a high precision 40ar 39ar age for hydrated impact glass from the dellen impact sweden the dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic however if the maximum potential of the terrestrial impact crater record is to be realized accurate and precise ages for crater forming events are critical here we report a high precision 40ar 39ar age for the dellen impact structure sweden previous radio isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach a detailed petrographical and mineralogical study provides a solid foundation for interpretation of step heating 40ar 39ar data culminating in a statistically robust age of 140 82 0 51 ma 2 full external precision for the dellen impact event for which data disfavour an inherited argon component primary hydration of the impact melt during cooling quenching and entrapment of molecular water promoted rapid loss of inherited 40ar from the impact melt of rhyolitic composition duplicate analyses of the water content and d of the glass give similar values for the former 1 9 0 1 mol mg 1 but unexpectedly low values for the latter 159 8 with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity this study highlights that the 40ar 39ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events less

late cretaceous cenozoic tectonic sedimentary evolution and u enrichment in the southern songliao basin abstract late cretaceous cenozoic tectonic sedimentary evolution is fundamental to u uranium migration and u enrichment in the southern songliao basin northeast china to establish the genetic connection between tectonic sedimentary evolution and u enrichment this paper presents a mineralogical study apatite fission track aft analysis and sem cl analysis of quartz grains two stages of mineralization were proposed including the u pre enrichment stage during the late cretaceous and the ore forming stage during the cenozoic based on the density plots of the aft ages paleoclimate studies and sem cl analysis of quartz grains it is inferred that the u pre enrichment was closely related to the rapid uplift of the great xing an range and the zhangguangcai range and the o2 rich atmosphere during 97 90 ma in the southern songliao basin the o2 rich atmosphere increased u mobility and led to the formation of red beds during the late cretaceous which widely developed u pre enrichment in the basins of northeast china the mineralogical study reveals that the qianjiadian uranium deposit developed in two stages the diagenetic process experienced the transition from early alkaline to late acidic conditions to promote the stage i mineralization u pre enrichment the dominant occurrence spaces for u were the dissolution pores of quartz tightly packed with calcite cement u enrichment during alkaline ambient conditions was mainly via adsorption with the increase of the buried depth the early alkaline diagenesis returned to acidic conditions due to the abundant organic material u vi was reduced to pitchblende due to the reducing capacity of the organic matter om stage ii ore forming stage mineralization was strictly related to the hydrocarbon re concentration according to the thermal history study of the cc1 and q2 boreholes the southern songliao basin experienced four episodes of rapid uplift during the cenozoic at 83 80 ma 68 52 ma 33 23 ma and 8 ma respectively the cenozoic uplifts led to hydrocarbon migration at 65 ma 23 ma and present the re concentration of hydrocarbon prompted the u mineralization in the qianjiadian uranium deposit in the southern songliao basin during the stable period of tectonic movements the reducing conditions caused by hydrocarbons promoted u mineralization the dominant spaces of coffinite were intergranular dissolved pores formed during the transformation from the feldspar to the kaolinite under acidic conditions the post rift phase and structural inversion phase directly controlled the u pre enrichment stage and the ore forming stage respectively overall the tectonic sedimentary evolution played an important role in controlling the u leaching from granites the dominant occurrence spaces and u recycling

the earthquake deformation cycle at the nankai trough southwest japan geodetic measurements made landward of the nankai trough site of great subduction zone thrust earthquakes in 1854 and 1946 provide a uniquely detailed picture of the strain buildup process supply constraints on the mechanism of strain accumulation and allow for improved estimates of earthquake recurrence provided the two most recent movement cycles are similar the observations dating from about 1890 may be used to reconstruct a single complete deformation cycle coseismic strain release postseismic transients interseismic strain accumulation very complete leveling and tidal gage data indicate that postseismic deformation extends more than 300 km inland from the plate boundary persists for at least 30 years and shows a clear tendency to become longer wavelength with increasing time the transient movements have two timescales the first of about a year or less corresponds to deformation largely uplift concentrated close to the coseismic fault and is most easily explained by aseismic slip or very localized deformation downdip of the earthquake rupture plane the second longer timescale is associated with a diffusion like spread of the deformation further landward an effect qualitatively similar to that first predicted by elsasser to be an expected consequence of faulting in an elastic plate overlying a viscoelastic asthenosphere cumulative uplift since 1890 correlates well with the distribution of uplifted marine terraces although average post 1890 tilt rates exceed late quaternary and holocene averages by at least a factor of three because of the nonlinearity of strain buildup and the significant permanent deformation simple recurrence calculations typically overestimate the true interval between great earthquakes by a factor of 2 to a factor of 3 strict application of the time predictable model assumed correct overcomes these difficulties provided the cumulative transient deformation and the proportion of permanent deformation per cycle can be estimated

