multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

an automated method for mapping geomorphological expressions of former subglacial meltwater pathways hummock corridors from high resolution digital elevation data abstract elongated tracts of hummocks or hummock corridors exposed on palaeo ice sheet beds are believed to represent former subglacial meltwater pathways here we present a method coded in matlab for automatically detecting and mapping hummock corridors from high resolution digital elevation models dems initially the dem is filtered to remove bed roughness outside the size range of hummocks a fast fourier transform is then performed to determine the dominant orientation of hummock corridors and remove misaligned features finally image segmentation is used to isolate and extract the hummock corridors as a binary mask the automated approach is tested visually and statistically against detailed manual mapping in three areas of canada and northern scandinavia results show that while the automated method does not perfectly reproduce the manual mapping it successfully captures the general configuration morphometry length and width and location of hummock corridors despite variation in expression across and between sites this technique is ideally suited to take advantage of newly available high resolution digital elevation data e g the arcticdem whose enormous volume makes large scale manual mapping prohibitively time consuming its application will enable efficient and comprehensive mapping of the spatial distribution of hummock corridors across palaeo beds that is necessary for deriving insights into their formation and the organisation of subglacial meltwater flow beneath ice sheets

the morphological characteristics of ribbed moraine abstract ribbed rogen moraines are large subglacially formed transverse ridges that cover extensive areas of the former laurentide scandinavian and irish ice sheets given their ubiquitous and conspicuous nature it is surprising that their characteristics are poorly understood to date most ribbed moraine studies have been spatially restricted and rely on small sample sizes thus published accounts of their characteristics are most likely unrepresentative of ribbed moraines generally this study addresses this deficit by producing the first representative data set on ribbed moraine size shape pattern and distribution and we describe their spatial characteristics various remote sensing and gis techniques were used to map ribbed moraines over a combined area of 81 000 km 2 in canada ireland and sweden producing a database of just over 33 000 individual landforms in all 25 082 ridges were mapped in two areas in central and northern quebec the lac naococane and river kaniapiskau regions 5637 in the lake rogen area of central sweden and 2500 in north central ireland in comparison against the published accounts of ribbed moraine we demonstrate that some widely held assertions are inaccurate or untrue and we show that ribbed moraine morphological characteristics are more complex than has been hitherto reported our data reveals they exist over a larger scale range than was previously stated this study concludes by presenting a list of ribbed moraine characteristics and argues that formational hypotheses must account for these if they are to remain valid explanations of ribbed moraine genesis

zircon radiation damage ages abstract the suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s in the original technique radiation damage was determined by x ray diffraction however this is not suitable for the small sample analysis needed in modern geochronology it is now possible to measure radiation damage and u and th contents on micron sized areas on the polished surface of zircon grains using raman spectroscopy and sims mass spectrometry this opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re examine the technique through its application to zircons from two granitoids from the archean yilgarn craton and ancient detrital zircons from the jack hills in western australia the three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence the 420 110 ma radiation damage age for the darling range granite is coincident with 500 400 ma biotite rb sr ages in the same region of the yilgarn craton likewise a tonalite from the youanmi terrane in the craton with a 1650 150 ma zircon radiation damage age lies within the domain of a 1600 ma event recorded by biotite rb sr ages the jack hills zircons have a 1120 130 ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite ar ar ages of about 1140 ma the positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events

radiation enhanced fission track annealing revisited and consequences for apatite thermochronometry apatite fission track aft analyses for granitoid and metamorphic bedrock samples from the western superior province ontario the churchill rae province melville peninsula and southampton islan

nano and micro geochronology in hadean and archean zircons by atom probe tomography and sims new tools for old minerals atom probe tomography apt and secondary ion mass spectrometry sims provide complementary in situ element and isotope data in minerals such as zircon sims measures isotope ratios and trace elements from 1 20 m spots with excellent accuracy and precision apt identifies mass charge and three dimensional position of individual atoms 0 3 nm in 100 nm scale samples volumes up to one million times smaller than sims apt data provide unique information for understanding element and isotope distribution crystallization and thermal history and mechanisms of mineral reaction and exchange this atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage and can test for intracrystalline element mobility nano geochronology is one application of apt in which pb isotope ratios from sub micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating based on sem imaging and sims analysis 11 needle shaped specimens 100 nm in diameter were sampled from one archean and two hadean zircons by focused ion beam milling and analyzed with apt the three dimensional distribution of pb and nominally incompatible elements y rees differs at the atomic scale in each zircon zircon jh4 0 4 007 ga jack hills western australia is homogeneous in pb y and rees in contrast pb and y and rees are clustered in sub equant 10 nm diameter domains spaced 10 40 nm apart in zircons arg2 5 2 542 ga grouse creek mountains utah and jh4 4 4 374 ga jack hills most clusters are flattened parallel to 100 or 010 u and th are not collocated with pb in clusters and appear to be homogeneously distributed in all three zircons the analyzed domains experienced 4 to 8 10 15 decay events mg due to u and th decay and yet all zircons yield u pb ages by sims that are better than 97 concordant consistent with annealing of most radiation damage the 207 pb 206 pb ratios for the 100 nm scale specimens measured by apt average 0 17 for arg2 5 0 42 for the jh4 0 and 0 52 for jh4 4 these ratios are less precise 10 18 2 due to the ultra small sample size but in excellent agreement with values measured by sims 0 1684 0 4269 and 0 5472 respectively and the crystallization ages of the zircons thus pb in these clusters is radiogenic but unsupported meaning that the pb is not spatially associated with its parent isotopes of u and th for the domain outside of clusters in jh4 4 the 207 pb

alpha recoil in u pb geochronology effective sample size matters displacement of the daughter isotope by recoil results in an open system on the nanoscale for a heterogeneous distribution of u and th this redistribution of intermediate and stable daughter isotopes results in subvolumes with a deficit of pb and others with an excess of pb whether such heterogeneities affect the analyzed u pb system depends on 1 the volume of the analyzed sample 2 the degree and scale of heterogeneity in the u and th distribution and 3 the analytical procedure spatial separation of parent and daughter through recoil affects the u pb systematics of leached samples where leaching gives access to domains less than 1 m wide anomalous data patterns originating from recoil induced parent to daughter fractionation are more important if there are strong heterogeneities in the u and th distribution whereby pb excess appears more pronounced than pb deficit fractionation of parent and daughter elements through selective dissolution of u ree rich growth zones in zircon and u inclusions in columbite as well as the presence of u th rich micro inclusions in silicates dated using a step leaching scheme may result in anomalous 207pbrad 206pbrad scattered 206pbrad 238u and 207pbrad 235u and reverse discordance the accumulated structural damage controls the leaching and dissolution behavior but may also influence the non stoichiometric element mobilization during sputtering or ablation in the analysis of u rich samples by shrimp and la mc icp ms

