sio2 was measured by energy dispersive spectroscopy and mass spectrometry it increased at pressures up to approximately 5 7 gpa and exhibited a constant value up to at least 12 gpa this implies that at these conditions the noble gas behaves as an incompatible element the result is completely different from those of previous lhdac studies which claimed that the increased solubility then dramatically drops above the pressure of about 5 gpa here we report the new details of experimentally determined pressure dependence of noble gas solubility by lhdac and discuss the experimental difficulties and the structure change of silicate melt under high pressures

helium and argon isotopes in rocks minerals and related ground waters a case study in northern switzerland abstract to address the problems related to the transfer of helium and argon isotopes from rocks into related aquifers the concentrations of u th li and k and helium and argon isotopes were measured in a sedimentary sequence from tertiary to permo carboniferous and in crystalline rocks in northern switzerland in addition to whole rock samples mineral separates have also been investigated the observed concentrations of 3 he 4 he and 40 ar in rocks minerals and groundwaters are compared to calculated values which would result from in situ radiogenic production in a closed system this comparison shows that the rocks and minerals have almost completely lost radiogenic in situ produced helium loss of 3 he and 4 he has been controlled by different retention capacities of the rock forming and accessory minerals and fillings therefore measured 3 he 4 he ratios in these rocks can differ from the calculated production ratios the deviation is generally less than a factor of 3 however in certain chemical sediments e g anhydrites elevated ratios of 3 he 4 he up to 15 10 8 which is 20 times the production ratio were observed it is postulated that 3 h the precursor of 3 he might be chemically bound in such sediments during the 12 3 years half life of 3 h and that the beta decay energy of 3 h is too small to liberate 3 he the calculated closed system concentrations of helium in groundwater exceed the measured ones by up to three orders of magnitude implying that the water rock system has not been a closed system since the time of sedimentation and that movable waters have removed helium to a discharge area and to the atmosphere stagnant old waters however could contain high concentrations of helium and supply it to younger movable waters aquitards with low permeability permian shales appear to be a more important source of helium isotopes than water bearing rocks permian sandstones high abundances of all parent elements in the shales almost complete loss of radiogenic helium and the enhanced production ratio of 3 he 4 he 7 2 10 8 in these rocks similar to the ratio observed in the neighbour permian aquifer 9 4 10 8 suggest the shale as a major source of helium isotopes for this aquifer and diffusion as a principal process for helium transfer from the stagnant porewaters in shales into the movable waters in sandstones some sedimentary rocks e g permian sandstones and shales contain more helium than could have been produced since the time of sedimentation while permian sandstones mainly have retained this excess rare gas component they have released radiogenic helium the difference in the retention of these two components most probably demonstrates that migration of radiogenic helium from damage tracks along crystal imperfections to grain boundaries is an important process controlling its loss the abundances of excess 3 he and 4 he in rocks cannot be predicted by in situ production calculations studies of rocks and minerals are required the average ratio of 40 ar 36 ar 385 in groundwaters collected from permian sediments from the weiach borehole is much higher than that in the aquifers from the crystalline basement below the sediments radiogenic 40 ar in permian sediments is mainly liberated by diagenetic water rock interactions the low contribution of radiogenic 40 ar in deep aquifers of the crystalline basement and the fact that chlorine concentrations in groundwaters from the basement are also much lower than in the overlaying permian aquifer eliminates a hypothetical upwards fluid flux from this study we envisage intrabasin sources for helium and argon isotopes in groundwaters with mainly lateral migration of waters through sedimentary layers at different rates and diffusion of helium from slower velocity older waters in more stagnant zones shales into flow passes with highervelocity younger waters in sections with higher permeability sandstones an external source for noble gases is not required in such a model