estimates of ar diffusion and solubility in leucite and nepheline electron microprobe imaging of ar distribution in a mineral leucite is unique among minerals that have been exploited for k ar and ar ar dating in that it exhibits a major phase change at 645 665 degrees c across which ar diffusion and solubility can be studied in addition well developed twinning occurs only in the low temperature form offering further opportunity to examine the effects of crystallographic change upon ar diffusion nepheline was studied to compare the effect of its i dimensional c axis oriented lattice channels with the 3 dimensional channels of leucite the amounts of ar introduced into leucite were far higher than those observed in feldspars allowing its analysis by electron microprobe spot traverses and x ray maps this has provided the first high spatial resolution 2 dimensional study of ar distribution in a k bearing mineral extreme changes in ar solubility were observed across the leucite phase transition with ar solubilities jumping from similar to 70 ppm at 1 kbar in the tetragonal low temperature form to similar to 750 ppm at i kbar in the cubic high temperature form argon penetration profiles were complex and estimates of the diffusion rates show that they are more rapid than in k feldspar in addition spikes of high ar concentration were observed in both forms suggesting extended defects or micro inclusions formed argon traps in the structure no concentration changes could be correlated to leucite twin planes suggesting that twin planes did not act as fast pathways for ar movement nepheline yielded a much lower ar solubility of 0 15 0 31 ppm at 1 kbar

textural characterization major and volatile element quantification and ar ar systematics of spherulites in the rocche rosse obsidian flow lipari aeolian islands a temperature continuum growth model spherulitic textures in the rocche rosse obsidian flow lipari aeolian islands italy have been characterized through petrographic crystal size distribution csd and in situ major and volatile elemental analyses to assess the mode temperature and timescales of spherulite formation bulk glass chemistry and spherulite chemistry analyzed along transects across the spherulite growth front glass boundary reveal major oxide and volatile h2o co2 f cl and s chemical variations and heterogeneities at a 5 m scale numerous bulk volatile data in non vesicular glass spatially removed from spherulitic textures reveal homogenous distributions of volatile concentrations h2o 0 089 0 012 wt f 950 40 ppm and cl 4 100 330 ppm with co2 and s consistently below detection limits suggesting either complete degassing of these volatiles or an originally volatile poor melt volatile concentrations across the spherulite boundary and within the spherulitic textures are highly variable these observations are consistent with diffusive expulsion of volatiles into melt leaving a volatile poor rim advancing ahead of anhydrous crystallite growth which is envisaged to have had a pronounced effect on spherulite crystallization dynamics argon concentrations dissolved in the glass and spherulites differ by a factor of 20 with ar sequestered preferentially in the glass phase petrographic observation csd analysis volatile and ar data as well as diffusion modeling support continuous spherulite nucleation and growth starting at magmatic emplacement temperatures of 790 825 c and progressing through the glass transition temperature range tg 750 620 c being further modified in the solid state we propose that nucleation and growth rate are isothermally constant but vary between differing stages of spherulite growth with continued cooling from magmatic temperatures such that there is an evolution from a high to a low rate of crystallization and low to high crystal nucleation based on the diffusion of h2o across these temperature ranges 800 300 c timescales of spherulite crystallization occur on a timescale of 4 days with further modification up to 400 years growth is prohibitively slow 400 c and would become diffusion reliant selective deformation of spherulites supports a down temperature continuum of spherulite formation in the rocche rosse obsidian indeed petrographic evidence suggests that high strain zones may have catalyzed progressive nucleation and growth of further generations of spherulites during syn and post emplacement cooling

ar diffusion in hydrous silicic melts implications for volatile diffusion mechanisms and fractionation the effect of dissolved water on the diffusivity of ar in glasses and melts of rhyolitic and albitic compositions was investigated experimentally at pressures up to 1500 mpa and water contents of 0 1 5 wt the data for water poor rhyolitic composition at 500 mpa can be described in the whole temperature range of 480 1102 c by a simple arrhenius relationship dar 2 14u10 36 m 2 s exp 318 883 t a 4 0 wt increase in water content increases the ar diffusivity by approximately one order of magnitude in both rhyolitic and albitic melts at 1000 c in contrast to viscosity and total water diffusion an exponential dependence of ar diffusivity on water content was observed for the rhyolitic composition in the whole range of water contents for water poor rhyolite ar diffusivity depends on pressure with an apparent activation volume of 13 15 cm 3 mol at pressures up to 800 mpa for water rich rhyolite v5 wt water there is no significant pressure effect at 1000 c in the range 500 1500 mpa combining our data with previous data from carroll m r carroll earth planet sci lett 103 1991 156 168 ar diffusivity in 10 312 m 2 s in rhyolitic melts can be expressed as