high resolution sequence stratigraphy from piezocone tests an example from the late quaternary deposits of the southeastern po plain abstract cone penetration tests are traditionally regarded as a major tool for geotechnical investigations this study based upon interpretation of 234 cone penetration tests with pore pressure measurements piezocone tests or cptu carried out in the late quaternary deposits of the southeastern po plain shows how cptu tests can also be used for sedimentological purposes including detailed facies characterization subsurface stratigraphic correlations and identification of the key surfaces for sequence stratigraphic interpretation the late quaternary depositional history of the southeastern po plain reconstructed on the basis of data from seventeen continuously cored boreholes includes 1 alluvial plain development during the late quaternary lowstand and the early stages of transgression 2 formation of a rapidly migrating barrier lagoon system during the late transgressive phases 8800 6000 y bp 3 construction and progradation of a wave dominated delta ancient po delta during the following sea level highstand 6000 800 y bp and 4 development of the present day alluvial plain following the delta lobe abandonment in the 13th century a d local execution of cptu tests in coincidence with drilling sites enables the calibration of borehole data with piezocone penetration profiles lithofacies characterization is based upon estimation of three major parameters corrected cone resistance q t sleeve friction f s and pore water pressure u plotting of q t versus the ratio of cone friction to cone bearing fr is adopted as the major tool for sediment texture classification eight major facies associations for the southeastern po plain are identified these are for decreasing q t values 1 fluvial channel sands 2 beach ridge sands 3 transgressive barrier sands 4 crevasse sands and silts 5 levee silts and sands 6 floodplain silts and clays 7 prodelta clays and 8 marsh clays and peats cptu profile interpretation also provides the basis for the identification of the three major key surfaces within the late quaternary 4th order depositional sequence 1 the transgressive surface ts marking the boundary between the locally pedogenized stiff pleistocene alluvial clays and the overlying holocene transgressive paralic deposits has a distinctive pore pressure response with very low u values and is characterized by a sharp downward increase in f s which is paralleled by a moderate increase in q t 2 the ravinement surface rs corresponding to the boundary between fine grained back barrier deposits and the overlying transgressive barrier sands is invariably marked by a sharp upward increase in q t and decrease in fr 3 the maximum flooding surface mfs which is not clearly recognizable on the sole basis of core data is identified within shallow marine prodelta clays in combination with minor peaks of q t which have been interpreted to reflect a laterally extensive fossil lag with transition in proximal areas to sand shell rich layers simplicity speed and comparatively low costs of cptu tests imply that an extensive use of this method when used in conjunction with core programs can be a very attractive alternative to economically less convenient methods for the geological mapping of alluvial coastal plain areas consisting of non gravel deposits

nanoscale occurrence of pb in an archean zircon abstract we report for the first time a direct atomic scale characterization of pb in zircon 4 4 3 1 ga from the early archean yilgarn craton in australia using high resolution haadf stem two forms of pb have been identified pb concentrated at 3 atom as a nanoscale patch in zircon structure and pb concentrated within the amorphous domain created by fission fragment damage the first result suggests that the pb atoms directly substitute for zr4 in the zircon structure and the latter observation demonstrates that pb diffusion can occur through amorphous regions created by radiation damage although volume diffusion is typically considered to be the dominant mechanism for pb diffusion beyond the first percolation point i e when the amorphous domains overlap and form a fully interconnected network of amorphous domains there is a new pathway for the diffusion of pb that is faster than volume diffusion through crystalline zircon

radiation damage in zircon a single zoned sri lankan zircon exhibits a range of microstructures from crystalline to nearly amorphous that are the result of radiation damage over a dose range of 2 1 10 1 1015 decay events mg 0 16 0 47 dpa the zones in the crystal were examined at a variety of length scales using optical microscopy micro raman spectroscopy electron microprobe analysis and transmission electron microscopy birefringence varies linearly with dose birefringence 4 71 10 18 g d 4 86 10 2 full width at half maximum fwhm measurements of the b1g 3 peak as determined by micro raman spectroscopy were used to estimate the extent of radiation damage in each zone the radiation dose calculated on the basis of u and th concentrations and sample age vs damage from the fwhm measurements of the b1g 3 peak relationship among zones was consistent with results for a suite of sri lankan single crystals suggesting that all of these zircon crystals have undergone a similar thermal history based on a comparison with zircon crystals that are considered to have undergone minimal annealing e g lunar zircon the sri lankan zircon crystals have accumulated less damage than is expected based on their calculated dose consistent with previous evidence for annealing to estimate the extent of annealing in sri lankan samples an equivalent damage dose i e the dose required to produce the same amount of damage in an unannealed sample was calculated for a given u and th concentration by determining the time of damage accumulation required to create an equivalent amount of damage in an unannealed zircon the dose vs damage relationship in sri lankan zircon crystals 560 ma is equivalent to that of a 375 ma unannealed zircon suggesting that approximately one third of the damage has been lost due to annealing the dose vs damage relationship is consistent with the direct impact model of damage accumulation transmission electron microscopy revealed zoning on a scale finer than could be identified optically 0 5 m and the presence of abundant zro2 nano particles at zone boundaries