helium diffusion and u th he thermochronometry of titanite we have explored the diffusivity characteristics of radiogenic he in titanite sphene and have developed analytical techniques for u th he dating of this mineral results of incremental he outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44 6 3 4 2 kcal mol and a frequency factor of 60 cm 2 s diffusivity is highly linearly correlated with the inverse square of the grain size indicating that the he diffusion domain in titanite is the crystal itself for typical titanite grains of 200 to 800 m minimum dimension the he closure temperature is in the range 191 to 218 c for a cooling rate of 10 c myr there is no indication in the titanites we studied that radiation damage plays a major role in he diffusion u th he ages of titanites from quickly cooled rocks yield ages with 5 8 1 reproducibility that are concordant with known ages and u th he ages of titanites from slowly cooled rocks are consistent with independently established cooling paths supporting the closure temperature estimates these experiments suggest that titanite u th he ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40 ar 39 ar dating but higher than apatite fission track or u th he dating

multiple origins of bondi cave and ortvale klde nw georgia obsidians and human mobility in transcaucasia during the middle and upper palaeolithic abstract using pixe four types of elemental compositions were found among obsidian artefacts from the bondi cave and ortvale klde middle to upper palaeolithic sites in nw georgia one of those types corresponds to obsidians from the chikiani source whose compositions were determined with a very good agreement by pixe and icp aes ms the composition of chikiani obsidians is remarkably constant despite k ar and 39ar 40ar extrusion ages from ca 2 4 and 2 8 ma the compositions of two other groups of obsidian artefacts are similar to source materials from eastern anatolia and armenia in particular ikisdere sarikamis gutansar and hatis obsidian is only a minority component in the lithic assemblages at the bondi cave and ortvale klde both neanderthal and modern human populations used obsidian in particular from chikiani considering that the shortest walking distance to this nearest source is at minimum of about 180 km and to other potential sources of more than 350 km it is suggested that this material reached these two sites mostly if not exclusively by a series of down the line exchanges

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

he diffusion in olivine helium diffusion in olivine dunite xenolith has been measured in the temperature range 1180 1460 c a linear arrhenius function was obtained with an activation energy of 120 27 32 kcal mole and a pre exponential factor d0 2 2 108 cm2 s diffusion mechanisms are not a viable means of degassing he from the mantle olivine phenocrysts can be expected to retain previously trapped he during cooling in extrusive basalts provided the flow units are thinner than 50 m xenoliths will retain mantle he signatures only if magma transport times are less than 50 years or if the he fugacity in the magma is high enough to prevent xenolith degassing the lower oceanic crust is probably substantially degassed of he trapped he will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 1014 c m y glass at several centimeters depth in a basalt flow near the spherulite zone will have cooling rates lower than this so he loss may be significant in many basalt glass samples

sem eds characterization of western mediterranean obsidians and the neolithic site of a fuata corsica during the neolithic obsidians of the monte arci sardinia volcanic complex were by far more used in the northern tyrrhenian area than those of the three other source islands lipari palmarola pantelleria in the western mediterranean it is shown that merely determinations of content for six major elements with a scanning electron microscope by energy dispersion spectrometry sem eds are sufficient to distinguish the four types of monte arci obsidians because of the compositional similarities between these obsidian types a multivariate analysis is recommended in provenance studies although sem eds electron microprobe wavelength dispersion spectrometry emp wds and particle induced x ray emission pixe give essentially concordant results in the determination of these six element contents subtle technique related biases prevent the combination of sem eds emp wds and pixe data on source samples for provenance purposes an sem eds test study reveals the first occurrence of obsidians of lipari for the a fuata middle to late neolithic site of nw corsica north of sardinia in addition to the usual monte arci obsidians similar to emp wds the sem eds technique requires only millimeter sized fragments