diffusion of ar in rhyolite orthoclase and albite composition glasses the diffusivity of ar has been determined in a natural rhyolite obsidian rhy and in glasses of naalsi3o8 ab and kalsi3o8 or composition at temperatures between 400 and 900 c at pressures from 1150 to 3700 bar the observed variations in the ar diffusion coefficient d with temperature can be described by the arrhenius relationshipd d0exp e rt with the following results for pressures from 1100 to 2500 bar d in cm2 s r in cal mole ab d 1 62 10 3 exp 32780rt or d 6 64 10 3 exp 29000rt rhy d 6 86 10 3 exp 34490rt the data although of limited extent also indicate a slight decrease ind with increasing pressure and the calculated activation volume of 36 8 cm3 mole for ar in rhy at 800 c agrees with the idea that the volume change associated with a diffusive jump is similar to the size of the diffusing species comparison of the ar results with cation tracer diffusivities indicates that ar diffusivity is similar to that of rubidium in rhy and ab argon diffusivity is several orders of magnitude slower than h2o att 900 c and exsolution of h2o to form bubbles might leave a residual rhyolitic melt enriched in ar and kr xe due to kinetic effects noble gases show lower activation energies for diffusion than are observed for tracer diffusion of cations with comparable radii a systematic increase in activation energy for diffusion with increasing size of the noble gas atoms and similarities in activation energies in rhyolitic through basaltic melts suggests that in basaltic melts he ne and probably ar can diffuse faster than co2 the major volatile component in basalts in contrast xe and possibly kr due to their estimated high activation energies may diffuse more slowly than co2 at temperatures below 1200 c this implies that kinetic factors may influence noble gas abundance patterns during separation of a co2 rich vapor phase from degasing basaltic magma during ascent

sulfur diffusion in rhyolite melts diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800 1100 c water contents of 0 7 3 wt and oxygen fugacities from the quartz fayalite magnetite buffer to air experiments involved dissolution of anhydrite or pyrrhotite into rhyolite melt over time scales of hours to days electron microprobe analysis was used to measure sulfur concentration profiles in the quenched glasses regression of the diffusion data in dry rhyolite melt gives dsulfur 0 05 exp 221 80rt which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as h2o co2 and cl diffusion of sulfur in melt with 7 wt dissolved water is 1 5 to 2 orders of magnitude faster than diffusion in the anhydrous melt depending on temperature sulfur is known to dissolve in silicate melts as at least two different species s2 and s6 the proportions of which vary with oxygen fugacity despite this oxygen fugacity does not appear to affect sulfur diffusivity except under extremely oxidizing conditions this result suggests that diffusion of sulfur is controlled by one species over a large range in oxygen fugacity the most likely candidate for the diffusing species is the sulfide ion s2 re equilibration between s2 and s6 in oxidized melts must generally be slow compared to s2 diffusion in order to explain the observed results in a silicic melt undergoing degassing sulfur will tend to be fractionated from other volatile species which diffuse more rapidly this is consistent with analyses of tephra from the 1991 eruption of mount pinatubo philippines and from other high silica volcanic eruptions

an 40ar 39ar geochronology on a mid eocene igneous event on the barton and weaver peninsulas implications for the dynamic setting of the antarctic peninsula the genesis of basaltic to andesitic lavas mafic dikes and granitoid plutons composing the subaerial cover on the barton and weaver peninsulas antarctica is related to arc formation and subduction processes precise dating of these polar rocks using conventional 40ar 39ar techniques is compromised by the high degree of alteration with loss on ignition as high as 8 in order to minimize the alteration effects we have followed a sample preparation process that includes repeated acid leaching acetone washing and hand picking followed by an overnight bake at 250 c after this procedure groundmass samples can yield accurate age plateaus consisting of 70 100 of the total 39ark released using high resolution heating schedules the different rock types studied on the barton and weaver peninsulas yielded almost coeval ages suggesting a giant igneous event in the weaver and barton peninsulas at 44 5 ma a compilation of newly published ages indicate that this event took place throughout the whole south shetland islands suggesting a dynamic incident occurred at this stage during the arc evolution history we related this igneous event to a mantle delamination mechanism during eocene times the delamination process began at 52 ma and the resultant upwelling of asthenosphere baffled the subduction of phoenix plate causing an abrupt decrease in convergence rate then multiple magmatic sources were triggered resulting in a culminating igneous activity during 50 40 ma with a peak at 45 ma along the archipelago the delamination also caused the extension regime indicated by the dike swarm plugs and sills all over the archipelago and the uplift of smith metamorphic complex and livingston island delamination process may have finished at some time during 40 30 ma leaving a weak igneous activity at that stage and thereafter the convergence rate then recovered gradually as indicated by the magnetic anomaly identifications this model is supported by seismic observation of deep velocity anomalies beneath the antarctic peninsula