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

glacial curvilineations found along the southern sector of the laurentide ice sheet and a hypothesis of formation involving subglacial slope failure in tunnel valleys and subglacial lakes 2018 john wiley sons ltd first discovered in poland glacial curvilineations gcls are enigmatic landforms comprising parallel sets of sinuous ridges and troughs of metres amplitude and around 150m wavelength found within kilometres wide valleys interpreted as being produced by meltwater flowing subglacially their morphological and sedimentary characteristics and association with tunnel valleys has been described for some prominent polish examples from these observations the existing hypothesis is that they form as a consequence of erosion by longitudinal vortices that develop in subglacial floods here we report for the first time gcls found along the southern sector of the laurentide ice sheet in three northern states of the usa using mapping and topographic analysis from high resolution digital elevation models we report observations on their morphological properties and landform associations we find aspects of their context and morphology difficult to explain using the existing hypothesis we instead suggest that these glacial curvilineations are produced by subglacial bank and slope failures that locally widen tunnel valleys or that occur near subglacial lake shorelines further investigation is required to test this hypothesis and to ascertain the mechanisms of proposed mass movements which may have occurred by rotational or translational slope failure or by creep deformation our preferred mechanism is that such movements occurred where subglacial water was emplaced over previously perma frozen ground under such circumstances sediment blocks thawed by the water may then easily glide over a frozen decollement at low slope angles analogous to subaerial active layer glides in permafrost environments permafrost spring sapping may have provided lines of weakness for slope failure if the requirement for permafrost is found to hold then gcls may become an important indicator of the palaeo distribution of permafrost

genesis of the glacial curvilineation landscape by meltwater processes under the former scandinavian ice sheet poland abstract dobrzyn plateau in central poland contains a suite of enigmatic glacial bedforms termed glacial curvilineations gcl these landforms occur within tunnel valleys forming a complex anabranched network gcl consist of a series of parallel ridges extending for as much as 5 km over their length gcl ridges are parallel to neighboring ridges and to tunnel valley margins internal gcl composition consists of sand gravel and till distributed randomly within ridges and in intervening troughs electric resistivity tomography surveys confirm the sediment distribution seen in field exposures truncated beds near the ridge forms and the heterogeneous nature and distribution of sediments suggest an erosional rather than accretionary origin for gcl in this case internal gcl composition is not related to gcl genesis it records proglacial and subglacial conditions prior to gcl shaping by erosion the terminal zone of the anabranched tunnel valley network is characterized by a broad apron like ice marginal sedimentary complex sand and gravel dominate sedimentary facies within this zone sediments record evidence of meltwater sedimentation associated with supraglacial melting near the ice margin and high energy subglacial outbursts emanating from tunnel valleys this interpretation suggests that the gcl surface is a regional unconformity representing an erosional landscape where sediments within landforms offer limited insights into landform genesis cf sharpe et al 2004 subglacial meltwater erosion is proposed as unifying model of landscape development as it accounts for both erosion of tunnel valleys and gcl and meltwater dominated sedimentation in the ice marginal sedimentary complex

erratum to metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage annealing of the radiation damage generated in decay events is a common phenomenon in natural zircons we have studied relationships between radiation damage age and radionuclide content of natural zircons on a micro scale the degree of metamictisation was estimated by means of confocal laser raman spectroscopic measurements raman band broadening in well ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth shows a nearly linear dependence on the fluence this dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage which may contribute to the understanding of the low t thermal history of their host rocks seven examples for the application are presented radiation damage may anneal heterogeneously with preferred re ordering in more damaged micro areas results suggest that single point defects do not anneal continuously over geologic periods of time but are in contrast a comparably stable type of radiation damage in contrast to recrystallisation processes structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic pb the most prominent example for this are concordant zircons from sri lanka although they are excellent standards for u pb dating sri lankan zircons should not be used as standards for structural radiation damage the experimental work is completed by monte carlo simulations of decay events in zircon

tracing the fluid evolution of the kiruna iron oxide apatite deposits using zircon monazite and whole rock trace elements and isotopic studies the ore genesis of the paleoproterozoic iron oxide apatite deposits in the vicinity of kiruna in northern sweden is poorly understood despite a century long mining history and 2500 mt of iron ore with grades of 30 to 70 wt fe produced in the region to date zircon grains from the ore recently dated at ca 1874 ma show very different appearances compared to zircon from surrounding host rocks ca 1880 ma and related intrusions ca 1880 and ca 1874 ma particularly an inclusion rich rim in contrast zircon from the host rocks and a proximal granite intrusion exhibit typical igneous growth zoning electron microprobe results show near stoichiometric composition for zr si and hf in the host rock zircon grains the ore zircon crystals have low analytical totals with significant concentrations of ca fe y and p and infrared spectroscopy showed several weight percent of water these ore zircon grains further show fe rich inclusions zones and or veins in elemental x ray maps and light rare earth elements lree enrichment transmission electron microscopy tem shows that the lree are not due to micro or nano inclusions in the zircon but are likely hosted as lree oxides in amorphous regions of the grains based on these characteristics the rims on ore zircon grains are interpreted to be of hydrothermal origin uranium pb in monazite from the ore measured by sims suggests a secondary event influencing the area at ca 1624 ma a period of known geologic activity in fennoscandia electron microprobe x ray mapping of these monazite grains shows no zoning and relatively low u and th concentrations stark contrasts are visible between the ore depleted mantle influence and host rocks crustal influence in the whole rock lu hf and sm nd data the depleted mantle signature of the ore could be related to the kiruna greenstone group as a potential source region for the iron the sm nd isotopic composition of monazite from the ore shows a crustal influence and indicates that the younger event has not disturbed the whole rock sm nd signature of the ore the hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source

on the formation of the tunnel valleys of the southern laurentide ice sheet abstract catastrophic releases of meltwater produced by basal melting and stored for decades in subglacial reservoirs at high pressure may have been responsible for eroding the broad deep tunnel valleys that are common along the margins of some lobes of the southern laurentide ice sheet we surmise that these releases began when the high water pressure was transmitted to the margin through the substrate the water pressure in the substrate at the margin would then have been significantly above the overburden pressure leading to sapping failure headward erosion of a conduit in the substrate piping could then tap the stored water resulting in the outburst in some situations development of a siphon may have lowered the reservoir below its overflow level thus tapping additional water following the flood the seal could have reformed and the reservoir refilled setting up conditions for another outburst order of magnitude calculations suggest that once emptied a subglacial reservoir could refill in a matter of decades the amount of water released during several outbursts appears to be sufficient to erode a tunnel valley we think that tunnel valleys are most likely to have formed in this way where and when the glacier margin was frozen to the bed and permafrost extended from the glacier forefield several kilometers back under the glacier as reservoirs would then have been larger and more common and the seal more robust and more likely to reform after an outburst