mass spectrometric identification of cosmic ray produced neon in terrestrial rocks with multiple neon components studies of cosmic ray produced nuclides in terrestrial rocks are expected to provide important geomorphological and glaciological information such as surface exposure ages erosion rates and extent of glacial cover in the case of the stable nuclide21ne the cosmic ray produced component may represent but one of several components and a component resolution based on three isotope correlations is essential we developed analytical techniques which permit accurate corrections for all interfering ions we show for our modified nier source that corrections for doubly charged species based on constant charge state ratios are not satisfactory the improved techniques permit isotopic studies of 106 atoms of21ne the spallation ratio 22ne 21ne c from si was calibrated using a quartz separate from an allan hills sandstone and yields a value of1 243 0 022 this ratio is expected to be essentially constant for terrestrial quartz samples and should be useful in component resolutions the stepwise release of ne at increasing temperatures shows that cosmic ray produced ne is released from quartz at rather low temperatures this permits an estimate of the activation energy for ne diffusion in quartz 90 10 kj mol

laboratory shock emplacement of noble gases nitrogen and carbon dioxide into basalt and implications for trapped gases in shergottite eeta 79001 abstract basalt samples have been analyzed mass spectrometrically for shock implanted noble gases n2 and co2 after exposure to 20 60 gpa 200 600 kbar shock in the presence of 0045 3 0 atm of ambient gas isotopically labelled gases were used in some samples to distinguish implanted atmospheric gases from contaminant or indigenous gases abundances of emplaced gases varied linearly with ambient gas pressure for constant shock pressure and gas emplacement was most efficient in the range of 35 50 gpa shock pressure uncrushed samples shocked in this range gave emplacement efficiencies number densities of gases in shocked samples number densities in ambient gas of 2 0 6 7 for heavy noble gases and nitrogen while powdered samples with higher effective porosity yielded 40 50 efficiencies indicating that 50 of noble gases and nitrogen available in porespaces was emplaced no elemental or isotopic fractionation was detected with ar kr xe or n2 helium and in some samples ne were lost by diffusion subsequent to shock emplacement efficiencies for co2 averaged a factor of 1 8 0 2 greater than those of n2 and noble gases in 4 samples with 20 50 gpa shock and yielded 3 2 times greater efficiency in a sample shocked to 60 gpa enhanced co2 emplacement is thought to be due to reaction with silicate materials data from shergottite eeta 79001 glass may be consistent with co2 efficiencies a factor of 2 3 greater than noble gas efficiencies trapped gases in eeta 79001 glass were probably emplaced by shock however apparent emplacement efficiencies are somewhat higher than even shocked powder samples possible explanations for the difference include atmospheric overpressure at the time of shock trapping of gas already in vugs by intruding melt material or collapse of gas filled vugs to form gas laden glass inclusions not enough is yet known about the behavior of vugs under shock to discount the latter possibility

experimental determination of crystal melt partitioning of ga and ge in the system forsterite anorthite diopside the crystal liquid partitioning of ga and ge has been measured experimentally between forsterite diopside anorthite and spinel and melts in the pseudoternary system forsterite anorthite diopside at one atmosphere pressure and 1300 c gallium is incompatible with forsterite and diopside d ga 0 024 and 0 19 respectively is only slightly incompatible in anorthite d ga 0 86 and is highly compatible in spinel d ga 4 6 the partition coefficient for ge is within a factor of two of unity for forsterite diopside and anorthite d ge 0 62 1 4 and 0 51 respectively but ge is incompatible in spinel d ge 0 1 the coefficients for the exchange of ga and al and the exchange of ge and si between minerals and melts generally are within a factor of two of unity as is expected from the geochemical coherence of these element pairs in natural samples the application of our results to the interpretation of natural basaltic and mantle samples from the earth and basalts from the moon and the shergottite parent body demonstrates that it is possible to discriminate between different mantle source compositions using ga al and ge si ratios the ge variation among lunar mare basalts may be indicative of a heterogeneous lunar mantle the substantial depletion of ge in chassigny relative to the other snc meteorites may be evidence of either a heterogeneous shergottite parent body spb mantle or of different geochemical behavior for ge in the spb