diagenesis and provenance of silurian quartz arenites in south eastern new york state abstract combined scanning electron microscopy cathodoluminescence and optical microscopy sem cl om of single quartz grains was used to determine the diagenetic history and provenance of framework grains in quartz arenites of the silurian shawangunk conglomerate and binnewater sandstone in south eastern new york state most sand sized sedimentary rocks of these two units experienced compaction and quartz cementation after deposition except for those with high matrix or pseudomatrix content no primary porosity is left today and secondary porosity is insignificant leaving the shawangunk conglomerate and binnewater sandstone with poor reservoir qualities the medium to coarse sand fraction of the shawangunk conglomerate and binnewater sandstone is dominated by quartz of plutonic origin of these usually sub rounded to rounded grains 10 15 carry remnants of inherited quartz cement indicating recycling from a sedimentary source such as quartz rich sedimentary rocks of the former iapetus passive margin in contrast polycrystalline vein quartz pebbles in the shawangunk conglomerate were most likely derived from a proximal crystalline source in the taconic orogen to the east formation of the shawangunk conglomerate and binnewater sandstone quartz arenites was likely supported by subtropical climatic conditions given the latitudinal position of eastern north america during the silurian with relatively high mean annual temperatures and high precipitation rates which accelerated the breakdown of unstable grains

sea level change and paleogeographic reconstructions southern vancouver island british columbia canada abstract forty eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern vancouver island british columbia sea level fell rapidly from its high stand of about 75 m elevation just before 14 000 cal bp 12 000 radiocarbon yrs bp to below the present shoreline by 13 200 cal bp 11 400 radiocarbon years bp the sea fell below its present level 1000 years later in the central strait of georgia and 2000 years later in the northern strait of georgia reflecting regional differences in ice sheet retreat and downwasting direct observations only constrain the low stand to be below 11 m and above 40 m analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the cascadia subduction zone however places the low stand at 30 5 m at about 11 200 cal bp 9800 bp the inferred low stand for southern vancouver island when compared to the sea level curve previously derived for the central strait of georgia to the northwest generates differential isostatic depression that is consistent with the expected crustal response between the two regions morphologic and sub bottom features previously interpreted to indicate a low stand of 50 to 65 m are re evaluated and found to be consistent with a low stand of 30 5 m submarine banks in eastern juan de fuca strait were emergent at the time of the low stand but marine passages persisted between southern vancouver island and the mainland the crustal uplift presently occurring in response to the late pleistocene collapse of the southwestern sector of the cordilleran ice sheet amounts to about 0 1 mm yr the small glacial isostatic adjustment rate is a consequence of low viscosity mantle in this tectonically active region

crystallization rates of shock melts in three martian basalts experimental simulation with implications for meteoroid dimensions abstract dynamic crystallization experiments have been performed on synthetic glasses representative of shock generated melts observed in los angeles sayh al uhaymir 150 and dar al gani 476 martian basalts on the basis of qualitative texture and quantitative fractal analysis results we show that melt pockets in los angeles cooled at a rate of 1040 1560 c h sayh al uhaymir 150 and dar al gani 476 melt pockets cooled at 780 c h conductive cooling models for a range of meteoroid diameters 10 50 cm indicate that the minimum meteoroid diameter was small on the order of 10 15 cm and that melt pockets cooled from post shock temperatures within minutes our results also have bearing on shock implanted martian atmospheric components because it is during cooling that the melt pockets have the potential to lose gases modeling of argon diffusion in a spherical melt pocket indicates that during cooling and quench crystallization 4 60 of trapped martian atmospheric argon may be lost from the melt pocket through diffusive transport

viscosity of the asthenosphere from glacial isostatic adjustment and subduction dynamics at the northern cascadia subduction zone british columbia canada 1 late glacial sea level curves located in the cascadia subduction zone csz fore arc in southwestern british columbia show that glacial isostatic adjustment gia was rapid when the cordilleran ice sheet collapsed in the late pleistocene gia modeling with a linear maxwell rheology indicates that the observations can be equally well fit across a wide range of asthenospheric thicknesses provided that the asthenospheric viscosity is varied from 3 1018 pa s for a thin 140 km asthenosphere to 4 1019 pa s for a thick 380 km asthenosphere present day vertical crustal motion predicted by the gia models shows rates of a few tenths of a millimeter per year consistent with previous analyses the model viscosities largely pertain to the viscosity of the oceanic mantle beneath the subducting juan de fuca slab but include a contribution from the mantle wedge above the slab for comparison effective viscosities for the upper mantle due to the tectonic regime subduction were computed using the strain rates and temperatures of an independent geodynamic model of the csz with a wet olivine power law rheology the effective viscosities agree well with gia model viscosities of 1019 pa s or less corresponding to an asthenosphere of 100 or 200 km thickness the agreement suggests a significant role for power law flow in the gia response regardless of the microphysical mechanisms responsible for the gia response the viscosity values inferred from gia can be applied to studies of the megathrust earthquake cycle because both processes take place on comparable time scales