evaluation of preconsolidation pressure and the overconsolidation ratio from piezocone tests of clay deposits in quebec the preconsolidation pressure and overconsolidation ratio profiles are the most important factors related to the mechanical behaviour of clay deposits they are interpreted on the basis of a limited number of laboratory tests but the near continuous information provided by the piezocone can allow local data to be extrapolated to an entire site numerous methods have been proposed for relating piezocone data to preconsolidation pressure and the overconsolidation ratio but their validity is still uncertain in this paper nine methods are compared on the basis of data collected on 31 sensitive clay sites in quebec whose preconsolidation pressure profiles are well known each method is briefly presented and the most promising are identified the simplest method which directly relates preconsolidation pressure to net tip resistance also appears to be the most effective key words clay preconsolidation pressure overconsolidation ratio piezocone correlations

influence of radiation damage on titanite he diffusion kinetics titanite is a common u th bearing accessory mineral that previous study suggested has a u th he closure temperature tc of 210 175 c although it has been recognized that radiation damage influences the he retentivity of apatite and zircon this effect has not been documented in titanite we acquired 51 the analyses from 11 archean basement samples across the kaapvaal craton south africa the the dates range from 20 to 1200 ma and are negatively correlated with their 10 970 ppm span of effective uranium concentration eu raman spectra acquired for 4 of these samples display peak broadening with increasing eu 4he diffusion experiments on the same 4 samples suggest tc values that vary by 175 c estimated alpha dose versus tc for our results and published the diffusion data document tc values of 150 210 c at alpha doses 50 1016 g with a sharp decrease in he retentivity above this threshold this damage diffusivity pattern is similar to zircon but the damage level at which the reduction in retentivity occurs appears to be lower for titanite because titanite typically has substantially lower eu than zircon for the same protracted thermal history the dates are likely to record higher temperature portions of the history than zircon u th he dates the results demonstrate that the dating can access a much larger temperature range and potentially be used to decipher more detailed time temperature paths than previously thought

reply to the discussion of the reply by r l romer and u kroner on geochemical signature of ordovician mn rich sedimentary rocks on the avalonian shelf 1 1appears in the canadian journal of earth sciences 49 11 1372 1377 doi 10 1139 e2012 049 romer and kroner 2012 stated the core of the harlech dome i in part is made up of ordovician deposits and ii contains tectonic repetitions in their comment schofield et al this issue d

weathering dynamics reflected by the response of riverine uranium isotope disequilibrium to changes in denudation rate abstract the ratio between the activity of dissolved 234 u and 238 u in river water 234 u 238 u riv shows promise for tracing weathering processes it has been suggested that preferential 234 u release generates a high 234 u 238 u riv under low weathering intensity while congruent weathering as expected for high weathering intensity would release uranium with a secular equilibrium 234 u 238 u riv of 1 however weathering intensity depends on the combination of exposure age of weathering interfaces and dissolution rate complicating the use of 234 u 238 u riv as a tool in weathering studies this work attempts to resolve the weathering dynamics reflected by 234 u 238 u riv using catchment scale denudation rate d as a first order approximation for the age of weathering interfaces a global dataset n 653 including 132 new measurements from chinese catchments indicates changing responses of 234 u 238 u riv to d higher values 1 5 and larger variability of 234 u 238 u riv are observed in catchments both with high 3 mm yr and low d 234 u 238 u riv are associated with catchments of medium to high d 0 3 2 mm yr the observed pattern can be explained by a model that involves competing uranium release from active and inactive weathering interfaces in response to a shifting weathering regime from kinetically limited to supply limited 234 u 238 u riv decreases with decreasing d under a kinetically limited weathering regime due to the increasing contribution of congruent weathering relative to the preferential dissolution of 234 u from radioactively damaged sites as d decreases in contrast 234 u 238 u riv increases with decreasing d under a supply limited weathering regime because the minerals that remain under these conditions continue to release 234 u to solution by recoil while contributing little to the dissolved 238 u because of their slow dissolution rate a much deeper depth of the weathering interface is suggested for catchments with high d due to higher weathering contribution from deep fractures relative to the poorly developed surface regolith a young age of 10 kyrs is constrained by the model for complete dissolution of exposed weathering active minerals which implies a rapid shifting from kinetically limited to supply limited weathering regime and limited influence of weathering on the calculation of comminution age based on 234 u 238 u in sediments a site specific dissolution rate that is consistent with those estimated from weathering profiles is also constrained which suggests that the discrepancy between field and laboratory weathering rate may largely be controlled by the difference in chemical affinity associated with stagnant micro pores low permeability and high rock fluid ratios rather than the passivation of weathering interfaces

elemental dispersion and stable isotope fractionation during reactive fluid flow and fluid immiscibility in the bufa del diente aureole ne mexico evidence from radiographies and li b sr nd and pb isotope systematics the 31 6 0 3 ma old bufa del diente alkali syenite ne mexico intruded a sequence of cretaceous limestones with intercalated sub horizontal chert layers the cherts acted as aquifers that facilitated transport of brines and pegmatitic melts within the shallow level 1 kbar contact metamorphic aureole fluid driven reactions between chert and marble wallrock and the influx of late melts and various fluids gave rise to distinct chemical and isotopic signatures within the aquifer and across the zones of infiltration and fluid driven reaction aqueous brines of magmatic origin produced thick wollastonite mantles around the chert layers wollastonite formation occurred at the expense of limestone and chert and generated co2 this co2 induced fluid unmixing into an aqueous brine and a low density co2 rich fluid which was lost to the overlying marble where it oxidized organic matter and caused 13c and 18o shifts in a zone some 5 10 cm wide after wollastonite formation the chert aquifers were locally intruded by pegmatite veins carrying alkali feldspar quartz aegirine augite eudialyte zircon and apatite aqueous fluids that exsolved during crystallization of the pegmatite veins escaped along late cross fractures and migrated along the inner and outer borders of the wollastonite margins chemical dispersion patterns of u al na k p s fe and ree across the chert to marble boundary and its metasomatic rims are shown by autoradiography and neutron induced radiography scavenging of cations at mineralogical contacts and cation transport into the marbles occurred only on the mm to cm scale isotopic data for pb and sr across a simple metachert marble boundary and for pb sr nd b and li across a metachert pegmatite marble sequence demonstrate the following 1 the pb and sr isotopic signature of early fluids was buffered by the carbonate wallrock only late fluids shielded from wallrock interaction by a wollastonite mantle variably preserved a memory of their initial magmatic signature 2 since the nd isotope signature of marble and chert is bound to calcite and clay minerals systematic shifts to unradiogenic nd in marble reflect loss of carbonate bound nd as the wollastonite margin is approached nd in the wollastonite margin is dominated by nd originally bound to clay minerals the later emplacement of the pegmatite which carried the nd isotope signature of its alkali syenite source had little effect on the nd isotopic composition of the wollastonite rim 3 although the li and b isotopic compositions reflect the alkali syenite source they are also affected by isotopic fractionation and partitioning between melt fluid and solids