recognition and significance of inherited quartz overgrowths in quartz arenites abstract inherited quartz overgrowths represent remnants of cement adhering to a grain s surface from a previous sedimentary cycle and can be recognized in petrographic studies on the basis of textural criteria they may occur as rounded double or penetrating overgrowths since these features occupy only a part of a grain s surface and are randomly positioned when studied in two dimensional sections the likelihood of their being transected and observed is significantly less than their whole rock abundance this has resulted in an underestimation of their importance as a provenance indicator probability that a random cut will transect a cap of inherited cement on any particular grain depends upon proportion of the grain s surface covered by the cap position of the cap on the grain s surfac and orientation of the cut through the grain an experiment was devised to analyze the relationship between size of a grain coating cap and likelihood that it will be transected in a random cut this involved using an eccentric stereonet one in which the origin is not at the projection sphere s center to project random planes results of the experiment were plotted as a curve and then derived mathematically since size of a coating cap is related to its projection angle measured from the sphere s center a second experiment was devised to determine the relationship between projection angles and equivalent apparent projection angles seen when a cut does not pass through a sphere s center it was found that the true projection angle equals the mode of apparent projection angles relati nships determined in these two experiments enable calculation of the whole rock proportion of grains with inherited overgrowths from petrographic data although at the present time results must be considered as estimates this capability leads to a more precise understanding of importance of inherited overgrowths as an indicator of sedimentary provenance the procedure was applied to samples of upper precambrian quartz arenites from the uinta mountain group the grand canyon supergroup and the belt supergroup resulting in calculated whole rock proportions of grains with inherited overgrowths that are in general twice the values observed in thin section this procedure may also be useful in a variety of other applications including studies of sediment distribution intrabasinal sedim nt maturation lithostratigraphic correlation and rates of grain surface abrasion

a new view of the he ar co2 degassing at mid ocean ridges homogeneous composition of magmas from the upper mantle abstract deep sea exploration is rapidly improving our understanding of volatiles geochemistry in mid ocean ridge igneous products it is also placing greater constraints on degassing processes of the earth s mantle with the result that degassing models based on vapour melt equilibrium are no longer able to explain the increasing number of data in fact such models force to postulate an upper mantle strongly heterogeneous at any scale and cannot account for the widespread carbon supersaturation of the recovered igneous products here we review the global he ar co 2 dataset of fluid inclusions in mid ocean ridge glasses using the framework of advanced modelling of multicomponent bubble growth in magmas we display that non equilibrium fractionations among he ar and co 2 driven by their different diffusivities in silicate melts are common in most of the natural conditions of magma decompression and their signature strongly depends on pressure of degassing due to the comparable ar and co 2 diffusivity magma degassing at low pressure fractionates both the he ar and he co 2 ratio by a similar extent while the slower co 2 diffusion at high pressure causes early kinetic effects on ar co 2 ratio and dramatically changes the degassing path on this ground the very different geochemical signatures among suites of data coming from different ridge segments mainly depend on the depth of the magma chamber where the melt was stored besides the variations inside a single suite highlight variable ascent speed and cooling rate of the emplaced lava the large variations in both the he co 2 and ar co 2 ratios at almost constant he ar displayed in glasses coming from the mid atlantic ridge 24 30 n segment and the rodriguez triple junction are therefore interpreted as a high pressure signature in contrast the simultaneous increase in both he co 2 and he ar of the east pacific rise pito seamount and south east indian ridge data sets suggests the dominance of low pressure fractionation implying that the shallow magma chambers are at a lower depth than those of the mid atlantic ridge 24 30 n and rodriguez triple junction our conclusions support the presence of a relationship between spreading rate and depth of high temperature zones below ridges and are consistent with the depth of magma chambers as suggested from seismic studies non equilibrium degassing explains the volatile systematics of mid ocean ridge basalts by starting from a single mantle derived magma dispensing with the supposed need for heterogeneities in abundance ratios of volatiles in the mantle below oceanic ridges

a laser probe 40ar 39ar investigation of poikilitic shergottite nwa 4797 implications for the timing of shock metamorphism spatially resolved argon isotope measurements have been performed on neutron irradiated samples of nw africa nwa 4797 shock heating of nwa 4797 completely melted and vesiculated precursor igneous plagioclase which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition ca k ratios using a focused ultraviolet laser beam is has been possible to distinguish between argon isotopic signatures from groundmass minerals igneous olivine pyroxene plagioclase and a shock vein this study focuses on the potential for this meteorite to shed light on shock ages of shergottites apparent 40 ar 39 ar ages of groundmass minerals show that there are large amounts of excess argon in this phase yielding a wide range of calculated ages from 690 30 ma to several apparent ages older than 4 5 ga a traverse of laser probe extractions across the 1 mm diameter shock vein in nwa 4797 yielded apparent 40 ar 39 ar ages younger than the groundmass a signature of the martian atmosphere identified by 40 ar 36 ar ratios of 1600 1900 was not found in the nwa 4797 shock vein this is distinct from other shergottites where the products of shock melting contain a nearly pure sample of martian atmosphere we attribute this to a distinct formation mechanism and hence gas trapping mechanism of the nwa 4797 shock vein we undertook 44 analyses of plagioclase areas identified by sem analysis ages ranged from 45 27 to 3771 109 ma and yield an average age of 375 77 ma considerably younger than ages obtained in this study from either the groundmass or the shock vein a plot of age v 37 ar 39 ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured older ages are correlated with higher ca k ratios of plagioclase indicating contamination from groundmass minerals rich in excess argon the youngest ages correlate to plagioclase extractions with the lowest ca k ratios interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a k rich mesostasis we see no evidence for multiple shock events in nwa 4797 rather we favour the interpretation that the cosmic ray exposure cre age of 3 0 0 5 ma obtained on nwa 4797 in this study using cosmogenic 38 ar approximates the timing of shock melting in this meteorite

recycled detrital quartz grains are sedimentary rock fragments indicating unconformities examples from the chhattisgarh supergroup bastar craton india abstract recycled detrital quartz grains are unequivocally identified if there are overgrowths on abraded overgrowths we argue that the presence of such recycled grains even in small quantities in successions of quartz arenites indicates derivation from one or more sedimentary units from below an intrabasinal unconformity or an extrabasinal unroofing surface as do detritus from characteristic rock fragments in polymict clastic rocks in fact detrital grains with inherited overgrowths are sedimentary rock fragments we apply this petrographic criterion to evaluate the presumed unconformities that bound the singhora chandarpur raipur and the kharsiya groups of the essentially mesoproterozoic chhattisgarh supergroup in the bastar craton a critical ur craton vis a vis columbia and rodinia optical examination of about 6000 grains in 12 thin sections reveal few recycled quartz grains in sandstone samples of the chandarpur group but about 4 recycled quartz grains with inherited abraded overgrowths in a sandstone unit of the raipur group characteristic rock fragments from a tuffaceous unit at the top of the raipur group are found in the basal sandstone of the kharsyia group the data indicate that the presumed unconformities below the kharsyia group and the raipur group are real but the one above the singhora group cannot be confirmed on the basis of recycled grains published age spectra of detrital zircons from these sandstones are compatible with the above inferences drawn from detrital quartz and rock fragment petrography

diffusion and redox reactions of sulfur in silicate melts although large scale transport in magmas is usually controlled by convection diffusion of sulfur in silicate melts nevertheless plays a crucial role in the kinetics of various magmatic processes the most important process is probably the degassing of magmas and melts initiated by an oversaturation of the melt with respect to dissolved volatiles such oversaturation can be caused in nature i e by decompression when a magma ascends to the earth surface but the degassing of melts is also of major interest for technical applications e g during fining of melts in industrial glass production the kinetics of volcanic eruptions and the fining of glass melts are basically controlled by the dynamics of bubble nucleation and growth and these properties are strongly affected by the diffusion of volatiles from the melt into bubbles nemec 1980a b sparks et al 1994 muller simon 2011 this volume hence understanding the diffusion of volatiles in melts provides a necessary tool for modeling bubble nucleation and growth however the diffusivities of volatiles such as h2o co2 so2 and h2s in the melt do not only affect the degassing rate they may also cause a fractionation of volatiles between gas phase and melt as well due to different diffusivities for instance in hydrous magmas the diffusion of h2o is usually much faster than co2 and sulfur diffusion baker et al 2005 zhang and ni 2010 so rapid disequilibrium removal of bubbles from the melt may cause an artificial enrichment of some components in the bubbles other processes which may be governed by sulfur diffusion in the melts are the dissolution and precipitation of minerals in glass manufacturing crucial problems are the kinetics of dissolution of the raw materials in the melt batch and of