complicated secondary textures in zircon record evolution of the host granitic rocks studies from western tauern window and otztal stubai crystalline complex eastern alps western austria abstract samples of metamorphosed and deformed granitic rocks were collected from two alpine complexes with well constrained metamorphic history western tauern window and otztal stubai crystalline complex zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques cathodoluminescence cl imaging and raman spectroscopy the aims were to describe and interpret complicated secondary textures and microstructures in zircon based on cross cutting relationships between secondary microstructures reconstruct the sequence of processes affecting zircon crystals link the evolution of zircon with the history of the host rocks the results indicate that zircon in the sampled granitic rocks forms growth twins and multi grain aggregates which are unusual for this mineral moreover various secondary textures have been found in the sampled zircon often cross cutting each other in a single crystal these include distorted oscillatory cl zoning with inner zones forming inward penetrating cl bright embayments which are the evidence of dry recrystallization via annealing lattice recovery cl mosaicism with no preservation of growth zoning but abundant nano and micro scale pores and mineral inclusions which are the evidence of recrystallization by coupled dissolution reprecipitation and or leaching embayed zircon boundaries filled with apatite monazite epidote and mylonitic matrix indicating mineral fluid reactions resulting in zircon dissolution and fragmentation overgrowth cl dark rims which contain nano pores and point to transport and precipitation of dissolved zircon matter we conclude that zircon in our meta granites is sensitive to metamorphism deformation events and was reactive with metamorphic fluids additionally we have found evidence of crystal plastic deformation in the form of low angle boundaries and bent grain tips which is a result of shearing and ductile deformation of the host rock we suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization prograde metamorphism peak of amphibolite facies metamorphism post peak cooling and exhumation formation of ductile shear zones and final cooling to 250 c

ice sheet dynamics and subglacial meltwater regime inferred from form and sedimentology of glaciofluvial systems victoria island district of franklin northwest territories on victoria island tunnel channels eskers and associated fans and extended deposits together constitute channelized glaciofluvial systems flutes and drumlinoid ridges interpreted as residuals left by erosive catastrophic subglacial meltwater sheet flows lie adjacent to these systems one tunnel channel is described in detail it exhibits deep scours a discontinuous thalweg sculpted margins and flutes on the downflow side of one wall features indicative of complex flow and possibly several flow events the tunnel channel is interpreted as the product of erosion by catastrophic subglacial meltwater flow in a combined ice substrate r n channel esker sediments and morphology are used to infer details of the depositional environment and meltwater regime a continuous esker with fans and extended deposits records seasonally controlled discharge events through an r channel these features may also suggest a grounding line environment thin ice and localized ice floatation events less well conn

closure to soil deformation and excess pore pressure field around a closed ended pile by juan m pestana christopher e hunt and jonathan d bray

glacial curvilineations new glacial landforms produced by longitudinal vortices in subglacial meltwater flows abstract the glacial landscape of the dobrzyn plateau in central poland contains a complex suite of enigmatic bedforms consisting of elongate and sinuous sediment ridges occurring within an anabranched network of tunnel channels terminating at a former ice margin marked by extensive glaciofluvial fans the term glacial curvilineations is proposed to describe these enigmatic bedforms that occur as fields of parallel sinuous ridges separated by troughs the regional pattern of glacial curvilineations replicates the morphology and pattern of tunnel channel margins and suggests a common genesis tunnel channels and glacial curvilineations are eroded in the interbedded diamictons and glaciofluvial sediments of the plateau we propose that tunnel channels and glacial curvilineations are the products of subglacial meltwater erosion curvilineations are erosional remnants produced by longitudinal vortices within the tunnel channel forming flows parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow tunnel channel margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel channel termini they are the depositional counterparts to the extensive subglacial fluvial dissection of the plateau understanding curvilineation genesis may shed light on glacier hydrologic processes tunnel channels develop abruptly in the plateau surface suggesting localized point sources of meltwater such as drainage of subglacial and or supraglacial lakes the extent of tunnel channels and lateral continuity of glacial curvilineations suggests broad sheet like flows as wide as a few kilometres

long term exhumation of an aegean metamorphic core complex granitoids in the northern menderes massif western turkey the egrigoz koyunoba and alacam plutons are located in the northern portion of the menderes massif a region characterized today by large scale extension to gain a better understanding of their exhumation history we acquired zircon ages geochemical analyses and cathodoluminescence cl images to search for evidence of micro to macro scales of deformation the combination is a powerful means to decipher their tectonomagmatic history in situ ion microprobe 238u 206pb zircon ages of the granitoids range from 30 0 3 9 ma to 14 7 2 6 ma 1 and indicate the plutons crystallized over 15 m y the dated zircons show cl zoning consistent with igneous crystallization and are only located adjacent to or as inclusions in biotite grains the youngest ages are dominated by a blue color in cl whereas a majority of older zircons are yellow higher th u contents are typically shown by zircons with black or dark green colors in cl most of the samples of these granitoids indicate they are magnesian calc alkalic and peraluminous granite to granodiorites but variations exist likely reflecting heterogeneity caused by magma mixing partial melting crustal contamination and post emplacement fluid interactions as evidenced by the cl images cl images show that all samples experienced magma mixing multiple episodes of brittle deformation and fluid mediated alteration sources for the generation of northern menderes massif granitoids include simple slab induced upwelling from the subduction of the african plate along the hellenic arc adiabatic decompression as the northern edge of turkey9s anatolide tauride block delaminates and upwelling asthenosphere as the continental lithosphere thins during extension we present a model in which western turkey is an amalgamation of stacked subduction zones that transition from north to south over time northern menderes massif granitoids document their complex geologic history in their ages chemistry and textures