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

argon diffusion in apollo 16 impact glass spherules implications for 40ar 39ar dating of lunar impact events abstract the 40 ar 39 ar technique applied to impact glass has been used to date both terrestrial and lunar impact events the ability to utilize the 40 ar 39 ar technique rests on the assumption that impact glasses are closed to the loss of daughter product 40 ar after formation diffusion experiments were performed on three apollo 16 lunar impact glasses and yielded activation energies for 39 ar of 17 to 20 kcal mol 1 and log 10 d 0 a 2 values of 5 2 to 6 0 s 1 the resulting diffusion coefficients are interpreted as minimum values and the apollo 16 glass is probably some of the least retentive of lunar glasses as the degree of non bridging oxygen is at one end of the range in lunar glasses at temperatures below the glass transition temperature i e 660 c the data can be explained by volume diffusion from a single diffusion domain modeling shows that apollo 16 composition glass could lose significant quantities of radiogenic argon 40 ar 90 100 over 20 40 myr assuming a diffusion domain size a of 75 m due to diurnal temperature variations on the lunar surface although 40 ar loss is highly sensitive to exposure duration and effective diffusion domain size modeling shows that loss from transient thermal events e g heating to 200 c for 10 2 yr duration can also cause partial resetting of apparent 40 ar 39 ar ages in small a 75 m glasses a maximum of 50 60 of 40 ar is lost over 4 ga when buried to depths corresponding to temperatures of 15 c results indicate that caution should be exercised in interpreting lunar impact glass 40 ar 39 ar ages as the assumption of closed system behavior may have been violated particularly in glasses with low fractions of non bridging oxygen

provenance analysis by single quartz grain sem cl optical microscopy abstract the integration of panchromatic scanning electron microscopy cathodoluminescence sem cl with optical microscopy analysis on single quartz grains is a new technique to interpret provenance of quartz rich sediments the combination of information gained from sem cl on textural features and cl response with information from optical microscopy allows distinction of different quartz types much more easily then with conventional microscopy or color cl analysis features visible by sem cl include zoning microcracks deformation fractures and lamellae diagenetic crushing recrystallization and strength and texture of cl response features visible by optical microscopy include embayments open fractures extinction behavior and mono poly and microcrystallinity the key is to integrate cl and optical properties of the same quartz grains in order to reduce ambiguities inherent in both techniques used on their own our technique is well suited for distinguishing between plutonic volcanic and metamorphic quartz in coarse to medium grained sand or sandstone this simple and inexpensive approach can easily be combined with other provenance techniques in this paper we outline the combined sem cl optical microscopy technique and demonstrate its application with examples from the south island of new zealand

variability of diffusion of argon in albite pyroxene and olivine in shocked and unshocked samples abstract we analyzed the argon diffusion parameters of albite and pyroxene as a function of shock as well as an olivine of unknown shock pressure in albite shock pressures of 30 60 gpa all have an equal effect on the diffusion parameters lowering the activation energy e from 45 to 25 kcal mol and the log d0 a2 from 4 to 1 s 1 in a mixture of low ca and high ca pyroxene similar shock pressures also have an equal effect lowering the e from 130 to 80 kcal mol and the log d0 a2 from 14 to 7 s 1 however given that some shocked and unshocked pyroxene splits have similar release patterns making a clear distinction between the two is difficult additionally splits of pure unshocked high ca pyroxene have an e of 100 kcal mol and a log d0 a2 of 10 s 1 intermediate to the values of mixed pyroxene when shocked and unshocked the olivine fa18 has an e of 45 kcal mol and a log d0 a2 of 1 s 1 somewhat similar to that of feldspar these results will be useful in interpreting complex argon release patterns from shocked multi mineral samples too fine grained for easy mineral separation such as chondrites

partitioning of excess argon between alkali feldspars and glass in a young volcanic system abstract argon system behavior in a young volcanic system has been investigated through analysis of alkali feldspar phenocrysts and glass separates in order to constrain the potential effects of excess ar upon 40ar 39ar ages on feldspars and the crystal melt partition coefficient the importance of syn and post emplacement processes was investigated by sampling glass and pumice clasts derived from varying post eruptive conditions including welded sections vapor phase altered sections and heavily weathered portions of the ignimbrite excess 40ar was detected in all glass samples at 1 10 ppb level yet despite this step heating of co existing alkali feldspar revealed a total gas age of 0 644 0 038 ma compared with the generally accepted age of 0 668 0 004 ma for the timing of emplacement of the arico formation this suggests that despite high concentrations of ar dissolved in the glass at quenching the alkali feldspar crystallizing from excess ar rich melt contain very little excess ar no correlation was observed between excess ar in the glass and total gas age obtained from the co existing alkali feldspar and a partition coefficient kd between feldspar and glass was constrained to be less than 10 3 a correlation between the concentrations of excess ar and atmospheric derived 36ar in the glass manifested as a restricted range of 40ar 36ar up to 320 suggests a uniform mechanism of incorporation of ar from these reservoirs measured in all samples prior to eruption in an assessment of post eruptive factors diffusional loss of ar was not significant during the elevated temperatures up to 670 c associated with welding processes at the relevant timescales over which welding occurs since diffusive exchange occurs between crystals and melt over long periods in the magma chamber ages determined on volcanic alkali feldspars reflect the eruption age as a result of the very low kd but in systems containing excess ar the ultimate accuracy of such ages will be limited by the actual value of kd we observe that biotite a mineral with a higher kd yields anomalously old ages in such systems an observation made previously for other volcanic systems diffusive loss of ar from volcanic glass occurs on timescales approximating that of volcanic eruptions suggesting that excess ar measured in the glass separates is representative of a degassed state and indicates even higher pre eruptive ar concentrations present in the magma chamber prior to eruption thus our measured kd is likely to be a maximum value the significance of physical contamination of feldspar by glass is also highlighted and quantified demonstrating that less than 0 01 by weight of adhered phonolite glass was required to elevate a measured feldspar age this is particularly significant for young samples where any such contamination effect would be magnified proportionally to the small amount of in grown radiogenic ar

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

petrography and geochemistry of the carboniferous triassic trinity peninsula group west antarctica implications for provenance and tectonic setting the carboniferous triassic trinity peninsula group is a metasedimentary sequence that crops out widely in the northern antarctic peninsula these are some of the most extensive outcrops in the area and hold the key to evaluating the connections of the antarctic peninsula in gondwana however they are still poorly understood here we present our provenance study of the trinity peninsula group using petrographic and geochemical approaches in combination with cathodoluminescence of detrital quartz in order to constrain its source characteristics and tectonic setting using differences in modal composition and quartz cathodoluminescence characteristics we define three petrofacies derived from the progressive uplift and erosion of a volcano plutonic continental arc which exposed the plutonic metamorphic roots as indicated by major and trace elements the source is felsic with a composition ranging from tonalitic to granodioritic the relatively unweathered condition of the source area points to a dry and cold climate at the time of deposition but this does not necessarily mean that it was glaciated deposition of the sediments occurred within an active continental margin relatively close to the source area probably along the south patagonia antarctic peninsula sector of gondwana strong chronological petrological and chemical similarities with the sediments of the duque the york complex in patagonia suggest that they were derived from the same source

argon retention properties of silicate glasses and implications for 40 ar 39 ar age and noble gas diffusion studies sized aggregates of glasses 47 84 wt sio2 were fused from igneous derived cohesive fault rock and igneous rock and step heated from 400 to 1 200 c to obtain their 39ar diffusion properties average e 33 400 cal mol 1 d o 4 63 10 3 cm2 s 1 at t 1 mm are analyzed glasses with compositions similar to ours may preserve their formation ages

diffusion of dissolved carbonate in magmas experimental results and applications the diffusivity of dissolved carbonate in sodium aluminosilicate 60 sio 2 30 na 2 o 10 al 2 o 3 and iron free basalt melts was measured by a 14 c thin source tracer technique at conditions of combined high temperature and high pressure ranging from 800 c to 1350 c and 0 5 to 18 kbar for the simple melt and 1350 to 1500 c at 15 kbar for the haplobasalt melt the two melts have similar dissolved carbonate transport properties expressed for the simple system at 0 5 kbar by d carbonate 3 559exp 46 600 rt the pressure dependence at 1200 c is given by d carbonate 4 13 10 7 exp 11 02p rt the activation energy of 46 6 kcal mole is low relative to expected values for small highly charged cations suggesting that the actual diffusing species is a larger carbonate anion this interpretation is supported by the 11 cm 3 mole activation volume which corresponds to twice the molar volume of oxygen anions because the 14 c diffusion gradients conform quite well to a constant diffusivity thin source model there can be no strong dependence of carbonate diffusivity upon dissolved carbonate content at least in the 0 0 2 wt range represented by the experimental profiles carbonate diffusion is markedly different from previously determined systematics for water being 1 2 orders of magnitude slower in granitic magmas and considerably faster in basalts this difference can result in diffusional fractionation of co 2 and h 2 o in magmas which may be most significant in small scale processes such as growth of vapor bubbles and formation of some types of glass inclusions in phenocrysts the new data also place constraints on the rate at which co 2 dominated fluid bubbles can actually grow under more or less isothermal conditions at 10 kbar the growth rate is on the order of 1 mm day depending upon the degree of supersaturation in co 2 when combined with knowledge of initial co 2 contents of magmas and observed characteristics of dispersed bubbles e g in ocean floor basalt glasses the diffusivity data may expedite the reconstruction of magma decompression paths