modeling piezocone cone penetration cptu parameters of clays as a multivariate normal distribution this study examines the possibility of modeling piezocone cone penetration cptu cone tip resistance excessive pore pressure behind the cone undrained shear strength and overconsolidation ratio of lightly overconsolidated clays as a multivariate normal distribution this is part of a continuing study to develop a multivariate distribution that could be used to simulate common soil parameters at a clay site this study compiles a large database consisting of 535 data points in which the cptu parameters undrained shear strength and overconsolidation ratio are simultaneously measured in close proximity a multivariate normal distribution is then used to capture the correlations between soil parameters of interest and to derive useful equations for bayesian inference this constructed multivariate normal distribution and equations are further validated by another independent database consisting of 594 data points as well as by empirical equations proposed in literature the most useful outcome of this s

improved accuracy of zircon u th he ages by rectifying parent nuclide zonation with practical methods abstract a major limitation in the accuracy of zircon u th he thermochronometry zhe is the alpha ejection correction the standard alpha ejection correction f t assumes a homogeneous u th distribution in the host crystal here two approaches are used to address parent nuclide heterogeneity often observed in natural zircons 1 a new methodology where offending zonation is mechanically removed and 2 custom alpha ejection corrections informed by a la icp ms depth profile f zac we present an empirical test of these methodologies on a suite of bedrock zircons from variscan orthogneisses of the central aegean which display marked parent nuclide zonation which has impacted the ability to obtain accurate and precise thermochronometric data during the progressive mechanical abrasion of enriched u th and he zones the change in grain diameter exhibits a parabolic relationship with respect to the apparent age where the youngest age should represent the true age of the sample the abrasion corrected weighted mean age for sample 08pa01 is 8 8 0 7 ma f t 1 a too young age possibly the result of parent nuclide enriched cores the findings also show that using f zac instead of f t may change single sample zhe aliquots up to 29 using f zac increases the weighted mean age 17 for sample 08pa01 from 9 2 0 7 ma f t to 10 8 0 9 ma f zac and 6 for sample 08pa71 from 10 9 0 8 f t ma to 11 5 0 9 ma f zac in this case the f zac correction improves the overall accuracy of the zhe data but does not impact the problem of over dispersed ages suggesting they may be separate issues the increase in zhe age brings the dataset into agreement with previous thermochronometric studies indicating that cooling of the paros footwall below 180 c may have occurred earlier than hitherto thought

microstructure of 24 1928 ma concordant monazites implications for geochronology and nuclear waste deposits the microstructure of monazite was studied using scanning electron microscopy sem electron microprobe analysis emp x ray diffraction patterns xrd and transmission electron microscopy tem four well characterized monazites were investigated having very different concordant u pb ages 24 to 1928 ma and up to 15wt tho2 and 0 94wt uo2 the sem and emp analyses of polished single crystal fragments reveal the absence of significant chemical zoning xrd and tem investigations show that the monazites are not metamict despite their old ages very high abundances of radionuclides and hence high time integrated radiation doses except for the youngest one the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains this structure is responsible for the presence of 1 mottled diffraction contrasts on the tem and 2 a second structural phase b with very broad reflections in the xrd patterns older monazites receive higher self irradiation doses and hence they contain higher amounts of this b phase for the 1928 ma monazite xrd reveals only the broad reflections of phase b implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice it is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because selfannealing heals the defects as they are produced by alpha decay damage the only memory of irradiation induced defects is the presence of distorted domains as the diffusion rate of pb in an undisturbed monazite lattice is extremely low pb loss due to volume diffusion out of the monazite lattice is virtually impossible this is considered as one reason why almost all monazites have concordant u th pb ages moreover as long term self irradiation effects are limited in monazite we consider this phase as a good candidate for the storage of high level nuclear waste under the aspect of its high resistance to irradiation

piezocone penetration testing in atlantic piedmont residuum piezocone soundings were performed in residual silts and fine sands of the u s atlantic piedmont geologic province to investigate penetration pore water pressures and subsequent dissipation characteristics in these nontextbook geomaterials piedmont residuum has been formed by the in place weathering of paleozoic schist gneiss and granite pore water pressures were measured at two positions midface of the cone tip u sub 1 and at the shoulder or behind the tip u sub 2 at the standard rate of 2 cm s penetration pore water pressures on the midface element are fairly high positive values u sub 1 600 kpa 200 kpa yet readings at the shoulder element were negative u sub 2 was approximately equal to 90 kpa at an increased penetration rate of 20 cm s midface and shoulder readings increased in magnitude and were positive u sub 1 was approximately equal to 800 kpa and u sub 2 was approximately equal to 200 kpa however measured tip resistance exhibited an unusual decrease per comparisons with data available in the literature upon halt of penetration both u sub 1 and u sub 2 decayed quickly and reached hydrostatic values within 1 2 minutes

retention of radiation damage in zircon xenocrysts from kimberlites northern yakutia abstract we have studied zircon xenocrysts from mesozoic kimberlites from the kuoika and ary mastakh fields in northern yakutia zircon xenocrysts are assumed to originate from crustal rocks our shrimp sensitive high mass resolution ion microprobe analyses yielded predominantly concordant u th pb ages up to 3570 ma paleoarchean that clearly predate kimberlite formation the general u th pb concordance observed excludes notable disturbance of the zircon xenocrysts u th pb isotope system during kimberlite ascent and emplacement in addition zircon xenocrysts were found to be significantly more radiation damaged than would correspond to damage accumulation only since the time of kimberlite formation this observation first indicates that zircon crystals were sampled by the kimberlite magma at comparably shallow depths not exceeding 10 12 km if in contrast zircon crystals originated from deeper levels of the earth s crust they would have been exposed to temperatures of 250 300 c or more this in turn would have caused long term thermal annealing of the radiation damage which was however not observed in our study second our observation contradicts the hypothesis that high temperatures experienced by zircon xenocrysts during kimberlite ascent will cause notable structural reconstitution by short term thermal annealing consequently zircon crystals cannot have spent more than a few hours at temperatures exceeding ca 700 800 c or more than a few days at temperatures exceeding ca 500 600 c this in turn suggests that i temperatures of the ascending kimberlite magmas were rather moderate and ii kimberlite ascent is a comparably short process followed by rapid cooling

thermal history of northwest africa 5073 a coarse grained stannern trend eucrite containing cm sized pyroxenes and large zircon grains we report on the bulk chemical composition petrology oxygen isotopic composition trace element composition of silicates and degree of self irradiation damage on zircon grains of the eucrite northwest africa nwa 5073 to constrain its formation and postcrystallization thermal history and to discuss their implications for the geologic history of its parent body this unequilibrated and unbrecciated meteorite is a new member of the rare stannern trend eucrites it is mainly composed of elongated zoned pyroxene phenocrysts up to 1 2 cm plagioclase laths up to 0 3 cm in length and is rich in mesostasis the latter contains zircon grains up to 30 m in diameter metal sulfide tridymite and ca phosphates textural observations and silicate compositions coupled with the occurrence of extraordinary fe rich olivine veins that are restricted to large pyroxene laths indicate that nwa 5073 underwent a complex thermal history this is also supported by the annealed state of zircon grains inferred from raman spectroscopic measurements along with u and th data obtained by electron probe microanalyses

incipient tunnel channels abstract glaciated terrains in east central alberta and south central michigan contain channels that have hummocks and transverse ridges separating depressions along their floors this association imparts a linked pothole appearance similar channels are often interpreted as tunnel channels or subaerial channels partly filled with sediment from a subsequent glacial advance a stagnating ice roof or slumped sediment from the channel margins however the truncation of sedimentary packages in the channel walls and intrachannel hummocks indicates that they are erosional landforms cut into glacial sediments till bedrock or gravel eskers overlie and are found within a few channels indicating that these channels formed before the final stagnation that produced the eskers these two characteristics combined with the observation that many channels have convex up long profiles indicate that the channels were eroded by pressurized subglacial water because the formative mechanisms for this type of channel are not clear and modern environments that could produce this type of landform are inaccessible we draw on several morphologic analogues to propose mechanisms for channel erosion we conclude that the erosion of these linked pothole channels incipient tunnel channels was the product of the complex interaction between complex turbulent flow structures and various scales of roughness elements

terrestrial ice streams a view from the lobe abstract the glacial landforms of minnesota are interpreted as the products of the lobate extensions of ice streams that issued from various ice sheds within the laurentide ice sheet low relief till plains trough shaped lowlands boulder pavements and streamlined forms make up the subglacial landsystem in minnesota that is interpreted as having been formed by streaming ice extremely uniform tills are created subglacially in a way that remains somewhat mysterious at the ice margins thrust moraines and hummocky stagnation topography are more common than single crested simple moraines if the ice lobes had repeated advances subglacial drainage features are obscure up ice but are present down ice in the form of tunnel valleys eskers spooner hills and associated ice marginal fans ice streaming may occur when basal shear stress is lowered as a result of high subglacial water pressure subglacial conditions that allow the retention of water will allow an ice lobe to extend far beyond the ice sheet as long as the ice shed also supports the advance by supplying adequate ice even with adequate ice flux however the advance of an ice lobe may be terminated at least temporarily if the subglacial water is drained through tunnel valleys or perhaps a permeable substrate thrust moraines and ice stagnation topography will result from sudden drainage although climate change is ultimately responsible for the accumulation of ice in the laurentide ice sheet the asynchronous advances and retreats of the ice lobes in the mid continent are strongly overprinted by the internal dynamics of individual ice streams as well as the interaction of ice sheds which obscure the climate signal

observations on the development of pore water stresses during piezocone penetration in clays piezocone data from 83 clay sites are reviewed to investigate first order trends between measured penetration pore water stresses ut and ubt and corrected cone tip resistance qt it is shown that the presence of fissures in clay deposits and of fissured crusts significantly affects the pore water stress response commercially available piezocones primarily favor the location of the porous element either 1 on the cone tip or face ut or 2 just behind the tip ubt the former ut provides optimal profiling while the latter ubt is required for correcting measured cone tip resistances for pore water stress effects acting on unequal areas of the cone the available data indicate that qt predominantly reflects penetration pore water stresses ut with measured ratios of ut qt on the cone face averaging in the order of 0 73 for most intact clays 0 64 specifically for leda clays and 0 46 for fissured clays behind the cone tip the ratio of ubt qt averages about 0 53 for intact clays 0 58 specific

isotopically heterogeneous initial pb and continuous 222 rn loss in fossils the u pb systematics of brachiosaurus brancai abstract diagenetic incorporation of uranium into bone material establishes a u pb system that provides a minimum age for the fossil loss of rn and ra from the 238u decay series recent mobility of u and pb and incorporation of radiogenic pb from an old basement which was available during diagenesis disturbed the u pb system all data fall in a 206pb 238u 207pb 235u concordia diagram to the right of the concordia while samples that do not experience redistribution of u pb 222rn and 226ra should define a linear array that intersects the concordia curve at the age of diagenesis and at the age of the source of inherited radiogenic pb loss of 222rn and 226ra has led to displacement of the present data set to lower 206pb 238u and therefore to younger intercept ages and to older 207pb 206pb ages the 207pb 204pb 208pb 204pb systematics demonstrates that 1 three out of 19 samples experienced significant postdiagenetic fractionation of u and pb and that 2 the diagenetic pb addition was isotopically heterogeneous the 235u 207pb 206pb 207pb and 206pb 238u 207pb 235u systematics combined demonstrate that 3 the samples experienced 5 to 18 222rn loss for a 1 9 ga old source of the diagenetically introduced pb and correspondingly more for a younger pb source and that 4 there is no unique solution for the age of the source of diagenetic pb since pb from a younger source appears in the 235u 207pb 206pb 207pb diagram in the same way as higher 222rn loss no precise age can be determined from the data although the most consistent solution falls in the range of 140 to 150 ma addition of radiogenic pb to the fossil bone is obvious only through its anomalous highly radiogenic character and the isotopic heterogeneity in 208pb 204pb no th i e no in situ 208pb growth the effect of unrecognized diagenetic additions of radiogenic pb is an overestimation of the age whereas unrecognized 222rn loss results in a too young discordant age since fossil material formed at times for which the discordia only shows a small curvature anomalies originating from diagenetic pb additions and 222rn loss may not always be recognized

dry annealing of metamict zircon a differential scanning calorimetry study we report the results of a differential scanning calorimeter dsc study of the annealing of a metamict sri lankan zircon raman measurements on most chips of the powdered zircon starting material sri lankan zircon wz19 showed no evidence of a crystalline structure whereas a few chips retained residual raman bands typical of highly radiation damaged zircon dsc runs on aliquots of the powdered sample were heated to 850 and 1000 c at rates of 2 and 10 c min and to 1500 c at a rate of 10 c min raman spectroscopy was used to investigate the crystallinity of grains at selected temperature stages exothermal peaks were observed at about 910 and 1260 c during the dsc run to 1500 c the 910 c peak was demonstrated by raman spectroscopy to mark the crystallization of tetragonal zirconia and the exothermic peak at about 1260 c was demonstrated to represent the reaction of zirconia and amorphous silica to form crystalline zircon the degree of crystallinity of these grains was almost identical to that of highly crystalline zircons from recent gem gravels from new south wales a small number of experimental chips from dsc analyses under 1000 c were found to have zircon raman bands that indicated they had undergone partial annealing the present experimental results suggest that reconstitution of amorphous zircon to the crystalline state by dry annealing will rarely occur in terrestrial geological settings even under extreme metamorphic conditions

age and evolution of the lower crust beneath the western churchill province u pb zircon geochronology of kimberlite hosted granulite xenoliths nunavut canada abstract zircon from granulite facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within archean cratons we present the first such records from beneath two archean crustal blocks that were affected by multiple episodes of regional scale proterozoic metamorphism and are located within the western churchill province nunavut canada zircon u pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption as confirmed with raman spectroscopy zircon was recovered from four metabasites grt cpx plg from the rankin inlet area of the chesterfield block and five metabasites grt cpx plg hbl opx and three metatonalite anorthosites plg cpx plg hbl opx from the repulse bay area of the rae craton sem cl imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization the latter are related to local high temperature recrystallization and micro fracturing and are spatially associated with zones of apparent pb loss and or intracrystalline pb variability evidence for the formation of paleoarchean mesoarchean to neoarchean crust is preserved in oscillatory zoned igneous cores having 207pb 206pb ages of ca 3 5 ga and ca 3 0 2 6 ga two periods of paleoproterozoic metamorphism and zircon growth followed with the oldest episode at ca 2 0 1 9 ga in the chesterfield block metamorphic zircon rims from all twelve xenoliths have an age range of ca 1 75 to 1 70 ga the trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores with the rims characterized by lower hree contents and low lu gd n suggesting that garnet crystallization occurred syn to pre rim growth at ca 1 75 ga the archean and paleoproterozoic ages correspond broadly to the upper crustal histories of the western churchill province however the prevalence of ca 1 75 1 70 ga metamorphic rims indicates regional scale metamorphic reworking of the lower crust across the region during a period of intracratonic extension and associated magmatism regional magmatic underplating and or remobilization of the lithosphere at ca 1 75 1 70 ga to an extent not widely appreciated represents a terminal stage in western churchill cratonization including stabilization of reworked archean crust relative to underlying diamondiferous mantle

raman micro spectroscopy on diamond graphite and other carbon polymorphs from the ultrahigh pressure metamorphic kimi complex of the rhodope metamorphic province ne greece raman micro spectroscopy was applied on carbon inclusions in garnet porphyroblasts from kyanite biotite garnet schists of the rhodope metamorphic province rmp ne greece diamond and cuboids of poorly to highly ordered graphite were identified either as single phase inclusions or as polyphase inclusions along with co2 and or carbonates calcite magnesian calcite questionable raman bands that may be assigned to other c phases nanodiamond lonsdaleite a different c polymorph have been observed the presence of diamond confirms beyond any doubt the ultrahigh pressure uhp metamorphism reported by mposkos and kostopoulos 1 e mposkos d kostopoulos diamond former coesite and supersilisic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established earth planet sci lett 192 2001 497 506 in the rmp cuboid graphite showing variable degree of disordering most probably formed after diamond the possible involvement of co2 and or c o h fluids in the formation of diamond is discussed

hadean age for a post magma ocean zircon confirmed by atom probe tomography the only physical evidence from the earliest phases of earth s evolution comes from zircons ancient mineral grains that can be dated using the u th pb geochronometer 1 oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established 2 3 chemical homogenization of earth s crust and the existence of a magma ocean have not been dated directly but must have occurred earlier 4 however the accuracy of the u pbzirconagescanplausiblybebiasedbypoorlyunderstood processes of intracrystalline pb mobility 5 7 here we use atomprobe tomography 8 to identify and map individual atoms in the oldest concordant grain from earth a 4 4 gyr old hadean zircon with a high temperature overgrowth that formed about 1gyrafterthemineral score isolatednanoclusters measuring about 10nm and spaced 10 50nm apart are enriched in incompatible elements including radiogenic pb with unusually high 207 pb 206 pbratios wedemonstratethatthelengthscales of these clusters make u pb age biasing impossible and that they formed during the later reheating event our tomography data thereby confirm that any mixing event of the silicate earth must have occurred before 4 4gyr ago consistent with

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

morphology internal composition and origin of drumlins in the southeastern part of the chelmno dobrzyn lakeland north poland abstract the sedimentology of four drumlins in north poland formed by complex basal ice flow along elongated glacial depressions in the southeastern part of the chelmno dobrzyn lakeland is examined in detail field data indicate that they developed initially by deformation and erosion of a variety of subglacial bed materials including till glaciofluvial and glaciolacustrine sediments with subsequent formation of proto drumlin nuclei models for formation of these nuclei due to complex pore water pressure perturbations on high and low permeability glacier beds are presented the proto drumlin nuclei that developed became relatively stable obstacles resistant to the shear stress imposed by overriding basal ice stabilised nuclei acted as points where till accumulation commenced proceeding by accretion of basal debris episodic erosion by subglacial meltwater produced discontinuous sandy intercalations within the accreting till beds

closure to piezocone penetration testing in atlantic piedmont residuum by kimberly a finke paul w mayne and rick a klopp

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

dating terrestrial impact structures u pb depth profiles and u th he ages of zircon we investigate the presence of epitaxial overgrowth rims and reset zircon complete loss of radiogenic lead pb from terrestrial impactites to constrain the occurrence of such phenomenon in impact environments and their possible use in dating impact events we also explore u th he dating of zircon to evaluate this geochronometer in accurately identifying impact ages particularly when no dateable melt sheet exists our results show that u th he ages of zircon from the brecciated and presumably shocked target can accurately date an impact event and provides another tool to determine impact ages when no melt sheet exists an alternative to problematic interpretations of commonly used apparent 40ar 39ar plateau ages no evidence of epitaxial overgrowth rims and or reset zircon was observed suggesting that zircons within shocked impactites have remarkably slow pb diffusion and possibly explaining the relatively few reset grains reported in terrestrial impactites

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes